Die Hard: Tardigrades Fired at Three Thousand Km / h (Biology)

Tardigrades are microscopic invertebrate animals whose excellent survival skills in extreme conditions are now well known. Among these, now reports a study published in Astrobiology, there is also the ability to withstand impacts at speeds up to 825 m / s. Led by two researchers from the University of Kent, the aim of the study was to evaluate the implications for panspermia of this resistance.

They are slow-paced animals, but they seem to have no problem with high speeds and the impact that can result from it. We are talking about tardigrades – a name composed of the Latin words tardus, ‘slow’, and degrees , ‘to walk’: tiny eight-legged invertebrate creatures that are champions of survival.

Low temperatures, emptiness, radiation, exposure to outer space, prolonged fasting of years and years … there is no extreme condition to which they have not been subjected and from which they have not come out “unharmed”. Out of the ordinary skills that have led scientists to hypothesize that they too may be among the phantom “seeds” which, according to the theory of panspermia , would have allowed the interplanetary transfer of life.

And precisely to explore this possibility – in particular the hypothesis of lithopanspermia , or the transfer of life forms from one planetary surface to another within rocks ejected into space by astronomical impacts – that Alejandra Traspas , researcher at the University of Kent (UK), and Mark Burchell , a professor at the same university, studied the ability of these tiny beings to survive impact shocks – the magnitude of which, in terms of speed and resulting impact pressures, is the limiting factor. of the theory, the researchers explain.

The Light Gas Gun facility at the University of Kent. Credits: research.kent.ac.uk

To do so, they literally shot individuals of Hypsibius dujardini – a species of tardigrade – at considerable speeds, knocking them into a sandy surface.

In the experiment, the microorganisms were first loaded into a bullet, which was then frozen for 48 hours in order to induce a state of cryptobiosis in individuals , a sort of metabolic lethargy in which tardigrades enter in response to adverse environmental conditions – one of the reasons why they are also called “water bears” ( waterbears , in English). Once the state of quiescence was induced, the bullets were inserted into a special two-stage gas “pistol”, from which six shots were fired at sand targets at speeds between 2000 and 3600 km / h.

After each shot, the target was poured into a column of water to separate the sand from other materials and thus isolate the tardigrades. The recovered individuals were finally observed over time to evaluate their survival after impact. As a control, 20 tardigrades were frozen and thawed without being fired with the gun. The result? These tough little creatures survived impacts up to 2,970 km / h , corresponding in their case to an impact pressure of about 1.01 gigapascals.

Tardigrades can therefore survive impacts at low to moderate speeds, although the researchers point out that their survival has not been observed at higher speeds.

In boxes ‘a’ and ‘b’, specimens of tardigrades before impact. In the box ‘there is a specimen after the collision at 2,620 km / h. In the box ‘d’ the fragments after the impact at over 3,000 km / h. Credits: Alejandra Traspas and Mark J. Burchell, Astrobiology, 2021

Since the results suggest that shock pressure peaks greater than 1.14 gigapascals kill tardigrades, their arrival on Earth via the impact of a celestial body is unlikely, according to the researchers, even for such resistant organisms. There are, however, other places in the Solar System, the researchers add, where biological material during transfer could impact at lower shock pressures and thus survive, for example during the impact-mediated ejection of terrestrial rocks on the Moon and the ejection of Mars rocks on the moon Phobos . Arguments, these, that the authors cover in the last part of their study – published this month in Astrobiology– together with the methods of collection, by spacecraft, of intact biological samples expelled from the plumes of the Jovian moon Europa and the Saturnian moon Enceladus .

The potential survival of tardigrades in impacts of terrestrial rocks on the Moon proved impossible for the average speed of the impact, the researchers point out. However, a substantial fraction, about 40 percent of this ejection, impacts at vertical velocities low enough to allow it to survive. Similarly, the impact of Martian rocks on Fobos at a typical impact velocity will not allow for the transfer of tardigrade-like organisms, but if a fraction of that material has a lower impact rate, survival may be possible. As for the collection of biological material from the plumes of the moons Europa and Enceladus,Determining the shock survival limit of around 1 GPa is critical to designing the appropriate mission scenarios and methods to be used to collect any biological material ejected from the two frozen water worlds, the researchers say. We can thus imagine that a mission involving the Europa flyby could attempt to collect any small vital organisms such as tardigrades using an airgel collector, while an orbiter on Enceladus could successfully use a solid collector as well. In this way, the researchers conclude, the hypothesis could be studied that these plumes may be responsible for the so-called ” icy satellite panspermia ” in their respective planetary systems.

Featured image: Tardigrade under the microscope. Credits: Ralph O Schill / Esa

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