Download Potential for Life on the Terrestrial Planets

1
CME activity of low mass M stars as an important
factor for the habitability of terrestrial
exoplanets, Part II: CME induced ion pick up
of Earth-like exoplanets in close-in habitable zones
Helmut Lammer1, Herbert I. M. Lichtenegger1, Yuri N. Kulikov2,
Jean-Mathias Grießmeier3, N. Terada4, Nikolai V. Erkaev5, Helfried K. Biernat1,6,
Maxim L. Khodachenko1, Ignasi Ribas7,
Thomas Penz1,6, Franck Selsis8
1
Space Research Institute, Austrian Academy of Sciences,
Schmiedlstr. 6, A-8042 Graz, Austria
( [email protected]; [email protected];
[email protected]; [email protected])
2
Polar Geophysical Institute (PGI), Russian Academy of Sciences,
Khalturina Str. 15, Murmansk, 183010, Russian Federation
([email protected])
3
Institute for Theoretical Physics, Technical University of Braunschweig,
Mendelssohnstrasse 3, D-38106 Braunschweig, Germany
([email protected])
National Institute of Information and Communications Technology,
4-2-1 Nukui-Kitamachi, Koganei, Tokyo,
and CREST, Japan Science and Technology Agency, Saitama, Japan
5
Institute of Computational Modelling, Russian Academy of Sciences,
Krasnoyarsk, Russian Federation
([email protected])
6
Institute for Physics, University of Graz, Universitätsplatz 5, A-8010 Graz, Austria
7
Institut d’Estudis Espacials de Catalunya (IEEC)
and Instituto de Ciencias del Espacio (CSIC), E-08034, Barcelona, Spain
([email protected])
8
Centre de Recherche en Astrophysique de Lyon (CRAL),
and Ecole Normale Supérieure (ENS), Lyon, France
([email protected])
Running Title: CME influence on planetary habitability
Corresponding Author:
Helmut Lammer
E-mail: [email protected]
Space Research Institute
Austrian Academy of Sciences
Schmiedlstr. 6, A-8042 Graz
Austria
Submitted to ASTROBIOLOGY
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ABSTRACT
The efficiency of atmospheric erosion of CO2-rich exoplanets, with the size and mass
similar to that of the Earth, due to Coronal Mass Ejection (CME)-induced ion pick up
within close-in habitable zones of active M-type dwarf stars is investigated. Since Mstars are active at the X-ray and EUV radiation wavelengths over long time periods we
have applied a thermal balance model at various XUV flux input values for simulating
the thermospheric heating by photodissociation and ionization processes, due to
exothermic chemical reactions and cooling by the CO2 IR radiation in the 15 µm band.
Our study shows that intense XUV radiation of active M-stars results in atmospheric
expansion and extended exospheres. Using thermospheric neutral and ion densities
calculated for various XUV fluxes, we applied a numerical test particle model for
simulation of atmospheric ion pick up loss from an extended exosphere arising from its
interaction with expected minimum and maximum CME plasma flows. Our results
indicate that the Earth-like exoplanets having no, or weak magnetic moments can lose
tens to hundreds of bars of atmospheric pressure, or even their whole atmospheres due to
the CME-induced O+ ion pick up at orbital distances ≤ 0.2 AU. We have found that
atmospheres with CO2/N2 mixing ratios lower then 96% and exposed to intense XUV
fluxes result in higher exospheric temperatures and more expanded thermosphereexosphere environments and, hence, suffer stronger atmospheric erosion, which may
result in the total loss of several hundred bars even if an exoplanet is protected by a
“magnetic shield” with its boundary located at one Earth radius above the surface.
Furthermore, our study indicates that magnetic moments of tidally locked Earth-like
exoplanets are essential for protecting their expanded due to intense XUV radiation upper
atmospheres against CME plasma erosion. Therefore, we suggest that larger and more
massive terrestrial type exoplanets may better protect their atmospheres against CMEs,
because the larger cores of such exoplanets would generate stronger magnetic moments
and their higher gravitational acceleration would constrain the expansion of their
thermosphere-exosphere regions and reduce atmospheric escape.
Keywords: stellar activity, CMEs, low mass stars, habitability, Earth-like exoplanets,
terrestrial planet finding missions
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1. INTRODUCTION
The recent discovery of a 7 Earth-masses exoplanet around the M-type dwarf star Gliese
876 by Rivera et al. (2005) showed that the search for terrestrial exoplanets within
circumstellar close-in habitable zones (HZs) will soon become feasible for M-type stars,
for which detection methods are more favorable than for solar-like G stars. The HARPS
telescope resolution of about 0.3 m s-1 for the radial velocity allows the detection of socalled “Super-Earth’s” and permits the determination of the mass of transiting exoplanets
within close-in HZs by assuming that high-resolution spectroscopy could be performed of
the reddish spectra of M stars. In the infrared (IR) (10 microns) and visible (600 nm)
light the star to planet contrast would be 25 and 50 times lower than the Sun to Earth
contrast, allowing faster direct detection with ESAs Darwin and NASAs TPF-C space
observatories.
For stellar masses below 0.6 MSun exoplanets orbiting in the HZ become tidally locked
within the first billion years (e.g., Kasting et al., 1993; Grießmeier et al., 2004;
Grießmeier et al., 2005). Using an energy balance model Haberle et al. (1996) showed
that a pure CO2 atmosphere with a surface pressure of about 150 mbar could be
sufficiently dense to allow the dark side of such an exoplanet to warm up above the
freezing point of CO2. Thus, because of the high temperature contrast between the
substellar point and the planetary nightside, CO2 atmospheres with surface pressures
exceeding 1 – 1.5 bars seem to be needed for supporting liquid water on the nightside of a
tidally locked Earth-like exoplanet to allow atmospheric heat transport from the dayside
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to compensate for radiative cooling on the dark side (Joshi et al., 1997; Joshi 2003; Joshi,
2004).
On the other hand, exoplanets residing within the HZs of M stars are exposed during long
time periods to high ionizing and dissociating radiation (e.g., Haisch et al., 1991; Smith et
al., 2004; Grießmeier et al., 2005; Ribas et al., 2005; Scallo et al., 2006; this issue) as
well as Coronal Mass Ejections (CMEs) from their host stars (e.g., Foing et al., 1989;
Houdebine et al., 1990; 1993a; 1993b; 1996; Ribas et al., 2005; Khodachenko et al.,
2006; this issue), which should result in enhanced atmospheric erosion due to various
thermal and non-thermal escape mechanism. Apart from ionization by X-rays and EUV
radiation (XUV), a neutral atmosphere can be ionized in the absence of an intrinsic
magnetic field by electron impact and charge exchange with CME particles, resulting in a
continuous loss of atmospheric constituents.
On Mars, similar atmospheric erosion
processes are currently studied by Mars Express with the ASPERA-3 particle experiment
(e.g., Lammer et al., 2003 and references therein; Lundin and Barabash, 2004; Lundin et
al., 2004) and will be studied at Venus in 2006 with Venus Express (ASPERA4/VEXMAG) (e.g., Lammer et al., 2006a and references therein).
Since the existence of liquid water on the nightside of a tidally locked exoplanet requires
the presence of a CO2-rich atmosphere with a surface pressure of at least one bar, it is
thus necessary to investigate the stability of planetary atmospheres which are exposed to
the emission of particle and high-energy radiation during the evolutionary stages of their
host stars. As the large majority of stars in the solar neighborhood belong to the M star
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domain (e.g., Scallo et al., 2006; this issue), the question regarding the influence of active
host stars on planetary atmospheres is crucial for terrestrial planet finding missions in
particular and for spreading of life in the Universe in general.
The aim of this paper is to study the possibility whether CO2-rich Earth-like exoplanets
under the influence of intense XUV radiation and CME plasma flows can retain a dense
atmosphere while orbiting within close-in HZs. As a first step, we consider only the nonthermal ion pick up loss process caused by the minimum and maximum CME plasma
flux derived by Khodachenko et al. (2006; this issue). A detailed investigation of other
important factors, like thermal (Jeans or hydrodynamic) escape, atmospheric sputtering,
atmospheric loss due to plasma instabilities, CME particle heating, etc. is beyond the
scope of this work and will be a subject of future studies. In Section 2 we discuss the
heating of planetary thermospheres by ionizing radiation which is connected with the
active phase of the host star. We apply a thermospheric model to a CO2-rich exoplanet
with a size and mass similar to the Earth and determine the thermospheric temperature, as
well as CO2 and oxygen number densities as functions of altitude for various stellar XUV
flux values. Using the calculated temperature, number densities and exobase altitude we
derive in Section 3 the exospheric neutral oxygen density as a function of planetocentric
distance. Further, we calculate ionospheric profiles as functions of various XUV flux
values by using the corresponding neutral density profiles and the ionospheric model of
Shinagawa et al. (1987). Number densities of energetic (hot) oxygen atoms which are
generated by dissociative recombination of O2+ molecular ions are obtained by means of
a two-stream Monte Carlo model of Lammer et al. (2000) and are added to the
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background atmosphere. In Section 4 we study the CME plasma interaction with nonmagnetic and magnetized Earth-like exoplanets using our modeled XUV-dependent
exospheric density profiles together with the minimum and maximum CME plasma
fluxes obtained by Khodachenko et al. (2006; this issue) for the determination of the
atmospheric ion pick up loss rates. These loss rate calculations are based on a numerical
test particle model (Lichtenegger et al., 2002) which includes ionization by the CME
plasma (charge exchange), electron impact and XUV radiation. The O+ ion pick up in a
CO2-rich atmosphere of an Earth-like exoplanet orbiting at a distance of 0.05, 0.1, and
0.2 AU from an M-type dwarf star is considered. Finally, in Section 5 the results of our
study and the implications for planetary habitability are discussed.
2. THERMOSPHERIC HEATING DUE TO X-RAYS AND EUV RADIATION
The principal radiation responsible for the heating of upper planetary atmospheres and
formation of planetary ionospheres from stellar sources is XUV radiation. According to
the thermal properties, an atmosphere of a planet can be divided into a number of regions.
The lowermost part of the atmosphere is the troposphere, where the primary heat source
is the planetary surface and heat is convected upward by turbulent motion, leading to a
convective or nearly adiabatic vertical temperature distribution. Thus, in the troposphere
the vertical temperature gradient ∂T/∂z depends on the planet's acceleration of gravity and
atmospheric composition and is given approximately by the equation ∂T/∂z = -g/cp,
where g is the acceleration of gravity and cp is the specific heat at constant pressure. The
troposphere terminates at the tropopause, where temperature decrease with the adiabatic
lapse rate ceases.
Above the tropopause the temperature distribution is governed
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primarily by radiative rather than convective processes and the temperature decreases
much more slowly (|∂T/∂z| < g/cp) or becomes nearly constant (∂T/∂z ≈ 0). In the
terrestrial stratosphere - the region above the tropopause, the temperature, after being
initially constant, increases with altitude due to the solar ultraviolet (UV) radiation
absorption by O3, reaching a temperature maximum at the stratopause. Above this level
the mesosphere begins, where ∂T/∂z < 0, reaching a temperature minimum at the
mesopause (on Earth at about 85 km altitude), mainly due to the presence of molecules
which provide a heat sink by radiating in the infrared (IR) wavelength range.
Above the mesopause there is the thermosphere, where XUV radiation is absorbed and a
substantial fraction of its energy goes for heating the thermosphere leading to a positive
temperature gradient ∂T/∂z > 0. In the lower thermosphere convection can play an
important role in the transport of heat, while in the upper thermosphere heat is
transported by molecular conduction, leading to an isothermal region (T=const). In the
region, called the exosphere, where the mean free path of the atmospheric species
becomes large and collisions become negligible, light atmospheric constituents whose
thermal velocity exceeds the gravitational escape velocity v∞, can escape from the planet.
The exobase is defined as an altitude level where the mean free path is about equal to the
local scale height H =kT∞ /mg of the gas, with k the Boltzmann constant, T∞ the
temperature at the exobase, g the gravitational acceleration, and m the mass of the main
atmospheric species. The most important heating and cooling processes in the upper
atmosphere of Earth are summarized as follows (e.g., Isakov 1971; Blum et al., 1972;
Dickinson 1972; Chandra and Sinha, 1974; Gridchin et al., 1977; Gordiets et al., 1978;
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Gordiets et al., 1981; Gordiets et al. 1982; Gordiets and Kulikov, 1985; Dickinson 1984;
Dickinson et al. 1987; Crowley, 1991; Bauer and Lammer, 2004):
•
heating due to N2, O2, and O photoionization by solar (or stellar) XUV radiation
(λ ≤ 102.7 nm),
•
heating due to O2 and O3 photodissociation by solar UV-radiation,
•
chemical heating in exothermic reactions with O and O3,
•
neutral gas heat conduction,
•
IR-cooling in the vibrational-rotational bands of CO2, NO, O3, OH, NO+, 14N15N,
CO, O2, etc.,
•
heating and cooling due to contraction and expansion of the thermosphere (to
model the thermosphere diurnal variations),
•
turbulent energy dissipation and heat conduction.
Gordiets et al. (1982) applied a numerical model to calculate the thermal budget of the
Earth’s upper atmosphere in the altitude range of 90 – 500 km. Their model includes the
main energy sources and sinks, such as IR-radiative cooling in the vibrational-rotational
bands of NO, CO2, OH, and O3, as well as heating and cooling arising from dissipation of
turbulent energy and eddy heat transport. The results of their simulations, which are in
agreement with observations (e.g., Jacchia, 1977; Crowley, 1991) revealed that the most
efficient heat source in the Earths’ thermosphere was due to photoionization by XUV
radiation and heating which arises from photodissociation of O2 (e.g., Gordiets et al.,
1982; Hunten, 1993). These thermospheric heating processes are balanced by the main
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cooling processes which include IR radiative cooling in the 1.27 – 63 µm wavelength
range and cooling due to molecular conduction (e.g., Gordiets et al., 1982).
The XUV heating of the Earths’ thermosphere yields an average exospheric temperature
of about 1000 – 1200 K (e.g., Jacchia, 1977; Crowley, 1991), while on a comparable
terrestrial type planet Venus, which is even closer to the Sun but has a 96 % CO2
atmosphere, the average exospheric temperature on the dayside is only about 270 – 290 K
(e.g., Nieman et al., 1979a; 1979b; von Zahn et al., 1980; Hedin et al., 1983). The main
reason for the “cold” Venusian thermospheric-exospheric environment compared to a
much “hotter” upper atmosphere of Earth is cooling by the IR radiation in the 15 µm CO2
fundamental band (e.g., Gordiets and Kulikov, 1985; Bougher et al., 1999; Bougher et al.,
2000; Kulikov et al., 2006).
This indicates that upper atmospheres of terrestrial
exoplanets having higher CO2 partial pressure are better protected from expansion due to
XUV heating and enhanced atmospheric loss arising from it.
Since M stars are active flare stars with fluxes of ionizing and dissociating XUV radiation
of up to about 100 times higher than that of the present Sun (e.g., Haisch et al., 1980;
Schmitt et al., 1995; Audard et al., 2000; Ciaravella et al., 2003; Smith et al., 2004; Ribas
et al., 2005; Khodachenko et al., 2006; this issue; Scallo et al., 2006; this issue), one has
to take into account the effects of thermospheric XUV heating of terrestrial type
exoplanets orbiting these stars at distances lying within the HZ.
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2.1 Numerical modeling of thermospheric heat budgets
The effective heat production QXUV in the upper atmosphere of a planet due to the
incoming XUV radiation and related heating processes is mainly balanced by the
divergence of the conductive heat flux in the thermosphere and the thermal energy loss
LIR per unit volume by emitted IR radiation and cooling owing to contraction and
expansion (e.g., Bauer and Lammer, 2004):
→ →
→
→
 ∂T → → 


