On Earth, temperature changes correspond to day and night, with the sun warming the planet by day and surface temperatures cooling after dark. One cycle of up and down per day is known as a "diurnal" pattern.
On Mars, this pattern happens twice each day, and the temperature can swing more than 58 degrees F within hours.
Using NASA's Mars Reconnaissance Orbiter and data collected by the Mars Climate Sounder, researchers were able to better understand atmospheric tides on Mars.
"We see a temperature maximum in the middle of the day, but we also see a temperature maximum a little after midnight," said Armin Kleinboehl of NASA's Jet Propulsion Laboratory, lead author of the report published in Geophysical Research Letters.
The semi-diurnal pattern on Mars was first seen in the 1970s, but until now it had been thought to appear just in dusty seasons, related to sunlight warming dust in the atmosphere.
“The discovery of a persistent semi-diurnal response even outside of major dust storms was quite unexpected and caused us to wonder what drove this response," Kleinboehl said.
The team discovered their answer in the water-ice clouds in the atmosphere. The ice-clouds are common on Mars through most of the year, usually six to 13 miles above its surface. The clouds absorb infrared light emitted from the surface of Mars during the day.
Though they appear similar to thin cirrus clouds on Earth, they absorb enough heat to radiate it back out into the middle atmosphere twice a day, with maximum temperature swings occurring away from the tropics.
"We think of Mars as a cold and dry world with little water, but there is actually more water vapor in the Martian atmosphere than in the upper layers of Earth's atmosphere," Kleinboehl said, "but the feedback of these clouds on the Mars temperature structure had not been appreciated."
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