导读
2018年，欧洲航天局火星快车轨道器上的火星地下和电离层探测高级雷达（MARSIS）在南极冰盖下方约1.5公里处探测到一块强反射区域。2021年发表于此的一项关于火星陨石的研究得出结论，它们的矿物组成能够产生足够的氢来支持微生物生存。
On Earth, deep subsurface life extends several kilometers down and may hold a comparable biomass to everything on the surface. Bacteria have been recovered from 5 kilometers deep in continental boreholes. The upper temperature limit for known life is about 122 degrees Celsius (°C).
Could the same be true of Mars? The Martian crust warms with depth at roughly 10–20 °C per kilometer. Starting from a mean surface temperature around −60 °C, the temperature ranges between about 0 and 120 °C at depths of 5 to 10 kilometers, potentially allowing life to survive in liquid water.
In 2018, the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on ESA’s Mars Express orbiter, detected a strongly reflective patch about 1.5 kilometers beneath the south polar cap. Its initial interpretation in terms of a subglacial lake of salty water was later replaced with the understanding that it is more likely a layered volcanic rock. It was also realized that perchlorate salts found by NASA’s Phoenix and Curiosity robotic missions, can pull water vapor from the air and form liquid films at night, a process Curiosity has indirectly detected. These are thin, briny, and fleeting layers, not an aquifer.
However, in 2024 the robotic NASA lander on Mars InSight, measured seismic velocities which revealed mid-crust beneath the lander, about 11–20 kilometers down, which is best explained by fractured rock saturated with liquid water. If that holds globally, the reservoir would be enough to cover Mars with a water depth of 1–2 kilometers, which would account for much of its missing ancient water in its hypothesized ancient lakes and oceans. The supporting evidence was presented in three papers here, here and here. The inference rests on one location and on model assumptions about rock porosity, so it is not conclusive without supporting data from more stations.
The deep Earth biosphere exists without sunlight or surface organics. Its microbes use hydrogen produced when water reacts with iron-rich minerals or when radiation from radioactivity splits water molecules in a process called radiolysis. The crust of Mars is basaltic, iron-rich, and radioactive enough that models find radiolysis alone could sustain microbial populations for billions of years. A 2021 study of Martian meteorites published here, concluded their mineralogy would generate enough hydrogen to support a deep biosphere on Mars comparable to Earth’s.
If life arose on the Martian surface during its wet period of 3.7–4 billion years ago, the deep crust is the one place it could have followed the water as the surface dried. Groundwater would have been connected to surface lakes then, so no migration across dry rock was necessary. In that case, the deep aquifer is not just habitable but is the one environment on Mars that has stayed habitable continuously.
In 2022, a rose-colored, 800-gram meteorite from Mars, labeled Teghaza 001, was discovered in the Sahara Desert in Mali. Its analysis was published here a few days ago and revealed that its age is at least 4.1 billion years old. This rock is a pristine piece of Mars’ primordial crust, providing a rare “Rosetta Stone” window into the Red Planet’s deep geological past. Microscopic analysis showed the rock’s minerals were heavily altered by liquid water about 4.1 billion years ago. This indicates that early Mars had a far more active subsurface water system than previously realized. Trapped hydrogen isotopes inside the rock show that even 4.1 billion years ago, Mars was already rapidly losing its lighter elements to space, hinting that its transition to a cold, dry world began early.
How feasible is it to drill Mars down to a depth of 5 to 10 kilometers in order to reveal whether it has life right now?
The deepest hole ever made is NASA’s Perseverance rover’s sampling cores at a few centimeters. InSight’s heat-flow probe, designed to reach 5 meters down, stalled at about 40 centimeters because the soil did not give it the friction it needed. ExoMars’ Rosalind Franklin rover, now scheduled for 2028, carries a 2-meter drill.
Terrestrial rigs that reach 5 kilometers weigh hundreds of tons and run on megawatts of power, with crews on site to change bits and handle casing. The Kola Superdeep Borehole took 19 years to reach 12 kilometers on Earth, with unlimited human attention and a surface base. Deep holes need to lift the rock debris out and to stop the walls from collapsing. On Earth the drilling mud is circulated by pumps and steel casing strings. Hard basalt erodes bits. Every bit change on Earth means pulling the entire string out and running it back in, which at 5 kilometers means days of work per change. A robot on Mars would have to do this thousands of times.
A simpler approach is to let geology do the digging. Impact craters, canyon walls, and fresh landslides expose material from kilometers down. The Martian Valles Marineris cliffs reach 7 kilometers, and some crater central peaks uplift rock from 5–10 kilometers. A rover at the base of such an exposure can sample deep crust without a drill. The downside is that the rock is dry and ancient by the time it is exposed, so it reveals the deep environment’s history, not whether water and life are there now. The challenge would be to sample deep fluids as soon as they surface. If the deep aquifer ever vents through fresh impacts, volcanic regions, or seepage, surface chemistry would carry its signature. A dedicated mission to a young volcanic or impact site, offers the best prospects for unraveling subsurface life on Mars.
The simplest way to confirm the aquifer is a network of seismic stations. A permanent human base with heavy equipment, local manufacturing of casing and bits, and years of continuous operation, could eventually accomplish the required drill and unravel not only water or life but also oil (as discussed here) that could power the local technological infrastructure.
