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Mysteries in Martian depths

NASA / JPL-Caltech / Cornell
NASA's Opportunity rover sent back this view of a Martian promontory at Victoria
Crater named Cape Verde. The picture is in soft focus due to the scattering effect
of dust on the camera's front window. Click on the image for a larger version.

The mysteries from the Red Planet just keep on coming: On the ground, NASA's Opportunity rover is carefully picking its way down a deep crater, sending back a stunning postcard along the way.

Meanwhile, high above, the European Mars Express orbiter has sent back curious evidence of equatorial deposits of material that go more than a mile beneath the Martian surface. Is it water ice? Dust? Volcanic ash? Scientists can't yet answer that question, but they really want to. If it's ice, that could help answer questions about Mars' past - and its future.

First, about NASA's rovers: For some weeks now, Opportunity and its twin, Spirit, have been focusing on long-term science projects. Spirit is looking at an intriguing layered rock formation nicknamed "Home Plate" that may shed light on ancient volcanic activity - and also looking for a safe, sunny place to spend the Martian winter.

On the other side of the Red Planet, Opportunity has driven down the inside slope of half-mile-wide Victoria Crater and is looking at a mysterious light-toned band of rock just below the crater's rim.

"We think it's made of the same stuff that all the other rock around here is made of, but something different happened to it during its history," Cornell astronomer Steve Squyres, the principal investigator for the rover missions, told me today.

The band is several meters wide, and consists of three "subbands" with different characteristics, Squyres said. The top band seems to consist of material from before the impact that created Victoria Crater, and the material may look different because it interacted with the Martian atmosphere millions of years ago, he said.

Could this show scientists whether the air on Mars was different back then? All Squyres would say is, "You could speculate like crazy."

Another hypothesis is that water seeped up from below and interacted with the rock, changing its texture and chemistry. If this suggestion is borne out, "the base of the bright band is effectively a bathtub ring," Squyres said.

"We saw something very much like it back at Endurance Crater," he said.

Scientists aren't yet close to figuring out exactly what caused the bright band to look the way it does. "It's a very laborious problem to try to solve this. ... Sometimes it's just grind-it-out science and it takes a while," Squyres said. The fact that Opportunity is sitting on a potentially perilous slope doesn't make the job any easier.

As of this week, Opportunity and Spirit have spent two full Martian years on the Red Planet, and they're both still going strong. To celebrate the milestone, NASA released a stunning picture of a promontory at Victoria Crater called Cape Verde. Those two years on Mars translate into nearly four years on Earth - not bad for a mission that was initially slated for just 90 days.

Deep deposits
Now for the results from the MARSIS radar altimeter aboard the Mars Express orbiter: For decades, scientists have been intrigued by an equatorial region known as Medusae Fossae, which marks a transition of sorts between the Martian highlands and lowlands. Even back in the 1970s, they suspected that there might be large deposits of water ice there, although they couldn't explain how those deposits got there.

ESA / ASI / NASA / Univ. of Rome / JPL / Smithsonian
This color-coded view shows the Martian surface
and subsurface in the Medusae Fossae region.
The MARSIS radar sounder found echoes from
the lowland plains buried by mysterious deposits.
The top arrows show the surface echo, and the
bottom arrows indicate the subsurface echo of
one of the hills made up of the deposits.

In a report published today on Science Express, the MARSIS team provides an estimate of just how deep the deposits go, based on radar sounding data. The answer? Pretty darn deep: about 1.6 miles (2.5 kilometers).

"If these materials are ice-rich, it's a significant amount of water that would be added to the inventory of water ice that we know about on Mars," Thomas Watters of the Smithsonian Institution, lead author of the Science study, told me today. "It would be something like a 36 percent increase in the total amount of water ice that we know about at the surface of Mars. Again, that's all qualified with a big if."

The "big if" relates to whether or not the deposits really do consist mostly of water ice. The radar readings indicate that the Medusae Fossae deposits have the density and electrical properties of water, but they also could conceivably consist of fluffy volcanic ash or dust. That doesn't seem likely: If the ash or dust is that deep, you would think it would compact into denser stuff. But the geology of Mars isn't like Earth's, and confirming the composition would require more detailed readings - by MARSIS or a higher-resolution radar imager aboard NASA's Mars Reconnaissance Orbiter, called SHARAD.

Even if it is water ice, the deep deposits appear to be covered with a layer of wind-sculpted soil that might be meters thick. "That could be the veneer or the covering that is insulating the thicker deposits that have ice in them," Watters said.

The presence of that top layer makes it harder to know for sure exactly what lies beneath. "We're really not going to be able to determine it definitively until we actually go there and sample below this desiccated outer layer," Watters said.

Past and future of the Red Planet
If it is water ice, that raises yet another question: How did all that water get there in the first place? Scientists believe the deposits are only a couple of million years old, based on the lack of cratering and the fact that they're sitting on a geologically recent lava plain.

Wind has sculpted the terrain in the Medusae Fossae region, as seen in
this view from the THEMIS imager
on NASA's Mars Odyssey orbiter.

"The fact that this exists at the equator is very intriguing, because there has to be some sort of climatic condition that allows accumulation and preservation of water ice in a tropical area on Mars," said Jeffrey Plaut, a planetary scientist at NASA's Jet Propulsion laboratory who is co-principal investigator for the MARSIS experiment and a co-author of the Science paper.

One hypothesis is that the tilt of Mars' axis was more pronounced millions of years ago.  "If the spin axis rotates to a high value, then you actually warm up the poles and cool down the midlatitudes and the equator," Plaut explained. Water ice at the poles might sublimate into vapor, make its way toward the equator and freeze out of the atmosphere as ice deposits.

Over time, ice crystals would mix in with soil deposits. As the planet's tilt became less oblique, temperatures would become warmer at the equator, and the ice near the surface would disappear - leaving that layer of soil on top to be sculpted by the wind.

It all makes for an intriguing story about Mars millions of years ago - but if the deposits really are ice-rich, that also could tell us something about the future exploration of the planet.

"The one advantage of having ice in the lowlands is that it's a much easier place to get to [than the poles]," Watters said. "The lowlands are an attractive place for robotic landers or human-piloted landers."

Plaut agreed, noting that his colleagues at NASA are already doing a lot of research into what it would take for humans to live off the land on Mars.

"Those folks are very interested in any evidence that there may be water ice reservoirs in these more temperate parts of Mars, because it's certainly easier to operate equipment in those regions of Mars than in the polar regions," he told me.

So it might be worth getting to know Medusae Fossae better in the years to come. It's definitely a weird-looking place, based on photos like this one, and this, and these. Oh, and this zoomable picture, too. To keep up with the saga of Mars exploration, be sure to check in on our special report, "Return to the Red Planet."

In addition to Watters and Plaut, authors of the Science study include Bruce Campbell and Lynn Carter of the Smithsonian Institution; Carl Leuschen of the University of Kansas; Giovanni Picardi and Roberto Orosei of the University of Rome;  Ali Safaeinili and Anton Ivanov of the Jet Propulsion Laboratory; Stephen Clifford of the Lunar and Planetary Institute; William Farrell of NASA's Goddard Space Flight Center; Roger Phillips of Washington University in St. Louis; and Ellen Stofan of Proxemy Research.