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Martian Moon Phobos from Mars Express

Explanation: Why is Phobos so dark? Phobos, the largest and innermost of two Martian moons, is the darkest moon in the entire Solar System. Its unusual orbit and color indicate that it may be a captured asteroid composed of a mixture of ice and dark rock. The above picture of Phobos near the limb of Mars was captured last month by the robot spacecraft Mars Express currently orbiting Mars. Phobos is a heavily cratered and barren moon, with its largest crater located on the far side. From images like this, Phobos has been determined to be covered by perhaps a meter of loose dust. Phobos orbits so close to Mars that from some places it would appear to rise and set twice a day, but from other places it would not be visible at all. Phobos' orbit around Mars is continually decaying -- it will likely break up with pieces crashing to the Martian surface in about 50 million years.

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Oh, I like this image, a lot!

I wonder why there appears to be a ripple along the horizon and surface of Mars? Is it just an artefact of the image?

There appears to be a large body of water along the left hand edge of the image...

APOD Robot wrote: Martian Moon Phobos from Mars Express

Explanation: Why is Phobos so dark? Phobos, the largest and innermost of two Martian moons, is the darkest moon in the entire Solar System.
Its unusual orbit and color indicate that it may be a captured asteroid composed of a mixture of ice and dark rock.

:: falls over laughing! ::

It's a pretty good sized chunk of rock. http://en.wikipedia.org/wiki/Phobos_(moon) It would be interesting to watch when in made landfall.

This is one of those pictures that, at least to me, makes me feel the greatness of our solar system and the importance of technological development to show it to us.
Greetings
César

What parameters made it possible for Mars to capture Phobus and Deimos ? Their small orbits are very circular with very small inclinations. Is it hypothesized that they were captured together? The other inner planets either have no captured asteroids or have suffered impacts. What makes Mars so special that it was able to capture at least two asteroids and possibly more that eventually broke apart and fell to its surface ?

Does computer modeling reveal that these moons had to come from the Asteroid Belt to be captured and/or were perturbed by Jupiter ?

Doug Ettinger
Pittsburgh, PA

Peter Smith wrote:I wonder why there appears to be a ripple along the horizon and surface of Mars? Is it just an artefact of the image?

The camera used to capture this image is a bit peculiar. It uses linear sensors, meaning the image is constructed as the object moves past the spacecraft (or by rocking the spacecraft itself). A consequence of this is that you can get odd distortions and motion artifacts. The camera wasn't designed to image nearby objects like Phobos, which obviously moves very fast compared with the surface of Mars. On an area array camera the artifact would be motion blur; on a line array camera motion blur manifests as spatial distortion.

I'd guess that we are seeing an effect caused either by the motion of the spacecraft, or by the reconstruction of the line-scan image during post-processing, required to geometrically correct the Phobos image (at the expense of the background).

dougettinger wrote:What parameters made it possible for Mars to capture Phobus and Deimos ? Their small orbits are very circular with very small inclinations. Is it hypothesized that they were captured together? The other inner planets either have no captured asteroids or have suffered impacts. What makes Mars so special that it was able to capture at least two asteroids and possibly more that eventually broke apart and fell to its surface ?

This assumes, of course, that the moons are captured bodies. That is far from a consensus opinion. As I have discussed in the past, a simple gravitational capture of one body by another is impossible, so you need an additional mechanism- a third body, or something else that can remove energy from the captured body. A multiple interaction isn't very likely in the case of Mars, which leaves aerobraking as the best candidate. The most convincing scenario I've seen involves a near miss by a binary asteroid, passing low enough into the Martian atmosphere to shed energy and end up in orbit- either circularized and low inclination, or enough so that millions of years of tidal action could finish the job.

My own view is that there are enough differences between the Martian moons and asteroids, and enough mineralogical similarities with Mars, that their origin is better explained by being debris from a large impact.

Chris, I prefer your view for the same reasons. Of course, an impact involves at least two bodies that can easily have radically different compositions. So the Martian moons could be debris from either Mars or the impactor.

