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，不受太阳风的影响，因为我们的星球有一个阻止太阳风流动的磁场。许多其他天体，如金星、火星和彗星，都直接暴露在风中。我的研究将考察这些天体如何受到这种持续不断的太阳粒子流动的影响。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，形成一个非常薄但非常宽广的大气层。这种气体通过几个过程带电，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，形成彗星的离子尾巴，就像油漆倒入水流中一样。这个过程被称为离子拾取，发生在离子加入现有气流的地方，在这种情况下，是太阳风。阳光使彗星离子发光，使尾巴可见。除了从彗星大气层中移除气体之外，这种吸收过程还会影响太阳风，主要是通过减缓太阳风的速度，有时甚至完全阻止太阳风。我的研究表明，微小的彗星影响太阳风的距离长得惊人--是太阳和地球之间距离的几倍。太阳风冲击金星和火星的大气层，将离子拖走，形成细小的离子尾巴，导致大气层失去气体。地球大小的金星一直保持着非常厚的大气层，但较小的火星很容易失去大气层。今天，火星的空气太稀薄，液态水无法在火星表面生存。很久以前，在大量天然气流失之前，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，现在正在进行令人兴奋的卡西尼号任务的研究。土星较小的卫星和光环也会释放出土星周围的物质。这究竟是如何发生的，以及它产生了什么影响，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多少气体，但在有大量气体的地方，例如沿彗星轨道，它可以对太阳风产生巨大的影响，这一点还没有被很好地理解。从外部流入太阳系的气体也会受到影响：它被转化为离子，然后再次被太阳风带走。虽然发生在太阳系这些不同地方的拾取过程相似，但也有许多不同之处。我这个项目的主要目的是帮助完成一个物体的离子吸收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，以及它的大气组成的图景。要做到这一点，我将使用探测单个离子的宇宙飞船仪器，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。现在是实施该项目的绝佳时机，每天都有来自火星快车和卡西尼号的数据，很快也会从金星快车收到数据。这些数据将提供火星、金星和土星卫星和光环的离子吸收细节。已经在彗星上进行的测量将与来自遥远地方的观测进行比较，我将制作计算机模型来帮助理解数据。我的工作将为离子拾取在其他地方的影响提供更好的预测，帮助为罗塞塔彗星任务做准备，该任务将观察彗星接近太阳时离子拾取过程如何变化，以及新地平线号前往冥王星的任务。利用来自新任务的独特数据，这个时机恰到好处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.