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Mars Had an Ancient Impact Like Earth
quixote9 writes "The BBC reports on a set of Nature articles showing that Mars had an impact about four billion years ago by a huge asteroid. This was about the same time that a much bigger object slammed into the Earth, throwing material into orbit around our infant planet. This material is thought to have coalesced to form the Moon. 'It happened probably right at the end of the formation of the four terrestrial planets — Mercury, Venus, Earth and Mars,' said Craig Agnor, a co-author on the Francis Nimmo study. 'In terms of the process of the planets sweeping up the last bits of debris, this could have been one of the last big bits of debris.' There's a theory that having a big moon is important to the development of life, because the much bigger tides create a bigger intertidal zone, but people used to think having a huge Moon like ours was a once-in-a-universe event."
Not sure if that was a joke, but Pluto's composition is far too different from Mars for that to have been the case. Besides, singling out Pluto isn't really rational. Pluto was demoted from planetary status precisely because Pluto isn't that special. It's just one of a whole bunch of similar objects out in the Kuiper Belt.
"People who think they know everything are very annoying to those of us who do."-Mark Twain
That bit in the summary about a 'huge' moon like ours and it being a once in a universe event is garbage/FUD added to the real story about an asteroid/small body 's collision with Mars.
People used to believe the world was flat but we don't need to throw that crap into every story about mapping or GPS.
IF you RTFA then you'll find no mention of some freaking 'Huge' moon being necessary to life. I suspect whoever wrote the Slashdot summary read too much Issac Asimov from the 60's, and wanted to seem 'SMRT'.
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The rock, the vulture, and the chain
The two colliding bodies generally merge. IE, "Earth" as we know it didn't really collide with some foreign object that went skipping off into space again. "Earth Mark I" was a somewhat different planet whose remains are still here with us, and it collided with a very large object (essentially a planet in it's own right) called Theia (or sometimes aka Orpheus). Those two would have merged and the collision throw up the debris that formed the moon (which is also made up of parts of both)
So there is no search for the object that hit Earth to form the moon. It's still here with us as part of the new Earth, and Theia/Orpheus is essentially our original planet as well; it's just easier to call the bigger planet involved in that collision "Earth" because, well, it was the bigger of the two.
The same is likely true of Mars - the object that struck it probably merged with it.
As an interesting note, it's also thought that a large impact must have struck Venus as well, and must have hit it hard enough to "flip" the planet, as Venus, compared to every other planet in the solar system, rotates "backwards". There were some BIG things floating around and colliding in the early Solar system.
"People who think they know everything are very annoying to those of us who do."-Mark Twain
"I guess the moral of the story is, don't paint your airship with rocket fuel." -- Addison Bain
Pluto and Charon aren't formed the same way. Charon really isn't even a "moon" at all, since rather than it orbiting Pluto, the center of mass is well between the two of them, and instead they orbit each other. They were almost certain separate bodies that simply became gravitationally bound to each other.
As well, Charon, in the inner solar system, wouldn't make much of a moon at all. The higher heat would essentially turn it into a comet that would melt away much of it's surface area, and reduce it's already pretty low mass.
Though I'm not sure about "once in a universe" odds, for 1 very, very large rock to become gravitationally bound in a STABLE orbit around another, without the two impacting (or, this system forming as a result of an impact), does seem like it'd be incredibly rare. Again though, "seeming" incredibly rare is indeed a guess on my part, and our sample size isn't large enough to really determine just how common it is.
"People who think they know everything are very annoying to those of us who do."-Mark Twain
Unlikely. Pluto's orbit doesn't come close to intersecting with that of Mars, and circularizing a Pluto-sized object's orbit after the huge Mars collision event would require ... another huge collision event. [Un]fortunately, most of the debris in the Sol system has been cleared out by the current set of planets, so most of this stuff will remain speculation because the evidence is gone by now.
I seem to recall that a mechanism had been proposed whereby the material for the colliding body collected at a Trojan point, from which is was dislodged once it reached sufficient size - after which collision with the proto-earth would be just about inevitable. If the mars collider formed in the same way then collision with a large body could be pretty much normal in our sort of system and the incidence of large moons might be very high indeed.
There are several things that our extremely large moon does for us that make life much more comfortable.
#1 is of course tides that are more extreme than the sun could generate
#2 rotational speed. The moon has kept our rotation from slowing as much as it should have. Shorter day/night cycles are important for environmental management.
#3 atmosphere. The moon helped reduce the density of proto-earth's atmosphere. Without it we probably would resemble Venus.
#4 plate tectonics. The tidal influence on the crust heats up the mantel and keeps the plates from sealing
#5 protection. The moon intercepts a large percentage of the impacts destined for earth. This reduces the disruptions caused my meteor impacts. This was more pronounced earlier, when the moon was closer and when it was even more important (more meteors around)
#6 increased metal content. Since the moon is almost entirely light minerals, most of the metals that were in the original impact were left on earth which artificially increased the concentration of metal in the crust.
While I can think of other things that could help with one or two of these, I can't think of something that would satisfy all of these consistently for a few billion years.
What do you suggest as an alternative?
Yeah, Venus must have had some fairly...unique event to make it rotate backwards, cause that's just flatly impossible to happen by itself. Debris further away from the sun is moving faster than debris closer, and hence when they merge they're always going to rotating in the same direction. At least on average of the billions of collisions that make the planet. So something really big must have hit the mostly-formed Venus and done something near the end.
Whether things brushed it 'the wrong way' enough to make it spin backwards, or if they hit the top or bottom of it and actually rotated it 180 degrees so its north became south, is unknown. I, like you, suspect the latter, that Venus was essentially flipped over like grabbing a spinning gyroscope and flipping it...which obviously takes a lot of force, even more than it would normally take because of gyroscopic action.
The first option is that it was bombarded with enough debris on the 'inside' and not on the 'outside'. But that's just sorta implausible, whereas the 'flip' theory just requires one really big hit.
If corporations are people, aren't stockholders guilty of slavery?
It's not the orbit that the huge moon stabilizes. It stabilizes the angle of the planet's axis of rotation. Life on planet earth would suck if the planet occasionally rolled over on its side so that the poles aimed right at/away from the sun.
You can have my SIG when you pry it from my cold, dead hands.
If you assume a circular orbit, then velocity is calculated as sqrt(G*M/r). Therefore, the radius is inversely proportional to the square of the velocity -- for example, something with an orbital radius 4x that of a closer object will be moving half as slowly as the closer object.
Note to self: Stop putting jokes in my insightful comments so I can get something other than +1 Funny!
Helles, Isidis, and Argyre, the other large impact basins on Mars, though orders of magnitude smaller don't have rims, just big holes blown out of the ground.
horror vacui
Depending on how it hit the debris may have been put into a retrograde orbit which is less stable.
Mars is significantly smaller so it is not able to hold onto it's satalites as easily...it could have escaped.
The collision is smaller than the earth-moon collision. There may have been less matter kicked up.
If it was very glancing then any debris would have more than escape velocity anyway...