Rare meteorite from Mars gives us a view into its ancient crust

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Crater counts tell us that most of the Martian surface is more than 3 billion years old, but pieces of Mars we have on Earth tell a different story. The three main classes of Martian meteorites—shergottites, nakhlites, and chassignites—are all 2.4 billion years old or younger. Until now, only two genuinely ancient Martian meteorites were known: Allan Hills 84001, found in Antarctica, and a jumbled rock from northwest Africa nicknamed Black Beauty. Now we have a third one.

陨石坑计数表明，火星表面的大部分区域已有超过 30 亿年的历史，但我们在地球上发现的火星碎片却讲述了不同的故事。火星陨石的三大主要类别——辉熔长无球粒陨石、辉橄无球粒陨石和纯橄无球粒陨石——其年龄都在 24 亿年或更短。到目前为止，人类已知的真正古老的火星陨石只有两颗：在南极洲发现的 Allan Hills 84001，以及在非洲西北部发现的一块被称为“黑美人”（Black Beauty）的杂乱岩石。现在，我们有了第三颗。

A team led by Yang Liu, a planetary scientist at NASA’s Jet Propulsion Laboratory, has shown that a Martian meteorite called Teghaza 001, found in the Sahara Desert in Mali in 2022, formed at least 4.1 billion years ago. “It’s very old and similar to some of the oldest rocks on Earth,” Liu says.

由美国国家航空航天局（NASA）喷气推进实验室的行星科学家刘洋（音译）领导的团队研究表明，2022 年在马里撒哈拉沙漠发现的一颗名为 Teghaza 001 的火星陨石，形成于至少 41 亿年前。刘说：“它非常古老，与地球上一些最古老的岩石相似。”

“We don’t fully understand why old Martian rocks are so rare on Earth,” Liu said. Part of the answer may lie in how a piece of Mars gets here. It takes a large impact to launch a rock off the planet, and not every rock can survive that. “If the impact hits into loose powder, it may not get cemented to become a rock and fly off Mars’ gravity,” Liu said. “So the natural impact already acts as a filter. Weak rocks do not make it off Mars.”

“我们并不完全理解为什么古老的火星岩石在地球上如此罕见，”刘说。部分原因可能在于火星碎片到达地球的方式。需要巨大的撞击才能将岩石从火星表面抛射出去，并非每一块岩石都能在撞击中幸存。“如果撞击发生在松散的粉末上，它可能无法凝结成岩石并脱离火星引力，”刘说。“因此，自然撞击本身就起到了过滤作用。脆弱的岩石无法离开火星。”

That left scientists with a biased collection. “Even to date, none of our collected Martian meteorites are similar to what we found on the surface,” Liu said. The partial exception is Black Beauty, which has a bulk chemistry that matches orbital measurements. But Black Beauty is a mix of soil and many small rock fragments. Teghaza 001 is a single, intact igneous rock.

这导致科学家们所收集的样本存在偏差。“即使到今天，我们收集到的火星陨石中，也没有一颗与我们在火星表面发现的岩石相似，”刘说。部分例外是“黑美人”，其整体化学成分与轨道测量结果相符。但“黑美人”是土壤和许多小岩石碎片的混合物。而 Teghaza 001 是一块完整、单一的火成岩。

Teghaza 001 was acquired through the desert meteorite trade, where hunters comb the Sahara are for dark stones that stand out against the sand. Its eight stones, weighing just under 800 grams in total, were bought by a private collector, who sent a piece to the University of New Mexico for classification. There, planetary scientists Carl Agee and Karen Ziegler measured its oxygen isotopes. “Each planetary body has a very characteristic oxygen isotope value,” Liu says. Teghaza 001’s values matched those of other Martian meteorites.

Teghaza 001 是通过沙漠陨石贸易获得的，陨石猎人在撒哈拉沙漠中搜寻在沙地上格外显眼的深色石头。这块陨石共有八块，总重量略低于 800 克，被一位私人收藏家买下，并送往新墨西哥大学进行分类。在那里，行星科学家卡尔·阿吉（Carl Agee）和凯伦·齐格勒（Karen Ziegler）测量了它的氧同位素。刘说：“每个行星天体都有非常独特的氧同位素值。”Teghaza 001 的数值与其他火星陨石相符。

At first, the rock was classified as a shergottite, the most common (and young) type of Martian meteorite. But Agee noticed some oddities and reached out to Liu. One of Teghaza’s minerals had been altered on the inside while its outer part stayed pristine, the reverse of how weathering on Earth works. The oxygen isotopes looked a bit strange, and there was some manganese-rich material similar to what Liu had studied in Black Beauty. “When I got to the sample, I looked around, I said, ‘That’s boring,’” Liu said. “The high-manganese material is actually terrestrial, but I quickly realized the sample is very different, very special.”

