Meteorites from Mercury? Options

[Guest_Enceladus75_*]

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?

[Guest_PhilCo126_*]

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?

[Guest_PhilCo126_*]

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

[AndyG]

But I can hardly believe it

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

[Guest_Enceladus75_*]

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.

[tty]

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?

[NGC3314]

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?

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...)

[nprev]

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.

[pjam]

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.

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

[pjam]

...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

[Explorer1]

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.

[ngunn]

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.

[pjam]

...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

[Antdoghalo]

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...)

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

[Explorer1]

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