MARTIAN ULTRAVIOLET AURORA: RESULTS OF MODEL SIMULATIONS

We present recent modeling results based on observations performed with the UV spectrographs on board the Mars Express and MAVEN missions.Two types of aurora are discussed: the localized and transient discrete aurora and the more stable diffuse aurora observed during periods of active solar periods.CODYMA

MARS OXYGEN GREEN LINE DAYGLOW FROM NOMAD/UVIS AND MODEL COMPARISON

The UVIS (UV and Visible Spectrometer) channel of the NOMAD (Nadir and Occultation for MArs Discovery) spectrometer onboard the ExoMars Trace Gas Orbiter performs limb observations of the dayside of the Mars atmosphere in both the visible and the ultraviolet domains since April 2019. The recently discovered visible emissions of the oxygen green line at 557.7 nm has here been investigated. The variations of the limb profile of this emission are studied over seasons. These average limb profiles are compared to photochemical model simulations with MAVEN/EUVM solar flux and the LMD GCM as inputs of the model. The global shape of the profile and the intensities are generally well reproduced. However, the peak altitude can sometimes be underestimated by the model and needs an adjustment of the CO2 density to reproduce the observations. We also compare the variations of the green line intensities over some individual UVIS limb tracking observations (observations of the atmosphere at a quasi-constant altitude) to model simulations and demonstrate a very good agreement. Finally, we show that the intensity and altitude of the lower emission peak are correlated with the solar Ly-α flux as expected from the theory of its production

A simple, autonomous, non-linear inversion method for the analysis of occultation observation of the dusty atmosphere of Mars.

editorial reviewedOzone (O3) is an important atmospheric specie of planet Mars, capable of absorbing ultraviolet (UV) radiation. Occultation of solar (or stellar) radiation and measurement of the extinction of UV photons by the atmosphere is a standard O3 remote sensing method. Both O3 and carbon dioxide (CO2) absorb UV photons in the 200 – 300 nm range, the O3 Hartley absorption band peaking near 250 nm. Dusts also contribute to, and sometimes dominate, the UV extinction by the atmosphere of Mars. The wavelength-dependent dust extinction coefficient (k) is often described using a power law k=k0 (λ0/ λ)α with reference value k0 at wavelength λ0. The ad-hoc α exponent stems from the properties of the dusts. We develop a simple autonomous, nonlinear method to retrieve the vertical profiles of CO2, O3 and dust properties from solar occultation profiles, under a spherical symmetry assumption. The gas concentration and dust reference extinction (k0) are represented using a combination of triangle functions of the radial distance (r), producing a piecewise linear profile. The α parameter is represented similarly using triangle functions of log(r). Slant line-of-sight optical thickness results from the Abel transform of these profiles, producing hypergeometric 2F1 functions for the dusts. The different parameters are retrieved by inverse Abel transform using a least squares minimization, which depends linearly on the CO2, O3 and k0 profiles, and non-linearly on α. The linear parameters are considered as functions of the α, reducing the fitting to a non-linear minimization over the α parameter profile only. This drastically reduces the number of dimensions of the parameter space. We show that this method allows efficient retrieval of all the parameters. Noise is however expected to be present when analyzing occultation data from the NOMAD-TGO instrument, which can reduce the ability to retrieve the minimization parameters. The k0 and O3 profiles can, nevertheless, be expected to be retrieved over about two orders of magnitude, while the CO2 density profile can be expected to be fairly retrieved at relatively low altitude

Mars Aurora: A Comparison of MAVEN/IUVS and EMM/EMUS Observations

peer reviewedMars' lack of a global magnetic field led to initial expectations of minimal auroral activity. Mars Express's SPICAM instrument nonetheless discovered an unusual form of aurora in 2005. The ultraviolet emissions were confined near Mars' strong crustal field region, showing that even weak magnetic fields can be responsible for aurora. These discrete aurora emissions were identified in 19 observations over SPICAM's decade of observations.  The MAVEN spacecraft arrived at Mars in 2014 carrying the Imaging UltraViolet Spectrograph (IUVS). Thanks to its high sensitivity and observing cadence, IUVS increased detections of discrete aurora twenty-fold. IUVS also discovered two new widespread forms of aurora. Diffuse aurora is a planet-engulfing phenomenon, caused by solar energetic protons and electrons directly impacting the entire unshielded planet. Proton aurora is caused by solar wind protons charge-exchanging into the atmosphere and causing Lyman alpha emission across the dayside. IUVS studies the aurora at mid- and far-UV wavelengths in both limb scans and nadir imaging. The Emirates Mars Mission (EMM) arrived in 2021 carrying the Emirates Mission UltraViolet Spectrometer (EMUS). EMUS quickly added to the menagerie of auroral phenomena thanks to its high far-UV sensitivity. Discrete aurora emissions were seen in a substantial fraction of nightside observations, and appear to take on new forms not seen by IUVS (sinuous, "non-crustal field", among others). Furthermore, EMUS detected a spatially-variable form of proton aurora called patchy proton aurora. EMUS studies the aurora through nadir imaging at far- and extreme-UV wavelengths. The net result of the tremendous influx of new observations is a lag in cataloguing and cross-comparing the types of observations made with different instruments at different wavelength ranges in different observing modes. We now have the perspective to identify the causes of these auroral phenomena, which gives a more physics-based nomenclature: suprathermal electron aurora: hot electrons from the Mars environment appear to be responsible for most forms of discrete aurora solar energetic particle aurora: SEP electrons and protons from the Sun cause the planet-wide diffuse aurora  solar wind aurora: solar wind protons charged-exchange into the atmosphere to cause dayside aurora This presentation seeks to give that broader context, highlighting what phenomena IUVS and EMUS observe, depending on their distinct instrumental capabilities whether they’re actually seeing the same phenomena or different ones,  how can one type of observation can complement the other,  where one’s capabilities are unique, and  what are the best directions for collaboration; how in situ measurements of particles and fields can contribute to the next stage of understanding of the conditions for particle precipitation A more coherent observational perspective, as outlined above, may grant a framework for developing a deeper physical understanding of Mars unexpected diverse auroral processes

Abel transform of exponential functions for planetary and cometary atmospheres with application to observation of 46P/Wirtanen and to the OI 557.7 nm emission at Mars.

Line-of-sight integration of emissions from planetary and cometary atmospheres is the Abel transform of the emission rate, under the spherical symmetry assumption. Indefinite integrals constructed from the Abel transform integral are useful for implementing remote sensing data analysis methods, such as the numerical inverse Abel transform giving the volume emission rate compatible with the observation. We obtain analytical expressions based on a suitable, non-alternating, series development to compute those indefinite integrals. We establish expressions allowing absolute accuracy control of the convergence of these series depending on the number of terms involved. We compare the analytical method with numerical computation techniques, which are found to be sufficiently accurate as well. Inverse Abel transform fitting is then tested in order to establish that the expected emission rate profiles can be retrieved from the observation of both planetary and cometary atmospheres. We show that the method is robust, especially when Tikhonov regularization is included, although it must be carefully tuned when the observation varies across many orders of magnitude. A first application is conducted over observation of comet 46P/Wirtanen, showing some variability possibly attributable to an evolution of the contamination by dust and icy grains. A second application is considered to deduce the 557.7 nm volume emission rate profile of the metastable oxygen atom in the upper atmosphere of planet Mars

A chemical survey of exoplanets with ARIEL

Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25–7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10–100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed – using conservative estimates of mission performance and a full model of all significant noise sources in the measurement – using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL – in line with the stated mission objectives – will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.Peer reviewedFinal Published versio