Mars

I have omitted the largest volcano in the solar system, Olympus Mons on Mars, in order to include the most spectacular canyon on that planet, Valles Marineris. At 3,000 km long, hundreds of kilometers wide and up to eight kilometers deep, it is best seen from space. If you were lucky enough to be on one edge, the opposite edge would be beyond the horizon.

It was probably initiated by fracturing when an adjacent volcanic region (called Tharsis) began to bulge upwards, but was widened and sunk deep by a series of catastrophic floods that reached their climax more than 3 billion years ago.

The Folded Mountains of Venus
We’re going to learn a lot more about Venus in the 2030s, when two NASA missions and one from ESA, the European Space Agency, arrive. Venus is about the same size, mass and density as Earth, so geologists wonder why it lacks Earth-like plate tectonics and why it has comparatively little active volcanism. How does the planet extract its heat?

I am reassured that at least some aspects of the geology of Venus are familiar to us. For example, the northern edge of the highlands called Ovda Regio is strikingly similar, apart from the absence of rivers running through the eroded and folded pattern, to Earth’s “folded mountains” such as the Appalachian Mountains, which are the result of a collision between continents.

Mercury’s piercings
I’m cheating a bit with my next example, because it’s both one of the largest impact basins in the solar system and an exploding volcano within it. Mercury’s Caloris Basin, 1,550 km in diameter, was formed by the impact of a large asteroid about 3.5 billion years ago, and its floor was flooded by lava shortly thereafter.

Shortly thereafter, a series of explosive eruptions blasted holes several kilometers deep in the solidified lavas near the edge of the basin, where the lava layer was thinnest. These holes sprayed volcanic ash particles for tens of kilometers. One of these deposits, called Agwo Facula, surrounds the explosive fumarole that I have chosen as an example.

Explosive eruptions are driven by the force of expanding gas, and are a surprising finding on Mercury. Its proximity to the Sun was expected to deprive it of such volatile substances (the heat would have made them boil).

Scientists suspect that there were in fact several explosive eruptions, possibly spaced out over a long time scale. This means that volatile gas-forming materials – the exact composition of which will remain uncertain until ESA’s BepiColombo mission begins work in 2026 – were repeatedly present in Mercury’s magmas.

The highest cliff
In land regions rich in soil or vegetation, cliffs offer the largest exposures of clean rock. Although dangerous to approach, they reveal an unbroken cross section of rock and can be great for fossil hunting.

Since geologists like them so much, I present to you the Verona Rupes escarpment, seven kilometers high. It’s a feature of Uranus’s small moon Miranda, which is often described as “the tallest cliff in the solar system,” including on a recent NASA website. In it, it is even commented that if one were careless enough to walk around the top, it would take 12 minutes to fall to the bottom.

This makes no sense, because Verona Rupes is far from vertical. The only images we have of it are from Voyager 2, taken during its pass by Uranus in 1986. It is undeniably impressive, since it is almost certainly a geological fault in which a block of Miranda’s icy crust ( the outermost “shell” of the planet) has moved down against the adjacent block.

However, the obliquity of the view is misleading, so it is impossible to be sure of the tilt of the face – it is probably tilted less than 45 degrees. If one trips at the top, I doubt they’ll slide to the bottom. The surface appears to be very smooth in the best (low resolution) image we have, but at Miranda’s -170°C daytime temperature, water-ice has high friction and is not at all slippery.

Titan’s Drowned Coast
For my last example I could have chosen just about anywhere on Pluto, but instead I’ve opted for an eerily Earth-like coastline on Saturn’s largest moon, Titan. Here, a large depression in Titan’s “ice bed” harbors a sea of liquid methane called Ligeia Mare.

The valleys carved by the methane rivers that flow into the sea have evidently been flooded as the sea level rises. This intricately indented coastline is very reminiscent of Oman’s Musandam Peninsula, on the southern side of the Strait of Hormuz. There, the local crust has been deformed downwards due to the ongoing collision between the Arabian and Asian lands.

Has something similar happened on Titan? We don’t know yet, but the way the coastal geomorphology changes around Ligeia Mare suggests that its drowned valleys are more than just a direct result of rising liquid levels.

Rock and liquid water on Earth, frigid water-ice and liquid methane on Titan: the difference is minimal. Their mutual interactions are the same; geology repeats itself on different worlds.