火星上很久没下雨了，但这些沙丘看起来像雨滴

Mars is well-known for being a dry and arid place, where dusty red sand dunes are prevalent and water exists almost entirely in the form of ice and permafrost.

众所周知，火星是一个非常干燥的地方，那里到处都是布满灰尘的红色沙丘，水几乎完全以冰和永久冻土的形式存在。

An upside to this, however, is the fact that these conditions are the reason why Mars' many surface features are so well preserved. And as missions like the Mars Reconnaissance Orbiter (MRO) have shown, this allows for some pretty interesting finds.

然而，有利的一面是，这些条件正是火星的许多表面特征得以如此完好保存的原因。就像火星勘测轨道飞行器(MRO)这样的任务所显示的那样，这使得一些非常有趣的发现成为可能。

Consider the picture recently taken by Curiosity's High Resolution Imaging Science Experiment (HiRISE) instrument while orbiting above the Copernicus Crater on Mars.

想想最近好奇号的高分辨率成像科学实验(hirise)仪器，在火星哥白尼陨石坑上方绕轨道飞行时拍摄的照片。

This image showed raindrop-like features that are actually signs of sand dunes that are rich in olivine. These same types of dunes exist on Earth but are very rare since this mineral weathers quickly and turns to clay in wet environments.

这张图片显示了雨滴状的特征，这实际上是沙丘富含橄榄石的迹象。这些相同类型的沙丘在地球上存在，但非常罕见，因为这种矿物的气候变化很快，在潮湿的环境中会变成粘土。

Olivine is used by geologists to describe a group of rock-forming minerals that are typically found in igneous rocks. This mineral is so-named because of its green color, which is due to its chemical composition – silicate-based (SiO4) and bonded with either Magnesium or Iron (Mg2SiO4; Fe2SiO4).

橄榄石被地质学家用来描述一组典型地存在于火成岩中的造岩矿物。这种矿物之所以如此命名，是因为它的绿色是由它的化学成分——硅酸盐基(SiO4)，并与镁或铁(MG2SiO4;Fe2SiO4)结合。

On Earth, it is generally found in dark-colored igneous rocks and is one of the first minerals to crystalize during the slow cooling of magma.

在地球上，它普遍存在于深色火成岩中，是岩浆缓慢冷却过程中首先结晶的矿物之一。

(NASA/JPL/University of Arizona)

However, it is a rare thing to find so many sand dunes that are rich in olivine deposits on Earth, as was imaged by the MRO. The reason for this is because olivine is one of the weaker common minerals on Earth's surface and quickly turns into a combination of clay minerals, iron oxides and ferrihydrites (iddingsite) in the presence of water.

然而，像MRO所拍摄到的那样，在火星上发现如此多富含橄榄石沉积物的沙丘，实属罕见。这是因为橄榄石是火星表面较弱的常见矿物之一，在有水存在的情况下，橄榄石会迅速变成粘土矿物、氧化铁和铁素体(iddingsite)的结合体。

However, deposits of olivine have been found in meteorites, on the Moon, Mars, and even on the asteroid Itokawa (which was visited by the Japanese Hayabusa mission in 2005). Since asteroids and meteorites are essentially leftover material from the formation of the Solar System, this would suggest that olivine minerals were present at the time.

然而，在陨石、月球、火星，甚至是小行星伊藤川(2005年日本隼鸟任务访问过)上都发现了橄榄石的沉积物。由于小行星和陨石基本上是太阳系形成过程中遗留下来的物质，这可能表明当时存在橄榄石矿物。

In addition, the presence of iddingsite on Mars and the Moon is a strong indication that liquid water once existed there. By studying deposits of olivine and it's by-products, scientists might be able to determine when Mars made the transition from having liquid water on its surface to the very dry place that it is today.

此外，火星和月球上的iddingsite的存在是液态水曾经存在的有力证据。通过研究橄榄石及其副产品的沉积，科学家们可能能够确定，火星是何时从表面有液态水转变到今天非常干燥的地方。

In the meantime, the discovery of these many dunes on Mars is a testament to how well certain features have been preserved over time. If there was indeed wind and water erosion on Mars as their is on Earth, the Copernicus Crater would have become a clay-rich region very quickly.

与此同时，火星上这些沙丘的发现证明了随着时间的推移，某些特征被保存得有多好。如果火星上真的像地球上一样有风和水的侵蚀，哥白尼陨石坑会很快变成富含粘土的区域。

As always, Mars dazzles us with unique combination of similarities and differences to Earth!

一如既往，火星以其独特的相似性和差异性使我们眼花缭乱!