Theoretical considerations and various lines of morphologic evidence suggest that, in addition to the normal seasonal and climatic exchange of H2O that occurs between the Martian polar caps, atmosphere, and mid to high latitude regolith, large volumes of water have been introduced into the planet's long term hydrologic cycle by the sublimation of equatorial ground ice, impacts, catastrophic flooding, and volcanism. Under the climatic conditions that are thought to have prevailed on Mars throughout the past 3 to 4 b.y., much of this water is expected to have been cold trapped at the poles. The amount of polar ice contributed by each of the planet's potential crustal sources is discussed and estimated. The final analysis suggests that only 5 to 15 pct. of this potential inventory is now in residence at the poles

Clifford, S. M.

English

NASA Technical Reports Server

6 Wor_hoponthePolarRegtonsofMars
extent. It is possible that these bins could be dominated by
data from the Argyre and Hellas Basins, which are within the
two longitude wedges represented at these times; further
study of the data in these Ls bins will be needed to resolve
this question.
Acknowledgments: This work was partially supported
by the NASA Planetary Atmospheres Program through Grant
NAGW 2337.
References: [1] James P. I3. et al. (1979) JGR, 84,
2889-2922. [2] Kieffer H. H. et al, (1976) Science, 193,
780-786. [3] Pollack J. B. et al. (1990) JGR, 95, 1447-1474.
WATER ON MARS: INVENTORY, DISTRIBUTION,
POSSH3LE SOURCES OF POLAR ICE. S. M. Clif-
ford, Lunar and Planetary Institute, 3600 Bay Area Blvd.,
Houston TX 77058, USA.
Theoretical considerations and various lines of mor-
phologic evidence suggest that, in addition to the normal sea-
sonal and climatic exchange of H20 that occurs between the
martian polar caps, atmosphere, and mid- to high-latitude
regolith (e.g., [1,2]), large volumes of water have been intro-
duced into the planet's long-term hydrologic cycle by the
sublimation of equatorial ground ice, impacts, catastrophic
flooding, and volcanism. Under the climatic conditions that
are thought to have prevailed on Mars throughout the past
3-4 b.y., much of this water is expected to have been col&
trapped at the poles. In this abstract the amount of polar ice
contributed by each of the planet's potential crustal sources is
discussed and estimated. The final analysis suggests that only
5--15% of this potential inventory is now in residence at the
poles.
Recent estimates of the inventory of water on Mars
suggest that the planet has outgassed the equivalent of a
global ocean at least several hundred meters deep. Evidence
for such a large inventory is provided by a long list of martian
landforms whose morphology has been attributed to the
existence of subsurface volatiles [3-6]. In particular, it is
supportedby the existence of the martian outflowchannels,
whose distribution, size, and range of ages suggest that a
significant body of groundwater was present on Mars
throughout much of its geologic history [4,5,7,8]. Based on a
conservative estimate of the discharge required to erode the
channels, and the likely extent of their original source region,
Cart [4,5] estimates that Mars may have outgassed the
equivalent of a global ocean of water 0.5-1 km deep.
On Mars thereare essentiallythreereservoirsinwhich
watercan reside:the atmosphere,perrcnialpolarcaps,and
near-surfacerust.Of these,theatmosphereisknown tocon-
tain-15precipitable micrometers of water averaged over the
planet's surface, while the quantity of water stored as ice in
the polar caps is equivalent to a global layer approximately
10 m deep. This leaves more than 98% of the suspected
global inventory of water on Mars unaccounted for--virtually
all of which is thought to reside as ground ice and groundwa-
ter beneath the surface.
Although mean annual surface temperatures are below
freezingeverywhereon Mars, observationsmade by theVi-
king Orbiter Mars Atmospheric Water Detectors (MAWD)
indicate a globally averaged frost point temperature of
-198 K. Therefore, given the present latitudinal range of
mean annual surface temperatures (-154-21 g K), any subsur-
face H20 is unstable with respect to the water vapor content
of the atmosphere at latitudes equatorward of+40 ° [9].
The survival of ground ice at equatorial and temperate
