The second of the planet Mars' moons is a small rocky world called Deimos. Yesterday I wrote about Mars' larger moon, Phobos, [here](https://steemit.com/space/@proteus-h/phobos-moon-of-mars) if you are interested. I thought it would be appropriate to cover Deimos, the second moon, shortly after Phobos.

Deimos is the smaller and more distant of Mars' two moons, and remains mostly unexplored like Phobos with no landers ever reaching the surface. It too could serve as a temporary base for future astronauts exploring the Mars system due to its extremely low gravity and escape velocity.

![](https://steemitimages.com/DQmXc36qJePKRMYYjWuvoXaifk2uZPBYUjWpMrF5UhFs7SU/image.png)
*The classic, color-enhanced picture of Deimos taken by the Mars Reconnaissance Orbiter*
*[Credit](https://en.wikipedia.org/wiki/Deimos_(moon))*

## Deimos

Deimos is a small rocky world 15 kilometers across at its widest point. It has a mass about ten times that of Mount Everest from the estimates I've seen (Deimos comes in at around 1.5e15 kilograms). Being so small and not very massive (for an astronomical body!), Deimos isn't pulled into a spherical shape by its own weak gravity, and as such is quite lumpy and like Phobos resembles your typical asteroid. 

It is hypothesized that Deimos was formed when a massive planet-sized object struck Mars a very long time ago, producing a debris field that coalesced into three moons: Deimos, Phobos, and an unmanned moon that no longer existed. The large unnamed moon eventually spiraled back into Mars, impacting it and erasing any trace of it from history. Deimos and Phobos remained to this day.

If you put a rock in space, it's just a rock and will retain its shape. Place a bunch of rocks in space, and they can coalesce together to form a big asteroid, weakly held together by their own mutual gravitational attraction. Put enough rocks into the asteroid, and the now quite massive object can collapse into a spherical shaped object, molded by its now much stronger gravitational force. A sphere is the most compact (lowest energy) shape for a blob of mass in space, so the mass "wants" to collapse into a sphere shaped world. This can only happen if the object contains enough mass. In order for this spherical-transition to happen, a rocky object must be at least several hundred kilometers across. Deimos, at just about a dozen kilometers wide, hardly fits the bill.

Deimos hangs above Mars, about 20,000 kilometers over the surface, orbiting the planet in a mostly circular orbit. This puts it much, much further away from the surface than larger Phobos. This distance coupled with Deimos' smaller size means that, while you could easily see Deimos from the surface, it would be difficult to make out many surface features. 

![](https://steemitimages.com/DQmWJaaac43MFrhGS8VVsc7A1ioGuxYL8W6yVkjjdN5LAnG/image.png)
*Another side of Deimos*
*[Credit](https://en.wikipedia.org/wiki/Deimos_(moon))*

In yesterday's post on Phobos I mentioned that Phobos' days are numbered due to its close distance to Mars. After some tens of millions of years, Phobos will either impact the Martian surface or be ripped apart by tidal forces and form a ring. Either way, Phobos has a temporary lifespan. Deimos on the other hand isn't at risk of being destroyed in this way. Its orbit never brings it anywhere near close enough to Mars to risk tidal destruction or long-term orbit shrinkage, so while it may be smaller than Phobos, Deimos will long outlive its bigger brother (assuming no external forces come into play, like an unexpected large asteroid flyby of Mars). 

## What's on Deimos?

I'd once again like to re-iterate that every light you see in the sky is a world of its own. Most of the lights are incomprehensibly massive giant burning balls of fusion-heated gas that you couldn't hope to walk on, some are dense clouds of hot gas with no surface, and some like Mars are rocky planetary worlds that you could walk or even live on. Even though Deimos is small, it still has a surface and is still a real place that you could walk on (barely) and explore.

But Deimos' tiny size means that there isn't much land there. Just under 500 square kilometers of surface exists on Deimos, coming it at around half the physical size of the city of San Diego, California. Walking here wouldn't really be easy - with a surface gravitational acceleration of just 3 millimeters per second per second, you would weigh about 300 times less if you tried to walk on Deimos. Several people on my post on Phobos mentioned Phobos' low escape velocity. Deimos' escape velocity is even smaller: At only around 5 m/s, a not-so-strong jump could send you permanently flying away from the moon, never to return. Any attempt to explore the surface would then require a tether (not USDT) or something to keep you pinned down to the surface. 

