A Martian rock has lots of carbon on it, and it’s not clear why

火星岩石上发现了大量碳，但来源尚不明确

NASA’s Perseverance rover has spent five years traversing Jezero Crater looking for the chemical leftovers of whatever processes were at work on Mars billions of years ago. The rover has found organic carbon, but it has mostly been inside rocks that had to be drilled or abraded to expose it. 美国国家航空航天局（NASA）的“毅力号”火星车已经在耶泽罗陨石坑（Jezero Crater）跋涉了五年，旨在寻找数十亿年前火星上各种地质过程留下的化学遗迹。火星车此前确实发现过有机碳，但大多存在于需要通过钻探或磨损才能暴露出来的岩石内部。

But now, at an outcrop on the edge of an ancient river channel named Neretva Vallis, Perseverance detected complex macromolecular carbon sitting right on the rock’s surface. “To our knowledge, that’s the shallowest detection of organic matter on Martian surface to date,” said Ashley E. Murphy, a researcher at the Planetary Institute in Tucson, Arizona, and lead author of the study of the rock, which was found at a site called Bright Angel. 然而现在，在一条名为内雷特瓦谷（Neretva Vallis）的古河道边缘的一处露头上，“毅力号”探测到了直接位于岩石表面的复杂大分子碳。“据我们所知，这是迄今为止在火星表面探测到的最浅层的有机物质，”亚利桑那州图森市行星科学研究所的研究员、该岩石研究报告的第一作者阿什利·E·墨菲（Ashley E. Murphy）表示。这块岩石是在一个名为“明亮天使”（Bright Angel）的地点发现的。

On Earth, this much macromolecular carbon usually suggests a biological origin. But to learn what this Bright Angel carbon is and where it came from, we might need to bring samples back to Earth. 在地球上，如此高含量的大分子碳通常暗示着生物起源。但要弄清楚“明亮天使”的这些碳究竟是什么以及它们从何而来，我们可能需要将样本带回地球进行分析。

Carbon on the rocks

岩石上的碳

The detection of Bright Angel carbon came from SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals), a UV Raman spectrometer fitted on Perseverance’s robotic arm. SHERLOC fires a deep-ultraviolet laser at a target and reads the light that bounces back at shifted energies, a signal that enables scientists to identify specific molecular bonds. 对“明亮天使”碳的探测来自“毅力号”机械臂上安装的紫外拉曼光谱仪——SHERLOC（用于有机物和化学品的拉曼及发光扫描宜居环境探测仪）。SHERLOC向目标发射深紫外激光，并读取反射回来的能量发生偏移的光，这种信号使科学家能够识别特定的分子键。

Between sols 1180 and 1218, the rover pointed this UV laser at four targets at Bright Angel. One, called Steamboat Mountain, was an ordinary rock the team used as a control. The remaining three (called Cheyava Falls, Apollo Temple, and Walhalla Glades) returned a spectroscopic signature of macromolecular carbon. This signal, called the graphitic band (G-band), indicates the presence of a tangled, cross-linked network of mostly reduced carbon atoms that is resistant to chemical and thermal breakdown. 在第1180至1218个火星日期间，火星车将这台紫外激光器对准了“明亮天使”的四个目标。其中一个名为“汽船山”（Steamboat Mountain）的岩石被团队用作对照组。其余三个（分别名为“切亚瓦瀑布”、“阿波罗神庙”和“瓦尔哈拉林地”）则返回了大分子碳的光谱特征。这种被称为石墨带（G-band）的信号，表明存在一种由还原碳原子组成的、纠缠且交联的网络，这种结构能够抵抗化学和热分解。

At least within the precision limits of the Perseverance’s instruments, the material roughly matches terrestrial kerogen. Using the word “kerogen,” though, was a no-go, the researchers decided. On Earth, kerogen is made almost exclusively of biological matter, mainly fossilized microbes that were buried millions of years ago. “The term kerogen implies biogenic source,” Murphy explained. “Macromolecular carbon implies we don’t know whether its origin is biotic or abiotic.” 至少在“毅力号”仪器的精度范围内，这种物质与地球上的干酪根（kerogen）大致吻合。然而，研究人员决定不使用“干酪根”这个词。在地球上，干酪根几乎完全由生物物质构成，主要是数百万年前掩埋的微生物化石。“‘干酪根’这个术语暗示了生物来源，”墨菲解释道，“而‘大分子碳’则意味着我们尚不清楚其起源是生物性的还是非生物性的。”

The material found on Martian rocks, Murphy’s team warns, might have originated from non-biological processes as well. A result like this usually invites two major questions, and the team immediately got busy trying to answer them. 墨菲的团队警告说，在火星岩石上发现的这种物质也可能源于非生物过程。这样的结果通常会引出两个主要问题，团队随即开始着手解答。

Artifacts and stowaways

人造伪影与“偷渡者”

