A global 3-D model of groundwater flow has been used to study possible behavior of groundwater on Mars and its role in creating fluvial features. Conclusions drawn from an earlier 2-D groundwater model are supplemented and expanded. Topical headings are discussed as follows: timescales of groundwater flow; wet areas on Mars and location of outflow channels; implications for valley networks; the enigma of Hellas; absence of fluvial or periglacial features on Syrtis Major; development of chaotic terrain and associated outflow channels; and structurally controlled valley networks

Howard, Alan D.

English

NASA Technical Reports Server

N92-10770
ROLE OF GROUNDWATER IN FORMATION OF MARTIAN CHANNELS
Alan D. Howard, Department of Environmental Sciences, University of Virginia,
Charlottesville, VA 22903
A global 3-D model of groundwater flow //] has been used to investigate possible behavior of
groundwater on Mars and its role in creating fluvial features. This report supplements and expands on
conclusion drawn from an earlier 2-D groundwater model [2], and is organized into topical headings:
Timescales of groundwater flow: Results from the 3-D model [11 confirm the timescales for
draining of near-surface aquifers inferred from the 2-D model [2]. The modeling indicates the difficulty
of sudden mobilization of large quantifies of groundwater to create outflow channels as suggested by Can"
[3]. Aquifers with high permeability (e.g., 103 darcies) would tend to drain to the lowest available
discharge point within 104 years, probably shorter than timescales for development of confining
permafrost. If this water were confined by a coherent permafrost, highest artesian gradients would not be
in the location of outflow channels but in the lowest basins on Mars, including those in the northern plains
and especially Hellas (Fig. 1).
Because of the distributed nature of inputs, recharge of aquifers from surface infiltration from
rain, snowmelt, or permafrost degradation occurs very rapidly, with timescales for equilibration with a
constant recharge rate of 1 to 10 cm/yr being on the order of 105 to 104 years. Spring-fed streams and
groundwater sapping processes would become active during relatively brief periods of climatic
amelioration due to orbital variations, volcanic activity, or impact events or due to regional changes in
geothermal heat flux due to volcanism.
"Wet" Areas on Mars and location of ouOqow channels: The global simulations imply which areas
were likely to have been characterized by the presence of near-surface waters or groundwater efflux
during past epochs. The groundwater model utilizes present topography (Fig. 1), except that most
simulations conceptually fill in fluvial channels in order to examine flow conditions contributing to their
formation. An accompanying abstract [1] argues that the present topography can be considered to be
indicative of conditions during channel development.
Figure 2 shows areas that have steady-state water levels less than 200 meters below average land
surface elevations under for areally uniform recharge and for a variety of values of the recharge to
permeability ratio (Q/k). Although the assumption of surface recharge may be invalid for most or all of
Mars' history, the simulations still show the likely flow patterns, relative depths, and exit locations of
water introduced into regional aquifers from any source, including melting of surficial permafrost layers.
Potential wet areas are primarily located in the lowest portions of the planet (Hellas and low portions of
the northern lowlands) but also in moderate-depth regional depressions (e.g., Argyre). Wet areas also
occur along lower portions of regional slopes, particularly in low areas intruding into highlands; such
areas are generally also locations of strong groundwater discharge. Outflow channels and some larger
valley networks are generally located in such areas (Fig. 1), such as AI Qahira, Ma'adim, lower Kasei,
Maja, and Vedra Valles, the possible source of Kasei adjacent to Hebes Chasma, and the chaotic terrain
of Margaritifer Sinus and associated outflow channels. The major exceptions are the channels emptying
into Deuteronilus and Protonilus Mensae (Auqakuh and Huo Hsing VaUes and Deuteronilus channel)
which according to present topographic maps are near the crest of the upland-lowland scarp, and Mangala
Valles, which is indicated to flow across a topographic nose.
Implications for Valley Networks: By contrast with the outflow channels, concentrations of valley
