**Ambient Pressure X-ray Photoemission Spectrometer for Chemical
Analysis of Planetary Surfaces**

P. Grunthaner^1^, C. Bryson^2^, M. Kelly^2^, F. Grunthaner^1^, V.
White^1^,

X. Amashukelli^1^, R. Quinn^3^

^1^Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, CA

^2^Apparati, Inc., Hollister, CA

^3^NASA Ames Research Center, Moffett Field, CA

We present a design and initial test results for a low-mass, low-power
ambient pressure X-ray photoemission spectrometer (XPS) for the
quantitative analysis of the composition and chemical state of elements
on planetary surfaces. This instrument targets high priority science
objectives for Mars such as understanding geochemical processes
especially with respect to the role of water, past habitability,
oxidants, organic degradation, and dynamic atmosphere/mineral
interactions. It is also suitable for other environments including
Europa and small bodies.

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  ![](media/image1.jpeg){width="2.5458333333333334in"   ![](media/image2.jpeg){width="2.58125in"
  height="2.7527777777777778in"}                        height="2.7875in"}

  ChemStat will probe the near-surface region of a      
  sample, including the atmospheric species interacting 
  with the surface. It will provide elemental analysis  
  and chemical state information for all elements       
  except H. The photo on the right-hand side shows the  
  electron energy analyzer breadboard. A diagram of the 
  complete instrument concept is shown on the left.     
  ----------------------------------------------------- ------------------------------------------

A novel approach to the electron energy analyzer and sampling
requirements are enabling for reducing the resource requirements of this
analytical technique. Commercial XPS systems use dispersive electron
energy analyzers, which are not amenable to miniaturization because
sensitivity and energy resolution depend upon the size of the concentric
hemispheres typically used. In contrast, the new electron energy
analyzer discussed here does not spatially disperse electrons according
to their energy, but instead uses high-pass and low-pass filters to
illuminate the detector with a narrow range of electron energies. This
approach is amenable to miniaturization without sacrificing performance
and was developed to the breadboard stage by Apparati, Inc. via the NASA
SBIR program.

The second enabling component is a dual-layer silicon-nitride structure
that isolates the spectrometer from the planet atmosphere. This
isolation membrane allows the sample to remain outside of the
spectrometer in the ambient environment of the planet, eliminating the
needs for sample preparation, mechanisms to transport the sample into
the spectrometer vacuum chamber, and the associated peripheral vacuum
pumps. Because the sample remains in its ambient environment, scientists
will be able to probe, in real time, dynamical changes on the surface of
minerals and regolith as a result of atmosphere/solid surface
interactions. This may provide insight into such phenomenon as
weathering processes, oxidant formation, and the photolytic or chemical
degradation of organic compounds.

The key strengths of XPS lie in the return of detailed chemical bonding
information, quantitative elemental analysis, and sensitivity to the
first 5 to 10 nm of the surface of the sample. Although the basic
principles of XPS are different from Mössbauer, both return information
on elemental oxidation states. However, while Mössbauer is primarily
suited to analysis of iron bearing materials, XPS can provide direct
chemical information on all elements, except H, in rocks and soils. In
addition to quantifying the distribution of oxidation states in a
sample, XPS provides direct information on the chemical bonding,
allowing the identification of chemical com­pounds and quantification of
distribu­tions.

This ambient pressure XPS has been dubbed *ChemStat*, a contraction in
reference to the chemical state information this technique will provide.
The table below compares the capability and resource requirements for
our *ChemStat* development with current commercial state-of-the-art XPS
systems.

+--------------------------+--------------+-------------+-------------+
| **Performance            | **Commercial | *           | *           |
| Characteristic**         | Laboratory   | *ChemStat** | *ChemStat** |
|                          | XPS**        |             |             |
|                          |              | **in        | **flight    |
|                          |              | de          | i           |
|                          |              | velopment** | nstrument** |
+--------------------------+--------------+-------------+-------------+
| **Mass (kg)**            | 1500         | 14          | 7           |
+--------------------------+--------------+-------------+-------------+
| **Power (W)**            | 1500         | 15          | 10          |
+--------------------------+--------------+-------------+-------------+
| **Operating Pressure at  | \< 10^-6^    | \<10^-6^ to | \<10^-6^ to |
| sample (Torr)**          |              | 3x10^3^     | 3x10^3^     |
+--------------------------+--------------+-------------+-------------+
| **Spectral Resolution    | 0.8          | 0.8         | 0.8         |
| (eV)**                   |              |             |             |
+--------------------------+--------------+-------------+-------------+
| **Spatial Resolution     | 100          | 100         | 100         |
| (µm)**                   |              |             |             |
+--------------------------+--------------+-------------+-------------+
| **Sample preparation     | yes          | no          | no          |
| required?**              |              |             |             |
+--------------------------+--------------+-------------+-------------+

> Part of this work was carried out at the Jet Propulsion Laboratory,
> California Institute of Technology, under a contract with the National
> Aeronautics and Space Administration and at Apparati, Inc., under a
> NASA Small Business Innovative Research contract.