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Lunar and Planetary Science XLVIII (2017)
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EXOSPHERIC ESCAPE: A PARAMETRICAL STUDY. R. M. Killen1 and M. H. Burger2, 1NASA Goddard
Space Flight Center ([email protected]), 2Space Telescope Science Institute ([email protected]).
Method: We assume that in a collisionless exosphere a
neutral species escapes if it achieves escape velocity defined
as:
vesc = (
2GM 1/2
8π
) = ( G ρ R 2 )1/2
R
3
where G is the gravitational constant, R is the object radius,
M is the object radius, and ρ is the object density.
where vth is the thermal speed defined as kT/m, k is Boltzmann's constant, T is the temperature, and m is the mass of
the neutral species. The temperature could refer to the surface temperature, the temperature of the vapor cloud produced by impact vaporization, or other process that results in
a thermal distribution (e.g., photon-stimulated desorption
ejects Na with a ~1200 K Maxwellian distribution [4] ().
We also consider a sputtering distribution in the form:
f (v) ∝
EU
(E +U)3
where U is the binding energy.
Results
Figure 1 shows the temperature at which 50% of an ejected
neutral species escapes from a body as a function of the object radius and species mass, assuming the body has a density
of 3 g cm-3; i.e., it is the temperature at which half the Maxwellian distribution is above the escape velocity. A small
body such as Phobos (radius=XX km) cannot hold any atmosphere at almost any temperature above 0 K.
Figure 2 shows the fraction of sputtered gas that escapes
from an object as a function of object radius and the species
mass, assuming the surface binding energy U=2 eV, consistent with Na bound to a regolith [5].
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Gravitational Escape Fraction, Test Mass = 18 AMU
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spheres, i.e., atmospheres that are collisionless down to an
object's surface, are the most common type of atmosphere in
the Solar System. They are known to occur at the Moon,
Mercury, and several outer planet moons. The primary
mechanisms for these exospheres vary for different objects
and exospheric species, but include micrometeoroid impact
vaporization, photon-stimulated desorption, ion sputtering,
and thermal desorption. Several objects, such as Enceladus
[1], Europa [2] and Ceres [3] show evidence of active venting of water. We have conducted a parametrical study of
exospheres to distinguish between the bound and escaping
components of exospheres (an escaping exosphere is commonly referred to as a corona). We describe a method for
determining whether an exospheric species will escape from
a body as a function of the object radius (mass) and surface
temperature, source velocity distribution, and the species
mass. This can be applied to extra-solar planets as well.
f (v) ∝ v 3 exp(−v 2 / vth2 )
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Gravitational Escape Fraction: Surface-bounded exo-
Neutral species are ejected from the surface with a Maxwellian flux distribution in the form:
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stand the long-term loss of volatiles from planetary bodies
due to interactions of planets, satellites, and small bodies
with the interplanetary medium (solar wind, meteors, and
dusk), solar radiation, and internal forces including diffusion
and outgassing. Recent evidence for water and OH on the
moon has spurred interest in processes involving chemistry
and sequestration of volatile species at the poles and in voids.
In recent years, NASA has sent spacecraft to asteroids including Vesta and Ceres, and ESA sent Rosetta to the asteroids Lutetia and Steins. OSIRIS-REX will return a sample
from a primitive asteroid, Bennu, to Earth. It is possible that
a Phobos-Deimos flyby will be a precursor to a manned mission to Mars. Exospheric particles are derived from the surface and thus reflect the composition of the body's regolith,
although not in a one-to-one ratio. Observation of an escaping exosphere, termed a corona, is challenging. We have
therefore embarked on a parametrical study of exospheres as
a function of mass of the exospheric species, heliocentric
distance, rotation rate of the primary, and composition of the
body (asteroid type or icy body). These parameters will be
useful for mission planning as well as quick look data to
determine the size and location of bodies likely to retain their
exospheres and observability of exospheric species. It is also
of interest to be able to determine the extent of contamination
of the pristine exosphere due to spacecraft sent to make
measurements.
Source Temperature (K)
Introduction: The study of exospheres can help us under-
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Figure 1. Gravitational escape fraction for a Maxwellian
distribution at source temperature vs. Object radius (km).
Ceres, the Moon and Earth are marked by the black lines.
Lunar and Planetary Science XLVIII (2017)
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Sputtering Flux Distribution, U = 2 eV
Ceres
Moon
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Species Mass (amu)
Phobos
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1341.pdf
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Figure 2. Gravitational escape fraction is shown here for a
sputtering velocity distribution with binding energy 2 eV.
References:
[1] (Porco C. C. et al. (2006) Science 311, 13931401. [2] Roth L. et al. (2014) Science, 343, 171-174.
[3] Kueppers M. et al. (2014) Nature 505, 525-527.
[4] Yakshinskiy B. V. and Madey T. E. (2004) Icarus
168, 53-59. [5] McGrath M. A., Johnson R. E. and
Lanzerotti L. J. (1986) Nature 323, 694-696.