+ v n ⋅ ∇ T  + p ∇⋅ v n − ∇⋅  K n ∇ T  = Q XUV − L IR − Lce ,


 ∂t

ρc v 
(1)
→
where ρ is the atmospheric mass density, cV the specific heat at constant volume, v n the
velocity and p pressure of the neutral atmosphere and Kn is the thermal conductivity.
In the present study we use a thermospheric model constructed by Gordiets et al. (1982)
and Gordiets and Kulikov (1985) which has been modified and adapted for a wide range
of the XUV flux values (Kulikov et al., 2006) in a “dry” 96 % CO2-rich atmosphere1 of
an Earth-like exoplanet.
Our thermospheric model solves the 1-D time-dependent
equations of continuity and diffusion (in z-direction), hydrostatic equilibrium and heat
balance, as well as the equations of vibrational kinetics for radiating molecules from the
mesopause level of about 90 km up to the exobase. The model is self-consistent with
respect to the neutral gas temperature and vibrational temperatures of IR-radiating
species and takes into account (Kulikov et al., 2006):
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•
heating due to the CO2, N2, CO, O2 and O photoionization by XUV-radiation
(λ ≤ 102.7 nm),
•
heating due to O2 and O3 photodissociation by solar UV-radiation,
•
chemical heating in exothermic 3-body reactions
O + O + M → O2 + M,
(2)
O + CO + M → CO2 + M,
(3)
O + O2 + M → O3 + M,
(4)
where M are CO2, O2 or CO molecules and O and He atoms. Also the model
includes:
•
neutral gas molecular heat conduction,
•
IR-cooling in the vibrational-rotational bands of CO2 (15 µm), CO, O3, and in the
63 µm O line,
•
turbulent energy dissipation and heat conduction.
The volume heating and cooling rates for the processes included in the simulations and
the heating rates due to photodissociation are discussed in detail by Gordiets et al. (1978;
1979; 1982) and Gordiets and Kulikov (1985). IR emission of CO2 in the 15 µm band is
the major cooling agent in the lower thermospheres of Venus, Earth and Mars (e.g.,
Dickinson 1972; Gordiets et al., 1982; Dickinson 1984; Gordiets and Kulikov, 1985;
Dickinson et al. 1987; Bougher et al., 1999; Bougher et al., 2000; Kulikov et al., 2006).
1
The mixing ratios of the main atmospheric species in our simulations at the mesopause altitude of 90 km
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Fig. 1 shows the modeled temperature profiles in a CO2-rich thermosphere of an Earthlike exoplanet as a function of altitude for various XUV fluxes. The exospheric
temperature on the planet for present-day XUV radiation (1 XUV) is about 290 K and is
in good agreement with the dayside exospheric temperature measured on Venus (e.g.,
Nieman et al., 1979a; 1979b; von Zahn et al., 1980; Hedin et al., 1983). For a 5 times
higher XUV flux our model yields an exospheric temperature of about 470 K and fluxes
of 10, 30, 50, 70, and 100 times higher than today yield exospheric temperatures of about
665, 1670, 3335, 5450 and 8500 K, respectively.
The short horizontal lines in Fig. 1 show the upward movement of the exobase from less
than 200 km altitude (for 1 XUV) to about 2000 km (for 100 XUV) as the XUV flux
increases.
The dotted line in Fig. 1 indicates the blow-off temperature for atomic
hydrogen (e.g., Chamberlain, 1963; Gross, 1972; Bauer and Lammer, 2004). Kulikov et
al. (2006) and Lammer et al. (2006b) showed that by considering a “dry” atmosphere
with lower CO2 and higher N2 mixing ratios during 70 – 100 times higher XUV radiation
periods one can obtain exospheric temperatures of more than 20000 K that can result in
still more expanded upper atmospheres and high Jeans loss rates even for heavy species
like oxygen, nitrogen and carbon atoms. The importance of a high CO2 abundance for
the survival of a young terrestrial type planetary atmosphere during its evolution thus
becomes evident.
are: CO2 = 0.9597, N2 = 0.0224, Ar = 0.0159, O2 = 0.0015, H2O= 1.46 × 10-7.
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3 EXOSPHERIC NUMBER DENSITY PROFILES
It is known from spacecraft observations and model simulations that dissociative
recombination of ionospheric O2+ ions is an important source of suprathermal atomic
oxygen in the exospheres of Venus and Mars (e.g., McElroy et al., 1982; Nagy et al.,
1981; Rodriguez et al., 1984; Ip, 1988; Lammer and Bauer, 1991; Zhang et al., 1993a;
Fox and Hac, 1997; Luhmann et al., 1997; Kim et al., 1998; Lammer et al., 2000;
Lammer et al., 2003; Lammer et al., 2006a). At present Venus and Mars these excited
products, O(3P), O(1D), O(1S), that is energetic atoms (O*), can reach much higher
altitudes than the “cold” background gas and are thus the main species in the exosphere.
For the ion pick up simulations the neutral gas number density of the main atmospheric
species is needed up to altitudes of several planetary radii, that is high above the exobase.
Since the exospheric number density of a constituent depends on the temperature and its
density at the exobase, these values must be known for exospheric density profiles
calculation. While the neutral gas temperature can be deduced from our thermospheric
model, the exobase density of the hot oxygen atoms should be determined by considering
the ionospheric recombination and excitation processes for the exospheric species.
For this we consider the four possible channels, through which the oxygen atoms can be
formed in the 3P, 1S and 1D states (e.g., Nagy and Cravens,1988; Fox and Hać, 1997; Kim
et al., 1998)
O2+ + e → O(3P) + O(3P)
∆E = 6.96 eV,
(5)
O2+ + e → O(3P) + O(1D)
∆E = 5.00 eV,
(6)
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O2+ + e → O(1D) + O(1D)
∆E = 3.02 eV,
(7)
O2+ + e → O(1D) + O(1S)
∆E = 0.80 eV,
(8)
and use the branching ratios for dissociative recombination of oxygen atoms by Kella et
al. (1997), O(3P) + O(3P) : O(3P) + O(1D) : O(1D) + O(1D) : O(1D) + O(1S) = 0.22 : 0.42 :
0.31 : 0.05.
The calculation of the O2+ ion density profiles as a function of various XUV flux values
in our study is based on the ionospheric model of Shinagawa et al. (1987), in which the
rate coefficients for chemical reactions have been updated with the data of Fox and Sung
(2001). Once the O2+ ion density is known, the energy density distribution of the O*
atoms at the exobase level is obtained by means of the Monte Carlo model of Lammer et
al. (2000). In this model, the collision probability, particle direction and energy loss after
each collision between a newly generated O* atom and the background gas particle is
simulated by generating random numbers. The produced O* atoms are assumed to
become eventually thermalized through a series of elastic hard sphere collisions with the
main background gases such as CO2 or atomic oxygen. Inelastic collision probabilities
are negligibly small at these low energies. After its release each hot O* atom may collide
with the neutral background gas particles, may change its direction, lose its energy, or
may travel long distances in the atmosphere without collisions. Those newly generated
O* atoms that move upwards are traced up to the exobase altitude where their
corresponding energy density distribution function is calculated.
As it is well known, the barometric law breaks down above the exobase and in the
collisionless exosphere the velocity distribution of the escaping high velocity particles is
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non-Maxwellian. For high exospheric temperatures the exosphere is expanding and the
matter is lost through a thermal process, corresponding to evaporative loss in the kinetic
theory (Chamberlain, 1963).
By using Liouville’s equation the exospheric number
density n as a function of a planetocentric distance r can be written as the product of the
barometric density and the sum of the partition functions corresponding to escaping ξesc,
ballistic ξbal, and satellite particle trajectories ξsat (e.g., Chamberlain, 1963; Bauer and
Lammer, 2004)
n( r ) = n c EXP − ( z / H )[ξ bal ( X c , X ) + ξ esc ( X c , X ) + ξ sat ( X c , X )],
(9)
where nC is the exobase density, Xc is the escape parameter at the exobase level r = rc and
X is given by
X (r ) =
GmM Pl
.
kT∞ r
(10)
Here G is the gravitational constant, MPl the planetary mass, m the particle mass, k the
Boltzmann constant, and T∞ the exospheric temperature. The ballistic particle trajectories
fraction is calculated from the following equation
(
)
η=
X2
,
X + Xc
2
2
2  3
 Xc − X
3

