Titan airglow spectra from Cassini Ultraviolet Imaging Spectrograph (UVIS): EUV analysis

peer reviewedaudience: researcher, professional, studentWe present the first UV airglow observations of Titan's atmosphere by the Ultraviolet Imaging Spectrograph (UVIS) on Cassini. Using one spectral channel in the EUV from 561-1182 Å and one in the FUV from 1115-1913 Å, UVIS observed the disk on 13 December, 2004 at low solar activity. The EUV spectrum consists of three band systems of N[SUB]2[/SUB] (b [SUP]1[/SUP]∏[SUB]u[/SUB], b' [SUP]1[/SUP]∑[SUB]u[/SUB] [SUP]+[/SUP], c[SUB]4[/SUB]' [SUP]1[/SUP]∑[SUB]u[/SUB] [SUP]+[/SUP] -> X [SUP]1[/SUP]∑[SUB]g[/SUB] [SUP]+[/SUP]), while the FUV spectrum consists of one (a [SUP]1[/SUP]∏[SUB]g[/SUB] -> X [SUP]1[/SUP]∑[SUB]g[/SUB] [SUP]+[/SUP]). Both the EUV and FUV spectra contain many N I and N II multiplets that are produced primarily by photodissociative ionization. Spectral intensities of the N[SUB]2[/SUB] c[SUB]4[/SUB]' [SUP]1[/SUP]∑[SUB]u[/SUB] [SUP]+[/SUP](v' = 0) -> X [SUP]1[/SUP]∑[SUB]g[/SUB] [SUP]+[/SUP](v'' = 0-2) progression from 950-1010 Å are resolved for the first time. The UVIS observations reveal that the c[SUB]4[/SUB]' [SUP]1[/SUP]∑[SUB]u[/SUB] [SUP]+[/SUP](0) -> X [SUP]1[/SUP]∑[SUB]g[/SUB] [SUP]+[/SUP] (0) vibrational band near 958 Å is weak and undetectable, and that N I multiplets near 953.2 and 964.5 Å are present instead. Magnetospheric particle excitation may be weak or sporadic, since the nightside EUV spectrum on this orbit shows no observable nitrogen emission features and only H Ly-β

Combined analysis of Far UV and Mid UV spectra obtained by the MAVEN IUVS instrument in a Stellar Occultation Mode

International audienceIn this presentation, we will focus on the results obtained by the Imaging Ultraviolet Spectrograph (IUVS) onboard the Mars Atmosphere and Volatile and Evolution (MAVEN) mission while performing stellar occultations observations. In the IUVS wavelength range, CO2 possesses a distinct and broad signature shortward of 200 nanometers which allows one to retrieve CO2 concentration and subsequently to deduce atmospheric pressure and temperature profiles from 30 to 150 km of altitude (upper troposphere up to the thermosphere) as well as the concentration of other atmospheric consitituents (clouds/aerosols, ozone and molecular oxygen). The occultation technique relies on the determination of atmospheric transmission at various altitudes above the surface. Only relative measurements are needed to infer species abundances, and thus the method is self-calibrated.The ratio of spectra taken through (close to Mars) and outside (far from Mars) the atmosphere gives an atmospheric transmission at each altitude. If any absorbing or/and scattering species is present along the optical path, photons are lost and resulting transmissions are lower than 1. The sampling rate yields a vertical resolution typically greater than 3 km on the vertical. For Mars, the sounded region inside which a quantity of atmospheric constituents can be derived lies generally between 20 and 150 km depending on the atmospheric state (dust loading). The compiled dataset has already yielded a variety of results, showing high concentrations of ozone in the deep polar night as well as the detection of a highly elevated aerosol layer potentially made of CO2 ice

