Evidence Detected of Lake Beneath the Surface of Mars (cnn.com)

For decades Mars has teased scientists with whispers of water's presence. Now they have some solid evidence. From a report: 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. CNN elaborates: Between May 2012 and December 2015, MARSIS was used to survey the Planum Australe region, which is in the southern ice cap of Mars. It sent radar pulses through the surface and polar ice caps and measured how the radio waves reflected back to Mars Express. Those pulses reflected 29 sets of radar samples that created a map of drastic change in signal almost a mile below the surface. It stretched about 12.5 miles across and looked very similar to lakes that are found beneath Greenland and Antarctic ice sheets on Earth. The radar reflected the feature's brightness, signaling that it's water. "We interpret this feature as a stable body of liquid water on Mars," the authors wrote in the study.

Comments

How do they know it's H2O?

[Lucas123]

According to the article, the Italian space agency's Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument works by transmitting pulses of low-frequency electromagnetic waves. 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, but it doesn't seem to have any spectrometer component.

So, and this is an honest (and not sarcastic) question, how could you possibly know what the liquid a mile below the surface is?