Martian Moons May Have Formed Like Earth's 50
sciencehabit writes: Astronomers have long believed that Mars snatched its two moons — Phobos and Deimos — from the asteroid belt. That would explain why the objects look like asteroids—dark, crater-pocked, and potato-shaped. But computer simulations by two independent teams of astronomers (abstract 1, abstract 2) indicated that Mars's moons formed much like ours did, after a giant space rock smashed into the planet and sprayed debris into orbit.
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Can you explain how they managed to kill people more than once?
Wou would have thought. (Score:4, Funny)
"Mars snatched its two moons — Phobos and Deimos — from the asteroid belt."
So there really _are_ (astronomical) body snatchers from outer space.
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Yeah, except TFA is about how Mars isn't (much of) one. Jupiter is a big body snatcher [wikipedia.org], and Neptune may have snatched a big body (Triton [wikipedia.org]), but it looks like Mars is innocent.
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But how exactly does a planet "snatch" another object? The smaller object starts out at a huge distance from the planet, falls towards it (increasing its kinetic energy), passes by the planet (if it doesn't crash into it), and then... converts its kinetic energy back to the amount of potential energy it started out with, right?
Now if two different bodies would collide while close to the planet, some of the debris might just end up in orbit around the planet. But why would one small object not simply leave t
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But why would one small object not simply leave the planet again?
There is no reason that doesnt defy physics for stable orbits to form in the manner suggested for "capture" of single bodies. A force other than gravity needs to be applied.
Not sure why it is so often suggested that "captures" work. They don't unless there is a 3rd body that can be given the energy difference. If this 3rd body then leaves the system then a stable orbit is possible for the "captured" object, but if it doesnt leave the system then its a 3 body problem where the initial conditions preclude
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In the case of small bodies (such as spacecraft) the atmosphere can substitute for the third body. I would assume that if it is small enough a debris cloud like Saturn's rings could substitute over a long period of time, but the upper limit on that would have to be pretty low.
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In the case of small bodies (such as spacecraft) the atmosphere can substitute for the third body.
No, it can't.
You have found a way to lose enough energy for the object to remain in the system, but have not found a way to then add the energy necessary to put the object into a stable orbit (one that doesnt intersect your "solution" atmosphere.)
To be quite clear: If the orbit intersects the atmosphere this time around, and you dont add energy at some point immediately after that, then it will again intersect the atmosphere the next time around, and the time after that.... it will only take a few orbi
Re:Wou would have thought. (Score:4, Interesting)
Basically you need a third object to get involved, for instance it's thought that Triton was one of a pair of similar sized objects in orbit around each other. Triton was (relatively) going backwards at the time of a close approach so landed up in orbit around Neptune, it's partner got slung away. That also seems to explain why Triton is in a retrograde orbit.
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"Mars snatched its two moons — Phobos and Deimos — from the asteroid belt."
So there really _are_ (astronomical) body snatchers from outer space.
I am full of Fear and Dread right now...
Aphelion vs Parhelion (Score:5, Informative)
Phobos:
9,234 km min
9,376 km average
9,518 km max
Deimos:
23,453 km min
23,458 km average
23,463 km max
Compared to our Moon:
384,400 km min
363,104 km average
405,696 km max
I can see how one could say that Phobos and Deimos, like our Moon, have extremely regular orbital distances, but given that the science that has stated that our Moon was caused by an impactor is still itself being both refined and challenged, I wonder if it's a little premature to conclude that based on orbital characteristics alone the two Martian moons derived from the same sort of event as our Moon. After all, many of the planets have orbits that are very near circular, but we do not interpret their existence in a similar fashion.
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Technically Aphelion and Perihelion refer to distance from the Sun for Sun-Orbiting bodies. I'm not sure what the term for min/max orbital distance for Mars orbits would be. Since greek for Mars is Ares, maybe aparion and periarion ?
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Don't tidal forces eventually produce near-circular orbits? Thus, if they were captured asteroids, over time a "lopsided" orbit should grow circular and roughly equatorial.
Density matches theory (Score:4, Interesting)
Here are some relevant space object mean densities. Mars 3.93 g/cm3, Phobos 1.87 g/cm3, Deimos 1.47 g/cm3, Mercury 5.42 g/cm3, Luna 3.34 g/cm3, Earth 5.51 g/cm3, Ceres 2.07 g/cm3, Vesta 3.45 g/cm3, Europa 3.01 g/cm3, Comet 67P/Churyumov–Gerasimenko 0.47 g/cm3. If Phobos and Deimos formed from a violent collision it might be expected that they would be dense rocky objects like our moon or Vesta. However it seems that these moons more resemble the icy object end of the density spectrum. Did they form during a wetter Martian era?
I think the take home message is that some exploring of the Martian moons is in order; a sample return mission would be much simpler than a Mars return with an interesting scientific purpose.
Re:Density matches theory (Score:4, Interesting)
The impact hypothesis nicely explains why the moon is less dense than Earth: the impact preferentially threw up light elements from the crust and upper mantle, not heavy elements that would have sunk to the core. The densities of Phobos and Deimos are also less than that of Mars, but because they're so small, and are probably more like orbiting gravel piles, their densities are also consistent with small asteroids.
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"many of the planets have orbits that are very near circular, but we do not interpret their existence in a similar fashion."
We do actually. It's pretty well accepted that the planets around the sun coalesced from a protoplanetary disc surrounding the young sun. The impact hypothesis for moon formation is similar: a big impact causes debris to be thrown into an orbiting disc around the planet and one or more moons then coalesce out of it. The alternative, capture of a separately orbiting body, isn't serio
Re:Aphelion vs Parhelion (Score:4, Informative)
I can see how one could say that Phobos and Deimos, like our Moon, have extremely regular orbital distances, but given that the science that has stated that our Moon was caused by an impactor is still itself being both refined and challenged, I wonder if it's a little premature to conclude that based on orbital characteristics alone the two Martian moons derived from the same sort of event as our Moon. After all, many of the planets have orbits that are very near circular, but we do not interpret their existence in a similar fashion.
The impactor theory for the origin of the moon is being refined, but AFAIK, it really hasn't been challenged seriously since the early 90's. Before that there were many competing theories for the origin of the moon from forming at the same time as the earth, captured by there earth, formed from impact, and a few others. Meanwhile there are various criteria such scenarios must meet dealing with angular velocity of the moon, composition, etc. In the early 90's computer modeling got to the point that they could do such for impactor theory and resulted with a model of an impact of another body of similar composition that would collide with the proto-Earth, split off a glob that would become the moon while leaving it's own iron core to explain the Earth's relative large one. At that point, while not perfect, the impactor theory was basically doing better over all in the criteria than the other options. I remember seeing the presentation and video of the computer simulation while an undergrad in physics in the early 90's. I've been keeping up with the subject when I see it, and it has been modified, mainly that two moons were formed and then they recombined in a rather low energy collision to form the moon. I have not heard any serious competition by the other theories since then.
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More detailed info:
https://www.psi.edu/epo/moon/moon.html [psi.edu]
Dumbing down (Score:2)
Is that the technical term?
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I had the same reaction.
I weep for my unborn grandchildren's generation.
cheese (Score:2)
You mean Phobos and Deimos are also made of cheese?
Phobos and Deimos (Score:1)
That also explains where the names of the characters came from in Quake 3 :)
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The Russians tried to send a sample return mission to Phobos. It failed to get out of Earth orbit. Eventually we'll probably do that though.
Computer Simulations (Score:2)