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5 Habitable zones and Planetary atmospheres
5.1 Introduction
Questions to answer:
 How does star affect the emergence and sustainability of life on a planet?
 What are the main properties of planetary atmospheres in the Solar system?
 How and what can we learn about atmospheres of extrasolar planets?
5.2 Habitable zones
Unique Earth’s properties enabling the life support:
 liquid water
 planetary environment (atmosphere, etc.)
A circumstellar habitable zone is defined as encompassing the range of distances from
a star for which liquid water can exist on a planetary surface.
Pure water exists as a liquid between 273 K and 373 K, unless the pressure is too low.
Therefore, the primary factor in determining a planet’s habitability is temperature.
The planet temperature is defined by the balance between
 absorption of stellar radiation (neglecting internal sources)
 re-radiation of it
The stellar energy absorbed by the planet is
Ea   R p2
L*
(1  a) ,
4 d 2
where L* is stellar luminosity, Rp is radius of the planet, d is distance between the planet
and the star, and a is albedo.
The amount of energy to be radiated by the planet can be expressed as
L p  4 R p2 Te4 ,
where Te is effective temperature of the planet.
The balance between Lp and Ea defines the effective temperature of the planet. The
effective temperature of the Earth is 255 K. It is lower than 273 K because the Earth is
not a perfect black body and it traps some energy because of the greenhouse effect .
If the Sun would be more luminous, to maintain the same effective temperature the
Earth should be further away, and vice versa:
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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Figure 1: Left: the distance to the habitable zone versus stellar luminosity calculated from the balance
between Lp and Ea. Right: the evolutionary track of a solar-type star starting on the Main Sequence and
continued through the Red Giant and Horizontal Branch phases to the Asymptotic Giant Branch. The
luminosity of the star increases by several orders of magnitude.
4 Gyr ago the solar luminosity was lower by 30%, as main-sequence stars move up
along the sequence and further to the red giant phases
 habitable zone was closer to the Sun
 habitable zone is moving away from the Sun
The continuous habitable zone (CHZ) is the region, in which a planet may reside and
maintain liquid water throughout most of a star’s life.
Figure 2: Continuous habitable zone: yellow region is the habitable zone in the beginning of the main
sequence, blue region is the habitable zone at late stages of the stellar evolution, and the green region is
an intersection of the other two.
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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Figure 3: Evolution of the solar luminosity (left) and the HZ (right). The most realistic model of the HZ
is depicted by long dashes (Kasting et al. 1993).
To estimate realistically the CHZ for the Sun, one needs to take into account
 albedo
 greenhouse effect
The inner edge of the CHZ is determined by photodissociation of water and loss of
hydrogen.
 0.95 AU
The outer edge of the CHZ is mainly defined by formation of CO2 clouds.
 1.15 AU
This contradicts however to the fact that surface of Mars was once carved by streams of
some flowing liquid. The used climate model should be modified by adding other
greenhouse gases (e.g. CH4) and perhaps by including a more dense cloud cover on
Mars.
Tidal heating of satellites around giant planets, such as Jupiter’s satellite Europa, raises
the possibility of liquid water existing below the surface of ice-covered satellites.
Sun in time
To understand the early phase of the planet formation and the evolution of the habitable
zone in the Solar system, we need to study the effects of the young Sun on planets. This
is possible with a sample of stars of about the same mass as the Sun but at different
evolutionary stages.
A study of such stars shows that the solar radiation has undergone dramatic variations
since it arrived to the Main Sequence. For instance, the X-ray luminosity of the young
Sun (~108 years) was 1000 times of the present level. Fig. 4 shows the X-ray luminosity
of solar-type stars versus the age. In Fig. 5 this is compared with the radiation in other
wavelengths.
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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Figure 4: The X-ray luminosity of solar-type stars versus the age. Note the decrease by three orders of
magnitude over 10 Gyr.
Figure 5: The X-ray, UV and total luminosity of the Sun versus age as deduced from solar analogues.
The young Sun had UV radiation factor of 10 to 100 higher than the present level, while its total
luminosity was in fact only 70% of the present.
Magnetic activity of the Sun (flares, coronal mass ejections, solar wind, etc.) was also
much more prominent as compared to the Sun today. These factors have significantly
influenced the planetary atmospheres, their composition and density.
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Effects of Solar magnetic activity on terrestrial planets
Mercury
0.39 AU from the Sun
Variation of the mean density with
diameter of the terrestrial planets as well
as the Moon (shown on the right) indicates
that Mercury has a much higher mean
density than expected given its size. This
is due to the relative size of its iron core
which is significantly larger than for any
other terrestrial planet.
