Atmospheric Escape Processes and Planetary Atmospheric Evolution

The habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, chemical and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our Solar system: it is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H-dominated thermosphere for astrophysicists, or the Sun-like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth's atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so-called Xenon paradox

Observing Mars from Areostationary Orbit: Benefits and Applications

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Toward More Realistic Simulation and Prediction of Dust Storms on Mars

Global dust storms have major implications for the past and present climate, geologic history, habitability, and future exploration of Mars. Yet their mysterious origins mean we remain unable to realistically simulate or predict them. We identify four key Knowledge Gaps and make four Recommendations to make progress in the next decade

Hydrogen escape from Mars is driven by seasonal and dust storm transport of water

Mars has lost most of its once-abundant water to space, leaving the planet cold and dry. In standard models, molecular hydrogen produced from water in the lower atmosphere diffuses into the upper atmosphere where it is dissociated, producing atomic hydrogen, which is lost. Using observations from the Neutral Gas and Ion Mass Spectrometer on the Mars Atmosphere and Volatile Evolution spacecraft, we demonstrate that water is instead transported directly to the upper atmosphere, then dissociated by ions to produce atomic hydrogen. The water abundance in the upper atmosphere varied seasonally, peaking in southern summer, and surged during dust storms, including the 2018 global dust storm. We calculate that this transport of water dominates the present-day loss of atomic hydrogen to space and influenced the evolution of Mars' climate.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Signatures of sputtering at Mars: the first evidence?

International audienceOne of the potential drivers of Mars' past atmospheric evolution is its escape to space following Mars' interaction with the solar radiation flux and solar wind. To understand how the solar mass and energy can lead to the erosion of the atmosphere is one of the main goals of the Mars Atmosphere and Volatile Evolution MissioN (MAVEN mission. Among the erosion processes that may have been dominant at Mars is sputtering, where neutrals can escape via collisions with heavy planetary ions that are picked up by the solar wind and precipitate back into Mars' upper atmosphere This flux of precipitating heavy ions has been clearly observed by MAVEN. However, so far, no signature of the expected effects of these precipitating ions has been identified in Mars' atmosphere or exosphere. A challenge in this regard is that present-day sputtering enhancement of the upper atmosphere is expected to be weak compared to the other sources including various heating episodes and photochemical processes, especially the dissociative recombination of ionospheric O2+. We will here describe a possible approach to disentangle the effect of the sputtering from other known sources of Mars' exosphere in Neutral Gas and Ion Mass Spectrometer (NGIMS/MAVEN) measurements of the exosphere by analyzing the height profiles of elements other than O. This novel approach could be applicable to future missions to other planetary atmospheres where sputtering is occurring, and if successful, would be the first observation of sputtering at Mars

Ar non-thermal component as seen by MAVEN after 4 years of observations?

International audienceThe dissociative recombination of O2+ in Mars' ionosphere is thought to be today major channel of Mars' neutral atomic oxygen escape to space. In Leblanc et al. (2018), we showed that the exospheric Ar can be measured by NGIMS/MAVEN up to 1200 km in altitude, highlighting the dichotomy of this exospheric population and the existence of a significant non-thermal component. Its main origin was shown to be the collisional interaction between the atmospheric Ar and the products of the dissociative recombination of the O2+ ions. We here present the whole set of NGIMS measurements of the atmospheric and exospheric Ar obtained so far

Photochemical escape of oxygen from Mars: first results from MAVEN in situ data

International audiencePhotochemical escape of atomic oxygen is thought to be one of the dominant channels for Martian atmospheric loss today and played a potentially major role in climate evolution. MAVEN is the first mission capable of measuring, in situ, the relevant quantities necessary to calculate photochemical escape fluxes. We utilize 18 months of data from three MAVEN instruments: LPW, NGIMS and STATIC. From these data we calculate altitude profiles of the production rate of hot oxygen atoms from the dissociative recombination (DR) of O2+ and the probability that such atoms will escape the Mars atmosphere. From this we determine escape fluxes for 815 periapsis passes. Derived average dayside hot O escape rates range from 1.2 to 5.5 x 1025 s-1 depending on season and EUV flux, consistent with several pre-MAVEN predictions and in broad agreement with estimates made with other MAVEN measurements. Hot O escape fluxes do not vary significantly with dayside solar zenith angle or crustal magnetic field strength, but depend on CO2 photoionization frequency with a power law whose exponent is 2.6 ± 0.6, an unexpectedly high value which may be partially due to seasonal and geographic sampling. From this dependence and historical EUV measurements over 70 years, we estimate a modern-era average escape rate of 4.3 x 1025 s-1. Extrapolating this dependence to early solar system EUV conditions gives total losses of 13, 49, 189, and 483 mb of oxygen over 1, 2, 3, and 3.5 Gyr respectively, with uncertainties significantly increasing with time in the past

First Retrieval of Thermospheric Carbon Monoxide From Mars Dayglow Observations

International audienceAs a minor species in the Martian thermosphere, Carbon Monoxide (CO) is a tracer that can be used to constrain changing circulation patterns between the lower thermosphere and upper mesosphere of Mars. By linking CO density distributions to dynamical wind patterns, the structure and variability of the atmosphere will be better understood. Direct measurements of CO can therefore provide insight into the magnitude and pattern of winds and provide a metric for studying the response of the atmosphere to solar forcing. In addition, CO measurements can help solve outstanding photochemical modeling problems in explaining the abundance of CO at Mars. CO is directly observable by electron impact excitation and solar resonance fluorescence emissions in the far-ultraviolet (FUV). The retrieval of CO from solar fluorescence was first proposed over 40 years ago, but has been elusive at Mars due to significant spectral blending. However, by simulating the spectral shape of each contributing emission feature, electron impact excitation and solar fluorescence brightnesses can be extracted from the composite spectrum using a multiple linear regression approach. We use CO Fourth Positive Group (4PG) molecular band emission observed on the limb (130 - 200 km) by the Imaging Ultraviolet Spectrograph (IUVS) on NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft over both northern and southern hemispheres from October 2014 to December 2016. We present the first direct retrieval of CO densities by FUV remote sensing in the upper atmosphere of Mars. Atmospheric composition is inferred using the terrestrial Atmospheric Ultraviolet Radiance Integrated Code adapted to the Martian atmosphere. We investigate the sensitivity of CO density retrievals to variability in solar irradiance, solar longitude, and local time. We compare our results to predictions from the Mars Global Ionosphere-Thermosphere Model as well as in situ measurements by the Neutral Gas and Ion Mass Spectrometer on MAVEN and quantify any differences