The total volatile inventory of Mars has been modeled using meteoritic and presumed primordial abundances in the early solar system. Evidence is presented which indicates that the elemental abundances of the noble gases on Earth and Mars are similar, and their ratios are comparable to those in average carbonaceous chondrites with the exception of xenon and krypton. In order to account for presently observed variations in gas abundances, two primordial sources were used. One was the solar composition similar to the solar wind, and the other of carbonaceous grains that were the source for trace exotic components. For Mars, a model in which the early, high solar EUV flux with continued hydrogen production by differentiation results in mass fractionation of the primordial atmosphere, early depletion of xenon, and later depletion of gases lighter than krypton. The result is that the primordial Mars water inventory may have been on the order of 20 to 30 km if spread over the planet

Pepin, R. O.

English

NASA Technical Reports Server

VOLATILE INVENTORY OF MARS-If: PRIMORDIAL SOURCES AND FRACTIONATING
PROCESSES. R. O. Pepin, School of Physics and Astronomy, University of
Minnesota, Minneapolis, Minnesota 55455.
The small differences in relative elemental abundances of noble gases in
the atmospheres of Earth and Mars are smooth functions of mass, and may imply
a fractionation relationship between them(l,2). Both planetary atmosphere pat-
terns are roughly similar to elemental noble gas ratios in carbonaceous chron-
drites, with the conspicuous exception of Xe: the meteoritic 130Xe/84Kr ratio
is _20x higher. The three reservoirs are isotopically distinct. Abundances of
the Xe isotopes on Earth and Mars are very different from AVCC (Av__erage _Carb°n-
aceous C_hondrite) Xe. Both appear to have been generated by gross mass fraction-
ation of meteoritic components: Mars from AVCC-Xe(3) and Earth from U-Xe, an
inferred composition closely related to AVCC-Xe but without a nucleogenetic
heavy-isotope contribution that is abundant in the carbonaceous chondrites(2,4).
Martian and terrestrial Kr compositions differ from each other and from AVCC-
Kr(3,5), and Ar on Mars has an extraordinarily low 36Ar/38Ar ratio(6). Finally,
relative elemental and isotopic abundances in the solar nebula, the presumed
primordial source for most of the noble gases in contemporary meteoritic and
planetary reservoirs, were different from the composition of these reservoirs
to varying degrees, based on estimates of "cosmic"(7,8) and solar wind(9-13)
abundances. It is clear that understanding the origin and evolution of noble
gas distributions in meteorites and planetary atmospheres, and their implica-
tions for the past and present volatile inventories of these reservoirs,
requires knowledge of the processes that generated these contemporary abundance
patterns from primordial sources, and identification of the astrophysical
environments in which they could reasonably have operated. A model that shows
some promise in this direction is outlined below.
Sources. Two primordial noble gas sources are assumed: the early solar
nebula with "cosmic" (_solar wind) composition, here called solar composition;
and pre-solar, probably carbonaceous grains carrying traces of exotic nucleo-
genetic noble gases (e.g., Ne-E, s-process Kr and Xe, H[CCF]-Xe and L-Xe).
Processes. Several reports in the literature have pointed out an apparently
fundamental process of mass fractionation in the take-up, presumably by adsorp-
tion, of ambient Ne, Ar, Kr and Xe on various carbonaceous substrates synthe-
sized in the laboratory in air or a noble gas atmosphere(14-16 and references
therein), or on natural terrestrial sedimentary materials such as shales(17-19).
The fractionation, which is strikingly uniform considering the wide range of
physical conditions under which it has been produced, is primarily elemental
--no consistent isotopic effects have been reported in nonplasma environments.
The noteworthy characteristic of this absorptive fractionation is that it
generates on the substrate, from an ambient atmosphere of solar composition,
elemental abundance ratios that begin to resemble the AVCC pattern, although
still too rich in Ne and Ar relative to Kr(14-19).
The other primary process invoked in the model is mass fractionation of
minor atmospheric species during early episodes of hydrodynamic hydrogen escape
from planetary bodies and large planetesimals. Hunten et al.(2) have recently
