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InSight – A Geophysical Observatory for Mars

Prof Tilman Spohn  |  Institute of Planetary Research  |  German Aerospace Center (DLR)  |  Berlin


On Nov 26th, 2018, the NASA InSight mission landed on Mars at Homestead Hollow, Elysium Planum as the first geophysical observatory on another terrestrial planet. The payload includes a seismometer SEIS, the Heat Flow and Physical Properties Package HP3 whose primary goal is to measure Mars’ geothermal heat flow, a package to measure the variations of the planetary rotation axis, a magnetometer and atmospheric pressure and temperature sensors. HP3 – built and operated by the DLR - consists of a mechanical hammering device called the “Mole” for penetrating into the regolith, an instrumented tether which the Mole pulls into the ground, an infrared radiometer (RAD) mounted below the lander deck to determine the surface brightness temperature, and an electronics box. The tether is equipped with 14 platinum resistance temperature sensors to measure temperature differences with a 1-σ uncertainty of 6.5 mK. Depth is determined by a tether length measurement device that monitors the amount of tether extracted from the support structure and a tiltmeter that measures the angle of the Mole axis to the local gravity vector. The Mole includes temperature sensors and heaters to measure the regolith thermal conductivity to better than 3.5% (1-σ).

The surface heat flow is calculated by multiplying the geothermal gradient and the thermal conductivity of the regolith. It is expected to vary across the surface of Mars, but modeling suggests that at Elysium Planitia, the surface heat flow is close to the average value.

The Mole is planned to penetrate to a depth of at least 3 m but at most  to 5 m. The requirement of a minimum depth of 3 m will help to significantly reduce errors introduced by the annual surface temperature variation.

By the time of writing, SEIS has recorded about 15 events of which one has been classified as a magnitude 3 quake with clear P and S wave arrivals. The mole on HP3 is – unfortunately – still stuck at a depth of 30-35 cm. The radiometer RAD was successfully deployed and has measured the thermal inertia and observed three Phobos eclispses. This is the first time that a Phobos eclipse has been observed in the infrared in-situ on Mars. The atmosphere pressure and temperature sensors have observed a large number of vortices and dust devils. The magnetometer data suggest a local magnetization 10 times stronger than suggested by previous orbiter data.  

A “rescue operation” for the Mole has recently been started after careful examination of the situation including listening in on the hammering with the seismometer. Of the most likely possible causes for the Mole’s being stuck is insufficient friction on the hull. Loading the surface with the arm is a possible remedy and will be tried later this month.