New gravity map provides best view yet inside Mars

Scientists have created the most detailed map of Mars' gravity to date using data from three NASA spacecraft, providing a glimpse into the hidden interior of the red planet.
"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," said lead author Antonio Genova of the Massachusetts Institute of Technology (MIT).
"Furthermore, the improved resolution of our gravity map will help us understand the still-mysterious formation of specific regions of the planet," Genova said.
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.

The team confirmed that Mars has a liquid outer core of molten rock by analysing tides in the Martian crust and mantle caused by the gravitational pull of the Sun and the two moons of Mars.
By observing how Mars' gravity changed over 11 years - the period of an entire cycle of solar activity - they inferred the massive amount of carbon dioxide that freezes out of the atmosphere onto a 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).
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.
"With this new map, we've been able to see gravity anomalies as small as about 100 kilometres across, and we've determined the crustal thickness of Mars with a resolution of around 120 kilometres," said Genova.

"The better resolution of the new map helps interpret how the crust of the planet changed over Mars' history in many regions," he said.
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.
The new map shows 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 study was published in the journal Icarus.