64 research outputs found
Ultraviolet and visible reflectance spectra of Phobos and Deimos as measured by the ExoMars-TGO/Nomad-UVIS spectrometer
Spectroscopic measurements are a powerful tool to investigate the surface composition of airless bodies and provide clues of their origin. The composition and origin of Phobos and Deimos are still unknown and are currently widely debated. We present spectroscopic measurements of Phobos and Deimos at ultraviolet and visible wavelengths (250–650 nm) made by the NOMAD-Ultraviolet and Visible Spectrometer (UVIS) on the ExoMars TGO mission. These new spectra cover multiple areas on Phobos and Deimos, and are of generally higher spectral resolution and signal-to-noise than previous spectra, and extend to lower wavelengths than most previous measurements. The UVIS spectra confirm a red-sloped spectrum lacking any strong absorption features; however, we confirm the presence of a previously identified absorption feature near 0.65 μm and tentative absorption near 0.45 μm. The observed Phobos and Deimos spectra are similar to D- and T-type asteroids, adding weight to the captured asteroid hypothesis for the moons' origins. We also find, however, that the UVIS Phobos reflectance spectra of Phobos' red unit is a relatively close match to the olivine-rich, highly shocked Mars meteorite NWA 2737, with a low overall reflectance, a red-sloped spectrum, and lack of olivine-associated absorption bands in the UVIS spectral range. This meteorite, however, exhibits spectral features at longer wavelengths that not observed in the Martian moon spectra, indicating a need for further investigation at longer wavelengths to interpret whether this material could inform our understanding of Phobos' origin
Calibration of NOMAD on ESA's ExoMars Trace Gas Orbiter: Part 2 – The Limb, Nadir and Occultation (LNO) channel
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).The Nadir and Occultation for MArs Discovery (NOMAD) instrument is a 3-channel spectrometer suite on the ESA ExoMars Trace Gas Orbiter. Since April 2018, when the nominal science mission began, it has been measuring the constituents of the Martian atmosphere. NOMAD contains three separate spectrometers, two of which operate in the infrared: the Solar Occultation (SO) channel makes only solar occultation observations, and therefore has the best resolving power (∼20,000) and a wider spectral region covering 2.2–4.3 μm. The Limb, Nadir and Occultation (LNO) channel covers the 2.2–3.8 μm spectral region and can operate in limb, nadir or solar occultation pointing modes. The Ultraviolet–VISible (UVIS) channel operates in the UV–visible region, from 200 to 650 nm, and can measure in limb, nadir or solar occultation modes like LNO. The LNO channel has a lower resolving power (∼10,000) than the SO channel, but is still typically an order of magnitude better than previous instruments orbiting Mars. The channel primarily operates in nadir-viewing mode, pointing directly down to the surface to measure the narrow atmospheric molecular absorption lines, clouds and surface features in the reflected sunlight. From the depth and position of the observed atmospheric absorption lines, the constituents of the Martian atmosphere and their column densities can be derived, leading to new insights into the processes that govern their distribution and transport. Surface properties can also be derived from nadir observations by observing the shape of the spectral continuum. Many calibration measurements were made prior to launch, on the voyage to Mars, and continue to be made in-flight during the science phase of the mission. This work, part 2, addresses the aspects of the LNO channel calibration that are not covered elsewhere, namely: the LNO ground calibration setup, the LNO occultation and nadir boresight pointing vectors, LNO detector characterisation and nadir/limb illumination pattern, instrument temperature effects, and finally the radiometric calibration of the LNO channel. An accompanying paper, part 1 (Thomas et al., 2021, this issue), addresses similar aspects for SO, the other infrared channel in NOMAD. A further accompanying paper (Cruz-Mermy et al., 2021, this issue) investigated the LNO radiometric calibration in more detail, approaching the work from a theoretical perspective. The two calibrations agree with each other to within 3%, validating each calibration method. © 2022 The Authors. Published by Elsevier Ltd.This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by the UK Space Agency through grants ST/V002295/1, ST/V005332/1 and ST/S00145X/1 and ST/R001405/1 and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant number 30442502 (ET_HOME). The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). SR thanks BELSPO for the FED-tWIN funding (Prf-2019-077 - RT-MOLEXO)
Explanation for the increase in high altitude water on Mars observed by NOMAD during the 2018 global dust storm
The Nadir and Occultation for MArs Discovery (NOMAD) instrument on board ExoMars Trace Gas Orbiter (TGO) measured a large increase in water vapor at altitudes in the range of 40‐100 km during the 2018 global dust storm on Mars. Using a three‐dimensional general circulation model, we examine the mechanism responsible for the enhancement of water vapor in the upper atmosphere. Experiments with different prescribed vertical profiles of dust show that when more dust is present higher in the atmosphere the temperature increases and the amount of water ascending over the tropics is not limited by saturation until reaching heights of 70‐100 km. The warmer temperatures allow more water to ascend to the mesosphere. Photochemical simulations show a strong increase in high‐altitude atomic hydrogen following the high‐altitude water vapor increase by a few days
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ExoMars TGO/NOMAD‐UVIS vertical profiles of ozone: Part 2: The high‐altitude layers of atmospheric ozone
