Various landforms possibly related to formation (growth), movement, or decay of ground ice have been identified on Mars, including fretted terrain (ft) and associated lobate debris aprons (lda), the chaotic terrain, concentric crater fills (ccf), polygonal ground, softened terrain, small domes that are possibly pingos, and curvilinear (fingerprint) features (cuf). Glaciers may also have been present. Some of these may involve ice derived from artesian groundwater. Topical areas of discussion are: Mars groundwater and the location of permafrost features; the ft, lda, ccf, and cuf; role of artesian groundwater in formation of fretted terrain, lobate debris blankets, and concentric crater fills; sources of glacial ice; and pingos and other pseudovolcanic structures

Howard, Alan D.

English

NASA Technical Reports Server

N92-10771
ROLE OF ARTESIAN GROUNDWATER IN I_DRMING MARTIAN PERMAFROST FEATURES
Alan D. Howard, I_partment of Environmental Sciences, University of Virginia,
Charlottesville, VA 22903
Various landforms possibly related to formation (growth), movement, or decay of ground ice have
been identified on Mars, including fretted terrain (fl) and associated lobate debris aprons (Ida) [1,2,12J3],
the chaotic terrain [3], concentric crater f'dls (ccjO [2], polygonal ground [4], "softened" terrain [5], small
domes that are possibly pingos [6], and curvilinear (fingerprin0 features (cuJ) [4,7]. Glaciers may also
been formerly present [8]. Some of these may involved ice derived from artesian groundwater.
Mars groundwater and the location of permafrost features: Several permafrost landforms
including ft, Ida, ccf, and, possibly, ca)Care located in narrow latitudinal belts [2,4,51 (Fig.l). The lack of
such features poleward of _)o latitude may result from ground temperatures too low for appreciable ice
flowage, and the equatorward limit at :_-30° is thought to result from climatic disequilibrium of permafrost
in equatorial regions. The permafrost features are widely distributed in the northern band, but occur in
the southern hemisphere only in the vicinity of Hellas and Argyre (Fig. 1). Squyres [21 suggests that the
the restricted southern distribution may be due to regolith properties in the vicinity of major impacts.
However, these permafrost features reveal a consistent topographic pattern (Fig. 1). The fretted terrain,
concentric crater fills, and lobate debris aprons (and, possibly, curvilinear features) occur along, and
particularly near the base of, high-relief regional slopes. Most are associated with the north-south
highland-lowland scarp, but some occur on the lower flanks of Elysium and in the Hellas and Argyre
depressions. The southern highlands lacking these features, and in the north these features are absent near
the tops of volcanic constructs and rare in the center of the major basins. Their absence poleward of
+60 ° may be related more to the absence of strong regional slopes than to reduced ice mobility.
Theft,/da, ccf, and cufoccur where groundwater may have discharged if it were unconfined [9].
Lowland areas are thus more likely to have developed permafrost with a completely saturated regolith
than highlands such as the southern hemisphere cratered terrain (which is likely to have been drained of
near-surface water during warmer epochs, during the early periods with higher geothermal heat or due to
episodes of ground heating by volcanism or impact cratering). Even if early climates were occasionally
warm enough for recharge from precipitation, water tables would have been deep in highlands [9].
In addition, the scarp-base locations of permafrost features favor artesian overpressuring. A
martian global groundwater model [10] was utilized to predict where artesian overpressuring might occur
(Fig. 2). In this simulation groundwater is assumed to be recharged (and discharged) in the latitude belt
:i:20 ° (groundwater could also be derived from melting of permafros0. Poleward of "t"90° a coherent,
impermeable permafrost was assumed to extend from the surface to a depth of 1 km at the equatorward
limit and to a depth of 4 km at the poles. Figure 2 shows contours of the artesian gradient, defined as (h -
hc)/(hs - he), where h is the hydraulic head, hs is the surface elevation, and h e is the elevation of the
base of the permafrost. Values greater than zero indicate artesian conditions (values less than zero are
not shown). The permafrost features generally occur in areas predicted to have artesian conditions. The
exceptions are portions of Deuteronilus, Protonilus, and Nilosyrtus Mensae, which extend high onto the
highlands-lowlands scarp. However, the distribution and magnitude of artesian gradients are highly
sensitive to the assumed distribution and thickness of ground ice, the assumed locations and magnitude of
recharge, uncertainties in the surface topography, and the hydrologic properties of the regolith.
Minimally the artesian conditions would assure development of permafrost with ice completely filling
voids. In addition, as detailed below and in [91, other effects may have occurred, including springs,
supply of water to glacial flows, development of segregated ice, intrusion of ice (or water sills) and
pingos, and rapid release of water derived from melting of segregated or intruded ice.
Softened terrain, possibly resulting from creep of ground ice [5], does not follow the above
distributional pattern. It is more widely distributed latitudinally and occurs on a variety of topographic
setting. If it is a permafrost feature (see dissents [11]) it probably does not involve artesian groundwater.
Role of artesian groundwater in formation of fretted terrain, lobate debris blankets, and
concentric crater fills: Theft, ida, and ccfare thought to result from slow flow of debris and ice outward
from steep scarps, with differences primarily resulting from variations in the topographic setting (narrow
