How the Moon’s two Grand Canyons formed in mere minutes

Both astronomically and geologically, our Moon has long been one of the most fascinating objects to all of humanity. With giant mountains, rich arrays of craters, ancient lava basins and more, it tells a very different history than Earth: one without volcanoes, oceans, weather, an atmosphere, or active plate tectonics to erase imprints on its surface. Unlike the Earth, where our surface is geologically very young, many features on the Moon are billions of years old, with some features dating back more than four billion years: to nearly the earliest moments in our Solar System’s history.

And yet, there are some similar-looking features between our world and our smaller companion world: giant canyons. Here on Earth, features like the Grand Canyon were carved by flowing water over many millions of years, with the release of huge stores of water — previously held back by dams of ice during our planet’s ice ages — responsible for the majority of the canyon’s width and depth. But on the Moon, there is no liquid water on the surface, and there likely never was. Yet many long, deep channels and valleys can be found there as well. Some of them meander irregularly, suggesting they were carved by lava flows. But two of them, located near the Moon’s south pole, Vallis Schrödinger and Vallis Planck, were formed in only about 10 minutes: from a large impact. Here’s the science behind how we figured it out.

This two-faced mosaic from NASA’s Lunar Reconnaissance Orbiter shows the near side (left) and the far side (right) of the Moon with modern technology. By looking at the ratios and sizes of craters found on the Moon with respect to the age of that portion of the Moon, Mars, Mercury, and Earth, we can learn how cratering rates have varied over the Solar System’s history. Now, with our first samples from the lunar far side having been returned to Earth, we might finally learn more about the Moon’s ultimate origins.

When you look at the Moon up close, either through a telescope or by taking a journey to the Moon itself, there are a number of striking differences that jump out at you, visually, compared to the Earth. While the Earth occasionally has craters on its surface — usually from relatively recent asteroid strikes — the Moon is littered with them.

• Some old, large craters have smaller craters inside of them, indicating that this cratering happened in stages, with newer craters often erasing and covering over older ones.
• Some low-lying areas are darker than the rest of the Moon, and have fewer craters in them overall: evidence for lava flows that filled in those low-lying areas at later times than when most of the cratering occurred.
• Some of the craters that we find have an array of rays coming out of them, as though the impactor that created them didn’t merely carve a giant depression into the ground, but kicked out that material for up to hundreds of kilometers in all directions.
• Some craters appear to have channels flowing out of them: looking akin to dried up riverbeds.
• And some craters, if we examine them closely, appear to form in linear-like chains.

There are good reasons for all of these features, even though they aren’t necessarily the same reason for each one.

This image shows Pierazzo crater and its prominent rays emanating from it. Once thought to be evaporated salt deposits, we know now that these rays are ejecta from the impact that created the crater itself, where the light color is due primarily due to oxidized chemicals, such as iron oxide, that are more prevalent than the uppermost material on the lunar regolith.

First up, let’s tackle the issue of crater rays. When a large, massive, rapidly moving object strikes a planet or moon, it doesn’t just make a crater, but causes an enormous excavation event. The impact itself compresses the solid ground beneath it, creating a depression, while the “walls” of the crater are pushed outward and often upward as well, creating a rim, or ridge, around the crater itself.

However, the reason we say “excavation event” is because that material that was previously where the crater now is must get ejected: kicked up and out of the crater itself, and then that material must land somewhere else that’s outside of the crater.

Most impact craters that have these rays send ejecta up to a few times the diameter of the crater itself, but a few of the largest and most prominent lunar craters — including Copernicus and Tycho — have rays of ejecta that can extend up to halfway round the Moon itself.

And in some cases, it’s thought that the impact craters themselves, way back when they were created, carved so deeply into the lunar regolith that they allowed liquid magma from inside the Moon’s interior to well up to the surface. When this occurs, there are ensuing lava flows that come out, carving lava channels in the Moon’s surface as they flow down to the region of lowest elevation.

This region of the Moon, known as Aristarchus Plateau, contains the largest rille on the Moon, Vallis Schröteri. Appearing snake-like, this valley, like most (but not all) lunar valleys, was formed by an ancient lava flow, and may be related to the nearby large impact crater seen at the lower-right.

