Electric Comets & X-rays Redux
How did a comet with next to no mass hold onto an extended atmosphere of dust and gas against a stiff solar wind?
As our friend
tusenfem has pointed out, the question is based on a false premise: In fact, the comet did not hold on to any extended atmosphere at all, nor does any comet. In the case of 17P/Holmes, we can consult for instance
Hsieh, et al., 2010, which presents an interesting & relevant abstract.
We present wide-field imaging of the 2007 outburst of Comet 17P/Holmes obtained serendipitously by
SuperWASP-North on 17 nights over a 42-night period beginning on the night (2007 October 22-23) immediately prior to the outburst. Photometry of 17P's unresolved coma in SuperWASP data taken on the first night of the outburst is consistent with exponential brightening, suggesting that the rapid increase in the scattering cross-section of the coma could be largely due to the progressive fragmentation of ejected material produced on a very short time-scale at the time of the initial outburst, with fragmentation time-scales decreasing from t
frag ~ 2 × 10
3 to ~1 × 10
3 s over our observing period. Analysis of the expansion of 17P's coma reveals a velocity gradient suggesting that the outer coma was dominated by material ejected in an instantaneous, explosive manner. We find an expansion velocity at the edge of the dust coma of v
exp = 0.55 +/- 0.02 km s
-1 and a likely outburst date of t
0 = 2007 October 23.3 +/- 0.3, consistent with our finding that the comet remained below SuperWASP's detection limit of m
V ~ 15mag until at least 2007 October 23.3. Modeling of 17P's gas coma indicates that its outer edge, which was observed to extend past the outer dust coma, is best explained with a single pulse of gas production, consistent with our conclusions concerning the production of the outer dust coma.
The coma brightens because of increased reflective surface area as ejected particles fragment, the measured expansion velocity shows that the comet is not holding on to any extended atmosphere, and the expansion of the coma is consistent with a single pulse of gas production, or in other words a single outburst. In short, there is nothing about the observed behavior of the comet that is contrary to the standard models of comets, certainly nothing about the observed behavior of the comet that will differentiate between a standard model versus an EU model, and likely nothing consistent with an EU model in any case. There are numerous additional papers to be found on comet 17P/Holmes, and as
tusenfem has also already pointed out, the coma of the comet is very tenuous, easily allowing sunlight to penetrate all the way to the nucleus.
One of the signs to look out for if the coma is indeed a double layer is to look for X-Rays.
Actually, that too is wrong. X-rays demonstrate either charged particles under very high acceleration (for continuum or bremsstrahlung emission) or charge exchange in highly ionized species or relaxation of excited nuclei (in the case of line emission, the former being more important around comets while the latter is more likely seen at the sun). While an exploding double layer might accelerate electrons to X-ray energies (I will leave that to
tusenfem), it is certainly not the only way to generate X-rays. Therefore, the mere presence of X-rays in fact says nothing at ll definitive about double layers. Rather, one must come up with some other means of distinguishing double layers as the source for cometary X-rays.
Meanwhile, let us consult
Christian, et al., 2010, which describes the observation of comet 17P/Holmes with the venerable
Chandra X-ray Observatory. They find that 90% of the X-ray emission is in the energy range 300-400 eV, which is really quite wimpy as X-rays go. In the paper, analysis shows that the bulk of the X-ray emission from 17P/Holmes can be attributed to solar wind charge exchange involving C
+4 and C
+5 ions. This is a good fit to the observed SED and also accounts for the lack of higher energy X-rays that would be due to charge exchange with more massive and more highly ionized species (comet 8P/Tuttle showed strong charge exchange X-rays from O
+6 and moderate charge exchange X-rays from Ne
+8 and Ne
+9 ions, amongst others). So in fact, an actual physical analysis of the X-rays seen in both comets 17P/Holmes & 8P/Tuttle rule out double layers as a source for the X-rays observed.
We have been over all of this before, so it is worth noting that
Sol88 and the electric comet crowd have nothing new to offer. All we see is a re-hash of ideas already debunked, falsified & otherwise intellectually laid to rest. See, for instance, my own
Electric Comets III: No Eu X-rays (21 June 2009),
Electric Comets II: References (20 June 2009, with more comet X-ray reference papers & physics) or
Electric Comets I, where I point out that the energetics of comet X-rays as observed are not consistent with the energetics of comet X-rays as predicted by EU hypotheses.
Or the interruption to the flow of the solar wind
Deep Space 1 took measurements with its plasma instruments between 90,000 kilometers (56,000 miles) and 2,000 kilometers (1,200 miles) away. These data show that the flow of ions around the comet's rocky, icy nucleus is not centered on the comet's nucleus as scientists expected before the Borrelly flyby. Ions in the turbulent flow are heated to about 1 million Kelvin (2 million degrees Fahrenheit).
LINK
The reference is to comet Hyakutake and goes along with the Hyakutake X-ray image posted by
Sol88. It is well known that the magnetic field that is carried along in the solar wind will drape over a comet coma (not the nucleus) as the magnetic field encounters the plasma of ions & free electrons in the comet nucleus. The draped magnetic field shapes the coma and near tail of the comet, and is a site of charged particle acceleration and therefore bremsstrahlung X-rays. The flow of ions around the comet will be controlled by the shape of the coma & draped magnetic field and not by the shape or location of the comet nucleus. And asymmetric outgassing of the comet will create an asymmetric coma and easily explain the "off center" flow, where "center" is presumed to be the comet nucleus. This is no problem for a standard model of comets.
Does anyone in the electric comet crowd have any
new ideas? Are we eternally doomed to re-debunk and re-falsify the same ideas already debunked & falsified before?