Mass transfer in our planetary system

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How and why are the atmospheres of Mars, comets, and other solar system bodies being eroded away? Our whole solar system is bathed in a flow of material from the Sun, travelling outwards at hundreds of kilometres per second. The Earth and its atmosphere are protected from this solar wind as our planet has a magnetic field that holds off the flow. Many other bodies, such as the planets Venus, Mars, and comets, are exposed directly to the wind. My research would examine how these bodies are affected by this relentless flow of solar particles. The simplest interaction is at comets. When a comet's icy nucleus nears the Sun, gas is released, forming a very thin, but very wide atmosphere. This gas gets electrically charged by several processes, forming ions. As soon as these ions are formed, they join the solar wind, and are carried away by it, creating the comet's ion tail, like paint poured into a stream of water. This process, called ion pickup, occurs where ions join an existing flow, in this case, the solar wind. Sunlight causes comet ions to glow, making the tail visible. As well as removing gas from a comet's atmosphere, the pickup process affects the solar wind, mainly by slowing it down and sometimes stopping it completely. My work has shown that tiny comets influence the solar wind over amazingly long distances - several times the distance between the Sun and Earth. The solar wind hitting the atmospheres of Venus and Mars drags ions away, forming wispy ion tails, and causing the atmospheres to lose gas. Earth-sized Venus has kept a very thick atmosphere, but smaller Mars loses its atmosphere quite easily. Today, Mars's air is too thin for liquid water to survive on the planet's surface. Long ago, before much of the gas had been lost, large lakes and rivers did exist there. So, this erosion is very important for learning how Mars's surface has changed. Exclusive measurements of this erosion are now being made by the Mars Express spacecraft. The thick atmosphere of Saturn's largest moon, Titan, also undergoes this erosion, and is now being studied by the exciting Cassini mission. Saturn's smaller moons and rings also release material around Saturn. Exactly how this happens, and what effects it has, is still a mystery. Ion pickup is very important elsewhere. Interplanetary dust doesn't produce much gas, but where there is a lot of it, such as along comet orbits, it can have big effects on the solar wind that aren't well-understood. The gas that flows into our solar system from outside is also affected: it is turned into ions and carried away again by the solar wind. Although the pickup process that occurs at these various places in the solar system are similar, there are also many differences. The main aim of my project is to help complete the picture of how the ion pickup process at a body is affected by solar wind conditions, how quickly ions are made, how big the body is, and the make-up of its atmosphere. To do this, I will use spacecraft instruments that sense individual ions, measuring their speeds, directions, and types, and use ion observations from telescopes in space and on the ground. This is a perfect time to carry out the project, with data arriving daily from Mars Express and Cassini, and soon also from Venus Express. The data will provide details of ion pickup at Mars, Venus, and at Saturn's moons and rings. Measurements already made at comets would be compared to observations from far away, and I would produce computer models to help understand the data. My work will offer better predictions of ion pickup's effects elsewhere, helping prepare for the Rosetta mission to a comet, which will observe how the pickup process changes as a comet nears the Sun, and the New Horizons mission to Pluto. Using unique data from new missions, this well-timed project will help us understand the extremely important process of ion pickup, and how it affects planets and comets nearby and at other stars.