ξ bal ( X c , X ) = 1/ 2 γ  , X  −
EXP (− η )γ  , X − η  ,
π  2 
Xc
2

(11)
where,
(12)
16
3 
and γ  , X  is the incomplete Γ function
2 
3
2
3


(13)
−1
γ  , X  = ∫ η 2 EXP (− η )dη.
The fraction of the atmospheric particles in satellite orbits can be calculated from
ξ sat ( X c , X ) =
2
π 1/ 2
(X
2
c
−X2
Xc
)
1/ 2
 3

EXP (− η )γ  , X − η ,

 2
(14)
3

where γ  , X − η  is
2

3

γ  , X −η  =
2

X −η
∫η
3
−1
2
EXP(− η )dη.
(15)
0
Finally, the partition function of the atmospheric particles with escaping trajectories is
(
1   3   3  X c2 − X 2
ξ esc ( X c , X ) = 1/ 2 Γ  − γ  , X  −
π   2   2 
Xc
)
1/ 2
  3  3
 
γ
,
X
η
−
−
Γ
 .



 2
 
   2
(16)
Given the XUV flux related density distributions and temperatures of the O and O* atoms
at the exobase level, the corresponding exospheric density distributions are obtained from
eqs. (9) – (16).
Fig. 2 shows the sum of the cold and hot oxygen number densities for various solar XUV
fluxes. As can be seen, the density contribution of O* atoms is primarily important for
the XUV flux values less than 50 times that of the present Sun. Due to the rise of the
exobase level (see Fig. 1) from 600 km at 50 XUV up to higher altitudes for larger XUV
17
fluxes the newly generated O* atoms collide many times on their way up to this elevated
exobase altitude and eventually become thermalized and incorporated into the main
background gas.
In the following Section we study the effect of these extended
exospheres on the non-thermal ion pick up loss process caused by the CME plasma flow.
4. ION PICK UP SIMULATIONS
4.1 Planetary obstacles
The following four kinds of planetary obstacles for the solar wind plasma flow are found
in the solar system (e.g., Bauer and Lammer, 2004):
•
Earth-like, as shown in Fig. 3. The atmosphere/exosphere environment is
protected from solar wind by a strong intrinsic magnetic field, which forms an
extended magnetosphere which balances the solar wind plasma flux at a
magnetospheric stand-off distance, the so-called magnetopause. For Earth the
magnetopause distance
RMP
 µ f 2Μ 2 
= 20 0

 8π ( ρv) sw,CME 
1/ 6
,
(17)
is located at about 10 Earth radii, where µ0 is the magnetic permeability, f0 = 1.16
a form factor for the magnetosphere (Voigt, 1995), M is the magnetic moment of
the planet, (ρv)sw,CME the mass flux of the deflected plasma flow (solar wind,
stellar wind or CMEs).
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•
Venus-like, illustrated in Fig. 4. The ionospheres of non-magnetic planets like
Venus and Mars are also able to deflect the solar wind plasma stream around
them. The boundary between the solar wind and the ionosphere is called the
ionopause and is determined by the pressure balance between the solar wind and
the ionospheric plasma. The subsolar ionopause distances RIP at Venus and Mars
are observed at altitudes of about 250 - 300 km. Variation of the ionopause
altitude is related to the variation of the solar wind mass flux (ρv)sw. On both
planets, RIP is located close but above the exobase altitude rc of about 200 – 240
km.
•
Titan-like. We know from the Voyager flyby (e.g., Neubauer et al., 2004) and
Cassini flybys (e.g., Waite et al., 2005) that Saturn’s corotating subsonic
magnetospheric plasma is deflected around Titan’s upper atmosphere at an
altitude below the exobase. The ionospheric pressure at Titan’s exobase altitude
(1450 km) is too weak to balance the incoming plasma flow from Saturn’s
magnetosphere and the ionopause is formed due to ionization of the collision
dominated neutral upper thermosphere at a distance of about RIP ∼ 1000 – 1200
km, i.e., below the exobase. In general, the atmospheric interaction with the
plasma flow past Titan is comparable to the Venus-case illustrated in Fig. 4, but
more neutral gas can be picked up from the upper atmosphere.
19
•
Moon-like. Bodies without any atmosphere like the Moon, Mercury or several
atmosphere-lacking satellites and asteroids interact with the solar wind plasma
directly at their surface. As a result they build thin exospheres due to surface
sputtering by the interacting solar wind plasma particles (e.g., Wurz and Lammer,
2003; Millilo et al., 2005).
The compression of exoplanetary magnetospheres by the stellar wind has been studied by
Grießmeier et al. (2004) and Grießmeier et al. (2005). Similarly, Khodachenko et al.
(2006; this issue) showed that the high plasma densities expected due to stellar CMEs can
strongly compress the weak magnetospheres of tidally locked Earth-like exoplanets
orbiting within close-in HZs of low mass M stars. Therefore, a substantial part of a
possible atmosphere on these planets may extend beyond the magnetosphere and will
thus be directly exposed to the CMEs and picked up by the plasma flow as illustrated in
Fig. 5.
4.2 Expected magnetic moments of Earth-like exoplanets within close-in habitable
zones
As shown by Grießmeier et al. (2005) and Khodachenko et al. (2006; this issue) the
expected magnetic moments of slow rotating Earth-like exoplanets orbiting close to their
host stars are much weaker due to tidal locking compared to a fast rotating planet like the
Earth at 1 AU. By using the scaling relations of Grießmeier et al. (2004) and Grießmeier
et al. (2005) we obtain possible intrinsic magnetic moments M for a tidally locked
exoplanet (with the size and mass of Earth) for a dwarf star with 0.5 MSun at an orbital
20
distance of 0.05 AU between 0.17 – 0.42, at 0.1 AU about 0.061 – 0.25, and at 0.2 AU
about 0.022 – 0.15 times that of the present Earth.
By using Eq. (17) and the minimum and maximum CME plasma parameters considered
in our study, one can calculate magnetopause distances as a function of magnetic
moments M.. We should note that the present M value for Earth is high enough to produce
a subsolar magnetopause distance RMP of about 10 Earth radii above the surface. Table 1
shows the required magnetic moments M for substellar RMp distances above the surface
for CME exposed Earth-like exoplanets at orbital distances of 0.05 AU, 0.1 AU and 0.2
AU. One can see that for weak CMEs (CMEmin) at 0.05 AU, an M value of about 0.25
times that of the present Earth will produce a standoff distance of about 1 Earth-radius.
For strong CMEs (CMEmax) the required M has to be about 2 times that of the present
Earth, which is too large and can not, therefore, be generated by a tidally locked Earthsize and mass exoplanet. At an orbital distance of about 0.1 AU the largest expected M
value could produce a substellar magnetopause radius RMp for weak CMEs only slightly
larger than 1 Earth-radius above the planetary surface. For strong CMEs and the largest
expected magnetic moment M at 0.1 AU the RMp value remains below 1 Earth-radius
above the planetary surface. At 0.2 AU the largest expected M can produce a magnetized
planetary obstacle for weak CME plasma fluxes close to 2 Earth radii above the surface
and for strong CMEs at distances less then 1 Earth radius.
21
In our ion pick up investigations we consider a CO2-rich Earth-like exoplanet at orbital
distances of 0.05 AU, 0.1 AU and 0.2 AU, respectively, and assume the following three
cases for planetary obstacles:
•
Case I: a weakly-magnetic (Fig. 5) or non-magnetic (Fig. 4) “Venus-“ or
“Titan”-like planetary obstacle, where we assume the XUV flux dependent
exobase altitude as the planetary obstacle boundary location. Note that we take
for the weakly magnetic exoplanets the exobase altitudes shown in Fig. 2 as the
planetary obstacles but will also compare the loss rates with an ionopause
obstacle similar than observed at Venus.
•
Case II: an intrinsic magnetic moment is strong enough to sustain a
magnetopause at 0.5 Earth-radii above the planetary surface.
•
Case III: an intrinsic magnetic moment is strong enough to sustain a
magnetopause at 1 Earth-radius above the planetary surface.
Neutral atoms and molecules above these obstacles (Figs. 4 and 5) can be transformed
into ions by XUV radiation, electron impact and charge exchange with the CME
particles (protons). These newly produced ions are picked up by the solar wind and
assumed to be lost from the planet.
4.3 Test particle model
For studying the ion pick up loss rates of ionized oxygen atoms above the defined
22
planetary obstacles, we use the gasdynamic model of Spreiter and Stahara (1980) to
calculate the plasma flow around the magnetopause/ionopause. The total loss rate is
obtained by calculating the oxygen ion production rate along the streamlines
(Lichtenegger and Dubinin, 1998; Lichtenegger et al., 2002). This model was also
successfully used to explain several characteristic features obtained by Pioneer Venus
(Luhmann, 1993; Lammer et al., 2006a) and by the Phobos 2 plasma measurements at
Mars (Lichtenegger et al., 1995; Lichtenegger and Dubinin, 1998).
Preusse et al. (2005) investigated stellar wind regimes of close-in exoplanets and found
that the stellar wind regimes at distances < 0.1 AU differ considerably from those for
planets at larger orbital distances in the Solar System. Their results indicate that in
contrast to our planets in the Solar System, some close-in exoplanets may build obstacles
in a sub-Alfvénic stellar wind plasma flow. This finding is in agreement with the
conclusion of Erkaev et al. (2005) that no bow shock like at Venus or other solar system
planets, which are exposed to the supersonic solar wind will form.
However, we do not consider the atmospheric interaction with the ordinary stellar wind,
which is less dense and much slower at orbital distances considered in our study
compared to the dense and fast CME plasma flow. As shown by Khodachenko et al.
(2006; this issue), Earth-like exoplanets within close-in HZs of M stars may be
permanently exposed to the dense and fast CME plasma flux. Because the average CME
velocity at orbital distances < 0.1 AU is about 490 km s-1, which is comparable to the
present solar wind velocity at 1 AU, a bow shock should form like at Venus and after the
23
shock the CME plasma will be deflected around the planetary obstacle (Erkaev et al.,
1995).
We apply our model in the present study only to ionized oxygen atoms, which is the main
exospheric species in our modelled CO2 atmospheres. The total production rate of O+
ions is the sum of the rates of the main ionization processes, i.e.,
•
photo-ionization by various XUV fluxes,
•
electron impact ionization,
•
charge exchange.
For the reaction of CME protons with the planetary O atoms, an energy dependent charge
exchange cross sections have been used (Kallio et al., 1997). To calculate the impact
ionization frequency, which involves both the electron temperature and density, we
assume that the CME electrons behave like the ideal gas of the gasdynamic model and we
approximate the electron temperature by the gasdynamic temperature (Zhang et al.,
1993b). For determination of the photoionization frequency we assume that the exosphere
is optically thin, i.e., we ignore any attenuation of the photon flux.
For the calculation of the ion pick up fluxes we use the total atomic oxygen neutral
number density profiles calculated in Section 3 and shown in Fig. 2. The simulation of
the particle fluxes is initialized by dividing the space around and above the chosen
obstacle of the Earth-like exoplanet into a number of volume elements ∆V. The
production rates of planetary ions are then obtained by first calculating the absorption of
24
the CME plasma flow along streamlines due to charge exchange with exospheric neutral
→
gas. The CME plasma flux ΦCME in a volume element ∆Vi at position ri with respect to
the planetary centre is given by
 si

