Límites de supervivencia de los tardígrados en impactos de alta velocidad: implicaciones para la panspermia y la recogida de muestras de los penachos emitidos por los mundos helados
Se investigó la capacidad de los tardígrados para sobrevivir a choques de impacto en el rango de kilómetros por segundo y gigapascales. Cuando las rocas impactan en las superficies planetarias, las velocidades de impacto y las presiones de choque se sitúan en el rango de los kilómetros por segundo y los gigapascales. Esta investigación probó si los tardígrados pueden sobrevivir en impactos típicos de los que ocurren naturalmente en el Sistema Solar.
Descubrimos que pueden sobrevivir a impactos de hasta 0,9 km s-1, lo que equivale a una presión de choque de 1,14 GPa, pero no pueden sobrevivir a impactos superiores. Esto es significativamente menor que el límite de presión estática y tiene implicaciones para la supervivencia de los tardígrados en los modelos de panspermia. La supervivencia potencial de los tardígrados en impactos de eyectos terrestres en la Luna se muestra imposible para la velocidad media de impacto lunar de dichos eyectos. Sin embargo, una fracción notable (alrededor del 40%) de dichos eyectos impacta a velocidades verticales lo suficientemente bajas como para permitir la supervivencia. Del mismo modo, los eyectos marcianos que impactan en Fobos, por ejemplo, a una velocidad de impacto típica no permitirán la transferencia viable de organismos similares a los tardígrados, pero si una fracción de ese material tuviera una velocidad de impacto más baja, la supervivencia podría ser posible.
También consideramos las implicaciones de esto para la recogida de muestras viables por parte de las naves espaciales que transitan por los penachos de mundos de agua helada como Europa y Encélado. Hemos comprobado que el límite de supervivencia de los choques se sitúa en torno a 1 GPa, lo que resulta decisivo para determinar los escenarios de misión y los métodos de recogida adecuados para la obtención de materiales viables.
https://www.liebertpub.com/doi/full/10.1089/ast.2020.2405
Conclusions
We have shown that tardigrades can survive low- to moderate-speed impacts and the involved shock pressures at speeds up to 0.728 km s−1, but then survival was not observed at 0.901 km s−1 (corresponding to peak shock pressures of 0.86 and 1.14 GPa, respectively). The statistics involved are low, and future experiments with larger numbers would be beneficial. Future experiments should also assess what happens to the tardigrades below the survival limit. Tardigrades under these circumstances took significantly longer than the control samples to recover, which suggests that a degree of internal damage has to be overcome. Furthermore, it is not clear whether the reproduction cycle can be undertaken by the survivors. This was not observed after any shot in the present study, but the sample numbers were small and the samples kept isolated; thus further study is needed. Similarly, collecting samples of tardigrade eggs, using them in the projectiles, and then assessing whether they can develop afterward would also be a fruitful area of study.
That complex structures undergo damage in shock events is not a surprise. Willis et al. (2006) showed that, even for the simplest of cells, cell wall delamination was a factor in lethality. Further, the results of Jerling et al. (2008) and Leighs et al. (2012) suggest that larger organisms such as seeds also suffered internal damage due to passage of shock waves. That the tardigrades studied here sustained internal damage that resulted in lethality at a similar shock pressure to seeds is therefore not a great surprise. However, it is clear that shock also causes internal damage at lower shock pressures, as indicated by the longer recovery time required to restore mobility for the shocked specimens in comparison to those that were simply frozen and revived directly from the tun state. The peculiarity here may be that recovery and survival is still possible until just before the impact events begin to break the tardigrades apart.
When considering the implications for successful transfer of tardigrades (or similar organisms) across space, the low shock pressure required rules out most of the common scenarios of interplanetary transfer that involve impact speeds well above 1 km s−1 and shock pressures of many gigapascals. However, as indicated in the discussion section, there are niche environments where such transfers may be possible. These include transfer from planetary surfaces to nearby moons (e.g., from Earth and Mars). Indeed, even when the average material involved is shocked above the survival limit, it may be possible that some experience a lesser shock, and survival may still occur. Similarly, if appropriate attention is given to the mission design (orbit or flyby) and collection method (solid collectors or underdense collectors such as aerogel), it may be possible to successfully sample the plumes of Europa and Enceladus for such life-forms. Indeed, the idea that these plumes may be responsible for icy satellite panspermia (e.g., see Burchell et al., 2003, for a discussion) in their respective planetary systems could be investigated. Czechowski (2018), for example, considered enceladean ejecta and found that, although material can escape Enceladus under the correct conditions, it is unlikely to escape the saturnian system, leaving it available to impact other saturnian satellites. A successful transfer of viable material, of course, would depend on the impact speed on another satellite of the parent planet (with a limit of around 1 km s−1).
Me parece horrible que los tardígrados sean disparados a altas velocidades. Creo desde este momento el movimiento TLM.
