Science from the Extended Mission of the Phoenix Mars Lander

美国宇航局的凤凰号着陆器于2008年5月25日降落在火星北纬68度的极地环境中，并近距离观察了火星表面。名义上的任务原定持续约3个月，但任务仍在进行，并已延长。它可能会一直持续下去，直到火星的冬天在2009年初结束。我们的工作将利用迄今为止的成果，并支持扩大的任务行动。凤凰号有一个机械臂，可以深入地下，并在地表以下5-10厘米处发现了冰。它还收集了用于分析的样本，并用气象传感器测量了当地的气候。总的目的是寻找冰从何而来的线索，土壤是由什么组成的，以及火星早期是否有微生物存在。我们对这项任务的贡献将从三个方面帮助我们了解火星。首先，我们将分析土壤颗粒的微观照片，并利用这些知识来建议着陆器在其扩展任务中应该在哪里收集更多的样本，这样我们就可以了解更多。我们将检查这些照片，以确定土壤是否曾经从湖泊或海洋的沉积物中沉积下来，被风带到那里，被小行星撞击抛起，还是来自火山岩石的侵蚀和分解。其次，凤凰号进行了一个实验，将机械臂铲起的土壤与水混合。土壤中的盐可以溶解，探针可以测量盐的类型，例如是否存在氯化钠，人们用在食物上的盐。到目前为止，实验发现土壤呈微碱性，与海水非常相似，并含有镁、钾和钠等成分。通过进一步分析数据并详细考虑，我们希望确定火星上的盐是在水干涸时留下的，还是来自最近的大气和土壤中的化学反应。第三，当凤凰号宇宙飞船从太空降落到火星表面时，由于与空气的摩擦，它的速度减慢了。着陆器减速的速度取决于空气密度和温度。我们已经使用减速测量来初步计算空气密度和温度如何随高度变化。我们发现，与计算机模型预测的类似地球上的天气相比，60公里以下的大气异常温暖。我们将进行更详细的分析，以了解这种差异，并了解有关气候的信息，例如，着陆时是否有云，或者空气中是否含有大量灰尘。我们还希望了解不寻常的大气层是否解释了为什么凤凰号会越过预定的着陆点。凤凰号上的显微镜站占了我们研究的很大一部分，它由一台可以拍摄彩色和紫外线照片的光学显微镜和一台被称为原子力显微镜（AFM）的高分辨率显微镜组成。原子力显微镜可以对比人类头发宽度还小的颗粒表面成像。这些表面为我们提供了颗粒如何分解的线索，有些还表明火山矿物腐烂成粘土。我们计划做一些实验室实验，看看我们能否在地球上复制火星上看到的一些特征。我们的研究是一个很好的机会，让英国以适度的成本为高调的国际火星任务做出贡献。因此，这项研究将有助于建立英国登陆火星的经验，并继续将火星探索带给英国公众。

NASA's Phoenix Lander landed on 25-May-2008 in the polar environment of Mars at 68 degrees north, and has taken a close-up look at Mars' surface. The nominal mission was set to last about 3 months, but the mission is still going and has been extended. It will probably keep going until Martian winter closes in, in early 2009. Our work will capitalize on the results to date and support the extended mission operations. Phoenix has a robotic arm, which digs into the ground, and has found ice lying just 5-10 cm below the surface. It has also scooped up samples for analysis and measured the local climate with weather sensors. The overall purpose is to look for clues about where the ice came from, what the soil is made of, and whether microbes might have lived in Mars' early history. Our contribution to the mission will help understand Mars in three ways. Firstly, we will analyze microscopic pictures of the soil particles and use this knowledge to suggest where the lander should scoop up further samples in its extended mission so that we can learn more. We will examine these pictures to determine whether the soil was once deposited from sediments in a lake or sea, carried there by the wind, thrown up by asteroid impact, or came from the erosion and break down of volcanic rocks. Secondly, Phoenix has an experiment where soil scooped up by the robotic arm is mixed with water. Salts in the soil dissolve and probes measure the type of salts, for example whether sodium chloride is present, the salt people use on food. So far, the experiment has discovered that the soil is slightly alkaline, very like that of seawater, and contains components such as magnesium, potassium, and sodium. By analyzing the data further and considering it in detail, we hope to determine whether the salts on Mars were left behind when water dried up or whether the salts came from more recent chemical reactions in the atmosphere and soil. Thirdly, when the Phoenix spacecraft fell to the surface of Mars from space it was slowed down by friction with the air. The rate that the lander decelerated depended on the air density and temperature. We have used the deceleration measurements for a preliminary calculation of how the air density and temperature changed with height. We found that the atmosphere below about 60 km height is unusually warm compared to predictions from the sort of computer models that are similar to those that predict weather on the Earth. We will do more detailed analysis to understand this discrepancy and learn information about the climate, for example, whether there were clouds present during landing or whether the air held lots of dust. We also hope to learn if the unusual atmosphere explains why Phoenix overshot its intended landing site. The microscopy station on Phoenix, that forms a large chunk of our research, consists of an optical microscope that takes colour and ultraviolet pictures and a very-high-resolution microscope called an atomic-force microscope, or AFM. The AFM can image the surfaces of particles smaller than the width of a human hair. These surfaces give us clues about how particles break down and some suggest the decay of volcanic minerals into clays. We plan to do some lab experiments to see if we can replicate on Earth some features seen on Mars. Our research is a great opportunity for a UK contribution to a high profile international Mars mission at modest cost. This research will therefore help to build up the UK's experience of landing on Mars and continue to bring Mars exploration to the UK public.