→
→
0
Φ CME  ri  = Φ CME  ri  EXP  − ∫ nO σ O ds ,


 
 
 ∞

(18)
where the integration is performed from the upstream CME plasma flow to the
→
0
is the unperturbed CME
corresponding point si at position ri on a streamline. Φ CME
plasma flow, nO the density of the neutral species (O atoms) as a function of altitude, σO
is the energy dependent charge exchange cross section between CME protons and the
exospheric O atoms.
The loss rates of CME protons lCME [cm-3 s-1] due to the interaction with exospheric
O atoms can be written as
H
lCME
= Φ CME nOσ O .
(19)
The corresponding planetary O+ ion production rates p due to charge exchange are
H
.
assumed to be equal to the corresponding loss rates of the CME protons, i.e., pOce+ = lCME
The rate of ion production by electron impact is given by pOei+ = νne nO , where ν is the
temperature dependent ionization frequency per incident electron and ne the electron
density. The total pick up ion production rate for O+ ions pOtot+ due charge exchange with
25
the CME plasma flow, electron impact ionization and photoionization and can be written
as
(20)
pOtot+ = pOce+ + pOei+ + pOλ + ,
where pOλ + is the production rate of planetary ions generated by the XUV radiation. To
determine the flux of the O+ ions from each volume element, a test particle, which is
considered to represent all particles in the volume, is launched and its trajectory followed
by integrating the equation of motion
→
d v q  → → →
=  E + v × B ,
dt
m