shown that this process, given an enhanced EUV flux from the young sun(20) and
the availability of large but not unreasonable amounts of hydrogen in the
accreted planets, could have generated the U-×e ++ Earth-Xe isotopic fraction-
ation and the apparent Mars ++ Earth noble gas elemental fractionation. This is
also the case for the AVCC-Xe++ Mars-Xe isotopic fractionation(3). Hunten et
al., while demonstrating the possible applicability of fractionation in hydro-
dy---namicescape to planetary atmospheres, did not attempt to develop a general
model to reconcile the various observations of fractionated mass distributions
https://ntrs.nasa.gov/search.jsp?R=19890001443 2020-03-20T06:06:54+00:00Z
I00
VOLATILE INVENTORY OF MARS
Pepin, R. O.
within the context of a single scenario for evolution of meteoritic and
planetary noble gases.
Astrophysical settings: [i] Planetesimals large enough (_I024-I025 g) to
acquire and retain atmospheres against thermal escape for at least limited
periods, probably 4106-107 years; and [2] the accreted terrestrial planets, q"ne
planetesimal atmospheres are solar in composition, possibly generated by inter-
nal outgassing but more probably captured from the surrounding nebula. (Gases
in dust grains comprising the planetesimals would probably have experienced
elemental fractionation during their earlier adsorption from the nebula, thus
leading to nonsolar abundances in atmospheres derived from grain outgassing).
This astrophysical setting is taken from a recent model by Donahue(21) for pro-
duction of noble gas distributions in planetary atmospheres. The Donahue model
involves accretion of the planets from two planetesimal populations, one with
atmospheres of solar composition and the other with atmospheric Ne and Ar de-
pleted by Jeans (thermal) escape. It can account reasonably well for relative
Ne:Ar:Kr elemental abundances and for Ne isotopic compositions in planetary
atmospheres. But because of the very steep mass dependence of Jeans escape,
predicted 36Ar/38Ar ratios are much too low. The model is unable to accommodate
the variations in Kr-Xe elemental and isotopic compositions in different plan-
etary reservoirs, nor does it speak to meteoritic elemental and isotopic com-
positions(21). In the present model, fractionation in Jeans escape is replaced
by fractionation during hydrodynamic escape of hydrogen, which is much less
sensitive to mass, in both planetesimal and planetary environments. The hydrogen
is assumed to be generated by reduction of H20 during impact or internal differ-
entiation, or by photodissociation of surficial H20.
Scenarios, assumptions, and results. Planetesimals, surfaced with carbon-
aceous grains carrying the nucleogenetic noble gas components, are warmed by
internal processes to at least the temperature of liquid water. In an episode
of hydrodynamic escape driven by an enhanced but declining solar EUV flux(20),
atmospheric species lighter than Xe are depleted and fractionated. The escape
episode is terminated, probably within _i0 _ years, by cooling and concomitant
cut-off of the hydrogen supply. Residual atmospheric gases experience a second
stage of elemental fractionation during adsorption on cooling grain surfaces
prior to final dissipation of the atmosphere. These adsorbed gases represent the
so-called "planetary" noble gas component. Together with the nucleogenetic com-
ponents, which are presumed not to have outgassed from pre-solar grains during
thermal/hydrothermal activity on the parent bodies, they comprise the noble gas
inventory of the carbonaceous chondrites. With appropriate choices for the
parameters of the hydrodynamic escape episode, and with certain assumptions
concerning isotopic compositions of solar noble gases in cases where they are
not well known (e.g., Ar, Kr and Xe), this scenario is capable of reproducing
the AVCC elemental and isotopic noble gas pattern.
A more complex sequence of events is required to account for the mass dis-
tribution of noble gases in planetary atmospheres. Here we focus on Mars; with
variation of parameters, the model is also applicable to Earth. (Venus needs
more thought, and perhaps a unique, solar-dominated gas component). AVCC gases
have contributed to the Mars volatile inventory, but a second component is man-
dated by the isotopic composition of martian [_SNC] Kr. A satisfactory choice
is simply a carrier of gases derived by adsorptive fractionation ("AF") of
solar gases (resulting in elemental but not isotopic fractionation), on dust
