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."
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