APOD: Phobos from Mars Express (2010 Mar 17)

[APOD Robot]

Phobos from Mars Express

Explanation: Why is this small object orbiting Mars? The origin of Phobos, the larger of the two moons orbiting Mars, remains unknown. Phobos and Deimos appear very similar to C-type asteroids, yet gravitationally capturing such asteroids, circularizing their orbits, and dragging them into Mars' equatorial plane seems unlikely. Pictured above is Phobos as it appeared during last week's flyby of ESA's Mars Express, a robotic spacecraft that began orbiting Mars in 2003. Visible in great detail is Phobos' irregular shape, strangely dark terrain, numerous unusual grooves, and a spectacular chain of craters crossing the image center. Phobos spans only about 25 kilometers in length and does not having enough gravity to compress it into a ball. Phobos orbits so close to Mars that sometime in the next 20 million years, tidal deceleration will break up the rubble moon into a ring whose pieces will slowly spiral down and crash onto the red planet. The Russian mission Phobos-Grunt is scheduled to launch and land on Phobos next year.

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[biddie67]

1. What factors made point "N" declared a north pole? How do you declare "north" on something like this? Is it on a line of rotation similar to earth's rotation?

2. Why would Phobos break up into pieces that would spiral down and crash onto Mars instead of staying together as one large chunk?

[cdcdent]

The photo shows grooves running around the "moon" which cannot be caused by asteriod hits. They look like erosion in sedimentary rock. Seems to me we are looking at a small fragment of a much larger planet which had sedimentary rock not unlike our planet earth. After this small part floated free (? after a cataclysmic collision?) it has been eroded by micrometeorites such as we have seen on the surface of our own moon. One day someone will scrape of drill the surface and maybe see signs of life. It seems to me that this photo lends strength to the theory that once there was a planet where the asterioids are now. Remember our earth and moon are a result of a collision between two proto-planets

Jeremy Rourke

[Amir]

1. What factors made point "N" declared a north pole? How do you declare "north" on something like this? Is it on a line of rotation similar to earth's rotation?

interesting, never thought of it before. i think the answer would be that they declare the north pole according to the magnetic field on that "Something", just like how they did on earth!

[DaveBone]

When I saw the grooves moving through the bottom of the craters, I immediately thought of sedimentary rock layers as Jeremy said. My problem with this is that the grooves should have been filled in when the craters formed and not just continue through as if they were put in after the crater. It is certainly a strange place. I hope the Russian mission can answer some of the questions about this "fearful" place

[CatchDude]

What I find interesting is that there is a strange snaking trail of small asteroid impacts (that being an assumption, it could be something else that created the holes) going from the large crater on the left to the bottom right in this picture. Even though there's multiple impact sites visible, this trail clearly stands out. While typing this, I also realise that it might be a straight line that only seems to 'snake' because of the curvature of the asteroid/moon.

What strange coincidende(?) could cause such a flawless trail? A brush in with a very narrow yet dense meteorite bundle? An object bouncing up and down across the moon because of it's very low gravity, like a balloon bobbing across the ground on earth? It makes me very curious...

[rstevenson]

The grooves have been pretty closely identified as originating at the leading point of Phobos as it orbits Mars (which happens to be close to Stickney Crater), fading out towards the trailing point. In other words, they are likely impact trails of multiple collisions from debris in orbit around Mars. It has been suggested that impacts on Mars have thrown debris up where Phobos could sweep it up, causing these crater trails.

Rob

[Chris Peterson]

1. What factors made point "N" declared a north pole? How do you declare "north" on something like this? Is it on a line of rotation similar to earth's rotation?

On a rotating body, "north" defines one end of the axis of rotation. When you look down on the north end of a rotating body you always see counterclockwise rotation.

2. Why would Phobos break up into pieces that would spiral down and crash onto Mars instead of staying together as one large chunk?

Because the tidal forces as it gets near Mars (that is, the differential force of gravity between the part of Phobos closer to Mars and the part farther away) are greater than the material strength of the moon itself. So it will get pulled apart. That will be a gradual, gentle sort of disruption, which is why Phobos will first become a sort of ring structure- not a sudden shower of rocks raining down onto the surface. Check out the idea of a Roche limit.

