In Ridley Scott’s terrible, but at the same time oddly enjoyable (in a car-crash kind of way) Alien prequel, Prometheus, life on Earth is kickstarted billions of years ago by the alien race of ‘Engineers’ and their never-adequately-explained ‘black goo’. For an equally never-explained reason, billions of years later, primitive human cultures left behind cave paintings venerating the extraterrestrial visitors. The whole thing is a weird and muddled mashup of Erich von Däniken and Looney Tunes (oh, c’mon, don’t tell me you didn’t think of Wile E Coyote as she ran in a straight line in front of the giant rolling spaceship!), but it does touch on one actual scientific theory: “panspermia”.
This theory, specifically, that life was ‘seeded’ on Earth by comets carrying the necessary molecular building blocks, has been floating around since the early 1960s. The discovery that living organisms on the outside of spacecraft are able to survive in space lent it some credence. But a new paper has pointed out a huge problem: surviving the absolute cataclysm of the carrier comet actually smacking into a planet.
The theory has always possessed a certain cinematic elegance. Comets are cosmic deliverymen, arriving from the furthest darkness with bags of amino acids, hydrogen cyanide and other ingredients for the primordial soup. The problem, naturally, is that the postal service involved is going several hundred kilometres per second and the goods are rather more fragile than the marketing department might admit.
A new study, published in the journal Scientific Reports, used computers to simulate just how likely the packages were to survive the cosmic equivalent of being tossed at the terrestrial porch from the galactic curb.
“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.
In 54 per cent of the simulations, no suitable impact occurred at all. In the remaining 46 per cent, there was at least one collision, although only a “tiny minority” produced as many as three. Anslow did not claim that success was mathematically impossible. A freakish strike at precisely the right place might have worked, but “precisely the right place” requires liquid water and a collection of other chemicals already waiting to participate in prebiotic chemistry. In other words, the comet would need to arrive at a location that had already become hospitable to life in order to deliver materials useful to life. The causal reasoning is hardly watertight.
“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 research concentrated on hydrogen cyanide, or HCN, a compound containing hydrogen, carbon and nitrogen that can help produce amino acids when combined with water. HCN has been detected in numerous comets, including Halley’s and Hale-Bopp, and has long been regarded as a useful precursor in the chemical drama leading towards proteins. Alas, the study found that comets generally arrived too quickly to deliver it intact.
If it wasn’t smashed to its component atoms in the collision, the chemical may have been dispersed beyond practical usefulness, especially in an ocean strike. And HCN alone would not constitute the beginnings of life. Compounds containing phosphorus and sulfur are also required: the only problem being that those are not routinely found on comets.
Undeterred, the panspermians also point to asteroids and even Mars as possible sites for life later seeded on Earth. Nor are they giving up their beloved comets just yet. Queenie Chan of Royal Holloway University of London, who was not involved in the research, noted that HCN might have reacted before impact, producing less volatile compounds capable of surviving the journey.
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.
Nicolle Zellner of Albion College likewise declined to consign comets to the dustbin of failed hypotheses. The researchers, she said, had at least “put some good parameters around the likelihood”.
So the black goo has been dislodged from at least one wing of the Enterprise. Comets may still have carried useful chemicals to Earth and an occasional freak impact may have achieved what their simulations failed to reproduce. But the romantic notion that these dirty snowballs acted as reliable planetary midwives is now looking considerably less robust. The universe, as ever, remains less hospitable to cheerful narratives than to, you know: evidence.
