Comets Carried The Ingredients For Life. The Problem Is They Were Likely Destroyed on Impact

Credit: ZME Science.

Comets may have brought some of the ingredients for life to Earth. The problem is that they probably destroyed them on delivery.

That’s the snag in a long-standing idea about how life originated on Earth. Scientists have considered comets possible suppliers of the organic molecules that helped kick-start life on our planet, including carbon- and nitrogen-bearing molecules that could have fed the chemistry that came before biology.

But new research led by the University of Cambridge, published in Scientific Reports, suggests that delivering those molecules in useful concentrations would have been exceptionally difficult and the odds are pretty much against it.

“Our study focused on whether comets were able to deliver a sufficient concentration of these key feedstock molecules on their own,” lead author Richard Anslow, of Cambridge’s Institute of Astronomy, told ZME Science.

Two bad outcomes

The appeal of comets is clear. If life’s raw ingredients arrived ready-made from space, early Earth wouldn’t have had to make them all locally. But a list of chemicals detected in a comet isn’t enough to say anything about whether such a scenario could have happened. The molecules must reach the surface intact, build up sufficient concentrations, and meet conditions that allow further reactions until something like some primitive RNA forms, ultimately facilitating the first cells.

To test that sequence, Anslow and colleagues combined models of comet populations and orbits with simulations of atmospheric entry and impacts. They focused on hydrogen cyanide (HCN), a starting material for many prebiotic reaction networks. HCN made a good test case because it occurs in comets at concentrations relevant to lab experiments, and its strong carbon-nitrogen triple bond should make it relatively resistant to impact destruction.

The trouble starts with size.

“Small comets typically disintegrate during atmospheric entry (in a destructive airburst)–spreading these molecules over a huge area of the Earth, and making it difficult to achieve the local concentrations required for subsequent prebiotic chemistry,” Anslow said.

Larger comets survive the atmosphere, only to slam into the ground at speeds that expose their cargo to extreme shock pressures and temperatures.

“Except for a small number of low-velocity impacts, this will cause the destruction of the majority of the comet’s feedstock molecules, again challenging their use in subsequent prebiotic chemistry,” Anslow said.

Other feedstock molecules fare even worse, Anslow added. They’re generally less abundant in comets and more vulnerable to impact.

What about one lucky comet? Maybe that’s all it takes, right?

After all, life only needed to get started once.

Anslow said the researchers acknowledged this. They deliberately chose optimistic assumptions about the early Solar System to give comets their best shot. Even so, the models produced an average of zero impacts delivering enough HCN for subsequent prebiotic chemistry.

The harder part is what would have had to happen next.

“For that event to contribute directly to the origins of life, all other feedstock molecules (not necessarily all found in comets) would need to be present in that same location, and the subsequent prebiotic chemistry would need to succeed on its first attempt,” Anslow said.

Comet impacts declined rapidly during Earth’s first 500 million years, and any HCN deposited wouldn’t have stayed concentrated forever. Taken together, these hurdles led the team to favor other explanations.

Mars was actually more likely to be seeded by comets

Run the same models on Mars, and the picture brightens. Comets typically hit Mars more slowly than Earth because the planet has a lower escape velocity and orbits farther from the Sun, where orbital speeds are lower. Slower impacts mean more HCN survives.

The Red Planet’s thin atmosphere also allows more comets to reach the ground intact. But today’s tenuous air may not reflect what the planet was like billions of years ago, so the team also ran simulations with an Earth-like, 1-bar atmosphere. Mars still produced many more successful impacts than Earth.

That makes Mars a more promising candidate for this delivery mechanism. At the same time, Mars is barren and, despite everyone’s hopes, most likely never hosted life in its geological history.

“I would however highlight that we stress throughout the manuscript that there are huge uncertainties in our understanding of early Mars surface environments,” Anslow said.

If comets were unreliable suppliers, where did the ingredients come from? Chemical synthesis after asteroid impacts, which were far more frequent than comet strikes, as well as local processes on Earth itself seem more likely avenues for biochemistry.

Comets aren’t out of the picture entirely. In earlier work, Anslow and colleagues proposed that closely spaced planets could slow comets through repeated gravitational encounters, letting them deliver organic molecules at gentler speeds.

For Earth, though, the study shifts the focus from what comets carried to what their cargo could actually do once it arrived. The ingredients were there. Bringing them together in a place where chemistry could become biology was the hard part.