Phobos and the Stickney Crater

The Stickney Crater on Phobos, a Moon of Mars


Phobos is the larger and closer of the two Martian moons. "Phobos" means "fear" in Greek. Phobos is small--only about 12 miles across (19 kilometers); and it's also very close to Mars, it's parent planet. How close? About 9377 kilometers (5,827 miles) from the center of Mars--which is closer than New York is to Japan, and about the same distance New York is to Tel Aviv, Israel. If we were Martians, it would be just as easy to go to the moon as it is to visit Tel Aviv here on Earth!

Phobos was discovered by astronomer Asaph Hall, Sr., at the U.S. Naval Observatory in Washington, DC on August 18, 1877.

Why Phobos? Why "fear"?

This is one of the most fascinating things I've come across in my various researches on space. The following is an excerpt from a letter to the journal Astronautics, dated February 1960, written by Fred Singer, who was then the science advisor to the US President, Dwight Eisenhower:

"My conclusion...that is the satellite is indeed spiraling inward as deduced from astronomical observation, then there is little alternative to the hypothesis that it is hollow and therefore Martian made..."

For many years, speculation was rife in the scientific community that this moon was an artificial object, and not man-made. Therefore, it had to be Martian-made . The astronomers of the time were correct in thinking that this moon was very light; and that it's orbit was spiralling inward. We still don't know for sure why this moon's density is so low, but the most recent pictures from the Mars Reconnaissance Orbiter, in March of 2008, gives us some ideas that perhaps Phobos is hollow, or perhaps has water trapped beneath the surface.

The astronomers of the 1960's, especially Russian astrophysicist Iosif Shklovsky, when studying the orbital pattern of Phobos, concluded that it was a hollow iron sphere about 16 kilometers (10 miles) across but less than 6 cm thick (2 inches thick), and also concluded that it was a Martian artifact .

We now have pictures of it. It isn't a Martian artifact, at all. It isn't a hollow iron sphere. Instead, it more closely resembles a pockmarked lump of rock.

The mars Reconnaissance Orbiter has given us photos of this object in space. Phobos is heavily cratered--it is pitted and scarred with craters. The largest crater is the Stickney crater, named after Chloe Angeline Stickney Hall, wife of the astronomer who discovered the moon. In 1878, Hall wrote:

"I might have abandoned the search for Martian satellites had it not been for the encouragement of my wife."

This crater has a crater within it. The Stickney crater is so large it takes up about half the moon's surface, and it is about 9 kilometers (5.6 miles) across. Grooves and chains seem to radiate from the Stickney crater. These have led to theories that the impact of the meteorite which formed this crater nearly destroyed the Phobos moon, itself. Some evidence from the Mars Express Orbiter indicates that those grooves extending outside the crater in streaks down its side, are unrelated to the initial impact and are formed by material ejected from other impacts on Mars. The crater has lined walls, caused by landslides of material falling into the crater.



Phobos's close orbit around Mars makes for some unusual effects. For one thing, Mars has two moons, and they aren't in synchronous orbit. So, as one moon is rising, the other moon is setting. Another thing--Phobos orbits Mars BELOW the synchronous orbit radius of Mars itself; meaning it goes around Mars faster than Mars rotates. Phobos rises and sets on Mars twice a day!

What this also means is that Phobos goes through its moon phases very rapidly. If you were standing on Mars, you would be treated to the sight of the new moon, quarter moon, half moon and full moon of Phobos three times per day! (Phobos's phases, as seen on Mars, take 0.3191 days), and a mere 13 seconds longer than the sidereal period of Phobos. If you were standing on Mars, you would see Phobos rise in the west, zip rapidly across the sky, and set in the east, about four hours later. And as it is zipping across the sky, you would see its phases change, from new to quarter to half to full. Because its orbit is so close to Mars, you couldn't see it at all above the horizon unless your latitude was greater than 70.4 degrees. So, for an Earth instance--Finland is about 70 degrees northern latitude. You would have to be further north on Mars than Finland is on Earth, in order to see this moon Phobos, at all.

And yes, the orbit of Phobos IS spiralling inwards. Because the Phobos orbit is shorter than a Martian day, "tidal locking" decreases the orbital radius by 20 meters in a century (about 65 feet). That means in 11 million years, Phobos will be sucked into Mar's gravity and BOOM! No more Phobos. It may, rather than impact Mars, break up into a planetary ring, prior to impact.

"Tidal locking" is a phrase used to describe the gravitational gradient which makes one astral body always face another--for example, the moon always presents one side towards the earth. The other side of the moon is dark.

So Phobos's short orbital period in combination with the gradient of gravity that produces tidal locking causes this spiral inwards of the moon's path.

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Comments 23 comments

Paradise7 profile image

Paradise7 5 years ago from Upstate New York Author

Thanks for the comment, Hop David.

