A miniature spectrometer is in the process of development for a future Mars mission, to measure the UV-VIS spectrum encountered at the martian surface. With an intended mass of ~100 g, the spectrometer is planned as part of the ESA ExoMars mission

Davies, A.

Husbands, A.

Leese, M. R.

Patel, M. R.

Towner, M. C.

Zarnecki, J. C.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsA miniature UV-VIS spectrometer for the surface ofMarsConference or Workshop ItemHow to cite:Patel, M. R.; Towner, M. C.; Zarnecki, J. C.; Leese, M. R.; Davies, A. and Husbands, A. (2004). A miniature UV-VISspectrometer for the surface of Mars. In: 35th Lunar and Planetary Science Conference, 15-19 Mar 2004, Houston,Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1247.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukA MINIATURE UV-VIS SPECTROMETER FOR THE SURFACE OF MARS .  M.R. Patel1, M.C. Towner1, J.C. Zarnecki1, M.R. Leese1, A. Davies2 and A. Husbands2, 1PSSRI, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K., email: m.r.patel@open.ac.uk, 2Anglia Instruments Ltd., PO Box 147, Soham, Ely,    Cambridgeshire, CB7 5QL, U.K.   Introduction:  A miniature spectrometer (~100 g) is in the process of development for future surface missions to Mars, aimed at measuring the UV-VIS spectrum encountered at the martian surface.  Respon-sivity of the unit covers the wavelength range 200-800 nm, with a resolution of 1.5 nm.  Great un-certainty lies in the derivation of optical properties of suspended dust in the martian atmosphere at UV wavelengths required for accurate modelling of the transfer of UV to the surface.  This instrument will provide surface spectra ranging from the UV to visi-ble, thus allowing the derivation of optical properties across a wide region of wavelength.  Presented here are initial results from laboratory experiments, verify-ing the concept of the spectrometer for its develop-ment as part of the ExoMars mission to Mars. General Spectrometry:  The study of solar spec-tral irradiance from planetary surfaces is important in a wide range of disciplines.  Solar irradiance is a key input parameter for atmospheric modelling, and gov-erns the photochemistry of planetary atmospheres.  UV and visible radiation are also important in the case of astrobiology – visible light is required for photosyn-thesis, but short wavelength UV can be extremely damaging to biological structures, in some cases plac-ing a limitation on the viability of micro-organisms.   In the case of Mars, measurements of solar irradi-ance as a function of wavelength have often been taken from orbit on numerous occasions (see [1] and references therein), and have always been incorpo-rated into the imaging systems of planetary landers through the use of narrow band filters (e.g. [2], [3]).  High resolution in situ spectroscopic measurements however have never been obtained, and remain an important measurement to describe accurately the electromagnetic spectrum encountered at the martian surface. Martian UV Spectra:  The presence of ubiquitous dust suspended in the martian atmosphere introduces a complex and highly variable element into theoretical simulations of surface fluxes.  In martian radiative transfer simulations, two optical properties of the sus-pended dust need to be known – the single scattering albedo (wbd) and the asymmetry parameter (gbd).  Due to the lack of in situ spectral measurements from the surface the nature of these optical properties still re-mains highly debated, and needs to be resolved to model accurately the UV region of the spectrum at the martian surface.  This contrasts the visible region, where many previous measurements of surface irradi-ance have been taken, and agreement is seen between different observations in the visible. Using the optical properties for martian dust as de-rived by [4], the UV spectrum encountered at the mar-tian surface was simulated, shown in Figure 1.  The model is a multi-layer atmospheric model, using the delta-Eddington approximation for radiative transfer flux.  This model is described in detail in [1] and [5], and the description of the model is omitted here.    Figure 1: Martian surface UV spectrum.  The contribution from scattering events with dust particles creates the diffuse irradiance, which is al-ways present in all martian spectra.  Another impor-tant modifier of the UV spectrum is O3.  O3 is a sea-sonally and geographically variable constituent of the martian atmosphere, with peak absorption in the UV near 250 nm.  Peak abundances tend to occur at high latitudes at northern winter, decreasing towards sum-mer [6].  Given high enough O3 abundances, an ab-sorption band should in theory be observed in martian surface UV spectra.   A UV-VIS Spectrometer for Mars:  Advances in optical technologies have drastically reduced the mass, power and volume requirements for spectrometers to extremely small levels.  