Radar Exploration of Mars: Recent Results and Progresses - Planetary Exploration and Science: Recent Results and Advances - page 83

SHARAD data contributed to investigate Martian pedestal craters, being able to
detect the base of their deposits. Pedestal craters, discovered by Mariner 9 mission,
are craters characterized by ejecta material more resistant to erosion compared to
the surrounding terrains. They distinguish from craters detected on the Moon by
sitting on a raised platform making a sort of pedestal. The erosion mechanism is
still debated: McCauley ( 1973 ) proposed eolian erosion as origin of the pedestal
craters, while Arvidson et al. ( 1976 ) suggested that the different erosion between
the pedestal and its perimetric scarp is due to different wind drag velocity threshold
for erosion. On the contrary, Head and Roth ( 1976 ) on the basis of the observed
azimuthal symmetry of pedestals, certainly not caused by the action of prevailing
winds, claimed that the mechanisms which control the pedestals origin are mostly
due to the ejecta themselves. Pedestals diameter varies between less than a kilometer
to hundreds of kilometers, while their height is generally less than 500 m.
Nunes et al. ( 2011 ), using SHARAD data, analyzed almost a hundred
pedestal craters. Due to the radar characteristics, radargrams of craters with
diameters <30 km are affected by clutter and, therefore, show basal reflection
only in few cases. Conversely, through SHARAD data it was possible to detect
the base of larger craters and to measure an average dielectric constant of 4,
compatible with either a mixture of ice and silicates or with a porous silicate,
possibly ash deposits.
Both MARSIS and SHARAD radars, with their radargrams, provided a useful
contribution to analyze the nature of the Medusae Fossae Formation (MFF). The
MFF is believed to be one of the youngest geologic deposits on Mars and has unique
characteristics. There is a special interest in investigating this region with a radar
sounder, since the eastern part of MFF correspond to the “Stealth” region on Mars,
so named because no echo was detected by radar operating from Earth, suggesting a
fine-grained composition of MFF deposits, such as volcanic ash or eolian sediments.
As Mars Express flied over MFF, MARSIS detected secondary echoes interpreted
as a subsurface interface between the MFF material and the basal substrate. On the
basis of MARSIS data, Watters et al. ( 2007 ) estimated for MFF deposits materials
a dielectric constant of about 2.9 ˙ 0.4, which suggests that MFF deposits could
either consist of dry highly porous or unconsolidated materials or of components
with higher dielectric constant mixed to ice. Carter et al. ( 2009 ), studying SHARAD
data, confirmed Watters results.
One of the main radar objectives is to probe the ice-rich polar layered deposits
of Mars on both south and north pole. On the south pole, only MARSIS has the
capabilities to penetrate deep into the deposits, which extends for few kilometers
beneath the surface. MARSIS data covering Martian south pole generally show, in
addition to the surface echo, a very bright secondary echo starting as the spacecraft
passes over the margin of south polar layered deposits (SPLD) (Fig. 5.5 ).
The area marked by the lower bright reflections extends from the margin of the
layered deposit to its thickest part ( 3.7 km), i.e., from 310 ı to 0 ı east longitude.
The interface is interpreted (Plaut et al. 2007 ) as the boundary between the base
of the ice-rich south polar layered deposits materials and the basal substrate.