Third Conference on Early Mars (2012)
7060.pdf
DID MARS EVER HAVE A LIVELY UNDERGROUND SCENE? Joseph. R. Michalski, Natural History Museum, London, UK and Planetary Science Institute, Tucson, AZ, USA. [email protected]
Introduction: Prokaryotes comprise more than
50% of the Earth’s organic carbon, and the amount of
prokaryote biomass in the deep subsurface is 10-15
times the combined mass of prokaryotes that inhabit
the oceans and terrestrial surface combined [1]. We do
not know when the first life occurred on Earth, but the
first evidence is found in some of the oldest preserved
rocks dating to 3.5 or, as much as 3.8 Ga [2]. While the
concept of a “tree of life” breaks down in the Archean
[3], it seems likely that the most primitive ancestors of
all life on Earth correspond to thermophile
chemoautotrophs. Perhaps these are the only life forms
that survived intense heat flow during the Late Heavy
Bombardment or perhaps they actually represent the
first life forms, which may have developed to take advantage of existing chemical gradients within the crust
[3-4].
The environments on early Mars and Earth may
have been similar (Figure 1). But, Mars cooled quickly
and since the Noachian, the surface has been cold, hyperarid, oxidizing, and probably inhospitable to life.
On Earth, life never developed photosynthesis until ~3
Ga (which corresponds to the end of the Hesperian)
[2], and probably didn’t colonize the land surface until
the Proterozoic (Amazonian) [5]. It is entirely possible
that even if life did form on Mars, it never colonized
the surface. Ongoing efforts to characterize the habitability of Mars by studying sediments that formed at the
surface might produce misleading results because they
are investigating environments that might never have
been inhabited on a planet that is very much habitable.
Spectroscopic results over the last 5-10 years have
revealed significant diversity, abundance, and distribution of alteration minerals that formed from aqueous
processes on ancient Mars (recently summarized by
Ehlmann et al. [6]). The mineralogy and context of
these altered deposits indicates that deep hydrothermal
processes have operated on Mars, and might have persisted from the Noachian into the Hesperian or later. In
this work, I consider the implications of recent results
for the habitability of the subsurface, the occurrence of
groundwater, and the possibility to access materials
representing subsurface biological processes.
Results: I carried out a survey of deep craters with
the intention of evaluating morphologic and mineralogical evidence for groundwater upwelling [7] in the
deepest basins on Mars – where groundwater would
have been most likely to emerge. A survey of deep,
ancient craters in the northern hemisphere shows that
most of the basins do not show any evidence for
groundwater upwelling. But, several craters in the
northwest Arabia Terra region do show such evidence,
which could indicate that a regional event occurred in
this area. The null results provide a way to constrain
the minimum depth below the surface of a saturated
groundwater zone, and craters that do show evidence
for upwelling allow for constrains on the slope and
absolute elevation of a past groundwater surface.
Figure 1: A concept diagram comparing the early histories of the Earth and Mars, with major events in the biological history of the Earth compared to epochs of alteration on Mars.
Third Conference on Early Mars (2012)
7060.pdf
regard to the habitability of ancient Mars. The types of
geologic processes that allowed life for form or survive
on the early Earth were also occurring on early Mars.
In order to truly characterize the habitability of Mars, it
will ultimately be necessary to focus on the geology of
the subsurface. Subsurface prokaryotes are not simply
extremophiles that could survive in the deep crust of
Earth or Mars. In fact, we are the extremophiles living
at the surface looking at the largest category of simple
life forms known, which occur at depth.
References: [1] Whitman, W. B. D. C. Coleman
(1998), and W. J. Wiebe, PNAS, 95, 6578-6583. [2]
Rothschild, L. J. and R. L. Mancinelli (2001), Nature,
409. [3] Martin, W. F. (2011), Biology Direct, 6:36 [4]
Parkes, R. J. et al. (2011), Geology, 39 (3), 219-22 [5]
Knauth, L. P. and M. J. Kennedy (2009), Nature, 460,
728-732. [6] Ehlmann, B. E. et al. (2011), Nature, 479,
53-60. [7] Andrews-Hanna, J. et al. (2010), JGR, 115.
[8] Clifford, S. M et al. (2010), JGR, 115, E07001. [9]
Niles, P. B. and J. Michalski, Nature, 2(3), 215-220.
escape
sublimation and evaporative loss
porosity
35%
T-gradient
a
1
modern cryosphere + brine
2
ag
m
m
5
2
intermediate Ca-Mg-brines
(sticky water?)
3
ient
d
nt gra
ancie
GEL 100 m
weak brine
at
is
recent grad
o
10 C/Km
GEL 200 m
c 1
acidic ice
clay formation
dilute water
n
tio
tra
ues
) seq
arly?
deep (e
GEL 300 m
m
o /Km
20 C
depth (km)
3
b
paleocryosphere
ient
hydrothermal
1%
zone
I propose a model of the subsurface geology of
Mars which includes 4 zones involving groundwater
[8]: 1) a surface cryosphere contain acidic ice deposits
within which, sulfates might form and below which,
clays may have formed [9]; 2) a shallow (1-2 km
depth) unsaturated zone through which transient meltwater from surface ice (or episodic rain) could have
passed, leaching the most mobile cations from basaltic
materials and remerging after short traverse distances
to deposit chloride salts; 3) a deep unsaturated zone
containing disseminated clays and other hydrous silicates, through which strong brines may have passed
and rarely or never reemerged at the surface; and 4) a
very deep (>4-6 km) saturated zone with dense brines
in limited pore space associated with highly altered
crust. The combined total of all of these reservoirs
could constitute a significant amount of Mars’ global
water budget, in subsurface fluid and structural water
in minerals.
The subsurface was hydrothermally active early in
Martian history [6]. Serpentinization in particular
would have been an important process to consider with
deep hydrothermal fluid
altered crust
4
10
locally elevated T-gradient?
-100
0
100
T (oC)
200
Figure 2: A conceptual diagram of the distribution and context of groundwater on Mars. At the left, a model of porosity as a function of depth (after [8]) shows that the deep crust could contain a significant amount of pore water. Two estimates of thermal gradients on Noachian Mars and in recent time show that the base of the cryosphere would have grown to significant depth since the
Noachian. But earlier, hydrothermal processes could have occurred in the deep crust (zone 4) or possible in the unsaturated zone
(zone 3). In zone 2, weak cryobrines could have traversed the crust only weakly altering basaltic material and becoming enriched
in Na and K that would have been deposited as chlorides upon emergence during upwelling events. Sulfates correspond to icedriven weathering processes in zone 1.