Little devils

2019-03-07 04:12:01

By Jeff Hecht SMALL whirlwinds called dust devils are so common on Mars that they load the Martian atmosphere with dust, making the sky look reddish from the ground. They may even seed the giant dust storms that sometimes engulf the planet, scientists suggest. Dust devils, which look like miniature tornadoes, form when sunlight warms air just above the ground. Small breezes can give a twist to the rising warm air, and once it has started spinning it can draw in more air, forming a whirlwind that picks up dust from the ground. Dust devils often develop in dry areas on Earth, whirling dust several kilometres up into the atmosphere. And the Viking spacecraft spotted a few much larger ones on Mars. “The perspective was that dust devils were some odd and uncommon event of huge proportions,” says Steve Metzger, a postgraduate student at the University of Nevada at Reno. But after studying terrestrial dust devils, Metzger had a hunch that smaller ones would be more common on Mars than anyone realised. “Martian surface conditions were ripe for their development in most regions,” he says. To test this, the team enhanced images from the Mars Pathfinder that landed on the Red Planet in 1997 and found five dust devils. They were up to 79 metres wide and 350 metres tall, and sometimes remained visible for nearly 7 minutes (Geophysical Research Letters, vol 26, p 2781). “They raised many tens of kilograms of dust to considerable heights in their brief lifespans,” says Metzger. The results show that Martian dust devils are the main source of the planet’s dusty haze. “Odds are that a couple of times a week, every square metre of ground at the Pathfinder site is covered by a dust devil,