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Habitability at the surface of a planet depends on having an atmosphere long enough for life to develop. The loss of atmosphere to space is an important component in assessing planetary surface habitability. Current models of atmospheric escape from exoplanets are not well constrained by observations. Atmospheric escape observations from the terrestrial planets are available in public data archives. We recast oxygen escape rates from Earth derived from an instrument on Dynamics Explorer‐1 as function of solar wind and compare them to similar data from Mars. Analysis demonstrates that oxygen escape rates from Mars are not as sensitive to variations in solar power components as those from Earth. Available data from Venus can confirm or refute the assertion that oxygen escape from magnetized planets is more sensitive than that from unmagnetized planets. Plain Language Summary: Habitability of a planet depends on having an atmosphere long enough for life to develop. NASA and ESA data archives contain information about atmospheric escape from the terrestrial planets. For these planets oxygen ions dominate atmospheric escape. The data archives are just beginning to be analyzed and presented in a form that allows comparison with, and validation of, models of the interaction of stellar winds with exoplanets. We derive oxygen escape rates from Earth as a function of solar power components from a recasting of Dynamics Explorer‐1 data and compare them to similar data from Mars. Our analysis demonstrates that oxygen escape rates from Mars are not as sensitive to variations in the solar power components as those from Earth. These data and similar data from Venus will prove to be important constrains on models of stelar wind/atmosphere interactions and atmospheric escape from exoplanets. Key Points: We recast oxygen escape rates from Earth derived from an instrument on Dynamics Explorer‐1 as a function of solar energy inputsWe compare escape rates for a magnetized planet (Earth) and an unmagnetized planet (Mars) as a function of solar energy inputsOxygen escape rates from Mars are not as sensitive to variations in the solar power components as those from Earth [ABSTRACT FROM AUTHOR]

Mars once had a dense atmosphere enabling liquid water existing on its surface, however, much of that atmosphere has since escaped to space. We examine how incoming solar and solar wind energy fluxes drive escape of atomic and molecular oxygen ions (O+ and O2+ ${\mathrm{O}}_{2}^{+}$) at Mars. We use MAVEN data to evaluate ion escape from 1 February 2016 through 25 May 2022. We find that Martian O+ and O2+ ${\mathrm{O}}_{2}^{+}$ both have increased escape flux with increased solar wind kinetic energy flux and this relationship is generally logarithmic. Increased solar wind electromagnetic energy flux also corresponds to increased O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape flux, however, increased solar wind electromagnetic energy flux seems to first dampen ion escape until a threshold level is reached, at which point ion escape increases with increasing electromagnetic energy flux. Increased solar irradiance (both total and ionizing) does not obviously increase escape of O+ and O2+ ${\mathrm{O}}_{2}^{+}$. Our results suggest that the solar wind electromagnetic energy flux should be considered along with the kinetic energy flux as an important driver of ion escape, and that other parameters should be considered when evaluating solar irradiance's impact on O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape. Plain Language Summary: Mars was once like Earth with a dense atmosphere enabling liquid water to exist on its surface. However, in the billions of years since then, Mars has lost much of its atmosphere to space. We study how energy inputs from the Sun and from the solar wind can drive escape of the ionized constituents of water from Mars' atmosphere. Ion escape is one of several processes of atmospheric loss, and it is a particularly effective process for removing species heavier than hydrogen and helium from terrestrial atmospheres. We find that previously unconsidered energy fluxes may play an important role in driving ion escape. Key Points: Increased solar wind electromagnetic energy flux increases escape of O+ and O2+ ${\mathrm{O}}_{2}^{+}$O+ and O2+ ${\mathrm{O}}_{2}^{+}$ have increased escape rates with increased solar wind kinetic energyUnclear dependence on increased solar irradiance for O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape [ABSTRACT FROM AUTHOR]

