The fate of the Earth?
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From: alien8er on 6 Sep 2009 19:22 On Sep 6, 8:23 am, dow <williamsdavi...(a)gmail.com> wrote: > On Sep 6, 1:54 am, alien8er <alien8...(a)gmail.com> wrote: > > > > > If Venus were tidelocked due strictly to tidal deformation one would > > expect, as I think you were implying, to see Mercury also perfectly > > tidelocked. > > > But we don't, and Mercury does have mascons: > > Mercury is tidelocked so it rotates three times in the time that it > takes to go around the sun twice. This situation is fairly stable > because Mercury's orbit is quite eccentric (elliptical). When it is > near perihelion, its angular orbital velocity is about 1.5 times the > average, and that is the time when the tidal effect of the sun on it > is the greatest. So the tidal effect makes the rotation speed 1.5 > times the average orbital speed. The mascons make this synchronization > exact. > > However, tidal friction is occurring, which is slowly removing energy > from Mercury's orbit (without altering the total angular momentum), > with the result that the orbit is very slowly becoming more circular. > Eventually, unless the sun engulfs Mercury first, the 3:2 tidelock > will be broken, and Mercury's rotation will be slowed by tidal > friction until it is exactly synchronized with its orbital motion. > > > > That's not happening. The Earth won't move away from the Sun until the > > > Sun loses a significant amount of its mass, when it goes into its red > > > giant phase. The Sun might puff out a large amount of matter as a > > > planetary nebula at that point, and it would thus reduce in mass, thus > > > Earth would move away from it. > > > Whoops, can't believe I wrote that. Of course Earth is slowly > > falling into the Sun. > > I don't think so. The sun raises substantial tides on the earth > (about 1/3 as high as the tides that are raised by the moon), so tidal > friction is occurring that is transferring angular momentum from the > earth's rotation to its orbital motion. As a result, the earth is, > VERY slowly, moving away from the sun. Yep, but tidal friction will eventually result in tidelocking as well, halting the transfer. > Also, the sun is losing mass as the solar wind takes material away > from it. This also causes the orbits of the earth and the other > planets to spiral outward. At present, this effect is very small, but > when the sun expands to become a red giant the solar wind will become > much stronger, causing more rapid spiralling. AIUI the timescale for the Earth becoming tidelocked is not nailed down all that well; it's entirely possible Earth will be tidelocked long before Sol goes Big Red. Mark L. Fergerson
From: Yousuf Khan on 6 Sep 2009 20:52 alien8er wrote: > On Sep 3, 9:00 am, YKhan <yjk...(a)gmail.com> wrote: >> On Sep 3, 2:39 am, alien8er <alien8...(a)gmail.com> wrote: >>> I think you have that slightly sideways; as I understand it the >>> greenhouse stopped plate tectonics, vulcanism, and the core >>> circulation that creates a planetary magnetic field by eliminating >>> (well, strongly suppressing) the usual temperature differential >>> between the core and the surface. >> The temperature at the surface of Venus is still nothing compared to >> the temperature at the core. The Earth's core temperature is estimated >> at between 5000-7000K, which would be presumably the estimate for >> Venus' core temperature since they are of nearly the same mass. Venus' >> surface temperature is only 750K. > > From: > > http://www2.ess.ucla.edu/~nimmo/website/paper25.pdf > > "The generation of a global magnetic field requires core convection, > which in turn requires extraction of heat from the core into the > overlying mantle. Plate tectonics cools the Earth�s mantle; on the > basis of elastic thickness estimates and convection models, it is > argued here that the mantle temperature on Venus is currently > increasing. This heating will reduce the heat flux out of the core to > zero over ~1 b.y., halting core convection and magnetic field > generation. If plate tectonics was operating on Venus prior to ca. 0.5 > Ga, a magnetic field may also have existed." Understood, but a build-up of heat in the mantle of Venus wouldn't be caused by Venus' greenhouse atmosphere. It would be caused by a crust that is insulating it too much. The heat at mantle-crust junction might be measured in the thousands of Kelvins, not in the hundreds of Kelvins like at the crust-atmosphere junction. It's assumed that a lot of heat in the Earth core is caused by Uranium decay, thus there is a large source of heat still burning. If Venus' nuclear core is finished, then it must've had much less Uranium than Earth's does. So the question would be why the big difference in Uranium between the two planets? If that is even the reason for it. >> Besides, it's highly unlikely that the atmosphere has enough power to >> stop plate tectonics. It's not even clear if plate tectonics on Venus >> has actually stopped as there seems to be volcanos on Venus, some of >> which may or may not be active. > > There are a _lot_ of volcanoes on Venus, more than on any other > planet in our system: > > http://volcano.oregonstate.edu/volcanoes/planet_volcano/venus/intro.html > > The above page also, like many others, says there is _no_ evidence > Venus ever supported plate tectonics. I can find no cites saying it > ever did. To me volcanoes would indicate an active crust. But then again, if the crustal plates don't move around on Venus, then the volcanoes would be the only way for heat to be released from the mantle. Volcanoes by themselves can spew enough magma to create continents. All of Venus' continents combined seem to be only large enough to fit into Africa, which may be the result of lack of plate tectonics. Just volcanic outcroppings turning into continents. If Venus had as much water as Earth, it would be 90% water, and only the tops of volcanic mountains would show through for the remaining 10%. It's still early to tell whether there was any plate tectonics