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Astronomy

Exploring the Mars atmosphere with ultraviolet spectroscopy

A state-of-the-art imaging spectrograph will make the first comprehensive measurements of how the composition and structure of the Mars upper atmosphere respond to its changing space environment.
21 February 2013, SPIE Newsroom. DOI: 10.1117/2.1201302.004733

NASA's MAVEN (Mars Atmosphere and Volatile Evolution) mission,1 scheduled to launch in late 2013, is designed to explore the planet's upper atmosphere and ionosphere and examine their interaction with the solar wind and solar ultraviolet radiation. Goals of the mission include assessing the current state of the upper layers of the Martian atmosphere and obtaining a comprehensive picture of the processes that control them. The Imaging Ultraviolet Spectrograph (IUVS, see Figure 1), one of MAVEN's instruments, is designed to measure the composition and structure of the atmosphere on a global scale. It will be the third ultraviolet spectrograph to orbit Mars, after the Mariner 9 Ultraviolet Spectrometer2 and the SPICAM, the Mars Express Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars.3


Figure 1. The Imaging Ultraviolet Spectrograph (IUVS) mounted on its handling fixture. The instrument measures 71×33×15cm and weighs 22.1kg.

The purpose of IUVS is to make spatial maps of upper-atmosphere neutral and ionized species (including hydrogen, carbon, oxygen, nitrogen, carbon monoxide, carbon dioxide, and ionized carbon dioxide4) and to measure their vertical distributions (100–225km). These constituents produce atomic-line and molecular-band emissions in the ultraviolet region of the electromagnetic spectrum (115–340nm wavelength range) that can be isolated by species using a spectrometer with an average resolving power R=λ/Δλ∼ 250, where λ is the radiation wavelength. Trace amounts of atomic deuterium are also present in the Mars atmosphere, and its abundance relative to atomic hydrogen is a measure of water content early in the life of the planet. The wavelengths of the hydrogen and deuterium resonance lines are 121.567nm and 121.533nm, respectively, requiring a minimum R ∼ 7400 to separate them.


Figure 2. IUVS cross-section view in the normal-incidence grating (N) configuration for nadir viewing. In the echelle configuration, N is rotated clockwise 90°(blue dashed outline) placing a prism-echelle grating combination (P-E) in the optical path. SM: Scan mirror. T: Telescope mirror. S: spectrograph entrance slit. M1: Spectrograph collimator mirror. M2: Spectrograph camera mirror. SPT: Beam splitter. MUV: Middle ultraviolet detector. FUV: Far ultraviolet detector. FOR: Field of regard.

Figure 3. (a) IUVS performs altitude scans of atmospheric emissions near periapsis. (b) By inverting these measurements, we can obtain density from the peak emission (∼140km) and temperature from the slope of the logarithmic profile.

Figure 4. IUVS disk maps. Images at specific wavelengths are diagnostic of the surface and atmosphere: (a) surface features are evident at 310nm, (b) attenuation from ozone absorption obscures the polar cap at 255nm and (c) atomic oxygen in the upper atmosphere is visible at 130nm. (d) The imaging strategy: IUVS uses a combination of spacecraft and scan mirror motion to construct global maps of the Mars atmosphere. (e) A color composite using the images above, which combines all these phenomena. (f) A ratio of the 255nm/310nm images, mapping ozone absorption. S/C: Spacecraft.

Figure 2 is a cross-section view of the instrument showing the dispersion plane. IUVS uses a single, spherical-mirror telescope to image the Mars atmosphere onto the 0.1°×11°entrance slit of a plane-grating spectrograph. To enable both altitude profiles and spatial maps, the instrument has two independent fields of regard (FOR), one designated as ‘limb’ (24°×11°) and the other as ‘nadir’ (55°×11°). These are individually selected by a plane scan mirror located in front of the telescope. The spectrograph is a modified Czerny-Turner5 design equipped with a toroidal camera mirror to eliminate astigmatism at the center of its focal plane. Separate gratings accommodate the two required resolving powers. One operates at near normal incidence and covers 110–340nm with R ∼ 250 and the other uses a prism cross-disperser and echelle grating6 to cover 120–131nm with R ∼ 19,000.