Thermal equilibrium in planetary atmospheres occurs at altitudes where the ion, electron, and neutral temperatures are equal. Thermal equilibrium is postulated to occur in the collision‐dominated ionosphere. This postulated altitude is above the lower boundary of all empirical models of planetary ionospheres. Physics‐based model predictions of the altitude cannot be validated due to a lack of adequate simultaneous observations of temperature profiles. This study presents temperature profiles from simultaneous observations on Atmosphere Explorer–C below 140 km and quiet‐time neutral observations from Thermosphere Ionosphere Mesosphere Energy and Dynamics/Global UltraViolet Imager over Millstone Hill. These are compared with profiles from physics‐based models with a discussion of their respective limitations. We conclude that there does not yet exist a quantitative understanding of the ion, electron, and neutral thermalization processes in low‐altitude planetary ionospheres. Progress on this topic requires an adequate database of simultaneous ion, electron, and neutral temperature profiles in the 110–140 km altitude range. Plain Language Summary: Solar radiation ionizes and heats the neutral atmosphere in the process of creating the ionosphere. Heating begins at the altitude where energy carried by photoelectrons produced by solar extreme ultra violet radiation is not absorbed locally. This is the altitude where the ionosphere and neutral atmosphere are no longer in thermal equilibrium. There are numerous ionospheric models predicting the neutral, ion, and electron temperatures (Tn, Ti, and Te) as a function of altitude. These models predict that, in the absence of other heating mechanisms thermal equilibrium where Ti = Ti = Tn occurs below ∼120 km. Temperatures in this altitude range are used as upper boundary conditions for models attempting to describe energy transfer from the upper atmosphere to the ionosphere. Our study shows that ionospheric models of neutral and plasma temperatures cannot be validated in the region of thermal equilibrium because existing observations are sparse and do not have the precision and/or required spatial and temporal resolution. Progress in understanding energy transfer from the atmosphere to the ionosphere requires validated models of the altitude of thermal equilibrium. Key Points: Thermal equilibrium is postulated to be attained at the lowest altitude heated by photoelectrons but has not been observedQuantitative understanding of thermal equilibrium requires observations that validate modelsThere is an ongoing community need for simultaneous neutral, ion, and electron temperature profiles below 140 km altitude [ABSTRACT FROM AUTHOR]

Electron temperatures are a key parameter for understanding the ionosphere‐thermosphere system. This study uses in situ data from the MAVEN mission to investigate the level of coupling between the Martian ionospheric electrons and the neutral thermosphere below the altitude of 190 km. Such coupling is responsible for significant diurnal variations and dawn/dusk asymmetries in both the neutral and plasma portions of the ionosphere‐thermosphere system. Earlier theoretical studies suggested that neutral species controlled the electron temperature by collisional cooling thus implying a diurnal trend in electron temperatures due to the diurnal variability of the neutrals. This hypothesis is evaluated across the dayside ionosphere in the present work. Observations of neutral densities confirm that neutral‐electron collisions are the primary process controlling the cooling of electrons in the Martian ionosphere below ~190 km. The consequence of this is that electron temperatures can be anticorrelated with the extreme ultraviolet (EUV) irradiance due to enhanced electron cooling brought about by an expansion of neutral densities. The results also indicate that in the afternoon sector, neutral densities are less responsive to changes in EUV. This might be associated with enhanced winds leading to dynamic cooling that mitigates the thermal expansion of the thermosphere in the afternoon region. Subsequently, the electron temperature as a function of EUV is relatively uncorrelated in the afternoon sector. Plain Language Summary: The upper atmospheres of planets are partially composed of an ionized plasma. The plasma component is important to understanding the behavior of this region as well as how radio transmissions travel through it. Plasma temperatures are especially important as they determine the rates of many chemical reactions in the upper atmosphere. This analysis confirmed that at Mars, plasma temperatures are made cooler by collisions with carbon dioxide. The upper atmosphere expands when the amount of solar ultraviolet light increases leading to increased amounts of carbon dioxide at any given altitude. Therefore, more solar ultraviolet radiation generally leads to cooler plasma in the upper atmosphere. We find that there is significant variation to this phenomenon across the planet's dayside with dawn, noon, and dusk each having a different atmospheric and plasma temperature response to solar ultraviolet light. Key Points: Electron temperature in the Martian ionosphere is anticorrelated with ultraviolet irradiance; this is consistent with neutral collisional coolingThe change in electron temperatures with ultraviolet irradiance is strongest near the morning and noon sectorsElectron temperatures exhibit little response to ultraviolet irradiance in the afternoon [ABSTRACT FROM AUTHOR]

The altitude and the local solar time dependence of the electron pressure and the resulting ambipolar electric field in the Martian ionosphere have been investigated by utilizing the electron density and temperature measurements obtained by the Langmuir Probe and Waves instrument aboard the Mars Atmosphere and Volatile Evolution mission (MAVEN) spacecraft. The magnitude of the ambipolar electric field is estimated to be 1–2 μV/m on the dayside—producing, on average, a total potential drop of up to 800 mV below 600 km. These calculations, when combined with the previous understandings on the significance of the Poynting flux in the Martian ionosphere, suggest that the ambipolar electric field and the Poynting flux are equally important in establishing a steady state in which the ions are transported from below the exobase to higher altitudes. Key Points: The critical altitude at which the ions transition from unmagnetized to magnetized varies between 150 and 210 km and shows an asymmetry with local solar timeThe total potential drop between the transition altitude and the topside ionosphere (600 km) can reach to values above 800 mVThe role of the draped magnetic field topology on the balance of the various terms in the ion momentum equation is explored [ABSTRACT FROM AUTHOR]

