On February 22, 2019, an unmanned
space probe was placed into orbit around the Moon with the objective of landing on its surface. This was a first, as no private spacecraft had ever landed on the lunar surface before. Moreover, the probe was carrying
tardigrades in a dehydrated and inactive, but viable form.
Everything was proceeding as planned when suddenly, on April 11, the probe encountered a propulsion issue as it began its descent. The velocity was too high to be adequately slowed, resulting in the spacecraft crashing onto our satellite at over 1864 miles per hour (about 3000 km/h).
The impact was tremendous and the probe was scattered over a range of about 328 feet (100 meters). This is known because the crash site was photographed by NASA's Lunar Reconnaissance Orbiter (LRO).
What happened to the tardigrades? Did they survive, and if so, could they colonize the Moon? Is the Moon contaminated?
Animals Resistant to (Almost) Everything
Tardigrades are microscopic animals, measuring less than 0.04 inches (1 mm) in length. Most have two eyes, but all possess neurons, a mouth at the end of a retractable trunk, an intestine containing a microbiome, and four pairs of non-jointed legs ended by claws. These animals share a common ancestor with arthropods like insects or arachnids.
The majority are found in aquatic environments, but they populate all habitats, even urban ones.
Emmanuelle Delagoutte, a researcher at CNRS, collects them from mosses and lichens in the Jardin des Plantes at the Museum in Paris. Tardigrades need to be surrounded by a film of water to remain active, feed on microalgae such as chlorella, grow, move, and reproduce. They reproduce either sexually or asexually through parthenogenesis, i.e., from an unfertilized egg, or hermaphroditism when an individual, possessing both male and female gametes, self-fertilizes. After hatching from the egg, an active tardigrade's lifespan varies from 3 to 30 months. In total,
1265 species have been described, including two fossil species.
Tardigrades are famed for their resilience to conditions nonexistent on both Earth and the Moon. Indeed, they can halt their metabolism, notably by losing up to 95% of their body water. Some species synthesize a sugar, trehalose, which acts as antifreeze, while others produce proteins thought to encapsulate cellular constituents in an amorphous "glass-like" network, providing resistance and protection to each cell.
Dehydration distorts the body, which can shrink by half in size. The legs disappear, leaving only the claws visible. This state, known as cryptobiosis, persists until conditions become favorable again.
However, depending on the species, individuals need more or less time to dehydrate, and not all specimens of the same species can return to active life.
Dehydrated adults can survive a few minutes at temperatures of - 457.6 °F (- 272 °C) or 302 °F (150 °C), and in the long term withstand high doses of gamma rays, 1,000 or 4,400 Grays (Gy) depending on the species. By comparison, a dose of 10 Gy is lethal for a human, and 40,000 to 50,000 Gy sterilizes any type of equipment. However, no matter the dose, irradiation kills the eggs. Moreover, the protection offered by cryptobiosis is not always clear, as seen in the species
Milnesium tardigradum, where irradiation oddly affects both active and dehydrated animals in the same manner.
Back to the Moon
What became of the tardigrades after the crash? Could some still be viable, buried under the regolith, the lunar dust varying in depth from a few meters to several tens of meters?
First, they would have had to survive the impact.
Laboratory tests showed that frozen specimens of the species
Hypsibius dujardini remained intact after a vacuum shock at about 1616 miles per hour (2600 km/h) on sand, but were mutilated beyond the speed of 1864 miles per hour (3000 km/h).
Next, they would need to withstand the lack of water and endure temperatures ranging from -274 to -310 °F (-170 to -190 °C) during the lunar night and 212 to 248 °F (100 to 120 °C) during lunar daytime. A lunar day or night lasts almost 15 Earth days. Even the probe was not designed to withstand such temperature extremes and was meant to cease all activity after only a few Earth days.
Finally, the Moon's surface is not protected against solar particles and cosmic rays, notably gamma rays. However, tardigrades could potentially resist them. Indeed, Robert Wimmer-Schweingruber, a Professor at the University of Kiel in Germany, and his team demonstrated that the
gamma ray doses striking the lunar surface were permanent but low compared to the previously mentioned doses. According to him, 10 years of exposure to gamma rays would correspond to a total dose of about 1 Gy.
Regardless, without water, oxygen, or microalgae, tardigrades could never reactivate. Thus, the colonization of the Moon by these animals is impossible. Yet, specimens are on the lunar surface, raising ethical questions as highlighted by
Matthew Silk, an ecologist at the University of Edinburgh. Among these questions, one scientific concern stands out. At a time when space exploration is expanding in every direction, could contaminating other planets prevent us from searching for extraterrestrial life?
The author extends his warm thanks to Emmanuelle Delagoutte and Cédric Hubas of the Museum of Paris, and to Robert Wimmer-Schweingruber of the University of Kiel, for their critical reading of the text and their advice.
Author: Laurent Palka