grains in the nebula or on a hydrogen-poor parent body whose solar atmosphere
was not fractionated by hydrodynamic escape.
A model scenario for Mars is then as follows. The AF and AVCC gas carriers
are supposed to have accreted into proto-Mars, in proportions such that _40%
VOLATILEINVENTORYOFMARS
Pepin, R. O. ioi
of the Kr was contributed by AVCCmaterial. Thesegases are at least partially
retained within the growing planet. The final stage of accretion involved
primarily AVCCcarriers, with impact energies sufficient to generate an AVCC-
like primordial atmosphere, rich in H2 or water vapor, by projectile outgassing.
At this stage the nebula has cleared, and the high solar EUV flux begins to
drive a prolonged (_108 year) and severe episode of hydrodynamic escape which
greatly depletes (Xe to a few % of its initial abundance, lighter gases to
larger extents) and mass-fractionates the primordial atmosphere. The residual,
severely fractionated AVCC-Xe generated in this episode remains in the contem-
porary martian atmosphere. Hydrodynamic escape may have been interrupted tempo-
rarily by exhaustion of the surficial H 2 supply, but resumes as H 2 produced in
internal differentiation is outgassed from the body of the planet(22), sweeping
with it the AF+AVCC noble gases incorporated at an earlier stage of accretion.
At this time the solar EUV flux is assumed to have decreased to the level where
only gases lighter than Kr are depleted and fractionated. This scenario, with
suitably chosen parameters for the escape episode, can account for all charac-
teristics of present-day noble gas abundances on Mars (including a low 36Ar/38Ar
ratio) except one: the low Xe/Kr ratio. Here it is necessary to introduce a
purely ad hoc assumption: that during differentiation processes under conditions
of high pressure in the martian mantle, Xe contributed by the AF+AVCC carriers
partitions very efficiently into mantle- or core-forming phases and is therefore
retained in the planet's interior. The same assumption is required for Earth.
For the particular choices of hydrodynamic escape functions and parameters
(2) that yield the requisite noble gas fractionations in this model, the
hydrogen losses needed to implement the process seem reasonable: the equivalent
of a few tens of meters of water in the planetesimal environment, and 20-30
kilometers of water on Mars. The latter loss estimate, while very large, is in
fact only about one-quarter of the initial martian water inventory if the
Dreibus-Wanke two-component mixing model for Mars(22) is correct.
REFERENCES. (1) Pepin R. O.(1986)Lunar and Planetary Science XVII, 656.
(2) Hunten D. M., Pepin R. O. and Walker J. C. G.(1986)Mass Fractionation in
Hydrodynamic Escape, preprint, submitted to Icarus. (3) Swindle T. D., Caffee
M. W. and Hohenberg C. M.(1986)GCA, in press. (4) Pepin R. O. and Phinney D.
(1978)Components of Xenon in the Solar System, preprint, submitted to The Moon
and Planets. (5) Becker R. H. and Pepin R. O. (1984)EPSL 69, 225. (6) Wiens
R. C., Becker R. H. and Pepin R. O.(1986)EPSL 77, 149. (7) Cameron A. G. W.
(1982), in: Essays in Nuclear Astrophysics, pp.23-43. (8) Anders E. and Ebihara
M.(1982)GCA 46, 2363. (9) Bochsler P. and Geiss J.(1977)Trans. Int. Astron.
Union XVIB, 120. (I0) Frick U. and Pepin R. O.(1981)Lunar and Planetary Science
XII, 303. (ll) Becker R. H., Rajan R. S. and Rambaldi E. R.(1983)Abstracts from
Workshop o_nnPast and Present Solar Radiation, Lunar and Planetary Institute,
Houston, TX, 2. (12) Becker R. H., Pepin R. 0., Rajan R. S. and Rambaldi E. R.
(1986)Light Noble Gases in Weston Metal Grain Surfaces, abstract submitted for
49th Meteoritical Society Meeting, New York, September. (13) Becker R. H. and
Pepin R. O.(1984)EPSL 70, 1. (14) Frick U., Mack R. and Chang S.(1979)Proc.
Lunar Planet. Sci. Conf. 10th, 1961. (15) Niemeyer S. and Marti K.(1981)Proc.
Lunar Planet. Sci. Conf. 12th, 1177. (16) Yang J. and Anders E.(1982)GCA 46,
861. (17) Canalas R. A., Alexander E. C. and Manuel O. K.(1968)J. Geophys. Res.
73, 3331. (18) Phinney D.(1972)EPSL 16, 413. (19) Bernatowicz T. J., Podosek
F. A., Honda M. and Kramer F. E.(1984)J. Geophys. Res. 89, 4597. (20) Zahnle
K. J. and Walker J. C. G.(1982)Rev. Geophys. Sa_ Phi. 20, 280. (21) Donahue
T. M.(1986)Fractionation of Noble Gases by Thermal Escape from Accreting Planet-
esimals, Icarus, in press. (22) Dreibus G. and Wfinke H.(1985)Meteoritics 20,367.


Volatile inventory of Mars-2: Primordial sources and fractionating processes

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server