This
story was updated on May 7, 2008.

A
space-based radar aboard a European Mars probe could help peer beneath the
surface of Earth's ice sheets, not to mention the frozen extraterrestrial seas
of moons like Europa and Titan.

The space
radar would take its cue from the Mars Advanced Radar for Subsurface and
Ionosphere Sounding (MARSIS) instrument, which has probed the Red
Planet's underground for evidence of water from the European Space Agency's
(ESA) Mars
Express spacecraft.

"I was
having discussions with scientists from MARSIS, and I saw that what they had
done could be applicable to Earth," said Florence Heliere, the ESA technical
manager heading the concept study.

Mars
Express wields a 131-foot (40-meter) long MARSIS
antenna boom that dwarfs the spacecraft's six-foot wide span, like a giant
whiplash antenna on a tiny car. That main antenna pings the Martian surface
with radar to try and detect underground water, but also receives a lot of
unwanted clutter signals from the Martian surface and surrounding underground
layers.

A second,
smaller antenna detects only the scattered clutter signals that do not come from
the target area directly below Mars Express. Scientists could theoretically
subtract the clutter signals from the main antenna signal to get the true target
reading, but the Mars Express team ultimately drew up a different solution that
did not require the second antenna.

Still, the
Mars
dry run has inspired the early development of the Advanced Concept for Radar
Sounder (ACRAS) that will set its initial sights on Earth's Antarctic ice
sheets.

"Antarctica
is an area where the ice is cold and dry, so you can penetrate up to 3-4
kilometers (1.8-2.5 miles) within the ice," Heliere told
SPACE.com.

Continuous
observation by ACRAS would help estimate the thickness of the Antarctic and
other polar locations, as well as determine the three-dimensional internal
structure of the ice sheets. Climate modelers could use data from the tool to
get a better idea of how the ice sheets change over time as ice melts and
reforms.

But the
ACRAS team faces several hurdles in adapting space-based radar technology
developed for Mars into an Earth-observing tool.

First, the
Mars Express radar works between 1.3 and 5 MHz on the electromagnetic spectrum,
but international regulations only give bandwidth around 435 MHz for Earth
observation. Scientists typically observe the Earth with airborne or other radar
systems that use around 60 MHz or 150 MHz, but Heliere hopes to test the 435 MHz
system through airborne trial runs starting next week.

Another
challenge has been wrestling with the clutter signals from a target's regional
surroundings. ACRAS will use a solution similar to MARSIS to screen out unwanted
clutter from the cross-track direction, or at right angles to the direction of
travel of its parent spacecraft. Although MARSIS used a secondary antenna to
pick up clutter signals, ACRAS will use just one antenna that can send out and
receive several different radar signals, Heliere noted.

A different
solution will screen out clutter coming from the direction of the spacecraft's
line of travel, taking advantage of the Doppler Effect where returning signals
meet the spacecraft flying head-on in their direction and appear to condense.
The same effect causes the drop in an ambulance siren's wail as the vehicle
speeds past, and would allow scientists to pick out the clutter in the
signals.

The third
challenge involves addressing distortion of the radar signal from the upper part
of the Earth's atmosphere, called the ionosphere. The German Space Agency (DLR)
has developed an autofocus technique that supposedly deals with ionosphere
distortion, and will publish results of their test simulations in the near
future, according to Heliere.

ACRAS could
see action by 2015 onboard ESA's Biomass Explorer mission, assuming that
satellite gets the green light, Heliere said. More distant possibilities include
deploying the radar to Jupiter's
moon Europa or Saturn's
moon Titan, with each suspected of harboring
oceans beneath their frozen crusts.

"This
[could be] applicable to Titan, Europa or Mars again to improve the capabilities
of radar sounding," Heliere said. "I originally worked on Earth as the main
subject, so [looking at MARSIS] has opened doors to other things."