For decades Mars has teased scientists with whispers of water's presence. Valleys and basins and rivers long dry point to the planet's hydrous past. The accumulation of condensation on surface landers and the detection of vast subterranean ice deposits suggest the stuff still lingers in gaseous and solid states. But liquid water has proved more elusive. Evidence to date suggests it flows seasonally, descending steep slopes in transient trickles every Martian summer. The search for a big, enduring reservoir of wet, potentially life-giving H 2 0 has turned up nothing. Until now.

The Italian Space Agency announced Wednesday that researchers have detected signs of a large, stable body of liquid water locked away beneath a mile of ice near Mars' south pole. The observations were recorded by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument—Marsis for short. "Marsis was born to make this kind of discovery, and now it has," says Roberto Orosei, a radioastronomer at the National Institute for Astrophysics, who led the investigation. His team's findings, which appear in this week's issue of Science, raise tantalizing questions about the planet's geology—and its potential for harboring life.

Marsis collected its evidence from orbit, flying aboard the European Space Agency's Mars Express spacecraft. It works by transmitting pulses of low-frequency electromagnetic waves toward the red planet. Some of those waves interact with features at and below the Martian surface and reflect back toward the instrument, carrying clues about the planet's geological composition. Conceptually, using the instrument to study Mars' polar regions couldn't be more straightforward: Just point it toward the ice and see what bounces back.

Artistic impression of the Mars Express spacecraft probing the southern hemisphere of Mars, superimposed to a color mosaic of a portion of Planum Australe. The study area is highlighted using a THEMIS IR image mosaic. Subsurface echo power is color coded and deep blue corresponds to the strongest reflections, which are interpreted as being caused by the presence of water. Davide Coero Borga/USGS Astrogeology Science Center/Arizona State University/ESA/INAF

In practice, though, it's a lot more complicated. Marsis spends relatively little time above Planum Australe, the southern polar plane of Mars and the focus of Orosei's team's investigation. That meant the researchers could only listen for echoes periodically. It would take many readings—and many years—to get a clear picture of what lies hidden beneath the planet's southern ice cap. So in May of 2012, on the heels of a software upgrade that enabled Marsis to acquire more detailed data, the researchers began their survey.

Three and a half years and 29 observations later, they had a radiogrammatic map of Mars' southern polar plane. When they cross-referenced all their measurements, something immediately seized their attention: Bright reflections in the radar signals, corresponding to what Orosei now calls "a well-defined anomaly" some 12 miles across and several feet deep, roughly one mile beneath the surface of the polar ice cap. The surface of an ice cap tends to reflect radar waves more strongly than the regions below it. But on multiple passes, Marsis had detected uncommonly strong echoes originating from beneath the southern pole.

Or rather: Uncommonly strong for a solid material.

Analyses of subglacial lakes on our own planet—like the ones beneath the Antarctic and Greenland ice sheets—have shown that water reflects radar more strongly than rock and sediment. And in fact, the radar profile of this region of Mars' southern pole resembles those of subglacial lakes here on Earth.





1 / 7 Chevron Chevron Alex Lutkus/ESA An artist’s rendering of the Mars Express orbiter, which Italian astronomers have used to detect a large body of liquid water beneath Mars’ south pole. The observations were gathered using the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument, the antenna for which is depicted here.

The researchers looked for other explanations for the bright signals. A layer of frozen carbon dioxide above or below the polar cap, for example, could conceivably produce readings like the ones they observed—though the researchers deemed this, and all other explanations that they considered, less likely than the presence of liquid water.