Dec 30th 2025|1 min read
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While the northern hemisphere of Earth makes its way through winter, the southern hemisphere of Mars is reaching the end of spring, and as a result it is blooming, with patches of darkness blossoming under the watchful eyes of orbiting satellites. Discovered in the early 2000s, they are of astrobiological interest because they indicate local conditions that could be more habitable than Mars’s surface is at other times.
In its winter, the high latitudes of Mars’s southern hemisphere become cold enough that carbon dioxide freezes out of the atmosphere to form a frosty dry-ice icecap roughly a metre thick. As spring comes, dust trapped within the opaque dry ice warms it from within, turning it translucent. Sunlight reaches the surface under the dry ice to warm it. The base of the dry ice turns back into gas. The pressure of that gas, trapped between the surface regolith and its translucent covering, builds up; the ice layer thins. Eventually the strength of the latter can no longer constrain the former, and pressurised gas bursts out in little fountains, carrying dark dust with it. The dust settles on the surface of the deflating icecap, forming a distinctive bloom.
The radial patterns that these processes leave behind look like stylised starbursts. Presumably because David Bowie neglected to write any songs about the starbursts from Mars, the patterns became known as spiders, and scientists now recognise a range of “araneiform” erosional features created by flows of pressurised dry ice in these parts of Mars. ■
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This article appeared in the Science & technology section of the print edition under the headline “Ziggy played guitar”
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