APOD: Phobos: Doomed Moon of Mars (2018 Nov 25)

[APOD Robot]

Phobos: Doomed Moon of Mars

Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. The origin of the Martian moons is unknown, though, with a leading hypothesis holding that they are captured asteroids. The larger moon, at 25-kilometers across, is Phobos, and is indeed seen to be a cratered, asteroid-like object in this false-colored image mosaic taken by the robotic Viking 1 mission in 1978. A recent analysis of the unusual long grooves seen on Phobos indicates that they may result from boulders rolling away from the giant impact that created the crater on the upper left: Stickney Crater. Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. The ultimate result will be for Phobos to break up in orbit and then crash down onto the Martian surface in about 50 million years. Well before that -- tomorrow, in fact, if everything goes according to plan -- NASA's robotic InSight lander will touch down on Mars and begin investigating its internal structure.

<< Previous APOD

This Day in APOD

Next APOD >>

[JohnD]

Gosh! Years ago, the origin of the grooves on Phobos were the subject of a long and sometimes bad-tempered (these are the moons of the War God) debate here on Staship Asterisk. Then just the the day another turned up that didn't, in fact only got a response from neufer: http://asterisk.apod.com/viewtopic.php? ... os#p287596

Previously, the grooves were ascribed to meteor chain impacts, but Ramsley and Head found that " the 'rolling boulder' model is quantifiably plausible." https://www.sciencedirect.com/science/a ... via%3Dihub Note, "plausible", within the bounds of probability, not proven.

Me, I find it implausible. Phobos is tiny, with a surface gravity of 0.0057m/s^2, nearly 2000 times less than that on Earth. For the test boulders they used in their simulation and guessing (not stipulated in the abstract) at 10000Kgs, escape velocity SQR(2GM/R) would be 0.014m/s. As Ramsey and Head suggested that the Stickney Impact ejected boulders at 6m/s, how come they ever came down again?

Has anyone read the correspondence that followed this paper? Or is it too recent to have been roasted?
John

[Chris Peterson]

Gosh! Years ago, the origin of the grooves on Phobos were the subject of a long and sometimes bad-tempered (these are the moons of the War God) debate here on Staship Asterisk. Then just the the day another turned up that didn't, in fact only got a response from neufer: http://asterisk.apod.com/viewtopic.php? ... os#p287596

Previously, the grooves were ascribed to meteor chain impacts, but Ramsley and Head found that " the 'rolling boulder' model is quantifiably plausible." https://www.sciencedirect.com/science/a ... via%3Dihub Note, "plausible", within the bounds of probability, not proven.

Me, I find it implausible. Phobos is tiny, with a surface gravity of 0.0057m/s^2, nearly 2000 times less than that on Earth. For the test boulders they used in their simulation and guessing (not stipulated in the abstract) at 10000Kgs, escape velocity SQR(2GM/R) would be 0.014m/s. As Ramsey and Head suggested that the Stickney Impact ejected boulders at 6m/s, how come they ever came down again?

Has anyone read the correspondence that followed this paper? Or is it too recent to have been roasted?
John

I think you are misunderstanding escape velocity. It is not dependent upon the mass of the escaping body, but the mass of the parent body (assuming the latter is much greater than the former). The value for M that you need to plug into the formula is the mass of Phobos, resulting in an escape velocity a bit over 11 m/s, which is the same for everything, be it sand grains or giant boulders.

[neufer]

Previously, the grooves were ascribed to meteor chain impacts, but Ramsley and Head found that " the 'rolling boulder' model is quantifiably plausible." https://www.sciencedirect.com/science/a ... via%3Dihub Note, "plausible", within the bounds of probability, not proven.

Me, I find it implausible. Phobos is tiny, with a surface gravity of 0.0057m/s^2, nearly 2000 times less than that on Earth. For the test boulders they used in their simulation and guessing (not stipulated in the abstract) at 10000Kgs, escape velocity SQR(2GM/R) would be 0.014m/s. As Ramsey and Head suggested that the Stickney Impact ejected boulders at 6m/s, how come they ever came down again?

