Red Planet Express: Mars spacecraft traces a watery tale

A Mars-orbiting spacecraft is providing new details about when and where liquid water, an essential ingredient for life, existed on the planet. The craft’s instruments have detected a long-sought group of ancient, water-bearing compounds on Mars and have provided the first peek beneath the Red Planet’s surface.

Over the past 3 decades, spacecraft and rovers have gathered compelling evidence that liquid water once existed in abundance on Mars. These data include images of channel networks gouged in the planet’s surface (SN: 5/28/05, p. 344: Roving on the Red Planet). But one hallmark of an aqueous era—surface minerals altered by water—remained elusive.

Researchers now report that a spectrometer aboard the European Space Agency’s Mars Express spacecraft has found such water-bearing minerals. The location of the minerals provides new evidence that liquid water resided on Mars for an extended period that probably ended about 3.8 billion years ago, report Jean-Pierre Bibring of Université Paris-Sud and CNRS in Orsay, France, and his colleagues in the Dec. 1 Nature.

Data collected by the spectrometer over the past 18 months reveal two classes of water-bearing minerals. Each class resides in several vast Martian areas.

One class forms when recently molten rock is in long-term contact with liquid water. Clays are among its compounds, which are called phyllosilicates. These were found only on the oldest terrains on Mars, which date to 3.8 billion years ago.

The other class, known as hydrated sulfates, was found only on younger terrain. These minerals require just brief exposure to water, but usually under acidic conditions.

The findings suggest that Mars has experienced at least two major climactic episodes—an early, moist phase and a later period that had a more acidic environment. Between these two episodes, the planet must have undergone a global climate change, says Bibring.

“My guess is that this is going to essentially rewrite the book on the history of water on Mars,” says planetary scientist Jim Head of Brown University in Providence, R.I.

Another detector on Mars Express, switched on in June, is providing researchers with a new dimension. The craft’s radar sounder has for the first time probed deep into the planet’s subsurface. In an upcoming Science, Jeff Plaut of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues describe their early results.

Radar echoes from surface ice that rings the north pole reveal that the roughly 3-kilometer-deep layer is at least 98 percent frozen water. Despite the weight of the ice, the underlying layer, the lithosphere, hasn’t sunk. One explanation is that the lithosphere is thick.

A more intriguing possibility, says Head, is that the polar-ice deposits haven’t made a dent because they haven’t been there long. Because Mars’ spin axis tilts dramatically on a cycle lasting about 20 million years, the current polar regions were warmer not long ago.

In another area, the radar detector found a buried impact crater, some 250 km across and 1.5 to 2.5 km in depth. The strong radar echo suggests that this crater in the northern lowlands is filled with ice, says Plaut. The crater sits in a region where several channels come together.

If the radar tool finds many other ice-filled craters, it may solve another mystery.

Plaut notes that Mars scientists are constantly asking where the water that once flowed on Mars has gone. Much of it undoubtedly has evaporated, but some of it may have headed underground, where it remains frozen in craters, he says.