I have a moon in my story that has a thin, deep gash or canyon that lies on its equator and spans the entire moon.

My original idea involved the sudden orbital decay of a ring system, caused by a sudden increase in mass from a few small-scale collisions. However, ring systems are invisible or nearly invisible as viewed from the side, which is the view it needs to be. The gash should be faintly visible with the naked eye.

My second idea was that the moon was captured by the planet and went through its ring system, but that causes the same problems as before.

I've done the calculations and the energy released from such small collisions from a ring system would not create craters large enough to be seen from far away.

What are some possible ways to explain this equatorial gash? Good answers will provide scientifically plausible theories of the gash's formation.

  • $\begingroup$ It spans the moon's entire equator? I mostly ask because that would be a ring-scar . . . I can only think that something started to rip the moon in half and failed, but the moon's structure was too rigid to settle back into a sphere . . . or maybe some structure ringing the moon exploded catastrophically? $\endgroup$
    – HA Harvey
    Commented Sep 14, 2019 at 6:05
  • $\begingroup$ If you wish to amalgamate your separate accounts, go to the bottom of any page and click in the "contact" link. A member of staff will sort it out for you. $\endgroup$ Commented Sep 14, 2019 at 6:20
  • $\begingroup$ Does it have to be natural? $\endgroup$ Commented Sep 14, 2019 at 10:45
  • $\begingroup$ So the anti-Iapetus? $\endgroup$ Commented Sep 14, 2019 at 18:41
  • $\begingroup$ Pretty much the anti-Iapetus, yes. But it's a thin gash you can only just make out with the naked eye. $\endgroup$
    – overlord
    Commented Sep 17, 2019 at 13:38

3 Answers 3


I see three ways to go about doing this: Go for something natural, but imperfect, like the Valles Marineris on Mars; cheat by using a small moon not in hydrostatic equilibrium, specifically a contact binary; or go for an artificial surface feature.

Valles Marineris and Rift Vallies

Valles Marineris is probably the most visually impressive rift valley in the solar system, being 4,000 km (2,500 mi) long, 200 km (120 mi) wide and up to 7 km (23,000 ft) deep and free of obscuring water or clouds it looks quite impressive. Rift valleys and structures suspected to be rift valleys have been found all over the solar system. Earth, Mars, Venus, Charon, Titania, Ariel, Oberon, Miranda, Tethys, and Luna have such structures.

While the martian Valles Marineris might be the best known and best looking example,

image of Mars with Valles Marineris from Wikipedia

Ithaca Chasma on Thetys might be closest to what you want, as it is up to 100 km wide, 3 to 5 km deep and 2,000 km long and runs approximately three-quarters of the way around Tethys circumference.

Ithaca Chasma on Thetys form Wikipedia

I would also not disregard Eaths mid-ocean-ridges as they too are quite long.

Contact Binary Asteroid-Moon

The concept of hydrostatic equilibrium has also become important in determining whether an astronomical object is a planet, dwarf planet, or small Solar System body. According to the definition of planet adopted by the International Astronomical Union in 2006, one defining characteristic of planets and dwarf planets is that they are objects that have sufficient gravity to overcome their rigidity and assume hydrostatic equilibrium. Such a body will normally have the differentiated interior and geology of a world (a planemo), though near-hydrostatic bodies such as the proto-planet 4 Vesta may also be differentiated. Sometimes the equilibrium shape is an oblate spheroid, as is the case with Earth. However, in the cases of moons in synchronous orbit, nearly unidirectional tidal forces create a scalene ellipsoid. Also, the dwarf planet Haumea is scalene due to its rapid rotation. It had been thought that icy objects with a diameter larger than roughly 400 km are usually in hydrostatic equilibrium, whereas those smaller than that are not. Icy objects need less mass for hydrostatic equilibrium than rocky objects. The smallest object that appears to have an equilibrium shape is the icy moon Mimas at 397 km, whereas the largest object known to have a non-equilibrium shape is the rocky asteroid Vesta at 525 km (573 × 557 × 446 km). However, Mimas is not actually in hydrostatic equilibrium for its current rotation. The smallest body confirmed to be in hydrostatic equilibrium is the dwarf planet Ceres, at 945 km, whereas the largest body known to not be in hydrostatic equilibrium is the icy moon Iapetus, at 1,470 km. Because the terrestrial planets and dwarf planets (and likewise the larger satellites, like the Moon and Io) have irregular surfaces, this definition has some flexibility, but a specific means of quantifying an object's shape by this standard has not yet been announced. Local irregularities may be consistent with global equilibrium. For example, the massive base of the tallest mountain on Earth, Mauna Kea, has deformed and depressed the level of the surrounding crust, so that the overall distribution of mass approaches equilibrium. The amount of leeway afforded the definition could affect the classification of the asteroid Vesta, which may have solidified while in hydrostatic equilibrium but was subsequently significantly deformed by large impacts (now 572.6 × 557.2 × 446.4 km). - From Wikipedia on hydrostatic equilibrium

