Glancing impacts on Earth

There is this guy who appears regularly on Twitter, trying to fill the shoes of Archimedes Plutonium, and the Expanding Earth guy, who asserts ad nauseam that the Puerto Rico back-arc basin and some structures in the Southern Andes are the product of glancing impacts by cosmic projectiles. He's a bit irritating - typically very dismissive of counter evidence, totally dismissive of plate tectonics possibly operating in "his" (I assume it's a "he" ; safe bet) area, but somewhat content that plate tectonics operate in the rest of the world. He's got a bit of and "Electric Universe" or "plasma physics" hang up too, associated with his putative impacts. He's probably also afraid of 5G telephones, and I think he's spoken approvingly of Expanding Earth ideas too.

A bog-standard, run of the Internet kook, in other words.

Well, I'm not saying that this piece's author is the same guy, or even knows of his existence, but this is the sort of thing that would greatly encourage the kook. I found this while checking for references about the potential "glancing impact" origin of Mars' Borealis (North Polar) basin.

On a Possible Giant Impact Origin for the Colorado Plateau

This is an Arxiv preprint, for a paper that was submitted to EPSL ("Earth and Planetary Science Letters" - a medium-hitter of a journal in the field) but with no mention of it actually being published. I'm not surprised.

So, firstly, it's a single-author paper. It would seem that Xiaolei Zhang hasn't managed to persuade any of his GMU (George Mason University, wherever that is) colleagues that his theory is valid. He's got a reasonable track record of publishing in "galaxy dynamics" (which he shoehorns into this paper too - confirming the identity of the author), but AFAICT, this is his only foray into geology/ planetary science. In itself, this isn't a disbarring factor, but it is a warning sign.

What is the theory? That about 750 Myr ago (in the Neoproterozoic era), the Earth suffered a "glancing blow" by an impactor to produce an astrobleme of about 640km diameter, which we now call the "Colorado Plateau". 

Aside : digging out the "area under discussion" was surprisingly hard - the author clearly thinks his audience has the same familiarity with the area that he does. There is a map - labelled as "Figure 1", but placed on P 52. That's an artefact of publishing conventions (let the journal arrange the figures into the text ; the author supplies them after the text) - annoying but not the author's fault. There's no legend on the map, which I think only shows volcanic rocks at surfaces versus "other" - which looks very single minded from a geologist's PoV.

Now, this is where the author departs from conventional cratering theory (to put it politely). Given the dimensions of the area considered "anomalous", and an estimate for how deeply the anomalies are incised into the Earth (estimate : 16km, but this seems to be derived from conventional cratering scaling laws, which this paper is rejecting for this "feature" - that point needs justification) then the author derives a simple geometrical estimate for the size of the "grazing" impactor as "Mars size". Actually, "3208km". The calculation used is the inverse of the well-known one for "distance to horizon from ships mast/ cliff, whatever". An allowance is made for drag changing the travel vector of the impactor by several degrees. But that's just about bonkers.

At this sort of scale, planets don't have significant strength. Their spherical shaoe is because the strength of the rocks is negligible compared to the hydrostatic forces due to their weight. Essentially, planets behave as strengthless drops of liquid. A contact like that would leave both objects with surfaces vibrating up and down by hundreds if not thousands of km until the energy is dissipated through most of the mass of the bodies.

It's also a very improbable contact. Even a small degree closer to head on, and the bodies would have merged, or generated so much ejecta that there would have been global secondary impacts, if not forming a moon of orbiting ejecta. And if the alignment had been 16km in the other direction (0.5% of impactor diameter) then it would have been a near miss.

It seems the geological data the astronomer is tieing onto is reporting of apparent horizontally directed shear at high levels in the crust of the Colorado plateau, combined with the Plateau's elevation. He's also relying on there being some great mystery about the so-called "Great Unconformity" observed in the Neoproterozoic of the Grand Canyon. (We have a "Great Unconformity" covering about the same Interval here in Scotland, but we don't blame it on wildly unusual events.)

The author ascribes the appreciable NW-SE elongation of the "Plateau" to the motion of the "impactor" ("grazer"?), but makes very little mention of the Sudbury structure in the Canadian Shield, which is generally accepted as being an impact structure that has been compressed on a NW-SE axis to have about twice the NE-SW dimension compared to the NW-SE dimension. The deformation of Sudbury is generally ascribed to continent-scale compression during the Grenville orogeny shortly after the impact. The more modest non-circularity of the posited Colorado Plateau structure is as easily explained by distortion since it's formation - regardless of intrinsic (e.g. mantle plume) or extrinsic (impact, the FSM's paintbrush) origin. (I emphasised elongation versus compression to avoid people thinking there was some stress field similarity - the elongation is similar, but the direction is opposite.)

Memo to astronomical dynamicists : you leave the geology alone, and I'll leave stirring the pot of star alone. OK?

November Slashdottery

Some new interesting things on Arxiv.

Avi Loeb doing his "'Oumuamua is an Alien Spaceship" thang, again.

