This graphic depicts the Mars Climate Sounder instrument on NASA's Mars
Reconnaissance Orbiter measuring the temperature of a cross section of
the Martian atmosphere as the orbiter passes above the south polar
region. Image credit: NASA/JPL-Caltech
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,
Marks on Martian Dunes May Reveal Tracks of Dry Ice Sleds
PASADENA, Calif. - NASA research indicates hunks of frozen carbon
dioxide - dry ice - may glide down some Martian sand dunes on cushions
of gas similar to miniature hovercraft, plowing furrows as they go.
Researchers deduced this process could explain one enigmatic class of gullies seen on Martian sand dunes by examining images from NASA's Mars Reconnaissance Orbiter (MRO) and performing experiments on sand dunes in Utah and California.
"I have always dreamed of going to Mars," said Serina Diniega, a planetary scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and lead author of a report published online by the journal Icarus. "Now I dream of snowboarding down a Martian sand dune on a block of dry ice."
The hillside grooves on Mars, called linear gullies, show relatively constant width - up to a few yards or meters across - with raised banks or levees along the sides. Unlike gullies caused by waterflows on Earth and possibly on Mars, they do not have aprons of debris at the downhill end of the gully. Instead, many have pits at the downhill end.
"In debris flows, you have water carrying sediment downhill, and the material eroded from the top is carried to the bottom and deposited as a fan-shaped apron," said Diniega. "In the linear gullies, you're not transporting material. You're carving out a groove, pushing material to the sides."
Images from MRO's High Resolution Imaging Science Experiment (HiRISE) camera show sand dunes with linear gullies covered by carbon dioxide frost during the Martian winter. The location of the linear gullies is on dunes that spend the Martian winter covered by carbon dioxide frost. The grooves are formed during early spring, researchers determined by comparing before-and-after images from different seasons. Some images have even caught bright objects in the gullies.
Scientists theorize the bright objects are pieces of dry ice that have broken away from points higher on the slope. According to the new hypothesis, the pits could result from the blocks of dry ice completely sublimating away into carbon-dioxide gas after they have stopped traveling.
"Linear gullies don't look like gullies on Earth or other gullies on Mars, and this process wouldn't happen on Earth," said Diniega. "You don't get blocks of dry ice on Earth unless you go buy them."
That is exactly what report co-author Candice Hansen, of the Planetary Science Institute in Tucson, Ariz., did. Hansen has studied other effects of seasonal carbon-dioxide ice on Mars, such as spider-shaped features that result from explosive release of carbon-dioxide gas trapped beneath a sheet of dry ice as the underside of the sheet thaws in spring. She suspected a role for dry ice in forming linear gullies, so she bought some slabs of dry ice at a supermarket and slid them down sand dunes.
That day and in several later experiments, gaseous carbon dioxide from the thawing ice maintained a lubricating layer under the slab and also pushed sand aside into small levees as the slabs glided down even low-angle slopes.
The outdoor tests did not simulate Martian temperature and pressure, but calculations indicate the dry ice would act similarly in early Martian spring where the linear gullies form. Although water ice, too, can sublimate directly to gas under some Martian conditions, it would stay frozen at the temperatures at which these gullies form, the researchers calculate.
"MRO is showing that Mars is a very active planet," Hansen said. "Some of the processes we see on Mars are like processes on Earth, but this one is in the category of uniquely Martian."
Hansen also noted the process could be unique to the linear gullies described on Martian sand dunes.
"There are a variety of different types of features on Mars that sometimes get lumped together as 'gullies,' but they are formed by different processes," she said. "Just because this dry-ice hypothesis looks like a good explanation for one type doesn't mean it applies to others."
The University of Arizona Lunar and Planetary Laboratory operates the HiRISE camera, which was built by Ball Aerospace & Technologies Corp. of Boulder, Colo. JPL manages MRO for NASA's Science Mission Directorate in Washington. Lockheed Martin Space Systems of Denver built the orbiter.
To see images of the linear gullies and obtain more information about MRO, visit:
For more about HiRISE, visit:
visit:
NASA
Guillermo Gonzalo Sánchez Achutegui
ayabaca@gmail.com
ayabaca@hotmail.com
ayabaca@yahoo.com
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