If that's solid water ice, and it were to evaporate, then vapour thickness at STP (yeah, right) would be about a thousand times as much, or three meters.
So yes, we do need a bigger comet. But not as much of a bigger comet as it might have appeared.
]]>http://en.wikipedia.org/wiki/Age_of_the_Earth#Early_calculations
based on the fact that the core is still hot. In fact, it is much older and the core is only molten because of decaying Uranium. Thus Mars failed to maintain a magnetic field and thus an atmosphere, and to develop plate tectonics, mainly because it has less Uranium rather than because it is smaller. Though its probably more complex, and it's smaller because it has less Uranium or something.
So, it's probably unlikely that Mars would really have enough Uranium for all that.
]]>Second of, Mars has about half the radius of Earth, so it has only (1/2)^3=12.5% of Earth's volume. Which would translate to 12.5% of the radioactive decay energy. OTOH, it has (1/2)^2=25% of Earth's surface area.
So there is much less heat energy for Mars, but not that much less surface area the heat energy is radiated from.
Err, hope I got that one right, biologist and physics, never the twain...
]]>What about a natural RBMK reactor? At least there'd be a positive void coefficient. While graphite is often of biological origin, lump graphite seems to be more or less abiogen, e.g. hydrothermal. Where this could also deposit uranium and thorium, especially if geochemistry and/or a deep biosphere help with it. Since we already have water, this'd be quite similar to a Chernobyl type reactor. Compared to the smaller man-made variant, the high pressure would mean it takes some time to get to the "positive void" part. One could also posit that first there is some other neutron absorbant slowly removed from the system.
As power output increases, we're closing in on the critical point, where small changes of temperature or pressure can lead to big changes in density:
http://en.wikipedia.org/wiki/Supercritical_fluid
Which might translate to a sudden decline in neutron absorption.
For the specific scenario, breeding materials like thorium, nuclear poisons like xenon created by the reactions and the exact timing might matter.
For the latter, imagine the reaction goes critical in the periphery, where pressure is low. Positive void coefficient means we get more energy, but not much because the assembly is destroyed in some small bang. The energy ledas to the material somewhat more inside becoming supercritical, too, this assembly is destroyed, too, but pressure etc. from the first reaction means there is a little bit more energy released and so on. In the end we get a shock wave going in, just like a implosion type nuclear weapon.
Just some musing, as already said, biologist and physics, never the twain...
]]>Another thing, condition like on Titan might mean no water, but plenty of hydrocarbons. Which might act as a neutron moderator:
http://en.wikipedia.org/wiki/Organically_moderated_and_cooled_reactor
]]>CANDU reactors also can have a positive void coefficient in the coolant, for similar reasons (marginal moderation behavior doing spectrum shifting in the presence of a stronger main moderator; for RBMK the graphite, for CANDU the D2O.)
Those only work in carefully constrained geometries; the distance between fuel elements has to be far enough for the moderation to work, but not too far or capture and moderation below the peak cross section neutron energies happens. Homogenous solutions of D2O, H2O, and Uranium salts are entirely different than a CANDU reactor, for example.
I don't think you can naturally match those conditions, and certainly not across a roughly 500-meter across ore body.
Solution reactors - liquid salt critical assemblies and the like - happen. A lot of criticality accidents happen that way, during processing. The big one in Japan a few years ago for example. But they all have negative void coefficients.
CANDU and RBMK are only positive coeficients so long as the main moderator (the graphite and the D2O) remain more or less intact. If you disrupt those, their reactivity drops right quick.
H2O is an OK moderator; D2O is often nearly ideal; Li-7 is ok (Li-6 is an absorber, but turns to Li-7 when that happens); He is ok but gas at any relevant pressure and temperature; C is ok, and various C-H materials have been used. Be is not a bad choice. O has been considered here and there (and moderating effects of oxygen in fuel oxide pellets are noted). F is not great but as a chemical constituent in other moderators works ok.
Finding a material and geometry on Mars that could give you a background moderator, in combination with a secondary moderator you could then boil off / remove / etc leading to positive void coefficient, seems difficult. But not impossible. Unlikely is not impossible..
]]>1) It's a two year old LPSC (Lunar and Planetary Science Conference) abstract. LPSC abstracts have not been peer-reviewed at all.