Hop David 5 years ago

Falsor Wing, a Mars is easier to build than an earth elevator because of Mars shallower gravity well and angular velocity. Well, Phobos has an even shallower well and higher velocity. A very powerful elevator could be built from Phobos that would require a small fraction of the resources a Mars elevator would. Besides being smaller, it also endure much less stress than a Mars elevator.

Here is a graphic of a Phobos elevator:

The foot of this elevator is moving .6 km with regard to Mars surface and thus could be reached from Mars' surface with only a small suborbital hop.

Paradise7 profile image

Paradise7 5 years ago from Upstate New York Author

Oh, duh, wiki, I followed the link and you're right!

Paradise7 profile image

Paradise7 5 years ago from Upstate New York Author

That's not what my research told me--just the opposite. What are your sources, may I ask?? (Very politely.) Perchance they are better than mine.

FactusMaximus 5 years ago

Sorry, but neither Phobos nor Deimos can be seen from the poles, unless your x-ray vision can see through the planet.


Both Phobos and Deimos have low-inclination equatorial orbits and orbit fairly close to Mars. As a result, Phobos is not visible from latitudes north of 70.4°N or south of 70.4°S; Deimos is not visible from latitudes north of 82.7°N or south of 82.7°S. Observers at high latitudes (less than 70.4°) would see a noticeably smaller angular diameter for Phobos because they are farther away from it. Similarly, equatorial observers of Phobos would see a noticeably smaller angular diameter for Phobos when it is rising and setting, compared to when it is overhead.

Paradise7 profile image

Paradise7 6 years ago from Upstate New York Author

Thanks, Kev, glad you enjoyed the hub. Yep, you guessed it--I'm a Martian!!!

Kevin Schofield 6 years ago

Hi Paradise7. Brilliant hub! I was totally engrossed. You write as though you were a Martian (you're not, are you?). I wonder how Mrs Hall felt about having a crater named after her! A wealth of fascinating information. Kindest regards, Kev.

Paradise7 profile image

Paradise7 6 years ago from Upstate New York Author

Thank you so much, satomko!

satomko profile image

satomko 6 years ago from Macon, GA

I'm a big fan of all your astronomical hubs. Keep up the good work.

Paradise7 profile image

Paradise7 6 years ago from Upstate New York Author

Thanks for the comment, Falsor Wing. Good point!

Falsor Wing profile image

Falsor Wing 6 years ago from Lodoss the Accursed Isle

Phobos is so low in orbit it would be the only major obstacle to building an orbital elevator on Mars, whose gravity is only 1/3g, because it would actually pass between Mars and the areostationary satellite at the "top" of the elevator.

Paradise7 profile image

Paradise7 7 years ago from Upstate New York Author

Thanks, Cosette. I really liked writing this one.

cosette 7 years ago

that is so cool! I had no idea the Red Planet had such a tiny moon. great hub.

Paradise7 profile image

Paradise7 7 years ago from Upstate New York Author

Thank you, Duchess. I'm glad you enjoyed it.

Duchess OBlunt 7 years ago

As always Paradise7 another good read. Great job sharing your learning with us. Easy to read and lots of information.

Paradise7 profile image

Paradise7 7 years ago from Upstate New York Author

Thank you, myownworld. The universe is a fascinating place. I'll never run out of topics. Thank you for reading.

myownworld profile image

myownworld 7 years ago from uk

ah..there we go...another fascinating hub! where do u come up with these ideas? makes you marvel at just how unbelievable our universe is! thanks for posting...

Paradise7 profile image

Paradise7 7 years ago from Upstate New York Author

Thank you both, Leop and dohn, for your comments. I'm always glad to share cool info I run across. It's fun!

dohn121 profile image

dohn121 7 years ago from Hudson Valley, New York

I thoroughly enjoyed reading this, Paradise7. While reading this, I felt like you were my tour guide or speaker at a lecture hall or planetarium! You made this a joy to read. Thank you for sharing this awesome well written hub.

Leop profile image

Leop 7 years ago

Great information here. Wow that is a lot, also well written. I don't remember reading something like this before this is a good learning experience for me. I love when I come across something like this. Very interesting material. Thank you Paradise7.

Paradise7 profile image

Paradise7 7 years ago from Upstate New York Author

Thank you, Veronica, for your comments. I like writing these and am so glad you enjoy them.

Thanks, advisor. Your support is appreciated.

advisor4qb profile image

advisor4qb 7 years ago from On New Footing

Cool hub!

Veronica Allen profile image

Veronica Allen 7 years ago from Georgia

Wow Paradise7, where do I begin? There were so many points in this hub that fascinated me. One being the distant comparison between phobos and its parent planet, second - the idea that this moon was once considered "Martian-Made", and third - how many times we would be treated to the Phobos moon (three times a day!) if we were standing on Mars. There is just no end to how fascinating and awe-inspiring the world in general is - out of space as well. Thanks for this great information.

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