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| Titel |
Three Dimensional Structure of the Mars North Polar Basal Unit from MARSIS data |
| VerfasserIn |
A. Frigeri, R. Orosei, M. Cartacci, A. Cicchetti, G. Mitri, S. Giuppi, R. Noschese, G. Picardi, J. Plaut |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063036
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| Zusammenfassung |
| Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is an orbital
subsurface sounder aboard ESA’s Mars Express spacecraft . It transmits a low-frequency
radar pulse that is capable of penetrating below the surface, and is reflected by subsurface
dielectric discontinuities. MARSIS has been used to probe both the south and the north polar
caps of Mars, revealing their thickness and structure. We report on the results of a
campaign of observations of the north polar ice cap of Mars that took place between
May and December 2011 in uniquely favorable conditions and produced data of
unprecedented quality. The focus of this work is the so-called Basal Unit, a dark, ice-rich,
complexely layered geologic unit lying stratigraphically between the polar layered
deposits and the Vastitas Borealis Formation, and extending beneath most of Planum
Boreum and Olympia Planitia. The objective of this work is the to study the full three
dimensional structure of the Northern Polar Deposit and in particular of the Basal Unit
(BU).
It was recently found that the BU consists of two markedly different units, called
the Rupes Tenuis unit and the Planum Boreum cavi unit. The Rupes Tenuis unit
appears to be older, horizontally layered, and lacking erosional contacts. It has
been thus interpreted as the result of precipitation and cold-trapping of dust-laden
volatiles. The Planum Boreum cavi unit displays cross-bedding, indicating dune
accumulation. Bright layers within it are interpreted as being made of ice-cemented
dust, while dark layers should consist of weathered basalt fines. It seems likely
that, in places, the Planum Boreum cavi unit rests directly on the Vastitas Borealis,
without the Rupes Tenuis unit in between. Because the two units in the BU have
formed much earlier than the north polar layered deposits, and at some interval from
each other, they bear evidence of past climatic conditions that were very different
from present, so that they “could potentially be a Rosetta Stone for the Martian
climate”.
Subsurface sounding radar investigations by both MARSIS and SHARAD revealed that
the BU has radar properties that are different from both the polar layered deposits and
the Vastitas Borealis Formation, probably because of a mostly icy composition,
but with a larger fraction of impurities than the polar layered deposits above. The
upper surface of the BU exhibits significant relief, with features appearing to be
erosional cutbacks and reentrants, indicating a complex accumulation history. Higher
dust content and the resulting stronger attenuation is thought to be the reason why
SHARAD radar signal could not penetrate through the BU and detect its bottom face.
However, such a volume fraction cannot be much larger than the polar layered
deposits, since MARSIS data revealed strong returns from the BU-Vastitas Borealis
Formation interface, implying a relatively low fraction of impurities within the
BU.
From the summer phase of the Polar Campaign of data acquisition we have selected 161
radargrams. The radargrams were processed in order to cancel the effect of the ionosphere
and to align the primary echo to a datum. The post-processed radargrams have
been ingested into a Free Open Source software stack for geophysical imaging and
interpretation.
We find weak echoes within the BU that appear to outline a two-layer structure, perhaps
corresponding to the Rupes Tenuis unit and the Planum Boreum Cavi unit. This was found
through visual inspection, however, because echoes within the BU are too sporadic to be
automatically picked, so further data processing and analysis is needed to confirm the result.
As first results we found that the BU has a dielectric constant significantly greater than that of
water ice, and thus that it contains a much larger dust fraction than the NPLD above
them. We find, however, that no single value can produce the correct topographic
relationship over the whole BU, which implies either that the topography of Planum
Boreum beneath the polar cap is not a regular continuation of the topography outside
the cap, or that the dielectric permittivity of the BU, and thus its dust content, is
laterally inhomogeneous. Work to verify these two hypotheses is currently ongoing. |
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