The first microwave image of the complete Moon

The first microwave image of the complete Moon was obtained thanks to the Chinese lunar satellite Chang'E-1. Global brightness temperature maps reveal radiation from the surface and deeper layers of the Moon and its diurnal variation. This will help astronomers to determine the detailed heat flow and, thus, the inner energy of the Moon. These exciting new results will be presented by Dr Yong-Chun Zheng and Dr Kwing L. Chan at the European Planetary Science Congress in Rome from Monday 20th to Wednesday 22d September.

Chang'E-1 (CE-1) is China's first scientific mission to explore planetary bodies beyond Earth. The stereo camera, one of the eight science instruments on board the spacecraft, has produced a state-of the-art global image of the Moon with unprecedented image quality and positioning precision. The Solar Wind Ion Detector (SWID) has discovered the acceleration of scattered solar wind protons close to the lunar polar terminator. And now, the Lunar Microwave Radiometer (MRM) made it possible, for the first time, to globally map the Moon in microwave frequencies.

Astronomers know that active radar observations of the Moon cannot provide thermal information, only passive observations in the infra-red and microwave regimes can achieve that. Furthermore, only microwave detectors can sense emission from below the lunar surface (down to tens of metres). Ground-based microwave observations are not the best choice in order to do this, because they cannot 'see' the far side of the moon neither can obtain accurate brightness temperature near the limb.

Before CE-1, there was no passive, multi-channel, microwave remote sensing of the Moon from a satellite. CE-1 had a polar orbit and, thus, was able to observe essentially every location of the moon with a nadir view. Thanks to the long lifespan of CE-1 (494-days), the MRM obtained brightness temperature data that cover the Moon globally eight times, during both lunar daytime and nighttime periods. This global, diurnal coverage provides extremely valuable data for studying the lunar regolith ('dust' and impact debris covering almost the entire Moon surface).

CE-1 was observing from an altitude of 200 km from the lunar surface, providing spatial resolution orders of magnitude better than any ground-based microwave observation can ever achieve on Earth. Indeed, the sensitivity (0.5K) and dynamical range (20-500K) of the observable brightness temperature obtained by CE-1 is unsurpassed. 'No future mission, from any country, has been planned with a comparable program in microwave measurement,' says Dr Zheng of the Chinese Academy of Sciences.

The CE-1 microwave observations have made several important breakthroughs. MRM passively measured microwave emission in four frequency channels: 3, 7.8, 19.35, and 37 GHz. The higher frequency emission comes from a layer just a little below the surface (a few centimetres), whereas the lower frequency emission can probe depths beyond a few metres. 'With such penetrative ability, the microwave data can be used to infer thermo-physical properties of the lunar regolith, as well as, to find out about the variation of regolith thickness across the lunar surface,' says Dr Chang from Hong Kong University Sci and Tech. Such information is useful for estimating the distribution and amount of helium 3, a promising nuclear fuel for in situ fusion energy production in the future human settlements on the Moon. (Helium 3 originated from the sun and is believed to have been implanted in the lunar regolith by the solar wind).

Using the MRM data, Dr Zheng and his team have constructed global brightness temperature maps of the Moon for different frequencies, and separately for day and night times. The results are particularly revealing. On the 37 GHz daytime map, the maria, which appear dark in visible light, become bright in microwave wavelengths to reflect the higher temperatures (due to stronger absorption in the solar visible spectrum). Geological features like craters and mountains are clearly visible, but the prominent bright areas correlate mainly with the surface abundance of titanium. The correspondent nighttime microwave image is even more striking: The nighttime moon appears dotted by dark (cool) areas that turn out to be associated with hot areas during lunar eclipses. 'This enigma will keep the theorists busy for a while!' says, Dr Zheng.

A sister orbital probe to CE-1, Chang'E-2, is scheduled to be launched in October 2010.