21)
→
→
where q is the particle charge, m is the particle mass, v is the particle velocity, B , is the
→
→
→
magnetic field and E = − v × B is the motion-induced electric field. We assume that the
magnetic field is frozen into the CME-plasma flow. The total O+ pick up ion flux
through an area ∆A, which originates inside the volume element ∆V (i ) , finally becomes
Φ
(i )
O+
=
p Otot+ ∆V ( i )
∆A
,
(22)
where pOtot+ has to be taken at the point of the particle’s origin. We perform the
calculation of the exospheric O+ pick up ion fluxes for minimum and maximum CME
plasma densities as a function of orbital distances (see Fig. 4: Khodachenko et al., 2006)
and for average CME velocities (Khodachenko et al., 2006 and references therein).
26
5. CME INDUCED ATMOSPHERIC EROSION DUE TO ION PICK UP AT
EARTH-LIKE EXOPLANETS ORBITING CLOSE-IN HABITABLE ZONES
As has been pointed out above, we consider in our simulations CO2-rich Earth-like
exoplanets orbiting within the HZs of M stars at 0.05 AU, 0.1 AU and 0.2 AU. We apply
our test particle model to the three planetary obstacle cases discussed above and use our
calculated exospheric profiles shown in Fig. 2. The minimum and maximum CME
plasma densities are taken according to Khodachenko et al. (2006; this issue) and
correspond to about 104 cm-3 and 7 × 104 cm-3 at 0.05 AU, to about 103 cm-3 and 7 × 103
cm-3 at 0.1 AU and to about 200 cm-3 and 1000 cm-3 at 0.2 AU, respectively. We use in
our simulations the average CME velocity observed for our Sun by the Solar and
Heliospheric Observatory (SoHO) of 490 km s-1 (see Khodachenko et al. 2006 and
references therein; this issue).
5.1 Model results
For the above input parameters Fig. 6 shows the results of our O+ ion pick up simulations
for a CO2-rich Earth-like exoplanet orbiting an M star within its HZ at a distance of 0.2
AU (Fig. 6a) and 0.1 AU (Fig. 6b) from the star. The accumulated total atmospheric
oxygen loss (in bars of atmospheric pressure) due to the CME plasma-atmosphere
interaction is presented as a function of time starting with 10 Myr after the star’s arrival
to the ZAMS for the three typical cases of a planetary obstacle described in the previous
section. The results are presented for the stellar XUV flux which is 70 times more than
the present solar flux value (XUVSUN). For convenience of analysis of oxygen loss in
27
different environments it is assumed that the initial reservoir of atmospheric oxygen on
the exoplanet is unlimited.
It can be clearly seen from Fig. 6a that the calculated atomic oxygen total loss due to ion
pick up for a specified orbital distance and XUV flux shows a strong dependence on the
planetary magnetic moment and CME plasma density. For an unmagnetized or weakly
magnetized exoplanet (case I) the loss is the largest, amounting after one Gyr continuous
CME exposure to several hundred bars for the maximum CME plasma density and to a
few tens of bars for the minimum CME plasma density. The effect of the CME plasma
density variation from the minimum to maximum value on the loss rate is also the
strongest for the case I planetary obstacle, being more than an order of magnitude (Fig.
6a). One can also see that CO2-rich Earth-like exoplanets with very weak magnetic
moments (exobase altitude is assumed as the magnetospheric stand-off distance) can lose
several tens of bars. On the other hand, the total atmospheric pick up loss from an
exoplanet having a substantial magnetic moment is greatly reduced due to magnetosphere
protection from the CME atmospheric erosion. Even for a relatively weakly magnetized
exoplanet having a magnetopause at 0.5 Earth’s radii above its surface (case II) the total
loss is reduced by more than 2 orders of magnitude as compared to the case I. For a
stronger magnetized exoplanet (case III) with a magnetopause located at 1 Earth radius
above the surface the total loss is reduced still further when compared with case I. From
a comparison of Fig. 6a and Fig. 6b one can see that when an exoplanet is placed closer
to its parent star (from 0.2 AU to 0.1 AU), the total loss due to the CME ion pick up from
a planet that has a non-negligible magnetic moment (cases II and III) grows higher, quite
28
as expected. Because of this the ratio of the atmospheric oxygen pressure lost from a
very weak or unmagnetized exoplanet at 0.2 AU to that lost from a magnetized one is
reduced by about an order of magnitude when a planet is placed at a closer distance of 0.1
AU.
Fig. 7 shows the atmospheric loss rates (in bar/Gyr) due to the CME erosion as a function
of M star’s XUV flux calculated for a CO2-rich Earth-like exoplanet orbiting at a distance
of 0.2 AU and for the considered planetary obstacle types (cases I-III). The panels (a) –
(c) show the results for the minimum (dotted-lines) and maximum (dashed-lines) CME
plasma densities, accordingly. One can see that the atmospheric CME induced ion pick
up loss rate, apart from the exoplanet’s magnetic momentum, quite dramatically depends
on the star’s XUV radiation flux. For the maximum XUV flux of 100 times that of the
present solar value considered in our study (100 XUVSun) and the minimum CME plasma
density the loss rate of about 10 bars per Gyr from a very weak magnetized exoplanet
(planetary obstacle: exobase altitude - panel a) and about bars per Gyr from an Earth-like
exoplanet having a non-zero magnetic moment (cases II and III) has been found (panels b
and c). If we assume that an Earth or Venus-like planet’s initial CO2 inventory was of the
order of 100 bars (as is the case for the present Venus and Earth) and the atmosphere was
exposed to the XUV flux that was 100 times that of the present Sun during 1 Gyr, then
for an unmagnetized or very weakly magnetized planet (case) its atmosphere could be
totally destroyed by the maximum CME plasma density flux (panel a). One can also see
that even a magnetized exoplanet with a magnetopause at 0.5 Earth radii above its surface
(case II, panel b) could lose 10 bars per Gyr. The strong rise in mass loss in case III
29
(panel c) for XUV flux values > 50 time that of the present Sun occurs due to the more
effective expansion of the heated upper atmosphere. CO2 atmospheres of magnetized
exoplanets (cases II and III) exposed at 0.2 AU to XUV fluxes ≤ 100 XUVSun and
minimum CME plasma density have a better chance to survive the expected CME
induced ion pick up erosion during a 1 Gyr period.
The loss rates due to the CME induced ion pick up atmospheric erosion as functions of
XUV radiation flux calculated for a closer orbital distance of 0.05 AU are shown in Figs.
8 and 9. Fig. 8 shows the time dependent loss rates over 1 Gyr as a function of minimum
(dashed lines) and maximum (dotted lines) expected atmospheric ion pick up mass loss
rates in units of bar as a function of XUV flux values for minimum (panel a) and
maximum (panel b) CME plasma flux and orbital distance at 0.05 AU, for a weakly
(dotted-lines: planetary obstacle at exobase altitude) and non-magnetized (dashed-lines:
planetary obstacle at present Venus ionopause at an altitude of about 300 km) CO2-rich
Earth-like exoplanets.
Fig. 9 show the loss rate for maximum (dashed lines) and minimum (dotted lines) CME
plasma flux for the similar but magnetized exoplanet for case II (panel a) and case III
(panel b). One can see that a very weak or non-magnetic exoplanet orbiting around an
M-star within its HZ (cases I, Fig. 8) suffers high atmospheric loss from XUV flux values
> 10 times that of the Sun. Even for relatively low M-star XUV radiation flux levels (∼
10 XUVSun) (e.g., Scalo et al., 2006; this issue) the atmospheric loss rates are of the order
of 1 bar/Gyr for the minimum CME plasma density (Figs. 8a) and about 10 bar/Gyr for
30
the maximum CME plasma density (Figs. 8b). These results also demonstrate a strong
CME plasma density effect on the atmospheric loss rate from an Earth or Venus-like
exoplanet. For high XUV fluxes (∼ 100 XUVSun) a case I Venus-like exoplanet at 0.05
AU may lose during 1 Gyr from several 100 bars of atmospheric pressure for the
minimum CME plasma density (Fig. 8a) up to the order of 104 bars for the maximum
CME plasma density (Fig. 8b). So, an unmagnetized Venus-like exoplanet orbiting an
M-star within its HZ will most likely lose its whole atmosphere due to the CME induced
ion pick up in less than 1 Gyr.
Our results shown in Fig. 9 indicate that magnetic moments which are expected at tidally
locked Earth-like exoplanets at 0.05 AU will not protect the atmosphere of its planet for
XUV fluxes ≥ 80 times than that of the Sun. A case II exoplanet may lose atmospheric
mass for 80 to 100 times XUVSun between tens and hundreds of bars over 1 Gyr CME
exposure (panel a). Depending on the CME plasma flux, even an exoplanet with a
magnetic moment which is strong enough to produce a magnetopause stand-off distance
at about 1 Earth-radius above its surface can lose for these XUV values atmospheric mass
from several bars up to 300 bars (panel b).
The results presented in Figs. 7 - 9 clearly demonstrate that higher XUV fluxes have a
very strong effect on the atmospheric loss rate, because due to substantial thermosphereexosphere expansion at high fluxes more neutral gas can be picked up by the CME
plasma flow past the planetary magnetosphere-atmosphere environment. The results also
show that for exoplanets that have a non-zero magnetic moment (cases II and III) the
31
atmospheric loss can decrease by many orders of magnitude due to a strong
magnetosphere protection effect.
Our simulations resulted in greatly reduced
atmospheric loss rates from magnetized exoplanets shielded by their magnetospheres
against the destructive CME plasma flow for low and moderately high XUV flux levels
of their parent stars when compared with non-magnetic exoplanets. However, at high
XUV fluxes (∼100 XUVSUN) this magnetosphere protecting effect becomes weaker and
the resulting loss rates show a dramatic increase.
For the case III exoplanets, for
example, the loss rate can reach from a bar/Gyr for the minimum CME plasma density at
0.2 AU to 300 bar/Gyr for the maximum CME plasma density at 0.05 AU.
Figs. 7 - 9 also show that variations of the planetary orbital distance and CME plasma
density have a strong effect on the loss rates for the three types of Earth-like exoplanets
considered here and, hence, should be taken into account when estimating a possibility to
preserve their atmospheres in the hostile radiation and plasma environment of an M-star.
