In a laboratory at the University of Kent in Great Britain, Alejandra Traspas and Mark Burchell shot frozen tardigrades from a gas-powered gun at 3,600 kilometers per hour – for science.
“We shot tardigrades at high velocity in a gun at sand targets, subjected them to impact shocks, and evaluated 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 if the microscopic animals could survive a cosmic impact, such as a Martian meteorite landing on Earth.
Their results – which ranged from slightly dazed to completely demolished tardigrades – suggest that it may be more difficult than expected for life to migrate between planets. But understanding the limits of microorganisms like tardigrades could provide some practical advice for future space missions hoping to obtain a sample of alien microbes from Europa’s water clouds.
What is new – Rocks are thrown into space, and from planet to planet, quite often. When an asteroid or comet hits a planet with enough force, the impact can blast rocky debris far into space. Some of these rocks eventually fall 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 the rocky debris (called ejecta) and eventually seed other worlds with life—an idea called lithopanspermia. It turns out that tardigrades (notoriously robust microscopic animals, also called water bears) can survive a meteor trip 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, like 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 unfortunate enough to be inside) to intense pressure—hundreds of thousands of times what you would feel if you were just standing at sea level on Earth. Only one tardigrade in 1,000 survived being shot into a wall of sand with similar force.
“It is likely that the arrival of a tardigrade on Earth, for example by a meteorite impact, is unlikely to be a viable means of successful transfer even for such hardy organisms,” Traspas and Burchell write 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 thrown from one place to another tend to land with less cell-membrane-shattering force, it turns out that space travel tardigrades . may just have a chance.
Here’s the background – Tardigrades have a hard-earned reputation as tough survivors. They usually live in fresh water here on Earth, but they can withstand extreme cold, harsh vacuum and cosmic radiation – which we know because they have run 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 entering the yard will pump almost all of the water out of its tiny body, lowering its metabolism to about 0.01 percent of its normal rate. When conditions improve—warmer or less radioactive, for example—the tardigrade can revive itself and continue 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 shot them into a sand target at different speeds.
At around 2,500 kilometers per hour, almost all the tardigrades survived the impact – although it took them between two and four times longer to recover than their luckier brethren, who had simply been frozen. That survival rate began to drop sharply at about 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 emerge from the yard and slowly begin walking around eating moss. But when frozen tardigrades were hurled into sand at 2,500 to 2,900 kilometers per hour, those lucky enough to survive took several times as long to recover: between 16 and 36 hours. It may be because the survivors of the accident had to contend with internal injuries along with the normal recovery process.
“In the higher velocity shots, only fragments of tardigrades were recovered,” write Traspas and Burchell, suggesting that the tardigrades had been broken apart by the high pressure (same, small tardigrades…same). The shock wave of pressure from an impact with high enough velocities can not only split a tardigrade apart, but it can also tear apart cell walls and wreckage structures in individual cells.
But it’s not all bad news for lithopanspermia. According to studies of Earth rocks found on the Moon, about 40 percent of them hit at speeds that a tardigrade could survive. The same may be the case for rocks blasted from the surface of Mars up to the moon Phobos. Most of these meteorites hit Phobos at speeds far too high for tardigrades to survive, but “if a fraction of such material had a lower impact speed, survival may be possible,” write Traspas and Burchell.
Of course, surviving the impact won’t seem like a relief for a tardigrade that finds itself stranded on the moon or Phobos – but it does suggest that microorganisms trapped in chunks of rock thrown from a planet to a relatively nearby moon might have a decent chance.
All in all, this tells us more about where and how to look for the origin of life, or for life bouncing around between habitable places in the solar system.
Why it matters – Currently, some of the most interesting possibly habitable places in our solar system are icy moons like Saturn’s moons Enceladus and Europa, whose hidden oceans vent into space through cracks in their icy crusts. The Cassini orbiter sampled 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 will need to be much more careful if they want to scoop up living microbes — or even recognizable parts — in their samples.
When a spacecraft like Cassini scoops up samples from something like Europa’s icy clouds, it’s not a gentle process. The sampled material slammed into Cassini’s metal sample collector at more than 11,000 miles per hour, according to Traspas and Burchell, which is more than enough to completely eject a tardigrade, or even individual cells.
According to Traspas and Burchell, future missions could solve that problem with some careful trajectory planning, or by using foamy, soft airgel for 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 cushion of foamy airgel can soften the blow enough to allow microscopic creatures like tardigrades to survive, so scientists back on Earth can do science with them.
What’s next – One of the biggest unanswered questions – which may yet make or break the panspermia hypothesis – is whether tardigrades that survive a meteorite trip can actually reproduce afterwards. Traspas and Burchell kept their tardigrades separate, so until someone allows the survivors to meet, we won’t have an answer. But it’s hard for a species to seed a planet with life if the potential pioneers just stagger out of the meteorite and eventually die out without leaving a second generation.
Traspas and Burchell also suggest that it might be interesting to load their tardigrade gun with tardigrade eggs, rather than adult tardigrades. 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 knocks it out into space – but do these eggs have a better chance of survival than a fully grown tardigrade?
There’s only one way to find out.