X-rays are constantly emitted from the Sun. They travel though space and interact with planetary surfaces. When they hit the Moon they are absorbed into the very upper few micron of the surface regolith, and are then ejected back out again. The ejected, or fluoresced, particles will now carry with them the characteristics of the material that they interacted with.

C1XS detects this emitted signature and the data will provide information as to the chemistry of the lunar soil. During normal solar conditions, C1XS will be able to detect elemental Mg, Al and Si. During strong solar flare events it may be possible to detect other elements too, such as Ca, Fe and Ti.

The on-board solar monitor will monitor the Sun simultaneously, as the X-ray flux detected from the surface of the Moon is dependent upon the amount of solar X-ray radiation striking it. This will enable the scientists at RAL to calibrate the C1XS data and determine absolute abundances of the elements they detect.

Lunar origin

Early theories for the origin of the Moon focused on three main ideas: co-accretion of the Earth and Moon from the same dust and gas reservoir; capture of the wandering Moon by the Earth's gravitational field, and fission (the splitting) of the Moon from the Earth due to the high angular momentum of the system. A fourth theory became generally more accepted after the Apollo missions, and that is the giant impact theory. Early in the formation of the Earth, a large body approximately half the size of the Earth struck the Earth, depositing large amounts of the Earth's crust and mantle into orbit that eventually solidified to form the Moon. This theory explains many features of lunar geochemistry, including its oxygen isotope composition and depletion in volatiles, as well as explaining the orientation and evolution of the Moon's orbit.

To understand the origin of the Moon we need to understand its dramatic early thermal history. After the initial impact event, there is strong evidence for further heating causing a lunar magma ocean to have formed early in the history of the Moon. It is possible that the accretion of the Moon occurred quickly enough that it was still hot on formation. Other heating may have occurred later due to radioactive heating and gravitational resettling. A detailed knowledge of the thermal history of the Moon is needed and has important consequences for establishing the size of the lunar core and the geochemistry of the lunar surface.

The lunar highlands

The lunar magma ocean hypothesis accurately predicts the formation of the low density crust we see in the form of the bright highland material on the Moon. The highlands represent the ancient crust that developed from the magma ocean prior to mare flooding. Therefore, the highlands provide an important record of the events that have occurred throughout lunar history. Measurements of the crust on a global scale are therefore directly applicable to theories of early lunar evolution.

The lunar maria

Current thinking suggests that the Moon underwent differentiation during the magma ocean phase, in other words, it separated out. The lighter material formed the crust while the denser material sank, creating the regions that later sourced the volcanism that formed the mare. These are the darker areas on the surface of the Moon, which can predominantly be seen on the near-side (the side facing the Earth). Although the mare basalts probably only account for ~1% of the lunar crustal volume, their study is very important for the understanding of the lunar mantle and chemical history of the Moon.

Lunar impact basins

Impact craters and impact basins offer an insight into the compositional structure of the crust, acting as bore-holes through which we can examine the layers directly beneath the surface. Put simply, the larger the crater, the deeper into the crust you can see. The South Pole-Aitken basin is the largest known impact structure in the Solar System with a diameter of 2500 km and a physical depth of up to 12 km. It is possible that this basin has excavated right through the crust exposing mantle material below. Although this possibility is still being debated, there is no doubt that the South Pole-Aitken basin has penetrated deeper into the lunar crust than any other impact.

Science Aims of C1XS

The primary science goal of C1XS is to produce high resolution maps (~20-25 km) of the lunar surface in the three main rock forming elements: Mg, Al and Si and also in Ca, Ti and Fe during solar flare events. The Moon was investigated by the Apollo X-ray experiment, however, C1XS will provide much greater coverage of the lunar surface and will produce absolute elemental abundances.

Al-abundance and distribution are critical factors in models of thermal lunar evolution and may provide a constraint for models of the global melting event this is assumed to have occurred early in the Moon's history. Global mapping of the lunar Mg-number (Mg/[Mg+Fe]) is of great importance in furthering our understanding of the evolution of the Moon. Recent work suggests that certain types of rock exhibit both primitive and evolved chemical signatures, but within those rocks only the Mg-number shows evidence of a primitive source. There are various models that could produce this difference, and each model predicts specific relationships between the Mg-number and other rock types. A more detailed characterisation of the Mg-number across the Moon would help enormously in our study of these relationships.

C1XS will help to solve important scientific questions about regional lateral geochemical variations in mare basalts (the darker regions of the Moon) and in the ancient highlands (the more predominant white areas) in order to constrains models of lunar evolution. Data will also allow the comparison of the major element geochemistry of stratigraphically distinct lava flows within the same geographical region (e.g. Procellarum, Imbrium, etc) as a probe of mantle evolution

A high-level goal of C1XS is the geochemical study of large impact craters, like the South Pole-Aitken basin. C1XS will be able to characterise the terrain and probe the stratigraphy of lunar crust (eg. Mg/Fe ratio with depth) by obtaining measurements of central peaks and ejecta blankets of a range of impact crater sizes. It is hoped that by combining our data with those of other missions, we will be able determine whether mantle material has been exposed or not.

Measure major element geochemistry of presumed pyroclastic dark halo craters, also as a possible probe of mantle evolution

In combination with information to be obtained by the other instruments on Chandrayaan-1 and the data already provided by the SMART-1, Clementine and Lunar Prospector missions, C1XS data will provide new insights into some of the fundamental questions that remain regarding the origin and evolution of the Moon.