NASA's Mars Reconnaissance Orbiter has revealed to scientists slender
dark markings -- possibly due to salty water – that advance seasonally
down slopes surprisingly close to the Martian equator.
"The equatorial surface region of Mars has been regarded as dry, free
of liquid or frozen water, but we may need to rethink that," said
Alfred McEwen of the University of Arizona in Tucson, principal
investigator for the Mars Reconnaissance Orbiter (MRO) High Resolution
Imaging Science Experiment (HiRISE) camera.
Tracking how these features recur each year is one example of how the
longevity of NASA orbiters observing Mars is providing insight about
changes on many time scales. Researchers at the American Geophysical
Union meeting Tuesday in San Francisco discussed a range of current
Martian activity, from fresh craters offering glimpses of subsurface ice
to multi-year patterns in the occurrence of large, regional dust
storms.
The seasonally changing surface flows were first reported two years
ago on mid-latitude southern slopes. They are finger-like features
typically less than 16 feet (5 meters) wide that appear and extend down
steep, rocky slopes during spring through summer, then fade in winter
and return the next spring. Recently observed slopes stretch as long as
4,000 feet (1,200 meters).
McEwen and co-authors reported the equatorial flows at the conference
and in a paper published online Tuesday by Nature Geoscience. Five
well-monitored sites with these markings are in Valles Marineris, the
largest canyon system in the solar system. At each of these sites, the
features appear on both north- and south-facing walls. On the
north-facing slopes, they are active during the part of the year when
those slopes get the most sunshine. The counterparts on south-facing
slopes start flowing when the season shifts and more sunshine hits their
side.
"The explanation that fits best is salty water is flowing down the
slopes when the temperature rises," McEwen said. "We still don't have
any definite identification of water at these sites, but there's nothing
that rules it out, either."
Dissolved salts can keep water melted at temperatures when purer
water freezes, and they can slow the evaporation rate so brine can flow
farther. This analysis used data from the Compact Reconnaissance Imaging
Spectrometer for Mars and the Context Camera on the MRO as well as the
Thermal Emission Imaging System experiment on NASA's Mars Odyssey
orbiter.
Water ice has been identified in another dynamic process researchers
are monitoring with MRO. Impacts of small asteroids or bits of comets
dig many fresh craters on Mars every year. Twenty fresh craters have
exposed bright ice previously hidden beneath the surface. Five were
reported in 2009. The 15 newly reported ones are distributed over a
wider range of latitudes and longitudes.
"The more we find, the more we can fill in a global map of where ice
is buried," said Colin Dundas of the U.S. Geological Survey in
Flagstaff, Ariz. "We've now seen icy craters down to 39 degrees north,
more than halfway from the pole to the equator. They tell us that either
the average climate over several thousand years is wetter than present
or that water vapor in the current atmosphere is concentrated near the
surface. Ice could have formed under wetter conditions, with remnants
from that time persisting today, but slowly disappearing."
Mars' modern climate becomes better known each year because of a
growing set of data from a series of orbiters that have been studying
Mars continually since 1997. That has been almost nine Martian years
because a year on Mars is almost two years long on Earth. Earlier
missions and surface landers have added insight about the dynamics of
Mars' atmosphere and its interaction with the ground.
"The dust cycle is the main driver of the climate system," said
Robert Haberle of NASA's Ames Research Center in Moffett Field, Calif.
One key question researchers want to answer is why dust storms
encircle Mars in some years and not in others. These storms affect
annual patterns of water vapor and carbon dioxide in the atmosphere,
freezing into polar ice caps in winter and replenishing the atmosphere
in spring. Identifying significant variations in annual patterns
requires many Martian years of observations.
The data emerging from long-term studies will help future human
explorers of Mars know where to find resources such as water, how to
prepare for hazards such as dust storms, and where to be extra careful
about contamination with Earth microbes.
Launched in 2005, Mars Reconnaissance Orbiter and its six instruments
have provided more high-resolution data about the Red Planet than all
other Mars orbiters combined. Data are made available for scientists
worldwide to research, analyze and report their findings.
NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the MRO
and Mars Odyssey missions for NASA's Science Mission Directorate in
Washington. Lockheed Martin Space Systems in Denver built both orbiters.
The University of Arizona Lunar and Planetary Laboratory operates the
HiRISE camera, which was built by Ball Aerospace & Technologies
Corp. of Boulder, Colo.
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