Mars Ozone Mapping with MAVEN IUVS

International audienceOzone (O3) on Mars is a product of the CO2 photolysis by ultraviolet radiation. It is destroyed with a timescale of less than ~1 hour during the day by the H, OH, and HO2 radicals. This tight coupling between O3 and HOx species makes ozone a sensitive tracer of the odd hydrogen chemistry that stabilizes the CO2 atmosphere of Mars, and ozone measurements offer a powerful constraint for photochemical models. Ozone is also expected to be anti-correlated to water vapour, the source of hydrogen radicals HOx. At high latitudes in winter, the absence of H2O prevents the production of HOx and the chemical lifetime of ozone may increase up to several days. In these conditions, the ozone column abundance usually reaches its largest values of the Martian year and ozone turns into a measurable tracer of the polar vortex dynamics.The Imaging Ultraviolet Spectrograph (IUVS) is one of nine science instruments aboard the Mars Atmosphere and Volatile and Evolution (MAVEN) spacecraft. In the apoapse imaging phase, the spacecraft motion carries the IUVS lines-of-sight across the Martian disk while the scan mirror is used to make transverse swaths. This observation mode allows mapping the ozone vertically-integrated column from its signature in the solar ultraviolet flux backscattered by the surface and the atmosphere.This paper will present an overview of the first year of ozone mapping by IUVS. We will describe in particular the last Mars northern winter (2015) when the largest ozone columns have been observed since the beginning of the MAVEN mission. The data will be compared to prior Earth-based observations and to the SPICAM and MARCI ozone datasets. We will also test our quantitative understanding of the Martian ozone by comparing the IUVS observations to our three-dimensional model with photochemistry

H Escape in 3D: MAVEN IUVS observations of the Mars corona

International audienceMars has lost a significant fraction of its initial water inventory to space, as evidenced by surface morphology and an enrichment of the heavy isotope deuterium relative to hydrogen. This loss continues today via thermal Jeans escape of neutral hydrogen from the extended H corona. The Mars Atmosphere and Volatile EvolutioN mission's Imaging Ultraviolet Spectrometer IUVS constrains this escape with high cadence, spatially resolved measurements of the corona in scattered sunlight in the Lyman alpha line of neutral H at 121.6 nm. These measurements indicate spatial variability in the corona never before recognized, with potential effects on reconstructing the escape history of water and on retrieved escape rates from previous spatially unresolved observations. I will present our current progress in retrieving escape rates and density distributions from the IUVS observations, including the degree to which observations of the H corona can constrain the thermospheric general circulation at Mars

The Martian ozone layer as seen by Mars Express and by MAVEN

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The Hot Oxygen Corona of Mars: Observations by MAVEN IUVS

International audienceObservation of the Martian hot oxygen corona is an important but difficult measurement in planetary science. These energetic oxygen atoms are thought to be primarily produced by dissociative recombination of O2+ in the planet's ionosphere. While the corona is dominated by gravitationally bound particles, the fraction capable of escaping the planet is expected to be the major pathway for oxygen escape from Mars at the present time. Thus, observing the hot oxygen corona provides valuable insight into the planet’s atmospheric evolution. Up until now, characterization of this population has been elusive due to its extremely tenuous nature. We present here altitude profiles of the hot oxygen corona collected by the MAVEN IUVS instrument as part of its investigation of atmospheric escape from Mars. These measurements confirm the presence of this long anticipated feature of the Martian exosphere, and offer an invaluable way of probing the mechanisms driving escape of atomic oxygen at the present day. Correlation with MAVEN EUVM measurements suggests a relationship between coronal density and solar photoionizing flux, supporting the expectation that dissociative recombination in the ionosphere is the primary source of hot oxygen at Mars. The quality and quantity of the dataset provides valuable constraints for the coronal modeling community, and preliminary comparison of the observed gross structure with a Monte Carlo model is presented

Mars Nitric Oxide Nightglow as observed by MAVEN/IUVS

International audienceWe report results from a study of nitric oxide nightglow over the northern hemisphere of Mars during winter. We present observations of the nitric oxide δ and γ band emissions between 190 and 270 nm by the Imaging UltraViolet Spectrograph on the MAVEN spacecraft. The emission reveals recombination of N and O atoms dissociated on the dayside of Mars, and is known to trace upper atmosphere hemispheric circulation patterns. We extend previous studies to higher latitudes, finding brighter emissions lower in the atmosphere as predicted from global circulation models. Statistically signicant variability suggests variation of the fluxes of N and O carried from the dayside thermosphere, as well as possible variations of the wind pattern in the nightside thermosphere and mesosphere. We report unexpected variability between the individual components of the nitric oxide band system. We provide statistical characteristics and evaluate possible explanations for this phenomenon

