 AMBIENT-PRESSURE X-RAY PHOTOEMISSION SPECTROMETER FOR CHEMICAL ANALYSIS OF PLANETARY SURFACES. P. J. Grunthaner1, C. Bryson2, L. DeFlores1, D. Gill2, F. J. Grunthaner1, M. Kelly3, R. Quinn4, C. L. Taylor4, V. White1, 1Jet Propulsion Laboratory, California Institute of Technology, MS 302-205, Pasadena, CA 91109 (paula.grunthaner@jpl.nasa.gov), 2Apparati, Inc. 221 Carpenter Dr., Hollister, CA 95023, 3Stanford University, 4SETI Institute, NASA Ames Research Center, MS 239-4, Moffet Field, CA 94035  Introduction:  We present an update to our development of a low mass, low-power ambient-pressure Xray photoemission spectrometer (XPS) for the quantitative analysis of the composition and chemical state of planetary surfaces. This instrument targets highpriority science objectives for Mars mineralogy, soil chemistry, astrobiology, and atmospheric chemistry.  It is especially well suited for investigating geochemical processes involving the role of water, past/present habitability, oxidants, organic degradation, and uniquely probes interactions between the soil/rock and ambient gases.  It is also suitable for other environments including Europa and small bodies. XPS provides both quantitative elemental and chemical state information through the energy analysis of photoelectrons emitted from a sample that has been irradiated with soft X-rays. X-rays, typically 1.5 keV Al Ka, excite the core level electrons of the constituent elements and produce photoelectrons with kinetic energies diagnostic of the core electron binding energy. The core level (e.g. 1s, 2p, 3d, 4f) binding energies are sufficiently unique for all elements for complete and unambiguous elemental analysis, as illustrated in the XPS survey in Fig. 1 (left).  The exact binding energy of a core level electron is dependent upon the chemical state of the element. The typical range of chemical shifts for a given element is 1-10 eV, and, given peak half-widths of ~1 eV, reasonably complex mixtures of chemical species can be reliably resolved.  For example, the 1s core electrons of oxidized carbon are shifted by many eV relative to those of reduced carbon species.  Thus, one can detect and distinguish reduced species such as hydrocarbons from increasingly oxidized species such as carboxylates and carbonates, as illustrated in the C 1s spectrum in Fig. 1 (right).  There is undoubtedly a large number of complex, photochemically driven oxidative processes on Mars involving interrelated atmospheric, aerosol, dust, soil, and organic chemical interactions.  Fig 2 illustrates another relevant example of how XPS can be used to quantitatively detect the full range of the chemical states of Cl from chloride (oxidation state Cl -1) through perchlorate (oxidation state Cl +7). Overall, XPS is well suited for identifying and distinguishing the various possible chemical states of elements of importance to understanding the Martian outcrops and chemical sediments such as O, N, P, Mg, Ca, Cr, S, Fe, Cl, Br, and C. Surface Sensitivity:  Photoelectrons are readily absorbed or scattered and lose their energy signature after passing through 5 to 10 nm of most materials. Thus, the technique is sensitive to only the first 5- 10 nm of the surface.  This surface sensitivity is an advantage for investigating Mars soil/rock chemistry related to weathering, oxidant formation, organic degradation, and other surface phenomena. Well-established tools such as the MER rock abrasion tool (RAT) can be used  Fig. 1: XPS survey of a rock collected from Lake Alchichica in Mexico (left).  Quantitative elemental analysis is routinely accomplished by XPS.  High-resolution C 1s core level data for this same rock (right) elucidates the chemical state of this element.  The XPS data was obtained on a surface of the rock with an exposed endolithic microbe community using our laboratory XPS. to remove rock surface layers to allow examination of the rock chemistry as a function of depth.  Similarly, the Viking and Phoenix approach of using a robotic arm with a scoop, rasp, or other end effector can be used to provide soil samples as a function of depth.  Ambient-Pressure XPS: For electrons of several hundred eV, typical of the photoelectrons analyzed here, the mean free path is in the millimeter range at millibar pressures. Thus, for application on Mars, we have elected to keep the sample in the Martian ambient to exploit the low-pressure conditions by inserting a silicon-nitride gas-isolation membrane that passes Xrays and photoelectrons while protecting the high vacuum environment of the spectrometer.  Fig. 3 illustrates the concept. The gas-phase molecules between the sample and the analyzer entrance will also be detected by ambientpressure XPS. Gas-phase molecules can be distinguished from those absorbed on the surface because several eV usually separates their binding energies. [1]  Important for Mars, CO2 (gas) is very well separated in binding energy from chemisorped CO2 and carbonates, so we expect to be able to study martian carbonates in the presence of a CO2 atmosphere. [2] Key Technologies: We utilize a novel analyzer that uses high-pass and low-pass filters to illuminate the detector with a narrow range of electron energies. [3] This approach is amendable to miniaturization and has high electron throughput, which has the added advantage of reducing the need for a high-power X-ray source.  This axially symmetric analyzer was initially developed in NASA SBIR Phase I and II programs by Apparati, Inc., one of the Co-Is of the effort proposed here (CB).  The design was selected because of its simplicity, ruggedness, and performance potential. The 2nd key technology is a duallayer gas isolation membrane that transmits both the incident X-rays to illuminate the sample as well as the photoemitted electrons. [4] Development Update:  We will describe the development progress to date for the integrated system (Fig 4). As of this writing, the detector noise characteristics are excellent, being essentially at the expected background radiation levels of 2-3 cts/sec with no signal present.  Characterization of the full system noise and resolution is in progress and will be presented.  Spectra obtained on a sample at Mars pressure are anticipated. Acknowledgements:  Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, and at the SETI Institute under NASA PIDDP funding; and at Apparati, Inc., under a NASA PIDDP contract and a Small Business Innovative Research Contract. References:  [1] Salmeron, M. (2008) Surface Science Reports, 63, 169-199; [2] Deng et al. (2008) Langmuir, 24, 94749478; [3] Bryson C. et al. (2006), SBIR report for Contract NNC04ACA20C, 28 Feb 2006; [4] Bryson C. et al. (2004) US Patent No. 6,803,570 B1  Fig. 2:  XPS spectra of the Cl 2p3/2-2p1/2 doublet showing good separation of Cl species in different oxidation states (ClO4- (perchlorate, Cl+7), ClO3- (chlorate, Cl+5), hypochlorite (ClO-, Cl+1), Cl- (chloride, Cl-1)).  Data taken on laboratory XPS used in this work.  Fig 3: Ambient XPS probes near surface region, including atmospheric species interacting with the surface.    Fig. 4: Ambient-pressure XPS with subsystems integrated. A future flight system is estimated to be 6 kg, 10 W. 
