SMART-1 aimed to flight
test electric propulsion and other deep-space technologies for the first
time, while at the same time performing scientific observations of the
Moon. D-CIXS was a miniature X-ray Spectrometer that used new
technology to reduce the mass and volume of the instrument. Its main
innovation was a purpose-designed matrix of Swept Charge Device (SCD)
X-ray sensors mounted behind collimators and filters.
The SCDs
were designed by e2v specifically as an energy resolving X-ray detector
that was capable of working with only passive cooling. To do this, they
incorporated a number of novel design features to maximise the X-ray
energy determination and reduce the impact of radiation damage.
First light on D-CIXS
Credit: STFC (Barry Kellett)
An
SCD works in a similar fashion to that of a conventional X-ray CCD; the
charge packets generated from the absorbed X-rays are transferred to an
output amplifier by voltages applied to clock lines, the clocking moves
the charge diagonally across the device so that the active area can be
clocked continuously and read out by a single readout channel.
D-CIXS's
primary aim was to demonstrate that the SCD and collimation techniques
could be used to produce a small X-ray spectrometer that would operate
using passive cooling for the detectors. This was the first time that
any mission had flown SCD detectors into space. The science goal was to
detect X-rays from the lunar surface and to then establish the elemental
abundances of the main rock forming elements – in particular Al, Mg and
Si.
Although insufficient data was obtained to generate abundance maps the instrument did achieve the following ‘firsts’:
- First direct detection of Ca on the lunar surface
- First direct detection of Ti on the lunar surface
D-CIXS Science
The SMART-1 mission will carry three remote sensing instruments: an infrared spectrometer, an imaging camera and D-CIXS, an X-ray spectrometer. Together, these instruments will provide a comprehensive view of the Moon in infrared, optical and X-ray wavelengths. Due to SMART-1's highly elliptical orbit, higher spatial resolution images will be obtained in the southern hemisphere of the Moon. The polar orbit means that D-CIXS will be able to get a global map of the Moon in X-rays, its primary objective for the mission. The Moon has previously been observed in X-rays by the Apollo 15 and 16 missions, but coverage was limited due to the short duration of each mission.
Introduction
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.
D-CIXS detects this emitted signature and the data will provide information as to the chemistry of the lunar soil. During normal solar conditions, D-CIXS 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 dependant upon the amount of solar X-ray radiation striking it. This will enable the scientists at RAL to calibrate the D-CIXS data and determine absolute abundances of the elements they detect.
Lunar Prospector Thorium Map of the Lunar Surface
Credit: Los Alomos
Cruise Science
During the mission cruise phase SMART-1 will spend 18 months traveling to the Moon which is an ideal opportunity to use D-CIXS to look at astronomical targets. To data, D-CIXS has made several observations of the Crab nebular, different comets, the bright Sco-X 1 x-ray source, Velax-1 and the Earth itself to study their emitted X-ray signature.
These observations have not only been important scientifically but have allowed instrument scientists to calibrate the D-CIXS instrument by carrying out analysis of previously well studied sources.
Science Aims of D-CIXS
The primary lunar science goal of D-CIXS is to produce a global lunar map of the three main rock forming elements: Mg, Al and Si. The Moon was investigated by the Apollo X-ray experiment, however, D-CIXS 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.
The South Pole Aitkin Basin
Credit: ESA
D-CIXS 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.
A high-level goal of D-CIXS is the geochemical study of large impact craters, like the South Pole-Aitken basin. D-CIXS will be able to characterise the terrain and study vertical changes in the impact structure. 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.
In combination with information to be obtained by the other instruments on SMART-1 and the data already provided by the Clementine and Lunar Prospector missions, D-CIXS data will provide new insights into some of the fundamental questions that remain regarding the origin and evolution of the Moon.
Long Live C1XS!
On Sunday, September 3rd 2006 at 05:42:21.759 (approximately!), SMART-1 successfully crashed into the Moon, bringing to an end a very eventful and challenging mission for ESA and all the scientists and engineers involved in the project (including staff at RAL).
The crash site appears to be a small mountain about 1.5 km high on the plain of the Lake of Excellence, close to a crater called Lehmann C. Data from the Canada-France-Hawaii Telescope (CFHT) on Hawaii clearly shows that the impact generated a dust cloud visible for some 130 seconds after the impact flash. The dust debris travelled more than 80 km from the original impact site.
SMART-1 launched very early on the morning of September 27th, 2003. Although this event brought an end to the mission it was also in a very real sense the beginning of the next lunar mission that RAL Space are involved in. On the very day SMART-1 crashed, a team from RAL Space travelled to India for an important (and ultimately successful) review of the C1XS instrument. C1XS is the direct descendant of D-CIXS (C1XS is pronounced "kicks" and stands for the Chandrayaan-1 X-ray Spectrometer). Chandrayaan-1 will be the first lunar mission launched by India and indeed the first Indian satellite to include ESA and NASA instruments. It is due for launch in early 2008.
Barry J. Kellett, Brian Maddison & Chris Howe for the C1XS and D-CIXS team.
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