『Abstract
Earth, Venus and Mars reached their final sizes in the first
100 Myr or so of solar system history. For part of that time the
growing planets and the materials forming them were immersed in
the Sun-like gases of the solar nebula, and so one would expect
that their early volatile inventories were acquired from the nebula.
But the compositions of atmospheres presently on these planets
are not solar, and therein lies a complex and fascinating
story of physical and chemical evolution over the past 4.5 Ga.
Records of physical processing survive most clearly in the chemically
inert noble gases, and data on the elemental and isotopic abundances
of these trace constituents, now from Mars and Venus as well as
Earth, point to atmospheric histories punctuated by enormous inputs
of energy from early astrophysical sources long since vanished.
Observational and theoretical advances during the past 30 years
underpin current evolutionary models in which primordial solar-like
atmospheric gases are fractionated by gravitational escape, driven
on Earth by a giant Moon-forming impact, on Mars by sputtering
at high altitudes, and on all three planets by adsorption of intense
ultraviolet radiation from the young Sun. Residual atmospheres
left behind after these outflows to space are augmented by planetary
degassing, including species generated in their interiors by radioactive
decay. Interplay over time of these mechanisms for loss and gain
of atmospheric gases can account for many of the details of contemporary
noble gas distributions. These of course are just models. However
they have predictive power for compositions as yet unmeasured,
particularly on Venus, and the modeling assumptions are in principle
testable by experiment or theory. The fundamental question of
whether nature actually shaped the atmospheres in this way is
still unanswered, but at least we have an outline of how it might
have happened.
Keywords: terrestrial planets; atmospheres; gravitational escape』
1. Atmospheres on (and in) the terrestrial planets: Why
are they interesting?
2. Tracers of atmospheric origin and evolution
3. In the beginning: Primordial atmospheres
4. Atmospheres now
5. Evolution from primordial states
5.1. Hydrodynamic escape
5.2. Xenology
5.3. Modeling earth
5.4. Venus and Mars
6. Summary, and a look ahead
Acknowledgments
References