Exploring the marvel that are mountains on the Moon
The near-instantly formed lunar mountains offer a peek into the Moon's interior, and improve our understanding of the solar system.
Unlike the millions of years it takes for most mountains on Earth to form, lunar mountains crop out near-instantly, 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 mountains several kilometers tall? For instance, Zeeman Mons on the Moon’s farside peaks as high as Mount Everest.
The answer lies in the one of the Moon’s most apparent features—craters.
Most lunar craters are small and bowl-shaped, formed when asteroids and comets impacted its surface. This shape persists for crater sizes up to about 20 kilometers but larger craters display variety.
Big asteroids and comets with high velocities can impart a tremendous force on the Moon’s surface. They will not just make a crater but compress the surface in and around the impact point enough to melt the crust. When the melted crust can’t be compressed any further, it bounces back and forms a central mountain upon cooling. This process is visualized in the GIF below.
Most mountains on the Moon are formed by this highly energetic process that takes a geologically negligible passage of time. The kilometer-plus high central peaks of the city-sized Aristarchus crater and 86-kilometer-wide Tycho crater stand tall as 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 mission will allow us to study the exposed Moon’s interior by the virtue of their central mountains.
For even larger craters or higher velocity impacts, the mechanics work such that the newly formed central peak splits into two before it can solidify. The 93-kilometer-wide Copernicus and 77-kilometer-wide King craters respectively host two distinct peaks, each towering more than six kilometers!
Apart from allowing scientists to study the Moon’s interior, visiting such places would be key to understanding exactly how impacts cratering takes place, not just on the Moon but across the solar system.
Put a ring on it
For even larger craters, the twin peaks widen to form a ring of mountains, like a liquid drop causing a ripple on still water. The 312-kilometer-wide Schrödinger crater on the Moon’s farside is a well preserved example, despite being almost four billion years old.
A mission to Schrödinger, such as the one NASA announced for 2024, can help solve fundamental mysteries about the Moon’s evolution, like if the Moon indeed was once fully covered in an ocean of magma—a hypothesis tied to its origin. Further, Schrödinger lies within the Moon’s largest impact crater, the 2,500-kilometer-wide South Pole-Aitken basin. The impact that created the basin excavated material from deep into the lunar crust, and perhaps even the mantle. Since Schrödinger formed later, its impact could’ve penetrated deeper and uplifted even more material, offering insights 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 has now worn down by Earth’s active weathering. As such, visiting the Schrödinger crater is our chance to better understand Chicxulub.
For craters larger than 500 kilometers, you get not one but multiple mountain rings. The 930-kilometer-wide ancient Orientale crater on the Moon’s farside boasts three mountain rings, most of which is preserved.
Missions to both Schrödinger and Orientale can tell us exactly when large asteroids and comets excessively bombard bodies in the solar system. This period of blistering impacts is particularly important as Earth is thought to have gotten its water, and possibly life-critical organics, from asteroids and/or comets during this time.
For some ancient craters, like Imbrium on Moon’s nearside, only parts of its outermost mountain ring are visible today. The basin’s interior has been drowned in lava, which you now see as dark regions on the Moon after the lava solidified. The prominent, arc-shaped mountain range of Montes Apenninus on Imbrium’s southeast border stretches 600 kilometers 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. Jupiter’s moon Callisto boasts the largest multi-ring basin in the solar system, called Valhalla, spanning 3800 kilometers wide.
The ubiquity of mountains formed by impacts across the solar system and their consistent patterns indicate common geological mechanisms at play. The Moon being so close to us presents an opportunity to study these fundamental processes in planetary science in great, testable detail.
Exploring the mountains
Lunar orbiters use remote sensing techniques to understand the composition of the lunar mountains. But to better understand their composition, structure and origin, surface missions are needed, especially sample return ones so as to determine precise ages. To that end, NASA had selected several of the above mentioned places as candidate landing sites for the now cancelled Constellation program to return humans to the Moon.
However, sending landing and roving missions to lunar mountains is a bit of an engineering hurdle. Most surface missions thus far have landed in the dark lunar plains—vast, solidified lava regions that provide a relatively uniform surface for spacecraft to land on. The rocky nature of the mountainous regions make it more difficult to safely touch down on. This may change with NASA’s upcoming Artemis missions. The Artemis program aims to explore the Moon’s poles in this decade, both robotically and with humans. The precision landing technologies required to touch down safely on the challenging polar terrain also enables missions to the lunar mountains.
Mountains on the Moon are a marvel that give us a peak (pun intended) 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.
Originally published in 2020, updated in 2021 to include context from NASA’s Artemis Moon exploration program. Republished by The Wire Science.