The Moon in the Microwave: Shedding New Light on the Lunar Farside

A new lunar map was produced by subtracting the nocturnal microwave radiance data from two frequency channels with different penetrative potential. Chang'E-2 microwave radiometer (MRM) data show areas of anomalous high differences relating to both known and unknown young impact craters, most of which are located on the lunar farside. Thirty-eight Features Of Interest (FOI) were selected and investigated by comparison with soil maturity maps (OMAT) and infrared-derived data (H-Parameter), a parameter relating to thermal inertia. In some cases, the surface area of the microwave feature reaches distances of over 40 units of crater radii from the rim before abruptly dropping to background level within less than 10 km. The origin of such microwave variations remains unclear but is likely related to terrain roughness and fine ejecta materials which produce higher signal loss ("loss tangent") at the uppermost surface layer. The observed discrepancy from the other datasets is due to the higher penetrative potential of microwave radiation, thus potentially carrying additional information on buried materials. The microwave-derived FOIs appear to be transient within a geological timeframe and, in principle, a relationship between temperature differences at night in the two microwave channels, along with infrared data analysis, could be employed to derive a formation time of the crater. However, the low spatial resolution of the available microwave data precludes a reliable quantitative derivation at present. Plain Language Summary Microwave radiation carries information about the properties of the lunar surface potentially down to a few meters in depth. In this work, we have used data from two different channels on the Chang'E-2 Microwave RadioMeter to generate a map showing spots where temperatures are very different between the deeper and the upper surface. We found that most of these correspond to young impact craters, which also stand out from the background in the visible and infrared maps. However, these temperature anomalies in the microwave are not always a match given that the radiation can be representative of deeper materials. We conclude that microwave data with higher spatial resolution will one day be very useful in estimating the time of the impact event. This will help us work out the rate of impacts in our Earth/Moon system as a function of time, with implications for a better understanding of the evolution of life on Earth.