THE CHEMICAL COMPOSITION OF AN ASTEROID SURFACE BY THE ALPHA X-RAY SPECTROMETER ON THE MUSES-C MISSION LANDER.

T. E. Economou, Laboratory for Astrophysics and Space Research

University of Chicago, 933 East 56th Street, Chicago, IL60637

tecon@tecon.uchicago.edu

Introduction: Probably one of the most important tasks on any asteroid mission is to determine the de- tailed chemical composition of the asteroid. The Japanese/NASA MUSES-C mission will undertake this task as its prime goal. While the main objective of the mission is to return a small amount of sample material from ML 1989 asteroid back to Earth, it will also conduct many other in situ investigations on the asteroid [1]. MUSES-C will be launched in July 2002 and it will return the asteroid samples in 2006. During the rendezvous period the orbiter will perform many in-vestigations from orbit to globally characterize the asteroid and then will drop a small rover (“nano-rover”) to perform in situ characterization of the as-teroid surface. On MUSES-C, the goal to obtain the complete elemental composition of the surface of the asteroid will be achieved by the Alpha X-ray Spectrometer (AXS).

The Alpha X-ray Spectrometer. The AXS is a dwarfish derivative of the APXS used on the Path-finder Mars mission in 1997. The principles of the AXS techniques are the same as for the Pathfinder’s APXS and they were described in detail elsewhere [2-4]. The AXS was reduced substantially in size from that of the Pathfinder APXS in order to fit into a 40x65x30-mm volume inside the nanorover box. The reduction was achieved mainly by reducing the sensor head dimensions, hybridizing the entire electronics, sharing some of the nanorover resources and eliminat-ing the proton mode. The proton mode provides re-dundant data to the alpha and the X-ray mode and therefore, the science loss will be minimal and will not affect the performance of the AXS. Although the AXS is now a very small instrument, it continues to provide unique capabilities. It will provide a detailed and accurate elemental composition of all elements (except hydrogen and helium) of the asteroid surface material present above a few tenths of atomic %. The alpha mode will still determine the abundance of the very light elements C, N, O that play an important role in organic matter and which cannot be deter-mined by the X-ray mode.

The nanorover will provide the AXS with mobility around the asteroid surface, sample selection, and deployment of the AXS, in the same manner the So-journer did on the Pathfinder mission. Since there are no batteries on the nanorover, the AXS will be operat-ing only during the day from the power

generated by the rover solar panels. It is expected that multiple samples will be analyzed during a 4-month period of operation on the asteroid surface. The in situ investigations by the AXS and other instruments on the rover (an APS camera and an IR spectrometer) will help to characterize the asteroid and also will help to establish any connection between the small bodies in our solar system and the extensive collection of the meteorites available in many laboratories around the world

Presently, we have assembled a prototype instru-ment (see Figure 1) that complies with all of the S/C and mission requirements. It is now being extensively tested under the extreme environmental conditions expected to be encountered on the asteroid surface. It is designed to operate reliably in the temperature range 100˚K–300˚K.


Fig. 1: Photograph of the AXS Prototype for MUSES-C Asteroid Mission

Table 1: AXS Characteristics for MUSES-C Mission:

Weight: 95 grams
Volume: 65 cm3
Power : <200 mW
Voltages: ±7.5 V DC
Radioactive Sources: 20 mCi of Cm-244
Ea: 5.8 MeV
T½: 18.1 years
Accumulation Time: 0.5-3 hours/ sample
Data Requirements : 10 kb / spectrum

    Laboratory work with AXS. The prototype instrument was also used in the laboratory to evaluate the performance of the AXS and to collect many alpha and X-ray spectra in vacuum. Responses from each element in alpha and X-ray mode were established and evaluated. Many terrestrial rocks and meteorites were measured and analyzed. Figure 2 shows the alpha and X-ray spectra from meteorite Murchison in vacuum. In the alpha spectrum, C, O, Al, Mg, Si and Fe are clearly discernable, while in the X-ray spec-trum many more major elements can be seen. Peaks from 0.2% Ti and 0.3% Cr are clearly visible.



    Fig. 2: Alpha and X-ray spectra from Murchison

    Meteorite obtained from the AXS prototype for MUSES-C mission.

    The AXS uses a monoenergetic beam of alpha particles from a radioactive source (Cm244). This type of excitation provides the best signal to background ratio and therefore the lowest detectability limit for any element. It is similar to the laboratory PIXE technique that uses charged particle accelerators for the excitation beam.

    Although the AXS does not quite have the sensitivity of the PIXE instruments, when it is com-pared to other types of instruments using electron or X-ray excitation, it has the best signal to noise ratio. That enables the AXS to detect many trace elements down to few hundreds of ppm level. As an example, Figure 3 shows the ability of the AXS to detect the element Zr. With enough statistics even 100 ppm of Zr in a light element matrix can be detected. Other trace elements, such as Y, Sr, Rb, Br, Pb, as it can be seen in Figure3 of a syenite rock, can be detected as well, although there is some interference from the scattered X-rays for some of them.



    Fig. 3: Zr detection limit from the AXS prototype

    The AXS for the MUSES-C mission is a more powerful tool than APXS was on the Pathfinder. Al-though the AXS is using about half of the Cm source intensity, it is 5 times more efficient in the X-ray mode and 30% more efficient in the alpha mode than the APXS. That means that now a sample can be ana-lyzed with the AXS in much shorter time than with the APXS.

    References: [1] Kawaguchi, J. et al (1999) IAF-99-IAA.11.2 02. [2] Turkevich, A., (1961) Science, Vol. 134, 672-674. [3] Economou, T. E., A. Turke-vich, (1976) Nucl. Instr. & Meth.134, 391-400., [4] Rieder, R., H. Wänke, T. Economou and A. Turke-vich. (1996) JGR, 102, 4027-4044.