Evidence of Recent Climate Change on Mars
The images were recorded by the Mars Orbiter Camera aboard NASA's unmanned Mars Global Surveyor. They show a unique surface terrain of pits and hummocks that appears to have been soil once impregnated by water ice. The ice has since evaporated, leaving a five-meter thick mantle of porous terrain. According to Mustard, these features are at most 100,000 years old, and may be even younger than that.
"The age is determined by the fact that there are no impact craters larger than 100 meters observed thus far on intact sections of the mantle," says Mustard. "This puts the maximum age at 100,000 years."
Because it is so difficult to obtain soil samples from other planets, scientists often use impact craters to date geological surface features. Knowing when and how often asteroids and comets bombard planets can help place surface features within a certain time frame. If a crater is superimposed on top of a feature, we know that the feature was there before the impact occurred. Likewise, if, for instance, a gully runs through the middle of a crater, we know that the gully came sometime after the impact.
Mustard, with graduate student Christopher Cooper and undergraduate Moses Rifkin, wrote about the findings in the July 26 issue of Nature.
Nathalie Cabrol, planetary scientist with the NASA Ames Research Center and NAI-member, agrees with Mustard's analysis of the martian images. Cabrol has been studying ice-related features that are in the same location as the hummocky terrain studied by Mustard.
"My own research strongly parallels Mustard's conclusion," says Cabrol. "The evidence is extremely compelling that what we are seeing is very young. There are no superimposed craters on the gullies, or on other young landforms such as mudflows or rock glaciers. In addition, dust is always blowing on Mars. If these features were ancient, then we could expect to see a lot of wind-deposited material in the gullies. But these features are neither filled nor very eroded, so they must be recent."
"Even more compelling evidence of a very recent climate change are some landforms that mimic terrestrial glaciers," says Cabrol. "The typical lifetime of a glacier is at most 100,000 years, and more usually tens of thousands of years."
While previous observations had shown that ice only had been present in the polar regions, the high-resolution images show evidence of water ice closer to the equator. The geologically young terrain observed by Mustard was in two bands north and south of the equator at 30 to 60 degrees latitude.
Due to climate change, it appears the icy region moved from the planet's poles to nearer its equator, and then retreated back to the poles. The geological features seen on the Global Surveyor images recorded the most recent of probably many such freeze/thaw cycles, Mustard said.
Other than climate change, chemical processes can create the type of geological features seen in the images. But Mustard says that there is no evidence that such chemical reactions are occurring on Mars.
"The mantle has been cemented," says Mustard. "There are two options for cement on Mars: chemicals or ice. There is no evidence from remote sensing for chemical cement, such as a sulfate. This leaves ice. And there is a fantastic analog on Earth in the Fox Permafrost tunnel in Alaska. It is in ice-permeated loess [fine-grained soil deposited by the wind], and when the ice sublimes [evaporates] from the loess in the tunnel, a weakly cemented mantle results with the same properties as we observe on Mars."
Mustard says that where the mantle is still intact, water ice still could be below the surface. But where the mantle is broken into pits and hummocks, the water is gone.
"There is a strong possibility that ice could be present very close to the surface if there are diffusive barriers to the sublimation of ice," says Mustard. "This would be easy to create in these soils, and if they exist, there may well be ice there. The analogy on Earth is 8 million year old ice in Antarctica beneath a one half meter mantle of soil."
According to Mustard, however, until we can actually test the martian soil, we can't be sure whether ice is still there.
"With climate change on Earth, a change in the soil temperature will have a lapsed response in the permafrost. The thicker the permafrost, the longer it takes for there to be a response time. On Mars, we know the permafrost is very thick. I think what we are seeing is a delayed response to a past climate change. The gullies we see are actually forming today because of this lag reaction time."
Some scientists have said that the location of the features in the mid-latitudes must mean that they are very old. The features are oriented on a south-facing slope, and at present, that region is much too cold to melt ice. According to Cabrol, however, this argument falls apart when you take into account the possibility of a lag reaction time to an earlier climate change.
Cabrol thinks these sites in the mid-latitudes would be a good place to search for either living or fossilized martian life.
"As the soil reacts to the temperature change and flows downhill, material from the martian subsurface may be brought up," says Cabrol. "Any sort of microbiological community hiding out underground would be brought up to the surface. Even if life does not exist on Mars today, perhaps these gullies are bringing up fossils of past life on Mars."
Cabrol says a priority for future missions to Mars will be to sample these gullies and rock flows to check for martian life, either living or extinct.
"As soon as we have solved the problem of precision landing on the planet, I will be working very hard to ensure that sampling these areas is one of the very first things that we do," says Cabrol.
In the meantime, Cabrol plans to present her observations of the glacier-like landforms at the American Geophysical Union meeting in San Francisco this December.
"The landforms are numerous and [their] material is still occupying part of the valleys," says Cabrol. "[Our observations of these landforms] very much support Mustard's conclusions."