networks (stippled in Fig. 1) generally occur on high portions of the cratered uplands and thus in areas
that are relatively "dry" (Fig. 2). That is, any recharge or release of water by melting of regolith ice
would tend to migrate vertically to a deep water table and then flow laterally to exit in the "wef' areas of
Mars. This implies that the suggestions of a sapping origin to these channels [4,5,6,7] are unlikely unless
the surface aquifer is very shallow (e.g. perched on permafrost or a duricrust) or water is released locally
by melting of permafrost due to volcanic intrusions [6] or impact-generated heat [7,81. However, the
amount of surficial materials eroded during formation of the networks suggests that more volume of water
was required than could be stored in the regolith at levels above the channels [2,9,10]. Thus the channels
may have originated by runoff erosion (or sapping erosion from perched aquifers) during a warmer and
moister Noachian climate [2,111.
The enigma of Hellas: The bottom of Hellas is the lowest location on Mars by a margin of about
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https://ntrs.nasa.gov/search.jsp?R=19920001552 2020-03-19T16:26:54+00:00Z
2km. It wouldthereforebetheultimatebaselevelforglobalgroundwaterandregionalsurfaceflow. A
varietyof evidencehasaccumulatedthatthenorthernlowlandshasservedasasinkfor sedimenteroded
fromthehighlands-lowlandsboundarywithcorrespondingwater,mud,or ice-coveredseas,andfeatures
suggestiveof shorelineshavebeenidentified[121. VallesMarinerishaslayereddepositsthatmay
representsubaqueousdeposition[131. The bottom of Hellas also has deposits that could be fluvial or
lacustrine, and it has received drainage from two outflow channels. However, cursory inspection of
images of the Hellas basin does not reveal the prominent shorelines that should have been formed if
Hellas has served as the baselevel for local and planet-wide groundwater flow. A variety of explanations
could be devised for the absence of lacustrine features (particularly impermeable rocks on the flanks of
Hellas, rapid evaporation, obliteration of shorelines by mass-wasting or eolian erosion, etc.), including the
possibility that pronounced regional groundwater flow has never occurred on Mars except under special
circumstances such as by geothermal melting of permafrost.
Absence of fluvial or periglacial features on Syrtis Major: Most of the strongly sloping
highland-lowland scarps exhibit either development of outflow channels or periglacial features such as
fretted terrain [141. The absence of either on the strong Syrtis Major slope is problematical, particularly
because Syrtis Major is a major re-entrant of the highland-lowland scarp, which in other circumstances
has generally encouraged formation of channels. One explanation might be the absence of late-Hesperian
or Amazonian volcanism, which has been argued to be instrumental in melting of ground ice to form the
circum-Tharsis outflow channels I3,15]. However, Syrtis Major has a very small contributing upland
area, and much of the subterranean water could have been diverted to the nearby and lower Hellas (Fig.
1). In fact, the simulations show that "wet" conditions are mostly limited to Isidis Planitia (where features
suggestive of a former periglacial and/or lacustrine environment abound 1161) and do not extend far up
the regional slope onto Syrtis Major (Fig. 2). The presence of thick volcanic plains in Syrtis Major may
also have contributed to a deep water table.
Development of chaotic terrain and associated ouO'low channels: A role of artesian groundwater
in development of chaotic terrain and its associated outflow channels has been suggested by Carr [3].
Carr's model, involving rupture of an artesian groundwater reservoir confined by permafrost, requires
very high regolith permeability, but this model has deficiencies discussed above. Chaotic terrain has also
been compared to collapse features resulting from permafrost degradation or to sudden mobilization
(liquefaction) of sediments with high void ratios [171. These models require mechanisms for generation
of large ice contents in the regolith. A mechanism is suggested here which combines aspects of the Carr
[31 and Nummendal and Prior [171 models. The chaotic terrain lies near the base of the highland-lowland
boundary in the Chryse trough (Fig. 1). This is a potentially wet area under unconfined conditions and an
area of high artesian pressures if confined by permafrost, as pointed out by Carr. Although permafrost is