Essentially an object must have a certain mass so gravity can force it into a spherical form. Below this ill-defined limit odd geometries are possible. If you look at the pictures of 2014 MU69 (Ultima Thule), 67P/Churyumov–Gerasimenko or 25143 Itokawa, for example, you'll see examples of contact binaries. Two asteroids which meat each other and came so close that they touched. I imagine the upper plausible size limit for this would be to take 2 metallic asteroid like 16 Psyche with a diameter of roughly 200 km and slap them together. This would be a sizable moon which could be stable over geological timescales. The caveat would be that the gash won't be that much of an obstacle, one could probably jump across it with some effort. Why the gash is equatorial and not polar as it would be in the beginning could bee explained by later impacts.

asteroids visited by spacecraft from blog.adafruit.com


One can get quite creative with reasons why a civilization would create a huge equatorial gash on a moon. Reasons could include

  • Because they can It is an art project or a demonstration of power.

  • Paraterraforming project The gash was created by excavation or detonations and then domed over to crate a moon-spanning garden-paradise.

  • Traces of a military operation Relativistic kill missiles, antimatter, gamma-ray lasers, star-powered particle beams, pick what you like to create a gash in a planet.

  • Mining It was a strip-mine. Down I the gash you'll find the remnants of a circumplanetary mass driver. This train to space used to export gigatons of minerals daily to support the economy of a K2 civilization. Maybe they were building a Dyson Swarm or some other megastructure which required such a strip-mine. One would expect to find many mineshafts branching out fro the central canyon, which was build to conveniently access the deeper mine-shafts.

  • 2
    $\begingroup$ This is an amazingly comprehensive answer! I like the contact binary best. $\endgroup$
    – Willk
    Commented Sep 15, 2019 at 19:47
  • $\begingroup$ if there is enough mass to make it spherical the two contacted objects will not leave a gap, so this only works if the moon is very very small. $\endgroup$
    – John
    Commented Apr 3, 2023 at 21:23

Space Weapons:

Several hundred years ago, a rogue warlord named Krizakr took his spacecraft to the moon and build a base to use as a staging post for running raids a nearby trading world. Because Krizakr was highly paranoid, he placed several powerful laser satellites in an low equatorial orbit.

Several years later Krizakr's died in a conflict with the Space Protection Fleet and on his starship the coordinates of his base were discovered. The SPF send a small fleet to access the base. It was determined that the armed sattelites were too dangerous to be left active and able to fire upon approaching vessels. Unfortunately they possessed no maneuvering thrusters, and approaching them with personnel was deemed too dangerous until they were disarmed - who knew what booby traps the notorious warlord had installed?

Fortunately they had recovered the firing control codes from Krizakr's ship so the satellites were pointed towards the (uninhabited) moons surface, and fired until their stores of hydrogen fluoride were depleted. With the primary weapons depleted, the satellites were left orbiting the moon, and a buoy was deployed warning passing vessels not to approach the satellites.

If you want craters rather than a single gouge, replace the Chemical Lasers with some sort of bomb or missile.

Rift Fault when the moon was forming

On mars there is the Valles Marineris: ttps://en.wikipedia.org/wiki/Valles_Marineris This is a canyon that is some 4000 km long, is plainly visible, and is roughly east-west. I'm no geologist and the wikipedia page goes a bit past my head, but it seems to have formed when the tectonics of Mars was a bit more active. I see no reason why this couldn't have happened on a moon somewhere.

Unexplained science

Something similar to whatever happened to Iapetus. If you don't need to explain it in detail, you can probably hand-wave it as a "mystery the scientists are still unsure about". A giant gash encircling a planet is something likely enough to be feasible and strange enough to puzzle scientists if there's only one of them in the solar system.


If your moon was captured by the planet, it could have been from further away in the orbit, even maybe a rogue (belonging to no stellar system) and had a lot of water that happened to be frozen because it's cold in space (like a comet).

Now when your moon gets too close to the star (either at your planet's orbit or before it was captured, passing nearer your home star, again, like a comet), all the water could have melted and/or evaporated.

Now it's up to you what happens to this water. If you want to say the rock is very porous and liquid water seeped into it and disappeared, that's cool. If you want to say it went up in gas, it probably requires more heat so the closer approach would be more likely, unless you prefer to say this happened before life developed on your planet and at that time the sun was stronger.

The point is, this ice would have acted like the glaciers did during the ice age forming lochs and generally destroying the landscape. You could even say that this used to be the polar region of your moon and this was a polar cap, before it was captured by your planet and a dramatic tilt shift happened. Fast forward a couple thousands (or million) years and through tidal forces your moon ended up being locked to your planet with a spin that wouldn't let you suspect its now-equator was once its pole.


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