Much like creeks flowing down a hillside here on Earth, those lava flows carve chasms — known as rilles — wherever they occur on the Moon. The largest rille on the Moon ever discovered is Vallis Schröteri, shown above as imaged by NASA’s Lunar Reconnaissance Orbiter. It’s an incredibly impressive valley:

• 160 kilometers in length,
• 11 kilometers in width at its widest,
• and almost 500 meters (half a kilometer) deep.

In other words, that was a lot of lava, possibly over a long amount of time or in multiple events, that flowed so long ago to create Vallis Schröteri.

These meandering shapes, where these channels flow downhill (to lower elevation) overall, represent the majority of rilles, or valleys, found on the Moon. Many of them are smaller, but still “snake-like” like Vallis Schröteri is, indicating that they, too, were likely formed by lava flows.

But it’s the exceptions that are often incredibly interesting to look at. Over near the Moon’s south pole, there’s a large and ancient impact crater: Schrödinger, named after the physicist famous for his equation in quantum mechanics. It has a wide, raised outer rim, but outside of it are two valleys leading away from it that don’t have those curved, snake-like features found in other lunar rilles.

This area of the Moon is centered near the South Pole, focused on a large impact crater known as Schrödinger crater. Two straight-line, valley-like features, Vallis Schrödinger (upper right) and Vallis Planck (lower right) are seen alongside the crater as well.

These two valleys are known as Vallis Schrödinger, which radiates away to the northwest, and Vallis Planck, which radiates away to the north. Schrödinger crater is a pretty big one: about 320 kilometers in diameter and around 4.5 kilometers deep. Based on cratering rates inside and outside of it, it’s estimated to be around 3.8 billion years old: very old, but about ~700 million years younger than the absolute oldest features found on the Moon.

So where did these two valleys that point away from Schrödinger crater come from?

They’re very unlike the valleys carved by lava flows on the Moon, as they don’t exhibit the meandering bends common to them. They don’t necessarily flow from high elevation to low elevation, as they appear to have no care for other pre-existing features such as mountains, valleys, or craters. And they aren’t necessarily completely continuous either, as there are large continuous valleys separated by other surface features, before additional craters and/or valley-like features reappear again following the exact same line. It might seem like a difficult problem, but there’s a hint from another class of features found commonly on the Moon: long crater chains, often found in-line with a large, giant crater.

This image shows a series of craters, varying in size, but all lined up. This is known as the Mendeleev crater chain, and was likely caused by a tidally torn-apart impactor striking the lunar surface long ago.

What could these crater chains, like Davy or Abulfeda (or Mendeleev, shown above) be indicative of?

We have to remember that the Moon is responsible for the tides on Earth: it exerts a slightly different gravitational force on different parts of the Earth. If the average force on the Earth is given by the gravitational force from the Moon on the Earth’s center, then we have to realize that the force on:

• the near side of the Earth will be slightly greater than average, toward the Moon,
• the far side of the Earth will be slightly less than average, toward the Moon,
• the “top” side of the Earth will be slightly directed downward compared to the average,
• the “bottom” side of the Earth will be slightly directed upward compared to the average,

and so on. This creates two tidal bulges on the Earth: one facing toward the Moon and one facing away from the Moon, while the “edges” of the Earth, as compared to the Earth-Moon line, are compressed inward toward the Earth’s center.

When this happens to an incoming impactor, however, the tidal forces that it experiences don’t just cause bulging; unlike the Earth, asteroids and comets are not held together significantly by the force of gravity. The tidal forces on them, as they approach a large, massive body like the Moon, can be enough to tear them apart completely, creating a “line” of objects that create a line of impacts, all in a row. This happened famously with Comet Shoemaker-Levy 9 as it broke apart on its collision course with Jupiter, and it clearly happened on the Moon to create the variety of crater chains that we see.

These views of the straightest, widest, deepest parts of Vallis Schrödinger (left) and Vallis Planck (right) show a series of lined-up craters all superimposed together. At bottom, a series of reconstructed impact craters allows scientists to determine the speed and size of the objects that could have created these scars on the Moon’s surface.

It’s with this in mind that scientists took a close, hard look at Vallis Schrödinger and Vallis Planck. We can compare these two long, narrow valleys with the Earth’s Grand Canyon, and learn the following.

• The Grand Canyon is 446 kilometers long, up to 29 kilometers wide, and up to 1.8 kilometers deep.
• Vallis Schrödinger is 270 kilometers long, up to 20 kilometers wide, and up to 2.7 km deep.
• And Vallis Planck is 280 kilometers long, 27 kilometers wide, and 3.5 kilometers deep, with a ray of secondary craters extending up to a total length of 860 kilometers.