The first concern was that the signal could have been light bouncing off SHERLOC’s own fused-silica front window. Bright Angel was the first site SHERLOC examined after a dust-cover anomaly disabled its focusing mechanism, forcing the team to adopt a new operating mode. To characterize the new mode, Kyle Uckert, SHERLOC’s deputy principal investigator at NASA’s JPL, and his colleagues collected spectra from spare flight optics in their own lab. They also pointed SHERLOC at nothing in particular on Mars and at known calibration targets. All these were used to confirm that SHERLOC was working properly. 第一个担忧是，该信号可能是光线从SHERLOC自身的熔融石英前窗反射回来的。“明亮天使”是SHERLOC在防尘罩异常导致其聚焦机制失效、迫使团队采用新操作模式后检查的第一个地点。为了表征这种新模式，NASA喷气推进实验室（JPL）SHERLOC的副首席研究员凯尔·乌克特（Kyle Uckert）及其同事在实验室中收集了备用飞行光学器件的光谱。他们还将SHERLOC对准了火星上并无特殊之处的区域以及已知的校准目标。所有这些测试都证实了SHERLOC工作正常。

The final confirmation of the data came when the team pointed it at Steamboat Mountain. “Other rock targets nearby do not exhibit the G-band spectral signal,” Uckert said. The Bright Angel signal was not coming from hardware. 当团队将仪器对准“汽船山”时，数据得到了最终确认。“附近的其他岩石目标并没有表现出G-带光谱信号，”乌克特说。这说明“明亮天使”的信号并非来自硬件本身。

The second concern was contamination—perhaps the rover itself dragged the organic material from Earth? Scientists pointed out that the abrasion bit the rover used to expose the rocks was sterilized before launch and has cut into other rocks across Jezero without ever producing a G-band this strong. Also, the Cheyava Falls rock was never touched by the hardware; the rover just blew the dust off its surface with a nitrogen puff. On top of that, the Steamboat Mountain scientist used as a control again came up empty. “It did not exhibit spectral evidence of organic matter,” Uckert explained. 第二个担忧是污染——也许是火星车本身从地球带去了有机物质？科学家指出，火星车用于暴露岩石的磨损钻头在发射前经过了灭菌处理，并且在耶泽罗陨石坑切割过其他岩石，从未产生过如此强烈的G-带信号。此外，“切亚瓦瀑布”岩石从未被硬件接触过；火星车只是用氮气喷射吹掉了其表面的灰尘。最重要的是，作为对照组的“汽船山”再次显示为空。“它没有表现出有机物质的光谱证据，”乌克特解释道。

Chemical company

化学成分

Once it was clear that the finding was most likely real, the team took a closer look at the chemistry of the material nearest to the Martian macromolecular carbon. “It suggests the carbon emplacement may have occurred during at least two different events over geologic time,” Murphy said. At Apollo Temple, the signal clustered with carbonate and sulfate minerals–the kind that precipitate out of water moving through older rock. At Walhalla Glades, the carbon instead sat within silicate-rich sediment. 一旦确定这一发现极有可能是真实的，团队便开始仔细研究火星大分子碳附近物质的化学性质。“这表明碳的沉积可能是在地质时间跨度内的至少两次不同事件中发生的，”墨菲说。在“阿波罗神庙”，信号与碳酸盐和硫酸盐矿物聚集在一起——这类矿物通常是从流经古老岩石的水中沉淀出来的。而在“瓦尔哈拉林地”，碳则位于富含硅酸盐的沉积物中。

Murphy sees that split as evidence for at least two separate windows in which carbon could have been locked into these rocks. First, as organic matter settled into mud at the bottom of an ancient lake and was buried alongside the sediment and again when groundwater later moved through this buried rock and left behind new carbonate and sulfate minerals. 墨菲认为这种差异证明了碳被锁定在这些岩石中的至少两个独立时期。首先，有机物质沉淀在古湖底的泥浆中，并与沉积物一起被掩埋；其次，当地下水后来流经这些被掩埋的岩石时，留下了新的碳酸盐和硫酸盐矿物。

In the end, though, the question of whether Bright Angel carbon is a remnant of ancient Martian life will remain open for quite a while. “The science payload of the Perseverance rover was not designed to distinguish between abiotic and biotic processes but to identify compelling rocks to be collected for possible return to Earth,” says Uckert. 不过最终，“明亮天使”的碳是否是古代火星生命的遗迹，这个问题在很长一段时间内仍将悬而未决。“‘毅力号’火星车的科学载荷并非旨在区分非生物过程和生物过程，而是为了识别出值得收集并可能带回地球的岩石，”乌克特说。

The unanswered question

未解之谜

“Perseverance rover has an incredible instrument payload, but those instruments pale in comparison to world-class techniques that could be used to analyze these samples when they get back to Earth,” said Kevin P. Hand, the Perseverance principal investigator at JPL. Hand is especially interested in the isotopic signature of Bright Angle’s carbon, which might provide some indications of life. Another thing he wants to look into is chirality—a preference for one type of molecular handedness over another that is strongly associated with biotic origin. “‘毅力号’火星车拥有令人难以置信的仪器载荷，但与样本带回地球后可使用的世界级分析技术相比，这些仪器就显得相形见绌了，”JPL的“毅力号”首席研究员凯文·P·汉德（Kevin P. Hand）表示。汉德对“明亮天使”碳的同位素特征特别感兴趣，这可能提供一些生命存在的迹象。他想研究的另一件事是手性（chirality）——即对某种分子旋向性的偏好，这通常与生物起源密切相关。