Variability of Martian Turbopause Altitudes

Abstract

The turbopause and homopause represent the transition from strong turbulence and mixing in the middle atmosphere to a molecular-diffusion-dominated region in the upper atmosphere. We use neutral densities measured by the Neutral Gas and Ion Mass Spectrometer on the Mars Atmospheric and Volatile EvolutioN spacecraft from February 2015 to October 2016 to investigate the temperature structure and fluctuations of the Martian upper atmosphere. We compare those with temperature measurements of the lower atmosphere from the Mars Reconnaissance Orbiter's Mars Climate Sounder. At the lowest Mars Atmospheric and Volatile EvolutioN altitudes we often observe a statically stable region where waves propagate freely. In contrast, regions from about 20 km up to at least 70 km are reduced in stability where waves are expected to dissipate readily due to breaking/saturation. We derive homopause altitudes from Neutral Gas and Ion Mass Spectrometer N 2 and Ar densities and find that it varies between 60 and 140 km. From the standard deviations in monthly averaged temperature profiles, we determine the wave turbopause altitude where applicable and find that these altitudes agree with the homopause altitudes. We show that the homopause variability is driven by order of magnitude changes in CO 2 densities, which drastically affect the altitude at which the eddy diffusion is equal to the molecular diffusion. These variations are observed as a function of local time, latitude, and season. That the turbopause/homopause does not track a fixed density level means that the eddy diffusion coefficient at the turbopause can vary significantly, suggesting differences in the dominant breaking waves and tidal modes. Plain Language SummaryThe gases that make up a planet's atmosphere are mixed together efficiently near the surface. But in the uppermost part of the atmosphere, gases separate by mass-heavier ones are held closer to the planet, while lighter ones are more abundant higher up. Because turbulence is responsible for the mixing in the atmosphere, the altitude where this transition occurs is called the turbopause. That turbulence is thought to be generated by atmospheric waves that lose energy (somewhat like ocean waves breaking at shore). In this work, we use measurements of the upper atmosphere from Mars Atmosphere and Volatile Evolution Mission and of the lower atmosphere from Mars Reconnaissance Orbiter to study the turbopause region during a full Mars year. Similar to Earth's atmosphere, we find that below the turbopause wave breaking is strong (which produces turbulence), but above waves can move freely. We also show that the turbopause varies significantly depending on the season and location on the planet. At Mars, gases can escape from the top of the atmosphere, so the relative amounts of escape of different gases depends on the turbopause. Understanding this altitude and why it varies is important for interpreting how the atmosphere evolved. Key Points:• Mars's turbopause is governed by the transition from strong wave dissipation ...

Seasonal and Latitudinal Variations of Dayside N2/CO2 Ratio in the Martian Thermosphere Derived From MAVEN IUVS Observations

“…Homopause altitudes inferred from IUVS under the assumption of an isothermal atmosphere and a fixed surface N 2 /CO 2 are estimated to vary between 100 and 130 km. The altitudes of inferred homopause and seasonal trends agree reasonably well with Slipski et al (2018). The variations of the homopause altitude could be the dominant driver of changes in N 2 /CO 2 ratios in the thermosphere.…”

“…(2017) and Slipski et al. (2018) reported substantial variations of the homopause altitude by using vertical profiles of N 2 / 40 Ar observed by NGIMS. The inferred homopause altitudes are located between 60 and 140 km.…”

Effects of the 10 September 2017 Solar Flare on the Density and Composition of the Thermosphere of Mars

“…Densities of the lower mass species change more gradually with altitude and as such have greater scale heights. Some mass dependence to this baseline density structure is expected, as the full range of altitudes from our observations are above the homopause (Slipski et al., 2018), where well‐mixed species separate by mass as molecular diffusion replaces eddy diffusion as the primary diffusive process regulating the atmosphere (Strobel, 2002).…”

“…Molecular diffusion affects the thermosphere's composition because the majority of the thermosphere lies above the homopause, which varies between ∼60 and 140 km (Slipski et al., 2018). Molecular diffusion in the thermosphere causes the physical process of diffusive separation, which separates elements and molecules (species) by mass.…”

Ion‐Neutral Coupling in the Upper Atmosphere of Mars: A Dominant Driver of Topside Ionospheric Structure

“…Simulations that include only photochemistry are representative of lower atmospheric plasma dynamics, and simulations that include both photochemistry and transport are more representative of upper atmospheric plasma dynamics (Mendillo et al, 2011). The neutral atmosphere of Mars expands and contracts with increasing and decreasing neutral temperature (e.g., Bougher et al, ; Slipski et al, ; Zhang et al, ). MAVEN's operation at Mars has overlapped with a time of relatively weak solar activity where the temperature in the thermosphere of Mars, on average, is relatively cool and therefore representative of a contracted atmosphere.…”