Curiosity has now been on Mars for two Martian years, allowing the rover to observe a pair of seasonal cycles. This has produced a highly detailed record of these changes on the Red Planet for the first time.

Having a second year of observation allows astronomers to differentiate between authentic seasonal changes and random weather conditions. One example was a significant release of methane detected by the spacecraft during its first year exploring Gale Crater. This event was not repeated the second year, and the cause behind the release remains a mystery.

Nevertheless, astronomers interpreting data from Curiosity have recorded a more subtle cycle of methane levels that do appear to be seasonal.

Other changes that appear to correlate with changes of the season include concentrations of water vapor, temperature, air pressure and the amount of ultraviolet light reaching the tawny surface.

"Curiosity's weather station has made measurements nearly every hour of every day, more than 34 million so far. The duration is important, because it's the second time through the seasons that lets us see repeated patterns," said Ashwin Vasavada of the Jet Propulsion Laboratory.

Temperatures recorded in Gale Crater fall as low as 148 degrees below zero Fahrenheit, and climb to a comfortable 60.5 degrees Fahrenheit during the height of a balmy summer day.

Like the Earth, the rotational axis of Mars is tilted, resulting in a yearly cycle of seasons. However, Mars follows a far more elliptical orbit than the Earth, resulting in exaggerated seasons in the southern hemisphere. The thin atmosphere of that alien world does not hold in heat, so temperatures at night plummet, even during the height of summer.

"Mars is much drier than our planet, and in particular Gale Crater, near the equator, is a very dry place on Mars. The water vapor content is a thousand to 10 thousand times less than on Earth," said Germán Martínez from the University of Michigan, Ann Arbor.

The Curiosity rover was launched from Earth in 2011, touching down on the alien surface a year later. Mars takes 687 Earth days to orbit once around the sun.

By analyzing seasonal changes on Mars today, researchers hope to better understand what the Red Planet was like billions of years in the past.

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