「双语太空播报」NASA新发现火星新证据：水流时间比想象的要长

NASA's MRO Finds Water Flowed on Mars Longer Than Previously Thought

MRO's View of Salt Deposits in Bosporos Planum: NASA’s Mars Reconnaissance Orbiter used its Context Camera to capture this image of Bosporos Planum, a location on Mars. The white specks are salt deposits found within a dry channel. The largest impact crater in the scene is nearly 1 mile (1.5 kilometers) across.

Mars once rippled with rivers and ponds billions of years ago, providing a potential habitat for microbial life. As the planet’s atmosphere thinned over time, that water evaporated, leaving the frozen desert world that NASA’s Mars Reconnaissance Orbiter (MRO) studies today.

It’s commonly believed that Mars’ water evaporated about 3 billion years ago. But two scientists studying data that MRO has accumulated at Mars over the last 15 years have found evidence that reduces that timeline significantly: Their research reveals signs of liquid water on the Red Planet as recently as 2 billion to 2.5 billion years ago, meaning water flowed there about a billion years longer than previous estimates.

The findings – published in AGU Advances on Dec. 27, 2021 – center on the chloride salt deposits left behind as icy meltwater flowing across the landscape evaporated.

While the shape of certain valley networks hinted that water may have flowed on Mars that recently, the salt deposits provide the first mineral evidence confirming the presence of liquid water. The discovery raises new questions about how long microbial life could have survived on Mars, if it ever formed at all. On Earth, at least, where there is water, there is life.

The study’s lead author, Ellen Leask, performed much of the research as part of her doctoral work at Caltech in Pasadena. She and Caltech professor Bethany Ehlmann used data from the MRO instrument called the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) to map the chloride salts across the clay-rich highlands of Mars’ southern hemisphere – terrain pockmarked by impact craters. These craters were one key to dating the salts: The fewer craters a terrain has, the younger it is. By counting the number of craters on an area of the surface, scientists can estimate its age.

MRO 对博斯普鲁斯平原盐沉积物的看法：美国宇航局的火星勘测轨道飞行器使用其背景相机拍摄了这张火星上的博斯普罗斯平原的图像。 白色斑点是在干燥通道中发现的盐沉积物。 现场最大的撞击坑直径近 1 英里（1.5 公里）。

数十亿年前，火星曾在河流和池塘中泛滥，为微生物生命提供了潜在的栖息地。随着地球的大气层随着时间的推移而变薄，水蒸发了，留下了美国宇航局火星侦察轨道器 (MRO) 今天研究的冰冻沙漠世界。

人们普遍认为，火星的水在大约30亿年前蒸发了。但两名研究MRO在过去15年中在火星积累的数据的科学家发现了显着缩短这一时间线的证据：他们的研究揭示了20亿至25亿年前红色星球上液态水的迹象，这意味着水流过比之前的估计长了大约十亿年。

该研究结果于2021年12月27日发表在AGU Advances 上，主要集中在流经景观的冰冷融水蒸发时留下的氯化物盐沉积物上。

虽然某些山谷网络的形状暗示水可能最近在火星上流动，但盐沉积物提供了第一个证实液态水存在的矿物证据。这一发现提出了新的问题，即火星上的微生物生命可以存活多久，如果它曾经形成的话。至少在地球上，哪里有水，哪里就有生命。

该研究的主要作者Ellen Leask 完成了大部分研究，作为她在帕萨迪纳加州理工学院博士工作的一部分。她和加州理工学院教授 Bethany Ehlmann 使用来自称为火星紧凑型侦察成像光谱仪 (CRISM) 的 MRO 仪器的数据来绘制火星南半球富含粘土的高地的氯化物盐地图——该地形被撞击坑坑坑洼洼。这些陨石坑是确定盐分年代的关键之一：一个地形上的陨石坑越少，它就越年轻。通过计算表面某个区域的陨石坑数量，科学家们可以估计它的年龄。