Has anyone read the correspondence that followed this paper? Or is it too recent to have been roasted?

I think you are misunderstanding escape velocity. It is not dependent upon the mass of the escaping body, but the mass of the parent body (assuming the latter is much greater than the former). The value for M that you need to plug into the formula is the mass of Phobos, resulting in an escape velocity a bit over 11 m/s, which is the same for everything, be it sand grains or giant boulders.

Stickney Impact ejected boulders at ~6m/s have only ~38% the energy needed to escape (into an adjacent Mars orbit) and would have formed a hemispherical cloud ~4.5 km high over the ~9 km wide Stickney crater (displaced somewhat based upon the incoming direction of offending meteor) . Such boulders then mostly crashed back down into Stickney crater and, presumably, bounced around until many of them rolled out over the craters lip and keep going.

[JohnD]

Chris,
OH! Doh! Thank you! Learn something every day (Socrates , or someone)!

neufer,
I'm sure you're right - bit like a dice game?

But I have to ask - the tracks of rolling rocks are so rare on other bodies as to be remarkable. They have been seen on the Moon and Mars. Why so many "tracks" on Phobos?
JOhn

[neufer]

Click to play embedded YouTube video.

Chris, OH! Doh! Thank you! Learn something every day
(Socrates , or someone)!

• http://mydailytruths.blogspot.com/p/quotes.html

neufer, I'm sure you're right - bit like a dice game?
But I have to ask - the tracks of rolling rocks are so rare on other bodies as to be remarkable. They have been seen on the Moon and Mars. Why so many "tracks" on Phobos?

The Phobos fight-or-flight response.

[Galaxian]

Phobos: Doomed Moon of Mars

Explanation: This moon is doomed.

The ultimate result will be for Phobos to break up in orbit and then crash down onto the Martian surface in about 50 million years.

"The ultimate result, absent Human intervention, will be for Phobos to break up in orbit and then crash down onto the Martian surface in about 50 million years"

There, I fixed it for you.

True, Human intervention is about as likely as a politician doing something altruistic as the age of manned off-world ships is just about over but it is within the powers of our current technologies to move that rock to a more stable orbit. We could even use it as an anchor for a Martian Orbital Tower.

It's a great pity that such things are fantasies and will never happen.

[Guest]

I noticed in this photo that hidden within the umbra in the lower portion of Phobos are three tiny points of light, two together on the left, and another further to the right by itself. Are these points of light merely aberrations of the camera, or something else? No mention is made of them in the comments.

[MarkBour]

Previously, the grooves were ascribed to meteor chain impacts, but Ramsley and Head found that " the 'rolling boulder' model is quantifiably plausible." https://www.sciencedirect.com/science/a ... via%3Dihub Note, "plausible", within the bounds of probability, not proven.

Me, I find it implausible. Phobos is tiny, with a surface gravity of 0.0057m/s^2, nearly 2000 times less than that on Earth. For the test boulders they used in their simulation and guessing (not stipulated in the abstract) at 10000Kgs, escape velocity SQR(2GM/R) would be 0.014m/s. As Ramsey and Head suggested that the Stickney Impact ejected boulders at 6m/s, how come they ever came down again?

Has anyone read the correspondence that followed this paper? Or is it too recent to have been roasted?
John

Thanks for bringing it up, John. This is all new to me. And thanks to Chris and Art for the further info, which nicely rescued the plausibilty. I had no idea there were so many lovely examples on our Moon.

So then, can y'all as easily rebut this skeptic's remark?

• If those are the tracks of rolling boulders, where are the boulders? I don't see any.

If there's a boulder or two on or at the end of a couple of these, then the question is pretty convincingly settled. But if there are no examples, then ...

Actually, looking at the lovely map at: https://upload.wikimedia.org/wikipedia/ ... on-Map.png , the whole region on one side of Stickney looks more like a massive lava flow. But I don't know half enough about this to put forth a theory of my own. I am kind of curious about the debate you referred to; probably some other good ideas were put forth?

