41st Lunar and Planetary Science Conference (2010)
1823.pdf
MODELING AN ICE-RICH LOBATE DEBRIS APRON IN DEUTERONILUS MENSAE. J L Fastook1, J W
Head2, J-B Madeleine3, F Forget3 and D Marchant4 (1University of Maine, Orono, ME 04469, fastook@maine.edu, 2Brown
University, Providence, RI, 3-LMD, CNRS/UPMC/IPSL, 4Boston University, Boston, MA.)
Introduction: Models of ice sheets on Mars have
helped to identify and interpret glacial deposits observed from orbit [1], while also testing hypothetical
scenarios that may have been responsible for their
formation [2, 3]. In many of these cases the ice sheets
themselves existed only in the past during periods of
different climate dictated primarily by dramatic
changes in obliquity [2, 4]. Here instead we are looking at lobate debris aprons (LDA) that have recently
been proven to contain hundreds of meters of relatively
pure water ice [5, 6, 7].
LDAs in the Deuteronilus Mensae region in the
fretted terrain along the dichotomy boundary [8] have
been recognized to involve significant amounts of water ice since Viking observations [9, 10], but controversy over the amount of water ice involved has ranged
from very low (~20-30%, ice assisted talus flow [11,
12]), medium (~30-80% rock-glaciers [13], and high
(>80% debris-covered glaciers [14, 15, 16, 17]. In addition, the low number of craters on the LDA surfaces
requires mid-to-late Amazonian ages for either formation, or at least, significant deformation [18]
Recent subsurface radar sounding from orbiting
spacecraft (SHARAD on MRO [19]) has confirmed
that many, if not most, of the LDAs contain relatively
pure water ice covered only by a thin layer of debris
shed from adjacent scarps [5, 6, 7]. Their requirement
that the valley floor extend undistorted beneath the
observed LDAs dictated a dielectric constant consistent with pure (<10% contaminant) water ice. In addition, they can constrain the surface debris layer to be
less than ~10 m due to the lack of a shallow soil-ice
interface in the radar data, but they point out that it
must be greater than 0.5 m to explain the lack of a hydrogen signal in gamma ray/neutron data [20, 21, 22].
Figure 1 shows a radargram and ground track from
[7] for a LDA at 39.1N, 24.2E in the Deuteronilus
Mensae region. We will focus on the feature in the lefthand side of the track, where it crosses between two
mesas. The profile in (b) of the figure shows a classic
convex profile indicative of viscous flow.
Modelings: UMISM, as used here, is an adaptation for the Martian environment [23, 24, 25] of a
thermo-mechanically coupled shallow-ice approximation terrestrial ice sheet model used for time-dependent
reconstructions of Antarctic, Greenland, and paleoicesheet evolution in response to changing climate on
Earth [26].
Figure 1. (from [7]) (a) as time delay, (b) assuming
water-ice dielectric constant, (c) simulated radargram
showing off-nadir reflectors, and (d) ground track from
The grid used in this exercise, with topography
from MOLA, is shown in figure 2, and spans 2 degrees
of latitude and longitude, centered on 40N, 23E. With a
MOLA spacing of 0.0078125 degrees, this yields a
256X256 grid with 65536 nodes, 65025 elements, and
approximately 400 m resolution While the surface
comes directly from MOLA, the bed is assumed to be
an extension of the surface outside the LDA and is flat
at an elevation of -3578 m (the nominal elevation of
the edge of the LDA). To preserve the mesas, only
those surfaces lower than -2738 m (the nominal elevation of the top of the LDA) are modified.
Experiments: Two hypotheses are tested in this
experiment: 1) alcove-only accumulation, and 2) collapse from a larger more extensive ice sheet.
41st Lunar and Planetary Science Conference (2010)
1823.pdf
305-317. [4] Madeleine et al. (2009) Icarus 203, 390405. [5] Holt et al. (2008) LPS XXIX, Abstract #2441.
[6] Plaut et al. (2008) LPS XXXIX, Abstract #2290. [7]
Plaut et al. (2009) Geophys. Res. Lett. 36, L02203. [8]
Sharp et al. (1973) J. Geophys. Res. 78, 4073-4083. [9]
Carr and Schaber (1977) J. Geophys. Res. 82, 40394054. [10] Luchitta (1984) J. Geophys. Res., 89,
B409– B418. [11] Squyres (1978) Icarus, 34, 600-613.
[12] Squyres (1979) J. Geophys. Res. 84, 8087-8096.
[13] Mangold et al. (2002) Planet. Space Sci. 50, 385401. [14] Colaprete and Jakosky (1998) J. Geophys.
Res. 103, 5897-5909. [15] Head et al. (2005) Nature
434, 346-351. [16] Head et al. (2006a) Geophys. Res.
Lett. 33, L08S03. [17] Head et al. (2006b) Earth
Planet Sci. Lett. 241, doi:10.1016/j.epsl.2005.11.016.
[18] Chuang and Crown (2005) Icarus, 179, 24– 42.
[19] Seu et al. (2007) J. Geophys. Res., 112, E05S05.
[20] Boynton et al. (2007) J. Geophys. Res., 112,
E12S99. [21] Feldman et al. (2004) J. Geophys. Res.,
109, E09006. [22] Mitrofanov et al. (2002) Science,
297, 78 – 81. [23] Fastook et al. (2004) LPS XXXV,
Abstract #1352. [24] Fastook et al. (2005) LPS XXXVI,
Abstract #1212. [25] Fastook et al. (2006) LPS
XXXVII, Abstract #1794. [26] Fastook (1993) Computational Science and Engineering 1, 55--67. [27] Marchant and Head (2004) LPS XXXV, Abstract #1405.
[28] Marchant and Head (2005) LPS XXXVI, Abstract
#1421. [29] Marchant et al. (2007) USGS OFR-2007,
1-4. [30] Shean et al. (2007) Antarctic Science 4, 485496. [31] Fastook et al. (2008) LPS XXXIX, Abstract
#1109. [32] Fastook et al. (2009) LPS XL, Abstract
#1144.
Figure 2: Surface and thickness of the modeled LDA.
Approximate groundtrack of Figure 1 is shown by the black
For the first case, for which the Earth analog is the
Mullins Glacier in the Dry Valleys of Antarctica [27,
28, 29, 30], a small catchment at the base of the scarps
is prescribed [31] with minimal ablation elsewhere. We
begin with no LDA and observe the formation time and
compare the modeled surface with the present. Figure
3 shows a sample of a possible velocity field where
alcove accumulation is 2.5 mm/yr and ablation elsewhere is -0.15 mm/yr.
For the second case, we assume a climate from a
GCM run at an obliquity of 35 degrees [4, 32] and
build a much more extensive ice sheet covering a
broader region. As this ice sheet collapses, we compare
remnants with the observed LDA.
References: [1] Head and Marchant (2003) Geology 31(7), 641-644. [2] Forget et al. (2006) Science
5759, 368-371. [3] Fastook et al. (2008) Icarus 198,
Figure 3: Velocity (mm/yr) for Alcove-accumulation case.