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New Gravity Map Gives Best View Yet Inside Mars
Jet Propulsion Laboratory
March 21, 2016

A new map of Mars' gravity made with three NASA spacecraft is the most detailed to date, providing a revealing glimpse into the hidden interior
of the Red Planet.

"Gravity maps allow us to see inside a planet, just as a doctor uses an
X-ray to see inside a patient," said Antonio Genova of the Massachusetts Institute of Technology (MIT), Cambridge. "The new gravity map will be
helpful for future Mars exploration, because better knowledge of the planet's gravity anomalies helps mission controllers insert spacecraft more precisely into orbit about Mars. Furthermore, the improved resolution of our gravity
map will help us understand the still-mysterious formation of specific
regions of the planet." Genova, who is affiliated with MIT but is located
at NASA's Goddard Space Flight Center in Greenbelt, Maryland, is the lead author of a paper on this research published online March 5 in the journal Icarus.

The improved resolution of the new gravity map suggests a new explanation
for how some features formed across the boundary that divides the relatively smooth northern lowlands from heavily cratered southern highlands. Also,
the team confirmed that Mars has a liquid outer core of molten rock by analyzing tides in the Martian crust and mantle caused by the gravitational pull of the sun and the two moons of Mars. Finally, by observing how Mars' gravity changed over 11 years - the period of an entire cycle of solar activity -- the team inferred the massive amount of carbon dioxide that freezes out of the atmosphere onto a Martian polar ice cap when it experiences winter. They also observed how that mass moves between the south pole
and the north pole with the change of season in each hemisphere.

The map was derived using Doppler and range tracking data collected by
NASA's Deep Space Network from three NASA spacecraft in orbit around Mars: Mars Global Surveyor (MGS), Mars Odyssey (ODY), and the Mars Reconnaissance Orbiter (MRO). Like all planets, Mars is lumpy, which causes the gravitational pull felt by spacecraft in orbit around it to change. For example, the
pull will be a bit stronger over a mountain, and slightly weaker over
a canyon.

Slight differences in Mars' gravity changed the trajectory of the NASA spacecraft orbiting the planet, which altered the signal being sent from
the spacecraft to the Deep Space Network. These small fluctuations in
the orbital data were used to build a map of the Martian gravity field.

The gravity field was recovered using about 16 years of data that were continuously collected in orbit around Mars. However, orbital changes
from uneven gravity are tiny, and other forces that can perturb the motion
of the spacecraft had to be carefully accounted for, such as the force
of sunlight on the spacecraft's solar panels and drag from the Red Planet's thin upper atmosphere. It took two years of analysis and computer modeling
to remove the motion not caused by gravity.

"With this new map, we've been able to see gravity anomalies as small
as about 62 miles (100 kilometers) across, and we've determined the crustal thickness of Mars with a resolution of almost 75 miles (around 120 kilometers),"
said Genova. "The better resolution of the new map helps interpret how
the crust of the planet changed over Mars' history in many regions."

For example, an area of lower gravity between Acidalia Planitia and Tempe Terra was interpreted before as a system of buried channels that delivered water and sediments from Mars' southern highlands into the northern lowlands billions of years ago when the Martian climate was wetter than it is today. The new map reveals that this low gravity anomaly is definitely larger
and follows the boundary between the highlands and the lowlands. This
system of gravity troughs is unlikely to be only due to buried channels because in places the region is elevated above the surrounding plains.
The new gravity map shows that some of these features run perpendicular
to the local topography slope, against what would have been the natural downhill flow of water.

An alternative explanation is that this anomaly may be a consequence of
a flexure or bending of the lithosphere -- the strong, outermost layer
of the planet -- due to the formation of the Tharsis region. Tharsis is
a volcanic plateau on Mars thousands of miles across with the largest volcanoes in the solar system. As the Tharsis volcanoes grew, the surrounding lithosphere buckled under their immense weight.

The new gravity field also allowed the team to confirm indications from previous gravity solutions that Mars has a liquid outer core of molten
rock. The new gravity solution improved the measurement of the Martian
tides, which will be used by geophysicists to improve the model of Mars' interior.

Changes in Martian gravity over time have been previously measured using
the MGS and ODY missions to monitor the polar ice caps. For the first
time, the team used MRO data to continue monitoring their mass. The team
has determined that when one hemisphere experiences winter, approximately
3 to 4 trillion tons of carbon dioxide freezes out of the atmosphere onto
the northern and southern polar caps, respectively. This is about 12 to
16 percent of the mass of the entire Martian atmosphere. NASA's Viking missions first observed this massive seasonal precipitation of carbon
dioxide. The new observation confirms numerical predictions from the Mars Global Reference Atmospheric Model - 2010.

The research was funded by grants from NASA's Mars Reconnaissance Orbiter mission and NASA's Mars Data Analysis Program. NASA's Jet Propulsion Laboratory,
a division of the California Institute of Technology in Pasadena, manages
the Mars Reconnaissance Orbiter and Odyssey missions.

For images and video, refer to:

http://www.nasa.gov/feature/goddard/2016/mars-gravity-map


Media Contact

William Steigerwald / Nancy Neal Jones
Goddard Space Flight Center, Greenbelt, Md.
301-286-5017 / 301-286-0031
william.a.steigerwald@nasa.gov / nancy.n.jones@nasa.gov

Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.w.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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