好奇号探测器暗示火星上的碳循环

Curiosity rover hints at the carbon cycle on Mars

The Mars Science Laboratory carried by the Curiosity rover is still functioning 10 years after landed. It includes a system aimed at scooping and drilling samples of soil and rock from the sedimentary strata deposited in the lake that once filled the crater about 3.5 to 3.8 billion years ago. The system on the rover is also capable of analysing the samples in various ways. A central objective of the mission was to obtain data on oxygen and carbon isotopes in carbon dioxide and methane released by heating samples, which uses a miniature mass spectrometer. In early 2022 a paper on Martian carbon isotopes was published in the Proceedings of the National Academy of Sciences (PNAS) (House, C.H. et al. 2022. Depleted carbon isotope compositions observed at Gale crater, Mars. Proceedings of the National Academy of Sciences, v. 119, article e2115651119; DOI: 10.1073/pnas.2115651119). PNAS deemed it to be one of the 12 most important of its articles during 2022.

好奇号火星车携带的火星科学实验室在登陆火星10年后仍然在运作。它包括一个系统，旨在从约35亿至38亿年前曾经填满火山口的湖泊沉积中挖出和钻取土壤和岩石样本。好奇号火星车上的系统还能够以各种方式分析这些样品。该任务的一个核心目标是获取加热样品所释放的二氧化碳和甲烷中的氧和碳同位素的数据，这需要使用一个微型质谱仪。2022年初，一篇关于火星碳同位素的论文发表在《美国国家科学院院刊》（PNAS）上（House, C.H. et al. 2022. Depleted carbon isotope compositions observed at Gale crater, Mars. Proceedings of the National Academy of Sciences, v. 119, article e2115651119; DOI: 10.1073/pnas.2115651119）。PNAS认为它是2022年期间其文章中最重要的12篇之一。

论文标题、作者和单位（编者加）

Oblique view of Curiosity's landing site in Gale crater on Mars, from which the rover has traversed the lower slopes of Mount Sharp. Credit: NASA-Jet Propulsion Laboratory

好奇号在火星盖尔火山口的着陆点的斜视图，探测器从这里穿越了夏普山的低坡。资料来源：美国宇航局-喷气推进实验室

Carbon isotopic analyses chart the type and degree of fractionation between carbon's two stable isotopesC and C. This is expressed by their relative abundances to one another in a sample and in a reference standard, signified by δC. The measure is a natural tracer of both inorganic and biological chemical processes: hence the potential importance of the paper by Christopher House and colleagues from the University of California, San Diego. The thin atmosphere of Mars contains both CO and traces of CH, so a carbon cycle is part and parcel of the planet's geochemical functioning. The 'big question' is: Did that involve living processes at any stage in the distant past and even now? Carbon held in various forms within Mars's ancient rocks and soils may provide at least a hint, one way of the other. At the very least it should say something about the Martian carbon cycle.

碳同位素分析显示了碳的两种稳定同位素C和C之间的分馏类型和程度。这由它们在样品和参考标准中的相对丰度来表示，用δC表示。该值是无机和生物化学过程的自然追踪器。因此，来自加利福尼亚大学圣地亚哥分校的Christopher House及其同事的论文具有潜在的重要性。火星稀薄的大气层包含二氧化碳和微量的甲烷，因此，碳循环是该星球地球化学运作的一部分和组成部分。“大问题”是：在遥远的过去甚至现在的任何阶段，这是否涉及生命过程？在火星的古代岩石和土壤中以各种形式存在的碳可能至少提供了一个线索，不管是哪种方式。至少它应该对火星的碳循环有所说明。

House et al. focus on methane released by heating 22 samples drilled from sandstones and mudstones traversed by Curiosity up a slope leading from the floor of Gale crater towards its central peak, Mount Sharp. The sampled sedimentary rocks span a 0.5 km thick sequence. Carbon in the expelled methane has δC values that range from -137 to +22 ‰ (per mil). Samples from six possibly ancient exposed surfaces were below -70 ‰. This depletion in C is similar to the highly negative δC that characterises carbon-rich sediments on Earth that were deposited at the Palaeocene-Eocene boundary. That anomaly is suspected to have resulted from releases of methane from destabilised gas hydrate on the sea floor during the Palaeocene–Eocene Thermal Maximum. Organic photosynthesis takes up 'light' C in preference to C, thereby imparting low δC to organic matter. In the case of the Mars data that might seem to point to the lake that filled Gale crater 3.5 to 3.8 billion years ago has contained living organisms of some kind. Perhaps on exposed surfaces of wet sediment primitive organisms consumed methane and inherited its δC. Some Archaean sediments of about the same age on Earth show similar C depletion associated with evidence for microbial mats that are attributed to the activities of such methanotrophs.

