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.