Astronomers spotted a huge hurricane on 27 April as it snaked around the North pole of Mars. Nearly four times larger than the state of Texas, the swirling cyclonic system, seen in two sets of Hubble Space Telescope images released today, dwarfs any storm previously observed on Mars.
Martian spiral storms, similar in size and shape to terrestrial hurricanes, were first observed in 1979 by the Voyager satellite. On Earth, cyclones are driven by sharp temperature contrasts, such as those found at the intersection of tropical and Arctic air masses. "We think that the same thing happens on Mars," says principal investigator Jim Bell of Cornell University in Ithaca, New York. On Earth, warm ocean currents heat the tropical air; on Mars, the heat comes from dark patches of soil near Mars' North pole. In the martian summer, the dark spots are heated up by the sun, explains Bell, while the polar cap stays cold, causing hurricanes like the one observed. But these theories are difficult to test, because so few storms have been studied in detail.
The team was very lucky to have found this storm, says team member Todd Clancy of the Space Sciences Institute in Boulder, Colorado. Although they cover a large area, martian cyclones appear extremely small from Earth; even Hubble can only see them when Mars is at opposition, its point of closest approach to our planet. Mars's northern summer coincides with opposition only once every 17 years, and the window of opportunity is further narrowed because the storms only last a day or two, says Bell.
The Mars Global Surveyor (MGS) spacecraft, now orbiting Mars, could watch for storms from a much closer vantage. Although a mechanical glitch kept it out of the fray this time, its polar orbit around the red planet allows MGS to keep an eye peeled for martian cyclones all year long. "Now that we have a spacecraft at Mars," says Clancy, "we expect to find many more of these storms."