in long-term disequilibrium in equatorial areas [18], import of water from groundwater flow would easily
compensate for slow dehydration by water vapor diffusion to the surface through the regolith. Thus a
coherent permafrost is likely to have formed in this area. Furthermore, segregated ice would probably
have been formed during permafrost growth, and intrusions of ice sills and laccoliths may have occurred
due to the artesian conditions. Thus the role of artesian groundwater may not have been a direct
dewatering of a large aquifer, but more indirect through slow development of thick ice layers which
might then have melted due to intrusions or enhanced geothermal heat flow. Another possibility is slow
intrusion but sudden breaching of water laccoliths emplaced at the base of the permafrost. In either case
the need for very high aquifer permeability is avoided.
Structurally-controlled valley networks: Some martian scarps are dissected by long, narrow,
reticulate valleys that are obviously structurally controlled. The best examples occur in and around the
bend of Kasei Valles at 10-33°N 70-80°W, including the Sacra Fossa and the arborescent network
dissecting the floor of Kasei. The angle, V, included between the tapering sidewalls is a function of the
ratio, R, of the scarp backwasting rate along the fracture to that of inter-fracture areas:
Sin V/2 = 1/R.
Measurements in Sacra Fossa suggests R values from 5 to more than 35. Recharge-fed sapping valley
networks on the Colorado Plateau show much less extreme structural control, even where fracture control
is best developed [191. The very strong structural control may be due to sapping by artesian upwellings
118
along fractures. This area may have been characterized by artesian groundwater capped by permafrost
with water derived from the Lunae Planum uplands [14]. The spacing between enlarged fractures (ca. 25
km) is probably much larger than the characteristic spacing of fractures in the plateau material,
suggesting control by flow through only the widest fractures coupled with inhibition of erosion along
intervening fractures due to drawdown effects of those fractures that have been enlarged. The eroded
debris could either have been removed by fluvial flow (although the valley walls lack the sinuous
undercutting that characterizes valleys such as Nirgal) or by ice flow, analogous to the fretted terrain.
References: [11 Howard, 1991, A martian global groundwater model [this volume]; [21 Howard,
1990, NASA TM 4210, 342-344; [31 Carr, 1979, J. Geophys. Res., 84, 2995-3007; [4] Pieri, 1980,
NASA TM 81979, 362 p.; [5] Baker, 1982, The Channels of Mars, U. Texas, Austin, 198 p.; [61
Brakenridge et al., 1985, Geology, 13, 859-62; [7] Brackenridge, 1990, J. Geophys. Res., 17289-308; [8]
Wilhems and Baldwin, 1988, LPSC XIX, 1270-1; [9] Howard, 1988, NASA SP 491, 1-5; [101 Goldspiel
and Squyres, 1991, Icarus, 89, 392-410; [111 Masursky, 1973, J. Geophys. Res., 78, 4009-30; McCauley
et al, 1972, Icarus 17, 289-327; [121 Parker et al., 1989, Icarus, 82, 111-45; [13] Nedell et al, 1987,
Icarus, 70, 409-41; [141 Howard, 1991, Role of artesian groundwater in forming martian permafrost
features [this volume]; [151 Tanaka and Chapman, 1990, J. Geophys. Res., 95, 14315-23; [16] Griffazi
and Schultz, 1989, Icarus, 77, 358-81; [17] Nummendal and Prior, 1981, Icarus, 45, 77-86; [18] Fanale,
1976, Icarus, 28, 179-202; Clifford and Hillel, 1983, J. Geophys. Res., 88, 2456-74; [19] Howard and
Kochel, 1988, NASA SP 491, 6-56; [20] Pied, 1976 Icarus, 27, 25-50; Carr and Clow, 1981, Icarus, 48,
91-117. _ _----------------o _ , _ t o "
_-i -_ f------,.-----------_
Fig. 1. Contours of generalized mars topography (kms) with areas of abundant small valley
networks stippled (from [201), major outflow channels as heavy lines or thin lines with arrows (Chryse
region), and chaotic terrain (cross-ruled).
clo_/
.
_- .._-._...
• " , _ o.i
__;_-__" ._..._-.:.
___.57:5 _--%- ,
Fig. 2. Wet and dry areas of Mars. Contours show areas with groundwater at steady state within
200 m of ground surface (including saturated areas) for various values of the ratio of vertical recharge, Q
(in cm/yr), to intrinsic permeability, k (in darcies). Particularly "wet" areas are stippled (Q/k < 0.5) and
"dry" areas shown by dashed pattern (Q/k > 0.2 for near-saturation conditions).
119


Role of groundwater in formation of Martian channels

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