I've been meaning for years to do a thread on the presumed origin of a big chunk of Earth's meteorites [The HED achrondites] from a collision with the asteroid Vesta. It's a well accepted theory that I've long felt needed closer re-examination for diverse, easy to grasp, reasons
Then we (well, NASA -- but I watched) launched a probe to Vesta... years later, it arrived (decent background material there) ... and after over a year of study, it left for the asteroid Ceres ... and six months later, I STILL hadn't begun to lay out my many doubts and questions, much less read the literature on Vesta's findings (admittedly, most of the data hasn't been published yet, much less the main body of academic discussion)
Tonight, I touched upon the subject in another thread, and it hit me that if I don't start a thread now, I'm unlikely to ever find a better time.
But first, a caution: my first post, transplanted from another thread (on orbit dynamics and Earth collisions which will be a WHOLE lot simpler and more fun than it sounds) was an unplanned late night rant, partly fueled by frustration at the loss of one post in previously composed series. Still, I'm pretty sure I have my details right (if deliberately simplified) -- and if I don't have them right, please correct me so I don't mislead anyone!
Consider the "Mystery of the HED meteorites" (Howardite-Eucrite-Diogenite). HEDs make up about 5% of all meteors found on Earth, and 60% of all "achondrite" meteors on Earth. Stony meteors fall into two basic types: achrondites appear to have been melted at some point, and chrondites don't. You see more large crystals in achrondites and more dust and chunks in chondrites -- HEDs can be very dissimilar. You wouldn't call most of them "the same kind of rock" if you had them in your hand or even tested them in a lab. The very name HED includes three distinct categories! They are similar *in principle" -- to astrophysicists.
Still, 5% of all Earth meteorites! That's a hefty chunk of mass, multiplied over eons [some HEDs show evidence of slowly crystallizing in the vacuum of space over up to 100 million years]. Where did it all come from? The answer was 'obvious': "From the moon, Alice. From the Moon!" -- it's not even one-millionth of the rock/ice in the solar system, but it's really close, so natural it dumps on us! Many astrophysics papers "proved" this origin, culminating in a "crown jewel" in the mid-60s (still acknowledged as an exceptionally elegant analysis by the professor who pointed me to it in the 1990s) Sadly, a week after that paper, NASA landed a sample probe on the moon, which completely disproved the entire edifice. Data will do that. Once we had moon rocks in our hands, we found that NO known meteorite was lunar. (We found our first in 1979, but it wasn't recognized until we found our second, some years later. All the lunar meteors EVER found wouldn't fill a grocery bag. That's not 5% -- or even 0.00000005%)
This sent astrophysicists scrambling for an alternative origin, and they soon settled on the asteroid Vesta, because it has an HED-like spectrum and experienced a major impact a billion years ago. This became dogma to the point that I found it hard to discuss with astro-folk: either they didn't care (not their field) or they were SO convinced that they just said "read up on it".
Alas there are MANY basic reasons to be skeptical. If the moon is a flyspeck, Vesta is a bacterium. Though mathematicians have found an elegant chaotic mechanism by which Jupiter sometimes perturbs an asteroidal fragment into a long sunward plunge, that falls short of explaining how Vesta could be the source of 5% of ALL Earth meteorites (plus, according to the accepted theory: all the Vesta group of asteroids, and almost all asteroids with a Vesta-like spectrum all around the solar system, including some near-Earth orbiters). The rest of the solar system has been smashing and perturbing too!
