[meteorite-list] Frosty Cold Nights Year-Round on Mars May Stir Dust

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=6564

Frosty Cold Nights Year-Round on Mars May Stir Dust
Jet Propulsion Laboratory
July 8, 2016

Some dusty parts of Mars get as cold at night year-round as the planet's 
poles do in winter, even regions near the equator in summer, according 
to new NASA findings based on Mars Reconnaissance Orbiter observations.

The surface in these regions becomes so frigid overnight that an extremely 
thin layer of carbon dioxide frost appears to form. The frost then vaporizes 
in the morning. Enough dust covers these regions that their heat-holding 
capacity is low and so the daily temperature swing is large. Daily volatilization 
of frost crystals that form among the dust grains may help keep the dust 
fluffy and so sustain this deep overnight chill.

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.

Here's what's new knowledge: 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, Pasadena, 
California, lead author of a report on these findings published online 
by the Journal of Geophysical Research: Planets. "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."

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.

Piqueux said, "These same regions that are coldest at night are the warmest 
during the day. It has to do with the nature of the material -- it's so 
fluffy. Think of when you're at the beach on a summer afternoon, where 
you step on the fine grain sand. You almost burn your foot, it's so hot 
at the surface, but just below the surface it's not as hot, and if you 
touch a boulder, it doesn't feel as hot. Then it's the opposite at night: 
The surface of the sand cools off quickly, while the boulder stays warm."

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 analyzing the infrared radiation oserved 
at the top of the atmosphere; this includes radiation from both the ground 
and the atmosphere. The Mars Climate Sounder instrument, by observing 
both sideways toward the horizon from orbit and downward, can record infrared 
emissions from a cross-section of the atmosphere, as well as from the 
planet's surface. Analysis then reveals the true -- colder -- ground temperature.

The same instrument also provides readings at multiple infrared wavelengths, 
yielding results consistent with the presence of microscopic-scale carbon 
dioxide frost crystals forming a layer no thicker than a few sheets of 
paper.

"If at night you form little frost crystals between the grains of dust 
on the surface, pushing the grains apart, then the frost crystal becomes 
a little puff of air in the morning, that might be helping to maintain 
the fluffiness of the surface," Piqueux said. "You prevent the cementation 
of grains, the locking together of grains into a more consolidated surface. 
It's a self-maintaining process: Where you keep the soil fluffy, you maintain 
the conditions to form frost at night."

"A cycle of carbon dioxide frost that happens every night could be related 
to other active processes on Mars," said Rich Zurek, JPL's chief Mars 
scientist. "This agitation of the soil would affect surface physical properties 
and could have implications for erosive processes and for the exchange 
of water vapor between the atmosphere and surface."

Many streaks on Martian slopes appear to be slides of dry material, with 
no liquid involved. The lubrication effect of carbon dioxide frost thawing 
directly into gas has been linked to such slides where winter frost thaws 
in spring. Daily frost cycles may have similar effects.

Another type of slope activity on Mars is called recurring slope lineae 
(RSL). These appear as dark streaks advancing downhill in a warm season, 
then fade away, then re-appear the next warm season. Hydrated salt has 
been confirmed at some of these sites, and they are considered the strongest 
evidence for the possible presence of liquid water on the surface of modern 
Mars. "Although RSL appear to start on steep, rocky slopes, the realization 
that overnight carbon dioxide frosts occur even during warm seasons adds 
another factor to be considered in RSL activity," Zurek said.

The science instruments on NASA's Mars Reconnaissance Orbiter have been 
examining Mars since 2006. JPL, a division of Caltech in Pasadena, manages 
the mission for NASA's Science Mission Directorate in Washington and built 
the Mars Climate Sounder. Lockheed Martin Space Systems of Denver built 
the orbiter and supports its operations.

News Media Contact
Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.webster at jpl.nasa.gov

2016-181