Mashable 04月22日 18:14
A NASA rover just exposed something on Mars that eluded orbiters
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NASA的好奇号火星车在火星岩石样本中发现了丰富的菱铁矿,一种铁碳酸盐矿物。这一发现为火星曾拥有浓厚二氧化碳大气层的理论提供了关键证据,可能支持了火星过去存在海洋、湖泊和河流的推论。研究表明,火星地壳中储存的碳含量比此前认为的要多。好奇号在盖尔陨石坑的夏普山钻取的岩石样本中发现了菱铁矿,表明这些岩石可能形成于平静的水域,如湖床。这一发现有助于科学家们了解火星的演化历史,以及其过去是否具备孕育生命的环境条件。

🔍 发现菱铁矿:好奇号火星车在盖尔陨石坑的岩石样本中发现了大量的菱铁矿,这是一种铁碳酸盐矿物,对研究火星大气层演化具有重要意义。

💧 菱铁矿的形成条件:菱铁矿在地球上通常在特定化学条件下形成,需要水、铁和二氧化碳。在火星上发现菱铁矿,暗示了火星过去可能存在液态水,以及富含二氧化碳的大气层。

⏳ 矿物形成的时间顺序:通过对岩石样本的分析,科学家们推断,菱铁矿可能形成于火星湖泊逐渐干涸的过程中。这一过程揭示了火星过去活跃的碳循环,以及大气层随时间推移的变化。

🌍 潜在的碳储存:如果在火星其他富含硫酸盐的区域也发现类似的碳酸盐矿物,可能意味着火星地壳中储存了大量的碳,甚至可能超过了今天火星大气层中的二氧化碳含量。

A NASA rover taking rock samples on Mars has uncovered a plentiful mineral that was invisible to orbiters studying the Red Planet from space.

Scientists say the discovery of siderite, a type of iron carbonate, could be crucial evidence to support the theory that Mars once had a thick carbon dioxide-rich atmosphere, allowing a warm enough environment to support oceans, lakes, and streams.  

Curiosity, a car-sized lab on six wheels, performed a chemical analysis of four rock samples drilled at different elevations of Mount Sharp, a mountain it has been exploring within Gale Crater. Three of the samples showed considerable amounts of siderite. Another sample, which had no significant traces of siderite, contained other iron-rich minerals that can form as siderite breaks down. 

This iron carbonate mineral is known to form on Earth under specific chemical conditions involving water, iron, and carbon dioxide. The study, published in the journal Science, suggests more carbon is stored in the Martian crust than previously thought. And if similar carbonates exist in other sulfate-rich regions, they could represent a hidden trove of Mars’ ancient atmosphere.

"The discovery of abundant siderite in Gale Crater represents both a surprising and important breakthrough in our understanding of the geologic and atmospheric evolution of Mars," said Benjamin Tutolo, lead author of the paper, in a statement.

NASA's Curiosity rover snaps a selfie image on lower Mount Sharp in Gale crater in August 2015. Credit: NASA / JPL-Caltech / MSSS

The results contribute to mounting evidence that ancient Mars had the right chemical and environmental conditions not only to have liquid water but also to trap and cycle carbon in the air — factors that may speak to the planet’s past habitability.

Scientists have had a long-standing theory that Mars used to have surface water. But for that to happen, the planet also would have needed to be warmer, with higher air pressure. That has led them to believe that though Mars' atmosphere is extremely thin today, it must have been thick and carbon dioxide-rich in the past. 

Volcanoes could have released large amounts of carbon dioxide into the air. Over time, some of that gas escaped into space, but enough probably stayed to support lakes and rivers. 

Over the past three decades, researchers have found lots of evidence that water flowed on ancient Mars. But up until now there's been a missing puzzle piece for the atmosphere within the rock record: Carbon dioxide in the air and water almost certainly would have reacted with rocks to create various carbonate minerals, so where are they?

At a Martian site nicknamed Ubajara, NASA's Curiosity rover discovers siderite, an iron carbonate mineral that might solve a mystery about how the planet lost its thicker atmosphere. Credit: NASA / JPL-Caltech / MSSS

After drilling less than 2 inches below the surface, Curiosity used its CheMin instrument to conduct X-ray diffraction analyses of rock and soil samples, according to the new paper. The presence of siderite in them means the rocks likely formed in calm water like lakebeds, not volcanoes or lava. On Earth, siderate tends to form in shallow lakes and swamps. 

Curiosity also detected sulfates, minerals that form when water evaporates. Geologists glean clues about a planet's past from the order in which minerals formed. That siderite came first in the sequence suggests a gradual drying of ancient Martian lakes, leaving behind these other minerals. The sample that didn't have siderite but had evidence of its breakdown materials supports the notion that Mars’ carbon cycle used to be active but became unbalanced over time.

"Drilling through the layered Martian surface is like going through a history book," said Thomas Bristow, a NASA research scientist and co-author of the paper. "Just a few centimeters down gives us a good idea of the minerals that formed at or close to the surface around 3.5 billion years ago."

If similar carbonates are found in other sulfate-rich layers across Mars, they could hold large amounts of carbon — perhaps equal to or even more than the carbon dioxide in Mars’ air today. Future observations could confirm these findings and illuminate how the planet changed as it lost its atmosphere.

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