Exospheres and Energetic Neutral Atoms of Mars, Venus and Titan

International audienceOur understanding of the upper atmosphere of unmagnetized bodies such as Mars, Venus and Titan has improved significantly in this decade. Recent observations by in situ and remote sensing instruments on board Mars Express, Venus Express and Cassini have revealed characteristics of the neutral upper atmospheres (exospheres) and of energetic neutral atoms (ENAs). The ENA environment in the vicinity of the bodies is by itself a significant study field, but ENAs are also used as a diagnostic tool for the exosphere and the interaction with the upstream plasmas. Synergy between theoretical and modeling work has also improved considerably. In this review, we summarize the recent progress of our understanding of the neutral environment in the vicinity of unmagnetized planets

Multipath Curved Planar Reformations of Peripheral CT Angiography : Diagnostic Accuracy and Time Efficiency

Objectives To compare diagnostic performance and time efficiency between 3D multipath curved planar reformations (mpCPRs) and axial images of CT angiography for the pre-interventional assessment of peripheral arterial disease (PAD), with digital subtraction angiography as the standard of reference. Methods Forty patients (10 females, mean age 72 years), referred to CTA prior to endovascular treatment of PAD, were prospectively included and underwent peripheral CT angiography. A semiautomated toolbox was used to render mpCPRs. Twenty-one arterial segments were defined in each leg; for each segment, the presence of stenosis > 70% was assessed on mpCPRs and axial images by two readers, independently, with digital subtraction angiography as gold standard. Results Both readers reached lower sensitivity (Reader 1: 91 vs. 94%, p = 0.08; Reader 2: 89 vs. 93%, p = 0.03) but significantly higher specificity (Reader 1: 94 vs. 89%, p < 0.01; Reader 2: 96 vs. 95%, p = 0.01) with mpCPRs than with axial images. Reader 1 achieved significantly higher accuracy with mpCPRs (93 vs. 91%, p = 0.02), and Reader 2 had similar overall accuracy in both evaluations (94 vs. 94%, p = 0.96). Both readers read mpCPRs significantly faster than axial images (Reader 1: 5′45″ based on mpCPRs vs. 7′40″ based on axial images; Reader 2: 4′41″ based on mpCPRs vs. 6′57″ based on axial images; p < 0.01). Conclusions mpCPRs are a promising 3D reformation technique that facilitates a fast assessment of PAD with high diagnostic accuracy.(VLID)357824

Evidence of Hot Hydrogen in the Exosphere of Mars

International audienceThis work is to study the energetic hydrogen atoms present in the exosphere of Mars. Usually this population is not discernible from the thermal H population on account of the large scale height of Mars' atmosphere. However, recent observations under certain conditions with the Hubble Space Telescope reveal the signature of the existence of such a population in the exosphere of Mars. The goal of this study is to understand an characterize the properties of this population and determine its effects on the escape of water from Mars

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

Four years of upper atmospheric exploration at Mars with MAVEN and IUVS

International audienceThe Mars Atmosphere and Volatile EvolutioN mission (MAVEN) is a Mars orbiter equipped with instruments to study the current state of the Mars atmosphere, atmospheric loss processes, and their fundamental drivers. Most instruments make in situ measurements of particles and fields in the Mars environment and upper atmosphere. IUVS is the only remote sensing instrument for the study of Mars' atmosphere and its interaction with the plasma environment. The instrument has two main channels for the study of the upper atmosphere at far-UV and Mid-UV wavelengths, plus an echelle channel capable of spectrally resolving hydrogen and deuterium Lyman alpha lines to measure the D/H ratio. The instrument uses a scan mirror to obtain limb scans or disk maps and is mounted on an Articulated Payload Platform (APP) which maintains Mars pointing while the spacecraft bus and solar arrays maintain sun-pointing. The spacecraft travels on an elliptical orbit allowing limb scans to be obtained at periapse and full-disk imaging at apoapse, and scans of Mars corona in between. Thanks to the scan mirror, APP, and routine planning, the instrument observes with >50% duty cycles in a variety of repeating modes. On a bimonthly basis, the instrument performs two-day stellar occultation campaigns to probe atmospheric structure

Four years of upper atmospheric exploration at Mars with MAVEN and IUVS

International audienceThe Mars Atmosphere and Volatile EvolutioN mission (MAVEN) is a Mars orbiter equipped with instruments to study the current state of the Mars atmosphere, atmospheric loss processes, and their fundamental drivers. Most instruments make in situ measurements of particles and fields in the Mars environment and upper atmosphere. IUVS is the only remote sensing instrument for the study of Mars' atmosphere and its interaction with the plasma environment. The instrument has two main channels for the study of the upper atmosphere at far-UV and Mid-UV wavelengths, plus an echelle channel capable of spectrally resolving hydrogen and deuterium Lyman alpha lines to measure the D/H ratio. The instrument uses a scan mirror to obtain limb scans or disk maps and is mounted on an Articulated Payload Platform (APP) which maintains Mars pointing while the spacecraft bus and solar arrays maintain sun-pointing. The spacecraft travels on an elliptical orbit allowing limb scans to be obtained at periapse and full-disk imaging at apoapse, and scans of Mars corona in between. Thanks to the scan mirror, APP, and routine planning, the instrument observes with >50% duty cycles in a variety of repeating modes. On a bimonthly basis, the instrument performs two-day stellar occultation campaigns to probe atmospheric structure

Loss of the Martian atmosphere to space: Present-day loss rates determined from MAVEN observations and integrated loss through time

International audienceObservations of the Mars upper atmosphere made from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft have been used to determine the loss rates of gas from the upper atmosphere to space for a complete Mars year (16 Nov 2014 – 3 Oct 2016). Loss rates for H and O are sufficient to remove ∼2-3 kg/s to space. By itself, this loss would be significant over the history of the planet. In addition, loss rates would have been greater early in history due to the enhanced solar EUV and more-active Sun. Integrated loss, based on current processes whose escape rates in the past are adjusted according to expected solar evolution, would have been as much as 0.8 bar CO2 or 23 m global equivalent layer of H2O; these losses are likely to be lower limits due to the nature of the extrapolation of loss rates to the earliest times. Combined with the lack of surface or subsurface reservoirs for CO2 that could hold remnants of an early, thick atmosphere, these results suggest that loss of gas to space has been the dominant process responsible for changing the climate of Mars from an early, warmer environment to the cold, dry one that we see today