The Mercury Gravity Field after the MESSENGER Low-Altitude Gravity Campaign

Abstract:

NASA’s MESSENGER spacecraft has collected more than 3.5 years of X-band radio tracking data in orbit around the planet Mercury. During its one-year primary mission, which started in March 2011, MESSENGER was in an eccentric, near-polar orbit of 12-hour period, and the periapsis altitude was actively maintained between 200 and 500 km. For its extended mission, the orbit period was reduced to 8 hours. As the orbit naturally evolved, in large part due to the third-body gravitational perturbation of the Sun, the periapsis altitude reached a maximum of ~450 km in March 2013 and then began to decrease.

An ambitious end of mission was designed to use the remaining fuel to delay impact and to observe the northern hemisphere for nearly a year at periapsis altitudes lower than 200 km, including four intervals of exceptionally low altitude (25–100 km). Periapsis passages are visible from Earth only for two of these intervals, in August and October 2014. These new data, the lowest-altitude radio tracking measurements to be acquired by MESSENGER, prompt an updated solution for the gravity field of Mercury.

In preparation for acquisition of the low-altitude (<100 km) data, we have reprocessed tracking data through 14 July 2014. These data already provide good coverage below 200 km over most longitudes. A preliminary gravity solution to degree and order 50 shows stronger gravity anomalies near the periapsis latitudes than in the most recent global solution, HgM005. To best capture the shorter-wavelength signals expected from the lowest-altitude passes, we are estimating a large number of local surface anomalies (arranged on a 1°x1° grid) in addition to a harmonic field.

We are also using the resulting gravity anomalies to update crustal thickness models and to explore the implications for gravity anomalies over basins and topographic rises and the modes of compensation of these features.