Our simulations also show that a strongly magnetized Earth-like exoplanets can possibly
preserve its CO2-rich atmosphere during 1 Gyr only if it orbits an M-star at orbital
distances ≥ 0.1 AU and XUV fluxes there are not much higher than ∼ 50-70 XUVSun. For
higher XUV fluxes and long time exposure an M star exoplanet may not keep its
atmosphere for all the cases and may not, therefore, evolve to a habitable world after the
X-ray luminosity saturation period of its parent star is over.
Moreover, the obtained loss rates corresponding to 80 – 100 times higher than XUVSun
flux and permanent long-time CME-atmospheric interaction may yield Mercury-type
exoplanets, although they orbit within the HZs of their host stars. It is of interest to
32
compare our results with a previous study by Lammer et al. (2005), which used like
Michel (1971) and Bauer (1983) a simple mass loading model based on momentum
balance considerations between the stellar wind plasma flux and planetary ions for
atmospheric loss estimations from close-in exoplanets due to CMEs and strong stellar
winds.
Their model neglected the XUV induced expansion of the thermosphere-
exosphere environment and higher ionisation rates due to intense XUV fluxes and
yielded, therefore, atmospheric loss rates, which are too low at orbital distances > 0.05
AU. However, their general conclusion that CMEs may represent a danger for Earth-like
exoplanets within close-in HZs, because they may erode more than 500 bar of
atmospheric gas pressure at orbital distances of about 0.03 AU are also in agreement with
the present study.
By comparing our estimates of possible expected magnetic moments and corresponding
magnetic planetary obstacle distances with the results of our ion pick up study shown in
Figs. 7 – 9, one can conclude that weakly-magnetized Earth-like exoplanets can lose their
whole atmospheres and, therefore, may not evolve into habitable worlds. Our study also
indicates that the most crucial parameter is the XUV flux exposure which results in the
upper atmosphere heating and expanded thermosphere-exosphere environments. As one
can see from Figs. 6a – 6b, XUV fluxes between 70 – 100 times that of the present Sun
can produce expanded exospheres on a planet at 0.05 AU, which would need magnetic
moments several times larger than that of the present unlocked Earth, to produce
magnetic planetary obstacles at atmosphere-protecting distances of more then 5 Earth
radii. Our study also shows that for active M stars with XUV radiation flux of more than
33
90 – 100 times that of the present Sun within their HZs the expected magnetic dynamos
are too weak for the generation of a magnetosphere with a magnetopause distance large
enough for protecting the atmosphere of an CME-exposed Earth-like exoplanet from an
ion pick up induced destruction.
5.3 Implications for planetary habitability
The results of our study, in which we considered only “one” of the atmospheric loss
process, namely, the ion pick up by CMEs, indicate that Earth-like exoplanets orbiting
inside their HZs between 0.05 – 0.2 AU around low mass M stars, having high CO2
mixing ratios (≥ 96 %) and strong magnetic dynamos, may preserve their atmospheres if
they are exposed to XUV fluxes which are less than 50 times that of the present Sun.
However, M stars which emit XUV radiation fluxes higher than 70 - 100 times the
present solar flux may present problems for atmospheric stability due to CME ion pick up
for Earth-like exoplanets. In fact a combination of intense XUV radiation and permanent
CME plasma interaction with extended thermosphere-exosphere regions and expected
weak magnetospheres of tidally locked Earth-like exoplanets can result in high nonthermal atmospheric loss rates of the order of 10 - 100, or even several 1000 of bars.
CO2-rich Earth-like exoplanets which are exposed to lower XUV radiation (≤ 50 times
the present Earth), in order to preserve their atmospheres should have strong magnetic
moments, because exoplanets with Venus-like plasma interaction may also lose their
atmospheres due to intense CME exposure.
34
One should also note that some other atmospheric loss processes which are not addressed
in this work will have an additional effect on the atmosphere. It is known from model
simulations for Mars and Venus, that atmospheric sputtering caused by solar wind
particles is also an important additional atmospheric loss process (e.g., Luhmann and
Kozyra, 1991; Luhmann et al., 1993; Jakosky et al., 1994; Johnson et al., 1994; Kass and
Yung, 1995; Chassefière, 1997; Leblanc and Johnson, 2001; 2002; Lammer et al., 2003
and references therein). One can expect that the dense CME plasma flow which interacts
with in the upper atmospheres of a terrestrial close-in exoplanet will act as an efficient
sputter agent and may enhance the atmospheric loss rates presented in this study.
Further, the CME plasma deposits a huge amount of energy in the upper atmosphere
which should result in additional thermospheric heating (e.g., Luhmann and Kozyra,
1991; Lammer et al., 1998; Chassefière and Leblanc, 2004) and may also trigger the
Kelvin Helmholtz-type plasma instability which results in additional atmospheric loss
due to so-called ionospheric clouds (Brace et al., 1982; Terada et al., 2002; Penz et al.,
2004).
Moreover, the expected XUV and CME-induced high exospheric temperatures will result
in large thermal escape rates of lighter atmospheric species like hydrogen and helium
(e.g., Hunten et al., 1987; Hunten, 1993; Chassefière, 1996a; 1996b). One should also
note that we assumed a dense Venus-like CO2 atmosphere for our model simulations,
because CO2 is the most efficient cooling species in the IR. Kulikov et al. (2005) and
Lammer et al. (2006b) showed that Earth- or Titan-like nitrogen atmospheres with low
CO2 but higher N2 contents will be much more affected by the XUV heating (see also
35
Scalo et al., 2006; this issue), atmospheric expansion and thermal and non-thermal loss
processes. By considering all these aspects, we conclude that it is very unlikely that
habitable “exo-Earth-twins” (of the same mass and size, atmospheric composition as the
present Earth, etc.) may be found within the HZs of M-type dwarf stars.
However, future studies should focus on larger and more massive “Super-Earths” or
“ocean planets” (Léger et al., 2004) inside close-in HZs of M dwarfs. Such exoplanets
with sizes of about 2 Earth-radii will have masses of about 10 times that of the Earth.
These exoplanets should have larger cores, which will result in stronger magnetic
moments compared to Earth-size exoplanets having lower CO2 mixing ratios, and their
higher mass will produce much less extended thermosphere-exosphere regions. Both,
stronger magnetic moments and less extended upper atmospheres would reduce the
damaging effect of the CME plasma bombardment of their exospheres compared to
Earth-size and mass exoplanets. Also, other atmospheric loss processes should be less
efficient for larger and heavier exoplanets.
6. CONCLUSION
High XUV radiation of an active dwarf star (more than about 50 XUVSun) results in
considerable expansion of the upper atmosphere of an Earth-like exoplanet orbiting it.
An unmagnetized exoplanet or an exoplanet having a weak intrinsic magnetic moment
and exposed to high XUV fluxes and CME impacts with the maximum estimated plasma
density during 1 Gyr period is in a real danger of being stripped of its whole atmosphere
even if it orbits its parent M star within a habitable zone at 0.2 AU.
36
High CO2 atmospheric mixing ratio results in enhanced IR cooling and inhibited
expansion of an atmosphere and, therefore, it leads to reduced non-thermal atmospheric
erosion due to CMEs. However, if an Earth-like exoplanet has no substantial magnetic
moment, its atmosphere has no chances to survive CME induced erosion under high
XUV radiation exposure of a M star even if its atmosphere has a high CO2 mixing ratio.
On the other hand, if an Earth-like exoplanet can generate a strong enough magnetic field
and has a high CO2 mixing ratio, its atmosphere may survive CME induced erosion if
XUV fluxes are not higher than about 50 − 70 times than that of the present Sun. For
XUV ≥ 70 XUVSun even for strongly magnetized CO2-rich planets the incident CME
plasma flows result in O+ ion pick up loss rates of more than several hundred or even
thousand bars of atmospheric pressure within a HZ < 0.05 AU. In such cases presently
estimated maximum possible planetary magnetic moments are too weak to protect
extended thermosphere-exosphere environments from total destruction by the CME
plasma.
Earth-type atmospheres having high nitrogen contents and little CO2 are in real danger of
rapid destruction due to their extensive expansion and resulting high loss rates. An
Earth-like atmosphere with high N2 and low CO2 mixing ratio can withstand the erosion
for more than 1 Gyr only if a planet has the strongest possible magnetic moment which
can be expected and XUV fluxes which are not higher than about 7 times that of the
present time Sun. However, such low XUV fluxes are not expected for M stars during
the early phases of their evolution (see Scalo et al., 2006; this issue). Consequently,
37
exoplanets having atmospheres and biospheres “similar” like that of the present Earth
may not be found within the HZs of M-type dwarf stars. Future studies of atmospheric
stability should be focused on larger and more massive terrestrial exoplanets, because
these bodies can generate stronger magnetic moments and their higher gravitational
acceleration will hold back their thermosphere-exosphere environments expansion.
Under such conditions atmospheric loss processes in general would be much less
efficient.
ACKNOWLEDGEMENTS
H. Lammer, H. I. M. Lichtenegger, Yu. N. Kulikov, H. K. Biernat, N. V. Erkaev and T.
Penz thank the “Österreichischer Austauschdienst” (ÖAD), which supported this work by
the projects I.12/04 and I.2/08. M. L. Khodachenko acknowledges support from the
Austrian “Fonds zur Förderung der wissenschaftlichen Forschung” (project P16919N08), ÖAD-RFBR Scientific and Technical Collaboration Program (project No.I.21/04),
and the ÖAD-Acciones Integradas Program (project No.11/2005). H. Lammer, H. I. M.
Lichtenegger, T. Penz, H. K. Biernat, and I. Ribas acknowledges support from the ÖADAcciones Integradas project No. 12/2005. H. K. Biernat and T. Penz also thank the
Austrian “Fonds zur Förderung der wissenschaftlichen Forschung” which supported this
study under project P17099-N08. The authors are also indebted to the the Austrian
