It is known, that some microorganisms like bacteria, Archaea, algae or fungi can survive in harsh conditions which are present on other planets and/or moons. But would it be also possible in respect of higher organisms, like animals? More precisely, a microscopic invertebrates - tardigrades?
Tardigrades, often called water bears or moss piglets, are the most fascinating animals known to science, near-microscopic animals with long, plump bodies and scrunched-up heads. They have eight legs, and hands with four to eight claws on each. While strangely cute, these tiny animals are almost indestructible and can even survive in outer space.
With all of these traits, it’s easy to imagine that this creature has been successful for a long time. Fossils of tardigrades have been found dating back hundreds of millions of years ago. It’s very likely that tardigrades have been around even longer than this.
The Earth has gone through many phases in the last billion years and has had a variety of climates, but through it all, the mighty water bear has survived. However, none of this explains why they were included as part of a mission on the space shuttle.
The major motivation for studying tardigrades on exoplanets is that tardigrade proteins could provide human DNA with protection from harmful cosmic radiation, which will be helpful in future space travel.
Research has found that tardigrades can withstand environments as cold as minus 328 degrees Fahrenheit (minus 200 Celsius) or highs of more than 300 degrees F (148.9 C), according to Smithsonian magazine. They can also survive radiation, boiling liquids, massive amounts of pressure of up to six times the pressure of the deepest part of the ocean and even the vacuum of space without any protection. A 2008 study published in the journal Current Biology found that some species of tardigrade could survive 10 days at low Earth orbit while being exposed to a space vacuum and radiation.
Main problem is, that they need a small percentage of oxygen. Would they find that on on an exoplanet? If not they enter anoxybiosis and soon die.
In their research, they used a similarity index technique for tardigrades to potentially survive on planets with water composition like Earth. The technique was previously used to find Earth-like and Mars-like planets outside our solar system (i.e. exoplanets). The team analyzed data from 3800 exoplanets from Planetary Habitability Laboratory – Exoplanet Catalog maintained by the University of Puerto Rico, Arecibo, which consists of raw data from CoRoT mission of ESA [France] and the Kepler mission from NASA [USA]. The detection of these exoplanets was done using radial velocity, transit, or occultation, gravitational microlensing, and direct imaging techniques.
Here, 57 rocky-water or water-gas exoplanets compositions, including Earth and Mars from our solar system, were chosen and a similarity index technique was applied to all these exoplanets (refer to our paper for technical details).
The geometrical means of the physical parameters used in the analysis were the radius, density, escape velocity, revolution period, surface temperature, and surface pressure. The weight exponent of each physical parameter was framed to the extreme survival ability of tardigrades. From the PHL-EC catalog, only rocky-water composition planets were chosen for the study. The two phases in which tardigrades could survive are namely: (a) Active Tardigrade Index (ATI) and (b) Cryptobiotic Tardigrade Index (CTI). The metric indices (i.e. ATI and CTI) were defined with minimum value 0 (tardigrades cannot survive) and maximum 1 (tardigrades will survive in their respective state). Values ranging from 0 to 1 indicate the percentage or chance of the active or cryptobiotic tardigrades surviving on a given exoplanet.
The results were obtained, with Mars as the threshold, indicating that Mars could be the only rock-water composition planet that could be more suitable for tardigrades than other considered exoplanets. Graphical representations of ATI and CTI are shown in the paper. The reason that no exoplanets are seen above the threshold value is due to the chosen sample having a much colder host star than our sun. To understand this cold temperature of the star, stellar classification needs to be followed. The stellar classification is such that we follow the temperature sequence in which the temperature (i.e. hot or cold) is denoted with respect to certain specified alphabetical characters. The letters from hot to cold stars are: O, B, A, F, G, K, M, and the mnemonic to identify this sequence is “Oh, Be A Fine Girl (or Guy), Kiss Me!” The cold stars are K and M type stars, and our Sun is a G-type star.
There may be water on Mars in small quantities. Main problem is, that they need a small percentage of oxygen. Would they find that on Mars? If not they enter anoxybiosis and soon die, within a few days, the state they enter in this case is not as resistant as the dessication state.
For tardigrades to potentially survive on Mars in an active state is 75% (i.e. ATI = 0.75) and in a Cryptobiotic state is 82% (i.e. CTI = 0.82). The next closest exoplanets below Mars range is around 60% [They are namely: Kepler-100 d, Kepler-48 d, Kepler-289 b, TRAPPIST-1 f, and Kepler-106 e].
The results with Mars as the threshold indicates that Mars could be the only rock-water composition planet that could be more suitable for tardigrades than other considered exoplanets.
Did Tardigrades Originate In Space?
Evolution has one simple rule, survival of the fittest. Animals best adapted to a specific set of criteria will survive and pass on their genes to the next generation.
Mutations happen from generation to generation that may give one branch of the family tree a slight advantage, which then leads to them dominating over the other parts of the tree. When applying this rule to the tardigrades, which we know are capable of surviving in the vacuum of space, then it must be likely that at some point in their history, these creatures were actually in this environment.
Perhaps tardigrades were riding on a rock through outer space. All of the tardigrades not capable of surviving these conditions died, and the ones that were went on to spread their unique traits to their offspring. What this theory is essentially saying is that tardigrades evolved the ability to survive in space because they may have actually come from space. Perhaps the tardigrades we see on Earth today are the descendants of true aliens, landing here millions of years ago.
Did Tardigrades Hitch An Asteroid Ride To Earth?
At first it may seem a little far fetched that tardigrades may have originated from off the Earth. But we have to remember that the early solar system was a volatile place.
There used to be a lot more debris orbiting the Sun and colliding with other planets and moons. We know for a fact that massive asteroids were raining down on all of the planets in the solar system. When this occurred, lots of material from the pummeled planet would be thrown into space. It is then possible for some of these rocks to exit the orbit of the planet and travel great distances before landing somewhere else in the solar system.
On Earth, there have been numerous meteorites found that have been confirmed to have come from the surface of Mars, confirming the possibility of this scenario. If a group of tardigrades were in the right spot during a large asteroid strike, they could have survived the immense heat of the initial collision, survived the frigid cold and empty vacuum of space, survived more extreme heat on reentry, and then survived the impact of hitting the surface.
These findings are described in the article entitled Tardigrade indexing approach on exoplanets, recently published in the journal Life Sciences in Space Research.