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"Thomas, I. R."
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Strong Localized Pumping of Water Vapor to High Altitudes on Mars During the Perihelion Season
Here we present water vapor vertical profiles observed with the ExoMars Trace Gas Orbiter/Nadir and Occultation for MArs Discovery instrument during the perihelion and Southern summer solstice season (LS = 240°–300°) in three consecutive Martian Years 34, 35, and 36. We show the detailed latitudinal distribution of H2O at tangent altitudes from 10 to 120 km, revealing a vertical plume at 60°S–50°S injecting H2O upward, reaching abundance of about 50 ppmv at 100 km. We have observed this event repeatedly in the three Martian years analyzed, appearing at LS = 260°–280° and showing inter‐annual variations in the magnitude and timing due to long term effects of the Martian Year 34 Global Dust Storm. We provide a rough estimate of projected hydrogen escape of 3.2 × 109 cm−2 s−1 associated to these plumes, adding further evidence of the key role played by the perihelion season in the long term evolution of the planet's climate. Plain Language Summary Studying the vertical distribution of the Martian atmosphere is crucial to understand what happened to the water presumably present in larger abundance on ancient Mars. We have analyzed the vertical profiles of three Martian Years during the Southern summer, revealing a strong vertical transport of water vapor to the upper atmosphere. This seasonal phenomenon seems to be repeated annually, although with variations in the location and time of the year. Our estimation of the associated upward hydrogen flux represents an important loss which could have contributed to the escape of water to space for at least the period in which Mars had its present orbital inclination. Key Points Latitudinal distributions of water vapor up to 120 km are analyzed in detail using Nadir and Occultation for MArs Discovery (NOMAD) observations with an improved retrieval scheme Water vapor injection during the perihelion localized around 50°–60°S in three consecutive Martian years Martian year 34 Global Dust Storm may have affected the driving mechanisms of the plume, delaying its appearance and reducing its magnitude
Journal Article
The Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment
The Diviner Lunar Radiometer Experiment on NASA’s Lunar Reconnaissance Orbiter will be the first instrument to systematically map the global thermal state of the Moon and its diurnal and seasonal variability. Diviner will measure reflected solar and emitted infrared radiation in nine spectral channels with wavelengths ranging from 0.3 to 400 microns. The resulting measurements will enable characterization of the lunar thermal environment, mapping surface properties such as thermal inertia, rock abundance and silicate mineralogy, and determination of the locations and temperatures of volatile cold traps in the lunar polar regions.
Journal Article
Laboratory emissivity measurements of the plagioclase solid solution series under varying environmental conditions
New laboratory thermal infrared emissivity measurements of the plagioclase solid solution series over the 1700 ∼ 400 cm−1 (6–25 μm) spectral range are presented. Thermal infrared (TIR) spectral changes for fine‐particulate samples (0–25 μm) are characterized for the first time under different laboratory environmental conditions: ambient (terrestrial‐like), half‐vacuum (Mars‐like), vacuum, and vacuum with cooled chamber (lunar‐like). Under all environmental conditions the Christiansen Feature (CF) is observed to vary in a systematic way with Na‐rich end‐member (albite) having a CF position at the highest wave number (shortest wavelength) and the Ca‐rich end‐member (anorthite) having a CF position with the lowest wave number (longest wavelength). As pressure decreases to <10−3 mbar four observations are made: (1) the CF position shifts to higher wave numbers, (2) the spectral contrast of the CF increases relative to the RB, (3) the spectral contrast of the RB in the ∼1200–900 spectral range decreases while the spectral contrast of the RB in the ∼800–400 spectral range either increases or remains the same and (4) the TF disappears. A relationship between the wavelength position of the CF measured under simulated lunar conditions and plagioclase composition (An#) is developed. Although its exact form may evolve with additional data, this linear relationship should be applied to current and future TIR data sets of the Moon. Our new spectral measurements demonstrate how sensitive thermal infrared emissivity spectra of plagioclase feldspars are to the environmental conditions under which they are measured and provide important constraints for interpreting current and future thermal infrared data sets. Key Points Emissivity measurements are sensitive to the environment conditions Different compositions of plagioclase feldspars can be uniquely identified Composition can be related to wavelength positions of spectral features
Journal Article
Detection of green line emission in the dayside atmosphere of Mars from NOMAD-TGO observations
The oxygen emission at 557.7 nm is a ubiquitous component of the spectrum of the terrestrial polar aurora and the reason for its usual green colour
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. It is also observed as a thin layer of glow surrounding the Earth near 90 km altitude in the dayside atmosphere
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,
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but it has so far eluded detection in other planets. Here we report dayglow observations of the green line outside the Earth. They have been performed with the Nadir and Occultation for Mars Discovery ultraviolet and visible spectrometer instrument on board the European Space Agency’s ExoMars Trace Gas Orbiter. Using a special observation mode, scans of the dayside limb provide the altitude distribution of the intensity of the 557.7 nm line and its variability. Two intensity peaks are observed near 80 and 120 km altitude, corresponding to photodissociation of CO
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by solar Lyman α and extreme ultraviolet radiation, respectively. A weaker emission, originating from the same upper level of the oxygen atom, is observed in the near ultraviolet at 297.2 nm. These simultaneous measurements of both oxygen lines make it possible to directly derive a ratio of 16.5 between the visible and ultraviolet emissions, and thereby clarify a controversy between discordant ab initio calculations and atmospheric measurements that has persisted despite multiple efforts. This ratio is considered a standard for measurements connecting the ultraviolet and visible spectral regions. This result has consequences for the study of auroral and airglow processes and for spectral calibration.
The oxygen emission at 557.7 nm, responsible for the green colour of auroras on Earth, is present in two layers at 80 and 120 km altitude on the dayside atmosphere of Mars. Simultaneous observations of the oxygen both in this visible line and in the ultraviolet could also constrain the elusive visible/ultraviolet intensity ratio of the auroral emission to a value of 16.5.
Journal Article
Martian Ozone Observed by TGO/NOMAD‐UVIS Solar Occultation: An Inter‐Comparison of Three Retrieval Methods
The NOMAD‐UVIS instrument on board the ExoMars Trace Gas Orbiter has been investigating the Martian atmosphere with the occultation technique since April 2018. Here, we analyze almost two Mars Years of ozone vertical distributions acquired at the day‐night terminator. The ozone retrievals proved more difficult than expected due to spurious detections of ozone caused by instrumental effects, high dust content, and very low values of ozone. This led us to compare the results from three different retrieval approaches: (a) an onion peeling method, (b) a full occultation Optimal Estimation Method, and (c) a direct onion peeling method. The three methods produce consistently similar results, especially where ozone densities are higher. The main challenge was to find reliable criteria to exclude spurious detections of O3, and we finally adopted two criteria for filtering: (a) a detection limit, and (b) the Δχ2 criterion. Both criteria exclude spurious O3 values especially near the perihelion (180° < Ls < 340°), where up to 98% of ozone detections are filtered out, in agreement with general circulation models, that expect very low values of ozone in this season. Our agrees well with published analysis of the NOMAD‐UVIS data set, as we confirm the main features observed previously, that is, the high‐altitude ozone peak around 40 km at high latitudes. The filtering approaches are in good agreement with those implemented for the SPICAM/MEx observations and underline the need to evaluate carefully the quality of ozone retrievals in occultations. Key Points We compare three different retrieval codes and different criteria to filter spurious detection of ozone on Mars The filtering criteria demonstrate the ozone detection in perihelion season is mostly spurious in both years The three retrieval methods produce consistent results with a maximum percentage difference of ∼30% for large ozone densities below 50 km
Journal Article