Such developments lead to the conclusion that with further engineering, the current technology could in theory be included in the payload for future missions to the martian surface. The unit under consideration here is the AvaSpec Spectrometer, manufactured by Avantes and supplied by Anglia Instruments UK.  The spectrometer incor-porates a 2048-element linear CCD array, and an op-tical bench with a focal length of only 75 mm.  This family of spectrometers cover the range 200-1100 nm, Lunar and Planetary Science XXXV (2004) 1247.pdfwith a resolution of up to 0.1 nm dependent upon con-figuration choice.   The dimensions of the unit are 175´110´44 mm, with a power consumption of only 1.92 W.  The ma-jority of this power consumption occurs in the process-ing electronics for the USB and RS-232 interfaces, and could be significantly reduced if removed.  The optical bench is based upon a symmetrical Czerny-Turner design, shown in Figure 2.  The detection range and resolution are set by the choice of diffrac-tion grating.  Figure 2: Optical bench design of the AvaSpec 2048.  (Key: 1 - Fibre optic input.  2 - Collimating mirror.  3 - Dif-fraction grating.  4 - Collimating mirror.  5 - CCD array).  Ruggedisation of the optical bench will drastically reduce the mass to an anticipated ~100 g.  The fibre optic input interface also allows the optical bench to be housed anywhere within the lander structure, with the fibre optic cable supplying the detected light from a suitable location on the lander. The development of this unit to space qualified standards will be undertaken through a collaborative agreement between the Planetary and Space Sciences Research Institute (PSSRI) and Anglia Instruments UK.  The optical bench will be ruggedised and re-duced in mass, and the electronics will be replaced with a qualified interface for integration onto a plane-tary lander.  Currently the spectrometer has a detec-tion range of 200-800 nm, with a resolution of 1.5 nm.  A higher resolution can be achieved by selecting a smaller wavelength detection range.  The unit will form part of a payload proposal for the ExoMars mis-sion in 2009, offering high resolution spectrometry directly from the surface of Mars for the first time, and requiring a mass allocation of only 100 g. Preliminary Results:  The preliminary results from exposure to a UV-VIS source are shown here.  A Xenon light source was used to irradiate the spec-trometer, across the full 200-800 nm range, to deter-mine the quality of the wide band response.  The re-sponse of the unit to the source is shown in Figure 3.  The response of the spectrometer is clearly defined, showing detailed structures and a distinct output curve.  The full spectrum is covered, giving good cov-erage over all the available areas.  The irradiance at 200 nm is extremely low, since the Xenon output at this wavelength is far lower with respect to the visible region.  This test however clearly demonstrates that the unit is viable for application on a planetary mis-sion, given its extremely low mass and power con-sumption.    Figure 3: AvaSpec 2048 response to Xenon lamp.  As well as the evaluation of dust optical properties in the martian atmosphere, this spectrometer will also in theory allow the detection of O3 in the martian at-mosphere.  The presence of O3 creates a ‘dip’ in the observed spectrum, which though comparison with model can yield the column abundance of O3.  Given a high enough signal to noise ratio, the detection of O3 should be possible.   Summary:  The problems remaining in the optical properties of martian dust at UV wavelengths has been shown, and a need for high resolution spectrometry exists on future surface missions to Mars.  High reso-lution in situ surface spectral data will help us under-stand better the biologically relevant UV radiation that is present at the martian surface, and the subsequent implications for astrobiology.  Accurate data on the scattering and absorption properties of martian dust will help refine future and existing radiative transfer models, which can then be used to investigate further various UV environmental conditions.  The instru-ment which could fulfill this requirement on the ExoMars mission is currently under development, and will offer an invaluable insight into the martian UV-VIS environment and answer the current question of the UV optical properties of suspended martian dust, for a mass of only 100 g. References: [1] Patel, M.R. et al (2002). Planet. Space Sci. 50 (9), 915-927. [2] Pollack, J.B. et al (1977) JGR 82 (28), 4479-4496. [3] Smith, P.H., et al. (1997) Science 278, 1758-1764. [4] Ockert-Bell, M.E. et al (1997) JGR, 102 (E4), 9039-9050. [5] Patel, M.R. et al (2003) Int. J. Astrobiology 2 (1), 21-34.  [6] Barth, C.A. et al (1973) Science 179, 795-796. Lunar and Planetary Science XXXV (2004) 1247.pdf

A miniature UV-VIS spectrometer for the surface of Mars

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A miniature UV-VIS spectrometer for the surface of Mars

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