From Newsgroup: sci.space.news

http://www.jpl.nasa.gov/news/news.php?feature=6512

Second Cycle of Martian Seasons Completing for Curiosity Rover
Jet Propulsion Laboratory
May 11, 2016

NASA's Curiosity Mars rover today completes its second Martian year since landing inside Gale Crater nearly four Earth years ago, which means it
has recorded environmental patterns through two full cycles of Martian seasons.

The repetition helps distinguish seasonal effects from sporadic events.
For example, a large spike in methane in the local atmosphere during the
first southern-hemisphere autumn in Gale Crater was not repeated the second autumn. It was an episodic release, still unexplained. However, the rover's measurements do suggest that much subtler changes in the background methane concentration -- amounts much less than during the spike -- may follow
a seasonal pattern. Measurements of temperature, pressure, ultraviolet
light reaching the surface and the scant water vapor in the air at Gale
Crater show strong, repeated seasonal changes.

Monitoring the modern atmosphere, weather and climate fulfills a Curiosity mission goal supplementing the better-known investigations of conditions billions of years ago. Back then, Gale Crater had lakes and groundwater
that could have been good habitats for microbes, if Mars has ever had
any. Today, though dry and much less hospitable, environmental factors
are still dynamic.

Curiosity's Rover Environmental Monitoring Station (REMS), supplied by
Spain's Centro de Astrobiolog|!a, has measured air temperatures from 60.5 degrees Fahrenheit (15.9 degrees Celsius) on a summer afternoon, to minus
148 F (minus 100 C) on a winter night. Seasonal patterns in temperature,
water vapor and pressure that Curiosity has measured in Gale Crater are charted in a new graphic at:

http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA20600

"Curiosity's weather station has made measurements nearly every hour of
every day, more than 34 million so far," said Curiosity Project Scientist Ashwin Vasavada of NASA's Jet Propulsion Laboratory, Pasadena, California. "The duration is important, because it's the second time through the seasons that lets us see repeated patterns."

Each Martian year -- the time it takes the Red Planet to orbit the sun
once -- lasts 687 Earth days. Curiosity landed on Aug. 5, 2012, (Pacific
Time; Aug. 6, Universal Time). It begins its third Martian year on May
11, 2016, during the mission's 1,337th Martian day, or "sol," since landing. Each Martian sol lasts about 39.6 minutes longer than an Earth day, and
a Martian year lasts 668.6 sols.

The similar tilts of Earth and Mars give both planets a yearly rhythm
of seasons. But some differences are great, such as in comparisons between
day and night temperatures. Even during the time of the Martian year when temperatures at Gale Crater rise above freezing during the day, they plummet overnight below minus 130 F (minus 90 C), due to the thin atmosphere.
Also, the more-elliptical orbit of Mars, compared to Earth, exaggerates
the southern-hemisphere seasons, making them dominant even at Gale Crater's near-equatorial location.

"Mars is much drier than our planet, and in particular Gale Crater, near
the equator, is a very dry place on Mars," said Germ|in Mart|!nez, a Curiosity science-team collaborator from Spain at the University of Michigan, Ann
Arbor. "The water vapor content is a thousand to 10 thousand times less
than on Earth."

Relative humidity is a function of both temperature and water-vapor content. During winter nights, Curiosity has measured relative humidity of up to
70 percent, high enough to prompt researchers to check for frost forming
on the ground. Other Mars landers have detected frost, but Curiosity has
not.

Curiosity's air-pressure measurements confirm a strong seasonal trend previously seen by other missions. "There are large changes due to the
capture and release of carbon dioxide by the seasonal polar caps," Mart|!nez explained. Most of the Martian atmosphere is carbon dioxide. During each pole's winter, millions of tons of this gas freeze solid, only to be released again in spring, prompting very un-Earthlike seasonal variations of about
25 percent in atmospheric pressure.

Other seasonal patterns measured by Curiosity and repeated in the rover's second Martian year are that the local atmosphere is clear in winter,
dustier in spring and summer, and windy in autumn. Visibility in Gale
Crater is as low as 20 miles (30 kilometers) in summer, and as high as
80 miles (130 kilometers) in winter.

For tracking changes in the concentration of methane in the air above
Gale Crater, researchers use the tunable laser spectrometer in Curiosity's Sample Analysis at Mars (SAM) suite of instruments. These measurements
are made less often than REMS measurements, though frequently enough to
tease out seasonal patterns. For most of the two Martian years, the rover
has measured methane concentrations between 0.3 and 0.8 parts per billion.
For several weeks during the first autumn, the level spiked, reaching
7 parts per billion. The mission checked carefully for a repeat of that
spike during the second autumn, but concentrations stayed at lower background levels.

"Doing a second year told us right away that the spike was not a seasonal effect," said JPL's Chris Webster of the SAM team. "It's apparently an episodic event that we may or may not ever see again."

However, the mission is continuing to monitor a possible seasonal pattern
in the background methane concentration. The background level is far less
than the spike level, but it appears to be even lower in autumn than in
other seasons. If this pattern is confirmed, it may be related to the
pressure pattern measured by REMS or to seasonal change in ultraviolet radiation, which is measured by REMS in concert with the rover's Mast
Camera.

"This shows not only the importance of long-term monitoring, but also
the importance of combining more than one type of measurement from a single platform," Webster said.

While continuing to study the modern local environment, Curiosity is investigating
geological layers of lower Mount Sharp, inside Gale Crater, to increase understanding of ancient changes in environmental conditions. For more information about Curiosity, visit:

http://mars.jpl.nasa.gov/msl


News Media Contact

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

Dwayne Brown / Laurie Cantillo
NASA Headquarters, Washington
202-358-1726 / 202-358-1077
dwayne.c.brown@nasa.gov / laura.l.cantillo@nasa.gov

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