Mass transfer in our planetary system

火星、彗星和其他太阳系天体的大气层是如何以及为什么被侵蚀的？我们的整个太阳系都沐浴在来自太阳的物质流中，以每秒数百公里的速度向外传播。地球和它的大气层受到保护，免受太阳风的影响，因为我们的星球有一个磁场，阻止了流动。许多其他天体，如金星、火星和彗星，都直接暴露在风中。我的研究将研究这些天体如何受到太阳粒子无情流动的影响。最简单的相互作用发生在彗星上。当彗星的冰核靠近太阳时，气体被释放出来，形成了一个非常薄但非常宽的大气层。这种气体通过几个过程充电，形成离子。一旦这些离子形成，它们就会加入太阳风，并被太阳风带走，形成彗星的离子尾，就像倒入水流中的油漆一样。这个过程称为离子拾取，发生在离子加入现有流的地方，在这种情况下，是太阳风。阳光使彗星离子发光，使尾巴可见。除了从彗星的大气层中移除气体外，拾取过程还影响太阳风，主要是通过减缓太阳风，有时甚至完全停止太阳风。我的工作表明，微小的彗星在惊人的长距离上影响太阳风-太阳和地球之间的距离的几倍。撞击金星和火星大气层的太阳风将离子带走，形成纤细的离子尾，导致大气层失去气体。地球大小的金星保持着非常厚的大气层，但较小的火星很容易失去大气层。如今，火星的空气太稀薄，液态水无法在行星表面生存。很久以前，在大部分天然气流失之前，那里确实存在大型湖泊和河流。因此，这种侵蚀对于了解火星表面的变化非常重要。目前，火星快车号航天器正在对这种侵蚀进行独家测量。土星最大的卫星土卫六的厚厚的大气层也经历了这种侵蚀，现在正在由激动人心的卡西尼使命任务进行研究。土星较小的卫星和环也会释放土星周围的物质。这究竟是如何发生的，以及它有什么影响，仍然是一个谜。离子拾取在其他地方非常重要。行星际尘埃不会产生太多的气体，但是在有很多气体的地方，比如沿着彗星轨道，它会对太阳风产生很大的影响，而这一点还没有得到很好的理解。从外部流入太阳系的气体也会受到影响：它会变成离子，并被太阳风再次带走。虽然在太阳系的这些不同地方发生的拾取过程是相似的，但也有许多差异。我的项目的主要目的是帮助完成一个物体的离子拾取过程如何受到太阳风条件的影响，离子产生的速度有多快，物体有多大，以及其大气的构成。为了做到这一点，我将使用航天器仪器来感知单个离子，测量它们的速度，方向和类型，并使用太空和地面望远镜的离子观测。这是执行该项目的完美时机，每天都有来自火星快车和卡西尼号的数据到达，不久金星快车也会到达。这些数据将提供火星、金星以及土星卫星和环上离子拾取的细节。已经在彗星上进行的测量将与遥远的观测进行比较，我将制作计算机模型来帮助理解数据。我的工作将提供更好的预测离子拾取的影响在其他地方，帮助准备罗塞塔使命的彗星，这将观察如何拾取过程的变化作为彗星接近太阳，和新视野使命冥王星。利用来自新任务的独特数据，这个适时的项目将帮助我们了解离子拾取的极其重要的过程，以及它如何影响附近和其他恒星的行星和彗星。

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.