latitudes was considered in detail by both Clifford and Hillel
[10] and Fanale et al. [11]. They found that, for reasonable
values of porosity and pore size, the near-equatorial crust has
probably been desiccated to a depth of 300-500 m over the
past 3.5 b.y., assuming that our present knowledge of the
quasiperiodic changes in martian obliquity and orbital ele-
ments is accurate (e.g., [12-14]. However, because the sub-
limation of H20 is sensitively dependent on temperature, the
quantity of ice lost from the regolith is expected to decline
with increasing latitude, falling to perhaps a few tens of me-
ters at a latitude of 35 ° [10,11].
By integrating the the likely pore volume between the
martian surface and the desiccation depths discussed above,
Fanale et al. [11] have estimated that over the course of mar-
tian geologic history as much as 2.7-5.6 x 106 km 3 of H20
(equivalent to a global ocean -20-40 m deep) may have been
sublimed from the equatorial regolith and cold-trapped at the
poles. However, as mentioned at the outset of this discussion,
the sublimation of equatorial ground ice is just one of several
potential processes that may have episodically introduced
large volumes of water into the atmosphere.
Perhaps the clearest evidence that the martian crust has
been a major source of atmospheric water are the outflow
channels. The abrupt emergence of these features from re-
gions of collapsed and disrupted terrain suggests that they
were formed by a massive and catastrophic release of
groundwater [7,8,15]. Channel ages, inferred from the density
of superposed craters, indicate at least several episodes of
flooding--the oldest dating hack as far as the Late Hesperian
(-2-3 b.y. ago), while the youngest may have formed as re-
cently as the Mid-to-Late Amazonian (i.e., within the last
1 b.y.) [16-20]. Based on a conservative estimate of how
much material was eroded to form the channels (-5 x
106kin 3) and the maximum sediment load that the flood
waters could have carried (40% by volume), Cart [5] has
estimated a minimum cumulative channel discharge of 7.5 x
106 kms of H20 (the equivalent of a global ocean -50 m
deep).
Impacts into the ice-rich crust may have been another im-
portant source of a_nospheric water. Assuming that the
thickness of permafrost on Mars averages about 2:5 km and
has an ice content of 20%, the volume of water excavated
https://ntrs.nasa.gov/search.jsp?R=19930010616 2020-03-17T08:26:09+00:00Z
LP! Technical Report 92-08, Part ] 7
and/or volatilized by an individual impact will range from
-34 km3 for a crater 10 ]an in diameter to in excess of 2.8 x
105 km3 for a major impact basin like Hellas (D ~ 2000 kin).
Given a global crater size-frequency distribution equivalent to
that preserved in the oratered highlands (e.g., [21]), the re-
sulting total volume of water that may have been injected into
the atmosphere by impacts over the course of martian geo-
logic history is roughly 1.5 x 107 km3 (-105 In). Note that,
because the crater statistics of [21] do not include any correc-
tion for obliteration or erosion, this estimate is likely a lower
bound.
Finally, as discussed by Greeley [22] and Plescia and
Crisp [23], volcanism has probably also introduced large
volumes of water into the martian atmosphere. For example,
if the water content of martian magmas is comparable to that
of terrestrial mafic to ultramafic lavas (-1% by weight),
Plescia and Crisp [23] estimate that the formation of the
volcanic plain_ in southeastern Elysium (5°N, 195°W) may
have alone exsolved between 103-104 lan 3 of H20. Gr_eeley
[22] has taken a more global perspective, calculating the total
volume of juvenile water released from the planet's interior
by estimating the extent and thickness of all volcanic units
visible on the planet's surface. However, the estimate of
extrusive magma production on which Greeley's [22]
calculation is based has recently been revised by Greeley and
Sclmeid [24]. Substituting this revised figure into Greeley's
[22] analysis suggests that volcanic processes have injected
-2.3 × l06 km3 (-16 m) of H20 into the atmosphere. It
should be noted, however, that in light of our inability to
accurately assess both the extent of plutonic activity and the
magnitude of ancient (>4 b.y. old) volcanism, this estimate is
