From Mars to the Earth: Studying ice beneath the surface (Forwarded)

ESA News
http://www.esa.int

17 April 2008

From Mars to the Earth: Studying ice beneath the surface

A technique being used by the European Space Agency at Mars could prove
invaluable for studying the stability of Antarctic ice sheets here on
Earth. In preparation, a new ESA study is seeking to refine the radar
method for use on our planet.

Funded by ESA's General Studies Programme, the new study is called
Advanced Concept for RAdar Sounder (ACRAS). It will assist both Earth
observation and planetary exploration by developing a technique that uses
orbiting radar at long wavelength to see beneath the surface of a
planetary body and return information about the subsurface environment.

Clearly, a method of obtaining information about the interior of a
planetary body from orbit removes the need for expensive landing and
drilling equipment. It also achieves global observations rather than
limited data derived from drilling at a few points dotted across the
planet. However, there are two complications.

The first is that the upper reaches of an atmosphere are populated by
electrically charged particles. Called the ionosphere, this layer distorts
the low frequency signal as it passes through. The second complication is
that the size of the radar beam covers a larger area than the target
region just below the spacecraft. Stray reflections from the surrounding
terrain can then bounce back and overwhelm the radar signal coming from
the sub-surface region. This is called clutter.

During the late 1990s, a mission to monitor Antarctic ice using radar was
proposed to ESA. However, whilst investigating the concept, ESA found that
it was impossible to suppress the confusing reflections from the surface
of the Earth that were masking the real signals coming from features below
the ground. Fortunately, the ACRAS study may have now found the solution.

The key is to treat the problem in two different ways. In the direction in
which the satellite is moving, the returning radar has its frequency
subtly altered by the Doppler effect. This is the same effect that makes
an ambulance's siren appear to change pitch as it passes. The Doppler
effect in the radar beam can be analysed to determine where the reflected
signals are coming from and reject the unwanted ones. In the 'cross-track
direction', at right angles to the direction of travel, the study has
determined that instead of sending out one radar beam, sending three with
slightly different properties can be used to determine which reflected
signals to keep.

ESA's Mars Express carries a pioneering radar experiment currently
investigating the Red Planet and searching for any evidence of sub-surface
reservoirs of water and ice. The Mars Advanced Radar for Subsurface and
Ionosphere Sounding (MARSIS) instrument works at 50 MHz. However, radar
using this frequency on Earth would disrupt radio communications and so
the ACRAS study is investigating a change to work at the frequency of 435
MHz.

This new study is meeting with success and paves the way for even more
precise instruments that could work around Earth. A space-borne
ice-sounding radar could accurately estimate the ice sheet thickness of
the Antarctic and other polar locations. It would give information about
the three-dimensional internal structure of the ice sheets, the shape of
the underlying terrain and the configuration of the bedrock. Repeated
observations could allow scientists to monitor the way in which the ice
sheets evolve with time. As ice melts and is discharged into the ocean, it
has a profound impact on the global climate.

In the realms of planetary exploration one of the biggest unanswered
questions is whether there is a global ocean of water under the icy crust
of Jupiter's moon Europa. A similar radar system on a mission further into
the future could map the thickness of the ice crust there and reveal the
ocean beneath.

Saturn's moon, Titan, is another natural target. As the Cassini spacecraft
found out just few weeks ago, this moon is also likely to hide an ocean of
liquid water beneath icy landforms and strange lakes of hydrocarbon fluid.
The combination of these chemicals and a heat source keeping the water
liquid provides the basic conditions for the appearance of life. The
confirmation of Cassini's discovery by a mission using an ACRAS-like
system would have profound implications in our search for life beyond our
planet.

The ACRAS study is set to conclude in October this year. Although a
fully-fledged satellite mission is a long way off yet, Florence Hélière,
the ESA study officer, says, "We hope to conduct an airborne test of the
technique."

The ACRAS study is in collaboration between ESA, eOsphere Ltd, British
Antarctic Survey, EADS Astrium, the German Space Agency (DLR), Max Planck
Institute (MPI), and Laboratoire de Planetologie de Grenoble (LPG), plus a
number of individual consultants.

For more information:

Florence Hélière
ESA/ESTEC Earth Observation Projects Department
EOP-PIM Microwave Instrument Section
Tel: +31 71 565 8699
Fax: +31 71 565 5301
Email: florence.heliere @ esa.int

Andrés Gálvez
General Studies Programme manager
Tel: +33 1 5369 7623
Email: andres.galvez @ esa.int

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