Is science sometimes in danger of getting tunnel vision? Recently published ebook author, Ian Miller, looks at other possible theories arising from data that we think we understand. Can looking problems in a different light give scientists a different perspective?

Planetary Update – September – November

Chemists are fairly adept at finding out what molecules are present in a sample, but what happens when the sample is light years away? Astronomers have worked out how to do some spectroscopy, but it is not exactly easy to do. One of the interesting reports recently was the announcement of the measurement of an exoplanet's atmosphere (Nature 526: 526 – 529). When starlight passes through the atmosphere, various absorption lines can be seen as long as the atmosphere is basically transparent. While the star is a strong source of light (somewhat too strong, since most of the light does not go through the planetary atmosphere, as the star is very much bigger) the path length of a giant planet's atmosphere is also somewhat longer than the average laboratory cell! In this case, the main signal detected was water, and it was noted that the level of heavier elements in the atmosphere relative to hydrogen was no greater than 700 times that of the star, as would be expected if the planet was a giant that accreted gas from an accretion disk. Not that there would be many other ways of making a gas giant.
Another study (Icarus 243: 39 – 47) considered the chemistry of cometary methanol during impacts. Impacts cause methanol to dissociate into CO and CH4, however the energies are such that methanol should survive accretion onto the large icy satellites, including Callisto, Ganymede, Titan, Ceres and Pluto, although cometary impacts following accretion will produce dissociation. Asteroid impacts onto Ceres would dissociate methanol. However, Callisto could have produced up to 10-2 bar during the late heavy bombardment, while Titan would have acquired 0.1 bar. Since it did not, the authors imply that the methanol concentration in the Saturnian system was much lower than that of comets, or alternatively, some unspecified conversion of CO to CH4 occurred. This supports my mechanism of planetary formation, in which comets were not the source of methanol or other carbonaceous material on the icy bodies. Titan would have contained methanol, but this would be converted to methane by geochemical processes. These authors also show that CH3OH and CH4 abundances on a persistently shadowed part of the moon cannot be of cometary origin.
One of the more difficult questions is what the original earth was like. The standard theory has it that the planet formed as a consequence of giant collisions that led to a magma ocean, but a recent publication (arXiv:1403.0806) throws up interesting constraints. The authors propose at least two giant impacts to generate a global magma ocean based on the ratios of 3He to 22Ne. The depleted mantle has a ratio at least 10, while a more primitive mantle has a ratio of 2.3 - 3. The solar ratio is 1.5.  In-gassing of gravitationally accreted nebular atmosphere will explain the 3, but to get to 10 it requires at least two episodes of atmospheric blow-off and magma ocean outgasssing. The preservation of the low ratio in a primitive reservoir sampled by plumes suggests that later giant impacts, including the moon-forming impact, did not generate a whole mantle magma ocean. Atmospheric loss episodes with giant impacts provide an explanation for Earth's subchondritic C/H, N/H and Cl/F elemental ratios, while preserving chondritic isotope ratios, but if so, a significant proportion of terrestrial water and other volatiles were accreted prior to the last giant impact, otherwise the fractionated elemental ratios would have been overprinted by the late veneer. What is most surprising here is that the collision that caused the moon to form was insufficient, yet the carbon, nitrogen and halogens were determined relative to hydrogen prior to the moon-forming event. That would require the current volatiles were degassed from the earth at a later date.
There were two big events in November. The first involved Philae landing on a comet, and apparently it has made a lot of measurements, and sent the data back to Earth. However, as yet we have no idea what was discovered. The fact it landed and ended up in the shade was bad news because the solar cells will not recharge the batteries adequately. For me the big disappointment was that the device that bored into the comet apparently struck something hard, and when the drill was withdrawn, apparently there was no sample. This is one of the difficulties with robots; whoever designs them has to know what the conditions would be. Why would there be no sample? One possibility is that the ice has clathrated or adsorbed gas in it, and the heat of the drill vaporized the gas, the pressure of which blew out the sample, however I guess we shall never know because "no sample" cannot be analysed.
The second big event involved the European Space Agency, who have studied the star, HL Tauri and found an accretion disk around it. The star is about 1 million years old, and the disk has rings in it, with dark gaps between them. The most obvious cause for such rings would be the formation of planets, although that does not mean there is a planet in every gap, because while a planet will clear out dust on its path, gravitational resonance will also clear out material. One problem is we cannot see the planets. Why would we? We can see four giant planets around the star HR 8799. These are newly-formed giants, and the gravitational energy of the gas falling onto the planet heats it to a yellow-white heat, hence they glow. These are all very much bigger than Jupiter. Similarly, there is a star LkCa 15 that is 3 million years old, and we see a planet much bigger than Jupiter, and significantly further from the star. Planetary growth should be faster the closer to the star, at least for the same sort of planet, because the density of matter increases as it falls into the star. Since we only see one giant, my theory requires there to be three other giants we cannot see, presumably because they are yet of insufficient size to glow sufficiently brightly for us to image them. So, if I am right, 1 My gets you giants of the size we have, and the longer the disk lasts, the bigger the giants get.
Posted by Ian Miller on Nov 30, 2014 8:58 PM Europe/London

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