NASA has released a new gravity map for Mars stitched together from telemetry data collected by a trio of spacecraft over the course of 16 years orbiting the Red Planet. The map has already led to the confirmation that Mars hosts a molten liquid outer core, and insights relating to the titanic transfer of atmospheric material to the polar regions of the Red Planet during their winter cycle.
The experiment relied on the transmission of Doppler and range tracking data sent from the agency's Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter spacecraft. The telemetry was received by a series of radio antennas collectively known as the Deep Space Network.
As the spacecraft orbited Mars, the bumps and ravines that characterize the surface of the Red Planet worked to create tiny deviations in the trajectory of the spacecraft as they passed over the terrain.
By observing orbital deviations from the planet's center of mass, a team of scientists at NASA's Goddard Space Flight Center were able to stitch together a highly detailed Gravity map of Mars far exceeding the resolution of any that had come before.
In order for the orbital deviations caused by Mars' uneven gravity to be detected, interference from a host of other sources had to be filtered out. This included minute forces such as the pressure exerted on the spacecraft's solar panels by particles emanating from our Sun, and drag generated via the interaction between the shallow-orbiting probes and Mars' tenuous atmosphere. The process of removing the interference took two years of work, involving the use of sophisticated computer models.
Gravity map of mars from the perspective of the Red Planet's southern polar region – red and white regions indicate high gravity areas, while blue represents low gravity areas
A similar technique was recently employed by the team handling ESA's Rosetta spacecraft in order to map the interior of the comet 67P/Churyumov-Gerasimenko. However, the Martian gravity mapping endeavor represented somewhat larger of a challenge.
One of the major insights allowed by the gravity map is the confirmation that Mars hosts an outer core consisting of liquid molten rock. The idea that Mars hosts a molten liquid outer core is not a new one, however previous gravitational studies had lacked the ability to confirm the discovery.
The superior resolution of the new map, which is capable of detecting gravity anomalies only 100 km (62 miles) across, has allowed scientists to confirm the theory by analyzing tides present in the mantle and outer crust of the planet created by the gravitational force of our star, and Mars' two moons Phobos and Deimos.
The map is already proving useful in deciphering the geological properties of an unusual region of the Martian surface located between the northern Martian lowlands and the southern highlands that are known to exhibit a below-average gravity signature.
Previous studies led scientists to theorize that the gravity depressions owed their existence to a network of subsurface channels carved out by running water billions of years in the past.
Gravity map of the Tharsis region – red and white regions indicate high gravity areas, while blue represents low gravity areas
However, NASA's newly-released high-resolution gravity map detected a number of the channel-like features running along topographical slopes in the Martian terrain rather than down them. In other words, the features did not correlate with the natural downhill flow of water required to create such a channel, essentially debunking the theory.
Instead, scientists are working on the premise that the current low gravity region was created when the lithosphere of the planet flexed and buckled under the titanic weight of the emerging Tharsis volcanoes.
Not all of the insights provided by the map were limited to the planet's surface. NASA scientists were able to infer the quantity of carbon dioxide that freezes out of the atmosphere each winter by analyzing how Mars' gravitational signature has alters over the course of a full cycle of solar activity (11 years worth of observations).
It is estimated that as each hemisphere enters its winter cycle, an astonishing 3 – 4 trillion tons of carbon dioxide settles in the polar regions, accounting for around 12 – 16 percent of the Red Planet's tenuous atmosphere. Furthermore, a significant amount of this mass is transferred from pole to poll as the hemispheres alternate through their respective winter cycles
In terms of operational uses, the map will be instrumental in allowing for a more accurate insertion of future spacecraft into Mars orbit.