NASA Mars Orbiters Reveal Seasonal Dust Storm Pattern
This graphic presents Martian atmospheric
temperature data as curtains over an image of Mars taken during a
regional dust storm. The temperature profiles extend from the surface to
about 50 miles up. Temperatures are color coded, from minus 243 degrees
Fahrenheit (purple) to minus 9 F (red).
Credits: NASA/JPL-Caltech/MSSS
This graphic shows Martian atmospheric temperature
data related to seasonal patterns in occurrence of large regional dust
storms. The data shown here were collected by the Mars Climate Sounder
instrument on NASA's Mars Reconnaissance Orbiter over the course of
one-half of a Martian year, during 2012 and 2013. The color coding
indicates daytime temperatures of a layer of the atmosphere centered
about 16 miles (25 kilometers) above ground level, corresponding to the
color-key bar at the bottom of the graphic.
Credits: NASA/JPL-Caltech
After decades of research to discern seasonal patterns in Martian
dust storms from images showing the dust, but the clearest pattern
appears to be captured by measuring the temperature of the Red Planet's
atmosphere.
For six recent Martian years, temperature records from NASA Mars
orbiters reveal a pattern of three types of large regional dust storms
occurring in sequence at about the same times each year during the
southern hemisphere spring and summer. Each Martian year lasts about two
Earth years.
"When we look at the temperature structure instead of the visible
dust, we finally see some regularity in the large dust storms," said
David Kass of NASA's Jet Propulsion Laboratory, Pasadena, California. He
is the instrument scientist for the Mars Climate Sounder on NASA's
Mars Reconnaissance Orbiter and lead author of a
report about these findings posted this week by the journal Geophysical Research Letters.
"Recognizing a pattern in the occurrence of regional dust storms is a
step toward understanding the fundamental atmospheric properties
controlling them," he said. "We still have much to learn, but this gives
us a valuable opening."
Dust lofted by Martian winds links directly to atmospheric
temperature: The dust absorbs sunlight, so the sun heats dusty air more
than clear air. In some cases, this can be dramatic, with a difference
of more than 63 Fahrenheit degrees (35 Celsius degrees) between dusty
air and clear air. This heating also affects the global wind
distribution, which can produce downward motion that warms the air
outside the dust-heated regions. Thus, temperature observations capture
both direct and indirect effects of the dust storms on the atmosphere.
Improving the ability to predict large-scale, potentially hazardous
dust storms on Mars would have safety benefits for planning robotic and
human missions to the planet's surface. Also, by recognizing patterns
and categories of dust storms, researchers make progress toward
understanding how seasonal local events affect global weather in a
typical Mars year.
NASA has been operating orbiters at Mars continuously since 1997. The
Mars Climate Sounder on Mars Reconnaissance Orbiter, which reached Mars
in 2006, and the Thermal Emission Spectrometer on Mars Global Surveyor,
which studied Mars from 1997 to 2006, have used infrared observations
to assess atmospheric temperature. Kass and co-authors analyzed
temperature data representative of a broad layer centered about 16 miles
(25 kilometers) above the Martian surface. That's high enough to be
more affected by regional storms than by local storms.
Most Martian dust storms are localized, smaller than about 1,200
miles (about 2,000 kilometers) across and dissipating within a few days.
Some become regional, affecting up to a third of the planet and
persisting up to three weeks. A few encircle Mars, covering the southern
hemisphere but not the whole planet. Twice since 1997, global dust
storms have fully enshrouded Mars. The behavior of large regional dust
storms in Martian years that include global dust storms is currently
unclear, and years with a global storm were not included in the new
analysis.
Three large regional storms, dubbed types A, B and C, all appeared in each of the six Martian years investigated.
Multiple small storms form sequentially near Mars' north pole in the
northern autumn, similar to Earth's cold-season arctic storms that swing
one after another across North America.
"On Mars, some of these break off and head farther south along
favored tracks," Kass said. "If they cross into the southern hemisphere,
where it is mid-spring, they get warmer and can explode into the much
larger Type A dust storms."
Southern hemisphere spring and summer on modern-day Mars are much
warmer than northern spring and summer, because the eccentricity of
Mars' orbit puts the planet closest to the sun near the end of southern
spring. Southern spring and summer have long been recognized as the
dustiest part of the Martian year and the season of global dust storms,
even though the more detailed pattern documented in the new report had
not been previously described.
When a Type A storm from the north moves into southern-hemisphere
spring, the sunlight on the dust warms the atmosphere. That energy
boosts the speed of winds. The stronger winds lift more dust, further
expanding the area and vertical reach of the storm.
In contrast, the Type B storm starts close to the south pole shortly
before the beginning of southern summer. Its origin may be from winds
generated at the edge of the retreating south-polar carbon dioxide ice
cap. Multiple storms may contribute to a regional haze.
The Type C storm starts after the B storm ends. It originates in the
north during northern winter (southern summer) and moves to the southern
hemisphere like the Type A storm. From one year to another, the C storm
varies more in strength, in terms of peak temperature and duration,
than the A and B storms do.
The longevity of NASA's Mars Reconnaissance Orbiter has helped enable
studies such as this of seasonal patterns on Mars. JPL provided the
Mars Climate Sounder instrument and manages the mission for NASA's
Science Mission Directorate. Arizona State University, Tempe, provided
the Thermal Emission Spectrometer for Mars Global Surveyor. Lockheed
Martin Space Systems, Denver, built both orbiters.
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