Mars atmospheric escape constrained using MAVEN IUVS coronal observations

Every planetary atmosphere is capped by a corona: an extended, extremely tenuous region where collisions are negligible and particles follow ballistic trajectories. At Mars, the corona is especially extended due to the low gravity of the planet, and a large number of coronal particles are on escaping trajectories. Such escape has played a critical role in the history of the Mars system, likely removing a substantial fraction of the water initially present on the planet, but the mechanism and magnitude of this escape remains poorly constrained. Currently in orbit at Mars, MAVEN's Imaging Ultraviolet Spectrograph (IUVS) is mapping the distribution of oxygen and hydrogen above 200 km at a high spatial and temporal cadence, revealing a dynamic corona in unprecedented detail. Results will be presented demonstrating that the H in the corona is not spherically symmetric in its distribution, and can potentially be used as a tracer of thermospheric general circulation; and that non-thermal "hot" O (in contrast with more spatially confined "cold" thermal O) is ionospherically sourced with a characteristic energy of 1.1 eV and responds to solar EUV forcing. These results will be interpreted in terms of their impact on our current understanding of how atmospheric escape operates today. We will also discuss how these processes may have acted in the past to deplete Mars' initial water inventory, potentially altering the redox balance of the planet and atmosphere through differential escape of H and O

Mars’ ultraviolet dayglow observations by IUVS/MAVEN: Structure and variability of Martian upper atmosphere

International audienceMars has been studied extensively at ultraviolet wavelengths starting from Mariner 6 and 7 (Barth et al. JGR, 1971; Stewart, JGR, 1972), Mariner 9 (Barth et al., Icaurs, 1972; Stewart et al. Icarus, 1972), and more recently by SPICAM aboard Mars Express (Leblanc et al., JGR, 2006). The results from these measurements reveal a large variability in the composition and structure of Martian upper atmosphere. However, due to the lack of simultaneous measurements of energy input in the atmosphere, such as solar electromagnetic and particle flux as well as limitations in the observation geometry and data itself, this variability is still not fully understood.We report a comprehensive study of Mars dayglow observations by the Imaging Ultraviolet Spectrograph (IUVS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite, focusing on upper atmospheric structure and seasonal variability. The dayglow emission spectra show features similar to previous UV measurements at Mars. The IUVS detected a second, low-altitude peak in the emission profile of OI 297.2 nm, confirmation of the prediction that the absorption of solar Lyman alpha emission is an important energy source there. We find a significant drop in thermospheric scale height and temperature between Ls = 218° and Ls = 337 - 352°, attributed primarily to the decrease in solar activity and increase in heliocentric distance. The CO2+ UVD peak intensity is well correlated with simultaneous observations of solar 17 - 22 nm irradiance at Mars by Extreme Ultraviolet Monitor (EUVM) aboard MAVEN. I will present and discuss the variability in Martian UV dayglow, its dependence on solar EUV irradiance, and the importance of IUVS observations in our current understanding of Mars’ thermosphere

Variability of D and H in the Martian Exosphere

International audienceThe IUVS instrument on MAVEN contains an echelle spectrograph with a novel optical design to enable long-aperture measurements of emission lines in the absence of continuum, intended primarily to measure the H and D Ly α emission lines from the martian upper atmosphere. The main scientific goal of the echelle channel is to measure the H and D Ly α emissions that result from resonant scattering of solar emission and to discover how the H and D densities, temperatures, and escape fluxes vary with location, season, topography, etc. The global D/H ratio of the martian atmosphere is roughly 5 times higher than in the terrestrial atmosphere due to the escape of a large volume of water into space, likely early in the history of Mars. Since H atoms escape faster than D atoms, the D/H ratio increases with time as more water is lost. Earth-based IR observations have indicated large variations in the HDO/H2O ratio in the lower atmosphere from location to location, and possible changes with the atmospheric seasonal cycles [Villanueva et al. 2015]. HST and MEX measurements of the H corona of Mars have shown large (order of magnitude) changes in the H exosphere and escape flux with changing seasons and/or heliospheric distance [Clarke et al. 2014; Chaffin et al. 2014]. A series of observations of D and H with the IUVS echelle channel now show a strong trend in the variation of both emissions, with order of magnitude changes in both species in the upper atmosphere. With the added data expected in Fall 2016, we will be able to determine this trend over a full range of martian solar longitude. These results and a comparison with proposed processes that might lead to the observed changes will be presented