These two “lunar Grand Canyons” are just as wide, even deeper, and nearly as long as the Grand Canyon found on Earth. Unlike the Grand Canyon, which was carved by water, or unlike the majority of lunar rilles and valleys, which were carved by lava, these two valleys were probably formed in the same event that created Schrödinger crater: a massive impact, where either:

• rocky debris from the collision,
• or tidally torn-apart components of the original impactor,

continued to strike the lunar surface even far away from the location of Schrödinger crater itself.

This two-panel animated graph shows a comparison of the depth and width of the Grand Canyon on Earth with Vallis Planck on the Moon. While these two geological features are comparably large in size and scale, the timescales for their formation are vastly different: millions of years for the Grand Canyon, but mere minutes for Vallis Planck.

According to the lead author of a new paper that uncovers the origin of these lunar Grand Canyons, USRA’s David Kring,

“Nearly four billion years ago, an asteroid or comet flew over the lunar south pole, brushed by the mountain summits of Malapert and Mouton, and hit the lunar surface. The impact ejected high-energy streams of rock that carved two canyons that are the size of Earth’s Grand Canyon. While the Grand Canyon took millions of years to form, the two grand canyons on the Moon were carved in less than 10 minutes.”

Interestingly, you can draw an “imaginary line” backward: from Vallis Schrödinger and Vallis Planck back toward Schrödinger crater, and you’ll find that the two imaginary lines, as shown below, intersect at roughly the very back of Schrödinger crater’s rim. This data is suggestive — but not conclusive — that it wasn’t a series of torn-apart fragments from the original parent impactor (that made Schrödinger crater) that created these two valleys, but rather secondary ejecta from the primary collision itself.

This orbital view of Schrödinger crater, alongside Vallis Schrödinger and Vallis Planck, suggests that the geometry of these three features are related. At right, if you trace back these long, straight valleys to Schrödinger crater itself, you see that all three align at the crater’s southern rim.

This is reflected in the different inferred speeds from the impactor that created the primary crater, at 15 kilometers-per-second, from the speed of the secondary impactors that created these two great valleys, which varies slightly between different parts of the valley: as low as 0.8 km/s for the larger fragments that are closer to the primary crater, but as high as 1.2 km/s for the smaller fragments at the greatest distances from the primary crater.

If these valleys were created the same way an array of crater chains gets created, then you would expect the impact speeds to all be comparable to not only one another, but to the speed of the main impactor as well. Instead, what we actually see is a huge difference in speeds: more than a factor of 10 slower, indicating that this wasn’t a torn-apart asteroid or comet that created a chain of overlapping craters. The most likely scenario, as determined by the team of researchers who conducted this study, is secondary ejecta — with more than 30 separate pieces ranging from ~1 kilometer up to about ~4-5 kilometers in size — were kicked up and then deposited hundreds of kilometers away from the initial, primary collision.

Based on the properties of the various craters found along a straight, deep valley that traces back to a crater, scientists can infer the size and speed of the various chunks of ejecta that collided with the Moon to create this valley. While it may technically be the result of a chain of craters superimposed atop one another, it’s very different in nature from other crater chains found on the Moon.

This has a profound implication for the Moon: the large basin at the South Pole, cleverly named the South Pole–Aitken basin, is likely not filled in with debris and ejecta arising from the impact that created Schrödinger crater and the associated Vallis Schrödinger and Vallis Planck. The geometry of this impact indicates a relatively shallow angle, and a direction that’s away from the lunar South Pole headed north-northwest. This is important: the South Pole-Aitken basin is the largest, oldest, and deepest basin on the Moon, while the Schrödinger impact basin is the second youngest basin on the lunar surface, occurring some 400-500 million years later.

Perhaps most impressively is the timescales on which all of this occurred. Whereas Arizona’s Grand Canyon was carved by the transport of water, ice, and material carried along by accompanying deluges over the last 5 to 6 million years, bringing together and integrating several other, older canyons (or paleocanyons) that took an estimated 70 million years to form, these two “Grand Canyons” on the Moon — Vallis Schrödinger and Vallis Planck — were carved by streams of secondary ejecta from an impact crater, or rock and rocky debris, in a timescale of under 10 minutes. Unlike the meandering valleys created by lava flows, these two lunar Grand Canyons were carved by massive impacts, and came fully into existence in a matter of mere minutes.