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Without doubt, Phobos is the grooviest moon of the Solar System. By that I mean, that it is covered with a multitude of parallel grooves.
Initially, it was thought that these markings radiated away from the largest crater on Phobos. Called Stickney, the crater has a diameter of 9 km and is the most obvious feature of the moon’s pockmarked surface. Some thought that the grooves were debris ejected across Phobos during Stickney’s creation. In other words, they were similar to the bright rays of material seen emanating from some craters on the Moon. Most thought that they were fractures in the moon, opened up by the mighty impact. But these hypotheses were based upon an incomplete picture of Phobos – literally and metaphorically.
This close-up of the grooves of Phobos was taken on 3 August 2008 at a distance from the moon’s centre of 656 km. Details as small as 6 metres across are shown in this image. Credit: ESA/ DLR/ FU Berlin (G. Neukum).
Once the High Resolution Stereo Camera on Mars Express had mapped the majority of the moon, a different point of view emerged. The images reinforced an often-overlooked fact that the grooves fall into 12 families, each of a different age and orientation. They also showed that the grooves do not radiate from Stickney at all but from the leading apex of Phobos, the part of the moon that always faces forward. This suggests a much more exotic origin. Instead of being ejecta from Stickney, they could be ejecta from Mars.
The current thinking is that if a large enough asteroid impacts the surface of Mars, it will throw debris into space. If the ejecta cloud crosses Phobos’s orbit it could strike the moon, like bugs on a car’s windscreen, creating the grooves. If this is the case, it leads to an intriguing possibility.
Collections of martian rocks could be lying on the surface of Phobos, perhaps even dating from all points in the Red Planet’s history. When the Russian mission Phobos-Grunt returns samples from the surface of Phobos in 2012, researchers may be in for a treat. In amongst the Phobos rocks, they may just find some from Mars too. -- Stuart
The top image shows a sketch map of the grooves of Phobos revealed by HRSC, MOC & Viking images. The bottom image shows a model of the grooves expected from Phobos ploughing through debris fields around Mars. Taken from: New evidence on the origin of Phobos’ parallel grooves from HRSC Mars Express by John B. Murray et al.
03-05-2010 • 10:59:59
Given that Phobos is destined to disintegrate into a ring in a few million years, and that that is a blink in geologic time, how many other moons may have been through that process and where would we find the debris (on mars, i assume). How long until ring material also has orbit decay and falls to mars.
03-05-2010 • 01:33:18
You can almost see the rivets.
25-04-2010 • 00:36:01
Inside sources tell me(richard Hoagland to name on) ESA is going to announce in 2010 that Phobos is 125% artificial on the inside with cubical room miles long, and furniture as well. Over a 100 scientist are working on the 3D images.
25-03-2010 • 01:35:30
Now that I understand why there's N and S poles (axis of rotation), I wonder why Phobos is not tidal locked with the fat end pointing at Mars. Which is relevent to the origin of "grooves", too. Look more like creases on a dried fruit.
02-04-2010 • 13:07:43
"not facing mars"
Because of a later impact perhaps? Altering it's axis?
23-03-2010 • 19:32:37
I agree with Gumbo, whilst we both agree that the grooves are indeed from Phobos ploughing through impact debris from Mars, they are not 'evenly' distributed. I understand that the Phobos grooves could be from as many as 12 different impact events on Mars.
Has actual 'bedrock' been identified on Phobos or is the entire surface covered in regolith? Also the boulders, can they be sourced, MGS did with MOC, MRO spied a few with HiRISE & Mars Express too, are they Stickney Crater ejecta, some from Mars, bit of both??
Great shame Mars Express cannot do these obsevations & use MARSIS with Deimos.
22-03-2010 • 14:28:07
Ive heard from different sources that Phobos has a declining orbit , and it is slowly falling into mars!!!!
Is'nt this therefore a violation of Newtonian laws of physics for stable bodies !
