SPECIAL SECTION: CHANDRAYAAN-1 CURRENT SCIENCE, VOL. 96, NO. 4, 25 FEBRUARY 2009 506 *For correspondence. (e-mail: mall@mps.mpg.de) Near Infrared Spectrometer SIR-2 on Chandrayaan-1 Urs Mall*, Marek Banaszkiewicz, Kjell Brønstad, Susan McKenna-Lawlor, Andreas Nathues, Finn Søraas, Esa Vilenius and Kjetil Ullaland Max-Planck-Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany Chandrayaan-1, the first Indian mission to the Moon, will provide an opportunity for in situ lunar observa- tions over a two-year period from a 100 km polar orbit. A comprehensive suite of onboard instruments will include the SIR-2 near-infrared grating spectrometer. SIR-2, a pointing spectrometer, will observe the Moon in the spectral range 900–2400 nm, with a unique spectral resolution of 6 nm over a wide range of phase angles. The high resolution SIR-2 observations, parti- cularly of the lunar far side and polar region, are expec- ted to have a large impact on our understanding of the mineralogy and composition of the Moon. Keywords: Chandrayaan, chemical composition, min- eralogy, Moon, near-infrared, spectroscopy. Introduction THE peculiar way in which the Moon’s surface reflects light incident from the Sun attracted the attention of early astronomers that led to frequent observations of lunar brightness. However, the first systematic attempts to measure the brightness variation over a lunation and also to record the brightness changes in selected lunar areas during a lunar day are generally attributed to W. F. Wisli- cenus. His measurements, completed in 1895 and pub- lished posthumously 1 , are notable because of the large number of observations which were uniformly distributed over all lunar phases and included measurements at large phase angles. Once basic photometric observational facts were established, the question as to how one could restrict the number of types of materials from which the lunar surface is made became an issue. This question led in a natural way to fundamental research, which aimed at determining the factors that govern the optical scattering characteristics of complex surfaces. Early, extensive, photometric studies to achieve this goal were published by Hapke and van Horn 2 . Through this type of research, multispectral imaging of the lunar surface to infer its composition through remote sensing was enabled. Despite the enormous progress made through ground-based imag- ing of the Moon, it was necessary to await the data from six American and three Russian sample return missions in order to arrive at the, much awaited, lunar ground truth. While samples were systematically collected on the Moon, the question remained as to whether, sourced as they were from a relatively narrow belt at mid-latitudes, the Apollo and Luna specimens represented all major lunar geological units. Nonetheless, the accessibility of these samples made it possible to generate experimentally measured which was usable as ground truth to support remote sensing observations. The availability of laboratory spectral data cleared the way for space-based observa- tions and investigations of the lunar mineralogical terrains on the far side of the Moon. Although Earth-based multi- spectral observations have been very successful with regard to observations of the lunar near side, one has had to wait for the lunar flybys of the Galileo mission in 1992 to obtain a glimpse of what lunar, multi-spectral, remote sensing information can deliver. Science objective Determinations of the chemical composition of the crust and mantle of a planet constitute the foremost important goals in planetary research. The terrestrial planets and the Moon are made up of rocks, which consist of minerals. These minerals provide a key to understanding the lunar rocks because their compositions and atomic structures reflect the physical and chemical conditions under which the rocks were formed. Thus lunar rocks provide informa- tion about the Moon’s origin, the evolution of its crust, and the timing of critical local events such as volcanism and meteorite bombardment. Unfortunately, direct physi- cal access to the Moon is presently limited to its crust, which constitutes only about 10% of its overall volume 3 . Because the Moon is not a uniform, homogeneous body, it consists of different groups of rocks formed in different ways at different times. Further, the rocks within particu- lar groups are not necessarily identical and differ in their mineral composition, shapes and sizes, as well as in their chemical composition. The best information about the composition of the lunar crust is undoubtedly gained from returned samples. Unfortunately, these samples cannot be taken from the bedrock. The uppermost few meters of the lunar crust consists of a layer of loose, highly porous,