NASA's Mars Reconnaisance Orbiter has found thin layer of carbon dioxide ice in some dusty parts of the red planet which get as cold at night year-round as the planet's poles in winter.
The surface in these regions becomes so frigid overnight that an extremely thin layer of carbon dioxide frost appears to form. The frost then vaporises in the morning.
Enough dust covers these regions that their heat-holding capacity is low and so the daily temperature swing is large.
Daily volatilisation of frost crystals that form among the dust grains may help keep the dust fluffy and so sustain this deep overnight chill, researchers said.
Carbon dioxide is the main ingredient of Mars' atmosphere. The planet also has large reserves of frozen carbon dioxide buried in the polar ice caps.
Seasonal buildup and thawing of carbon dioxide frost at high latitudes on Mars have been studied for years and linked to strange phenomena such as geyser - like eruptions and groove - cutting ice sleds.
Scientists found the presence and extent of transient overnight carbon dioxide frosts, even at middle and low latitudes.
Infrared-wavelength observations of dust-covered regions by the Mars Climate Sounder instrument on NASA's Mars Reconnaisance Orbiter not only indicate cold-enough nighttime surface temperatures for carbon dioxide frost to form, they also detect a spectrum signature at night consistent with a trace of frost.
"The temperature gets so low, you start freezing the atmosphere onto the surface," said Sylvain Piqueux of NASA's Jet Propulsion Laboratory in the US.
"Once you reach that temperature, you don't get colder, you just accumulate more frost. So even on the polar caps, the surface temperature isn't any colder than what these lower-latitude regions get to overnight," Piqueux said.
Three middle- and low-latitude areas in the Tharsis, Arabia and Elysium regions of Mars have nightly temperatures cold enough for carbon dioxide frost year-round or nearly year-round. Each of the three is bigger than Texas.
All three are dust-covered to the extent that surface temperatures change much quicker than in areas with exposed-bedrock surfaces.
"These same regions that are coldest at night are the warmest during the day," said Piqueux.
Unlike the polar regions, at lower latitudes the atmosphere is warmer than the ground at night.
A critical step in understanding just how cold the ground in these areas gets at night was correcting observations of the planet's surface for slightly warmer atmospheric temperatures.
Temperatures are determined from orbit by analysing the infrared radiation observed at the top of the atmosphere; this includes radiation from both the ground and the atmosphere.
The research was published in the Journal of Geophysical Research: Planets.
