宇航员在火星上发现幽灵沙丘

环绕着火星飞行的机器人轨道飞行器已经得到了幽灵沙丘的照片。它们是一些坑洞,科学家们相信在那里曾经出现过这个红色荒漠世界里的高大月牙形沙丘。

火星是一个荒漠般的星球,非常像地球上的一些沙漠,但是比地球寒冷很多。火星的沙漠拥有大量的沙丘,从小沙丘到高耸的、峭壁般的、被细腻的沙粒覆盖的沙丘。但是——虽然大量种类的沙丘已经从轨道飞行器和火星漫游者里看见了,最近仍然十分活跃,正在逐步成为火星的风景——现在另外一种沙丘也在火星上被发现了。科学家们把它们称作幽灵山丘,它们极其古老。他们在2018年7月10日的«地球空间»里报告了这个发现。新的研究论文刚刚在地球物理研究杂志中发表。

这些沙丘在今天并不是活跃的沙丘。更确切地说,它们是以前古老的沙丘在其被侵蚀后留下来的、在地上像坑洞般的洼地。大量这样新月形的坑洞被发现了,每个大约有美国国会大厦那么大。西雅图华盛顿大学的行星地貌学家、做新研究的作家麦肯齐·戴解释道:

这些坑洞的任意一个都不足以告诉你那是一个沙丘或者它是从覆盖着大片古老沙丘的地方来的。可是当你将它们放在一起时,它们有太多火星上和地球上的沙丘的共同点以至于你必须运用一些极好的解释方法来去说明它们不仅仅只是沙丘。

幽灵沙丘是怎样形成的呢?在地球上,幽灵沙丘有可能已经部分地被熔岩或者用水传播的泥沙掩埋。对于这种火星的幽灵沙丘,当岩浆或泥沙变硬时,它们保持沙丘的轮廓。然后,沙丘剩下的上部就会被风侵蚀并将其冲刷走,只留下原来的沙丘“模型”般的轮廓。现在它们看起来像有着坚固边缘的坑洞。

火星上这些沙丘的存在对它很久以前是什么样的状况提供了更多的证据,特别是风。正如戴指出的:

关于幽灵沙丘的一件很酷的事情是它们能够确切地告诉我们火星上的风在形成时的古老过去是和现在截然不同的。那时的风与现在的不同,这一事实告诉我们火星上的环境状况在长时间里不是一成不变的,在过去的20亿年中它们已经改变了,我们需要知道这些事情去解释火星上的地质。

火星幽灵沙丘是人们在希腊平原流域和诺克提斯迷宫的轨道照片中被发现的。它们和2016年在爱达荷州东部发现的蛇江平原相似。戴和合著者大卫·凯特林仅仅在轨道里拍的诺克提斯迷宫的图片中就发现有多于480个潜在的幽灵沙丘模型,在希腊平原上的轨道图片发现有多于300个的沙丘模型。诺克提斯迷宫是一个在太阳系已知最大的峡谷,位于水手号谷西部混乱无序的高原区域。希腊平原是一个位于火星南半球的巨大火山口。它跨越的距离多于1678英里(2700公里)。它的年龄为40亿,是撞击而成的。

火星上的幽灵沙丘的形状是月牙形,就像地球上的新月形沙丘,这意味着原来的沙丘可能会和火星和地球上最普遍的类型新月形沙丘很相像。月牙形的“角”或尖端指着盛行风的方向。这种沙丘往往会在几乎或完全没有植被的平坦地势上形成。每个地点都有那么多坑洞的事实预示着:它们是曾经活跃的沙丘场的遗迹。正如戴指出的:

因为沙丘都是在同一个风的体系里形成、移动,所以你会预料到沙丘都是朝同一个方向的。因此形状和大小就能告诉我们这些是它们来自于远古的沙丘系统的特点。

对火星上幽灵沙丘的分析表明原始的沙丘是很庞大的——在诺克提斯迷宫里大约有130英尺(40米)高,在希腊平原上的大约有246英尺(75米)高。通过比较,好奇号探测器在盖尔撞击坑的夏普山山脚附近研究了一系列的沙丘。纳米布沙丘大概有16英尺(5米)高。

沙丘除了提供极关于火星上古老的环境状况,它们也可能是一个寻找过去生命证据极好的地方。正如在新论文的概要中提到的:

火星上两处古老的沙丘被部分地掩埋,然后被风侵蚀而去,留下保留关于古老环境信息的沙丘状的洞穴。这些坑洞的边缘有可能含有古老沙丘的砂岩,能够成为一个寻找古代生命的证据的好地方。洞穴的形状也告诉我们以前的风是如何运动的。

正如戴也指出的:

我们知道地球上的沙丘能够维持生命,而且地球上的沙丘和火星上的很相似。有一个问题是火星有的但是地球没有的,这是地表的辐射。如果你在一个沙丘里面或者它的底部,而且你是一个有微生物特征的生命,那个沙丘正在保护你接触过多的辐射。现在,有可能没有任何东西生活在那个地方。可是如果火星上曾经有过什么东西,这是一个比普通的地方更好的观察地点。

最下面的一行:沙丘不仅在地球上很常见,它们也被在火星、金星、土星最大的卫星泰坦,甚至在格拉西缅科彗星上。现在另外一种沙丘被人们在火星上发现——形如洞穴的古老‘鬼沙丘’。它们曾经是活跃,大部分都是被侵蚀的沙丘的遗迹。它们和今天的沙丘有极强的相似性,甚至有可能拥有这颗红色行星过去生命的证据。

本文出处:Astronomers find ‘ghost dunes’ on Mars

Robot orbiters circling Mars have acquired images of ghost dunes. They’re pits where, scientists believe, tall crescent-shaped sand dunes once existed on this red desert world.

