Wikipedia:Reference desk/Archives/Science/2011 June 21

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June 21

Is there an estimate available for the fraction of the Earth's mass that is dark matter?

Also, what about the fraction of stars made of dark matter? If a hefty fraction, wouldn't it greatly change the way stars work? Thanks, Rich Peterson24.7.28.186 (talk) 05:17, 21 June 2011 (UTC)

Although the total mass of dark matter is estimated to be nearly five times as much as the mass of normal matter in the universe, most of it is thought to exist outside planets and stars. I suppose it is a matter of opinion whether the billions of neutrinos passing through the earth make up part of the earth's mass, but I think it would be a very small proportion. Perhaps an expert can make a calculation? Dbfirs 06:21, 21 June 2011 (UTC)

That's right. The fraction of mass composed of dark matter is negligible. Dauto (talk) 06:51, 21 June 2011 (UTC)

Rarer than the total amount of ununoctium present in the whole earth? Plasmic Physics (talk) 06:59, 21 June 2011 (UTC)

Not that rare. But read dark star for an interesting speculative possibility that dark matter played an important role on the behavior of the very first stars of the universe. The OP's instinct that dark matter might change the way stars work is correct for these stars (If they turn out to actually exist). Dark matter plays little role on the behavior of regular stars like the sun. Dauto (talk) 07:06, 21 June 2011 (UTC)

Also read dark matter#Indirect detection experiments where the possibility of measuring dark matter trapped within the earth or the sun is briefly mentioned. Dauto (talk) 07:16, 21 June 2011 (UTC)

I was under the impression that the phrase "dark matter" is simply a term used as a catch-all label for "that which we cannot observe/identify/explain" much like "terra incognita" was used on old maps. Roger (talk) 07:19, 21 June 2011 (UTC)

Whilst the term was invented for this "missing mass" we didn't know anything about, since significant research has been done it's more like a known unknown. 'Dark' is a useful word because it implies we can't observe it, or at least technology-wise, it is difficult to directly observe. There are other 'dark' things theorised, Dark energy, Dark fluid and Dark flow for example. Since we have little idea of what dark matter actually is, then it may well have more than one constituent part, so 'dark matter' can be consider a grouping of sorts, but it seems to be a useful and well defined one. Grandiose (me, talk, contribs) 07:25, 21 June 2011 (UTC)

Here I get a density of 0.3–0.39 GeV/cm³. Multiplied by the volume of the Earth that's 2–3 kg, or 3–4 million kg in the volume of the Sun. In either case, that's about one part per septillion. It would be a bit odd to include that mass in the "composition" of the Earth/Sun, because the dark matter is a gas of rapidly moving, barely-interacting particles (like neutrinos, which are one component of it), and there just happens to be that much of it passing through the Earth/Sun at any given time. -- BenRG (talk) 08:19, 21 June 2011 (UTC)

The content of the link reads "Error 404 - Not Found". Do you need a login there?

BenRG probably meant this [1] - it looks like he hand-typed the URL and missed out the 'r' in 'dark'. CS Miller (talk) 12:58, 21 June 2011 (UTC)

Thank you. 5BYv8cUJ (talk) 14:06, 21 June 2011 (UTC)

I copied and pasted it. I have no idea how that happened. (Now fixed.) -- BenRG (talk) 20:36, 21 June 2011 (UTC)

Dark matter is subject to gravitation, so one would expect it to aggregate inside stars and planets. With this, it should be possible to calculate some limits on its temperature (the stuff (to avoid the word "particle") would have to be faster than the escape velocity, and with some nifty observations it should be possible to calculate a lower limit for the interaction probability dart to normal matter and dark to dark matter, too, but I havn't seen anything yet. 5BYv8cUJ (talk) 11:52, 21 June 2011 (UTC)

Unlike ordinary matter, dark matter cannot cool (radiatively), therefore it is not expected to aggregate, at least not to anything like the density contrast represented by stars and planets. Dark matter is distributed like a tenuous gas with very little, if any, self-interaction. --Wrongfilter (talk) 14:14, 21 June 2011 (UTC)