Black lines in Figure 2 show the light path for normal-incidence grating. Light enters the spectrograph through the entrance slit. It is collimated by a spherical mirror, dispersed by the grating into two diffraction orders (first order: 180–340nm and second order: 110–190nm), and re-imaged by a toroidal camera mirror toward a quartz area-division beam splitter. The beam splitter transmits wavelengths greater than 180nm to the middle ultraviolet (MUV) detector that consists of an image intensifier equipped with a cesium telluride photocathode. The output from the intensifier is coupled to a CMOS array detector by a fiber-optic taper. The beam splitter reflects first-order and second-order light toward the far ultraviolet (FUV) detector, which is identical to the MUV device except that its photocathode is cesium iodide. Because cesium iodide is solar blind (i.e. is very insensitive to photons with wavelengths greater than 200nm), the FUV instrument essentially detects only the second order wavelength while excluding MUV radiation emitted by the atmosphere and solar continuum radiation reflected from the surface of the planet. A stepper-motor-driven mechanism configures the high resolving power mode by rotating the normal-incidence grating approximately 90° out of the collimator beam, illuminating a prism-echelle-grating combination (see dashed lines in Figure 2). The FUV detector records the resulting spectrum.

After reaching Mars in September 2014, MAVEN will enter a 4.5 hour long orbit around the planet with a periapsis altitude of 150km and an apoapsis altitude of 6250km. Whenever the spacecraft altitude is less than 500km (approximately 23 minutes duration centered on periapsis), IUVS is oriented to place the center of the limb FOR perpendicular to the spacecraft velocity vector and approximately 12° below horizontal. Small rotations of the scan mirror project the spectrometer slit onto the limb of the planet so that it traverses the 100–225km altitude range with a sampling interval of about 4km: see Figure 3(a). At each slit altitude, IUVS records FUV and MUV emissions from the atmosphere. These are inverted to yield atmospheric densities and temperatures. Figure 3(b) shows a measurement example, simulated using models constructed from Mariner 9 observations.

As a complement to periapse vertical profiles, disk maps near apoapsis provide global images of the upper atmosphere with no detailed altitude information. In this case, the IUVS views the planet through the nadir FOR with the spectrograph slit oriented perpendicular to the spacecraft orbit's major axis. We use the combination of spacecraft and scan mirror motions to build up images of the atmosphere with resolution of about 160km: see Figure 4(d). UV images are sensitive tracers of dynamical processes over a wide range of altitudes because the transmission of the upper atmosphere varies from nearly transparent at wavelengths near 340nm to completely opaque for wavelengths less than 180nm (due to ozone and carbon dioxide absorption). Figure 4 illustrates this by showing simulated images that we constructed using models and Mariner 9 data.2

Between the periapsis and apoapsis orbit segments, the entrance slit is oriented perpendicular to the spacecraft orbit. IUVS uses its normal-incidence and echelle gratings to measure energetic hydrogen, deuterium, and oxygen atoms that are escaping from the atmosphere. It also characterizes the state of the lower atmosphere (altitude range 30–100km), which is inaccessible to all other MAVEN instruments. For these measurements, the IUVS is oriented so that a star is imaged into one of two ‘keyholes’ located at either end of the spectrograph slit. As the spacecraft orbits Mars, the atmosphere passes in front of the star absorbing its light. Determining the amount of starlight that is absorbed as the star sets or rises provides a sensitive measure on the concentrations of carbon dioxide (the major constituent in the atmosphere), molecular oxygen, ozone, and aerosols.

By virtue of its two resolution modes and split-middle and far-UV detectors, MAVEN's IUVS will be the most capable ultraviolet spectrograph ever sent to another planet. Its comprehensive measurements of the Martian atmosphere will allow us to determine the role that loss of volatile species from the atmosphere to space has played in shaping the history of the Mars climate, liquid water, and potential habitability.


William McClintock, Nicholas Schneider, Ian Stewart, Gregory Holsclaw
Laboratory for Atmospheric and Space Physics
University of Colorado
Boulder, CO

References:
1. http://lasp.colorado.edu/home/maven/ Website for Laboratory for Atmospheric and Space Physics. Accessed 4 January 2013.
2. C. A. Barth, C. W. Hord, A. I. Stewart, A. L. Lane, Mariner 9 ultraviolet spectrometer experiment: initial results, Science 175, p. 309-312, 1972. doi:10.1126/science.175.4019.309
3. J.-L. Bertaux, O. Korablev, D. Fonteyn, E. Guibert, S. Chassefière, F. Lefèvre, E. Dimarellis, Global structure and composition of the martian atmosphere with SPICAM on Mars express, Adv. Space Res. 35, p. 31-36, 2005. doi:10.1016/j.asr.2003.09.055
4. C. A. Barth, The atmosphere of Mars, Annu. Rev. Earth Planetary Sci. 2, p. 333-367, 1974. doi:10.1146/annurev.ea.02.050174.002001
5. M. Czerny, A. F. Turner, Über den Astigmatismus bei Spiegelspektrometern, Zeitschrift Phys. 61(11--12), p. 792-797, 1930. doi:10.1007/BF01340206
6. G. R. Harrison, The production of diffraction gratings: II. The design of echelle gratings and spectrographs, J. Opt. Soc. Am. 39(7), p. 522-527, 1949. doi:10.1364/JOSA.39.000522