The local atmosphere in Mars is clear in winter, dustier in spring and summer, and windy in autumn, NASA scientists, including one of Indian-origin, have found.
NASA's Curiosity Mars Rover, which has completed its second Martian year since landing inside Gale Crater nearly four years ago, recorded environmental patterns through two full cycles of Martian seasons, scientists said.
The other seasonal patterns measured by Curiosity and repeated in the Rover's second Martian year are that the local atmosphere is clear in winter, dustier in spring and summer, and windy in autumn, they said.
The repetition helps distinguish seasonal effects from sporadic events. For example, a large spike in methane in the local atmosphere during the first southern-hemisphere autumn in Gale Crater was not repeated the second autumn, they added.
"Curiosity's weather station has made measurements nearly every hour of every day, more than 34 million so far," said Curiosity Project Scientist Ashwin Vasavada of NASA's Jet Propulsion Laboratory in California.
"The duration is important, because it is the second time through the seasons that lets us see repeated patterns," he said.
It was an episodic release, still unexplained. However, the Rover's measurements do suggest that much subtler changes in the background methane concentration - amounts much less than during the spike - may follow a seasonal pattern, the scientists said.
Measurements of temperature, pressure, ultraviolet light reaching the surface and the scant water vapour in the air at Gale Crater show strong, repeated seasonal changes, they said.
Curiosity's Rover Environmental Monitoring Station (REMS) has measured air temperatures from 15.9 degrees Celsius on a summer afternoon to minus 100 degrees Celsius on a winter night.
"Mars is much drier than our planet, and in particular Gale Crater, near the equator, is a very dry place on Mars," said German Martinez from University of Michigan.
"The water vapour content is a thousand to 10 thousand times less than on Earth," said Martinez.
Each Martian year - the time it takes the Red Planet to orbit the sun once - lasts 687 Earth days. Curiosity landed on August 5, 2012.
It began its third Martian year on May 11 this year during the mission's 1,337th Martian day, or "sol" since landing.
Each Martian sol lasts about 39.6 minutes longer than an Earth day, and a Martian year lasts 668.6 sols, scientists said.
Relative humidity is a function of both temperature and water-vapour content. During winter nights, Curiosity has measured relative humidity of up to 70 per cent, high enough to prompt researchers to check for frost forming on the ground, they added.