I suppose the boosters on Phobos are no longer functioning after thousands of years without care takers around
22-03-2010 • 08:51:53
There is a new idea on the formation of the grooves. It is set out in the UMSF forum,Unmanned Spaceflight.com > Mars & Missions > Orbiters > Mars Express & Beagle 2, in the thread on Phobos, starting on page 11. The nub of it is that the grooves were cut when Phobos intersected rings which formed at the same time as Phobos, and which for some time were circular and coplanar with the eccentric orbit of the main body. What else but rings could score such long, straight and regular grooves? And yes, it is my idea.
The grooves on Ganymede are nothing like those on Phobos. They appear to be tectonic fractures, caused by stretching and shear of the icy surface.
@Gumbo. I agree, impact debris would pepper the moon with craters, not score grooves. Rayed craters on the moon and elsewhere don't show rays that regular or straight.
28-03-2010 • 22:16:55
Thanks to bk_2 for the link to the interesting discussion on the UMSF site about these grooves. The ring idea is compelling.
I am wondering just how much of a delta V would be required between Phobos and any intercepted ringlets to form the grooves. Perhaps a few hundred metres per second or less might be sufficient. It wouldn't necessarily have to be many kilometres per second.
I think a more critical point is the near 90 degree crossings of the grooves on the leading orbital hemisphere. The principle axis of orientation of Phobos in its orbital locked state is with the long 26.8km diameter aligned radially towards the centre of Mars. There is no way that Phobos could have rotated about an axis aligned with the direction of its orbit in order that the leading hemisphere grooves be crossed over as observed. The only way that I can see this happening is if there was a large polar axial inclination (say 45 degrees) which then precessed over time. This would then present the leading hemisphere at a variety of orientations over time with respect to any ring plain,
An interesting fact about Phobos is that it is not far from disintegrating into a ring itself. It seems destined to do this in just a few million years as its orbit steadily decays under the tidal influence of Mars. From what I have been able to glean it seems to be loosely consolidated and have a deep dusty regolith. With such low gravity a collision with a ring would indeed be peculiar. The ring impact ejecta could very well reform once Phobos had passed through as Phobos would not be able to hold onto it. I would envisage individual ringlets rather than a ring disk. They would cut just as good a groove when met edge on. Each ‘grooving’ event would be more like sand blasting than anything.
I am sure there will be much more to this story in the coming years. It's a fabulous puzzle.
21-03-2010 • 16:51:35
I don't buy ejecta. NO ejecta will create parallel trace evidence. One only has to look at our own moon to see this. And the moon is in such an orbit as to be the BEST possibility of creating parallel ejecta traces and there are NONE.
I think the best possible theory are either erosion (minimal chances) or some kind of temperature variance during the cooling process after a cataclysmic collision event.
That or its canals built by Marvin to launch his Eludium PU36 explosive Space Modulator?!?
20-03-2010 • 07:19:53
The pattern of the groves can be interpreted as Martian impact ejecta being intercepted by the leading orbital side of Phobos in its low orbit of Mars.
The fact that they should be grooves at all is still not obvious. It does not seem that Phobos ploughed through evenly dispersed pepperings of impact debris in the process of being hurled upwards from Mars or falling back towards Mars.
Surely the impact ejecta must have been distributed in some radial fashion from the impact site so that Phobos would then intercept closely spaced chains of objects in order for grooves to be formed.
Is this a well understood property of the behavior of impact debris?
24-03-2010 • 18:11:37
If these grooves are to be explained by ejecta, then are we to assume that the impact craters pre-date this event as the grooves follow the crests and dips of each crater. Surely the grooves would not have survived a subsequent impact?
20-03-2010 • 01:32:22
I wonder if the grooves aren't the result of differential erosion. When a small object hits solid rock, it will launch ejecta free from something as small as Phobos. If it hits a crack, or close to one, it will fragment more material than an impact on solid rock. Over time, impacts near the cracks have ejected more material than impacts farther away.
19-03-2010 • 15:26:31
http://ciclops.org/view/4386/Ganymede_-_Ridges_Grooves_Craters_and_Smooth_Areas_of_Uruk_Sulcus_Region?js=1
sez otherwise!