A representative image of the Moon. Photo: Mike Petrucci/Unsplash.
Unlike the millions of years it takes for most mountains on Earth to form, lunar mountains crop out near instantaneously – geologically speaking.
Earth’s mountains primarily form when two colliding plates of the Earth’s crust lift up volumes of rock, slowly creating an elevated landform. Over millions of years, wind, water and gravity erode these uplifted sections, wearing their surface to make the mountains we are familiar with today.
But the Moon has no plate tectonics, atmosphere or running water. How then does it boast of mountains several kilometres high? For example, Zeeman Mons on the lunar farside peaks as high as Mt Everest.
The answer lies in the one of the most apparent features on the Moon: craters.
Most lunar craters are small and bowl-shaped, formed when asteroids and comets impact the surface. This shape persists for crater sizes up to about 20 km. Larger craters display more complexity.
Larger asteroids and comets, especially those with high velocities, can impart a tremendous amount of force on the Moon’s surface. In such cases, in addition to a crater being formed, the surface in and around the impact point is compressed further. This causes the crust to melt. When the melted crust can’t be compressed any further, it bounces right back up, forming a central mountain.
Most lunar mountains are formed by this highly energetic process that takes a geologically negligible amount of time. The kilometre-plus high central peaks of the young, city-sized Aristarchus crater and 86-km-wide Tycho crater are fine examples.
Aristarchus crater was one of the candidate landing sites for the now-cancelled Apollo 17+ missions. Visiting Aristarchus or Tycho in a future surface mission will allow us to study the lunar interior exposed by virtue of their central mountains. Even larger craters offer two central peaks instead of just one.
Specifically, for larger crater sizes or more energetic impacts, the newly formed central peak splits into two before it can solidify. The 93-km-wide Copernicus and 77-km-wide King craters respectively host two distinct peaks, each towering more than six kilometres high!
Apart from allowing scientists to study the lunar interior, such places are key to understanding mechanics of impacts that form such features, found not just on the Moon but across the Solar System.
Put a ring on it
For even larger craters, the twin peaks widen into a ring of mountains, like a liquid drop causing a ripple on still water. The 312-km-wide crater of Schrödinger on the Moon’s farside is a well preserved example, despite being almost four billion years old.
A mission to Schrödinger can help solve fundamental mysteries about the Moon’s evolution, like if the Moon indeed had a global magma ocean – a hypothesis tied to its origin. Further, Schrödinger lies inside the Moon’s largest impact crater, the 2,500-km-wide South Pole-Aitken basin. The impact that created the basin excavated deep into the lunar crust, and perhaps even the mantle. Since Schrödinger formed later, its impact could have penetrated deeper and uplifted more materials, offering insights – literally and figuratively – into the lunar interior.
The Chicxulub crater on Earth, linked to the extinction of dinosaurs, is also thought to have formed as a ringed crater, but wore down to its original form due to Earth’s active weathering. As such, Schrödinger offers an analog to better understand Chicxulub.
For craters bigger than 500 km, you get not one but multiple rings of mountains. The ancient crater of Orientale on the Moon’s farside, over 900 km wide, boasts three mountain rings, most of which remain intact.
Missions to both Schrödinger and Orientale can tell us exactly when large asteroids and comets excessively bombarded bodies in the Solar System. This period of blistering impacts is particularly important as Earth is thought to have got its water in this time.
For some ancient craters, like Imbrium on Moon’s nearside, only parts of the outermost mountain ring are visible today. The rest of the interior has been drowned in lava, visible as dark regions on the Moon. The prominent, arc-shaped mountain range of Montes Apenninus, forming Imbrium’s southeast border, stretches 600 km long.
Multi-ring impact basins exist on many other worlds in the Solar System, like Caloris on Mercury, an unnamed basin on Jupiter’s moon Ganymede, Evander on Saturn’s moon Dione and more. Callisto, one of Jupiter’s natural satellites, boasts the largest multi-ring basin of the Solar System, called Valhalla. It is 3,800 km wide.
The ubiquity of mountains formed by impacts across the Solar System and their consistent patterns indicate common underlying geological mechanisms. Being so close to us, the Moon presents us with an opportunity to study these fundamental processes in planetary science.
Exploring the mountains
Moon orbiters use remote sensing techniques to understand the composition of lunar mountains. But to better understand the structure and origin as well, we need surface missions, especially sample-return missions that bring back some of the lunar soil so researchers can more precisely determine their age. To that end, NASA selected several of the above mentioned places as candidate landing sites for the now-cancelled Constellation programme, to return humans to the Moon.
However, sending landing and roving missions to lunar mountains presents a considerable engineering hurdle. Most surface missions of the past landed in the dark lunar plains – vast, solidified lava regions with a relatively uniform surface. The rocky nature of the mountainous regions make it more difficult to safely touch down on. However, things may change with NASA’s upcoming Artemis missions.
The Artemis programme aims to explore the lunar poles in this decade, both robotically and with humans. This requires developing precision landing technologies for safe descent on the challenging polar terrain, as a result also enabling surface missions to lunar mountains. But there is a type of mountain that spacecraft can visit even with present-day technologies.
The Moon has experienced several distinct periods of volcanism in the last four billion years. Lava slowly oozing out of openings on the surface during such times have formed volcanic domes. These domes aren’t as tall as the impact-created mountains and have gentle slopes. China’s third Moon landing mission, Chang’e 5, to be launched at the end of 2020, is targeting the largest such lunar dome, Mons Rümker.
Mons Rümker, 70 km wide, is thought to have formed less than two billion years ago. The Chang’e 5 mission aims to bring samples back from the dome to determine its exact age. Since scientists use the Moon to calibrate the timing of events across bodies in the Solar System, data from analysis of Mons Rümker’s dome would be an invaluable addition to our knowledge of happenings in the geologically recent past.
Mountains on the Moon are a marvel that give us a peek into the lunar interior, help discern the chain of events in the Solar System’s evolution and improve our understanding of the physical processes that shape airless worlds everywhere.
This article was originally published on Jatan Mehta’s blog and has been republished here with permission, with light edits for style.