Bacteria riding on an incoming meteorite may be able to survive the violent shockwave created when it crash-lands on a planet. Their cell walls have been seen to rapidly harden and relax after a sudden shock compression, enabling them to bounce back even after an extreme collision.
“When you are exposing life to such extreme conditions, it is a surprise when they survive quite well,” says Rachael Hazael at University College London
Microbes can withstand extreme environments on Earth, including the crushing pressure of the deep ocean or deep beneath the ground. This suggests that life forms could thrive on distant worlds in similar high-pressure environments.
But few people have studied what happens to microbes under dynamic “shock compression”, which is a very short-lived high-pressure environment.
To find out, Hazael and colleagues subjected a hardy, metal-eating microbe called Shewanella oneidensis to varying levels of sudden, extraordinarily high pressures. After each blast in increasingly high-pressure experiments, the team cultured the microbial survivors and found they fared better in a sudden high-pressure environment than in a long-lasting high-pressure condition.
To withstand these conditions, the microbes were adjusting the rigidity of their cell, says Hazael.
“They don’t deform as you would expect, and they behave very rigidly, which protects the inner materials in the cell,” says Hazael. “They adopt this glassy cellular behaviour, which allows them to maintain their integrity.”
Bacteria could have small genetic mutations that would enable them to survive, says Hazael, or they might have been using certain chemicals or fats in a way that safeguarded their delicate interiors.
“It’s that very fast timescale that allows them to adopt these glassy responses. We’ve held them at pressure for an hour, and they have a different response. We don’t get as many survivors,” she says.
The rigid glass-like state is similar to what happens in a non-Newtonian fluid like oobleck, says Barry Herschy, an astrobiologist at the University of South Florida in Tampa who studies extremophile bacteria. This mixture of flour and water hardens into a cohesive shape when thrown against a wall, but simply looks like wet flour when at rest.
However, he notes that the experiments were conducted at low temperatures, but the conditions that would cause a sudden shock compression would also cause extreme heating. It’s unclear how heat would have changed the microbes’ survival, he says.
“It does indicate that survival of bacteria would be possible on a meteorite or a comet,” Herschy says. “It actually shows life would be able to survive on a planet even if it was under bombardment.”
Beating the blast
Hazael found that the microbes not only survived short blasts of pressure, but went on to reproduce in colonies. On Earth prior to and during the Late Heavy Bombardment 3.8 billion years ago, when the planet was hammered by meteorites, this type of bacteria could have not only survived but thrived.
This also means that bacteria might survive a spacecraft landing – or even crashing – on other planets. That would lend weight to the panspermia theory, in which comets or meteorites could potentially deliver life to otherwise sterile planets. It also means it is something to keep in mind for space agencies and private companies exploring the solar system and beyond.
“Every time we explore a new environment, we find life is already there, so I’m never surprised when I find out these things,” Herschy says. “Any niche that becomes available, life finds a way to adapt to it.”