起初，这块岩石被归类为辉熔长无球粒陨石，这是最常见（且年轻）的火星陨石类型。但阿吉注意到了一些异常情况，并联系了刘。Teghaza 的一种矿物内部发生了改变，而外部却保持完好，这与地球上的风化作用正好相反。氧同位素看起来有些奇怪，而且其中含有一些类似于刘在“黑美人”中研究过的富锰物质。“当我拿到样本时，我环顾四周，说：‘这太无聊了，’”刘说。“高锰物质实际上是地球上的，但我很快意识到这个样本非常不同，非常特别。”

Teghaza 001 is made mostly of feldspar, a pale mineral common in the crusts of rocky planets. It has darker pyroxene crystals mixed in; according to Liu, those resemble some rocks exposed in the lunar highlands. Its crystals are rather big, up to a millimeter across, which scientists interpret as an indication it cooled down slowly. “For slow-cooled rock, they tend to have time to grow bigger,” Liu said. By comparing the crystals with lab experiments and terrestrial rocks, the team estimated Teghaza 001 cooled by roughly 0.01° Celsius per year, indicating it formed from magma that never erupted but instead solidified inside the crust.

Teghaza 001 主要由长石组成，这是一种在岩石行星地壳中常见的浅色矿物。其中混合了颜色较深的辉石晶体；据刘介绍，这些晶体类似于月球高地上露出的一些岩石。它的晶体相当大，直径可达一毫米，科学家认为这表明它冷却得很慢。“对于缓慢冷却的岩石，它们往往有时间长得更大，”刘说。通过将这些晶体与实验室实验和地球岩石进行比较，研究小组估计 Teghaza 001 的冷却速度约为每年 0.01 摄氏度，这表明它是由从未喷发、而是在地壳内部凝固的岩浆形成的。

The team dated the rock in several independent ways. Zircon grains gave about 4.11 billion years. Phosphate minerals and rubidium-strontium dating gave similar ages. “Doesn’t matter what type of zircon we are analyzing or what mineral we are analyzing, we get this very consistent age. And we didn’t expect that,” Liu said. It is notoriously hard to reset the age of zircons, and the impact that launched the rock, while strong enough to forge high-pressure minerals, wasn’t hot enough to do it. “To change it, you really require some kind of hydrothermal fluid,” Liu said.

研究小组通过几种独立的方法对这块岩石进行了测年。锆石颗粒显示其年龄约为 41.1 亿年。磷酸盐矿物和铷-锶测年法得出了相似的年龄。“无论我们分析的是哪种类型的锆石或哪种矿物，我们都得到了非常一致的年龄。这是我们没想到的，”刘说。众所周知，锆石的年龄很难重置，而将岩石抛射出来的撞击虽然足以形成高压矿物，但其温度不足以改变锆石的年龄。“要改变它，确实需要某种热液，”刘说。

Sodium-rich rims on feldspar grains and sodium trapped in the zircons support the idea that hot water percolated through the rock shortly after it formed. So 4.1 billion years marks its last soaking, the youngest of what were probably several alteration events. “The crystallization age would be older than 4.1 billion years,” Liu said.

长石颗粒上富含钠的边缘以及锆石中捕获的钠，支持了岩石形成后不久就有热水渗入的观点。因此，41 亿年标志着它最后一次被浸泡，这可能是多次蚀变事件中最晚的一次。“结晶年龄应该比 41 亿年更早，”刘说。

Teghaza 001 is also unusually rich in oxygen-18, a heavier form of oxygen, with about two parts per thousand more than other Martian meteorites. Neither crystallization nor gas escaping from magma can produce a difference that big. Because water that reacts with rock at low temperatures near the planet’s surface forms minerals rich in heavy oxygen, the team thinks the magma either swallowed bits of crust that had already interacted with surface water or reacted directly with fluids that had picked up the same heavy signature.

Teghaza 001 还异常富含氧-18（一种较重的氧同位素），其含量比其他火星陨石高出约千分之二。结晶作用或岩浆中逸出的气体都无法产生如此大的差异。由于在行星表面附近低温下与岩石反应的水会形成富含重氧的矿物，研究小组认为，岩浆要么吞噬了已经与地表水相互作用的地壳碎片，要么直接与携带了相同重氧特征的流体发生了反应。

This implies that “There was active circulation of water through Martian crust,” Liu said. “When this rock was forming, we do have a hydrosphere on Mars, and that hydrosphere is reacting with Martian crust intensely.” The water must have been circulating through the crust and reacting with magma before it had even finished cooling. “It’s actually more dynamic and more active than we thought,” Liu said. She argues that this finding fits the picture of a wet early Mars and points to potentially habitable environments in its crust.

这意味着“火星地壳中存在活跃的水循环，”刘说。“当这块岩石形成时，火星上确实存在水圈，而且该水圈正在与火星地壳发生剧烈反应。”水一定是在岩浆冷却完成之前就在地壳中循环并与岩浆发生了反应。“它实际上比我们想象的更具动态性和活跃性，”刘说。她认为，这一发现符合早期火星潮湿的图景，并指向其地壳中潜在的宜居环境。

Teghaza 001 also reveals details about Mars’ interior. The rock crystallized near the surface, but the magma it formed from came from much deeper. Ratios of trace elements that resist alteration, like the zirconium to titanium ratio, differ from those in every other Martian meteorite. The team’s melting models matched the rock’s chemistry best when its mantle source contained garnet, a mineral that is stable in the Martian mantle on.

Teghaza 001 还揭示了有关火星内部的细节。这块岩石在靠近地表的地方结晶，但它形成的岩浆来自更深处。抗蚀变的微量元素比例（如锆钛比）与所有其他火星陨石都不同。当研究小组的熔融模型假设其地幔源含有石榴石（一种在火星地幔中稳定的矿物）时，模型与岩石的化学成分最为吻合。