Solar occultations performed by the Nadir and Occultation for MArs Discovery (NOMAD) ultraviolet and visible spectrometer (UVIS) onboard the ExoMars Trace Gas Orbiter (TGO) have provided a comprehensive mapping of atmospheric ozone density. The observations here extend over a full Mars year (MY) between April 21, 2018 at the beginning of the TGO science operations during late northern summer on Mars (MY 34, Ls = 163°) and March 9, 2020 (MY 35). UVIS provided transmittance spectra of the Martian atmosphere allowing measurements of the vertical distribution of ozone density using its Hartley absorption band (200 – 300 nm). The overall comparison to water vapor is found in the companion paper to this work (Patel et al., 2021). Our findings indicate the presence of (1) a high-altitude peak of ozone between 40 and 60 km in altitude over the north polar latitudes for at least 45% of the Martian year during mid-northern spring, late northern summer-early southern spring, and late southern summer, and (2) a second, but more prominent, high-altitude ozone peak in the south polar latitudes, lasting for at least 60% of the year including the southern autumn and winter seasons. When present, both high-altitude peaks are observed in the sunrise and sunset occultations, suggesting that the layers could persist during the day. Results from the Mars general circulation models predict the general behavior of these peaks of ozone and are used in an attempt to further our understanding of the chemical processes controlling high-altitude ozone on Mars
First Observation of the Oxygen 630 nm Emission in the Martian Dayglow
Following the recent detection of the oxygen green line airglow on Mars, we have improved the statistical analysis of the data recorded by the NOMAD/UVIS instrument on board the ExoMars Trace Gas Orbiter mission by summing up hundreds of spectra to increase the signal to noise ratio. This led to the observation of the OI 630 nm emission, a first detection in a planetary atmosphere outside the Earth. The average limb profile shows a broad peak intensity of 4.8 kR near 150 km. Comparison with a photochemical model indicates that it is well predicted by current photochemistry, considering the sources of uncertainty. The red/green line intensity ratio decreases dramatically with altitude as a consequence of the efficient quenching of O(1D) by CO2. Simultaneous observations of the green and red dayglow will provide information on variations in the thermosphere in response to seasonal changes and the effects of solar events
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 colour1. It is also observed as a thin layer of glow surrounding the Earth near 90 km altitude in the dayside atmosphere2,3 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 CO2 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
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Total column ozone climatology from MY34 to MY36 from measurements by the NOMAD-UVIS spectrometer
Near continuous radiance measurements of the martian atmosphere in the 200-650 nm wavelength range by the Ultraviolet and VISible spectrometer (UVIS) (Patel et al., 2017) as part of the NOMAD instrument on the ExoMars Trace Gas Orbiter (TGO) (Vandaele, et al., 2018) provides a powerful tool for investigating the ozone climatology, the water cycle (from the well established photochemical anti-correlation between water vapour and ozone), and by extension photochemical reactions in the martian atmosphere.
Previous observations have shown that, spatially, ozone is observed in three main regions (1) at high northern latitudes (50° - 90° N) through the northern autumn, winter and spring seasons, (2) at equivalent high southern latitudes during the aphelion season, and (3) at low latitudes between 30°S - 30°N from Ls = 30° - 120°, coinciding with observations of the aphelion cloud belt (Bertaux et al., 2000; Clancy et al., 2016; Holmes et al., 2018). Entrapment of ozone in deep depressions, such as Hellas basin (Clancy et al., 2016), has been observed and associated with the meridional transport of O-rich air from northern latitudes and from south polar air being transported to equatorial regions after southern winter.
In this study, radiative transfer modelling, that includes multiple scattering from aerosols and surface reflectance, has been used to model the UVIS radiances in the wavelength range 220-310 nm to retrieve the O3 column abundances for the latter half of Mars Year (MY)34 through MY36. We report the spatial distribution of ozone as measured by UVIS, including the observed entrapment and diurnal cycle of ozone within Hellas basin
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Density and Temperature of the Upper Mesosphere and Lower Thermosphere of Mars Retrieved From the OI 557.7 nm Dayglow Measured by TGO/NOMAD
The upper mesosphere and lower thermosphere of Mars (70–150 km) is of high interest because it is a region affected by climatological/meteorological events in the lower atmosphere and external solar forcing. However, only a few measurements are available at this altitude range. OI 557.7 nm dayglow emission has been detected at these altitudes by the limb observations with Nadir and Occultation for Mars Discovery (NOMAD) aboard the ExoMars Trace Gas Orbiter (TGO). We develop an inversion method to retrieve density and temperature at these altitudes from the OI 557.7 nm dayglow limb profiles. We demonstrate that the atmospheric density around 90 and 140 km and temperature around 80 km during the daytime can be retrieved from the TGO/NOMAD limb measurements. The retrieved densities show a large seasonal variation both around 90 and 140 km and reach maximum values around perihelion period. This can be explained by temperature variation in the lower atmosphere driven by the dust content and Sun-Mars distance. Temperature around 80 km is higher than predicted by general circulation models, which is tentatively consistent with the warm atmospheric layer recently discovered in nighttime. The temperature retrieval relies on the temperature dependence of the quenching coefficient of 1S oxygen by CO2. Further validation of this coefficient in the range of the Mars upper atmosphere is needed for the verification of the retrieved high temperature
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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
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