“We have fired tardigrades at high speed in a gun onto sand targets, subjecting them to impact shocks and evaluating their survival,” write Traspas and Burchell, in possibly the most compelling opening to a scientific paper we’ve ever seen. The two planetary scientists wanted to know whether the microscopic animals could survive a cosmic impact, such as Martian meteorite landing on Earth.
Their results — which ranged from slightly dazed to completely demolished tardigrades — suggest it might be more difficult than expected for life to migrate between planets. But understanding the limits of micro-organisms like tardigrades could yield some practical advice for future space missions hoping to scoop up a sample of alien microbes from Europa’s watery plumes.
What's New — Rocks get flung into space, and from planet to planet, fairly often. When an asteroid or comet hits a planet with enough force, the impact can blast rocky debris all the way out into space. Some of those rocks eventually end up falling onto the surface of another planet or moon. That’s why there are chunks of Earth on the Moon, and why a surprisingly high percentage of Phobos’ surface actually originated on Mars.
For centuries, scientists have debated whether microscopic organisms could hitch a ride on that rocky debris (called ejecta) and eventually seed other worlds with life — an idea called lithopanspermia. It turns out that tardigrades (notoriously sturdy microscopic animals, also called water bears) could survive a meteor ride from one world to another, but only under exactly the right circumstances, according to Traspas and Burchell’s recent experiment.
Microbes probably couldn’t hitch a ride between planets, such as Earth and Mars. When a Martian meteorite lands on Earth, it smashes into the ground at several thousand kilometers per hour. The impact briefly subjects the meteorite (and any microscopic creatures unlucky enough to be inside) to intense pressure — several hundred thousand times what you’d feel just standing at sea level on Earth. Only about one tardigrade in 1,000 survived being shot into a wall of sand with comparable force.
“It is likely that arrival of a tardigrade on Earth, for example by way of a meteorite impact, is not likely to be a viable means of a successful transfer even for such hardy organisms,” write Traspas and Burchell in their paper. But elsewhere in the Solar System, where planets and their potentially-habitable moons are closer together than Earth and Mars, and meteorites flung from one place to the other tend to land with less cell-membrane-shattering force, it turns out that spacefaring tardigrades just might have a chance.
Here’s The Background — Tardigrades have a hard-earned reputation as tough survivors. They normally live in freshwater here on Earth, but they can endure extreme cold, hard vacuum, and cosmic radiation — which we know because they’ve ridden on the outside of spacecraft several times.
The key to tardigrades’ incredible resilience is their ability to put themselves into a dormant state, called “tun.” A tardigrade going into tun will pump nearly all the water out of its tiny body and slow its metabolism down to about 0.01 percent of its usual rate. When conditions get better — warmer or less radioactive, for instance — the tardigrade can revive itself and get on with doing tardigrade things.
Digging Into The Details — Traspas and Burchell loaded two or three frozen tardigrades at a time into a projectile made of ice, then fired them into a sandy target at various speeds.
At around 2,500 kilometers per hour, nearly all the tardigrades survived the impact — although it took them between two and four times longer to recover than their more fortunate brethren, who had only been frozen. That survival rate began to drop off sharply at around 2,900 kilometers per hour. At impact speeds of 3,600 kilometers per hour, there were no survivors.
Frozen tardigrades usually take about eight or nine hours to recover from tun and start slowly ambling around and eating moss. But when frozen tardigrades got slammed into sand at 2,500 to 2,900 kilometers per hour, the ones lucky enough to survive took several times as long to recover: between 16 and 36 hours. That might be because the crash survivors had to contend with internal damage along with the normal recovery process.
“In the higher-speed shots, only fragments of tardigrades were recovered,” write Traspas and Burchell, who suggest that the tardigrades had been broken apart by the high pressure (same, little tardigrades ... same). The shockwave of pressure from an impact at high enough speeds can not only dismember a tardigrade, but it can also rip apart cell walls and wreck structures within individual cells.
But it’s not all bad news for lithopanspermia. According to studies of Earth rocks that have been found on the Moon, about 40 percent of them hit at speeds that a tardigrade could survive. The same may be true for rocks blasted from the surface of Mars up to its moon Phobos. Most of those meteorites hit Phobos at speeds much too high for tardigrades to survive, but “if a fraction of such material had lower impact speed, survival may be possible,” Traspas and Burchell write.
Of course, surviving the impact won’t seem like such a relief to a tardigrade that finds itself stranded on the Moon or Phobos — but it does suggest that micro-organisms trapped in chunks of rock flung from a planet to a relatively nearby moon might stand a decent chance.
All in all, this tells us more about where and how to look for the origins of life, or for life bouncing around between habitable places in the solar system.
Why It Matters — At the moment, some of the most interesting possibly habitable places in our Solar System are icy moons like Saturn’s satellites Enceladus and Europa, whose hidden oceans vent to space through cracks in their icy crusts. The Cassini orbiter grabbed samples of Enceladus’ plumes back in 2015, hoping to detect chemicals associated with the building blocks of life, or at least habitability. But Traspas and Burchell say their experiments suggest that future missions need to be much more careful if they want to scoop up live microbes — or even recognizable pieces — in their plume samples.
When a spacecraft like Cassini scoops up samples from something like the icy plumes of Europa, it's not a gentle process. The sampled material slammed into Cassini’s metal sample collector at more than 18,000 kilometers per hour, according to Traspas and Burchell, which is more than enough to completely splatter a tardigrade, or even individual cells.
According to Traspas and Burchell, future missions could solve that problem with some careful orbital planning, or by using foamy, cushy aerogel for their sample collectors, instead of metal. At the speeds required to orbit Enceladus, a spacecraft could get away with a metal sample collector. But around Europa, a 1.5 to 15-centimeter thick pad of foamy aerogel could soften the blow enough to let microscopic creatures like tardigrades survive so researchers back on Earth could do science to them.
What’s Next — One of the biggest unanswered questions — which may yet make or break the panspermia hypothesis — is whether tardigrades who survive a meteorite ride-along can actually reproduce afterward. Traspas and Burchell kept their tardigrades separated, so until someone lets the survivors meet up, we won’t have an answer. But it’s hard for a species to seed a planet with life if the would-be pioneers just stagger out of the meteorite and eventually die off without leaving a second generation.
Traspas and Burchell also suggest that loading their tardigrade gun with tardigrade eggs, instead of adult tardigrades, might be interesting. It’s easy to imagine a crop of tardigrade eggs (or the alien equivalent) being trapped in the nooks and crannies of a rock when a stray comet impact blasts it into space — but do those eggs have a better chance of survival than a fully-grown tardigrade?
There’s only one way to find out.