Just over five years ago, on February 22, 2019, an unmanned space probe was launched into orbit around the moon. It was called Beresheet and built by SpaceIL and Israel Aerospace Industries. It was intended to be the first private spacecraft to perform a soft landing.
The probe’s payload included tardigrades, known for their ability to survive even in the harshest climates.
The mission encountered problems from the start, as the star tracker cameras, intended to determine the spacecraft’s orientation, failed to properly monitor the engines.
Budgetary constraints had imposed a more austere design, and while the command center was able to work around some problems, things became even trickier on April 11, the day of the landing.
On the way to the moon, the spacecraft had been traveling at high speed and had to be slowed down slowly to make a soft landing. Unfortunately, a gyroscope failed during the braking maneuver, blocking the primary engine.
At an altitude of 150 m, Beresheet was still traveling at 500 km/h, far too fast to stop in time. The impact was severe: the probe shattered and its remains were scattered over a distance of about a hundred meters. We know this because the location was photographed by NASA’s LRO (Lunar Reconnaissance Orbiter) satellite on April 22.
Animals that can withstand (almost) anything
So what happened to the tardigrades traveling on the probe? Given their remarkable ability to survive situations that would kill virtually any other animal, could they contaminate the moon? Worse yet, could they reproduce and colonize it?
Tardigrades are microscopic animals that are less than a millimeter long. They all have neurons, a mouth opening at the end of a retractable proboscis, an intestine containing a microbiota, and four pairs of non-articulated legs ending in claws, and most have two eyes. As small as they are, they share a common ancestor with arthropods such as insects and arachnids.
Most tardigrades live in aquatic environments, but they can be found in any environment, even in cities. Emmanuelle Delagoutte, researcher at the CNRS, collects them in the mosses and lichens of the Jardin des Plantes in Paris. To be active, feed on microalgae such as chlorella, and move, grow and reproduce, tardigrades must be surrounded by a layer of water.
They reproduce sexually or asexually via parthenogenesis (from an unfertilized egg) or even hermaphroditism, when an individual (possessing both male and female gametes) fertilizes itself. Once the egg has hatched, a tardigrade’s active life lasts from 3 to 30 months. A total of 1,265 species have been described, including two fossils.
Tardigrades are known for their resistance to conditions that exist neither on Earth nor on the moon. They can shut down their metabolism by losing up to 95% of their body water. Some species synthesize a sugar, trehalose, that acts as an antifreeze, while others synthesize proteins that are thought to incorporate cellular constituents into an amorphous ‘glassy’ network that provides resistance and protection to each cell.
During dehydration, the tardigrade’s body can shrink to half its normal size. The legs disappear and only the claws are visible. This condition, known as cryptobiosis, persists until conditions for active life become favorable again.
Depending on the species of tardigrade, individuals require more or less time to dehydrate and not all specimens of the same species manage to return to active life.
Dehydrated adults survive for a few minutes at temperatures as low as -272 degrees Celsius or as high as 150 degrees Celsius, and in the long term at high doses of gamma radiation of 1,000 or 4,400 Gray (Gy). By comparison, a dose of 10 Gy is fatal to humans, and 40-50,000 Gy sterilizes all types of material.
Regardless, radiation kills tardigrade eggs regardless of the dose. Furthermore, the protection provided by cryptobiosis is not always clear, as in the case of Milnesium tardigradum, where radiation affects both active and dehydrated animals in the same way.
Moon life?
So what happened to the tardigrades after they crashed on the moon? Are there any still viable, buried beneath the moon’s regolith, the dust that ranges in depth from a few meters to a few tens of meters?
First of all, they must have survived the impact. Laboratory tests have shown that frozen specimens of the Hypsibius dujardini species traveling at 3,000 km/h in a vacuum were fatally damaged when they hit the sand. However, they survived collisions of 2,600 km/h or less – and their ‘hard landing’ on the moon, unwanted or not, was much slower.
The surface of the moon is not protected against solar particles and cosmic rays, especially gamma rays, but here too the tardigrades could resist. In fact, Robert Wimmer-Schweingruber, professor at the University of Kiel in Germany, and his team showed that the doses of gamma rays hitting the moon’s surface were permanent but low compared to the doses mentioned above – 10 years of exposure to lunar gamma rays would correspond to a total dose of approximately 1 Gy.
But then there is the question of “life” on the moon. The tardigrades should be able to withstand a lack of water and temperatures ranging from -170 to -190 degrees Celsius during the lunar night and 100 to 120 degrees Celsius during the day.
A Monday or night is long, just under 15 Earth days. The probe itself was not designed to withstand such extremes and even if it had not crashed, it would have ceased all operations after just a few Earth days.
Unfortunately for the tardigrades, they cannot overcome the lack of liquid water, oxygen and microalgae – they would never be able to reactivate, let alone reproduce. Their colonization of the moon is therefore impossible.
Yet inactive specimens exist on lunar soil, and their presence raises ethical questions, as Matthew Silk, an ecologist at the University of Edinburgh, notes. Furthermore, at a time when space exploration is going in all directions, contaminating other planets could mean losing the chance to detect extraterrestrial life.
Laurent Palka, conference leader, Museum national d’histoire naturelle (MNHN)
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