[JohnD]

Thank you, Mark.
After being so kindly slapped down by Chris exposing my ignorance, it may be inappropriate for me to raise any other objections to the Rolling Rocks theory, but...

They are continuous.
Many pics show the grooves passing down into craters and back up the other side. See: https://www.space.com/33885-phobos-mars ... tures.html Picture 3. How will a rolling rock do that?
Moreover, when a groove crosses a crater, it often does so on a chord, not a diameter, so that the course of a rolling rock would be diverted by the slope inside the crater, yet the groove stays straight.
A pic from HiRISE shows a rolling boulder on Mars that traversed a crater. Launched by the rim, it skipped over the crater, landed on the far lip and continued. See https://www.universetoday.com/wp-conten ... bounce.jpg The Phobos grooves appear completely different

They are straight.
Pictures of rolling rock trails on the Moon and Mars show they are irregular. This is not suprising: a rock is a very approximate sphere and its irregular faces will cause it to twist and turn as it rolls. See https://www.bing.com/images/search?view ... ajaxhist=0

They aren't grooves!
Many are clearly lines of craters, some so close together as to appear as a groove, but most are obviously not. In some cases, a 'groove' peters out and continues as isolated craters, all on the same line. See https://www.space.com/10733-mars-moon-p ... hotos.html Follow the biggest groove in the middle of the image, down to the left (seven o'clock) as it crosses a double crater. Small craters continue the line.


This "Rolling Rocks" theory is one of many, but all the others depend on tidal disruption. A storm of weak bodies, captured by Mars, broken into strings that impact on Phobos, or else the debris of Stickney, also captured and disrupted by Mars coming back at Phobos to leave the grooves. In either case as crater chains. OR, Mars' tides are pulling Phobos itself apart, and these grooves are fissures and the craters regolith subsidence.

John

[neufer]

I noticed in this photo that hidden within the umbra in the lower portion of Phobos are three tiny points of light, two together on the left, and another further to the right by itself. Are these points of light merely aberrations of the camera, or something else? No mention is made of them in the comments.

The first small crater to the right of Limtoc (at ~38º W)

https://upload.wikimedia.org/wikipedia/ ... on-Map.png

(reflecting back-scatter from bright Limtoc?) seems to be the primary point of light:

[Chris Peterson]

This "Rolling Rocks" theory is one of many, but all the others depend on tidal disruption. A storm of weak bodies, captured by Mars, broken into strings that impact on Phobos, or else the debris of Stickney, also captured and disrupted by Mars coming back at Phobos to leave the grooves. In either case as crater chains. OR, Mars' tides are pulling Phobos itself apart, and these grooves are fissures and the craters regolith subsidence.

You raise some good questions, worth considering. I would note, however, that this is a very different body from the Moon or Mars. The latter two have high gravity and deeper craters. Rocks rolling on their surfaces may have their paths dominated by that gravity- deviations in course, for instance, due to ground shape. On Phobos, with very low gravity, the momentum of the rock may be dominant. The rocks might not even be rolling, but simply sliding (with an independent rotation or tumble).

I have no opinion about what caused the grooves. But I would think that very straight grooves are better explained by rolling or sliding objects on the surface than by tidal disruption, which I'd expect to produce more fractal cracks, not perfectly straight. Of course, there may be other explanations that are better than either.

[neufer]

---

So then, can y'all as easily rebut this skeptic's remark?

• If those are the tracks of rolling boulders, where are the boulders? I don't see any.

If there's a boulder or two on or at the end of a couple of these, then the question is pretty convincingly settled. But if there are no examples, then ...

Good point, Mark.

Hey, kids...
embiggen the Laputa Regio map here:
http://wenamethestars.inkleby.com/feature/14868

and be the first in your neighborhood to locate a fossil Rolling Stone
(and win a Little Orphan Annie Secret Decoder Ring).

