Mars Express radar collects first surface data

Press Release N° 38-2005
Paris, 5 August 2005

Mars Express radar collects first surface data

Marsis, the sounding radar onboard ESA's Mars Express spacecraft, is
collecting the first data about the surface and ionosphere of Mars.

This radar started its science operations on 4 July, the same day as its
first commissioning phase ended. Due to the late deployment of Marsis, it
was decided to split the commissioning, originally planned to last four
weeks, into two phases; the second will take place in December. It has thus
been possible to begin scientific observations with the instrument earlier
than initially planned, while it is still Martian night-time. This is the
best environmental condition for subsurface sounding, as in daytime the
ionosphere is more 'energised' and disturbs the radio signals used for
subsurface observations.

As from the start of commissioning, the two 20m-long antenna booms have been
sending radio signals towards the Martian surface and receiving echoes back.
"The commissioning procedure confirmed that the radar is working very well
and that it can be operated at full power without interfering with any of
the spacecraft systems," says Roberto Seu, Instrument Manager for Marsis, of
University of Rome 'La Sapienza', Italy.

Marsis is a very complex instrument, capable of operating at different
frequency bands. Lower frequencies are best suited to probing the
subsurface, the highest frequencies are used to probe
shallow subsurface depths, while all frequencies are suited to studying the
surface and the upper atmospheric layer of Mars. "During commissioning we
worked to test all transmission modes and optimise the radar's performance
around Mars," says Professor Giovanni Picardi, Principal Investigator for
Marsis, of University of Rome 'LaSapienza'. "The result is that since we
started the scientific observations in early July, we have been receiving
very clean surface echoes back, and first indications about the ionosphere."

The Marsis radar is designed to operate around the orbit 'pericentre', when
the spacecraft is closer to the planet's surface. In each orbit, the radar
is switched on for 36minutes around this point, spending the middle
26minutes on subsurface observations and the first and last fiveminutes of
the slot on active ionosphere sounding.

Using the lower frequencies, Marsis has been mainly investigating the
northern flat areas between the 30° and 70° latitudes, at all longitudes.
"We are very satisfied with the way the radar is performing. In fact, the
surface measurements taken so far match almost perfectly the existing models
of the Mars topography," said Prof. Picardi. Thus, these measurements have
proved to be an excellent test. The scientific reason for concentrating on
flat regions with the first data analysis is the fact that the subsurface
layers are in principle easier to identify, though the task is still a
tricky one. "As the radar appears to work so well for the surface, we have
good reason to think the radio waves are also propagating correctly below
the surface," added Prof. Picardi.

"The bulk of our work has just started, as we now have to be sure to clearly
identify and isolate the echoes coming from the subsurface. To do this, we
have to carefully screen all data and make sure that signals which could be
interpreted as coming from different underground layers are not actually
produced by surface irregularities. This will keep us occupied for a few
more weeks at least."

The first ionospheric measurements performed by Marsis have also led to some
interesting preliminary findings. The radar responds directly to the number
of charged particles composing the ionosphere (plasma). This has at times
been shown to be higher than expected."We are now analysing the data to find
out if such measurements may result from sudden increases in solar activity,
such as the one observed on 14July, or if we have to put forward new
hypotheses. Only further analysis of the data can tell us," said Jeffrey
Plaut, co-Principal Investigator, from the NASA Jet Propulsion Laboratory,
Pasadena, USA.

Marsis will carry on sending signals that hit the surface and penetrate the
subsurface until the middle of August, when the night-time portion of the
observations will have almost ended. After that, observation priority will
be given to other Mars Express instruments that are best suited to operating
in daytime, such as the HRSC camera and Omega mapping spectrometer. However,
Marsis will continue its surface and ionospheric investigations in daytime,
with ionospheric sounding being reserved for more than 20% of all Mars
Express orbits, under all possible Sun illumination conditions.

In December, the Mars Express orbit pericentre will enter night-time again.
By then, the pericentre will have moved closer to the south pole, allowing
Marsis to carry out optimal probing of the subsurface once again, this time
in the southern hemisphere.

Note to editors

The first commissioning phase was given over to testing the Marsis
electronics and software and the two 20m-long antennas (dipole). The second
commissioning phase, lasting about ten days, will be spent calibrating the
7m 'monopole' antenna. This antenna is to be used in conjunction with the
Marsis dipole to correct any surface roughness effects caused by the radio
waves emitted by the dipole and reflected by an irregular surface. The
monopole will find its best use during investigations of areas where surface
roughness is greater.

The Marsis instrument was developed within the framework of a Memorandum of
Understanding between the Italian Space Agency (ASI) and NASA. It was
developed by Alenia Spazio under ASI management and the scientific
supervision of University of Rome 'La Sapienza', in partnership with the
Jet Propulsion Laboratory (JPL) and the University of Iowa. JPL provided the
antenna manufactured by Astro Aerospace. It is the first instrument designed
to actually look below the surface of Mars.

Its major goals are to characterise the subsurface layers of sediments and
possibly detect underground water or ice, conduct large-scale altimetry
mapping and provide data on the planet's ionosphere.

For subsurface probing, Marsis must operate between 300 km and 800 km from
the Martian surface, while for ionospheric sounding, it has already provided
satisfactory results from a distance of up to 3000 km. Radar vertical
resolution is about 150m (in free space), while horizontal resolution, in
the range of a few kilometres, depends on the spacecraft's altitude.

The joint Italian and American Marsis team is also largely involved in the
Sharad radar, a facility instrument provided by ASI for NASA's Mars
Reconnaissance Orbiter (MRO), due for launch in August. Marsis and Sharad
are two radars designed to provide complementary information about the
Martian subsurface. Marsis can penetrate to an average depth of 5km, while
Sharad will concentrate on layers closer to the surface.


For more information:

Giovanni Picardi,
Marsis Principal Investigator, Infocom Dept. - Universita' di Roma "La
Sapienza"


Jeffrey Plaut
Marsis co-Principal Investigator, NASA/JPL


Roberto Seu,
Marsis Instrument Manager and Sharad Team Leader, Infocom Dept. -
Universita' di Roma "La Sapienza"


Agustin Chicarro
ESA, Mars Express Project Scientist


Fred Jansen,
ESA, Mars Express


Enrico Flamini
ASI, Program Manager for Italian contribution to Mars Express



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