I'm not thinking of density but of gravitation. The article linked states a speed of 300km/s, this is far less than the escape velocity of the sun, so the dark matter would be trapped in the gravity well. With ever so small interactions it would start to aggregate. Might well be the aggregation is too small to detect, but from the principle I don't see what's wrong with that. 5BYv8cUJ (talk) 14:52, 21 June 2011 (UTC)

Yes, that sort of calculation is routinely done. Just as an example, read the abstract of that paper. Dauto (talk) 14:58, 21 June 2011 (UTC)

Thank you. And I see that as long as the results are negative they won't appear in the headlines, and so people like me wouldn't notice this sort of research at all. 5BYv8cUJ (talk) 15:31, 21 June 2011 (UTC)

Any average particle speed like 300km/s is given at infinity, where the escape velocity is zero. The particles will speed up as they approach a gravitating object and slow down as they leave, always exceeding the local escape velocity. Some kind of dissipative interaction (i.e., friction) is always necessary for capture. Frictional effects on dark matter are small but nonzero, so some capture is expected, but not a lot. -- BenRG (talk) 20:36, 21 June 2011 (UTC)

I see your point. A particle moving at 300km/s (not really matter what direction) would be accelerated towards a gravity well and thus gain more speed to overcome the escape velocity. Are there any mathematical models that account for that? 5BYv8cUJ (talk) 20:54, 21 June 2011 (UTC)

I'm not sure I understand your question, but in the absence of friction the particle's speed at a point x will be $v(x)={\sqrt {v_{\infty }^{2}+v_{e}(x)^{2}}}$ where $v_{\infty }$ is the speed at infinity and $v_{e}(x)$ is the escape velocity at the point x. This is because the total energy ${\tfrac {1}{2}}mv(x)^{2}-{\tfrac {1}{2}}mv_{e}(x)^{2}$ is constant and $v_{e}(\infty )=0$. The "escape velocity of the Sun" is the escape velocity at the Sun's surface. -- BenRG (talk) 03:49, 22 June 2011 (UTC)

As Dauto points out, this is a standard problem (could be a homework problem for astrophysics students). The DM particles have some scattering cross-section for interactions with nuclei, so you can calculate the capute rate by the Earth. For the typical DM masses that one considers, these then settle in the Earth's core. They then get into thermal equilibrium with the Earth's core, which then allows you to compute the self-annihilation rate. This then yields a differential equation for the total DM content of the Earth (capture minus self annihilation is the increase per unit time). On the very long term you would then get a dynamical equilibrium between self-annihilation and capture. However, in case of the Earth that would take hunderds of billions of years (for the typical cross-sections that are assumed for DM).

The Sun captures so much more DM than the Earth, that the self annihilation signal from the Sun (in the form of neutrinos) is expected to be larger than from the Earth. Count Iblis (talk) 15:40, 21 June 2011 (UTC)

• • Thanks to all of you for all the thoughtful helpful discussion. Followup questions:Since dark matter does aggregate, perhaps minimally, to the extent that it forms haloes around galaxies, i'm thinking that its average velocity isn't much greater, and probably less than, the average galactic escape velocity(I think someone above also alluded to that in a link I haven't gone to yet)which is around a few hundred km/sec. But IF dark matter turned out to be neutrinos, isn't that a remarkable slowing down of the usual near lightspeed of neutrinos? Given they hardly ever collide with anything we know about, and usually elastically, wouldn't that mean some unknown inelastic collision exists? And is it possible that as neutrinos slow down, their cross section for some reason increases so that they collide with themselves? (I'm dreaming someday of a box with a pound of neutrinos inside with average speed l meter/year) Thanks again.-Rich Peterson199.33.32.40 (talk) 23:39, 21 June 2011 (UTC)

That is the difference between hot dark matter (HDM) which is made of particles traveling close to speed of light and cold dark matter (CDM) made of particles moving at much slower speeds. Neutrinos are an example of HDM. Cosmological models seem to require quite a bit of CDM in order to match the observed large scale structure of the universe. Dauto (talk) 00:33, 22 June 2011 (UTC)

Primary 5 Science

What will be the coloured water level in the tube that is inserted into a enclosed tank topped soda lime which a mouse is in it? And explain why? — Preceding unsigned comment added by 220.255.1.83 (talk) 07:46, 21 June 2011 (UTC)

Please do your own homework.

Welcome to Wikipedia. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know.