[Guest]

Has anyone considered the possibility of the grooves being caused by large volumes of regolith (including tiny rock fragments) being ejected outward from the Stickney impact point, and then falling back onto the surface, creating long straight lines of ejecta across the surface? Since these lines transverse smaller craters without any deviation, it would make more sense to me that they were caused by ejecta, and not by rolling boulders.

[JohnD]

Indeed they have, as I referred to above. See Nayak &Asphuag, or read this: https://www.space.com/33895-phobos-groo ... olved.html


John

[BDanielMayfield]

Gosh! Years ago, the origin of the grooves on Phobos were the subject of a long and sometimes bad-tempered (these are the moons of the War God) debate here on Staship Asterisk. Then just the the day another turned up that didn't, in fact only got a response from neufer: http://asterisk.apod.com/viewtopic.php? ... os#p287596

Ah yes John, the good olden days when conversations were keenly interesting. I wasn't a member back then but I've reviewed the archive ... "all our rowdy friends have rowdied on down."

Phobos: Doomed Moon of Mars

Explanation: This moon is doomed.

The ultimate result will be for Phobos to break up in orbit and then crash down onto the Martian surface in about 50 million years.

"The ultimate result, absent Human intervention, will be for Phobos to break up in orbit and then crash down onto the Martian surface in about 50 million years"

There, I fixed it for you.

True, Human intervention is about as likely as a politician doing something altruistic as the age of manned off-world ships is just about over but it is within the powers of our current technologies to move that rock to a more stable orbit. We could even use it as an anchor for a Martian Orbital Tower.

It's a great pity that such things are fantasies and will never happen.

Perhaps not in this age of politicians, but things might change. Never say never.

I've been enjoying the National Geographic channel's docu-drama Mars.

[Galaxian]

Gosh! Years ago, the origin of the grooves on Phobos were the subject of a long and sometimes bad-tempered (these are the moons of the War God) debate here on Staship Asterisk. Then just the the day another turned up that didn't, in fact only got a response from neufer: http://asterisk.apod.com/viewtopic.php? ... os#p287596

Ah yes John, the good olden days when conversations were keenly interesting. I wasn't a member back then but I've reviewed the archive ... "all our rowdy friends have rowdied on down."

Phobos: Doomed Moon of Mars

Explanation: This moon is doomed.

The ultimate result will be for Phobos to break up in orbit and then crash down onto the Martian surface in about 50 million years.

"The ultimate result, absent Human intervention, will be for Phobos to break up in orbit and then crash down onto the Martian surface in about 50 million years"

There, I fixed it for you.

True, Human intervention is about as likely as a politician doing something altruistic as the age of manned off-world ships is just about over but it is within the powers of our current technologies to move that rock to a more stable orbit. We could even use it as an anchor for a Martian Orbital Tower.

It's a great pity that such things are fantasies and will never happen.

Perhaps not in this age of politicians, but things might change. Never say never.

I've been enjoying the National Geographic channel's docu-drama Mars.

I should, perhaps parenthetically, have noted that "human intervention" was not the sole, exclusive possibility. Accidental bumping by intersecting rocks or ice cubes from other parts of the System is also possible in the long term. Mars, Mercury and Earth's very own Moon tell us that impacts by falling rocks are not impossible.
Indeed, Earth herself has been known to gobble up the occasional ex-asteroid.
Also, technological intervention need not be human. We haven't found any but there could be other beasties with tools and Starships out there in the deep, deep dark. If we can imagine Man sculpting planetary systems in his colonisation of the galaxies, it's not too much of a stretch of that imagination to substitute Man with Those Other Men.
So far, nothing in the Solar Ssytem has been touched by other hands but Man may not last forever and other manipulators could find Phobos to be a useful blob of stuff at some time in the not too near future.

The moon is even small enough to be converted into a useful habitat in which aliens, or us, could go roaming among the stars.

Or maybe we may force the poor thing to accelerate its doom and drop it onto Mars as a source of carbon and other stuff. That's a plan that has been novelised in SF a couple of times and which is almost feasible at our current level of technologies.

Phobos's future is not quite as certain as some of us would prefer.