[Edited to add: Juno, the fourth largest asteroid, has a fresh impact crater, too. For that matter, if we're talking billions of years, what about ... I dunno, the impact that kicked the Moon off of Earth? That EASILY kicked up enough molten basaltic rock, in a compatible orbit, to still be falling as slow-cooled HED. Hey, one chunk of that impact (the moon) is 30,000x as massive as the Rheasylvian impact on Vesta, and MORE mass would've become dust and 'rocks' (but much of that may have re-accreted)]
Don't get me wrong: I can accept that Vesta may be *related* to many (perhaps most) HEDs on Earth. I'm willing to accept that Vesta (the 2nd largest asteroid) may be the largest fragment of a much larger parent body which was shattered long ago (Vesta and other asteroids display differentiation [separation of materials] that might be most easily explained by the fracture of a planet with enough gravity to settle out in molten layers) but the literature usually bluntly says "HEDs came from Vesta", and IMHO that's grossly overstating it. I've mentioned some of my reasoning in earlier EtU posts, but I have much more evidence than I've ever posted. I intend to make a full thread on it.
Well, actually, I'm overstating things a bit myself: the literature HAD been mostly blunt that Vesta and its fragments were the sole origin of HEDs, but about 5 years ago, we launched a probe to Vesta, and during its long multi-year trip, I noticed that more scientists, perhaps emboldened by the prospect of prompt vindication (vs indefinite decades of dismissal) were pointing out problems with the Vesta hypothesis and positing alternate origins -- but health/personal issues kept me from monitoring the literature as I had planned to after waiting a decade for the repeatedly-canceled Dawn spacecraft to finally reach Vesta orbit in July 2011
[Dawn has since left Vesta without touching it or even chemically analyzing its orbital dust -- it was never designed to do anything but "look at" Vesta with cameras and spectrometers from 1000s of km away (it did briefly pass closer than 100 km), highlighting the difference between astrophysics and the earth sciences! The Dawn space probe website remains a bastion of unquestioning HED=VESTA dogma. It even labels several earth meteorite pictures "Rocks from Vesta", leading many readers to believe that those are actual samples of Vesta and not "best guesses" of what Vesta rocks are like.]
I do know one Dawn result: the gigantic Rheasilvia crater, supposedly the source of all the HEDs (and Vesta group asteroids and V-types, etc.) is as showy but shallow as some Las Vegas dancers. It looks impressive from Earth, but it has now been measured, up close, to only equal about 1% of Vesta's tiny mass. True, that's 2.5 million million kg but divide that by roughly a billion years since the impact and it's just be 2.5 tons/year (mostly dust) if every speck came to Earth! Worse, consider that Vesta's contribution would have been tapering off over a billion years. A billion years ago the sky would've had to be "all Vesta, all the time" -- day and night, on Earth, Venus, Mars and Mercury!
Why Hyacinth Bucket would be positively apoplectic "Vesta lights up four planets on a budget of just 5500 pounds a year?! That's positively unseemly! And hardly sporting!" [What, I can't make foreign sitcom puns? I'm a licensed cosmopolitan.]
I've seen countless theories backed by astrophysical research/analysis implode quickly under new data. As fancy as space probes are, I couldn't diagnose a hangnail using NASA's full armory of space observation tech, but astrophysicists "live in that murk" and have never experienced the level of hard evidence that more earthbound sciences take for granted. It's almost all interlinked deduction.
There is a HUGE tendency in science to mistake a compelling narrative that strings together data and studies (which may be of quite high quality) for proof, but that's not even science--it's lore! I've always said that future centuries will consider ST:TNG brilliant for making Lore the (evil) brother of Data. Lore, the firstborn of the same creative and analytic impulse was flawed by human impulses that Data was said to lack; he considered himself superior and refused to be cast aside. Most scientists feel the same way about THEIR lore. Remember that professor who pointed me at that "crown jewel" paper "proving" that HEDs came from the Moon? He was basically arguing "See how clever-but-wrong we were before? We wouldn't be cleverly wrong again!" :sphericalcow:
Scientists are only human, after all. In every field of human endeavor, we're still telling tales around the campfire. Science is not immune to "urban legends" and mythic narrative. I can list more examples in medicine than astrophysics, though physicians each get up close with thousands of "test cases" and insist they "know what they see". Modern humans are merely the first stage past cavemen -- and will forever be seen as such by our successors.
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