Mars Water-Ice Clouds Are Key to Odd Thermal Rhythm

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Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
guy.webster@jpl.nasa.gov

Dwayne Brown 202-358-1726
NASA Headquarters, Washington
dwayne.c.brown@nasa.gov

News release: 2013-201 June 12, 2013

Mars Water-Ice Clouds Are Key to Odd Thermal Rhythm

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.php?release=2013-201&cid=release_2013-201

PASADENA, Calif. -- Researchers using NASA's Mars Reconnaissance Orbiter have found that
temperatures in the Martian atmosphere regularly rise and fall not just once each day, but twice.

"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
in Pasadena, Calif., who is the lead author of a new report on these findings.

Temperatures swing by as much as 58 degrees Fahrenheit (32 kelvins) in this odd, twice-a-day
pattern, as detected by the orbiter's Mars Climate Sounder instrument.

The new set of Mars Climate Sounder observations sampled a range of times of day and night all
over Mars. The observations found that the pattern is dominant globally and year-round. The
report is being published in the journal Geophysical Research Letters.

Global oscillations of wind, temperature and pressure repeating each day or fraction of a day are
called atmospheric tides. In contrast to ocean tides, they are driven by variation in heating
between day and night. Earth has atmospheric tides, too, but the ones on Earth produce little
temperature difference in the lower atmosphere away from the ground. On Mars, which has only
about one percent as much atmosphere as Earth, they dominate short-term temperature variations
throughout the atmosphere.

Tides that go up and down once per day are called "diurnal." The twice-a-day ones are called
"semi-diurnal." 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.

"We were surprised to find this strong twice-a-day structure in the temperatures of the non-dusty
Mars atmosphere," Kleinboehl said. "While the diurnal tide as a dominant temperature response
to the day-night cycle of solar heating on Mars has been known for decades, 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."

He and his four co-authors found the answer in the water-ice clouds of Mars. The Martian
atmosphere has water-ice clouds for most of the year. Clouds in the equatorial region between
about 6 to 19 miles (10 to 30 kilometers) above the surface of Mars absorb infrared light emitted
from the surface during daytime. These are relatively transparent clouds, like thin cirrus clouds
on Earth. Still, the absorption by these clouds is enough to heat the middle atmosphere each day.
The observed semi-diurnal temperature pattern, with its maximum temperature swings occurring
away from the tropics, was also unexpected, but has been replicated in Mars climate models
when the radiative effects of water-ice clouds are included.

"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. "Water-ice clouds have been known to form in regions of cold temperatures, but the
feedback of these clouds on the Mars temperature structure had not been appreciated. We know
now that we will have to consider the cloud structure if we want to understand the Martian
atmosphere. This is comparable to scientific studies concerning Earth's atmosphere, where we
have to better understand clouds to estimate their influence on climate."

JPL, a division of the California Institute of Technology in Pasadena, provided the Mars Climate
Sounder instrument and manages the Mars Reconnaissance Orbiter project for NASA's Science
Mission Directorate, Washington.

For more about the Mars Reconnaissance Orbiter, visit: http://www.nasa.gov/mro .

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