Martian Upper Atmospheric Thermal Structure, Composition, and Water and their Significance for Atmospheric Escape and Evolution
Publisher
The University of Arizona.Rights
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction, presentation (such as public display or performance) of protected items is prohibited except with permission of the author.Abstract
Mars has lost the majority of its atmosphere to space over the last 4 billion years, leaving the planet cold, dry, and oxidized. The NASA Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft arrived at Mars in 2014 to investigate the Mars-near space environment and the processes leading to atmospheric escape today, with the goal of understanding the evolution of Mars' atmosphere and climate through time. The Neutral Gas and Ion Mass Spectrometer (NGIMS) is a quadrupole mass spectrometer onboard MAVEN which directly measures the neutral and ionic composition of the Martian upper atmosphere as MAVEN flies through it. Using data obtained from NGIMS between 2014 and 2020, we investigate the thermal structure and neutral composition of this region. In addition, we report on the discovery of water in the upper atmosphere. Each of these three research efforts provides insight into the escape of the Martian atmosphere to space. The thermal structure of the Martian upper atmosphere is important for the determination of atmospheric escape rates because temperature is a measure of the distribution of velocities of the atoms that can escape to space and it determines the rate of many chemical reactions which alter the composition of the upper atmosphere. Using NGIMS measurements of Ar, N$_2$, and CO$_2$ densities, we calculate vertical profiles of temperature by assuming hydrostatic equilibrium and using the ideal gas law. We find the thermal structure of the upper atmosphere is highly variable, with large diurnal variations, and is consistent with a 1D energy balance model which includes solar ultraviolet and near infrared heating, thermal conduction, and radiative cooling by the CO$_2$ ν$_2$ 15 µm band. Determination of the composition of the upper atmosphere of Mars is necessary to understand the chemistry and energy balance in this region and ultimately what species escape to space. NGIMS measures the abundances of CO$_2$, Ar, N$_2$, O, He, and H$_2$. These data are the first vertical profiles of H$_2$, an important source of escaping H, in the Martian upper atmosphere. We investigate variations in the composition of the upper atmosphere with latitude, local time, and season. Transport-driven polar and nightside enhancements are identified in the ratios to CO$_2$ of N$_2$, O, He, and H$_2$. We also observe seasonal variations in the Ar mixing ratio associated with the seasonal deposition and sublimation of CO$_2$ at the polar ice caps. The discovery of water in the upper atmosphere of Mars has a direct impact on the desiccation of the Martian atmosphere and surface, because H$_2$O transported high into the upper atmosphere is rapidly destroyed by ions to ultimately produce H and O atoms which escape to space. We use NGIMS measurements of water-related ions, namely H$_2$O$^+$ and H$_3$O$^+$, to determine the water abundance in the upper atmosphere. The H$_2$O abundance varies with season, peaking in southern summer when Mars is closest to the Sun. Regional and global dust storms lead to a sudden splash of additional water into the upper atmosphere. We demonstrate that the proportion of escaping H atoms produced from H$_2$O is comparable to that from H$_2$, the classical source of escaping H, during most of the Martian year. H$_2$O becomes the dominant source of escaping H during global dust storms.Type
textElectronic Dissertation
Degree Name
Ph.D.Degree Level
doctoralDegree Program
Graduate CollegePlanetary Sciences
Degree Grantor
University of ArizonaCollections
Related items
Showing items related by title, author, creator and subject.
-
Titan's haze at opposite seasons from HST-STIS spectroscopyWe present an analysis of three new image cubes of Titan by the Space Telescope Imaging Spectrograph taken in 2017, 2018, and 2019, half a Titan year after previously analyzed image cubes. Both data sets probe periods when Titan's seasonal north-south-asymmetry switched. The new observations show that the new reversal came exactly half a Titan year after the previous opposite reversal. On the other hand, the phase lag of the reversals with respect to Titan's equinoxes was different indicating that the seasonal variation is close to harmonic and does not follow variations due to Saturn's orbital eccentricity. The reversal had two components, a major one at altitudes below 80 km reversing two years after a minor one above 150 km. The observations further revealed small temporary deviations of <10% of the seasonal amplitude. The new observations provide an improved seasonal model of Titan that gives accurate constraints for future global circulation models.
-
The PHOENIX Exoplanet Retrieval Algorithm and Using H− Opacity as a Probe in Ultrahot JupitersAtmospheric retrievals are now a standard tool to analyze observations of exoplanet atmospheres. This data-driven approach quantitatively compares atmospheric models to observations in order to estimate atmospheric properties and their uncertainties. In this paper, we introduce a new retrieval package, the PHOENIX Exoplanet Retrieval Algorithm (PETRA). PETRA places the PHOENIX atmosphere model in a retrieval framework, allowing us to combine the strengths of a well-tested and widely-used atmosphere model with the advantages of retrieval algorithms. We validate PETRA by retrieving on simulated data for which the true atmospheric state is known. We also show that PETRA can successfully reproduce results from previously published retrievals of WASP-43b and HD 209458b. For the WASP-43b results, we show the effect that different line lists and line profile treatments have on the retrieved atmospheric properties. Lastly, we describe a novel technique for retrieving the temperature structure and e(-) density in ultrahot Jupiters using H- opacity, allowing us to probe atmospheres devoid of most molecular features with the James Webb Space Telescope.
-
Carbon photochemical escape rates from the modern Mars atmosphereWe provide a comprehensive update of photochemical escape rates of atomic carbon from the present-day Martian atmosphere using a one-dimensional photochemical model and a Monte Carlo escape model. The photochemical model incorporates new results relevant to carbon photochemistry at Mars, including new cross sections for photodissociation of CO2 into C and O2 (Lu et al. 2014) and electron impact dissociation of CO (Ajello et al. 2019). We find the newly included channel of CO2 photodissociation to be the largest contributor to C escape, at 34%–58%. CO photodissociation and CO+ dissociative recombination, which have been discussed extensively in the literature, also show up as significant sources of hot C atoms, with respective contributions of 15%–23% and 7%–10%. Electron impact dissociation of CO2 (11%–15%) and photoionization of CO (6%–20%) are also important channels. Overall, escape rates vary over 3–11×1023 s−1, with an increase of 70% at perihelion compared to aphelion, and a much larger increase of 133% at solar maximum compared to solar minimum. While these present escape rates give a total integrated escape of only 1.3 mbar of CO2 when multiplied by 3.6 billion years, the better characterization of carbon photochemistry and escape from this study will enable us to more reliably extrapolate backwards in time to when conditions of the Martian atmosphere were significantly different from those of today.
