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Exploring the Mars atmosphere with ultraviolet spectroscopy

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 The purpose of IUVS is to make spatial maps of upperatmosphere 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 D = 250, where 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 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 Figure 1. The Imaging Ultraviolet Spectrograph (IUVS) mounted on its handling fixture. The instrument measures 71 33 15cm and weighs 22.1kg.