Also, you're obviously missing some numbers. You can't get numbers out of a calculation without first putting some numbers in. StuRat (talk) 07:58, 21 June 2011 (UTC)

Sounds like this old riddle: "If a car is driving west at 50 miles per hour, what time is it in Chicago?" ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:44, 21 June 2011 (UTC)

Are you about 10 years old? (That's about right for Primary 5 in the Scottish education system), but this seems far harder than questions for 10 year-olds. Anyway, the mouse will breathe in oxygen and exhale (breathe out) carbon dioxide. The carbon dioxide is then absorbed by the soda water, reducing the volume of air. Assuming the coloured water is part of a manometer then the liquid will move towards the tank. You will need to know

1. The mouse's rate of oxygen consumption
2. How long the experiment is running for

From this you can calculate how much oxygen the mouse has used. Divide this by the volume of air in the tank to determine the percentage of air used, and thus the pressure drop in the tank. Assuming the other end of the the manometer is sealed, then it is simple to work out how much the coloured water will move.

If you don't know how much oxygen the mouse uses, then it can be calculated if you know its basal metabolic rate, and assume that its burning glucose.

If you get stuck, tell us how far you've got and we can help you further. CS Miller (talk) 11:51, 21 June 2011 (UTC)

Atmospheric content of nitric oxide

Does anybody know where I can find an exhaustive list of Earth's atmospheric content? Specifically I have had some trouble finding information of the average content of nitric oxide aka nitrogen monoxide aka NO. Does anybody know the value or where to find it?

cheers. Denito (talk) 08:57, 21 June 2011 (UTC)

You may be interested to look at [2] and [3] only released in the last week. Also Atmosphere of Earth gives some information. But not NO. nitric oxide is oxidised in air to nitrogen dioxide which is listed. Graeme Bartlett (talk) 11:13, 21 June 2011 (UTC)

metastasis

6cm adenocarcinoma in bile duct, grown for almost 5.5 years. What is the probability that the cancer cells have metastasized? — Preceding unsigned comment added by 199.224.149.10 (talk) 10:13, 21 June 2011 (UTC)

If that's a request for medical advice, we're not allowed to answer it. If it's a homework question... we're not supposed to answer it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:31, 21 June 2011 (UTC)

Any 2011 news on the 2000 year-old Judean date palm?

I havnt been able to find any 2011 news, only blog posts that refer to information of years earlier. Is there any 2011 news yet? Has it flowered, or is it male? 2.97.210.205 (talk) 12:20, 21 June 2011 (UTC)

Our article which you linked to includes information from 2010 which is not years earlier. A quick search easily finds [4] with comments from 2010 suggesting the sex is undetermined. It also suggests your best bet may be to just contact the person involved. I was suprised that they didn't just try to determine the sex genetically but from [5] and [6] it seems the Sex-determination system in date palms is only beginning to be understood. If you do contact the person involved you may want to consider suggesting they get in touch with the team studying the sex determination system in date palms as it would seem both would be interested in each other's work but it's possible they may not be aware of each other. Nil Einne (talk) 14:56, 21 June 2011 (UTC)

"Our article which you linked to includes information from 2010 which is not years earlier". Who said it wasnt? You first link links to something dated 2009. 2.101.2.152 (talk) 17:21, 21 June 2011 (UTC)

The OP said they could only find blog posts from years earlier. Since our very own article includes information from 2010, it's not clear why blog posts form years earlier are relevant. Also the link I included above includes comments from 2010 (regardless of when the blog post itself was dated) as I clearly said in my first post. Nil Einne (talk) 11:30, 22 June 2011 (UTC)

Rereading, perhaps there was some confusion. My impression when the OP said 'years earlier' was they meant several years ago. Perhaps however the OP was trying to say from previous years which would include info from 2010. If there was such confusion I apologise although I do feel this needed clarification since info from several years ago is quite different from info from 2010. In any case, I find no evidence of any info from 2011. Since the info in our article from 2010 is unsourced (and was added by an anonymous IP who only edit those times) and the only source was have for info from 2010 so far comes from direct communication with the person involved I would again suggest this is the best course of action. It may be there is more info in Hebrew sources but I suspect if the tree had flowered it would have made it to English sources by now. Nil Einne (talk) 11:39, 22 June 2011 (UTC)

I suggest you read the question and heading. 92.24.177.159 (talk) 13:36, 22 June 2011 (UTC)

How does a dry "dead" seed come to life?

