Free Astronomy Magazine May-June 2018

9 MAY-JUNE 2018 PLANETOLOGY and sturdy enough to support the solidified rock, it can collapse to the floor of the tube it- self, revealing the exis- tence of the tunnel when viewed from above. A rille is born. However, the roof of the lava tube does not always collapse. In this case, a long tunnel re- mains that is almost in- distinguishable from the surrounding sur- face when viewed from above. The width of this tunnel depends on the mass and, there- fore, the gravity of the planetary body at the surface of which the tunnel formed. Mathe- matical models indicate that on Earth, a lava tube can be up to about 30 meters wide; on Mars, the limit is close to 250 meters; on the Moon, there could exist lava tubes up to 5 km wide and hundreds of km long. A typical over- crusted lava tube on the north- ern side of Arsia Mons in the Thar- sis volcanic prov- ince of Mars. These structures are sky- light openings to an underground conduit. [NASA/JPL/ University of Ari- zona] Below, a hy- pothetical under- ground view of the same lava tube. Until a decade ago, the existence of tunnels outside of the Earth was still a matter of de- bate, but between 2009 and 2010 there was a turning point. Indeed, Kaguya (JAXA) and Lunar Reconnaissance Orbiter (NASA) probes succeeded in photographing cavities that give access to vast spaces in the lunar subsoil. More than 200 such cavities, called “sky- lights”, are known today and are thought to be the result of the col- lapse of short sec- tions of the roofs of numerous lava tubes. In some, if not all cases, the collapse could have been triggered by the impact of a slightly massive meteorite, a dy- namic that would explain the typi- cally circular or moderately elon- gated appearance of many skylights. Simultaneous with the discovery of skylights on the Moon, some have also been observed on Mars, above all thanks to the Mars