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 mountains several kilometers high? For instance, Zeeman Mons on the lunar farside peaks as tall as Mount 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 kilometers. Larger craters display more complexity.
The force imparted on the Moon’s surface by larger asteroids and comets, especially those with high velocities, can be tremendous. 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. This process is visualized in the GIF below.
Most lunar mountains are formed by this highly energetic process that takes a geologically negligible passage of time. The kilometer-plus high central peaks of the young, city-sized Aristarchus crater and 86-kilometer-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 exposed lunar interior by the virtue of their central mountains. Craters larger than these offer two central peaks.
The mechanics work such that for larger crater sizes or more energetic impacts, 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 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-kilometer-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-kilometer-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’ve 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 of its original form due to Earth’s active weathering. As such, Schrödinger offers an analog to better understand Chicxulub.
For crater sizes beyond 500 kilometers, you get not one but multiple rings of mountains. The 930-kilometer-wide ancient crater of Orientale 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 did 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 during 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, which you see as dark regions on the Moon. The prominent, arc-shaped mountain range of Montes Apenninus, forming 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 of 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. And the Moon being so close to us 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 the lunar mountains. But to better understand the composition, structure and origin, surface missions are needed, especially sample return ones 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 of the past 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. However, things may change with NASA’s upcoming Artemis missions.
The Artemis program 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 the 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, launching end of 2020 is targeting the largest such lunar dome, Mons Rümker.
The 70-kilometer-wide Mons Rümker is thought to have formed less than two billion years ago. The Chang’e 5 mission aims to bring back samples from the dome and determine its exact age. Since scientists use the Moon to calibrate the timing of events across bodies in the Solar System, analysis of Chang’e 5 samples would be an invaluable addition to happenings in the geologically recent past.
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.
Republished by The Wire Science.