This is definitely a much better or more plausible idea than an impactor re-assembling itself to make the Earth's moon.

I have a question for the asteroid expert. What happens if an asteroid somehow assumes an almost identical orbit (almost same elipse and plane) and direction as a planet such as Mars or Earth ? Does the asteroid eventually over millions of years become captured in an orbit, collide, or be ejected from the neighborhood ?

Doug Ettinger
Pittsburgh, PA

dougettinger wrote:Chris, I prefer your view for the same reasons. Of course, an impact involves at least two bodies that can easily have radically different compositions. So the Martian moons could be debris from either Mars or the impactor.

The former is much more likely. Impactors tend to be completely destroyed, and excavate orders of magnitude more volume than they themselves contain.

This is definitely a much better or more plausible idea than an impactor re-assembling itself to make the Earth's moon.

I'm not sure I follow this. The idea that the Moon formed from a collision between the Earth and another planet sized body, through the reassembly of ejected material, is so well supported by both observation and theory that few doubt it. That mechanism is, by far, the most plausible explanation for the formation of the Moon.

I have a question for the asteroid expert. What happens if an asteroid somehow assumes an almost identical orbit (almost same elipse and plane) and direction as a planet such as Mars or Earth ? Does the asteroid eventually over millions of years become captured in an orbit, collide, or be ejected from the neighborhood ?

It might end up co-orbiting at a Lagrange point. Otherwise, such an orbit is likely to be unstable, and the body will be perturbed to a different orbit. In general, the orbits of most small bodies can be considered unstable over millions of years due to perturbations, mainly from Jupiter. So such bodies end up either colliding with a planet or the Sun, getting ejected completely, or being relocated to a stable resonance zone in the asteroid belt.

Chris Peterson wrote:

dougettinger wrote:Chris, I prefer your view for the same reasons. Of course, an impact involves at least two bodies that can easily have radically different compositions. So the Martian moons could be debris from either Mars or the impactor.

The former is much more likely. Impactors tend to be completely destroyed, and excavate orders of magnitude more volume than they themselves contain.

Then I should assume that any sizable ejecta that is flung beyond escape velocity after a major impact is from the impacted body. So what happens to the material of the impactor ? Does it vaporize and become part of a temporary atmosphere ? Does some of the material impregnate the interior of the parent body ? Can we assume that 3.9 billion year ago the Earth was molten and much softer under its developing crust when the impact occurred ?

This is definitely a much better or more plausible idea than an impactor re-assembling itself to make the Earth's moon."
I'm not sure I follow this. The idea that the Moon formed from a collision between the Earth and another planet sized body, through the reassembly of ejected material, is so well supported by both observation and theory that few doubt it. That mechanism is, by far, the most plausible explanation for the formation of the Moon.

I question the miracle of how the Moon's final resting place is in an orbit with the same ecliptical plane. And why was only one moon formed ? Should there not exist some larger collisional debris that did not form into a spherical shape ? Why does the moon have a comparatively large iron core? Why does not the Moon crustal materials match those of the Earth's mantle and crust ? And where does such a large impactor come from in the early solar system ?

It might end up co-orbiting at a Lagrange point. Otherwise, such an orbit is likely to be unstable, and the body will be perturbed to a different orbit. In general, the orbits of most small bodies can be considered unstable over millions of years due to perturbations, mainly from Jupiter. So such bodies end up either colliding with a planet or the Sun, getting ejected completely, or being relocated to a stable resonance zone in the asteroid belt.

How does Jupiter actually perturb the long standing orbits of asteroids ?

Doug Ettinger
Pittsburgh, PA

dougettinger wrote:Then I should assume that any sizable ejecta that is flung beyond escape velocity after a major impact is from the impacted body.

Yes, except for a case like the formation of the Moon, where you have two huge bodies of similar size. The collision dynamics in that case are very different. I'm assuming we're talking about "typical" collisions, where an asteroid a few km in diameter or smaller strikes a rocky planet. In that case, the impactor is vaporized, along with a large volume of the material around the impact site. Some of this vaporized material may be ejected to space, but most will probably remain in the atmosphere and rain back down over time. There will be a region a little outside the impact point where planetary material may avoid vaporization but still get enough energy transferred to it to escape the planet. That is how we have Martian meteorites on Earth.

