EPSC Abstracts,
Vol. 3, EPSC2008-A-00156, 2008
European Planetary Science Congress, © Author(s) 2008
PEDESTAL CRATERS IN UTOPIA PLANITIA AND MALEA PLANUM: EVIDENCE FOR A PAST
ICE-RICH SUBSTRATE FROM MARGINAL SUBLIMATION PITS.
S. J. Kadish (1), J. W. Head (1), N. G. Barlow (2), and D. R. Marchant (3)
(1) Brown University, Rhode Island, USA ([email protected] / Fax: +1-401-863-3978), (2) Northern
Arizona University, Arizona, USA, (3) Boston University, Massachusetts, USA.
Introduction: Pedestal craters (Pd) are a subclass of
impact craters unique to Mars [1] characterized by a crater
perched near the center of a pedestal (mesa or plateau) that
is surrounded by a quasi-circular, outward-facing scarp. The
marginal scarp is usually several crater diameters from the
crater rim (Figs. 2,4,5), and tens to over 100 meters above
the surrounding plains (Fig. 2). Pd have been interpreted to
form by armoring of the proximal substrate during the
impact event. Hypotheses for the armoring mechanism
include an ejecta covering [e.g., 3], increased ejecta
mobilization caused by volatile substrates [4], distal
glassy/melt-rich veneers [5], and/or an atmospheric
blast/thermal effect [6]. Subsequently, a marginal scarp
forms by preferential erosion of the substrate surrounding
the armored region, most commonly thought to involve
eolian removal of fine-grained, non-armored material [e.g.,
3]. An understanding of the distribution of Pd, which form
predominantly poleward of ~40°N and S latitude [7-9] (Fig.
1), and the role of redistribution of ice and dust during
periods of climate change [e.g., 10-11], suggests that the
substrate might have been volatile-rich [8-9, 12-14]. As such,
some researchers [e.g., 8-9] have proposed a model for Pd
formation that involves impact during periods of higher
obliquity, when mid- to high-latitude substrates were
characterized by thick deposits of snow and ice [e.g., 15].
Subsequent sublimation of the volatile units, except below
the armored regions, yielded the perched Pd. Thus, this
model predicts that thick deposits of snow/ice should
underlie Pd. This is in contrast to the eolian model [3],
which calls primarily for deflation of sand and dust.
Here, we show the results of our study [8,16] that has
documented and characterized 2461 Pd on Mars
equatorward of ~65° N and S latitude (Fig. 1) in order to test
these hypotheses for the origin of pedestal craters. In
particular, we report on the detection of 50 Pd in Utopia
Planitia and 21 Pd in Malea Planum that have pits in their
marginal scarps [17]. We interpret these as sublimation pits
(Fig. 3), providing evidence for snow/ice deposits preserved
below the protective cover of the Pd.
Marginal Pits in Pedestal Craters: Pedestal craters with
marginal pits are a newly identified crater morphology in
which one or more pits exist along the marginal scarp of a
Pd (Figs. 2,4,5). The ejecta deposit surface (top of the
pedestal) is perched ~100 m above the surrounding terrain
(Fig. 2), about twice as high as a typical Pd crater. At the Pd
plateau edge, the marginal scarp slopes down to the
surrounding terrain, except where it is interrupted by a pit.
The pits have a typical depth of ~20 m, often contain
isolated mesas (Fig. 2), and are elongated, generally
spanning <3 km in length (measured tangential to the
pedestal margin) and <1 km in width (measured normal to
the pedestal margin). In some cases, pits appear to coalesce
to form larger pits (Fig. 5), and can yield a marginal, moatlike depression along a significant part of the pedestal
circumference. Altimetry data from MOLA indicate that pits
form in the side of the pedestal scarp; they do not extend
below the elevation of the surrounding substrate (profiles in
Fig. 2).
Pd containing scarp pits identified thus far occur poleward
of 48°N in Utopia Planitia and 58°S in Malea Planum
(orange dots in Fig. 1). Pits are similar in morphology to
dissected terrain [11,18] and pits on the floors of some
ancient outflow channels [19], both thought to represent
sublimation of an ice-rich substrate. They are also similar to
formerly ice-rich and now beheaded pits in the proximal part
of debris-covered glaciers on Earth [20] and Mars [21] (see
also [22]). Both of the regions in which we observe Pd with
marginal pits also exhibit scallop-shaped depressions,
indicative of sublimation of interstitial ice [e.g. 23-25].
Climate models show that these specific regions are both
predicted to have high seasonal water-ice accumulations
during periods of high obliquity [26,27].
Discussion: The morphologic similarity between the
marginal pits associated with Pd and ice sublimation pits
leads us to favor an origin of preferential sublimation of
ice/snow from the Pd scarp. In this interpretation, an impact
crater forms in a thick (~10s to ~100s m) regional highlatitude deposit of ice and snow, mixed with dust. The area
around the crater (the future pedestal surface) is armored by
proximal ejecta and distal sintering effects of impact melt
and atmospheric blast/thermal effects accompanying crater
formation [5-6]. Following crater formation, obliquitydriven climate change leads to removal of the intervening
snow and ice, leaving the Pd perched. Over time, the
volatile-rich scarp margins, where the armoring tapers off,
undergo continued sublimation to produce the pits, while the
heavily armored Pd surface inhibits/prevents sublimation of
underlying volatiles (Fig. 3). Ice-rich layered substrates are
thus interpreted to be preserved under Pd.
On the basis of our analysis, Pd represent the remnants of
a past extensive, layered, climate-related deposit, similar to,
but thicker than the latitude-dependent mantle emplaced in a
recent ice age [11,18]. Due to the large number and
widespread distribution of Pd (Fig. 1) [8,9,16], we believe
that this climate-related deposit persisted for a considerable
part of the recent past, implying that obliquity was relatively
higher than at present during a significant portion of the
Amazonian period of the history of Mars.
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