Wednesday, July 1, 2015

One may be regarded as a misfortune; two begins to look like carelessness.

This is a post not so much about planetary science, as about the sociology of science. If you have been following recent developments in the astronomy of the solar system at all, you will know that the comet 67P-Churyumov-Gerasimenko has a funny shape. The standard working explanation for this is that it has a funny shape because it has been formed from two comets that have collided and stuck together, a ‘contact binary’, and explanations that ascribe its shape to deformation or ablation of an initially more spherical object are marginalised.

I have not understood why, but now I think I do.

First of all, I should explain that my biases are all the other way around. I would be inclined to go through the most elaborate mental gymnastics to avoid having to suppose a body was formed  from two things running into each other. This is based on two things: firstly, common to all people who do not get their picture of space primarily from space opera, my perception of space is of someplace that is very big and very empty where things move very fast. I do not expect to be hit by a comet any time soon. If I am, I expect the relative difference in our velocities to be such that we will not end up as a ‘contact binary’, but as a mass of superheated comet and minor scientist-derived dust. Secondly, in my particular case, I am used to thinking about chemical reactions, and I know that almost all collisions of molecular bodies in the gas phase do not end up with them sticking together; without a third body to take excess kinetic energy away, they are too energetic to stick together, and fly apart again. I know very well that comets are not very much like molecules, and there is no reason to behave in exactly the same way, but that is the intuitive bias I have about things running into each other in a big empty space: chances are that if they not smashed to flinders they will bounce off one another, and never come within coo-ee of each other ever again.

I have been drawn inexorably into thinking about comets by Andrew Cooper and Marco Parigi, and I interpret the shape of 67P-Churyumov-Gerasimenko as they do, in terms of the ‘Malteser’ model I have outlined below: a brittle, dense crust has developed on the comet surrounding a deformable core, and at some time in the past this crust has ruptured and the gooey interior has stretched. If this were true, unless it happened a very long time ago indeed, the neck of the comet should be less depleted in volatiles, and that is where most of the water sublimation should be coming from. This seems to be the case.
67P-Churyumov-Gerasimenko, being awesome.

Yesterday I found out, being a slow newcomer in the comet world, that 67P-Churyumov-Gerasimenko is not the only comet with a funny shape. There is 103P-Hartley 2. It has a relatively smooth neck, and this neck is also associated with direct sublimation of water, as would be expected if it had a fresher surface that had been exposed for a shorter time.

103P-Hartley2. The bright light from the rough end is coming predominantly from chunks breaking off a frangible surface and subliming, if I have understood correctly.
And – ahem – it was also suggested that 103P-Hartley 2 is a contact binary. To quote Lori Feaga and Mike A’hearn of the University of Maryland*, 8th October 2011: "The heterogeneity between lobes is most likely due to compositional differences in the originally accreted material. We are speculating that this means that the two lobes of the comet formed in different places in the Solar System.  They came together in a gradual collision and the central part of the dog-bone was in-filled with dust and ice from the debris."

I might be prepared to grant one contact binary, since the universe is a strange place, and all manner of things can happen. But two from such a small sample size? I don’t think so.

But as I said at the beginning, this isn’t supposed to be a post about planetary science. It finally struck me yesterday why so many people would cling to the idea that funny comets are formed by bodies sticking together.

Paper after paper refers to comets as ‘primitive’; relics of the primordial Ur-cloud out of which the solar system accreted, from which we can learn about the nature of the Ur-cloud. If comets are not primitive – if they have suffered all sorts of physical and chemical transformations as a result of repeated annealing by swinging by the sun – then they don’t tell us anything about the Ur-cloud.

I think a lot of people who study comets are very interested in the Ur-cloud. They got into comets in the first place as the best way they could approach this beast, whose nature is so important for understanding the overall history of the solar system and all those other solar systems out there. They are not necessarily interested in comets as ends in themselves. The contact binary model is the one way that observed heterogeneous, oddly-shaped and oddly-behaved comets can be reconciled with observable comets as primordial bodies. So, if you are interested in the antiquity of the solar system, what are you going to do? Accept that ‘you can’t get there from here’, or grab at this improbable lifeline that leaves the road unblocked?
*: In the interests of full disclosure, I was once a resident of Maryland, when my father was in the army. We moved back to Arizona when I was 2.