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Atmospheric Escape at Mars and Venus: Past, Present and Future

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Atmospheric Escape at Mars and Venus: Past, Present and Future Exoplanets in our Backyard 2020 (LPI Contrib. No. 2195) 3067.pdf ATMOSPHERIC ESCAPE AT MARS AND VENUS: PAST, PRESENT AND FUTURE. S. M. Curry1 and J. G. Luhmann1, 1University of California, Berkeley, Space Sciences Laboratory ([email protected]), 7 Gauss Way, Berkeley, CA 94720 Introduction: Earth, Venus and Mars formed at similar exoplanetary atmospheres are unlikely in our lifetime, it times, yet their atmospheres have evolved drastically is critical to use the natural planetary laboratory in our differently. Venus’s upper atmosphere, or exosphere, own back yard to understand how atmospheres, and wa- hosts several atomic species such as hydrogen, helium, ter, evolve and influence potential habitability. With the oxygen, carbon, and argon, some of which are energized upcoming James Webb Space Telescope (JWST) mis- in the upper atmosphere to escape energies or ionized sion, future observations of Earth-sized planets will and carried away from the planet. Mars’ atmosphere has only increase our understanding of atmospheric compo- a similar mainly carbon dioxide makeup, but the present sition and the relationship a planetary atmosphere has day atmospheric pressure is significantly less than that with its host star. of Venus. So how is atmospheric escape at different at Venus and Mars? Both Venus and Mars lack a global dipole magnetic field, which may expose their atmospheres to scavenging more so than a planet like Earth [1]. How- ever at Venus, as opposed to Mars, virtually all signifi- cant present day atmospheric escape of heavy constitu- ents is in the form of ions. At Mars, photochemical es- cape is currently the main channel for atmospheric es- cape [2]. This is the process where molecules are pho- todissociated and obtain enough energy to exceed the escape velocity. However, Venus is roughly 7x more massive than Mars, and subsequently its gravitational well impedes photochemical escape. The sheer differ- ence in their size and proximity to the sun have pro- found effects on their atmospheric evolution, which we will discuss at length. Image courtesy of Darby Dyar, Nature Relevance to Exoplanets. Throughout the inner helio- Results: We will present Pioneer Venus Orbiter (PVO) sphere, the evolution of the solar or stellar wind interac- and Venus Express (VEX) observations of Venus’ ion tion is a critical topic for terrestrial planets since the so- loss throughout solar cycle 22, 23 and 24. We will also lar (stellar) wind interacts directly with the planetary at- present the most recent observations during the Parker mosphere to drive atmospheric escape. A star’s activity Solar Probe (PSP) flybys of Venus. We will compare plays a critical role in the evolution of terrestrial atmos- the Venusian picture of atmospheric escape to that at pheres, with extreme EUV and X-ray fluxes, as well as Mars by presenting Mars Express (MEX) and Mars At- a more intense solar wind and higher occurrences of mosphere and Volatile Evolution (MAVEN) measure- powerful solar transient events. At CO2 planets, if you ments. Using a composite of in-situ observations at follow the oxygen, you can follow the water. Evidence Mars and Venus, we will construct a global picture of of liquid water is present at Earth, Mars and Venus, and atmospheric escape, ion precipitation, and magnetic to- the presence of liquid water directly contributed to hab- pology throughout the last three solar cycles. We will itable conditions at Earth (see Figure 1). Additionally, extrapolate these results to conditions when our sun was volcanism has been active at all three of these terrestrial younger and more active, serving as an analogue . planets, which serves as an atmospheric source as well as a catalyst for the water and carbon cycle. Understand- References: ing how the atmospheres of our terrestrial neighbors [1] Curry S.M., Luhmann J., Ma Y., Liemohn M., evolve with respect to our sun may tell us how and when Dong C., Hara T. Comparative pick-up ion distributions a rocky planet may be able to sustain an atmosphere at Mars and Venus: Consequences for atmospheric with sufficient pressure to maintain liquid water. deposition and escape. Planetary and Space Science. We currently have multiple assets at both Mars and 2015;115:35-47. Venus that are able to observe atmospheric dynamics [2] Jakoskey et al., (2018). Loss of the Martian at- and escape. Because in-situ measurements of mosphere to space: Present‐day loss rates determined Exoplanets in our Backyard 2020 (LPI Contrib. No. 2195) 3067.pdf from MAVEN observations and integrated loss through time. Icarus, 315, 146– 157. .
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