[neufer]

1. What factors made point "N" declared a north pole?
How do you declare "north" on something like this?
Is it on a line of rotation similar to earth's rotation?

The angular momentum vector of the entire solar system points in a certain direction.

If the dot product of that vector with the angular momentum vector of Phobos is positive then Phobos's vector points to its north pole.

If the dot product of that vector with the angular momentum vector of Phobos is negative then Phobos's vector points to its south pole.

"The north pole is that pole of rotation that lies on the north side of the invariable plane of the solar system".

This definition means that an object's axial tilt is always 90° or less, but its rotation period may be negative (retrograde rotation). Venus rotates in the opposite direction to the other planets, and Uranus has been knocked on its side and rotates almost perpendicular to the rest of the solar system.

Some bodies in the solar system, including Saturn's moon Hyperion and the asteroid 4179 Toutatis, lack a stable geographic north pole. They rotate chaotically because of their irregular shape and gravitational influences from nearby planets and moons, and as a result the instantaneous pole wanders over their surface, and may momentarily vanish altogether (when the object comes to a standstill with respect to the distant stars).

[Chris Peterson]

interesting, never thought of it before. i think the answer would be that they declare the north pole according to the magnetic field on that "Something", just like how they did on earth!

North on Earth (just like on all rotating objects) is defined strictly in relation to the axis and direction of rotation. There is no connection to the magnetic field at all. (Additionally, on Earth, the north magnetic pole is quite far from the actual north pole. Phobos has no magnetic field.)

[neufer]

2. Why would Phobos break up into pieces that would spiral down and crash onto Mars instead of staying together as one large chunk?

It is generally assumed that large solid bodies act more or less like fluids globally.
Thus the rotating earth is oblate in roughly the shape that Isaac Newton calculated for a rotating fluid sphere.

Smaller lunar bodies that are rubble piles act like rigid quasi-fluids.

Phobos has already passed within the Roche limit for a stable orbiting fluid moon.
Phobos is about 72% outside of the the Roche limit for a stable orbiting rigid rubble pile.

When exactly Phobos breaks up as its orbital radius shrinks can only be guessed at:

Code: Select all

    Satellite    (Orbital Radius)/(Roche limit)
________________      (rigid)  (fluid)
Earth      Moon         41:1    21:1
Pluto      Charon       25:2    13:2
Mars       Deimos       451%    234%
Jupiter    Thebe        254%    128%
Neptune    Larissa     ~218%   ~113%
Saturn     Epimetheus   200%     99%
Uranus     Juliet      ~199%   ~102%
Saturn     Janus        195%     97%
Uranus     Desdemona   ~194%   ~100%
Uranus     Cressida    ~191%    ~98%
Jupiter    Adrastea    ~188%    ~95%
Jupiter    Metis       ~186%    ~94%
Uranus     Bianca      ~183%    ~94%
Jupiter    Amalthea     175%     88%
Mars       Phobos       172%     89%
Saturn     Pandora      167%     83%
Uranus     Ophelia     ~166%    ~86%
Saturn     Prometheus   162%     80%
Saturn     Atlas        156%     78%
Uranus     Cordelia    ~154%    ~79%
Neptune    Galatea      153%     79%
Neptune    Despina     ~152%    ~78%
Neptune    Thalassa    ~145%    ~75%
Saturn     Pan          142%     70%
Neptune    Naiad       ~139%    ~72%

[jjohnson]

The grooves are not as unusual as claimed, in that similar constant-depth grooving is observed on rocky bodies with no or only tenuous atmospheres generally.

This includes Mercury, our moon, and smaller moons of planets generally. Although resolution of photos of comet nuclei, asteroids, and bodies from Pluto outward has been too low to reveal details like this, I suspect that they will be found there, too.