probably a minimum. Note also that, unlike water derived
from other crustal sources (which simply undergoes an
exchange from one volatile reservoir to another), water
released by volcanism represents an actual addition to the
planet's outgassed inventory of H20. However, once this
water has been introduced into the atmosphere, its fate is
governed by the same processes that affect water derived
from any other crustal source---leading to a slow but
inexorable transfer of water from equatorial and temperate
latitudes to the pales [10,1 l].
Indeed, even if we consider only those sources of atmos-
pheric H20 for which there is unambiguous evidence (i.e.,
volcanism and catastrophic floods), it suggests that Mars
should possess a polar inventory of H20 roughly an order of
magnitude greater (-1 × 107 km3) than that which is pres-
ently observed in the caps. If the potential contribution fi'om
impacts and the sublimation of equatorial ground ice is also
included, it could easily increase this disparity by an addi-
tional factor of three.
Potential solutions to this mass balance problem include
(1) the amount of water released to the atmosphere by vol-
canis_ catastrophic floods, and other processes may have
been significantly smaller than presently believed; (2) the
geographic location of the spin axis of Mars has wandered
over geologic time due to changes in the planet's moment of
inertia, thereby redistributing ice_ cold-trapped at the poles
over a much larger area of the planet [25]; or (3) the bulk of
the ice deposited in the polar regions has been reintroduced
into the crust through the process of basal melting [26]. A
more detailediscussionofpolardeposition,basalmelting,
and thepolar mass balance iscontainedinClifford[27].
References: [I]JakoskyB. (1985)Space Sci.Rev.,41,
131-200.[2]Zent A_ P. etal.(1986)Icarus,67, 19-36.
[3]Rossbacher L. A. and. Judson S. (1981) Icarus, 45,
39-59. [4] Cart M. H. (1986) Icarus, 68, 187-216. [5] Cart
M. H. (1987) Nature, 326, 30-35. [6] Squyres S. (1989)
Icarus, 79, 229-288. [7] Sharp R. P. and Malin M. (1975)
GSA Bull., 86, 593--609. [8] Baker V. (1982)The Channels
ofktars, Univ. of Texas, Austin. [9] Farmer C. B. and Doms
P. E. (1979) JGI_ 84, 2881-288g. [10] Clifford S. M. and
Hillel D. (1983) JGR, 88, 2456-2474. [11] Fanale F. P. et al.
(1986) Icarus, 67, 1-18. [12] Ward W. (1979)JGt_ 84,
237-241. [13] Toon O. B. et al. (1980) Icarus, 44, 552-607.
[14] Bills B. G. (1990)JGR, 95, 14137-14153. [15] Cart
M.H. (1979) JGI_ 84, 2995-3007. [16] Neukum O. and
Wise D. (1976) Science, 194, 1381-I387. [17] Cart M. H.
and Clow G. (1981) Icarus, 48, 91-117. [18] Tanaka K.
(1986) Proc. LPSC 17th, in JGI_ 91, E139-E158. [19] Par-
ker T. J. et al. (1989) Icarus, 82, 11 !-145. [20] Rotto L. and
Tanaka K. L. (1991) In LPI Tech. Rpt. 92-02, 111-112.
[21] Barlow N. (1990) JGI_ 95, 14191-14201. [22] Greeley
R. 0987) Science, 236, 1653-1654. [23] Plescia J. B. and
Crisp J. (1992) LPI Tech. Rpt. 92-02, 102-103. [24] _eeley
R. and Sclmeid B. (1991)LPSXXII, 489-490. [25] Schultz
P. H. (1985) $ci. Am., 253, 94-102. [26] Clifford S. M. (1987)
JGI_ 92, 9135-9152. [27] Clifford S. M. (1992)JGR, sub-
mired.
t',l-9  os-- P
RHEOLOGY OF WATER-SILICATE MIXTURES AT ---
LOW TEMPERATURES. William B. Durham, Lawrence
Livermore National Laboratory, Livermore CA 94550, USA.
Our laboratory studies of the effects of hard particulates
on the rheology of ice have been mainly directed at the evo-
lution of the Galilean satellites, but yield results that may be
applicable to the theology of the martian polar caps. Our ex-
periments have explored the ductile theology as well as brit-
tle behavior of water + particulate (mainly quartz) mixtures
in particulate volume fi'actions _ ranging from 0.001 to 0.56,
particulate sizes 1-150 tan, temperatures 77-224 K, and de-
formation rates 3.5 x 10-7 to 3.5 × 10-4 s -1, under confining
pressures of 50-100 MPa. Particulates act mainly to
strengthen the material in the ductile field, although work by
others has shown that very close to the melting temperature
hard particulates can actually cause softening (possibly by
impeding grain growth). So-called dispersion hardening by


Water on Mars: Inventory, distribution, and possible sources of polar ice

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server