New study casts doubt on theory about the role comets played in kick-starting life on Earth

The popular theory that comets were crucial in kick-starting life on Earth billions of years ago may not hold up, according to new computer simulations that counter decades of scientific thinking.

The finding suggests the fragile organic molecules supposedly carried by the icy objects would have been destroyed on impact, upending an established origin of life theory that dates back to at least 1961 . The broader idea that the ingredients for life arrived from space, known as panspermia , is even older and was scientifically postulated in 1908 .

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Life on Earth requires complex organic molecules, but it’s unclear whether our early planet had enough of the raw materials needed for prebiotic chemistry — the chemical reactions that produce the building blocks of life. Comets are made of dust and ice but are also known to contain organic material. This composition, which has earned the celestial bodies the nickname “dirty snowballs,” prompted scientists to suggest that comets could have deposited water and organic molecules on Earth’s surface.

Early in its history — over 4 billion years ago — Earth may have experienced a period of heavy bombardment from more than 100,000 comets larger than 100 meters (328 feet), according to the new study, published Wednesday in the journal Scientific Reports . Those impacts, however, were mostly too heavy for life-starting molecules to withstand.

The team used the latest models of comet formation and evolution to simulate this early planetary period 100,000 times. “Given what we know about surface environments on the early Earth, on average, our simulations found that there were no successful impacts,” said lead study author Richard Anslow, who conducted the research while completing his doctorate at the Institute of Astronomy of the University of Cambridge in the United Kingdom.

The comets contained large concentrations of key molecules needed for life, but the simulation found the objects were coming in too fast to successfully deposit them. “The typical velocities of these comets when they impact the Earth meant that actually very few, if any, would have been able to deliver intact organic molecules, so comets actually don’t seem to be as useful for prebiotic chemistry as was previously thought,” Anslow said.

More precisely, 54% of the simulations showed no impact at all, while the remaining 46% had at least one and a “tiny minority” returned up to three collisions, Anslow said. “We can’t rule out confidently that there was never a successful impact — there could have been one of these fluke impacts.” However, such flukes had to occur in a “perfect location” that already met several important conditions, including the presence of liquid water and other chemicals needed to support life. That combination makes the overall process unlikely, he added.

The study focuses on a naturally occurring chemical called hydrogen cyanide or HCN, an important precursor in prebiotic chemistry that has been detected in dozens of comets, including the notable Halley’s and Hale-Bopp comets.

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Composed of hydrogen, carbon and nitrogen, HCN can lead to the formation of amino acids — molecules that make up proteins — when combined with water; other studies have previously identified HCN as a potential key ingredient for the origin of life on Earth.

“When comets strike Earth, they’re shocked to high temperatures and pressures, and recent studies have found that a lot of these precursor molecules can be destroyed by this high pressure and temperature as well as the following impacts,” Anslow said.

The study authors argue that even if a successful impact occurred, it’s unlikely that it would have delivered the chemical in a useful concentration, and in the case of an ocean impact, it would have quickly dissolved.

NASA Jet Propulsion Laboratory/California Institute of Technology

To set the stage for prebiotic chemistry to occur, many other precursor molecules are also needed, such as compounds that contain phosphorus and sulfur, but these are not routinely found in comets, Anslow said. “Comets are really good for delivering hydrogen cyanide, but given it’s random where a comet impacts onto Earth, we’re not going to be guaranteed to have everything else that we need for this chemistry right in that spot.”

If comets didn’t deliver the building blocks of life on Earth, Anslow said there are several other promising avenues to explore. Another long-held , competing theory is the possibility that Earth’s primordial soup could have synthesized the molecules needed for life on its own. An alternative cosmic source of organic material could have been asteroids, which typically struck Earth more slowly and in greater numbers. Anslow pointed to recent findings on samples from asteroids Bennu and Ryugu, which contain the majority of protein-building amino acids.

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The recent study also raises another intriguing possibility: While Earth may not have had the ideal conditions to foster HCN from comet impacts, ancient Mars might have been better suited . “Because Mars is much smaller than Earth and further away from the sun, the typical impact velocity of comets is much lower, so while these impacts on Earth were destroying most of the HCN, this wasn’t the case so much for Mars,” Anslow said.

This idea could add another dimension to the long-standing question of whether the red planet once supported life.

“It’s just another perspective to have in the back of our minds when we’re trying to understand the early evolution of Mars in more detail,” Anslow said.

While the authors closely analyzed how HCN could have survived comet impact, the chemical may have undergone reactions before the icy bodies entered Earth’s atmosphere, said Queenie Chan, an associate professor of Earth sciences at Royal Holloway University of London, who was not involved with the study. This process could have created biologically important compounds, such as those protein building blocks, that are less volatile and more likely to survive.

“Therefore, the inefficiency of delivering intact HCN does not completely rule out a potential role for comets in supplying other important organic compounds that may have contributed to Earth’s prebiotic organic inventory before the emergence of life,” Chan wrote in an email.

Nicolle Zellner, a professor of physics at Albion College in Michigan, said in an email that she wasn’t surprised by the findings. While she is also reluctant to eliminate comets as a possible contributor to the emergence of life, she acknowledged that “the authors have definitely put some good parameters around the likelihood.”

