Hot stuff: darker areas heat up in the sunshine.NASA

Mars is getting hotter. Measurements of the brightness of the planet's surface show that the thermometer has ratcheted up some 0.65 °C over a few decades.

Lori Fenton at the Carl Sagan Center, Mountain View, California, and colleagues looked at maps of Mars's 'albedo', a measure of how much light reflects off a surface. By comparing a map from 1976-78 with one from 1999-2000, they found "some pretty dramatic changes", says Fenton. In particular, the southern highlands region of Mars had darkened significantly.

The darkening is thanks to the clearance of light-coloured dust that covers the planet's darker bedrock, they propose. When the Sun's light hits dark rock it warms the surface, and the heat is kept in by the atmosphere. This warming kicks up winds, which swirl any dust around and can even make dust devils. This sweeps the light-coloured dust into pockets, revealing more bedrock and causing further heating.

"The two processes that lift dust are being enhanced by the darkening of the surface, and those are the processes that darken the surface in the first place," says Fenton.

Fenton then used a model of the planet's climate to infer the temperature change caused by this darkening. These models are very similar to those used to predict the weather on Earth, but with fewer parameters: Mars has no oceans or vegetation, and the effects of clouds are nowhere near as important as they are on Earth.

Here today, gone tomorrow

Using these results, one might come to the conclusion that in 500 or so years the martian polar ice-caps will be completely gone, notes Phil Christensen, a planetary scientist at Arizona State University, Tempe. But, he says: "I don't think that's likely. They're looking at a piece of the cycle, other processes could turn this around to a place where the ice-caps start growing again."

A major dust storm that engulfs the entire planet, for example, could redistribute dust more evenly around the planet and instigate cooling. "Dust storms are like a reset mechanism," says Fenton. Such storms were seen on the planet in the 1970s.

Fenton's work shows nicely how conditions on Mars have changed on decadal timescales, says Christensen. But the results shouldn't be taken further than that: "You can't take 10 years of data and extrapolate out to 1,000 years," he says.

Big change

David Paige, a planetary scientist at the University of California, Los Angeles, says that Mars's albedo will surely affect the climate. But he is sceptical about the magnitude of the temperature change calculated by Fenton: "That's a big change," he says of 0.65 °C.

Paige notes that the data for the study come from two different sources: the albedo map from the 1970s was produced by the Infrared Thermal Mapper onboard the Viking mission, and the more recent map comes from the Thermal Emission Spectrometer aboard Mars Global Surveyor. Perhaps differences between these two instruments might make it inappropriate to compare their data directly, he says.

Paige is currently waiting to analyse the latest albedo data from the Mars Climate Sounder on Mars Reconnaissance Orbiter.

The warming on Mars is likely to be seized by climate-change sceptics here on Earth - if Mars is hotting up even without any cars or pollution, then perhaps the Sun or some other natural, Solar-System-wide factor is to blame. But to infer that would be "crazy" says Christensen (see 'Hot times in the Solar System').

"The more we learn about Mars, the more intuition it gives us about Earth, but the systems are fundamentally different," he says.

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• References

  1. Fenton I., Geissler P. E. & Haberle R. M.. Nature, 446 . 646 - 649 (2007). | Article |