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在地球上，深层地下生命可以延伸到地下数公里深处，其生物量可能与地表生物总量相当。人们曾在大陆钻孔5公里深处发现过细菌。已知生命存在的最高温度约为122摄氏度（°C）。
火星的情况是否也如此？火星地壳随深度增加而升温，大约每公里升温 10–20 摄氏度。平均表面温度约为 -60°C，在 5 至 10 公里的深度，温度范围约为 0 至 120°C，这可能使生命能够在液态水中生存。
2018年，欧洲航天局火星快车轨道器上的火星地下和电离层探测高级雷达（MARSIS）在南极冰盖下方约1.5公里处探测到一块强反射区域。其初步解释最初认为它是冰下咸水湖的观点后来被另一种观点所取代，即它更可能是层状火山岩。人们还意识到，NASA的凤凰号和好奇号火星探测器发现的高氯酸盐可以从空气中吸收水蒸气，并在夜间形成液膜，好奇号间接探测到了这一过程。这些液膜很薄，含盐量高，而且转瞬即逝。含水层。
然而，2024年，美国宇航局的火星探测器“洞察号”（InSight）测量了火星地壳中部的地震波速度，揭示了着陆器下方约11-20公里处存在地壳中层，这最合理的解释是富含液态水的裂隙岩石。如果这一解释在全球范围内成立，那么这个水库足以覆盖火星表面，形成1-2公里深的水层，这将解释火星上大部分缺失的古代地层。在其假定的古代湖泊和海洋中发现了水。相关证据发表在三篇论文中（链接1、链接2和链接3）。该推断基于一个地点和关于岩石孔隙度的模型假设，因此在没有更多站点的支持数据之前，尚不能得出确切结论。
地球深层生物圈在没有阳光或地表有机物的情况下存在。其中的微生物利用水反应产生的氢气生存。火星地壳富含铁质矿物，或者放射性物质产生的辐射会分解水分子，这一过程称为辐射分解作用。火星地壳由玄武岩构成，富含铁，且放射性极强，模型显示，仅辐射分解作用就足以维持微生物种群数十亿年。2021年发表于此的一项关于火星陨石的研究得出结论，它们的矿物组成能够产生足够的氢来支持微生物生存。火星上存在与地球相当的深厚生物圈。
如果生命起源于37亿至40亿年前火星表面湿润时期，那么随着地表干涸，深层地壳是唯一能让生命沿着水源迁徙的地方。当时的地下水与地表湖泊相连，因此生命无需穿越干燥的岩层。在这种情况下，深层含水层不仅适宜生命生存，而且是唯一可能孕育生命的地方。火星上持续保持宜居状态的居住地。
2022年，一块重800克的玫瑰色火星陨石，被命名为Teghaza 001，在马里撒哈拉沙漠被发现。几天前，相关分析结果在此发表，显示其年龄至少为41亿年。这块岩石是火星原始地壳的一块完整碎片，为我们了解火星提供了一扇罕见的“罗塞塔石碑”。火星深厚的地质历史。显微分析表明，这块岩石中的矿物在大约41亿年前曾被液态水强烈蚀变。这表明早期火星的地下水系统比之前认为的要活跃得多。岩石内部捕获的氢同位素表明，早在41亿年前，火星就已经开始迅速地向太空释放较轻的元素，这暗示着……它向寒冷干燥世界的过渡很早就开始了。
将火星钻探至 5 至 10 公里深处，以揭示火星上是否存在生命，这种可行性如何？
迄今为止最深的孔是NASA“毅力号”火星车采集的岩芯样本，深度仅几厘米。“洞察号”火星车的热流探测器设计深度为5米，但由于土壤摩擦力不足，在约40厘米处停止了探测。它需要的钻探设备。ExoMars 的罗莎琳德·富兰克林号火星车，目前计划于 2028 年发射，它携带了一个 2 米的钻头。
可达5公里的陆地钻机重达数百吨，耗费兆瓦级电力，现场配备人员更换钻头和处理套管。科拉超深钻孔耗时19年才钻至12公里深，期间需要大量人力维护，并设有地面基地。深井需要提升……清除岩石碎屑并防止岩壁坍塌。在地球上，钻井泥浆由泵和钢套管循环输送。坚硬的玄武岩会侵蚀钻头。在地球上，每次更换钻头都意味着要将整套套管拔出并重新安装，5公里长的套管意味着每次更换都需要数天时间。火星上的机器人则需要重复数千次这样的操作。
更简单的方法是让地质学来完成这项工作。撞击坑、峡谷壁和新近发生的滑坡暴露了数公里深的物质。火星水手谷的悬崖高达7公里，一些撞击坑中心的山峰将5-10公里深的岩石抬升起来。探测车在这样的暴露区域底部无需钻探即可采集深层地壳样本。缺点是，岩石暴露出来时已经干燥且年代久远，因此它揭示的是深层环境。关键在于探寻火星地表下是否存在水和生命，而非其历史。挑战在于如何尽快采集到深层流体的样本。如果深层含水层通过新近形成的撞击坑、火山区或渗漏点喷发，地表化学成分将带有其特征。专门前往年轻的火山或撞击坑的探测任务，是揭开火星地下生命之谜的最佳途径。
最简单的方法是为了确认含水层是否为地震台网，需要建立一个配备重型设备的永久性人类基地，并在当地生产套管和钻头，并进行多年的持续作业。这样，最终或许能够完成所需的钻探，不仅能找到水源或生命迹象，还能找到石油（如本文所述），从而为当地的技术基础设施提供动力。
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https://lweb.cfa.harvard.edu/~loeb/
https://avi-loeb.medium.com/https://www ... sorAviLoeb
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原文链接：https://avi-loeb.medium.com/is-there-li ... 3c783224a5
作者：Avi Loeb（哈佛大学天文系教授）