One possible explanation is that Mercury’s
lighter mantle/crust was eroded away by
the strong (~1,000 times present values)
winds and the early Sun’s higher extreme
ultraviolet fluxes. The eroded and ionized
material has been carried away with the
Figure 6: The mean density of terrestrial planets
solar wind.
versus the diameter. Mercury significantly deviates
from the common tendency.
Venus
0.71 AU from the Sun
No water or oxygen. The explanation is in
photochemistry/photoionization effects:
 Venus has a slow rotation period (P= 243 days)
and a very weak magnetic dynamo
 Venus is thus not protected from the Sun’s
plasma by planetary magnetic field
Perhaps the young Sun’s enhanced activity and UV
flux played a major role: e.g. Photodissociation of
water H2O  H + OH, and subsequent loss of H.
Earth
1 AU from the Sun
A Young active Sun may have played a major role in the evolution of the Earth’s
atmosphere and possibly the origin and evolution of life.
 Destruction of methane (CH4) by the early Sun’s strong UV radiation
 Formation of ozone (O3) via photoionization of O2
 Photochemical reactions leading to the formation of organic molecules
 Erosion of the atmosphere due to enhanced solar wind: loss of H+, O++, N+
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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Mars
1.52 AU from the Sun
Today, Mars is a cold dry planet with a thin
atmosphere rich in CO2. Mars also possesses a very
weak magnetic field. There is also geological
evidence of running water and possibly a permanent
layer of permafrost. It is important to study the
effects of the active young Sun on Mars:
 Loss of water and atmosphere
 Soil oxidation
 Possible early life
Early Mars: liquid iron core produced a magnetic field strong enough to protect the
young Martian atmosphere and surface water from the punishing effects of the young
Sun’s intense solar wind. (Lammer et al. 2003).
Roughly 3.5 Billion years ago, Mars’ core solidified, shutting down the Martian
magnetic dynamo. Consequences:
 Without a magnetic field, the outer Martian atmosphere was subjected to the
ionizing effects and strong winds of the sun, and began to erode.
 At this time, water disassociates into 2H+O, where the lighter hydrogen is lost to
the space while the heavier oxygen combines with iron on its surface
Habitable zones around other stars
In principle low-mass stars (dwarfs) are prime candidates for searches of planets in
habitable zones:
 Long life times on the Main Sequence (>10Gyr, see figure below)
 Very abundant in the solar neighborhood (>70%)
 Better contrast star/planet for detection
4
1.0 M.;1.0
G2M
1.4 M.; F4-5
1.4
M
F 4-5
3
G2
log L/L .
2
“stable” lifetime
~2.5 Gyr
1
“stable” lifetime
~8 Gyr
0
-1
-2
0.0
“stable” lifetime >20 Gyr
0.70.7
MM.;
, KK2-3
2-3
“stable” lifetime >40 Gyr
0.40.4
MM.;
, MM1-2
1-2
5.0e+9
1.0e+10
Age (years)
1.5e+10
2.0e+10
Astrobiology: 5. Habitable zones and Planetary Atmospheres
S.V. Berdyugina, University of Freiburg
Figure 7: Variations of
the total luminocity for
stars of different masses
and spectral classes.
Low-mass K-M dwarfs
have long periods of
constant luminosity and
are therefore prime
candidates for searches
of planets in habitable
zones.
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Stars later than F0 have main sequence lifetimes exceeding 2 Gyr. They are
therefore potential candidates for harboring habitable planets. The HZ around an
F star is larger and occurs farther out than for our Sun. The CHZ of F stars are
narrower (log distance) than for the Sun because they evolve more rapidly.
the HZ around K and M stars is smaller and occurs closer to the star.
Nevertheless, the widths of all of these HZs are approximately the same in log
distance.
the CHZs around K and M stars are wider (in log distance) than for our Sun
because these stars evolve more slowly. Planets orbiting late K stars and M stars
may not be habitable, however, because they can become trapped in
synchronous rotation as a consequence of tidal interaction.
Lower UV flux of M dwarfs implies smaller planetary atmosphere erosion.
However, young M dwarfs are extremely active and stay active for longer
periods of time! Potential for very severe erosion of atmospheres due to X-rays,
flares, etc.
Mid-to-early K stars should be considered along with G stars as optimal
candidates in the search for extraterrestrial life.
Figure 8: Diagram showing the zero-age main-sequence habitable zone (solid curves) as a function of
stellar mass. The long-dashed lines delineate the probable terrestrial planet accretion zone. The dotted line
represents the distance at which an Earth-like planet in a circular orbit would be locked into synchronous
rotation within 4.5 Gyr (Kasting et al. 1993).