Can we assume that 3.9 billion year ago the Earth was molten and much softer under its developing crust when the impact occurred?

Not molten, but certainly hotter, and with very different crustal composition. But I don't think that matters in the slightest. The energy released from two planets colliding is so huge that the result will be the same whether those planets have molted cores or are frozen bodies.

I question the miracle of how the Moon's final resting place is in an orbit with the same ecliptical plane.

Why is that a miracle? The collision was between two bodies that likely both lay near the ecliptic. And there are very strong tidal forces that will seek to flatten the lunar orbit. Keep in mind that dynamically, the Moon isn't even in orbit around the Earth, it is actually in orbit around the Sun.

And why was only one moon formed ? Should there not exist some larger collisional debris that did not form into a spherical shape?

The more bodies you have, the more unstable the system is- especially if the bodies are in similar orbits. As collisional material coalesced, you'd expect the largest body to act as a sort of vacuum cleaner, sweeping up or ejecting other material. If the amount of ejected material was much less, that might be different. But what formed was more of a binary planet than a planet-moon system.

Why does the moon have a comparatively large iron core?

Because both bodies involved in the collision were differentiated planets with iron cores. The iron that was ejected settled to the core of the Moon as that body differentiated and cooled.

Why does not the Moon crustal materials match those of the Earth's mantle and crust?

In most important respects, it does. Of course, it has undergone some modification through the Moon's own early volcanism, and the Earth's crust is continually being modified.

And where does such a large impactor come from in the early solar system?

Most models that attempt to describe the formation of the Solar System have more planets than we see today. Gravitational interactions can result in rapid and substantial shifting of orbits (that this can happen is a fact; whether it actually happened in our own system is a matter of uncertainty).

How does Jupiter actually perturb the long standing orbits of asteroids?

All realistic orbits in a system with more than two bodies are unstable. An asteroid is orbiting the Sun, but it is also subjected to a varying force from Jupiter and the other planets. Resonances are particularly effective at altering asteroid orbits, and occur when perturbing forces peak in the same place during each orbit. That's why the asteroid belt looks like it does, with big gaps where there is no material (and Saturn's rings, as well).

Some trivia: A cube spanning the volume of Lake Superior would neatly circumscribe Phobos.

I don't understand the statement that "Phobos orbits so close to Mars that from some places it would appear to rise and set twice a day, but from other places it would not be visible at all".

I understand that the closer a body orbits, the higher the velocity required to stay in orbit, but why would it be invisible from some places? Are you saying that a black moon would be invisible in a night sky?

Brucet wrote:I understand that the closer a body orbits, the higher the velocity required to stay in orbit, but why would it be invisible from some places? Are you saying that a black moon would be invisible in a night sky?

I think the International Space Station provides an analogy here. It never climbs above the horizon near our north or south poles.

Brucet wrote:I don't understand the statement that "Phobos orbits so close to Mars that from some places it would appear to rise and set twice a day, but from other places it would not be visible at all".

I understand that the closer a body orbits, the higher the velocity required to stay in orbit, but why would it be invisible from some places? Are you saying that a black moon would be invisible in a night sky?

No. It's so low that it is actually below the horizon if you are within about 20° from either pole.

In the explanation it states the image was captured last month, thus in November. In information brought up through the "captured" link in the explanation it states the image was taken during orbit 7982 of the Mars Express. I like to know the date when APOD images were taken but I have not been readly able to find when orbit 7982 was. The Mars Express site does not seem to let that information be found (if it does it is difficult to find). I would be grateful if someone could let me know when orbit 7982 was or at least direct me to where I can find it out. Thanks.

I vote this picture one of the best of 2010.

APOD Robot wrote: Martian Moon Phobos from Mars Express

Phobos is a heavily cratered and barren moon,

Heavily catered & Baron Moon-chosen

I saw it on U-tube once again - the human face of Mars. Doug