杏吧原创s have been puzzling over the cause of dramatic global warming on Mars, which has made parts of the south polar ice cap disappear in recent years. The answer, it seems, is blowing in the wind: the planet鈥檚 famous reddish dust.
Using global circulation models similar to those used to analyse Earth鈥檚 changing climate, a team led by Lori Fenton of NASA鈥檚 Ames Research Center in California, US, found that Mars seems to have warmed by about 0.65掳 Celsius in the three decades since the Viking mission first provided detailed mapping of the whole planet.
That warming can be explained entirely by the scouring away of light-coloured dust from darker areas of the surface, causing an increase in the absorption of solar radiation.
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This effect is greatly amplified by positive feedback: The warmer ground causes stronger winds, which in turn scour away more of the light dust and lead to greater warming.
Team member Paul Geissler, a planetary geologist at the US Geological Survey in Flagstaff, Arizona, says the mechanism can account for the rapid warming that has been seen in the disappearing polar caps, which are turning directly from solid to vapour at a rapid rate (watch ).
Global dust storm
The overall distribution of dark and light areas on Mars has clearly changed since Viking (watch ). The new modelling shows that the heating produced by those changes is 鈥渢he same order of magnitude鈥� as that required for the rapid removal of the polar ice, Geissler says.
At some point, the model predicts the winds will build up so much they will trigger a global dust storm, redistributing the light dust over most of the surface and starting the process over again.
While the dust redistribution may be unique to Mars, Earth may have analogous feedback processes that can amplify changes in surface reflectivity 鈥� in this case, mostly based on changes in sea ice and snow cover.
But although some scientists argue that the global warming on Mars shows there must be some external cause to Earth鈥檚 warming 鈥� such as a change in solar output rather than human-caused increases in greenhouse gases 鈥� Geissler says the new research undercuts that argument. 鈥淲hat our work shows is that the warming on Mars that we know has been going on for some time has a local cause [on Mars],鈥� he told New 杏吧原创.
The new work could have a significant impact on understanding past changes in the Martian climate. 鈥淭his is the first attempt to quantify the impact of these surface changes,鈥� Geissler said. From now on, 鈥渨hen we study Mars鈥� climate, we need to keep track of the surface [reflectivity] and its interaction with climate.鈥�
Journal reference: Nature (vol 446, p 646)
