ASTEROIDAL
ORIGIN
OF
EARTH'S
IMPLICATIONS FOR MARTIAN WATER
OCEANS:
J.I. LUNINE1 A. MORBIDELLI2
1
LPL, U. Arizona, Tucson AZ 85721 USA
Obs. Côte d'Azur, Nice France
2
Morbidelli et al. (M&PS 35, 1309, 2000) quantified the delivery of water to
the forming Earth from a number of solar system sources, and concluded that
planetary embryos in the primordial asteroid belt constitute the largest
source-- one consistent with the deuterium-to-hydrogen ratio (D/H) in
chondrites and Earth's ocean. In the present paper we explore the implications
of our work for the delivery of water to Mars during that planet's formation,
and assess whether the results are consistent with constraints on the early
D/H of Martian water. While the D/H of the present Martian atmosphere
almost certainly reflects strong fractionation during loss of the atmosphere
over geologic time, the SNC meteorite record of the ancient Martian crust
provides important constraints on the original water inventory. Leshin (GRL
27, 2017, 2000) obtained a D/H ratio twice that of Standard Mean Ocean
Water (SMOW) in the meteorite QUE94201, and interprets this as the
signature of an early, rapid episode of atmospheric doubling of D/H from the
SMOW value. However, an alternative view would be that the 2 x SMOW
value of D/H is a primordial value and that Mars received a different mix of
high vs. low D/H water during its formation, relative to the Earth. If so,
where does the high D/H water come from, and with how much water might
the crust of Mars have been initially endowed?
We consider three reservoirs of water in the solar system. The first is the
primordial asteroid belt, with some three orders of magnitude more material
than is in the current belt, and with D/H, based on the mean for chondrites,
very close to the SMOW value. The second source is water cold trapped at a
nebular condensation front located somewhere in the 3-5 AU region, a
natural consequence of the nebular temperature gradient at the time of the
formation of the solar system's present cohort of planets. Up to 20 Earth
masses of water ice might have existed there. The D/H ratio of this material
depends on its exposure as water vapor to high temperatures, but in principle
would be lower than for molecular cloud ices that had never evaporated.
Finally, 50 Earth masses or more of ice in the form of cometary material
would have been present in the outer solar system. If this material was largely
formed of molecular ices than D/H would have been twice SMOW or even
higher. In the presentation we will use the results of our published
simulations, which delivered appropriate amounts and isotopic compositions
of water to Earth, to estimate the amount and D/H composition of water
delivered to Mars. While it is possible to bring enough material from outer
solar system alone to formally satisfy the geologic constraints on Martian
water--and such material would be biased toward high D/H--the early arrival
of this material mitigates against its retention.