Now, I would strenuously like to point out that the vast majority of abstracts submitted to LPSC are perfectly good science written by legitimate scientists and students of science. Most of them are ideas in progress that may or may not be later developed as papers, and some are presented at the conference as posters or talks. Some of them get shot down, some pass scrutiny - that's how science works (I submitted and presented a few myself while I was at university - some worked out, some didn't).
However, a handful of kooky ones do slip through every now and then (because they're not vetted after they're submitted) - I remember reading one abstract a few years back suggesting that the sun had accreted around a neutron star - it was clearly nonsense written by someone who had no idea about star formation or anything. This nuclear reactor abstract is one of those kooky abstracts, and here's why:
2) The first reason to be suspicious is usually if the author isn't associated with a university. It's not always the case, but many of those who submit from a 'company' or independently are "on the fringe", so to speak. In this case, a bit of digging reveals that the author is (or was) a Cydonia nut - the only other thing I found by him was a "paper" written about how the Face on Mars was built by an alien civilisation. While this doesn't necessarily mean that the science presented here is bad, the author's credibility is already suspect.
3) Obviously, the proper way to proceed is to analyse the science. Unfortunately, even disregarding the author's credibility, the science in the paper is pretty terrible and full of holes. A few issues I found: a) he says that the "reactor" was "tamped" by the overlying rock but doesn't provide any calculations to support this (and for all I know forgot that Mars has lower gravity than Earth, so pressure is lower at a given depth). That's a fairly critical part of the scenario that we just have to take his word for. b) He also doesn't explain how uranium ore forms and concentrates on Mars in the first place. c) He doesn't provide any evidence for this supposed explosion beyond "it looks like the interesting stuff is concentrated around a depression" which could have been caused by a number of other means. Occam's Razor seems nowhere to be found. d) We know of precisely one natural nuclear reactor on Earth, which implies that they're somewhat unlikely to form... and it didn't blow up. And yet there was supposedly one on Mars that did? Seems like a lot of unlikely coincidences would have to line up to make that happen on the next planet over from us. e) And he spends a lot of time telling us his interpretation of the data, and not a lot of time just objectively describing the data and saying what other options could explain it. f) if this happened so long ago, why would there be evidence left on the surface after a billion years of aeolian deposition and erosion and redeposition?
And this is before I even get to the nuclear physics side of it... (I'll leave others to do that)
So all in all I'd say the "evidence for a nuclear reactor on Mars" is actually pretty darn sketchy!
]]>"C/2013 A1" in google gets me all the info I need ....
]]>a) uranium is only soluable when there is some oxygen around, which means it takes some time with developing biosphere and such.
b) there has to be lots of fissile uranium isotopes around, which means it has to be early in planetary history, else the isotopes decay.
Which, first of, explains why natural reactors are somewhat rare, and second of, means some problems for Mars with the oxygen. Now there are quite some salts with oxidation numbers of chlorine too high to be comfortable on Mars, and there might be oxygen thanks to geochemistry or chemolithoautotroph organisms deep below. Still, something of a bummer.
]]>I think that I mentioned some expertise in this. In this case, tamping is the correct term of art for material intimately in contact with the fast fission "pit" or mass of reacting fissile and fissionable material, which does not significantly act to reflect neutrons back into the bulk material, but does act to inertially retard pit expansion and disassembly.
Materials which return (some) neutrons to the core are reflectors, those that do both efficiently are tamper/reflectors.
Tamping works via two mechanisms:
First, by being in intimate physical and thermal transfer contact with the pit, it moves the surface expansion wave start point further out from the critical radius.
Second, to some degree inertially retarding the direct bulk expansion within the pit.
See for example the Nuclear Weapons FAQ and tamper-effects adjusted Serber efficiency equation.
On this one point, the useage is correct ( or appears so; the papers' details on the model are sketchy ). Not sure if the effect would matter for a fast fission reaction in a hundreds of meter wide zone if the conditions were to set one up; time and geometric size will be in play mostly. But it is tamped.
That does not change any of the other criticisms leveled so far.
]]>If that's the case, I would expect to see some numbers and calculations to demonstrate (a) that this actually is at kilometre depth, (b) whether the depth is actually enough to "inertially confine" the reaction, and (c) why it's such a big explosion that it penetrates the surface from kilometres so that it can spread everything across the planet. There is none of that in the abstract.
]]>This one doesn't hold up to informed examination.
]]>