Mars is pockmarked by towering mountains such as Olympus Mons – three times the height of Mount Everest – and craters 9 km deep. The mass differences in the crust mean spacecraft experience tiny changes in gravitational tug as they orbit Mars. These wobbles are measurable and can be used to calculate the changing gravity and mass. For instance, they revealed that the mass of the CO2 icecaps varies seasonally by 4 trillion tons.
Scientists pooled measurements from three orbiters to build this gravity map of the planet in 2016. Low- gravity canyons like Valles Marineris in blue (centre) stand out from the high- gravity reds and whites of Tharsis Montes, the three aligned volcanoes left of centre and Olympus Mons, above and left. This ‘gravity’ map will help future craft chart their orbit with greater precision.