Unfortunately, no photographs taken from the surface exist. No spacecraft has ever landed here. The only spacecraft to even try was the second of the Soviet Phobos landers, which would have landed on Deimos had the first spacecraft succeeded. Neither made it to either of Mars' moons, leaving both of them unexplored. All of our knowledge of Deimos then comes from various flybys executed by several different Mars orbiting spacecraft.

![](https://steemitimages.com/DQmdYYE71ckWyW7CVy1Kzuq4ZhbkFWiJD3RvyoGCdqguGMG/image.png)
*US Geological Survey map of Deimos*
*[Credit](http://solarviews.com/cap/mars/deimoscyl3.htm)*

The surface of Deimos is mostly smooth with scattered craters littered the surface, indicating a somewhat soft dusty surface. The moon bears the scars of previous meteoroid impacts, as the craters can't be eroded by anything other than other impacts due to the lack of atmosphere. Interestingly, Deimos lacks the strange trenches that cross the surface of Phobos. 

![](https://steemitimages.com/DQmXwXFTtpZAHdq5SHVriLd1EjyuLLzVvENfLK4iS47FAPu/image.png)
*Close-up of the surface from an orbiter*
*[Credit](https://mars.nasa.gov/images/mep/allaboutmars/d2.gif)*

As you can see, many of the craters are filled with dust. When something hits Deimos, most of the ejected dust and rock doesn't fall back to the surface because of the ridiculously low escape velocity (~5 m/s). So, this dust escapes into a circular orbit around Mars very similar to that of Deimos. However, Mars' escape velocity is extremely high compared to Deimos', so the dust remains in orbit. Eventually, Deimos re-collides with the dust when their trajectories overlap, depositing the dust on the surface and filling the craters.

![](https://steemitimages.com/DQmZ1SpVXYdn2Ztz8f6bZ5pE6tmAf5GDsLj6sq9zSinoEVs/image.png)
*Another view of Deimos from the Viking 2 orbiter*
*[Credit](https://mars.nasa.gov/images/mep/allaboutmars/d3.gif)*

Living on Deimos would be much like living on a space station in orbit, but with more to do since you would have several hundred square kilometers of unexplored land around you. The extremely minimal gravitational force on the surface would make you feel almost weightless, and you would require a sealed, pressurized habitation spacecraft to survive in the total lack of atmosphere. The reason that Deimos lacks an atmosphere is its low escape velocity, which is directly caused by its low mass. If a significant amount of gas molecules at the surface temperature of an object have enough velocity to escape the object, then the object cannot hold an atmosphere. This is the reason why Earth has very little hydrogen and helium in its atmosphere but can hold things like oxygen: The lighter elements move faster at the same kinetic energy and therefore can escape more easily. Tiny Deimos, with a pretty high (for space) temperature of around -40 Celsius, has no chance keeping any gases on its surface, and as such has no atmosphere at all.

I very much hope to see a lander here some day, as it would be very interesting to see what this neighbor of Mars is hiding buried away in its craters.

![](https://steemitimages.com/DQmTvuqJ95Dqbr568FLD6BMrXjSLeuDnxTyEsxW5wHKvMUD/image.png)
*Rendition of a manned base on Deimos. I fear for this astronaut's safety without a tether to hold him/her to the surface!*
*[Credit](http://www.thespacereview.com/article/2631/1)*

Unfortunately isn't a huge amount of additional information to share outside of various specifications like temperature and composition. This is, of course, due to the lack of landing spacecraft, or spacecraft dedicated solely to studying Deimos (all previous visits have been from primarily Mars-focused spacecraft). Hopefully sometime soon a vehicle is dedicated to exploring this interesting micro-world.

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Let me know if you have any questions or comments. To facilitate discussion I'll try to upvote all legitimate non-fluff/spam posts with my little bit of voting power. 

*Thanks for reading!*

**Sources Utilized:**
[AstronomyNow: Martian moons formed by giant impact](https://astronomynow.com/2016/07/05/mystery-solved-martian-moons-formed-by-a-giant-impact/)
[NASA Deimos Overview](https://mars.nasa.gov/allaboutmars/extreme/moons/deimos/)
[Astronomer Mike Brown Dwarf Planets (References diameters necessary for spherical objects)](http://web.gps.caltech.edu/~mbrown/dps.html)
[Deimos Wikipedia Entry](https://en.wikipedia.org/wiki/Deimos_(moon))