I assume that the seed is not metabolising at all, yet somehow it awakens. Animal cells never do this. 2.97.210.205 (talk) 12:22, 21 June 2011 (UTC)

There is an article on germination. And animals can do similar things, see Tardigrade. 5BYv8cUJ (talk) 12:30, 21 June 2011 (UTC)

I don't know the answer, but some animals do this too. Look at http://en.wikipedia.org/wiki/Cryptobiosis and http://en.wikipedia.org/wiki/Anhydrosis Zzubnik (talk) 12:33, 21 June 2011 (UTC)

Seeds can be alive or dead. A seed's probability of being able to germinate diminishes with time, but it depends on how it's kept. Cold storage will keep individual seeds alive longer. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:52, 21 June 2011 (UTC)

I can think of two different concepts of "alive or dead". One is that alive means having an active metabolism, the second is having the potential for having an active metabolism (when water is present, for example). 5BYv8cUJ (talk) 20:45, 21 June 2011 (UTC)

A live seed can sprout under the right conditions. A dead seed cannot sprout under any conditions. That's the difference. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:12, 21 June 2011 (UTC)

"I can think of two different concepts of 'alive or dead'. One is that alive means having an active metabolism, the second is having the potential for having an active metabolism". So Jesus only rose from the dead(1).--188.28.167.165 (talk) 10:35, 22 June 2011 (UTC)

I'm wondering how on a molecular level a dessicated cell can restart its 'machinery'. 92.29.113.106 (talk) 23:09, 21 June 2011 (UTC)

Sometimes it can't. My calling them "dead" or "alive" probably oversimplifies things. A better way to say it would be "capable of germination" or "not". According to the Germination article, moisture is the key. It absorbs into the seed and its presence triggers various chemical reactions in the embryo that result in the beginning of germination. Other conditions are also required, of course, in order for germination to continue. There's an article called imbibition which describes, at least in general terms, how the moistening process works. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:20, 22 June 2011 (UTC)

A very important trick is trehalose, a simple sugar which has a remarkable tendency to preserve proteins and other cellular constituents as if water were present. As water leaves and trehalose takes over, there is very little change in these components, but things become more and more solid. Essentially everything just stops, without other changes. Provided no other damage occurs, there's no reason for it not to resume as the water solvent is added back. I'm more familiar with it in animal systems, but apparently it has some important role in Arabidopsis seeds also.[7] Wnt (talk) 02:34, 22 June 2011 (UTC)

Does the same "trick" work in humans? why or why not. --188.28.167.165 (talk) 10:47, 22 June 2011 (UTC)

Yes (sort of) - see PMID 16417738 and [8]. Trehalose helps cells survive freezing as well as desiccation, again by replacing water with something less vulnerable to change in the environment. Wnt (talk) 13:50, 22 June 2011 (UTC)

As a side-note (kind of), the use of a gibberellic acid (GA3) supplement can aid in the germination of older dormant seeds that would otherwise have difficulty germinating. This acid I believe was used to germinate a Judean date palm in 2005. Juliancolton (talk) 20:17, 24 June 2011 (UTC)

comets

Have any retrograde orbit comets been observed? As in a comet which goes around the sun in the opposite direction that earth does within roughly 30deg of inclination of earth's orbit? I would like to clarify that I am speaking only in terms of periodic comets. Googlemeister (talk) 14:57, 21 June 2011 (UTC)

Quite a few. Halley's Comet is on a retrograde orbit, for instance. Its inclination is 162.3°, so I guess that conforms to your constraint (180° would be exactly retrograde). See also Retrograde motion. --Wrongfilter (talk) 15:41, 21 June 2011 (UTC)

So what causes the retrograde orbits of comets when all planets and moons in the solar system, as well as the vast majority of asteroids have a standard orbit that I assume is caused by the formation of these objects from the sun? Is the material from comets not from this system? Googlemeister (talk) 19:15, 21 June 2011 (UTC)

