When we talk about amazing geological features, we often confine ourselves to Earth. But as a geologist, I think it’s crazy – there are so many structures on other worlds that can excite and inspire, and that can put the processes on our own planet into perspective.
Here, in no particular order, are the five geological formations in the Solar System (excluding Earth) that strike me the most.
the largest valley
I’ve left the Solar System’s largest volcano, Olympus Mons, on Mars, so that I can include that planet’s most spectacular canyon, Valles Marineris. Being 3,000 km long, hundreds of kilometers wide and up to eight kilometers deep, it is best viewed from space. If you were lucky enough to stand on one rim, the opposite rim would be far beyond the horizon.
It was probably initiated by fracturing when an adjacent volcanic field (called the Tharsis) began to bulge upward, but was widened and deepened by a series of devastating floods that climaxed more than 3 billion years ago.
Read more: Plate tectonics: New findings fill 50-year-old theory that explains Earth’s terrain
mountain of venus
We’re going to learn a lot more about Venus in the 2030s when two NASA missions and one from Essa (European Space Agency) will arrive. Venus is roughly the same size, mass, and density as Earth, leading geologists to wonder why it lacks Earth-style plate tectonics and why (or indeed) it has relatively few active volcanism. How does the planet extract its heat?
I find it reassuring that at least some aspects of the geology of Venus are familiar. For example, the northern edge of the highlands named Ovada Reggio looks surprisingly similar, apart from the lack of rivers through eroded, fold-like patterns, “folding mountains” on Earth such as the Appalachians, which result of one. Confrontation between continents.
I’m cheating a bit with my next example, as it is one of the largest impact basins in the Solar System and has an erupting volcano within it. Mercury’s 1,550 km diameter Caloris Basin was formed by a large asteroid impact about 3.5 billion years ago, and its floor was filled with lava shortly thereafter.
Not long after, a series of explosive eruptions eroded kilometer-deep holes through the solidified lava near the edge of the basin, where the lava cap was thinnest. These volcanoes spread ash particles over a radius of tens of kilometers. A deposit named Agvo Facula surrounds the explosive vent that I have chosen as my example.
Explosive explosions are driven by the force of gas expansion, and there is a surprising finding on Mercury, which had previously been expected to starve such volatile substances from its proximity to the Sun – the heat would have boiled them off. Scientists suspect that there were actually several explosive eruptions, which were probably over a long period of time. This means that gas-forming volatile material (the composition of which would remain uncertain until work on Essa’s BepiColombo mission in 2026) was repeatedly available in Mercury’s magma.
In soil or vegetation-rich areas on Earth, reefs provide the greatest exposure to clean rock. Although dangerous to reach, they reveal an uninterrupted cross-section of rock and can be great for fossil hunting. Because geologists love them so much, I give you the seven kilometer high Verona look. It is a feature on Uranus’ small moon Miranda that is often described as the “tallest rock in the Solar System”, including recently on NASA’s website. It has even been said that if you were careless enough to fall from the top, it would take you 12 minutes to fall down.
Read more: Mysterious red spots on Mercury get names – but what are they?
This is bullshit, because Verona Rups is nowhere near vertical. The only images we have of this are of Voyager 2, taken while flying by Uranus in 1986. It’s undeniably impressive, almost certainly a geological fault where a section of Miranda’s icy crust (the planet’s outermost “shell”) has driven downward against an adjacent block.
However, the skew of the view is deceptive, making it impossible to be sure of the stability of the face—it probably slopes at less than 45 degrees. If you stumble at the top, I doubt you’ll slip at the bottom as well. The face appears very smooth in the best, rather low resolution image we have, but at Miranda’s -170 °C day temperature, the water-ice has a high friction and is not slippery at all.
Titan’s submerged coastline
For my last example I could happily choose anywhere on Pluto, but instead I’ve opted for a ghostly Earth-like beach on Saturn’s largest moon, Titan. Here, a large depression in Titan’s water-ice “bedroll” hosts a sea of liquid methane called Ligeia Mare.
The basins created by methane rivers flowing into the ocean are apparently flooded as sea levels rise. This intricately indented beach reminds me strongly of Oman’s Musandam Peninsula, which is on the south side of the Strait of Hormuz. There, the local crust has been deformed downwards due to an ongoing collision between the Arabian and Asian mainlands. Has something similar happened on Titan? We don’t know yet, but the way the coastal geomorphology changes around Lijia Mare suggests to me that its submerged basins are more than a direct result of rising liquid levels.
Read more: Titan: First cosmic map reveals secrets of Saturn’s potentially habitable moon
Rock and liquid water on Earth, cold water—ice and liquid methane on Titan—make little difference. Their mutual interactions are similar, and so we see geology repeating itself in different worlds.