Academy of Sciences, “Verwaltungsstelle für Auslandsbeziehungen” and the Russian
Academy of Sciences for supporting working visits to the PGI/RAS in Murmansk and to
38
the ICM/RAS in Krasnoyarsk, Russian Federation. I. Ribas acknowledges also the
support from the Spanish Ministerio de Ciencia y Tecnología through a Ramón y Cajal
fellowship. Further, the authors thank the Austrian Ministry for Science, Education and
Culture (bm:bwk) for supporting the CoRoT project. This study was supported by the
International Space Science Institute (ISSI) and was carried out under the framework of
the ISSI Team “Evolution of Habitable Planets”.
REFERENCES
Audard, M., Güdel, M., Drake, J. J., Kashyap, V. L. (2000) Extreme-Ultraviolet Flare
Activity in Late-Type Stars, Astrophys. J., 541, 396 - 409.
Bauer, S. J. (1983) Water on Venus: lack or loss? Ann. Geophys., 1, 477 - 480.
Bauer, S. J., Lammer, H. (2004) Planetary Aeronomy: Atmosphere Environments in
Planetary Systems. Springer Verlag, Heidelberg.
Biernat H. K., Erkaev, N. V., Farrugia, C. J. (1999) Aspects of MHD flow about Venus,
J. Geophys. Res., 104, 12617 - 12626.
Biernat, H. K., Erkaev, N. V., Farrugia, C. J. (2001) MHD effects in the Venus
magnetosheath including mass loading. Adv. Space Res., 28, 833 - 839.
Blum, P., Harris, I., Priester, W. (1972) The physics of the neutral upper atmosphere, in:
COSPAR International Reference Atmosphere. Akademie Verlag Berlin, p.399.
39
Bougher, S. W., Engel, S., Roble, R. G., Foster, B., (1999) Comparative terrestrial planet
thermospheres 2. Solar cycle variation of global structure and winds at equinox, J.
Geophys. Res., 104, 16,591 - 16,611.
Bougher, S. W. Engel, S., Roble, R. G., Foster, B. (2000) Comparative terrestrial planet
thermospheres 3. Solar cycle variation of global structure and winds at solstices. J.
Geophys. Res., 105, 17669 - 17692.
Brace, L. H., Theis, R. F., Hoegy, W. R. (1982) Plasma clouds above the ionopause of
Venus and their implications. Planet. Space Sci., 30, 989 - 992.
Chamberlain, J. W. (1963) Planetary coronae and atmospehric evaporation. Planet.
Space. Sci., 11, 901 – 996.
Chandra, S., Sinha, A. K. (1974). The role of eddy turbulence in development of selfconsistent models of the lower and upper thermospheres. J. Geophys. Res., 79, 1916 1921.
Chassefière, E. (1996a) Hydrodynamic escape of oxygen from primitive atmospheres:
applications to the cases of Venus and Mars. Icarus, 124, 537 - 552.
Chassefière, E. (1996b) Hydrodynamic escape of hydrogen from a hot water-rich
atmosphere: the case of Venus. J. Geophys. Res., 101, 26039 - 16056.
40
Chassefière, E. (1997) Loss of water on the young Venus: the effect of a strong primitive
solar Wind. Icarus, 126, 229 – 232.
Chassefière, E., Leblanc, F. (2004) Mars atmospheric escape and evolution; interaction
with the solar wind. Planet. Space Sci., 52, 1039 - 1058.
Ciaravella, A., Raymond, J. C., van Ballegooijen, A., Strachan, L., Vourlidas, A., Li, J.,
Chen, J., Panasyuk, A. (2003) Physical Parameters of the 2000 February 11 Coronal
Mass Ejection: Ultraviolet Spectra versus White-Light Images. Astrophys. J., 597, 1118 1134.
Crowley, G. (1991) Dynamics of the Earth’s thermosphere – A review. Rev. Geophys.
Supp., 29, 1143 - 1165.
Dickinson, R. E. (1972) Infrared radiative heating and cooling in the Venusian
mesosphere. I: Global mean radiative equilibrium. J. Atmos. Sci., 29, 1531-1556.
Dickinson, R. E. (1984) Infrared radiative cooling in the mesosphere and lower
thermosphere. J. Atmos. Terr. Phys., 46, 995-1008.
Dickinson, R. E., Roble R. G., Bougher S. W. (1987) Radiative cooling in the NLTE
region of the mesosphere and lower thermosphere – global energy balance. Adv. Space
Res., 7, (10)5-(10)15.
41
Dickinson, P., Bolden, R., Young, R. (1974) Measurement of atomic oxygen in the lower
ionosphere using a rocket-borne resonance lamp. Nature, 252, 289 - 291.
Erkaev, N. V., Farrugia, C. J., Biernat, H. K., Burlaga, L. F., Bachmaier, G. A. (1995)
Ideal MHD flow behind interplanetary shocks driven by magnetic clouds. J. Geophys.
Res., 100, 19919 – 19932.
Erkaev, N. V., Faruggia, C. J., Biernat, H. K. (1999) Three-dimensional, one-fluid, ideal
MHD model of magnetosheath flow with anisotropic pressure. J. Geophys. Res., 104,
6877 – 6888.
Erkaev, N. V., Penz, T., Lammer, H., Lichtenegger, H. I. M., Wurz, P., Biernat, H. K.,
Griessmeier, J.-M., Weiss, W. W. (2004) Plasma and magnetic field parameters in the
vicinity of short periodic giant exoplanets. Astrophys. J., 157, 396 – 401.
Foing, B. H. (1989) Stellar flare spectral diagnostics: Present and future. Solar Phys.,
121, 117 - 133.
Forget F., Pierrehumbert R. T. (1997) Warming early Mars with carbon dioxide clouds
that scatter infrared radiation. Science, 278, 1273 - 1276.
Fox, J., L., Hac, A. (1997) Spectrum of hot O at the terrestrial planets. J. Geophys. Res.,
102, 24005 – 24011.
42
Gordiets, B. F., Markov, M .N., Shelepin, L. A. (1977) The role of infra-red radiation in
the energy balance of the upper atmosphere. Kosm. Issled., 15, 725 - 735.
Gordiets, B. F., Markov, M. N., Shelepin, L. A. (1978) Infra-red radiation of the upper
atmosphere. Planet. Space Sci., 26, 933 – 948.
Gordiets, B. F., Kulikov, Yu. N. (1981) Numerical modelling of the terrestrial
thermosphere thermal structure. Kosm. Issled., 19, 249 – 260.
Gordiets, B. F., Kulikov, Yu. N., Markov, M. N., Marov, M. Ya. (1982) Numerical
modelling of the thermospheric heat budget. J. Geophys. Res., 87, 4504 – 4514.
Gordiets, B. F., Kulikov, Yu. N. (1985) On the mechanisms of cooling of the nightside
thermosphere of Venus. Adv. Space Res., 5, 113 -117.
Grießmeier, J.-M., Stadelmann, A., Penz, T., Lammer, H., Selsis, F., Ribas, I., Guinan, E.
F., Motschmann, U., Biernat, H.-K., and Weiss W. W. (2004) The effect of tidal locking
on the magnetospheric and atmospheric evolution of “Hot Jupiters”. A&A, 425, 753 –
762.
Grießmeier, J.-M., Stadelmann, A., Motschmann, U., Belisheva, N. K., Lammer, H.,
Penz, T. (2005) Cosmic ray impact on extrasolar Earth-like planets in close-in habitable
zones. Astrobiology, in press.
43
Gridichin, G. V., Zhadin, E. A., Ivanovsky, A. I., Marchevsky, V. A. (1975) Mechanisms
of formation of the daily average state of the mesosphere and thermosphere. Geomagn.
Aeron., 15, 93 - 100.
Haisch, B., Strong, K. T., Rodono, M. (1991) Flares on the sun and other stars. ARA &A,
29, 275 - 324
Haberle, R., McKay, C., Tyler, D., Reynolds, R. (1996) Can synchronously rotating
planets support an atmosphere? in: Circumstellar Habitable Zones. Travis House, Menlo
Park, CA.
Hedin, A. E., Nieman, H. B., Kasprzak, W. T., Seiff, A., (1983) Global empirical model
of the Venus thermosphere. J. Geophys. Res., 88, 73 - 83.
Houdebine, E. R., Foing, B. H., Rodonò, M. (1990) Dynamics of flares on late-type dMe
stars. I. Flare mass ejections and stellar evolution. A&A, 238, 249 - 255.
Houdebine, E. R., Foing, B. H., Doyle, J. G., Rodonò, M. (1993a) Dynamics of flares on
late-type dMe stars. II. Mass motions and prominence oscillations during a flare on AD
Leonis. A&A, 274, 245 - 264.
44
Houdebine, E. R., Foing, B. H., Doyle, J. G., Rodon`o, M. (1993b) Dynamics of flares on
late-type dMe stars. II. Kinetic energy and mass momentum budget of a flare on AD
Leonis. A&A, 278, 109 - 128.
Houdebine, E. R., Mathioudakis, M., Doyle, J. G., Foing, B. H. (1996) Observation and
modelling of main sequence star chromospheres. V. Ultraviolet excess emission in active
M dwarfs. A&A, 305, 209 - 228.
Hunten, D. M. (1993) Atmospheric evolution of the terrestrial planets. Science, 259, 915
- 920.
Hunten, D. M., Pepin, R. O., Walker, J. C. G. (1987) Mass fractionation in hydrodynamic
escape. Icarus, 69, 532 - 549.
Ip, W.-H. (1988) On a hot oxygen corona of Mars. Icarus, 76, 135 –145.
Izakov, M. N. (1971) On theoretical models of the structure and dynamics of the Earth’s
thermosphere. Space Sci. Rev., 12, 261 - 298, 1971.
Jacchia, L. G. (1977) Thermospheric temperature, density and composition: New models.
Spec Rep. 375, Smithson. Inst. Astrophys. Obs., Cambridge Mass., USA.
45
Jakosky, B. M., Pepin, R. O., Johnson, R. E., Fox, J. L. (1994) Mars atmospheric loss and
isotopic fractionation by solar-wind induced sputtering and photochemical escape.
Icarus, 111, 271 - 288.
Johnson, R. E., Schnellenberger, D., Wong, M. C. (2000) The sputtering of an oxygen
thermosphere by energetic O+. J. Geophys. Res., 105, 1659 - 1670.
Joshi, M. M., Haberle, R. M. (1996) The Atmospheres of Synchronously Rotating
Planets. Bull. American Astron. Soc., Vol. 28, 1114.
Joshi, M. M., Haberle, R. M., Reynolds, R. T. (1997) Simulations of the atmospheres of
synchronously rotating terrestrial planets orbiting M dwarfs: Conditions for atmospheric
collapse and the implications for habitability. Icarus, 129, 450 - 465.
Joshi, M. M. (2003) Climate model studies of synchronously rotating planets.
Astrobiology, 3, 415 - 427.
Kallio, E., Luhmann, J. G., Barabash, S., (1997) Charge exchange near Mars: The solar
wind absorption and energetic neutral atom production. J. Geophys. Res., 102, 22,183 22,197.
Kasting J. F., Whitmire D. P., and Reynolds R. T. (1993) Habitable zones around main
sequence stars, Icarus, 101, 108 – 128.
46
Kass, D. M., Yung, Y. L. (1995) Loss of atmosphere from Mars due to solar-wind
induced Sputtering. Science, 268, 697 - 699.
Khodachenko, M. L., Ribas, I., Lammer, H., Grießmeier, J.-M., Leitner, M., Selsis, F.,
Eiroa, C., Hanslmeier, A., Biernat, H. K., Farrugia, C. J., Rucker, H. O. (2005) CME
activity of low mass M stars as an important factor for the habitability of terrestrial
exoplanets, Part I: CME impact on expected magnetospheres of Earth-like exoplanets in
close-in habitable zones. Astrobiology, this issue, submitted.
Kim, J., Nagy, A. F., Fox, J. L., Cravens, T. E. (1998) Solar cycle variability of hot
oxygen atoms at Mars. J. Geophys. Res., 103, 29339 – 29342.
Kulikov, Yu. N., Lammer, H., Lichtenegger, H. I. M., Terada N., Ribas, I., Kolb, C.,
Langmayr, D., Lundin, R., Guinan, E. F., Barabash, S., Biernat, H. K. (2005)
Atmospheric and water loss from early Venus. Planet. Space Sci., in press.
Lammer, H., Bauer, S. J. (1991) Nonthermal atmospheric escape from Mars and Titan.
J. Geophys. Res., 96, 1819 – 1825.
Lammer, H., Stumptner, W., Bauer, S. J. (2000) Upper limits for the Martian exospheric
number density during the Planet B / Nozomi mission. Planet. Space Sci., 48, 1473 –
1478.
47
Lammer, H., Lichtenegger, H. I. M., Kolb, C., Ribas, I., Guinan, E. F., Bauer, S. J. (2003)
