Two other Mars missions heating up
Radar mapping polar caps, impact craters
By Tony Fitzpatrick
Washington University in St. Louis
March 1, 2006
Two Mars orbiter missions - one from NASA, the other
from the European Space Agency (ESA) - will open new vistas in the
exploration of Mars through the use of sophisticated ground-penetrating
radars, providing international researchers with the first direct clues
about the Red Planet's subsurface structure.
Roger Phillips, Ph.D., Washington University in St. Louis Professor of
Earth and Planetary Sciences in Arts & Sciences, is participating in
both the Mars Express (ESA) and NASA's Mars Reconnaissance Orbiter
missions by lending his expertise in radar. Phillips says that the
combination of the radars on the two missions will provide important
unique data sets that will directly map the structure of the upper
portions of the interior of Mars.
The Mars Reconnaissance Orbiter (above) is poised to go into orbit
around Mars in March, then spend about six months aerobraking to place
the spacecraft in a low circular orbit by this fall. Roger Phillips,
Ph.D., WUSTL professor of earth and planetary sciences and director of
the university's McDonnell Center for the Space Sciences, is lending
expertise in radar for both the MRO and for the European Space Agency's
Mars Express features an instrument called the Mars Advanced Radar for
Subsurface and Ionospheric Sounding (MARSIS); Mars Reconnaissance
Orbiter's radar instrument is called the Shallow Subsurface Radar
(SHARAD). Both instruments have been built primarily by the Italian
Space Agency, and, says Phillips, they are complementary.
Mars Express went into orbit on Christmas Day of 2003, but the radar
developed some technical problems and didn't start operating until the
summer of 2005. One of the problems was making sure that the craft's
long antennas would unfold without damaging the spacecraft. This
necessitated a full stop, followed by many hardware and computer
simulations until confidence was gained that the antennas could be
deployed safely. The first data were published in the journal Science
late last year, with Phillips a co-author along with approximately 30
European and American colleagues.
"One of the spectacular results of that paper is the fact that we've
sounded the northern polar cap of Mars, the radar signals penetrating
all the way to the bottom of the icy cap and bouncing back so that we
can see right down to the cap's base, nearly two kilometers deep,"
Phillips says. "This result tells us that we will eventually be able to
map the volume of both the northern and southern ice caps, which will
provide a much better understanding of the origins and evolution of
these features and the amount of water that is tied up frozen in the
caps. The radar is looking inside the planet directly - that's never
been done before on Mars."
The northern cap data also provide the Mars Express team with the first
direct observation of how the load of the ice cap deforms the planet's
underlying crust - a phenomenon called flexure. There is in fact no
flexure observed within the error limits of the radar data, which means
that the crust beneath the northern polar cap is very strong, says
"That also tells us that the heat output of the planet at present is
quite low. There have been theoretical models predicting this, but
a direct observation until now. "
Finding unfrozen subsurface water is a possibility, too; this is the
Holy Grail of MARSIS, Phillips says.
"At the boundary of a water-rich environment we should see a very
reflection. Whether we find such a reflector remains to be seen - we're
in the early days of gathering data. Right now MARSIS is carrying out a
campaign to map the structure of the southern polar cap."
Phillips says that the MARSIS radar also has mapped buried impact
craters, which should revise the theory of how old the martian crust is
and how it evolved.
SHARAD will likely not probe as deeply as MARSIS, but it has ten times
the vertical resolution, allowing for opportunities to map detailed
subsurface stratigraphy, says Phillips, who is also director of
Washington University's McDonnell Center for the Space Sciences
The MRO spacecraft is now poised to go into orbit around Mars in March,
and will then spend about six months aerobraking to place the
in a low circular orbit by this fall. This will mark the start of a
two-year mapping campaign that NASA calls the primary science phase.
Phillips is Deputy Team Leader for the MRO SHARAD experiment. The Team
Leader is Roberto Seu, Ph.D., of the INFOCOM Department at the
University of Rome, La Sapienza, Italy.
"I think that SHARAD will be an excellent mapper of the sedimentary
layers on Mars, and that will help us better understand the water
history of the planet, "Phillips says. "One of my hopes is to connect
what SHARAD maps in the subsurface in the Terra Meridiani area to the
sulfate-rich stratigraphy that has been seen there by the Mars
Exploration Rover (MER) Opportunity. This will help place the local
stratigraphy at the MER landing site into a more regional context and
help refine hypotheses for the origins of these enigmatic sedimentary