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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5.3 Planetary atmospheres: Solar system
General remarks
The Solar system contains four major planets of the terrestrial type and four of the
jovian type. The jovian planets are orbited by tens of satellites. Some of these have sizes
that would easily classify them as planets were they to orbit the Sun directly. Pluto is
now classified as a minor body.
Substantial atmospheres are found on all major planets except Mercury, and on Titan,
Saturn’s largest satellite. Thin atmospheres are found on Mercury, Pluto and Triton,
Neptun’s largest satellite.
The atmospheres are divided on three types:
 highly oxidized Earth-like atmospheres (CO2, O2)
 mildly reduced atmospheres of Titan, Pluto and Triton (CO/ CH4 ~ 0.1-0.01)
 highly reduced atmospheres of the giant planets (CO/ CH4 ~ 10–6)
The element is called oxidized, when it saturates all its chemical bounds with oxygen
(CO2).
The element is called reduced, when it saturates all its chemical bounds with hydrogen
(CH4).
Correspondingly, the atmosphere is called oxidized or reduced and the conversion
process is oxidation or reduction.
Three most abundant species of planetary atmospheres:
1’
Oxidized atmospheres:
Venus
Earth
Mars
Mildly reduced atmospheres:
Titan
Pluto
Highly reduced atmospheres:
Jupiter
Saturn
Uranus
Neptune
CO2 (0.96)
N2 (0.78)
CO2 (0.95)
N2 (0.95)
N2 (0.98)
H2 (0.864)
H2 (0.885)
H2 (0.85)
H2 (0.85)
2’
N2 (0.035)
O2 (0.21)
N2 (0.027)
Ar (<0.07)
CH4 (0.010.001)
He (0.136)
He (0.115)
He (0.15)
He (0.15)
3’
SO2 (0.00015)
Ar (0.0093)
Ar (0.016)
CH4 (0.04)
CO (0.001)
CH4
(0.00181)*
CH4 (0.005)*
CH4 (0.02)*
CH4 (0.02)*
* H2O is probably more abundant than CH4 below the visible clouds.
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None of the known atmospheres resembles the ISM or comets
Most massive (jovian) planets have preserved their initial composition, as it is
impossible even for the lightest H to escape in significant quantities
The atmospheres of the smallest planets are subject to the escape of part of the
lightest gases
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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The escape process has important consequences for the time evolution of the
composition the planetary atmospheres.
The difference in atmospheres of terrestrial and jovian planets roots its origin in
the planet formation history:
o larger jovian planets were able to retain gases from a solar nebula
o smaller terrestrial planets were unable to secure a significant amount of
gases since the solar wind from the nearby sun blew the hydrogen and
helium gases away
o subsequent volcanism, outgassing and comet bombardment continuously
replenished and added gases: H2O, CO2, N2, H2S, SO2.
Atmospheric structure

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troposphere: T  with altitude
stratosphere: T  with altitude
mesosphere: T  with altitude
thermosphere: T  with altitude
Figure 9: Atmospheric structure: the variation of temperature with altitude for (a) Venus, (b) Earth, and
(c) Mars. Note that only Earth has a stratosphere.
Heating
 internal sources
 sunlight
 greenhouse effect
Cooling
 molecular processes (ionization, dissociation, emission)
 convection
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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Venus
The escape process influenced the formation and chemical
evolution of the atmosphere on Venus (and other terrestrial
planets). On early Venus liquid water was evaporated and
transported to higher altitudes, where it was photodissociated.
Most of the H atoms escaped, but the oxygen remained in the
atmosphere and reacted with other species. The decrease of
amount of liquid water reduced the capability to store carbondioxide (on Earth most CO2 is stored in the lithosphere as
carbonates). An increasing CO2 abundance raised the surface
temperature, as well as the rate of evaporation. This processes
entered a runaway state, resulting in the observed dry CO2 rich
atmosphere.
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thick atmosphere
CO2 resides in the atmosphere
chemical composition is determined by
o equilibrium chemistry in the low atmosphere and at the surface (0–60
km)
o photochemistry in the upper atmosphere (60–110 km)
thick and global cloud deck (concentrated sulfuric acid + others)
slow atmospheric cycle of sulfur
strong greenhouse effect
surface pressure of 90 bars (1bar=105 Pa)
surface temperature of 730 K
no organic molecules (not even methane)
Earth
Venus and Earth are very similar in terms of size and density.
