What does the MASCOT radiometer MARA measure?
A radiometer is a device for determining the radiation emitted by a surface. Its objective is to measure the temperature of the surface without touching it. When MASCOT separates from the Hayabusa2 probe on 3 October 2018, the MASCOT radiometer MARA will already be switched on, and will measure the surface temperature of the asteroid Ryugu during the descent, landing and rest periods. At first glance, measuring surface temperature sounds rather mundane and not particularly exciting. But what can the temperature tell us about the asteroid? In order to gain a better understanding, we have to consider the factors that determine the surface temperature on a body that lacks an atmosphere. Naturally, sunlight is an important factor – as are the properties of the surface material, known as the regolith. The key factor here is the speed at which the heat dissipates into the ground, a property known as thermal inertia.##markend##
The thermal properties of the surface can also be determined from measuring the surface temperature. These are, in turn, largely determined by the grain size and porosity of the surface material. As such, we can determine key surface properties using relatively simple means. The images taken by the MASCOT camera, which has the surface area being measured by the radiometer within its range of vision, also help to interpret the measurements.
If we are lucky and there are large boulders in MARA’s filed of view, we will be able to learn even more, and together with the properties of the loose material we will also be able to determine the thermal properties of the very boulders that served as the raw material for the loose regolith. During the Hayabusa1 mission, there was a discrepancy: the telescopically determined thermal properties of the asteroid Itokawa did not exactly match the observations made by the Hayabusa1 probe. The thermal readings and low thermal inertia measured suggested that the surface of Itokawa should have been predominantly covered by regolith, but the camera images from Hayabusa1 showed a surface largely covered in rocks. Until now, it has not been possible to determine whether these rocks were solid chunks or porous agglomerates. MARA’s measurements could be instrumental in solving this mystery, at least for Ryugu.
Comparing MARA's measurements with those taken by the TIR infrared instrument on board the Hayabusa2 probe may also bring new findings. Due to Hayabusa2’s stationary position at the subsolar point, TIR will only be able to observe Ryugu’s sun-facing side. MASCOT, on the other hand, will observe the landing site for an entire day-night cycle, so that the MARA data will partially complement that of TIR. As night-time temperatures are particularly indicative of the thermal properties of the regolith, the MARA data from this period will serve as a means of in-situ calibration for the TIR data. A joint interpretation of the data will ultimately enable an overall characterisation of the thermo-physical properties of Ryugu.