Planetary scientists have known since the 1970s that a huge depression on Mars, centered on its north pole, dominates the planet's northern hemisphere. And they've mapped gigantic flood channels cut into the ruddy landscape, proof that water once gushed freely and drained into that vast lowland area.
So was a third of ancient Mars once covered with an ocean?
In the late 1980s, geologist Tim Parker (now at NASA's Jet Propulsion Laboratory) led a team that used high-resolution images from the Viking orbiters to identify strings of landforms encircling the northern basin. To their eyes these looked like wave-cut terraces and other features common along terrestrial shorelines, and they concluded that the big basin must have brimmed with water at least twice in the distant past. But the case for this enormous "Oceanus Borealis" was never ironclad — in fact, other scientists used the even-better views from later spacecraft raised serious doubts about the shorelines' existence.
You might think that getting a definitive answer would simply require taking more and better pictures with ever-sharper optics, or perhaps chemical analyses from landers and rovers. But some of the best recent evidence has come instead from two lesser-known instruments —a laser and a neutron spectrometer — looking down on the planet from orbit.
Each is a great detective story in itself, but for now here's a recap: The laser, at the heart of an orbiting altimeter, traced the Red Planet's highs and lows with extreme accuracy and revealed exactly where "sea level" should have been. The neutron spectrometer sensed that hydrogen, present in H2O ice, is abundant in the topmost surface layers surrounding both poles but especially so up north.
The latest word comes from four researchers at the Institute of Planetology in Grenoble, France. Led by Jérémie Mouginot, they've analyzed radar soundings from the Mars Advanced Radar for Subsurface Ionosphere Sounding. MARSIS is one of several instruments aboard the European Space Agency's Mars Express orbiter, a very capable spacecraft that hasn't been in the spotlight much since its arrival in late 2003.
The radar's waves can probe the Martian surface to depths of up to 2½ miles (4 km) if the ground is loose or full of ice, or to several hundred meters if it's solid rock. During the last half of 2011, MARSIS was in the midst of a dedicated campaign to probe the vast northern basin, and Mouginot's team summarizes those findings in a recent issue of Geophysical Research Letters.
The researchers home in on a property of solid matter called the dielectric constant, essentially its ability to hold an electric charge. The MARSIS observations show that dense volcanic materials on Mars exhibit values near 10. But those surrounding the north and south poles are closer to 3 — implying that they're highly porous, chock full of ice, or both.
Interestingly, in the south, the region of low dielectic constant maps closely with the limit of where water ice near the surface should be stable in the current Martian climate (very cold and very dry).
However, it's a different story up north. There MARSIS mapped a region of low values that here and there extend far beyond the presumed limits of ice's current stability — it's a poor fit. But the match is much closer to the presumed boundaries of the ancient Martian ocean, and in particular to the "Deuteronilus" shoreline traced out by Stephen Clifford and Parker back in 2001.
This coincidence falls short of outright proof, though Mouginot and his colleagues help make the case further by showing that their map of low dielectric constant also matches geologic maps of sedimentary deposits in Mars's great northern basin. Taken together, they suggest that the evidence in hand "can only be explained by the widespread deposition of (now dessicated) aqueous sediments or sediments mixed with massive ice."
It's a fascinating development in a long-running debate. You can read the European Space Agency's press release, but I recommend that you check out the excellent explanation by S&T contributing editor Emily Lakdawalla in her Planetary Society blog.