A little over five years ago, on February 22, 2019, the Beresheet space probe, built by SpaceIL and Israel Aerospace Industries, entered the orbit of the Moon. Beresheet was to be the first private spacecraft to make a soft landing on the lunar surface. Among the probe’s payload were thousands of tardigrades, tiny animals known for their extraordinary ability to survive in the most extreme conditions.
The mission had problems from the beginning: the tracking cameras, which allowed determining the orientation of the ship and controlling its engines, failed. Budget constraints dictated a stripped-down design, and while the command center was able to fix some problems, things became even more complicated on April 11, the day of the moon landing.
On the way to the Moon, the ship had traveled at high speed and it was necessary to slow it down for a soft landing. Unfortunately, during the braking maneuver a gyroscope failed, blocking the primary motor. At 150 m altitude, Beresheet was still moving at 500 km/h, too fast to stop in time.
The impact was violent: the probe shattered and its remains were scattered about a hundred meters away. We know this because NASA’s LRO (Lunar Reconnaissance Orbiter) satellite photographed the impact site shortly after the accident, on April 22.
Animals that can withstand (almost) everything
What happened to the tardigrades that traveled on the probe?
Given their extraordinary ability to survive situations that would kill almost any other animal, could they have contaminated the Moon? Worse yet, could they reproduce and colonize it?
Tardigrades are microscopic animals that measure less than a millimeter in length. They all have neurons, an open mouth at the end of a retractable proboscis, an intestine containing a microbiota and four pairs of unarticulated legs ending in claws, and most have two eyes. However small 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 urban. Emmanuelle Delagoutte, a researcher at the French National Center for Scientific Research, collects them from 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 need to be surrounded by a film of water. They reproduce sexually or asexually through parthenogenesis (from an unfertilized egg) or even hermaphroditism, when an individual (which has male and female gametes) self-fertilizes. Once the egg has hatched, the active life of a tardigrade lasts 3 to 30 months. 1,265 species have been described, including two fossils.
Powers from another world
Tardigrades are famous for their resistance to conditions so extreme that they do not even exist on Earth or the Moon. They can lose up to 95% of their body water to disrupt their metabolism. Some species synthesize a sugar, trehalose, that acts as an antifreeze, while others synthesize proteins that are believed to incorporate cellular constituents into an amorphous “glassy” network that offers 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 remain visible. This state, known as cryptobiosis, persists until the conditions for active life become favorable again.
Depending on the species of tardigrade, individuals need 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°C or as high as 150°C, and in the long term they withstand high doses of gamma rays of 1,000 or 4,400 Gray (Gy). For comparison, a dose of 10 Gy is fatal to humans and 40 to 50,000 Gy sterilizes all types of material. However, whatever the dose, the radiation kills tardigrade eggs.
Furthermore, the protection offered by cryptobiosis is not always clear, as in the case of Milnesium tardigradum, a species for which radiation affects active and dehydrated animals in the same way.
The Milnesium tardigradum species in its active state. E. Schokraie, U. Warnken, A. Hotz-Wagenblatt, MA Grohme, S. Hengherr, et al. (2012)., CC BY
Moon life?
What happened to tardigrades after they crashed on the Moon? Are any of them still alive, buried under the lunar regolith, the dust that varies in depth from a few meters to several tens of meters?
For this to be possible, they would have to have survived the impact in the first place. Laboratory tests have shown that frozen specimens of the species Hypsibius dujardini, traveling at 3,000 km/h in a vacuum, suffer fatal damage when crashing into sand. However, they survive impacts of 2,600 km/h or less, and their “crash landing” on the Moon was much slower.
The surface of the Moon is not protected from solar particles or cosmic rays, particularly gamma rays, but in this case too the tardigrades would be able to resist. In fact, Robert Wimmer-Schweingruber, a professor at the University of Kiel (Germany), and his team have shown that the doses of gamma rays that hit the lunar surface are permanent, but low compared to the doses mentioned above: 10 years of exposure to lunar gamma rays would correspond to a total dose of about 1 Gy.
But there is still the key question of whether “life” on the Moon is possible. Tardigrades would have to endure a lack of water, as well as temperatures ranging between -170 and -190 °C during the lunar night and between 100 and 120 °C during the day. A lunar day or night lasts a long time, just under 15 Earth days. The probe itself was not designed to withstand such extreme temperatures and, even if it had not crashed, it would have ceased all activity within a few Earth days.
Unfortunately for tardigrades, they cannot overcome the lack of liquid water, oxygen and microalgae, so they would never be able to reactivate, much less reproduce. Therefore, their colonization of the Moon is impossible.
Even so, it is foreseeable that there are inactive specimens on lunar soil and their presence raises ethical questions, as pointed out by Matthew Silk, an ecologist at the University of Edinburgh. Furthermore, at a time when space exploration is taking off in all directions, contaminating other planets could mean we would miss the opportunity to detect extraterrestrial life.