When NASA’s Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) lander touches down on Mars in November 2018, it will become the planet’s most advanced geophysical monitoring station. From its landing site on the plains of Elysium Planitia, the craft will attempt to probe the inner depths of the planet with its suite of instruments.
InSight’s goal is to reconstruct how rocky planets like Mars—and Earth—form. One of its most important objectives (to be performed with the Heat Flow and Physical Properties Probe developed at the German Aerospace Center) will be measuring how much heat rises from the planet’s mantle to the surface. This heat, produced by the decay of radiogenic elements, has been building and escaping to the surface since Mars was forged in the early solar system. Knowing the planet’s global average heat flux will help scientists determine the composition and structure of its interior and constrain different models of planet formation.
But to make a truly global measurement, InSight will need some help. It’s not a rover: It will remain stationary, and thus, its heat flux readings will be heavily biased if, for example, it happens to land atop an enormous mantle plume stretching out below the Elysium Mons volcano, roughly 1500 kilometers to the north. To generalize its findings to the rest of the planet, scientists must rely on computer models that simulate how heat flows up through the mantle and crust to the surface.
To that end, Plesa et al. have produced the most detailed simulations to date. They’re the first to use 3-D thermal evolution models with crustal thickness changes across the planet based on gravity and topographical data. These models are combined with an inference of residual radioactivity in the rock of the crust, which also emits heat that makes its way to the surface. Such residual radioactivity wouldn’t be unprecedented: Patches of radioactivity near the Apollo 15 landing site caused surface heat flux readings to be an estimated 2–4 times higher than elsewhere on the Moon.
The models’ results are good news for InSight. The simulations do produce mantle plumes, where the heat flux can be roughly twice as much as the average across the Martian surface and 3 times higher than the coldest points on the surface. However, these plumes are quite narrow. On the basis of their simulations, the authors believe that if a mantle plume exists under Elysium, it probably extends outward for only 800 kilometers, hundreds of kilometers away from InSight’s landing site. Even better, the heat flux at the landing site seems to be similar to the global average.
The study also suggests that variations in the level of radioactivity will not present huge errors in InSight’s readings. Measurements from previous Mars orbiters indicate that radioactive elements are distributed more evenly across Mars’s surface than on the Earth and Moon, and their lateral variations have a much smaller effect on the surface heat flux. Instead, the simulations show that variations in the amount of heat flowing to Mars’s surface seem to be mostly dependent on the thickness of the crust. (Journal of Geophysical Research: Planets, doi:10.1002/2016JE005126, 2016)
—Mark Zastrow, Freelance Writer