Possibly, though quite likely complex gravitational interactions working on a comet over many millions of years, coupled with an already eccentric orbit, could alter such an orbit to do strange things. These things are literally unpredictable given the number of gravitational interactions that a comet has to contend with during its lifetime. (not just hard to calculate, but literally impossible, see n-body problem). --Jayron32 20:23, 21 June 2011 (UTC)

You might be interested in the articles Scattered disk and Oort cloud for the origins of many long-period comets. 67.169.177.176 (talk) 20:26, 21 June 2011 (UTC)

And also the article Centaur (minor planet). 67.169.177.176 (talk) 20:27, 21 June 2011 (UTC)

Planets, moons and other bodies of the inner solar system were formed from a spin disk of dust. Comets come from the Oort cloud which has a separate origin. Dauto (talk) 07:31, 22 June 2011 (UTC)

spin altering

http://www.sciencedaily.com/releases/2011/06/110605132431.htm 'because photons at the right frequency make the antiatom's spin flip up or down.' why do photons at the right frequency make the antiatom's spin flip up or down? — Preceding unsigned comment added by 82.38.96.241 (talk) 20:37, 21 June 2011 (UTC)

Photons, when treated as a particle, are carriers of spin. You can read about spin and how it participates in atomic interactions. When a photon and an atom "interact", there is often a transfer of energy, momentum, and spin. One such interaction causes a change of the atom's spin (or, changes the spin of one or more electrons around the atom). In simple quantum interactions, the spin can only take one of two possible values; this is usually notated "up" and "down." In more complicated quantum mechanical systems, the spin can take on other values. Nimur (talk) 21:22, 21 June 2011 (UTC)

Bugonia, bee mimics

I am currently doing research for the bugonia article. One of the sources has a theory that the so-called bees were actually flies that looked remarkably similar to bees, possibly Eristalis tenax. It seems the author did not know the technical term for Batesian mimicry. So I was wondering where I could find a more comprehensive list of species meeting these critera:

• looks like a bee
• lays it's eggs in carrion
• widespread in the ancient Mediterranean region
• has a maturation time of approximately 10 days, or
• has a maturation time of approximately 30 days, or
• has a maturation time of approximately 10 days on a 20 day old corpse

Thanks! Craig Pemberton (talk) 21:33, 21 June 2011 (UTC)

I am not aware of any species that meets your criteria that is not a hoverfly. Many species of hoverfly are common in their ranges. However, even the ancients may have noticed that bees have four wings, whereas all dipterans have two. If you have access, you can probably find some good articles by searching for 'syrphidae' in a biological review journal, such as entomological review. There may be a few bee-looking hymenopteran saprotrophs, but I can't readily find any indication of such. SemanticMantis (talk) 22:27, 21 June 2011 (UTC)

The zoological knowledge of the ancients was very sporadic. Aristotle (c 350 BC) knew enough to distinguish between queens ("kings"), workers ("bees"), and male reproductives ("drones"), but even much later we have fellows like Aelian who thought that vipers would leave their venom on shore, swim out to sea, mate with moray eels, and then return and swallow their poison once again. So it's fully plausible that a mimic could fool at least most of the people. Craig Pemberton (talk) 00:26, 22 June 2011 (UTC)

Sure, many modern people get confused too :) Another interesting angle is that most hoverflies are good pollinators. Though they don't provide honey, I think they would generally be beneficial to a ancient Greek farms. Anyway, this practice seems like it could attract a wide variety of local hoverfly species, but I'm not sure how to easily find a list of what those specific species are. SemanticMantis (talk) 03:07, 22 June 2011 (UTC)

• Am I the only one who finds it a hilarious thing that the name of the concept of finding bugs on ya is called "bugonia". --Jayron32 03:42, 22 June 2011 (UTC)

On a related note, I'm surprised we don't have a page on bee mimics. Craig Pemberton (talk) 15:20, 22 June 2011 (UTC)

Habitable planet/moon pair.

I know this has been asked here before, but I want to add some things to it.

Would it be possible for both elements of a planet/moon system (probably more like a double-planet system in this case) to be capable of supporting human life? When this question was asked before, I think the asker (for lack of a better term) intended for both bodies to be more or less Earth like, but let's say the worlds were each on opposite extremes of the habitability spectrum. So the primary body would be a large terrestrial planet but still capable of supporting human life comfortably, while the smaller object would be a barely habitable Mars-sized planet with a thicker atmosphere. Probably the only way such a system could exist is through capture, or maybe through co-accretion, but then again, I'm no expert.

