Scientists using ground-penetrating radar from an orbiting spacecraft have discovered what appears to be a 20-kilometre-wide lake underneath one of the Martian ice caps.
The discovery was made with an instrument on the European Space Agency’s Mars Express spacecraft, which found a strong radar reflection from beneath 1,500 metres of ice.
“Normally, the echo from such a large depth should be weak,” says Roberto Orosei, a planetary scientist at the National Institute of Astrophysics in Bologna, Italy. “But surprisingly in this particular area it was stronger than the echo from the surface.”
The most likely source of such a strong subsurface reflection, he says, is a layer of liquid water — i.e., a lake. “On Earth that would be almost a given.”
On Mars, however, it’s not that easy.
The problem is that polar winters on Mars get so cold that carbon dioxide freezes out of the air into a layer of dry ice — something that doesn’t happen even in the coldest parts of Antarctica. Because this ice is very transparent to radar, a deposit of it might allow more of the radar signal to penetrate, rather than reflecting off the top. More of the radar pulse would therefore reach the bottom, producing a reflection that might mimic a lake.
By careful modelling, however, Orosei’s team was able to rule out this possibility (along with several others), coming to the conclusion that they had found a lake.
It’s an important find, says Anja Diez, a glaciologist at the Norwegian Polar Institute in Tromsø, Norway. Previously, scientists had seen enigmatic dark streaks on sun-warmed parts of Mars where soil might be dampened by tiny rivulets of water from melting ice — although it is not certain that these dark streaks have anything to do with water.
“This is the first time a stable body of water was found [on Mars],” Diez says. “The radar data was collected over three years, and in all these years, [there is] this area of high reflections, indicating water.”
The depth of the lake cannot be determined from the radar signal. Nor can the composition of the water, including the amount of sediment it contains. But it is probably highly briny, Diez says, because the temperature beneath the Martian ice cap hovers around -68°C, too cold for fresh water to remain liquid, even under the pressure of 1.5 kilometres of ice.
Shallow or deep, however, the existence of a water beneath the Martian ice cap is important for glaciologists trying to reconstruct changes in the Martian climate. Typically, this is done by studying layers in the ice cap — layers that are themselves visible in other radar studies. “Water under the ice influences the flow of the ice and therefore needs to be taken into account,” Diez says.”
Jeffrey Plaut, a geologist at NASA’s Jet Propulsion Laboratory, Pasadena, California, agrees that it’s an important and “robust” find. “But it’s not quite a slam dunk,” he says. “I would like to see some more lines of collaborating evidence.”
One way to do this would be to discover more lakes. That’s something Orosei would also love to see because it would also suggest that subglacial lakes may have existed throughout the history of Mars. If there ever was life in the Red Planet’s warm, wet past, he says, these lakes might have provide refugia for it to continue to survive, all the way to the present.
Scouring the icecap for more such lakes, however, is a time-consuming process that requires using the Mars Express radar in a manner different from the way it was designed. “That limits the capability of the radar,” Orosei says, “so we have very limited coverage, on the order of a few percent.”
Worse, Mars Express is an ageing spacecraft that’s already been in orbit for nearly 15 years. “We don’t expect it to last more than a few more years,” he says.
Orosei and Diez both reported their findings in this week’s issue of Science.