Abstract: On 10 September 2017, irradiance from a magnitude X8.2 solar flare impacted Mars while the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter was characterizing the Mars upper atmosphere. This solar flare was the largest to occur during the MAVEN mission to date, nearly tripling the ionizing irradiance impacting Mars in tens of minutes, and provides an opportunity to study the planet's response to extreme irradiance changes. This letter reports in situ observations of the Mars topside ionosphere's response to this flare above 155 km made 1.67 hr after the flare soft X‐ray peak. The observed plasma density increase is higher than expected based solely on increased ionization, and the electron temperature decreases below 225 km; both effects can be explained by an expanded neutral atmosphere, which efficiently dissipates any flare‐induced heating of the thermal electrons at altitudes where CO2 is the dominant species. Further, the ion density and composition change significantly at both fixed altitude and pressure level, which can be explained by a change in the O:CO2 density ratio, highlighting the importance this ratio has in determining ionospheric structure. [ABSTRACT FROM AUTHOR]

Abstract: Prior to the Mars Atmospheric Volatile EvolutioN (MAVEN) mission, altitude profiles of the electron temperature in the Martian thermosphere were measured only twice. Because the rates of several geophysically important processes depend strongly on the electron temperature, models of the Martian thermosphere and atmospheric escape rates have not been well constrained. In this paper, we use densities and temperatures measured by MAVEN instruments and the one‐dimensional model of Matta et al. (2014, https://doi.org/10.1016/j.icarus.2013.09.006) to test our understanding of the processes that determine the electron temperature. Our analysis is limited to inbound orbits where the magnetic field is within 30° of horizontal and the satellite is within 30° of the subsolar point at altitudes from 120 to 250 km. We introduce empirically adjusted electron temperatures below 180 km, where the MAVEN electron temperature measurements are known to be biased high. We introduce the concept of a local electron heating efficiency, which we define at a given altitude as the ratio of electron heating from photoionization to the total extreme ultraviolet energy deposited. Our analysis shows that MAVEN observations are consistent with the one‐dimensional model below ~210 km if the electron heating efficiency varies with altitude, and the electron temperature is within the empirical bounds below 180 km we introduced. It indicates that above ~210 km electron heat conduction dominates extreme ultraviolet heating in determining electron temperature. Our analysis also suggests that in the subsolar region electrons and neutrals are in thermal equilibrium below 120 km. [ABSTRACT FROM AUTHOR]

Abstract: We present Mars Atmosphere and Volatile EvolutioN observations of large‐amplitude magnetosonic waves propagating through the magnetosheath into the Martian ionosphere near the subsolar point on the dayside of the planet. The observed waves grow in amplitude as predicted for a wave propagating into a denser, charged medium, with wave amplitudes reaching 25 nT, equivalent to ∼40% of the background field strength. These waves drive significant density and temperature variations (∼20% to 100% in amplitude) in the suprathermal electrons and light ion species (H+) that correlate with compressional fronts of the magnetosonic waves. Density and temperature variations are also observed for the ionospheric electrons, and heavy ion species (O+ and O 2 +); however, these variations are not in phase with the magnetic field variations. Whistler waves are observed at compressional wave fronts and are thought to be produced by unstable, anistropic suprathermal electrons. The magnetosonic waves drive significant ion and electron heating down to just above the exobase region. Ion heating rates are estimated to be between 0.03 and 0.2 eVs−1 per ion, and heavier ions could thus gain escape energy if located in this heating region for ∼10–70 s. The measured ionospheric density profile indicates severe ionospheric erosion above the exobase region, and this is likely caused by substantial ion outflow that is driven by the observed heating. The effectiveness of these magnetosonic waves to energize the plasma close to the exobase could have important implications for the long‐term climate evolution for unmagnetized bodies that are exposed to the solar wind. [ABSTRACT FROM AUTHOR]

Energetic O+ and O [ABSTRACT FROM AUTHOR]

Abstract: Statistical correlations are reported between measured electron temperatures and total electric field wave power (in the 2–100 Hz frequency range), at Mars' subsolar point ionosphere. The observations, made by the Mars Atmosphere and Volatile EvolutioN spacecraft, suggest that electric field wave power from the Mars‐solar wind interaction propagates through the Martian ionosphere and is able to heat ionospheric electrons by over 1000 K. Such heating can account for a substantial (but likely not complete) fraction of previously reported discrepancies between modeled and observed electron temperatures in Mars' upper ionosphere. Wave power is typically less than observable thresholds below altitudes of about 200 km, suggesting that energy is deposited into the ionosphere above this. Observed total wave powers range between 10−12 and 10−9 (V/m)2 and decrease with increasing integrated electron density (or decreasing altitude). [ABSTRACT FROM AUTHOR]