Loss of water from Mars: Implications for the oxidation of the soil. Icarus, 165, 9 - 25.
Lammer, H., Kulikov, Yu. N., Penz, T., Leitner, M., Biernat, H. K., Erkaev, N. V. (2005)
Stellar-planetary relations: atmospheric stability as a prerequisite for planetary
habitability. Cel. Mech. Dyn. Astron., 1 –13, DOI 10.1007/s10569-005-0004-4.
Lammer, H., Lichtenegger, H. I. M., Biernat, H. K., Erkaev, N. V., Arshukova, I. L.,
Kolb, C., Gunell, H., Lukyanov, A., Holmström, M., Barabash, S., Zhang, T. L.
Baumjohann, W. (2006a) Loss of hydrogen and oxygen from the upper atmosphere of
Venus. Planet. Space Sci., in press.
Lammer, H., Kulikov, Yu. N., Lichtenegger, H. I. M. (2006b) Thermospheric X-ray and
EUV heating by the young Sun on early Venus and Mars. Space Sci. Rev., in press.
Leblanc, F., Johnson, R. E. (2001) Sputtering of the Martian atmosphere by solar wind
pick-up ions. Planet. Space Sci., 49, 645 - 656.
Leblanc F., Johnson, R. E. (2002) Role of molecules in pick-up ion sputtering of the
Martian atmosphere. J. Geophys. Res., 10209/2000JE001473.
Léger, A.,
Selsis, F., Sotin, C., Guillot, T., Despois, D., Mawet, D., Ollivier, M.,
Labéque, F. A., Valette, C., Brachet, F., Chazelas, B., Lammer, H., (2004) A new family
of planets? “Ocean Planets’’, Icarus, 169, 499 - 504.
48
Lichtenegger, H. I. M., Schwingenschuh, K., Dubinin, E. M., Lundin, R. (1995) Particle
simulation in the Martian magnetotail. J. Geophys. Res., 100, 21,659 - 21,667.
Lichtenegger, H. I. M., Dubinin, E. M. (1998) Model calculations of the planetary ion
distribution in the Martian tail. Earth Planets Space, 50, 445 - 452.
Lichtenegger, H. I. M., Lammer, H., Stumptner, W. (2002) Energetic neutral atoms at
Mars: III. Flux and energy distributions of planetary energetic H atoms. J. Geophys.
Res., 107, 1279, doi:10.1029/2001JA000322.
Luhmann, J. G., Kozyra, J. U. (1991) Dayside pickup oxygen ion precipitation at Venus
and Mars: Spatial distributions, energy deposition and consequences. J. Geophys. Res.,
96, 5457 – 5467.
Luhmann, J. G., Johnson, R. E., Zhang, M. G. H. (1992) Evolutionary impact of
sputtering of the Martian atmosphere by O+ pick-up ions. Geophys. Res. Lett., 19, 2151 2154.
Luhmann, J. G. (1993) A model of the ionospheric tail rays of Venus. J. Geophys. Res.,
98, 17,615 - 17,621.
Luhmann, J. G. (1997) Correction to “The
ancient oxygen exosphere of Mars:
implications for atmosphere evolution by Zhang et al. J. Geophys. Res., 102, 1637.
49
Lundin, R., Barabash, S. (2004) Evolution of the Martian atmosphere and hydrosphere:
Solar wind erosion studied by ASPERA-3 on Mars Express. Planet. Space Sci., 52, 1059
– 1071.
Lundin, R., Barabash, S., Andersson, H:, Holmström, M., Grigoriev, A., Yamauchi, M.,
Sauvaud, J.-A., Fedorov, A., Budnik, E., Thocaven, J.-J., Winningham, D., Frahm, R.,
Scherrer, J., Sharber, J., Asamura, K., Hayakawa, H., Coates, A., Linder, D. R., Curtis,
C., Hsieh, K. C., Sandel, B. R., Grande, M., and 23 co-authors (2004) Solar wind
induced atmospheric erosion on Mars: First results from ASPERA-3 on Mars Express.
Science, 305, 1933 – 1936.
Michel, F. C. (1971) Solar wind-induced mass loss from magnetic field-free planets.
Planet. Space Sci., 19, 1580 – 1583.
Nagy, A. F., Cravens, T. E., Yee, J.-H., Stewart, I. F. (1981) Hot oxygen atoms in the
upper atmospheres of Venus. Geophys. Res. Lett., 8, 629 – 632.
Neubauer, F. M., Gurnett, D. A., Scudder, J. D., Hartle, R. E. (1984) Titan's
magnetospheric interaction, in: Saturn. University of Arizona Press, 760 – 787.
Nieman, H. B., Hartle, R. E., Kasprzak, W. T., Spencer, N. W., Hunten, D. M., Carignan,
G. R., 1979a. Venus upper atmosphere neutral composition: Preliminary results from
Pioneer Venus orbiter, Science, 203, 770 - 772.
50
Nieman, H. B., Hartle, R. E., Hedin, A. E, Kasprzak, W. T., Spencer, N. W., Hunten, D.
M., Carignan, G. R., 1979b. Venus upper atmosphere neutral composition: First
observations of the diurnal variations, Science, 205, 54 - 56.
Penz, T., Erkaev, N. V., Biernat, H. K., Lammer, H., Amerstorfer, U. V., Gunell, H.,
Kallio, E., Barabash, S., Orsini, S., Milillo, A., Baumjohann, W. (2004) Ion loss on Mars
caused by the Kelvin-Helmholtz instability. Planet. Space Sci., 52, 1157 - 1167.
Preusse, S., Kopp, A. Büchner, J., Motschmann, U. (2005) Stellar wind regimes of closein extrasolar planets. A&A, in press.
Ribas, I., Guinan, E. F., Güdel, M., Audard, M. (2005) Evolution of the Solar Activity
over Time and Effects on Planetary Atmospheres: I. High-energy Irradiances (1-1700
Ǻ). Astrophys. J., 622, 680 – 694.
Rivera, E. J., Lissauer, J. J., Butler, R. B., Marcy, G. W., Vogt, S. S., Fischer, D. A.,
Brown, T. M., Laughlin, G. (2005) A ∼ 7.5 Earth-Mass Planet Orbiting the Nearby Star,
GJ 876. Astrophys. J. Let., in press.
Rodriguez, J. M., Prather, M. J., McElroy, M. B. (1984) Hydrogen on Venus: Exospheric
distribution and escape. Planet. Space Sci., 32, 1235 – 1255.
Scallo, J., Segura, A., Fridlund, M., Ribas, I., Selsis, F., Kulikov, Yu. N., Odert, P.,
Leitzinger, M., Grenfell, J. L., Rauer, H., Kaltenegger, L., Khodachenko, M. L., Eiora,
51
C., Kasting, J., H. Lammer (2006) M-type dwarf stars and their relevance for terrestrial
planet finding missions. Astrobiology, this issue, submitted.
Smith, D. S., Scalo, J., Wheeler, J. C. (2004) Transport of ionizing radiation in terrestriallike exoplanets atmospheres, Icarus, 171, 229 – 253.
Schmitt, J. H. M. M., Fleming, T. A., Giampapa, M. S. (1995) The X-Ray View of the
Low-Mass Stars in the Solar Neighborhood, Astrophys. J., 450, 392 - 400
Spreiter, J. R., Stahara, S. S. (1980) Solar wind flow past Venus: Theory and
comparisons, J. Geophys. Res., 98, 17,251 - 17,262.
Terada, N., Machida, S., Shinagawa, H. (2002) Global hybrid simulation of the KelvinHelmholtz instability at the Venus ionopause. J. Geophys. Res., 107,
doi:10.1029/2001JA009224.
Von Zahn, U., Fricke, K. H., Hunten, D. M., Krankowsky, D., Mauersberger, K., Nier, A.
O., (1980) The upper atmosphere of Venus during morning conditions, J. Geophys. Res.,
85, 7829 - 7840.
Waite Jr., J. H., Nieman, H., Yelle, R. V., Kasprzak, W. T., Cravens, T. E., Luhman, J.
G., McNutt, R. L., Ip, W.-H., Gell, D., Haye, V. De. La., Müller-Wordag, I., Magee, B.,
Borggren, N., Ledvina, S., Fletcher, G., Walter, E., Miller, E., Scherer, S., Thorpe, R.,
52
Xu, J., Block, B., Arnett, K. (2005) Ion neutral mass spectrometer results from the first
flyby of Titan. Science, 308, 982 - 985.
Zhang, M. G:, H., Bougher, S. W., Nagy, A. F. (1993) The ancient oxygen exosphere of
Mars: Implications for atmospheric evolution. J. Geophys. Res., 98, 10915 – 10923.
Zhang, M. H. G., Luhmann, J. G., Nagy, A. F., Spreiter, J. R., Stahara, S. S. (1993b).
Oxygen ionization rates at Mars and Venus: relative contributions of impact ionization
and charge exchange. J. Geophys. Res., 98, 3311 - 3318.
53
Figures and Figure Captions:
Fig. 1:
Modelled temperature profiles in a “dry” 96% CO2-rich thermosphere of an Earth-like
planet as a function of altitude for different XUV flux values. The short horizontal lines
mark the exobase altitudes, and the dotted line shows the blow-off temperature for
atomic hydrogen.
54
Fig. 2:
Total (“cold” and “hot”) oxygen number density as a function of altitude and different
XUV flux values. The short horizontal lines mark the exobase altitudes, which are used
for the planetary obstacle boundary in case I. The dashed line (case II) and the dotted line
(case III) correspond to magnetized planets having obstacle boundaries at about 0.5 (case
II) and one (case III) Earth-radius used in the ion pick up study.
55
Fig. 3:
Illustration of the CME plasma interaction with a strongly magnetized planet (the Earthlike). The grey area represents the near-planetary space where the exospheric gas density
is negligible. In such a case the atmosphere is protected against dense stellar/solar winds
and the CME plasma interaction. The average RMP distance for Earth is at about 10 Earth
radii.
56
Fig. 4:
Illustration of the CME plasma interaction with a “Venus-like” or “Martian-like” nonor weakly magnetized exoplanet. The neutral atmosphere above the ionopause can be
eroded by the stellar/solar wind and CME plasma flux.
57
Fig. 5:
Illustration of the CME plasma interaction with a compressed magnetized exoplanet and
an extended thermosphere-exosphere environment. In such a case, the exospheric gas
which is above the magnetopause distance can be eroded stellar wind and CME plasma
flows.
58
Fig. 6:
Time dependent atmospheric loss as function of minimum (lower lines) and maximum
(upper lines) CME plasma flux for XUV radiation 70 times more intense than that for the
present time Earth at an orbital distance of 0.2 AU (a) and 0.1 AU (b).
59
Fig. 7:
a
b
c
Time dependent ion pick up loss as a function of minimum (lower dotted lines) and
maximum (upper dashed lines) CME plasma flux and orbital distance at 0.2 AU, for
various XUV value values compared to that of the present Sun. Case I: weaklymagnetized Venus-like interaction; Case II: magnetopause at 0.5 Earth radii; Case III:
magnetopause at 1 Earth radii above the planetary surface.
60
Fig. 8:
a
b
Time dependent ion pick up loss as a function of minimum (dashed lines) and maximum
(dotted lines) expected atmospheric mass loss rates in units of bar over 1 GYR as a
function of XUV flux values for minimum (upper panel) and maximum (lower panel)
CME plasma flux and orbital distance at 0.05 AU, for a weakly (dotted-lines: planetary
obstacle at exobase altitudes) and non-magnetized (dashed-lines: planetary obstacle at
present Venus ionopause at an altitude of about 300 km) CO2-rich Earth-like exoplanets.
61
Fig. 9:
a
b
Time dependent ion pick up loss as a function of minimum (lower dotted lines) and
maximum (upper dashed lines) CME plasma flux and orbital distance at 0.05 AU, for
various XUV value values compared to that of the present Sun. Case II: magnetopause at
0.5 Earth radii; Case III: magnetopause at 1 Earth radii above the planetary surface.
62
Table 1
0.05 AU
0.1 AU
0.2 AU
CMEmin CMEmax CMEmin CMEmax CMEmin CMEmax
RMP [REarth]
M [×MEarth]
M [×MEarth]
M [×MEarth]
0.5
0.11 …… 0.88 0.048 …… 0.25 0.021 …… 0.073
1.0
0.25 …… 2.10 0.11 …… 0.60
0.051 …… 0.17
2.0
0.85 …… 7.07 0.38 …… 2.03
0.17 …… 0.58
3.0
2.01 …… 16.8 0.90 …… 4.82
0.41 …… 1.38
5.0
6.79 …… 56.6 3.05 …… 16.30
1.37 …… 4.67
Required magnetic moments M for the generation of magnetopause distances RMP related
to minimum and maximum CME plasma flux values at an orbital distance of about 0.05,
0.1 and 0.2 AU.