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Posted by: Enceladus75 Oct 28 2008, 07:09 PM

I see in the current issue of Astronomy magazine (November 2008) that there is a very interesting article about a class of meteorites that are theorised to have possibly originated on Mercury. We now know of meteorites that came from the Moon and Mars, and there might also be some from Venus, but I think that it is fascinating that we may have, right here on Earth, pieces of the innermost planet at hand.

Would it take a sample return mission from Mercury to prove these meteorites came from the planet or could the Messeneger results in the coming years clinch the question?

What do others think?

Posted by: PhilCo126 Oct 29 2008, 08:46 AM

How would these meteorites end up in an orbit towards Earth... from the Moon to Earth is a short trip, even from Mars inwards towards the Sun I can understand meteorites travel past Earth... but from Mercury outwards to Earth?

Posted by: PhilCo126 Oct 29 2008, 09:01 AM

Intrigued I started an Internet search and came up with “Sky & Telescope” article:

Free rock samples from the first rock from the Sun?
Past studies assumed that rocks knocked off Mercury weren't getting away with much more than its escape velocity of 2.6 miles (4.2 km) per second. That's too slow to climb away from the Sun and make it out to Earth. Mercury, the Sun's innermost planet speeds through space with a mean velocity of 48 km per second. Furthermore, impactors (comets & asteroids) travel fast and could strike the planet at speeds 5 to 15 times its escape velocity, and ejecta can rocket off the surface traveling much faster than had been assumed…


But I can hardly believe it

Posted by: AndyG Oct 29 2008, 11:12 AM

Then again...

Mercury -> Venus = 4.5 km/s (best case)
Mercury -> Venus = 9.9 km/s (worst case)
Mercury -> Earth = 7.2 km/s (best case)
Mercury -> Earth = 12.8 km/s (worst case)

Given that Mars -> Earth is ~2.6 km/s, perhaps some proportion of these Mercurial meteorites came via Venus?

Andy

Posted by: Enceladus75 Oct 29 2008, 12:34 PM

Thanks for the replies guys. It seems like the orbital dynamics go strongly against the easy passage of material from Mercury to Earth, but it is possible. The class of meteorites in the Astronomy article are called angrites, and they have a suite of unusual properties that hint that they originated much closer into the Sun than conventional asteroidal material. However, the author of the article is sceptical that angrites originated on Mercury, and says that the Messenger mission analysis of Mercury's surface will clarify the issue.

But I suppose some day meteorties from Mercury and perhaps also Venus will be found somewhere here on Earth.

Posted by: tty Oct 29 2008, 07:35 PM

I think Venus is if anything less likely the Mercury. Escape velocity is high and the atmosphere is extremely dense. Only an impactor large enough to punch a hole right through the atmosphere for the ejecta to escape through would be any good. We are probably thinking Chicxulub size or up here. Question for Emily if you read this: Is there any young crater big enough on Venus?

Incidentally it is a bit odd that nobody has ever reported any Terran meteorite. A lot of the material ejected by impacts on Earth must eventually return here. Perhaps all the stuff from Chicxulub and the Eocene/Oligocene impacts was swept up long ago?

Posted by: NGC3314 Oct 29 2008, 08:22 PM

As I understand the consensus these days, Terran ejecta that gets swept up immediately makes tektites. But that doesn't say much about any bits that don't get melted during ejection. (Mental note to watch for Cretaceous fossils with fusion crust...)

Posted by: nprev Oct 29 2008, 10:05 PM

Wouldn't it be a bit hard to identify a Terran meteorite? There are many processes that can mimic fusion crust (ex: 'desert glazing'), and the isotope ratios would obviously not differ from Terrestrial norms. It'd probably just look like an unremarkable metamorphic rock.

With respect to Mecurian & even Venusian meteorites, one exciting thing to think about is that we KNOW they're here, if quite rare. It's a statistical certainty that N>0 no matter what. For all we know, any of us may have kicked one at some point in our lives. The trick is to identify them chemically, and that really does seem to require ground truth from both bodies.

Posted by: pjam Mar 22 2010, 06:15 PM

It's true that we can never say `never' for the possibility of meteorites from Mercury, Venus or the Earth. But it's really, really unlikely:

Mercury: Hard to get out of the sun's gravity well, so meteorites not only need to be ejected from Mercury (relatively easy) but they need to get out to the Earth too. Hey, maybe that Venus flyby would help!

Venus: Big planet and thick atmosphere + interior to Earth.

Earth: Here's the rub -objects liberated from the Earth have a dynamical lifetime of some few millions of years, after that they are nearly all swept up or otherwise removed from orbit. So... for any reasonable possibility of terran meteorites, we would need to have had a large impact capable of liberating meteoroids from the Earth sometime, say, within the last 10 million years. There are no such young, large impacts known.

...but I think there is still an interesting possibility of locating older terran *fossil* meteorites, if one could demonstrate the presence of a fusion crust on a fossil meteorite recovered from rocks that shortly postdate known major impacts. The point is, these meteorites would not be falling anymore now, but they might exist in the sedimentary rock record.

-pjam

Posted by: pjam Mar 22 2010, 06:28 PM

...as an afterthought, meteorite delivery from Mars was also once considered to be impossible until it became clear in the '80s that there were definitely martian meteorites! Then, their survival during launch from Mars was expored in more detail and a viable mechanism for doing it was discovered.

