The European Space Agency (ESA) has provided more evidence that suggests the surface of Mars was once home to an ocean. Featuring ground-penetrating radar capabilities, the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) radar aboard the ESA's Mars Express spacecraft has detected sediments like that seen on an ocean floor.
In 1877, with the aid of a 22 cm (8.6 in) telescope, Italian astronomer Giovanni Sciaparelli produced the first detailed map of Mars, which featured what he called canali. Although canali actually means "channels" in English, it was popularly mistranslated as "canals," which, along with books by Percival Lowell, helped foster the popular notion of water and life - including Martians - on the Red Planet's surface. Although these "canals" were later proven to be an optical illusion, these myths weren't dispelled until NASA's Mariner missions in the 1960's.
Yet more recent mapping efforts still point to there being liquid water on the planet's surface at some point in its history. It is within the boundaries of features tentatively identified in images from various spacecraft as shorelines that MARSIS detected sedimentary deposits reminiscent of an ocean floor.
"MARSIS penetrates deep into the ground, revealing the first 60 - 80 meters (197 - 262 ft) of the planet's subsurface," says Wlodek Kofman, leader of the radar team at the Institut de Planétologie et d'Astrophysique de Grenoble (IPAG). "Throughout all of this depth, we see the evidence for sedimentary material and ice."
The sediments detected by MARSIS are areas of low radar reflectivity, which typically indicates low-density granular materials that have been eroded away by water and carried to their final resting place.
"We interpret these as sedimentary deposits, maybe ice-rich. It is a strong new indication that there was once an ocean here," says Jérémie Mouginot, from IPAG and the University of California, Irvine.
Two oceans at different times in Mars' history have been proposed - one 4 billion years ago when warmer conditions prevailed, and one 3 billion years ago when geothermal activity may have caused subsurface ice to melt and flow into areas of low elevation.
Dr Mouginot estimates that this latter ocean would have lasted only a million years or less, with the water either being frozen in place underground again, or turned into vapor and released into the atmosphere.
"I don't think it could have stayed as an ocean long enough for life to form," he says, suggesting astrobiologists would have to look further back into Mars' history when liquid water existed for much longer periods.
But the ESA says the MARSIS findings provide some of the best evidence yet that large bodies of water once existed on the surface of Mars and that liquid water played a role in martian geological history.
"Previous Mars Express results about water on Mars came from the study of images and mineralogical data, as well as atmospheric measurements. Now we have the view from the subsurface radar," says Olivier Witasse, ESA's Mars Express Project Scientist. "This adds new pieces of information to the puzzle but the question remains: where did all the water go?"
The ESA says the Mars Express spacecraft, which was launched in 2003 and has been granted five mission extensions - the latest until 2014 - will continue its investigations with the hope of providing an answer.