In fact the total amount of CO2 is also similar for both planets,
but
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CO2 resides in the lithosphere (carbonate rocks)
many others are dissolved in water (NaCl in the
oceans)
the only planet with liquid water on the surface
H2O cycle: interaction between the oceans and the
atmosphere
surface pressure of 1 bar
surface temperature 288 K
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Mars
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very thin atmosphere
the surface pressure is about 6 mbar
surface temperature varies from 170 K to 290 K
transparent to all solar radiation
no confirmed detection of any organic species
chemical composition is similar to that of the Venus’
atmosphere

Methane is not expected to be present in large amounts. If
detected, it either originates from outgassing, biological
sources, or both. Local enhancement of the methane abundance should be
searched for. Outgassing events can trigger a whole series of chemical reactions
in the presence of the UV radiation leading to synthesis of other organic
molecules, such as formaldehyde and methanol.
Titan
Titan is one of the most interesting and complex objects in the
Solar System when it comes to organic molecules in the
atmosphere. Being a moon of Saturn, it is actually larger than both
Mercury and Pluto.
The atmosphere of Titan has some similarities to the Earth’s.
 surface pressure of 1.5 bar
 N2 is the main atmospheric constituent
 CH4 is the second most important molecular species
It is believed that the composition of the current Titan atmosphere is similar to that of
the Earth’s atmosphere before the appearance of life.
Crucial differences to the Earth:
 low surface temperature on Titan, 94 K
 water vapor is kept out of the atmosphere.
 solid body is less massive by a factor of about 44
 the atmosphere of Titan is less bound to the planet
 atoms and molecules escape easier
Specific features:
 In the higher stratosphere, UV sunlight causes photolysis of methane and
subsequent chemical reactions which lead to the formation of more complex
molecules
 In the lower stratosphere and upper troposphere, low temperatures result in the
condensation of organics onto the haze particles.
 A global haze layer exist in the lower stratosphere
 Organics could be transported down to the surface
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The presence of areas (lakes, ponds) of liquid methane/ethane is possible
Methane is important greenhouse gas. If its supply stopped, the atmosphere
could collapse.
The present abundance of methane and the rate of its destruction in the upper
stratosphere requires a permanent source of methane, e.g.
o evaporation from lakes
o diffusion from a sub-surface ocean
o cryovolcanism
There could be a methane cycle in the atmosphere (clouds, rains)
The Huygens space probe (NASA/ESA/ISA) landed on Titan
on14 January 2005, collecting data as its parachutes slowed it
down. The mission only lasted about 3 hours - 2.5 hours to
descend to the surface, and then another half-hour on Titan’s
surface before the batteries ran out. During its descent,
Huygens' camera captured many images, and its other five
instruments sampled Titan's atmosphere to determine its
composition. This is the first probe that has ever been landed in
the outer solar system. Once Huygens landed, it measured the
wind, weather, energy flux and surface features, relaying the
information back to the Cassini spacecraft.
Figure 10: Image of the Titan surface
transmitted by the Huygens space probe.
Pluto and Triton
Pluto and Triton are very similar in terms of mass and composition, and are located in
the outer, colder parts of the Solar System.
Pluto’s atmosphere was discovered in 1980s, from stellar occultation measurements.
 surface pressure of 10100 bar
 surface temperature 36 K
 large quantities of frozen nitrogen on the surface
 N2 in the atmosphere
 destruction of methane and ethane can produce more organics
Triton’s atmosphere was detected by Voyager 2 in 1989
 N2 and methane are main components
 similar pressure as on Pluto
 photochemical hazes throughout the atmosphere
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Giant planets
Observable parts of the giant planet’s atmospheres:
 methane is the most abundant organic molecule
 other organics if formed by UV photolysis of methane in the upper stratosphere
 molecular hydrogen is a background gas supplying hydrogen for chemical
reactions
 the main stable products are ethane, acetelyne and polyacetelynes (C2nH2)
 vertical motions in the atmosphere transport the molecules to different layers
 at deeper layers (troposphere) the complex molecules transformed back to
methane
 clouds and hazes composed of hydrocarbons
 clouds of ammonia ice and water
The two smaller giants, Uranus and Neptune, have the highest methane abundances.
These giants have accreted less of the solar composition gas after the formation of their
ice-rich cores. The enrichment in the heavy elements is therefore much higher than for
Jupiter and Saturn.
Jovian system
The icy satellites of Jupiter embedded in the energy-rich Jovian radiation belt complex
are potential reservoirs of organic material.
Detected molecules:
Io
Europa
Ganymede
Callisto
SO2, SO3, H2S?, H2O?
