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Caving…. on the Moon!

Lunar Volcanism

The Moon may not look like it at first glance, but instead of being home to giant and violent volcanoes like on Earth, it is in fact covered by plains of lava, known as “maria” (singular being “mare”) meaning “seas” in Latin (mistaken for seas by early astronomers [1]), along with a few smaller features like domes, cones, dark mantling deposits (deposits from more explosive eruptions [2]) and sinuous rilles (channels which lunar lava used to flow down, resembling rivers [3]).

However, volcanoes on the Moon don’t work quite like they do back on Earth: the Moon’s gravity is only a sixth of what it is on Earth, meaning there’s less force to move the lava, and yet there are massive lava plains, suggesting a very thin and runny lava was used to make this happen. Also the lunar volcanoes are less explosive, since what makes Earth’s volcanoes explosive is water: something that there is little of on the Moon (although it’s not devoid of it).

The forces driving it are also distinctly different, with no plate tectonics like Earth has, instead using the impacts of meteorites to create thinner crust where the lava can escape from. The overall thickness of the crust could in fact drive where lunar volcanic plains occur, with only 2% of the farside (the side not facing Earth) covered by maria, whilst a third of the nearside is covered by them, possibly because of the thinner crust on the nearside because of Earth’s gravitational pull [4].

And where the domes form reflects this, being several kilometres across, formed from slow cooling lava from lunar eruptions, but most importantly in clusters (where the crust is thinnest) [5]

Volcano Complex Mons Rümker in Oceanus Procellarum

Volcano Complex Mons Rümker in Oceanus Procellarum. Credit: NASA

So When’s the Next Eruption?

Well, there may never be another one (although we can’t be sure). This is because the magma, from when the Moon first formed, is cooling down, like on Earth. However, when it cools down, it gets thicker (more viscous), and with this thickening comes the increased difficulty to have eruptions.

Magma needs a pressure difference between it (the less dense material) and the rock above (the denser material), leading to the magma coming out of the lunar surface (as happens on Earth), like juice through a straw. In the Apollo 14 mission however, samples of lunar rock collected contained titanium-rich glass, formed from titanium-rich magma: something of a similar density to the Moon’s surface rock, leading to it being harder to have eruptions.

That said, the the cooling down of the Moon would start to change its composition, making it a less dense substance and so letting the less dense magma make its way up to the surface (as hypothesised by a team of Dutch earth scientists) [6].

Orange Taurus-Littrow soil, orange because of microscopic glass beads from early lunar volcanism

Orange Taurus-Littrow soil, orange because of microscopic glass beads from early lunar volcanism. Credit: NASA

Volcanic Solar System

It’s not just the Earth and Moon that are/were volcanically active; oh no! Most major rocky bodies, from planets to moons, have some form of volcanic past. In fact, there’s so many that I’m going to cut it down to three: Mars, Io and Pluto: three very different and interesting volcanic bodies.

Mars, one of our closest neighbours, is home to the tallest planetary mountain in the solar system: Olympus Mons [7], 21km in height (nearly 2.5x Mount Everest) [8] and 600km in diameter [7]. But how does something get this big?

Earth’s volcanoes are relatively short lived (generally), lasting a maximum of a few million years, but Martian volcanoes last for more than a billion years (but living a slower paced life). This long life means that layers of solidified lava can be built up over time, growing into what is seen on Mars today [9].

Olympus Mons

Olympus Mons. Credit: NASA/Viking 1

Io is the next stop in our volcanic tour. It’s the third largest Galilean moon (easily visible through binoculars) and has a distinctly ‘cheese-like’ look about it. This colour, however, is not because of cheese (sorry Wallace!), but frozen sulphurous molecules (apart from the dark regions) reflecting the Sun’s rays. And all these colours are because of one thing: volcanism.

This small body is littered with hundreds of volcanoes (which form those dark regions), some standing out not because they visibly look like volcanoes (they do), but because of infrared radiation from the heat the magma is giving out. It might look like it had an active past, but it’s still active today, not just shown by the heat from volcanoes, but from the ever-changing amount of heat given off (detectable from Earth). This activity is not just because of the pressure difference between the magma and crust, but the gravitational pull of its host planet, Jupiter. Yes, the gravitational pull from Jupiter is literally dragging the magma out of Io, shown by the difference in infrared radiation between either side of the moon [10].


Io. Credit: NASA/Galileo Spacecraft/JPL/University of Arizona

Pluto is our final, and most peculiar, stop in this quick tour. This is because the volcanoes don’t emit lava but, since they’re so far from the Sun, ice! Well, icy slush made of substances like water, nitrogen, ammonia or methane [11], but emitted from volcanoes which look rather like ones on Earth (apart from their size, at up to a massive 6km in height) [12], formed in a similar way to on Mars [11], creating mountains of frozen substances such as nitrogen (normally a gas back on Earth).

Yet cold Pluto (at an average of -232ºC [13]) needs heat to drive geologic processes like volcanoes (even if they do emit something like nitrogen slush). Heat could come from an underground ocean bringing energy up to the surface (in the form of energy know as latent heat, given out during the freezing process, thereby turning the surface into a liquid). Yes, a hidden liquid ocean - something we have on Earth, the mantle, but working in a very different way [14][15].


Pluto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Lunar Caving

But back to the Moon, because ESA are planning to send a mission to it to explore lunar caves created by its volcanic past. They’re more accurately described as ‘skylights’, since they’re collapsed parts of large systems of lava tubes running under the lunar surface, where lava once flowed [16]. The aim of this mission will be to both discover more about the Moon’s geology, but also to find out if these underground areas could be used as shelters for human visitors [17].

