The rock that went to Mars and back

Robin

Penultimate Amazing
Robin
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This has bugged me for a while. Way back when that Martian rock was in the news, you know, the one with the possible evidence of life – a well known physicist wrote in an Australian newspaper that it may well have originated from Earth. The idea was that if we have Martian rocks on Earth there are just as likely Earth rocks on Mars and that one could have been involved in two catastrophic impacts – one that blew it to Mars, and one that blew it back to Earth.

I promptly wrote to the Australian Skeptics and nominated him for a bent spoon award for this comment. They wrote back and said they could see nothing wrong with the idea.

I would have thought that in the first place Earth rocks on Mars are going to be a lot less common than Mars rocks on Earth, due to the fact that the journey to Mars is against the gravity of the Sun and Mars is a much smaller target. Also the fact that it will take a much larger impact to blow something clear of Earth’s gravity in the first place.

So the whole scenario of an impact dislodging an Earth rock from Mars and sending it hurtling back to Earth seemed to me to be so highly improbably as not to be worth even mentioning.

I would be interested in comments.
 
It'd be a BIG stinking rock to survive reentry, and even then it'd leave a crater.

But to think that a rock would gain sufficient speed and altitude to leave Earth's orbit, and be large enough to survive the heat of such, and we just HAPPENED to find it on out tour...

Well, I'm sorry, but it stretches the bounds of my believability. COULD have? Maybe. But I wouldn't bet on it.
 
I agree an impact to send a rock to Mars, survive on the surface, then be the target of another impact (that did not destroy it but placet is in a earth intercepting trajectory) is highly improbable.
 
I thought the usual objection was that the things found in the Mars rock were too tiny to contain organelles.
 
Robin said:
This has bugged me for a while. Way back when that Martian rock was in the news, you know, the one with the possible evidence of life – a well known physicist wrote in an Australian newspaper that it may well have originated from Earth. The idea was that if we have Martian rocks on Earth there are just as likely Earth rocks on Mars and that one could have been involved in two catastrophic impacts – one that blew it to Mars, and one that blew it back to Earth.

I promptly wrote to the Australian Skeptics and nominated him for a bent spoon award for this comment. They wrote back and said they could see nothing wrong with the idea.

I would have thought that in the first place Earth rocks on Mars are going to be a lot less common than Mars rocks on Earth, due to the fact that the journey to Mars is against the gravity of the Sun and Mars is a much smaller target. Also the fact that it will take a much larger impact to blow something clear of Earth’s gravity in the first place.

So the whole scenario of an impact dislodging an Earth rock from Mars and sending it hurtling back to Earth seemed to me to be so highly improbably as not to be worth even mentioning.

I would be interested in comments.
Click to expand...

I don't think the meaning of this was that THAT particular rock came from earth, but that the signs of life (if they were) could nevertheless have originated on Earth, and subsequent generations later come back.

There is no significantly greater difficulty in going to Mars from Earth than the other way. A little higher velocity in one path perhaps, but not so if a boosted swing by Venus is contemplated, for example.
 
There was a thread about this in the old forum. I gave the same uphill versus downhill reply, but I admit I had not considered a venus or perihelion slingshot orbit. Still, the probability has to be low that everything would be just right.

Back in the early days of the system when a lot of rocks were zooming madly in all directions, the odds are a lot higher of Earth-Mars travel by rock- but was there life on either world then? (My guess is yes, for a very broad definition of "life"- ie molecular replicators, possibly inorganic.
I suppose the guy's point was that saying life on Earth originated on Mars solves nothing. We might as easily say Martian life originated on Earth. The question of interest is not "where? ", but "how? ".
 
Just to get one thing clear: There in no uphill/downhill thing in orbits. To get something to from earth to Mars, you have to accelerate it enough to rise to the Mars orbit (once you have gotten it clear of Earth). To get somedhing from Mars to Earth, you have to decelerate it enough to descend to the Earth orbit. In space, accelerating and decelerating comes at the same cost.

Hans
 
There's an issue here of the timing of such matter transfer.

During the accretion period there would have been matter transferrence on an almost continuous basis as the protoplanets gradually stabilised into their now recognisable forms. However, there was almost certainly too much instability for life to have formed and survived during this time.

There are some scientist who believe that the Earth-Mars-Moon bodies had some major collisions and major matter transferrence after the accretion period, perhas leading to life even arriving here after having started on what we now know as Mars. Evidence for this is, at best, sketchy.

However, the long and short of all this is that it is very much possible for large rocks to have survived these kind of interchanges, no matter how unlikely the specified event may be.
 
I'm curious about this whole accretion thing. What causes clouds of rock and dust to besome solid structures? Is it the gradual work of gravity? If I put two rocks somewhere out in space and wait a billion years, will they merge?
 
It all boils down to the uneven distribution of mass.

The universe itself was distributed unevenly from the chaos of it's inception, hence there are areas of high relative density and thus higher gravity.
This can be extrapolated into the existence of galactic superclusters and the galaxies themselves, the solar systems and so on, down the scale of size to the level of planets and moons.
A very basic/simplified description, I know, but I hope it paints the picture.
 
David Swidler said:
I'm curious about this whole accretion thing. What causes clouds of rock and dust to besome solid structures? Is it the gradual work of gravity? If I put two rocks somewhere out in space and wait a billion years, will they merge?
Click to expand...

I'm no expert, but my understanding is "no" - but it depends on how big the rocks are. If you put a couple of bowling-ball-size rocks out in space, away from any other influences, say ten metres apart, and at rest relative to each other, their mutual gravitational attraction would cause them to move together, possibly bounce a couple of times, and then just come to rest in contact with each other - there's no 'magic' that causes them to meld together.

However, a couple of Everest-size rocks, starting say ten kilometres apart, would come crashing together at such speed and thus with such huge kinetic energy that they would either shatter into millions of little rocks, or simply melt. After a while, the original two rocks would effectively have ceased to exist, and the resulting system would be a new, roughly spherical asteroidy-type thing.
 
Well, you asked "If I put two rocks somewhere out in space and wait a billion years, will they merge?" the answer to that is almost certainly not, no - there's probably not enough mass/matter.

But you also asked how accretion works - hope I at least began to explain it in a vaguely understandable fashion.
 
Soapy Sam said:
There was a thread about this in the old forum. I gave the same uphill versus downhill reply, but I admit I had not considered a venus or perihelion slingshot orbit. Still, the probability has to be low that everything would be just right.

Back in the early days of the system when a lot of rocks were zooming madly in all directions, the odds are a lot higher of Earth-Mars travel by rock- but was there life on either world then? (My guess is yes, for a very broad definition of "life"- ie molecular replicators, possibly inorganic.
I suppose the guy's point was that saying life on Earth originated on Mars solves nothing. We might as easily say Martian life originated on Earth. The question of interest is not "where? ", but "how? ".
Click to expand...

The where could possibly be determined if dating is clear, but then we are left with the question of the outer planet moons, and so on. On the other hand it might be clear that it is a frequently repeatable occurrence. Time will tell.

On probability, the type of "signs" of life being debated would apply from 4 billion years ago or more, and hardly more than one piece of rock. I see them falling weekly when I am outside looking at the night sky.
 
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