H2O, SO2, CO2, sulfate salts, H2O2, H2SO4
H2O, O2, CO2, CH, SO2, O3
H2O, SO2, SH, CO2, CH
5.4 Planetary atmospheres: Exoplanets
Detection
Atmospheres of extrasolar planets can be detected during transit events:
Figure 11: The brightness of the star is reduced
during the transit of the planet over the stellar
disk.
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Some of the stellar light is blocked by the planet
Some light goes through the planetary atmosphere
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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Figure 12: The light passing through the planetary atmosphere during the transit is partly absorbed and
can be detected as an excess of line absorption in the stellar spectrum compared to the one outside the
transit.
Figure 13: Detection of the atmosphere on HD209458b by measuring the absorption excess in Na I lines
(Charbonneau et al. 2000).
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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Figure 14: Detection of the absorption excess in various spectral lines due to planetary atmosphere. Left:
Charbonneau et al. (2000), right : Schneider (2005).
Day and night side of extrasolar planets
NASA's Spitzer Space Telescope has made the first measurements of the day and night
temperatures of a planet outside our solar system. The infrared observatory revealed that
 Andromedae b, a hot Jupiter circling very close to its host star with an orbital period
of 4.6 days, is always as hot as fire on one side, and potentially as cold as ice on the
other.
Scientists believe the planet is tidally locked to its star. This means it is rotating slowly
enough that the same side always faces the star, just as the same side of Earth's tidally
locked moon always faces toward us, hiding its "dark side." However, since this planet
is made of gas, its outer atmosphere could in principle be circulating much faster than
its interior.
The observed temperature difference between the two sides of  Andromedae b is
extreme  about 1,400 degrees Celsius. This is unlike Jupiter, which is eventemperatured all the way around. Such a large temperature difference indicates the
planet's atmosphere absorbs and reradiates sunlight so fast that gas circling around it
cools off quickly, i.e. reemission is faster than the circulation than the time scale it takes
to evenly distribute temperature within the atmosphere.
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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Figure 16: The top graph consists
of infrared data from NASA's
Spitzer Space Telescope. It shows
that  Andromedae b, always has a
giant hot spot on the side that faces
the star, while the other side is
cold and dark. All data points are
actually the results of averaging
observations over many orbits,
always taken at the same phase
(error bars indicated). The artist's
concepts above the graph illustrate
how the planet might look
throughout its orbit if viewed up
close with infrared eyes. The
bottom graph and artist's concepts
represent what astronomers might
have seen if the planet had bands
of different temperatures girdling
it, like Jupiter, with no difference
between the average temperatures
of the sunlit and dark sides to
detect.
Evaporation of the planetary atmospheres
Detection of an extended hydrogen envelope on HD209458b in Lyman alpha line with
Hubble Space Telescope (HST):
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atmosphere extends over 200,000 km
hydrogen is unbound to the planet and escapes with velocity ~ 100 km/s
loss rate ~ 107 g/s
explains very few detections of planets close to the parent star. Those planets
should quickly evaporate, or become hydrogen-poor Neptune-mass planets.
Figure 15: Hot Jupiters being extremely close to the host star loose their atmospheres at high rate and
most probably look like comet-like objects (artist impression).
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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Predicted spectra of hot Jupiters
Infrared spectra of hot Jupiters with solar metallicity are dominated by CO, H2O, CH4.
The features in the visible are alkali absorption lines Na I and K I (see figure below).
Higher metallicity of the planet increases
the absorption of the incoming stellar flux
leading to
 warmer atmospheres
 Ti-condensation is not possible
 gaseous TiO (and VO) is free in the
atmosphere
 Strong absorption by TiO/VO leads
to a stratosphere
Temperature profiles in the planetary
atmosphere for different metallicity ratios
are shown on the right.
Spectra of the planet with a stratosphere
are dramatically different. They are still
dominated by H2O and CO in the
infrared.
BUT: all molecular features are seen in
emission, including strong CO emission
at 4.5 μm (figure on the right):
 The planet is brighter if it has a
stratosphere
 Planets with stratospheres are
most probable to be detected
directly
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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References:
Roos-Serote, M. “Organic molecules in planetary atmospheres”, in Astrobiology:
Future Perspectives, 2004, Kluwer
Roush, T.L., &
organics”, ibid.
Cruikshank, D.P. “Observation and laboratory data of planetary
Gilmour I., & Sephton M.A. An introduction to astrobiology, 2003, Cambridge
Astrobiology: 5. Habitable zones and Planetary Atmospheres
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