Astronomical gadfly Avi Loeb has summarised his arguments why he thinks interstellar object 1I/2017U1 'Oumuamua is an artefact, and since we didn't make it (or did we? That'll get the conspiracy nuts foaming at the mouth.) then it proves the existence of alien civilisations, and probably a lot of them.

Well, there aren't many other serious astronomers who'd go that far, but 'Oumuamua remains a very peculiar object. 2I/Borisov in contrast was a very much more normal cometary object. And everyone in the asteroid-hunting game is looking to put their own handle on 3I/YourNameHere.

There's nothing particularly new here - 'Oumuamua has disappeared into the dark depths of space and we're never going to see it again or acquire more data on it (unless a Star Trek scriptwriter decides to take a plunge, V'ger style). But if you're looking for a quick summary of why it's considered weird, this is as good as any half-dozen of the 130-odd other papers on the subject hosted on Arxiv. 

Planet and asteroid formation histories

A couple of Russian astronomers have contributed an appendix for a forthcoming book on the Chelyabinsk meterorite of 2013. One would hope that such a chapter would be a reasonably even-handed description of modern theories (note : plural) of how the medium to large bodies in the inner Solar system accreted, but really the authors have beaten their own drum for a minority position and have paid short shrift to the majority opinion. (They dislike the "giant impact" scenarios, and prefer forming a debris disc from multiple small impacts, then assembling a Moon from that ; moons of various asteroids are proposed to form by analogous events.) Well, it's a position, but I'd hope they at least put in another appendix on the Giant Impact family of theories, since they're considerably the more popular. If they want this to become a textbook, they're doing their readership a disservice.

I didn't read this closely. In the introduction they make several significant errors (they assert that the asteroid belt has about the same mass as the Moon - it's about 4% of the mass of the Moon ; they question "where did the 99.9% of the original matter of the asteroid belt go to", when again the majority opinion within mainstream planetary science is that it was scattered by early migrations of Jupiter (and Saturn) in the "Grand Tack" model). With these problems, I didn't think it worth more than a cursory read. As a model for the formation of asteroid satellites and/ or double asteroids, their ideas have some merit, but as an appendix to what aspires to be a textbook, this chapter has some serious problems.

Was Venus Ever Habitable?

In popular science, Venus is often described as "Earth's twin", with an implicit subtext about "why isn't it habitable like Earth?" In reality, it is much more dubious if Venus was ever habitable, and this paper describes some quite detailed modelling of the evolution of the planet's mantle chemistry and atmosphere to arrive at the current situation, from an inferred formation of chondritic composition (with several Earth-oceans of water). They find there is a significant chance of the planet going directly from a magma ocean state to a runaway greenhouse without any appreciable period of habitability.

Dull stuff, but it's important to remember when people are getting excited about terraforming planets. Venus remains potentially easier to terraform than Mars (there aren't enough volatiles on Mars to make a useful atmosphere - you'd have to move essentially all the volatiles of the asteroid belt onto the surface of Mars - which is going to take millennia during which the surface is going to be a very dangerous place to be. Stick that in your Musk and smoke it.), but it is quite dubious whether it has ever been habitable.

This was making me think last night. 

The Faint Young Sun paradox has always been a challenge over why Earth has remained habitable for so long. I wondered, how much geological evidence would be left by a close approach (not an impact!) by, say, Mercury migrating inwards at (say) 2Gyr, to move Earth (and potentially Venus too) out by, say, a tenth of an AU. I don't think I've seen any work trying to address early rearrangements of the inner solar systems, while there have been a number of studies projecting the Solar system forwards to the Sun's red giant stage, which produce a few percent chance of Mercury and Venus interacting to eject Mercury from the system - potentially even to impact the Earth! If that could happen in the future, then I don't see a clear reason why it couldn't have happened in the past.

That's some special pleading to try to resolve the FYS paradox : might be completely unnecessary ; might be a part of the "Rare Earth" question.

Could Mercury be the core of whatever hit Mars to make the "hemispheric dichotomy"? Not seriously - Mercury is half Mar's mass, but the hypothesised "Borealis Impactor" is, IIRC, a few % of the mass of Mars. (Off to check - SETI talks, Martian Dichotomy, Margarita Marinova. Paper : Nature v453 p1216, 2008 ; where they look at impactors 1600-2700km diameter. Mars is 6792km (nominal diameter), so diameter ratio 0.23 to 0.4 and volume ratio to 1.3 to 6%. It's not so simple to go to masses, because there is compression in the cores, but it's not a huge effect at these small masses.) 

Nice idea. Doesn't even begin to work.

Comets can be active a LONG way out.

Jewitt and Bouziani study the observation that comet C/2017 K2 had visible gas emission while out beyond Uranus. Who ordered that !? That's not the normal story. But yeah, it happened. They make a model of heat conduction into the body of a comet after perihelion, and it's effect on CO pressure, suggesting that as the warming front propagates into the body (but it is already past perihelion) then pressure can build up under the surface crusts and migrate to the surface producing outgassing potentially out as far as 150AU. That's ... a lot further out than expected - for example C1/Halley could be outgassing all the way to aphelion.