I guess my question then is: can a two-planet system consisting of two (relatively) large "Earthlike" planets exist naturally?

EDIT: added signature. 70.246.235.69 (talk) 21:42, 21 June 2011 (UTC)

I don't see why not. Just bring Pluto and Charon (moon) into the Earth's orbit and increase their sizes a bit and you would have it. One thing that doesn't seem right, though, is a Mars-sized planet with a thick atmosphere. I would expect such a small planet to lose most of it's atmosphere to space. StuRat (talk) 00:28, 22 June 2011 (UTC)

That doesn't bother me so much, as in my scenario (yes, I'm using this for fiction as you might have guessed), the smaller body is partially terraformed. In its natural state, it was uninhabitable, much like Mars, but it was artificially brought to a minimally habitable state. 70.246.235.69 (talk) 01:30, 22 June 2011 (UTC)

Ursula Le Guin's book The Dispossessed is set in such a two planet system. I know this doesn't answer your scientific question, but it might help you artistically, to see what she did with it. Her book won several awards, and I rather enjoyed it. It could serve as a source of inspiration. Cheers. ~ Mesoderm (talk) 18:30, 22 June 2011 (UTC)

However, Mars may not have lost most of its atmosphere to space. One hypothesis is that its atmospheric gasses have mostly chemically combined with its rocks as happens continually on the Earth, but unlike the Earth it has no active plate tectonics to recycle those gasses back into the atmosphere via vulcanism. This may be because it cooled quicker than Earth by virtue of being smaller (or may be for other reasons), but proximity to a larger primary might provide sufficient energy for such a planet/satellite to maintain vulcanism through tidal friction, as with Io. {The poster formerly known as 87.81.230.195} 90.201.110.203 (talk) 01:24, 22 June 2011 (UTC)

Could two Earth-sized planets form in close proximity without coalescing? Not an expert either, but that seems rather doubtful to me. Capture seems the best bet. Clarityfiend (talk) 00:44, 22 June 2011 (UTC)

My limited understanding of the current state of theories of planetary formation (see under Nebular hypothesis) is that we don't yet know, but certainly can't rule it out. However we seem to be on the threshold of discovering enough about extrasolar planetary systems that we might soon have more definite answers. My own gut feeling as a lapsed astronomer (with friends who used to computer-model this kind of stuff) is that it's possible. {The poster formerly known as 87.81.230.195} 90.201.110.203 (talk) 02:05, 22 June 2011 (UTC)

Note they need not form together. They could form separately, and then come together - like Asteroid capture, but with a planet. Note that it need not be a gentle do-si-do. The Earth-Moon system is thought to have formed when an Earth-sized and Mars-sized planet collided. One could easily imagine two larger planetoids colliding in a slightly different fashion (with respect to orientation and velocity) resulting in two Earth-sized planets orbiting each other. -- 174.24.222.200 (talk) 17:30, 25 June 2011 (UTC)

Some factors to consider here are tidal locking and the Roche limit. The Roche limit sets the absolute restrictions to how big and how close the satellite can be. But tidal locking can be just as fatal - Pluto and Charon are tidally locked, and the Moon is tidally locked. Monthlong days and nights make ordinary biology a lot more difficult. The answer to tidal locking is to bring the moon really close so it revolves in just a single day, i.e. geosynchronous orbit. Now there are posts about this on various boards around the internet, with claims that you could have the Moon in geosynchronous orbit without breaking the Roche limit [9] but I haven't calculated this myself and I have no idea what sort of side effects would be seen beyond the mathematics (such as seismic activity from any slight variation from a perfectly circular orbit?) I think the effect on gravity on the near side of the moon would be so intense, it would be quite remarkable.