Maybe a similar thing will happen with Terran meteorites. It will seem impossible or at least very unlikely until someday one is discovered.
-pjam

Posted by: Explorer1 Mar 22 2010, 11:21 PM

We do known there are some massive impacts on Mercury. Caloris Basin is the most well known, for instance.
If any impact had the ability to throw something to Earth, Caloris would have.

Posted by: ngunn Mar 22 2010, 11:58 PM

I'm sure I've read somewhere that although it's hard to get chunks of Mercury out to the other planets because they have a hill to climb it's also more likely because asteroid impacts on Mercury happen at higher velocity.

Posted by: pjam Mar 23 2010, 02:10 AM

...An oldie but goodie paper that considers that Mercury delivery problem is:

B. Gladman, J.A. Burns, M. Duncan, P. Lee, and H. Levison. The exchange of impact ejecta between terrestrial planets. Science, 271, 1387-1392 (1996)

non-subscriber link is:
http://www.sciencemag.org/cgi/content/abstract/271/5254/1387

-pjam

Posted by: Antdoghalo Mar 24 2010, 02:21 AM

Actually id rather watch for Permian fossils in recollected ejecta from the unconfirmed huge 2500 Km
large Gulf of Mexico (not Chixulub the GOM was already there when it formed) comet impact that occured 250 mya and destroyed Pangea and killed 95% of all terrestial life in the Permian-Triassic boundary.

Any sweeped ejecta from a terrestial impact would be shatter cones with burnt crust from reentery

One thing thats weird is why are there no rocks orbiting entirly inside Mercurys orbit

Posted by: Explorer1 Mar 24 2010, 02:31 AM

I imagine Mercury would sweep them up pretty quick, in astronomical terms. It certainly has enough craters for that!

Posted by: machi Mar 24 2010, 12:16 PM

Extinction on Permian/Triassic boundary is probably caused by massive volcanic eruptions (but maybe combined with some impact event).
But Terran meteorites older than 3.5 mld. years can answer the question about beginning of life.

Posted by: Rob Pinnegar Mar 24 2010, 02:37 PM

Hi, Phil! Nice to see you here.

I guess there is one other thing for us to consider here besides the initial escape velocity of the material, and that is that any material getting knocked off Mercury would inevitably remain in a Mercury-crossing orbit. Things like the advance of perihelion could keep it away from Mercury for a while, but sooner or later, it would have to encounter Mercury again. If it did so in the "right way", presumably this could give it a bit of an energy boost.

Getting it straight from an initial Mercury-crossing orbit to an orbit that intersects Earth's may be too much to ask for. Alternatively, it could land in a Venus-crossing orbit, giving it the chance to get kicked even further out. But now we're getting into some serious hypotheticals... the odds of any given particle encountering Mercury in a way that just happens to get it back to Mercury, then just happens to get it to Venus, then just happens to get it to Earth? I can picture this happening from time to time -- surely many of the meteorites we pick up here on Earth have experienced multiple planetary encounters -- but it makes things more difficult.

Posted by: tty Mar 24 2010, 07:08 PM

The largest (known) young meteor craters are

Kara-Kul, 52 km, c. 5 million years
El'gygytgyn, 18 km, 3-4 milllion years
Zhamanshin, 14 km, 1 million years
Lake Bosumtwi, 10 km, 1 million year

It seems that at least Kara-Kul might be large enough for ejecta to reach escape velocity, particularly as it is at 3900 meters altitude, that means a lot less atmospheric drag (though I guess it was probably rather lower back then). Interestingly it is situated inside the Tien-Shan, so any ejected rocks would be fairly young sedimentary stuff (though I suppose the impact would turn them into instant metamorphics).

As for really old terran meteorites, I've pointed out before that the best (only?) chance to find out anything about what happened on Earth during the first 500 million years or so is probably to look for terran meteorites on the Moon. It seems that the Late Heavy Bombardment wiped out almost every trace of what happened before c. 3800 million years ago.

Posted by: pjam Mar 30 2010, 04:46 AM

Hey, that's a good point about Kara-Kul being youngish and fairly large. It's worth a good study, to see if a crater that size and at that elevation could produce ejecta that would escape the Earth. To assess the possibility of Kara-Kul as a possible source for terran meteorites, we need a better understanding of its age and crater diameter, though. It might not be as young or as large as advertised! Unfortunately, it is difficult to do geological fieldwork in Tajikistan these days...

The ejected blocks from Kara-Kul would not be metamorphosed in the normal geological sense by the shock event, they would just be variably shocked pieces of the target rock. A big issue here would be that only the most competent rocks would survive ejection -at least, this is the current argument for why we have only igneous rocks as martian meteorites. The sedimentary rocks on Mars' surface more likely get destroyed during ejection. ...but for Kara-Kul as the terrestrial analogue, maybe limestones are competent enough to act like a basalt and survive ejection during the impact event.

-pjam
Phil McCausland

Posted by: stevesliva Apr 12 2013, 05:35 PM

News from the Mercury meteorite front:

Could be:
http://www.planetary.org/blogs/emily-lakdawalla/2013/03211549-lpsc-hermean-meteorite.html

But maybe not:
http://www.npr.org/2013/04/11/176714430/origin-of-meteorite-is-a-puzzle-to-scientists

Posted by: TheAnt Apr 13 2013, 02:21 PM

Indeed the age is from the very formation of the solar system, which make it quite unlikely that the rock come from Mercury.
The lack of sulphur is another.

As already have been pointed out, it's unlikely but not impossible for a piece of Mercury to get here, so lets see until another candidate turns up.