After getting ideas for this mission in 2019, ESA recently selected five of those ideas to become potential phases of a lunar mission. Firstly, the University of Würzburg, in Germany, came up with a spherical probe which could be lowered into these ‘skylights’ to create a 3D model of the lava tube, investigate its entrance, and find places with stable levels of radiation and temperature for humans to survive.

Then there’s the University of Oviedo, in Spain, who came up the idea of using a group of robots which use WiFi (and wireless charging) to connect with a lander on the lunar surface, in order to investigate what’s happening within the lunar tunnels [16].

The University of Bremen and the Robotics Innovation Centre of the German Research Centre for Artificial Intelligence (DFKI) (catchy!) also put forward their proposal, in which a semi-autonomous rover is lowered into the tube system via a cable, before being released from it and using it as a recharging and communication station, whilst mapping the tube system between charges [18].

Then there’s the Canadian aerospace company Canadensys, who proposed a scouting mission using a radar and a gravimeter (a sensor to detect how much gravity there is, changing by slight amounts depending on what is underneath the surface) mounted on rovers, to look underground at the tube system and see if it’s interesting enough to explore [19].

And then there’s the University of Manchester (saving the best ‘till last!), which uses the unconventional jumping robot (or rather fleet of jumping robots), instead of the rover, to move in the tube system, since there’s so much rubble inside it from its collapse (which made the ‘skylight’ which is needed to enter the tube system). And that’s not all: these small hopping rovers could be ejected from the lander, where they fall onto the lunar surface surrounded by an air bag, before they then eject from that air bag to hop in the hole using another air bag to protect themselves from a second fall, then escaping that one to jump around inside the lunar caves [20].

Marius Hills Pit (a 'skylight' possible landing sight for the mission)

Marius Hills Pit (a 'skylight' possible landing sight for the mission).

Credit: NASA/GSFC/Arizona State University

Human Settlement

As described before, these tubes could be used for human settlement. They would serve as places free from the large temperature swings (up to 127ºC by day, and as low as -173ºC by night [21]) to the radiation from the Sun (with no atmosphere or magnetic field to protect humans from harmful radiation such as X-ray and ultraviolet light) to micrometeorites (not burnt up like they are in our atmosphere, instead raining down onto the surface) to the dust kicked up by the rocket engines of spacecraft leaving the lunar surface.

Then there’s the possibility of water within them, in the form of ice, surviving in this state instead of evaporating away because of the caves acting as places of eternal darkness. This means they could be a source of water for people living within the tube system.

Transport is also a possibility, since they run across maria, possibly becoming the gateways to exploration on an even larger scale than seen before, and by humans [22].

But there are lots of challenges, all to be addressed in ESA’s mission to investigate a lunar volcanic tube system, from the question of whether they’re stable, to that of whether they’re actually able to protect from temperature swings and dangerous radiation.

With all this progress, hopefully one day we will have humans living permanently on the Moon, possibly using this tube system as a natural habitat. But even if it doesn’t turn out to be viable, there’s still a lot of interesting science to be conducted to see just what lunar volcanism was like, mapping these enormous and extensive tube systems.

Artist Impression of Moon Colony from 1986

Artist Impression of Moon Colony from 1986. Credit: NASA/Dennis Davidson

by George Abraham, ADAS member.

Click here for the previous news article

Click here for the next news article

Click here to watch the video on the Canadensys' proposal

Click here to watch the video on the University of Manchester's proposal

Click here to watch the video on the University of Oviedo's proposal

Click here to watch the video on the University of Würzburg's proposal

Click here to watch the video on the University of Bremen's and the DFKI's proposal

Click here to have a look at a map of a possible area to be explored by ESA for this mission

Click here to look at Olympus Mons on a map of Mars



  1. "Lunar maria: a complete guide to the seas of the Moon". BBC Sky at Night Magazine. Archived from original on 27th February 2021.

  2. "Dark Mantling Deposits". Oregon State University. Archived from original on 27th February 2021.

  3. "Sinuous Rilles". Oregon State University. Archived from original on 27th February 2021.

  4. "Volcanism on the Moon". Oregon State University. Archived from original on 28th February 2021.

  5. "All the planets and moons known to have volcanoes". BBC Earth. Archived from original on 28th February 2021.

  6. "Why the Moon has no active volcanoes". Wired. Archived from original on 28th February 2021.

  7. "The Tallest Mountains in the Solar System". Smithsonian Magazine. Archived from original on 28th February 2021.

  8. "How did the largest volcanoes in the solar system grow on Mars?" Natural History Museum. Archived from original on 28th February 2021.

  9. "Io: a guide to Jupiter's volcanic moon". BBC Sky at Night Magazine. Archived from original on 28th February 2021.

  10. "New Horzions: Pluto may have ice volcanoes". BBC News. Archived from original on 28th February 2021.

  11. "Icy Volcanoes May Erupt on Pluto". Archived from original on 28th February 2021.

  12. "Pluto". NASA Solar System Exploration. Archived from original on 28th February 2021.

  13. "If Pluto is Frozen, How Is It Generating Heat?" Science Channel, YouTube. Archived from original on 28th February 2021.

  14. "The Mystery of Pluto's Beating Heart". Archived from original on 28th February 2021.

  15. "ESA plans mission to explore lunar caves". ESA. Archived from original on 28th February 2021.

  16. "Seeking innovative ideas for exploring lunar caves". ESA. Archived from original on 28th February 2021.

  17. "Hopping rovers for lunar exploration". ESA. Archived from original on 28th February 2021.

  18. "What is the Temperature on the Moon?" Archived from original on 28th February 2021.

  19. "Living Underground on the Moon: How Lava Tubes Could Aid Lunar Colonisation". Archived from the original on 28th February 2021.

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