[meteorite-list] MARSIS Completes Measurement Campaign Over Martian North Pole (Mars Express)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 15 Dec 2011 09:41:39 -0800 (PST)
Message-ID: <201112151741.pBFHfdrQ002424_at_zagami.jpl.nasa.gov>

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=49771

MARSIS completes measurement campaign over Martian North Pole
European Space Agency
14 Dec 2011

The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS)
instrument on board Mars Express has recently completed a subsurface
sounding campaign over the planet's North Pole. The campaign was
interrupted by the suspension of science observations several times
between August and October due to safe modes and to anomalies in the
operation of the spacecraft's Solid-State Mass Memory (SSMM) system. As
MARSIS best observes in the dark, which for the North Pole only occurs
every few years, it was among the first instruments to resume
observations once a partial work-around for the problems had been
implemented.

The primary objective of MARSIS is to map the distribution of water and
ice in the upper layers of the Martian subsurface. Using techniques
similar to oil prospecting on Earth, the instrument analyses the
reflection of radio waves down to a few kilometres in the subsurface; it
is able to distinguish between dry, frozen and wet soil.

The polar regions of Mars are of particular interest because climate
variations affect the quantities of water ice and dust found in the
polar deposits.

The North Pole measurement campaign lasted from June to November 2011,
taking place during orbits 9500 to 10 100. The observations extended
from the pole out to just beyond 45?? N. Data acquisition was affected by
solar events, as well as the technical problems with the spacecraft.
During the main part of the campaign, around 40 per cent of the
available orbits were lost, with roughly a quarter of the losses being
attributable to solar activity and three quarters to the suspension of
observations.

The velocity of Mars Express at pericentre is extremely high and the
fly-overs of the north polar cap lasted only between three and seven
minutes per orbit. The accumulated observing time over ~ 600 orbits was
about 3000 minutes. Mars Express therefore spent a total of about two
days over the north polar cap in the whole observing season. About 25
hours were spent acquiring data while the pole was in darkness, and
another 25 hours observing the pole while it was in sunlight. MARSIS can
observe the subsurface with maximum sensitivity only when the pole is
not illuminated, so the best observations were made between June and
September. The pole was still observable until late November, but by
then it was partially illuminated, so the measurements were of lower
quality.

The presence of an ionosphere also impacts the MARSIS measurements with
MARSIS signals being disturbed or even completely attenuated when free
electrons are present in the Martian atmosphere. There is always an
ionosphere on the dayside of the planet, created by solar ultraviolet
photons and energetic particles interacting with the thin atmosphere. It
is, therefore, greatly preferably to observe on the nightside, where, in
principle, there is no ionosphere. In practice, during periods of high
solar activity an active ionosphere can be present on the nightside as well.

The careful scheduling of MARSIS measurement campaigns for polar
observations is crucial to their success. Mars Express has an elliptical
polar orbit, so it passes over the polar caps during every orbit;
however, the altitude of the spacecraft over the poles varies as the
orbit pericentre drifts with time. In the period from June to November,
the spacecraft flew over the North Pole at very low altitudes ??? less
than 1000 kilometres. Having the spacecraft in a low orbit over the
target area is a requirement for operating the radar; no signal would be
received at higher altitudes.

"This campaign to investigate the subsurface of Mars's North Pole is
one of the highlights of the extended Mars Express mission," notes
Olivier Witasse, Mars Express Project Scientist at ESA. "Despite the
temporary suspension of operations during the campaign we have been
able, with the excellent support of our colleagues in spacecraft
operations, to complete this campaign as expected. The data that were
acquired are now being analysed by the MARSIS team and we are eagerly
anticipating the results."
      
Contact

Olivier Witasse
Mars Express Project Scientist
Research and Scientific Support Department
Directorate of Science & Robotic Exploration
ESA, The Netherlands
Email: owitasserssd.esa.it
Phone: +31 71 5658015
Received on Thu 15 Dec 2011 12:41:39 PM PST