| Based on the geomorphic interpretation of a wide variety of Martian landforms, and a
conservative estimate of the volume of water required to erode the outflow channels, Carr [1,
2] has estimated that, at the time of peak outflow channel activity (~2-3 Ga), Mars possessed
a planetary inventory of water equivalent to a global equivalent layer ~0.5–1 km deep. As the
outflow channels significantly post-date the period when the most efficient mechanisms of
water loss (impact erosion and hydrodynamic escape) were thought to be active (>4 Gya) [3],
it is expected that the bulk of this water still survives on Mars today, 90-95% of which is
believed to be stored in the subsurface, as either ground ice or groundwater [2,
4].
However, a recent theoretical investigation of the evolution of subsurface H2O on Mars
[5] suggests that the equatorial regolith should have been completely desiccated, just a few
hundred million years following the inferred transition from a warm to cold early climate, ~
3.7-3.9 Ga.
Yet there is evidence of significant, and geologically recent, outflow channel activity
at several equatorial locations, including: Mangala Valles (~0. 2-1 Ga [7], Kasei
and Echus Chasma (~0.07 – 1 Ga [8, 9], and Cerberus/Athabasca Valles (~2 Ma
[10-12]).
Recent atmospheric methane observations also appear to indicate a low-latitude
subsurface source [13, 14]. Since there is no obvious evidence of local volcanism, the most
plausible origins of this methane appear to be either the serpentization of olivine or the
presence of methanogenic bacteria – both of which require the presence of a significant
subsurface reservoir of liquid water. Thus, if the methane is recent, that reservoir of liquid
water should still exist.
However, if the methane is old (i.e., formed during the Noachian and preserved to the
present day as gas hydrate by the diffusion limiting properties of the regolith) then it implies
that the vast majority of the H2O that existed in the equatorial subsurface at the time of peak
outflow channel activity should still be there (either as ground ice or groundwater). This
follows because the diffusion coefficient of methane is similar to that of H2O – implying
more restrictive subsurface diffusive conditions than generally assumed by most theoretical
models.
Thus, the prediction that equatorial subsurface H2O should have experienced rapid
desiccation is in conflict with several independent lines of observational evidence, including
the occurrence of Mid- to Late Amazonian outflow channel activity and recent emissions of
methane, which can only be explained by the survival of an equatorial reservoir of H2O to the
present day.
References: [1]Â Carr, M. H. (1986) Icarus 68, 187-216. [2] Carr, M. H. (1996) Water on
Mars, Oxford University Press. [3] Tanaka, K. (1986) JGR 91, 139-158. [4] Clifford, S.
M. (1993) JGR 98, 10973-11016. [5] Grimm, R. and S. Painter (2009), GRL, 36,
L24803. [6] Basilevsky et al. (2009), Planetary andSpace Science 57, 917–943. [7]
Chapman et al. (2009), EPSL 294, 238–255. [8] Neukum et al. (2009), Earth and
Planetary Science Letters 294, 204–222 [9] Hartmann, W. and D. Benman (2000),
J. Geophys. Res. 105, 15011–15026. [10] Burr et al. (2002), Icarus 159, 53–73.
[11] Plescia, J. (2003), Icarus 164 (2003) 79–95. [12] Formissano et al. (2004),
Science 306, 1758 (2004). [13] Mumma et al. (2009), Science 323, 1041-1045. |