The other day, I came across a funny meme on Instagram.
I laughed, I gave it a like, and I moved on with my life- kind of. As I continued with my day, one thought kept creeping up in the back of my mind- “Why does Mars’s moon look like that?” It doesn’t even look like a moon, from the standards of as average Earth-dweller, who is so used to the beautiful, spherical, and almost perfect moon that we have been spoiled by. Why is Earth’s moon so perfect, while Mars’s moon looks like it just hopped off the nearest asteroid belt into Mars’s orbit?
Mars has two moons, Phobos and Deimos. There is no absolute consensus among the scientific community on how these two moons came to be, but there are two main theories. The first theory, known as the Capture Theory, is that Mars’s moons are captures asteroids, meaning debris created during the early formation of the solar system were pulled into Mars’s orbit by the planet’s gravitational pull. The fact that the two moons are so small and irregularly shaped supports the Capture Theory, along with the fact that they are visibly much darker in color than the Red Planet. The opposing theory is known as the Impact Theory, proposes that Mars underwent some impact from some collision during its early years, uplifting debris that aggregated into two moons. Recent photographic evidence has suggested that Deimos is composed of a rocky material, very similar to what makes up Mars, which supports the Impact Theory and opposes the Capture Theory, which would expect the moons to be made of a more carbon-rich material. There is also debate to whether or not these two moons are even made of the same material, or moreover, were even formed in the same event.
There are a great deal of unknowns about how Mars’s moons were formed, but however they came to be, they sure look a lot different from our moon. While formation process may play a role in this, the main reason for this difference, is mass. Our moon has an approximate mass of 7.348 × 1022 kilograms, while Mars’s larger moon, Phobos, has a mass of only 1.060×1016 kilograms. While these both seem like huge numbers, the masses of these two moons are on vastly different scales, with Phobos being about 0.000014% the mass of Earth’s moon. In summary, Mars’s moons are tiny, and this accounts for the fact that they look so nonuniform.
When the Earth’s moon was forming, it became rounded due to the same principle that causes every other sizable celestial body to become spherical- self gravity. Self-gravity is the gravitational force that an object exerts on itself. Once a body accumulates enough mass, this force will pull in towards its center, forming a spherical shape. This final stage is known as hydrostatic equilibrium. Every object that has mass has some amount of self-gravity, even people! But of course, we do not have nearly enough mass for our self-gravity to pull us into a sphere. So that’s the main reason Mars’s moons are so misshapen; they’re too small to exert a strong enough self-gravitational pull on themselves. But, there is one more piece to the puzzle. Mass isn’t the only things that cause celestial bodies to become rounded. The other secret ingredient is material.
For bodies to reach a spherical state, their self-gravity must be strong enough to warp and squish the physical material that they are composed. For example, a hypothetical body made of only liquid water would not need much mass in order to achieve hydrostatic equilibrium, because water molecules can be moved around with very little force. There are other materials, such as iron, that resist gravitational pulls even more, and require even more mass to become spherical. The smallest known celestial body that has reached hydrostatic equilibrium is Saturn’s moon, Mimas, which has a diameter of only 396 km. This moon has a very low density, and is composed of mainly ice, and a small amount of rock. This composition gives Mimas a low resistance to its own self-gravity, hence allowing it to become spherical, even with its small mass.
While the answer to my original question turned out to be pretty simple, topics involving space always have a way of becoming more complex. I hope you were able to learn something from this article, or maybe find a topic that you would like to learn more about. There are still so many unknowns in this topic, including exactly what Mars’s moons are even made of, but there is also new and exciting research coming out every day.
Sources
Coffey, D. (2023, November 13). Why are things in Space round?. LiveScience. https://www.livescience.com/space/why-are-things-in-space-round#:~:text=Gravity’s%20rounding%20effect%20is%20a,into%20a%20sphere%2Dlike%20shape.
Giant impact may explain origin of martian moons phobos and deimos. Smithsonian Insider. (n.d.). https://insider.si.edu/2010/12/giant-impact-may-explain-origin-of-martian-moons/
Jonti Horner Professor (Astrophysics). (2024, March 5). I’ve always wondered: Why are the stars, planets and Moons Round, when comets and asteroids aren’t? The Conversation. https://theconversation.com/ive-always-wondered-why-are-the-stars-planets-and-moons-round-when-comets-and-asteroids-arent-160541
Metcalfe, T. (2023, May 18). A new look at Mars’ Moon Deimos highlights its mysterious origin. Astronomy Magazine. https://www.astronomy.com/space-exploration/a-new-look-at-mars-moon-deimos-highlights-its-mysterious-origin/
