Escape to space or return to venus : ion flows measured by venus express

Abstract: The present-day Venusian atmosphere is crushingly dense, extremely hot and arid. Yet, in its early history, Venus presumably had a massive amount of water, which, if spread evenly over the surface, provided a water depth of 10s to 100s of meters. Therefore, over the course of the atmospheric evolution, the water must have been removed from Venus. The main processes responsible for water loss can be catagorised into either diffusion into the surface materials or escape to space, where the focus of this thesis is the latter. Determining the contribution on the atmospheric evolution from each of these processes can help us understand how planetary atmospheres evolve, both here in our Solar System and in extra-solar systems, and tell us why Venus became so dry.The water escape to space is determined by several processes, where the main processes are a consequence of the interaction between the Venusian atmosphere and the solar wind. As Venus does not have an intrinsic magnetic field, its atmosphere interacts directly with the solar wind, and creates a, so called, induced magnetosphere. The interaction causes part of the solar wind energy and momentum to be transferred to the upper atmospheric particles. The additional momentum may allow the ions to reach above escape energy and escape the planet. Therefore, the interaction between the atmosphere and the solar wind is important to study to determine the rate of escape of atmospheric constituents to space.In this thesis, the escape of atmospheric constituents to space is investigated through measurements of the H+ and O+ ion flows. These ion flows were measured by the Ion Mass Analyser (IMA) on board the Venus Express spacecraft, which orbited Venus during 2006-2014. Using IMA measurements near the North Pole ionosphere, the ionospheric ion flows were shown to have a strong dusk-to-dawn component along the terminator, inside the collisional region of the atmosphere. From ion flow measurements in the magnetotail, the rate of escape of atmospheric H+ and O+ ions were shown to be affected by the solar cycle, with an average escape rate ratio near two, the stoichiometric ratio of water. The change is mainly attributed to the decrease in the net escape rates of H+, which is a result of the increase in return flows, i.e. ions that flow back towards Venus in the magnetotail. Furthermore, the O+ net escape rate increases as the amount of energy available in the upstream solar wind increases. The increase indicates, as expected, that a portion of the available energy in the upstream solar wind is transferred to the escaping ions. However, the total portion of energy transferred from the solar wind to the escaping ions decreases as the available upstream energy increases. Using the simple relation between the O+ escape rate and the upstream solar wind energy flux, the total atmospheric escape was extrapolated backwards in time, by accounting for the evolution of the solar wind parameters. The resulting total escape over the past 3.9 Ga can be translated into a global equivalent water depth of 0.02-0.6 m. This result cannot explain the massive historical water content on Venus.