• Like on Earth, Moon and Mars also have volcanic features – such as lava tubes. 
• Lava tubes are basically lava caves formed in one of two ways: either through the crusting over of lava channels, or more rarely, from pahoehoe flows where the lava is moving under the surface. After the lava leaves, it can leave behind a tunnel. 
• Sometimes, the lava might re-emerge and might be injected into its old tunnels (due to the low resistance); once there, it expands and leaves behind even larger tunnels, sometimes reaching impressive sizes.
• Sometimes, complex tunnel systems are formed, connected to each other and to the surface.
• By far the largest known lava tubes in the Solar System are on Venus. However, such tubes have also been discovered on the Moon, and especially on Mars.
• Considering the innumerable lava flows and lava channels on the flanks of Olympus Mons, it makes a lot of sense to expect truly gargantuan lava tube systems on the Red Planet. 
• Scientists now believe that these tunnels can be used to shield future explorers from the massive amounts of radiation they would be exposed to on the surface.
• Researchers can detect these tunnels remotely, through gravitational remote sensing. Things stand like this: we talk about the gravitational pull of a celestial body (let’s say, the Earth), but the gravitational field isn’t uniform. 
• The same goes for the Moon, Mars, and everything else — nothing is really uniform.
• By studying localized gravitational anomalies, underground cavities – such as lava tunnels – can be discovered. But researchers are also working on more advanced methods. 
• A concept for a radar system specifically designed to detect lava tubes on the Moon from orbit was presented. 
• The idea is to send an electromagnetic pulse towards the planet and then detect a specific signature associated with these tunnels. 
• Although it’s still early stages, there are promising results which may be used in the near future.
• The studies we have developed show that a multi- frequency sounding system is the best option for detecting lava tubes of very different dimensions. 
• The electromagnetic simulations show that lava tubes have unique electromagnetic signatures, which can be detected from orbit irrespective of their orientation to the radar movement direction. 
• Therefore, a mission carrying this instrument would enable a crucial step towards finding safe habitats on the Moon for human colonisation. 
• Geologists have also learned that the distribution of such systems can be generous enough to facilitate the establishment of large stations, or perhaps even entire cities.
• On Earth, they can be up to thirty metres across. In the lower gravity environment of Mars, we see evidence for lava tubes that are 250 metres in width. 
• On the Moon, these tunnels could be a kilometre or more across and many hundreds of kilometres in length,. 
• Lava tubes are environments shielded from cosmic radiation and protected from micrometeorites flux, potentially providing safe habitats for future human missions. 
• They are also, potentially, large enough for quite significant human settlements – you could fit most of the historic city centre of Riga into a lunar lava tube.

Source:Economic Times