Mars is a desert planet, a lot like some deserts on Earth, but much colder. Also just like Earth, the Martian deserts have vast dunes, ranging from small sand ripples (technically not dunes) to towering, cliff-like true dunes of fine sand. But – while various types of dunes have been seen from orbit and up close by Mars rovers, currently still active and gradually making their way across the landscape – now another kind of dune has been found on Mars as well. Scientists call these ghost dunes, and they are very ancient. They reported the finding in GeoSpace on July 10, 2018. The new research paper was just published in the Journal of Geophysical Research: Planets.

These ghost dunes are not active dunes today. Rather, they are the remains of previous ancient dunes that left pit-like depressions in the ground after they eroded away. Hundreds of these crescent-shaped pits have been discovered, each about the size of the U.S. Capitol building. As Mackenzie Day, a planetary geomorphologist at the University of Washington in Seattle and an author of the new study, explained:

Any one of these pits is not enough to tell you that it’s a dune, or from an ancient dune field, but when you put them all together, they have so many commonalities with dunes on Mars and on Earth that you have to employ some kind of fantastic explanation to explain them as anything other than dunes.

How do ghost dunes form? On Earth, ghost dunes may have been partially buried by lava or water-borne sediments. For these Martian ghost dunes, when the lava or sediments hardened, they preserved the contours of the dunes. The remaining top portions of the dunes were then eroded away by winds, which scoured them out, leaving only the “mold” outlines of the former dunes. Now they look like pits with hardened edges.

These dunes’ existence on Mars provide more clues as to what conditions were like billions of years ago, in particular, winds. As Day noted:

One of the cool things about the ghost dunes is that they tell us, for sure, that the wind on Mars was different in the ancient past, when they formed. The fact that the wind was different [when the ghost dunes formed] tells us that the environmental conditions on Mars aren’t static over long time scales, they have changed over the past couple billion years, something we need to know to interpret the geology on Mars.

The Martian ghost dunes were found in orbital images of Hellas Planitia basin and Noctis Labyrinthus. They are similar to ones discovered in the Snake River Plain in eastern Idaho in 2016. More than 480 potential dune molds were discovered in orbital images of Noctis Labyrinthus alone, and more than 300 in Hellas Planitia, by Day and co-author David Catling. Noctis Labyrinthus is a region of jumbled plateaus just west of Valles Marineris, the largest-known canyon in the solar system. Hellas Planitia is a massive 4-billion-year-old impact crater over 1,678 miles (2,700 km) across in the southern hemisphere.

The shapes of the ghost dunes on Mars are crescents, just like barchan dunes on Earth, meaning that the original dunes would have been very similar to barchan dunes, the most common type on both Mars and Earth. The “horns” or tips of the crescents point in the direction of the prevailing wind. This type of dunes tends to form on flat terrain where there is little or no vegetation. The fact that there are so many of these pits in each location points to them being the remains of once-active dune fields. As Day noted:

They are all going the same way, which you would expect for dunes because they are all migrating and forming in the same wind regime. So just the shape and size tell us that these are features that are coming from an ancient dune system.

Analysis of the ghost dunes on Mars indicates that the original dunes were quite large – about 130 feet (40 meters) tall at Noctis Labyrinthus and 246 feet (75 meters) tall at Hellas basin. By comparison, the Curiosity rover has studied a series of dunes near the base of Mount Sharp in Gale Crater. The striking Namib Dune is about 16 feet (5 meters) tall.

As well as providing fascinating clues about ancient environmental conditions on Mars, these ghost dunes may also be a good place to search for evidence of past life. As mentioned in the summary in the new paper:

Ancient dunes in two places on Mars were partially buried and then eroded away, leaving behind dune-shaped pits that preserve information about the ancient environment. These pits may contain ancient dune sandstones around the edges of the pits and could be a good place to look for evidence of ancient life. The shapes of the pits also tell us how the winds behaved in the past.

And as Day noted also:

We know that dunes on Earth can support life, and dunes on Earth are very similar to dunes on Mars. One problem that Mars has that Earth doesn’t is the surface radiation. If you are inside a dune, or at the bottom of a dune, and you are microbial life, the dune is protecting you from a lot of that radiation. There is probably nothing living there now. But if there ever was anything on Mars, this is a better place than average to look.

Bottom line: As well as being common on Earth, dunes have also been found on Mars, Venus, Titan and even comet 67P. Now another type of dune has been discovered on Mars – “ghost dunes,” the pit-like remains of ancient, once-active dunes which have mostly eroded away. They bear a strong similarity to present-day dunes and may even hold clues to past life on the red planet.

 

 

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