troughs, scarp margins, and crater interiors). Mechanisms that have been proposed are gelifluction [121,
rock-glacier fl0w with ice derived from seasonal frosts incorporated in mass-wasted debris [2], and flow
120
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of ice-rich layers beneath the uplands, which thereby erodes the scarp margin [13]. As pointed out by
Squyres [2], the shape of the debris blankets suggests bulk movement rather than surficial movement of
the debris, casting doubt on gelifluction. Lucchitta [1J] has criticized seasonal frost _apping as being
insufficient to generate a high enough ice/rock ratio to permit bulk fow. In addition, the debris blankets
_ha_ ,,or,, !i_e v___sfion with _ect, whirh wn.ld nrahahlw ¢_r.r i¢ d_-u_eit/nn and ¢hlat/_n _}" e,_ae_nal
frosts were involved. In turn, the mechanism of flow of ice-rich layers beneath the plateau units is also
deficient. Emplacement of a high ice content beneath the plateau units must be accomplished (filling
voids and development of segregated or inu'uded ice from artesian groundwater is one possibility).
However, ff ice-rich layers in the plateau material were involved in bulk flow, lowering (as the ice flowed
out) and tilting (due to areal variations in ice content) of the plateau surface should occur, as well as
break-up and separation into small blocks due to tensional stresses on coherent upper layers. This
breakup would be accentuated by upward displacement of geothermal isotherms below plateaus (see
below). Although Lucchitta [13] points out an instance of such effects along a fretted channel margin
there is little evidence for downdrop, tilting, or relative movement of residual upland blocks and mesas.
A somewhat different mechanism for generation and flow of ice relies on artesian water pressures.
The height of fretted terrain scarps is on the order of 1-2 km, about the same order of magnitude as the
permafrost thickness. Therefore the bottom of the permafrost is displaced upwards below the scarp (Fig.
3A), and the thi,nest permafrost is found near the scarp base. If the sub-permafrost aquifer were
saturated and under artesian pressure, the gradient through the permafrost would be greatest near the
scarp base, and this, coupled with the tendency for the saturated regolith to flow downwards and outwards
could lead to a steady-state situation in which outward flow of ice and debris from the base of the scarp
(aided, perhaps, by development of segregated ice and ice intrusions) would be balanced by replacement
of ice by flow and freezing of groundwater (Fig. 3B). Depending upon the locus of the lower ice-water
contact, greater or lesser amounts of regolith would be incorporated in the outward glacial flow (with
corresponding variation in backwasting rates). This speculative scenario requires modeling of the heat
budget, groundwater flow, ice movement, and regofith response to demonstrates its feasibility.
Source of glacial ice: The former presence of glaciers has been suggested based upon
identification of possible moraines or ice-carved features [8]. Although groundwater as a water source
for such glaciers has been suggested [8], objections have been raised. Cart [14] notes that springs in
Arctic areas do not form large masses of ice, but flow for long distances before freezing. Lucchitta [8]
feels that the confined form of the putative glaciers occupying portions of Mars' outflow channels is not
the domical shape that might form from freezing of water reaching the surface. However, glaciers may
have been fed by a mechanism similar to that suggested for the lobate debris blankets (Fig. 3B) in which
groundwater does not have to reach the surface. A variety of phenomena might accompany such
glaciation, including rapid headcutting of the scarps (Fig. 3B), creation of ice-dammed lakes with the
potentiality of catastrophic draining, and jokulhlaups created by episodic draining of sub-ice lakes created
by intrusion of groundwater.
Pingos and other pseudo-volcanic structures: Numerous small domes, some with cen_al
depressions, occur on the southern portion of the northern lowlands. The predominant explanation is that
they are small volcanic domes [15], although an origin as pingos has also been suggested [6], based upon
the very strong resemblance in size and form of open-system pingos to small volcanic cones [17]. An
origin as either closed- or open-system is possible, with the position near the highland-lowlands boundary
possibly providing the requisite artesian pressures for the latter type.
References: [1] Sharp, 1973a,b, Y. Geophys. Res., 78, 4063-72, 4073-83; [2] Squyres, 1978,
Icarus, 34, 600-13; Squyres, 1979, J. Geophys. Res., 84, 8087-96; [3] Cart, 1979, ./. Geophys. Res., 84,
2995-3007; Nummendal and Prior, 1981, Icarus, 45, 77-86; [4] Rossbacher and Judson, 1981, Icarus, 45,
39-59; [5] Squyres and Cart, 1986, Science, 231, 248-52; Squyres, 1989, Icarus, 79, 229-88; [6]
Lucchitta, 1981, Icarus, 45, 264-303, Costard and Dollfns, 1986, LPI Tech. Rpt. 87-02,211-2; [7] Parker
et al., 1989, Icarus, 82, 111-45; [8] Luchitta, 1982, J. Geophys. Res., 87, 9951-73; [9] Howard, 1991,
Role of Groundwater in Formation of Martian Channels [This volume];.[lO] Howard, 1991, A Martian
Global Groundwater Model [This volume]; [11] Zimbleman eL al., 1988, LPSC XIX, 1321-2; Moore,
1990, Y. Geophys. Res., 95, 14279-89; [12] Cart and Schaber, 1977, J. Geophys. Res., 82, 4039-54; [13]
Lucchitta, 1984, J. Geophys. Res., 89, B409-18; [14] Presentation at 1989 Fall AGU; [15] Frey and
Jarosewich, 1982, Y. Geophys. Res., 87, 9867-70; [16] Muller, 1959, Medd. om Gronland 153(3), 127p.
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Role of artesian groundwater in forming Martian permafrost features

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