Constant-depth grooving is exemplified by plasma machining in industrial applications on earth, where near vertical walls can be obtained, and depth is a function of duration of force. I would suggest electrodynamic forces are a logical and straightforward explanation for phenomena of this type - sometimes termed "rilles", and sometimes "canyons". Electrodynamic removal processes, like plasma phenomena in general, can be scaled up and down over many orders of magnitude, from microscopic to cosmic.

Long, glancing blows by wandering body impacts do not create constant-depth grooves at any scale.

[DonAVP]

Here is a thought regarding the grooves. As noted by others this moon is very loosely held together and it is below the Roche limit. The grooves show were Mars gravitational forces are distorting the existing fracture lines of the object. If a meteor were to impact this moon today in a few orbits around Mars we would see a groove passing through it as is see in the ones now. The impact would have to have been were an existing fracture was.

I like the idea of a small meteor bouncing along the surface. For this to have happened the meteor would have be going almost at the same speed as the moon (a little slower). I would have thought that we would see the meteor setting on the surface of the moon were it stopped bouncing, and as far as I can tell it is not there. Unless something came along and knocked it off. Just my 2 cents worth.

It does look like a snow cone that fell out of its cup however.

[Vaccum Explorer]

If a moon was bombarded headon, coudn't it be hit by a glancing blow also. So we have valleys and grooves?

[BMAONE23]

On the subject of the "N" pole.
We use North to typically denote the part of the planetary body that faces Polaris. The fact that this object has an "N" also denotes that it doesn't tumble but rather has an equitorial rotation relative to Mars' rotation as it orbits.

[JohnD]

Sigh. We have been here before.
"craterchains", you stalwart of interplanetary war, cosmic strafing runs and hollow planets, where are you?

All the 'grooves' are crater chains, down to the limit of resolution of the pic, whereupon they seem to have smooth sides.
This closer-up pic from MarsExpress reveals even more crater chains, that previously were grooves, with some even smaller 'grooves'.
http://www.esa.int/images/3_h7915_phobos_nadir_H.jpg
Why adopt an extraordinary and unlikely argument for the origin of these markings when the answer is plain on Phobos' face?

John

[tdevore@collinsco.com]

With all the talk about the moons of other planets in our solar system I was wondering why all have names but Earths moon does not?

[BMAONE23]

But it does...We call her Luna

[JohnD]

In the British Isles there are EIGHT rivers called "Avon".
And the word means ................. river.

So much for the originality of the great English language, or its speakers.
Same with "Moon" - they're all 'moons' but the is only one Moon.

John

[owlice]

We call her Luna

This triggered "We call him Flipper! Flipper! Faster than lightning!" on the radio in my head.

The angular momentum vector of the entire solar system points in a certain direction.

Interesting, and thanks, and now, I'm off to Google "angular momentum vector." (So very much to learn, no matter how much learning has gone before!)

[Chris Peterson]

On the subject of the "N" pole.
We use North to typically denote the part of the planetary body that faces Polaris.

Only for the Earth. Not for anything else.

[Amir]

On a rotating body, "north" defines one end of the axis of rotation. When you look down on the north end of a rotating body you always see counterclockwise rotation.

North pole right ascension: 18 h 11 min 2 s
North pole declination: 67.16°

the coordinates show a spot just 23° away from earth's north pole which roughly points at this star: USNO J1811031+671504
earth's N & Venus's N are in the same hemisphere, while Venus rotates clockwise. so if you look down on the north end of Venus it rotates clockwise.
did i make any mistake?

[bystander]

@amir

you forgot axial tilt, 177.3°

[Amir]

axial tilt shows the direction of rotating, it doesn't mean that north pole will rotate too.
the fact is that if you stand on the spot that is named "N" on Venus & look downward you'll see it rotating clockwise!

[bystander]

Planet Venus has an axial tilt of 177.3 degrees because it is rotating in retrograde direction, opposite to other planets like Earth. North pole of Venus is pointed 'downward' (our southward). Planet Uranus is rotating on its side in such a way that its rotational axis and hence its north pole is pointed almost in the same direction of its orbit around the Sun. Hence axial tilt of Planet Uranus is 97 degrees.

Looking down on Venus north pole, Venus would be rotating ccw.