CHANG E RESULTS

The scientific results of the Chang e mission were substantial. Data return was 1.4 ТВ of raw data, transformed into 4 ТВ of science data, but, more importantly, they enabled China to start building its own repository of scientific knowledge.

The first objective was for China to compile its own Moon map, essential for its subsequent rover and sample return missions. The map combined both the imaging system and the altimeter, whose purpose was to give a precise topography. To do this, the imaging system made 589 tracks of data (or complete photographic passes), matched against 9.16m altimeter measurements.

The definitive map from the Chang e mission was published in December 2009 in Science in China and the Chinese Science Bulletin, authored by Jinsong Ping, Qian Huang, Chao Chen, and Qing Lieng. This made China the third country to publish its own Moon maps, after the Soviet Union and the United States, with the bonus of three-dimensionality. It was a 1:2,500,000 map with contours at every 500 m. It provided a full topography with elevation – moreover one that revealed fresh details of the Moon, such as a possible fault structure along the Apennines Uke one of the Earth’s tectonic plates. Analysts put forward the idea that the Apennine chain was a fault line similar to the Himalayas on the Earth [5]. Five tectonic maps were also in preparation for later publication. A bilingual Chinese-English atlas was published in 2010.

The map refined existing maps and found new features. A new impact basin was identified – the 470-km-wide Guanghangong basin – and a new crater, 190 km across – Wugang. There was a new volcano, Yutu, 2 km tall and 300 km across in

the Ocean of Storms, with another volcano nearby – Guisho. New craters were named after leading Chinese scientists: Cai Lun, after the first-century вс inventor of paper; Bi Sheng, after the eleventh-century inventor of movable type; and astronomer Zhang Yuzhe (1902-86). Following representations from China, the International Astronautical Union approved the naming of 14 features on the Moon after Chinese scientists – a small but growing Chinese proportion of the 1993 named features of the Moon [6].

The use of the altimeter enabled a much-improved knowledge of lunar topography, with an accuracy of at least 31 m. It determined that the Moon was

more spherical than the Earth by a factor of 1/963.7256 compared to the Earth’s 1/298.257. The radius of the Moon was re-measured as 1,737.013 km. Mare Ignii was identified as the biggest mass concentration (mascon). Profiles were published of individual features, such as Mare Moscoviense. Several years later, a critical account of the imaging system praised its definition (“better than the American Clementine”), but criticized its handling of bright light levels [7].

The gamma-ray spectrometer enabled Chinese scientists to make a chemical map of the Moon. The gamma-ray spectrometer scanned the Moon every three seconds between 27th November 2007 and 25th July 2008 and sent over 2.4m spectra to Miyun and Kunming ground stations. A global map of uranium, iron, titanium, and KREEP abundance was published. Chang e provided the most detailed iron and titanium maps since the American Clementine probe. Such maps were especially important in reconstructing the history of the Moon: iron was generally found on the top of lava flows and its presence indicated the order in which the lunar seas were flooded. The imaging interferometer compiled an iron and titanium abundance map of 84% of the Moon between 70°N and 70°S and a cross-section of the Mare Crisium was published. Further analysis showed the distribution of orthopyroxines, clinopyroxenes, olivine, pigeonite, and plagioclase across highland and mare areas, with more detailed studies made of Copernicus, Zucchius, Mare Orientale, Ariastarchus, Tsiolkovsky, and Tycho. A new model of these processes was put forward by Lu Yangxiaoyi, showing how melts of basaltic lava reached the mare from deep in the lunar interior in three periods of lunar history over 2bn—4bn years ago [8].

The amount of Helium 3 was recalculated downward from 5bn tonnes to lbn tonnes: 658,000 tonnes on the near side and 286,000 tonnes on the far side. The presence of Helium 3 was closely connected to levels of solar wind, the age of the lunar surface, and the presence of titanium. Chinese scientists then turned to the long-standing problem of whether there might still be water ice on the Moon. Chang e’s four-channel microwave radiometer first made a temperature map of the south polar region. Then it focused on Cabeus crater, where the American LCROSS mission had already impacted and offered comparative data. It found that the temperature in the bottom and permanently shaded part of Cabeus was 70 К and suggested a water ice content there of 2.8% [9].

Iron and titanium displayed on the lunar globe by Chang e. Courtesy: Wu Yunzhao.

The microwave sounder led to a detailed knowledge of the regolith, published by the Lunar and Planetary Research Centre, in “Methods and Advances on Lunar Soil Thickness” (Acta Minneralogica Sinica, 27(1) (2007)). Based on the temperature measurements of the microwave sounder, an algorithm was devised that made it possible to calculate the depth of the regolith all over, dividing it into a dust layer, regolith layer, and bedrock. The regolith was found to be much thinner than previous assumptions, but thicker on the far side:

Uranium of the Moon by Chang e. Courtesy: COSPAR China.

• mare regolith ranged from 1.2 m to 11 m, the average being 4.5 m;

• it was thinnest in the Mare Imbrium, thickest in the Sea of Fertility and Mare Nectaris;

• highland thickness was 1-15 m, averaging 7.6 m;

• far-side thickness was thicker, at more than 8 m.

Using the same instrument, a temperature brightness map of the Moon was published, the equatorial regions showing up as bright red, with greens appearing the farther away one went and, at the poles themselves, the blues of shaded craters. The temperature brightness was higher in mare than in uplands, higher at the equator than at the poles, and higher on the near side than on the far side [10].

Chinese scientists compared their results with earlier American results (notably Clementine and Apollo), Russian findings (Luna), and their contemporaries (Chandrayan of India and Kaguya of Japan), especially to iron out differences between them due either to the calibration of instruments or interpretation of data. They specifically compared results to data gathered at Apollo landing sites, Apollo 16 being the benchmark. They later corrected their altimeter findings when a discrepancy of 145 m was identified. They also re-photographed the landing sites for Apollo 12, 14, and 15 [11]. Finally, the solar wind detector found few disturbances and low temperatures in the solar wind, except when the Moon passed through the Earth’s magnetotail.

Chang e achieved its four objectives of making a map, analyzing the chemistry and thickness of the lunar surface and characterizing the lunar environment, as well as overcoming the technically demanding trajectory used to reach the Moon in the first place. Chinese scientists should have been more than satisfied with the results and now had important data and analysis to share internationally.