根据新的计算机模拟结果，彗星在数十亿年前地球生命起源中发挥关键作用的流行理论可能站不住脚，这与数十年来科学界的认知相悖。

这一发现表明，冰体携带的脆弱有机分子在撞击时会被破坏，从而颠覆了至少可以追溯到1961年的既定生命起源理论。而认为生命成分来自太空的更广泛的观点，即泛生论，则更为古老，早在1908年就被科学界提出。

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地球上的生命需要复杂的有机分子，但我们尚不清楚早期地球是否拥有足够的原材料来进行生命起源前的化学反应——即产生生命基本组成部分的化学反应。彗星由尘埃和冰组成，但也含有有机物质。这种成分使彗星获得了“脏雪球”的绰号，也促使科学家们提出，彗星可能在地球表面沉积了水和有机分子。

根据周三发表在《科学报告》杂志上的一项新研究，地球在其早期历史中——超过40亿年前——可能经历过一段遭受超过10万颗直径超过100米（328英尺）的彗星猛烈撞击的时期。然而，这些撞击的威力大多过大，以至于生命起源分子无法承受。

研究团队利用最新的彗星形成和演化模型，对早期行星时期进行了10万次模拟。该研究的主要作者理查德·安斯洛（Richard Anslow）表示：“根据我们对早期地球表面环境的了解，平均而言，我们的模拟结果表明，当时没有发生过成功的撞击事件。”安斯洛在英国剑桥大学天文研究所攻读博士学位期间开展了这项研究。

这些彗星含有大量生命必需的关键分子，但模拟结果显示，它们的飞行速度太快，无法成功地将这些分子输送到地球。“这些彗星撞击地球时的典型速度意味着，实际上几乎没有彗星能够携带完整的有机分子，因此，彗星对生命起源前的化学反应的作用似乎并不像之前认为的那样重要，”安斯洛说道。

更准确地说，54%的模拟结果显示完全没有发生撞击，而其余46%的模拟结果显示至少发生了一次撞击，极少数模拟结果显示发生了多达三次撞击，安斯洛说。“我们不能完全排除从未发生过成功的撞击——可能存在过一次偶然的撞击。”然而，这种偶然事件必须发生在“完美地点”，该地点已经满足了几个重要条件，包括存在液态水和其他维持生命所需的化学物质。他补充说，这些因素使得整个过程发生的概率很低。

该研究重点关注一种名为氰化氢或 HCN 的天然化学物质，它是生命起源前化学的重要前体，已在数十颗彗星中被探测到，其中包括著名的哈雷彗星和海尔-博普彗星。

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科学家们重新进行了一项实验，该实验揭示了地球生命可能的起源。他们发现了一种新的可能性。

氰化氢（HCN）由氢、碳和氮组成，与水结合可生成氨基酸（构成蛋白质的分子）；其他研究此前已将氰化氢确定为地球生命起源的潜在关键成分。

“彗星撞击地球时，会受到高温高压的冲击，最近的研究发现，许多彗星前体分子会在这种高温高压以及随后的撞击中被破坏，”安斯洛说。

研究作者认为，即使撞击成功，也不太可能将化学物质以有用的浓度输送到海洋，而且如果撞击发生在海洋中，化学物质会很快溶解。

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安斯洛表示，为了使生命起源前的化学反应得以发生，还需要许多其他前体分子，例如含磷和硫的化合物，但这些物质通常不会在彗星中发现。“彗星非常适合输送氰化氢，但考虑到彗星撞击地球的位置是随机的，我们无法保证在撞击点就能找到进行这种化学反应所需的所有其他物质。”

安斯洛表示，如果彗星没有带来地球生命的基本组成部分，还有其他几个很有前景的途径值得探索。另一个由来已久的竞争理论是，地球的原始汤可能已经自行合成了生命所需的分子。另一种可能的宇宙有机物质来源是小行星，它们通常撞击地球的速度较慢，但​​数量更多。安斯洛指出，最近对贝努和龙宫小行星样本的研究发现，这些样本含有构成蛋白质的大部分氨基酸。

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科学家称，历史性小行星样本揭示“生命的组成要素实际上起源于外星”。

最近的这项研究还提出了另一种引人入胜的可能性：虽然地球可能不具备彗星撞击产生HCN的理想条件，但远古火星或许更适合。“由于火星比地球小得多，而且距离太阳更远，彗星撞击地球的典型速度要低得多。因此，尽管地球上的彗星撞击会破坏大部分HCN，但火星的情况并非如此。”安斯洛说道。

这一想法可能会为火星是否曾经存在生命这一长期存在的问题增添新的维度。

“这只是我们在试图更详细地了解火星早期演化时，需要牢记的另一个视角，”安斯洛说。

伦敦大学皇家霍洛威学院地球科学副教授陈倩雯（Queenie Chan，她并未参与这项研究）表示，尽管研究人员仔细分析了HCN如何在彗星撞击后幸存下来，但这种化学物质可能在彗星进入地球大气层之前就已经发生了反应。这一过程可能生成了具有重要生物学意义的化合物，例如蛋白质的组成单元，这些化合物的挥发性较低，更容易存活。

“因此，彗星无法完整输送HCN，但这并不能完全排除彗星在提供其他重要有机化合物方面发挥潜在作用的可能性，这些化合物可能在生命出现之前就对地球的益生元有机物储备做出了贡献，”陈在一封电子邮件中写道。

密歇根州阿尔比恩学院的物理学教授妮可·泽尔纳在一封电子邮件中表示，她对这一发现并不感到惊讶。虽然她也不愿完全排除彗星作为生命起源可能因素的可能性，但她承认“作者们确实为这种可能性设定了一些合理的参数”。