Lava tubes on Earth, Moon and Mars: a review on their size and morphology revealed by comparative planetology
Authors: Sauro Francesco , Pozzobon Riccardo , Massironi Matteo , De Bernardinis Pierluigi , Santagata Tommaso , De Waele Jo 
Published: October 2020

Sinuous collapse chains and skylights in Lunar and Martian volcanic regions have often been interpreted as collapsed lava tubes (also known as pyroducts, [1]). This hypothesis has fostered a forty years debate among planetary geologists trying to define if analogue volcano-speleogenetic processes acting on Earth could have created similar subsurface linear voids in extra-terrestrial volcanoes. On Earth lava tubes are well known thanks to speleological exploration and mapping in several shield volcanoes, with examples showing different genetic processes (inflation and overcrusting [1, 2, 3]) and morphometric characters. On the Moon subsurface cavities have been inferred from several skylights in maria smooth plains [4], and corroborated using gravimetry and radar sounder [5, 6] while on Mars several deep skylights have been identified on lava flows with striking similarities with terrestrial cases [7]. Nonetheless, a clear understanding of the potential morphologies and dimensions of martian and lunar lava tubes remains elusive.Although it is still impossible to gather direct information on the interior of martian and lunar lava tube candidates, scientists have the possibility to investigate their surface expression through the analysis of collapses and skylight morphology, morphometry and their arrangement, and compare these findings with terrestrial analogues. In this work we performed a morphological and morphometric comparison with lava tube candidate collapse chains on Mars and the Moon.By comparing literature and speleological data from terrestrial analogues and measuring lunar and martian collapse chains on satellite images and digital terrain models (DTMs), this review sheds light on tube size, depth from surface, eccentricity and several other morphometric parameters among the three different planetary bodies. The dataset here presented indicates that martian and lunar tubes are 1 to 3 orders of magnitude more voluminous than on Earth and suggests that the same processes of inflation and overcrusting were active on Mars, while deep inflation and thermal entrenchment was the predominant mechanism of emplacement on the Moon. Even with these outstanding dimensions (with total volumes exceeding 1 billion of m3), lunar tubes remain well within the roof stability threshold. The analysis shows that aside of collapses triggered by impacts/tectonics, most of the lunar tubes could be intact, making the Moon an extraordinary target for subsurface exploration and potential settlement in the wide protected and stable environments of lava tubes.&#160;References&#160;[1] Kempe, S., 2019. Volcanic rock caves, Encyclopedia of Caves&#160;(Third edition). Academic Press, pp. 1118-1127[2] Calvari, S. and Pinkerton, H., 1999. Lava tube morphology on Etna and evidence for lava flow emplacement mechanisms. Journal of Volcanology and Geothermal Research, 90(3-4): 263-280.[3] Sauro, F., Pozzobon, R., Santagata, T., Tomasi, I., Tonello, M., Mart&#237;nez-Fr&#237;as, J., Smets, L.M.J., G&#243;mez, G.D.S. and Massironi, M., 2019. Volcanic Caves of Lanzarote: A Natural Laboratory for Understanding Volcano-Speleogenetic Processes and Planetary Caves, Lanzarote and Chinijo Islands Geopark: From Earth to Space. Springer, pp. 125-142.[4] Haruyama, J., Morota, T., Kobayashi, S., Sawai, S., Lucey, P.G., Shirao, M. and Nishino, M.N., 2012. Lunar holes and lava tubes as resources for lunar science and exploration, Moon. Springer, pp. 139-163.[5] Chappaz, L., Sood, R., Melosh, H.J., Howell, K.C., Blair, D.M., Milbury, C. and Zuber, M.T., 2017. Evidence of large empty lava tubes on the Moon using GRAIL gravity. Geophysical Research Letters, 44(1): 105-112[6] Kaku, T., Haruyama, J., Miyake, W., Kumamoto, A., Ishiyama, K., Nishibori, T., Yamamoto, K., Crites, S.T., Michikami, T. and Yokota, Y., 2017. Detection of intact lava tubes at marius hills on the moon by selene (kaguya) lunar radar sounder. Geophysical Research Letters, 44(20).[7] Cushing, G.E., 2012. Candidate cave entrances on Mars. Journal of Cave and Karst Studies, 74(1): 33-47