An image of Mars taken during a dust storm, with atmospheric temperature data included. The temperatures are colour coded, ranging from –153°C (purple) to –23°C (red). Image credit: NASA/JPL-Caltech
NASA Mars orbiters have confirmed a pattern of three regional dust storms occurring on the Red Planet at the same times each Martian year.
The storms were detected over six Martian years by measuring the temperature of the planet’s atmosphere.
Previous studies over the decades had attempted to discern seasonal patterns in dust storms by observing the dust itself, but analysing atmospheric temperature has proved a more fruitful technique.
This is because dust kicked up by Martian winds is directly linked to atmospheric temperature.
The dust absorbs sunlight and so dusty air is hotter than clean air, sometimes with differences of over 35°C.
The heat also has an effect on Martian wind distribution, producing downward motions that warm the air outside the dust-heated regions.
This means that Mars orbiters can conduct temperature observations capturing both the direct and indirect effects of dust storms.
Mars has an eccentric orbit, meaning it is not a perfect circle, and this has a bearing on its seasons.
Southern hemisphere spring and summer are warmer than northern spring and summer, because the planet is closest to the Sun towards the end of southern spring.
This warmer period has long been identified as the dustiest part of the year on Mars, with many global dust storms occurring.
Martian dust storms are mostly localised, smaller than about 2,000km across and dissipate within days.
The study has identified three different types of large regional storms called types A, B and C.
When a type A storm from the north of the planet drifts into the southern hemisphere spring, sunlight warms the Martian atmosphere, causing the winds to increase in speed.
These stronger, faster winds lift more dust and cause the storm to expand.
Type B storms, however, originate close to the south pole before the beginning of southern summer and are thought to come from winds generated at the edge of the south polar carbon dioxide ice cap as it retreats.
Type C storms begin after type B storms end, originating in the north during northern winter and and moving to the southern hemisphere like the type A storm.
A type C storm varies more in strength than types A and B.
The results are a step toward fully understanding the cycle of dust storms on Mars, which would be beneficial to future robotic or human missions to the Red Planet.
"When we look at the temperature structure instead of the visible dust, we finally see some regularity in the large dust storms," says lead author David Kass of NASA's Jet Propulsion Laboratory.
"Recognising a pattern in the occurrence of regional dust storms is a step toward understanding the fundamental atmospheric properties controlling them.
We still have much to learn, but this gives us a valuable opening."