House等论文重点研究了好奇号从盖尔火山口底部通往中央山峰夏普山的斜坡上钻出的22个砂岩和泥岩样品加热后释放的甲烷。这些采样的沉积岩跨越了0.5公里厚的序列。释放的甲烷的δC值在-137到+22‰之间。来自六个可能的古代出露表面的样品低于-70‰。这种亏损的C与地球上沉积在古新世-始新世边界的富碳沉积物的高度负δC的特征相似。这种异常现象被怀疑是在古新世-始新世极热事件期间海底不稳定的天然气水合物释放甲烷所致。有机物的光合作用吸收了"轻"C而不是C，从而使有机物的δC很低。对火星数据而言，这似乎表明35至38亿年前填充盖尔火山口的湖泊含有某种生物体。也许在潮湿沉积物的暴露表面，原始生物消耗了甲烷并继承了其δC。地球上一些年龄差不多的太古代的沉积物显示出类似的亏损的C，与微生物席的证据有关，这些微生物席被归因于这种甲烷营养体的活动。

Geologic context of samples included in this study. (A) Stratigraphic column with labels for each of the MSL drill sites. (B) Moray_Firth Mastcam mosaic (mcam14053) from Sol 2685 showing Greenheugh pediment near the location of the EB drill hole, which was drilled on top. (C) HU drill hole in the Glasgow member of the Murray formation just below the Greenheugh pediment. (D) HF drill hole in gray-colored Jura member Murray mudstone at the top of the VRR. (E) Namib dune of the Bagnold dunes where the GB sample was taken. (F) Yellowknife Bay locality where the CB drill hole was drilled into mudstone of the Sheepbed member of the Bradbury group rocks.

本研究中包括的样品的地质背景。(A) 带标签的地层柱，为每个MSL钻探点。(B) Sol 2685的Moray_Firth Mastcam（mcam14053），显示了在EB钻孔位置附近的Greenheugh岩层，该钻孔是在上面钻的。(C) 在Greenheugh岩层的Glasgow段的HU钻孔，就在Greenheugh岩层下面。(D) 位于VRR顶部的灰色Jura段Murray组中的HF钻井。(E) 采集GB样本的Bagnold沙丘的Namib沙丘。(F) Yellowknife Bay地点，CB钻井在Bradbury组岩石的Sheepbed段的泥岩中钻探。（编者加）

Before exobiologists become too excited, no images of possible microbial mats in Gale crater sediments have been captured by Curiosity. Moreover, there are equally plausible scenarios with no recourse to once-living organisms that may account for the carbon-isotope data. Extreme depletion in C is commonly found in the carbon within meteorites, almost certainly inherited from the interstellar dust from which they accreted. It is estimated that the solar system passes through giant molecular clouds every 100 Ma or so: the low δC may be inherited from interstellar dust. Alternatively, because Mars has an atmosphere almost entirely composed or CO – albeit thin at present – various non-biological chemical reactions driven by sunlight or electrically charged particles may have reduced that gas to form methane and other compounds based on C-H bonds. Carbon dioxide still in Mars’s atmosphere is highly enriched in C, suggesting that earlier abiotic reduction may have formed C-depleted methane that became locked in sediments. Yet such an abundant supply of inorganic methane may have encouraged the evolution of methanotrophs, had life emerged on early Mars. No one knows …

在外星生物学家们过于兴奋之前，好奇号还没有捕捉到盖尔陨石坑沉积物中可能的微生物席的图像。此外，还有另外同样合理的可能的情形，不需要求助于曾经的生物体，就可以解释碳同位素数据。陨石中的碳普遍存在着极度亏损的C，几乎可以肯定的是，它们是从星际尘埃中吸积而来的。据估计，太阳系每隔一亿年左右就会穿过巨大的分子云：低δC可能是从星际尘埃中继承的。另外，由于火星的大气层几乎完全由二氧化碳组成--尽管目前很稀薄--由阳光或带电粒子驱动的各种非生物化学反应可能已经减少了这种气体，形成甲烷和其他基于C-H键的化合物。仍在火星大气中的二氧化碳在C中高度富集，这表明早期的非生物还原可能已经形成了C耗尽的甲烷，并被圈闭在沉积物中。然而，如果早期火星上出现了生命，如此丰富的无机甲烷供应可能会促进甲烷营养体的进化。没有人知道...

Notional geologic history for Gale crater progressing from left to right starting with (A) a lacustrine environment. (B) After deposition, the lacustrine mudstones and sandstones were later exposed and eroded to produce the unconformity between the Mount Sharp group rocks and the Stimson formation. (C and D) The Stimson sandstone has also been eroded to produce a paleosurface.

盖尔火山口的可能的地质历史，从左至右，从（A）湖泊环境开始。(B) 沉积后，湖泊泥岩和砂岩后来被暴露出来并被侵蚀，造成了夏普山群和Stimson组之间的不整合。(C和D）Stimson砂岩也被侵蚀，产生了古平面。（编者加）

It's becoming a cliché that, ‘We may have to await the return of samples from the currently active Perseverance rover, or a crewed mission at some unspecifiable time in the future. The Curiosity carbon-isotope data keep the lamp lit for those whose livelihoods have grown around humans going to the Red Planet.

这已经成为一种陈词滥调，"我们可能不得不等待目前正在活动的 "毅力号 "火星车的样本返回，或者在未来某个不确定的时间进行载人任务。好奇号的碳同位素数据为那些以人类前往红色星球为生的人保留了一盏明灯。

英文来自博客https://earthlogs.org/ 日期2023年1月18日。特此致谢。中文由编辑部组织翻译，水平有限，敬请见谅。

为更好地为地学同行服务，对本公众号有关的意见与建议，欢迎大家通过沉积之声邮箱：chenjizhisheng@126.com与我们交流、交换意见。

主编 | 胡修棉

作者 | Steve Drury

美编 | 陈凤婷