Note that the Moon reportedly could hold an atmosphere for about 100,000 years (or something like that - I see this figure on forums but I'm not finding reliable sources!), which would be more than enough for your scenario. I wonder what the surfing would be like there... Wnt (talk) 02:27, 22 June 2011 (UTC)

Would a planet with the same pressure as Mars have a breathable atmosphere if it was 100% O2? Googlemeister (talk) 13:17, 22 June 2011 (UTC)

Isn't 100% O2 kinda dangerous and deadly? --Ouro (blah blah) 13:44, 22 June 2011 (UTC)

This would have to be a non-smoking planet. Googlemeister (talk) 13:49, 22 June 2011 (UTC)

I'm not sure it would be dangerous or deadly. For SCUBA, at least, you are interested not in the relative proportion of oxygen, but only in the partial pressure. So if you have an atmosphere of ~200mbar pure oxygen, you should be fine (from a breathing point of view - you might have other trouble). Water boils at ~60°C at 0.2 bar, so you would not even evaporate away ;-). --Stephan Schulz (talk) 15:45, 22 June 2011 (UTC)

Agreed. See what happens in a pressured pure oxygen environment here: Apollo_1#Fire. The pressurized cabin also ensured that they couldn't open the inward opening hatch once the fire started. Whoever designed that test should have been subjected to it, IMHO. StuRat (talk) 16:22, 22 June 2011 (UTC)

The problem is, Mars has a really weak atmosphere. You can't just seed it with algae and convert all the CO2 to O2 and be done - it's just a tiny pressure, less than 1% of Earth's. I think that the pressure of pure oxygen needed is about the partial pressure of oxygen in Earth's atmosphere, i.e. about 1/5 of Earth's atmospheric pressure. Wnt (talk) 16:31, 22 June 2011 (UTC)

You are right. 200mbar of 100% O2 would work fine for an atmosphere. A StuRat mentions, it's used in some manned spacecraft (Apollo 1 had high pressure 100% O2 for takeoff, which was very dangerous and was redesigned after the fire, but would have dropped down to about 350mbars after takeoff). --Tango (talk) 17:01, 22 June 2011 (UTC)

Isn't Mars more like 10 mbar though? Googlemeister (talk) 18:55, 22 June 2011 (UTC)

Then you just need to have 2000% oxygen. Duh! --Stephan Schulz (talk) 18:56, 22 June 2011 (UTC)

The limit for human life (with considerable difficulty) is a partial pressure of about 65 millibars of O2. The total Martian atmosphere is about 6 millibars (at mean surface elevation). So no, the Martian atmosphere would still be much too thin to support humans even if it were 100% oxygen. Dragons flight (talk) 19:22, 22 June 2011 (UTC)

Foil pan heat conductivity of a disposable grill

How good can the foil pan of a disposable grill conduct heat? Would it work better if it were better of worse heat conductor? Wikiweek (talk) 22:15, 21 June 2011 (UTC)

Depends on what you want. For quick, but potentially uneven heating, you want a good conductor. For slower, more even heating, a poor one. StuRat (talk) 00:33, 22 June 2011 (UTC)

I'm probably misunderstanding something, but if it was a good conductor, wouldn't there be little or no heat? I thought it was the poor conduction that caused the heat in the first place. Matt Deres (talk) 13:41, 22 June 2011 (UTC)

You must be thinking of electrical conductivity of something carrying electricity. We are talking about thermal conductivity with no electricity involved. StuRat (talk) 16:26, 22 June 2011 (UTC)

No, Matt is correct, the grill would be marginally more efficient if the foil were a heat insulator (place it on rock-wool or something similar to achieve this), but so much heat is generated by the charcoal that the small amount conducted away by the thin foil (to the air and whatever the foil is resting on) doesn't make a big difference, but the answer to the question is that it would work marginally better if foil were a worse conductor of heat because then there would be marginally more radiant and convected heat to cook the food. The same amount of charcoal placed in a very large and thick iron container would be noticeably less efficient because of heat conducted away from the food. Aluminium (aluminum for American readers) has a fairly high thermal conductivity but a thin sheet will not conduct very much heat away. Dbfirs 19:18, 22 June 2011 (UTC)

If you have a poor thermal conductor between the flame and the food, then most of the heat will go around both and up into the air. (Since the OP said "grill", I'm assuming it's open on top, so that heat escapes.) StuRat (talk) 19:54, 22 June 2011 (UTC)

No, StuRat is correct and I am wrong; I read the question incorrectly. Matt Deres (talk) 20:27, 22 June 2011 (UTC)

Perhaps we are talking about different designs of disposable grill. I assumed that the question was about the base pan. Dbfirs 09:24, 23 June 2011 (UTC)

Irish hurricanes

Is it even possible for a hurricane to hit Ireland I know we get tail ends but is it possible for full blown tropical hurricanes to hit Ireland. The reason I ask this because of the warm current of the gulf stream should enough to keep a hurricane going. --109.78.92.74 (talk) 23:00, 21 June 2011 (UTC)

None has ever done so. The nearest is Hurricane Debbie (1961), which was downgraded to a tropical storm before it hit Ireland. European windstorm lists other major storms. We can only speculate about what weather might result from the changing climate. -- Finlay McWalter ☻ Talk 23:10, 21 June 2011 (UTC)

Maybe the cold northern Atlantic waters tend to sap a hurricane's strength? ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:09, 22 June 2011 (UTC)

Hurricane and Gulf Stream explain why this is extremely unlikely. The Gulf Stream is only "warm" on the western side of the Atlantic relative to the deep cold of the rest of the surrounding waters. It isn't the 26.5+ degrees C that a hurricane needs. Bielle (talk) 00:25, 22 June 2011 (UTC)

Hurrican Faith hit the Faroe Islands as a cat 1 hurricane so it is possible for one to hit Ireland while still a hurricane, but it would be quite rare. Googlemeister (talk) 13:11, 22 June 2011 (UTC)

The gulf stream can keep a tropical storm or hurricane going at relatively northern latitudes, such as in the case of Alex 2004. However, keep in mind a tropical cyclone generally needs waters of 81F (though there are certain exceptions) to form and sustain itself, and even then you're not going to get anything major spinning up. The gulf stream only stays so warm for so long as it heads into the far north, and even if a hurricane is moving at exceedingly fast speeds, it'll inevitably dissipate or transition into an extratropical storm before coming anywhere close to Ireland. The only shot Ireland has at claiming a Cat 1 landfall is if a dwindling TC can hold on to just enough tropical characteristics before striking the island to justify the National Hurricane Center maintaining advisories on it, but that's pretty unlikely. Juliancolton (talk) 18:09, 23 June 2011 (UTC)

Why can I see clearly when I wear two pairs of glasses?

Why can I see clearly when I wear two pairs of my own prescription glasses, one on top of the other? Shouldn't the outer pair distort the correction produced by the inner pair? I think that if a person with perfect vision wore my glasses, his own vision would be blurred. So if one pair corrects my vision, shouldn't the second pair blur it? --82.31.133.165 (talk) 23:32, 21 June 2011 (UTC)

Not necessarily. This has to do with Lens (optics) and the fact that several lens may(depending on the lens curvature and refraction) change the focal point to better aim at your retina.Smallman12q (talk) 23:45, 21 June 2011 (UTC)

I just tried putting my prescription sunglasses over my prescription glasses (essentially same values), and it was blurry as hell. So this would not be a general phenomenon. Looie496 (talk) 00:01, 22 June 2011 (UTC)

It could be that your prescription is not very strong. However, if you can actually see better with two pairs than with one, it's probably time for your annual checkup. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:07, 22 June 2011 (UTC)

It depends on what your prescription spectacles are correcting.

If it's purely long- or short-sightedness (which your description suggests) then all that will change is the depth of field the lens combination is optimum for. This is exactly how reading glasses work: they either add some positive dioptre value to your otherwise unspectacled eyes (bringing the near limit of your focussed depth of field nearer to you, to a comfortable reading position) or - if you have separate prescription spectacles for longer distances - their lenses are the same formula as those distance lenses plus a few dioptres to achieve the same effect; you can therefore achieve exactly the same result by wearing appropriate-strength simple reading glasses over your distance prescription glasses, and I've constructed clip-on reading glasses (by cannibalising and combining cheap reading glasses and cheap clip-on sunglasses) which are sometimes more convenient to use than swapping to the separate prescription reading glasses I also have (to my optician's amusement but not disapproval).

If however you had other sight aberrations, such as astigmatism (like myself), wearing two pairs of your usual prescription spectacles would over-correct your eyes by as much as they're undercorrected with no spectacles at all. {The poster formerly known as 87.81.230.195} 90.201.110.203 (talk) 01:56, 22 June 2011 (UTC)