on Venus or not. We'll really need to land there eventually to really know. >>> Worse, neither Venus nor Earth have large mascons like Earth's moon >>> and Mars do. >> I'll assume you were comparing Venus and _Mercury_ against Earth and >> Mars. > > No. Tidelocking is usually assumed to be due mostly to tidal > deformation of the smaller body and resistant to dragging of the > resulting bulges, but the presence of mascons, as on Earth's Moon, can > rapidly accelerate the process. So what you're talking about when you're talking about a "mascon" is a "mass concentration"? >> This one shows the relative size of the Moon versus the rocky planets >> (but doesn't show Phobos or Deimos): >> >> http://www.saintjoe.edu/~dept14/environment/rogero/core5/solar_system... > > Phobos and Deimos are both tidelocked to Mars. I don't know if we've > found mascons in either. I think since neither of them are spherical moons, we can tell their mascons simply by looking at their shapes. >> That's not happening. The Earth won't move away from the Sun until the >> Sun loses a significant amount of its mass, when it goes into its red >> giant phase. The Sun might puff out a large amount of matter as a >> planetary nebula at that point, and it would thus reduce in mass, thus >> Earth would move away from it. > > Whoops, can't believe I wrote that. Of course Earth is slowly > falling into the Sun. Yeah, it probably won't have a chance to even fall into the Sun until well after the Sun's expected lifespan. > I also don't agree with the Gaia hypothesis (or its variants) which > claim that life regulates the planet's surface environment to its > benefit. That, as far as I'm concerned, is largely blind luck on > life's part. Add enough energy and despite all the tricks life can do, > it will get baked out of existence, even the extremophiles. Really? Why not? Some of the mildest versions of the theory are stating nothing more controversial than that life evolves to take advantage of the environment it's given, and second that life alters the environment around it. Yousuf Khan
From: dow on 6 Sep 2009 22:41 On Sep 6, 7:22 pm, alien8er <alien8...(a)gmail.com> wrote: > On Sep 6, 8:23 am, dow <williamsdavi...(a)gmail.com> wrote: > > > > > > > On Sep 6, 1:54 am, alien8er <alien8...(a)gmail.com> wrote: > > > > If Venus were tidelocked due strictly to tidal deformation one would > > > expect, as I think you were implying, to see Mercury also perfectly > > > tidelocked. > > > > But we don't, and Mercury does have mascons: > > > Mercury is tidelocked so it rotates three times in the time that it > > takes to go around the sun twice. This situation is fairly stable > > because Mercury's orbit is quite eccentric (elliptical). When it is > > near perihelion, its angular orbital velocity is about 1.5 times the > > average, and that is the time when the tidal effect of the sun on it > > is the greatest. So the tidal effect makes the rotation speed 1.5 > > times the average orbital speed. The mascons make this synchronization > > exact. > > > However, tidal friction is occurring, which is slowly removing energy > > from Mercury's orbit (without altering the total angular momentum), > > with the result that the orbit is very slowly becoming more circular. > > Eventually, unless the sun engulfs Mercury first, the 3:2 tidelock > > will be broken, and Mercury's rotation will be slowed by tidal > > friction until it is exactly synchronized with its orbital motion. > > > > > That's not happening. The Earth won't move away from the Sun until the > > > > Sun loses a significant amount of its mass, when it goes into its red > > > > giant phase. The Sun might puff out a large amount of matter as a > > > > planetary nebula at that point, and it would thus reduce in mass, thus > > > > Earth would move away from it. > > > > Whoops, can't believe I wrote that. Of course Earth is slowly > > > falling into the Sun. > > > I don't think so. The sun raises substantial tides on the earth > > (about 1/3 as high as the tides that are raised by the moon), so tidal > > friction is occurring that is transferring angular momentum from the > > earth's rotation to its orbital motion. As a result, the earth is, > > VERY slowly, moving away from the sun. > > Yep, but tidal friction will eventually result in tidelocking as > well, halting the transfer. > > > Also, the sun is losing mass as the solar wind takes material away > > from it. This also causes the orbits of the earth and the other > > planets to spiral outward. At present, this effect is very small, but > > when the sun expands to become a red giant the solar wind will become > > much stronger, causing more rapid spiralling. > > AIUI the timescale for the Earth becoming tidelocked is not nailed > down all that well; it's entirely possible Earth will be tidelocked > long before Sol goes Big Red. > > Mark L. Fergerson- Hide quoted text - > > - Show quoted text - Neglecting the expansion of the sun, the earth will become tidelocked to the moon, so each body keeps one face toward the other. (Pluto and Charon are already in this configuration). The sun will continue to produce tidal friction, so the whole earth-moon system will lose angular momentum. The moon will therefore spiral inward toward the earth, and the rotations will speed up. Eventually, the moon will get so close to the earth that it will be broken up, forming a ring. The ring will spread. Material from its outer edge will be lost to space, and material from the inner edge will fall into the earth. This will add more angular momentum to the earth, which will then be spinning very fast. When the ring has disappeared, tidal friction from the sun will slow the earth's spin, until it is synchronized with its orbital motion. So, over a period of many billions of years, the earth's rotation will slow down, then speed up, then slow down again. dow
From: Yousuf Khan on 7 Sep 2009 19:57 dow wrote: > On Sep 6, 7:22 pm, alien8er <alien8...(a)gmail.com> wrote: >> AIUI the timescale for the Earth becoming tidelocked is not nailed >> down all that well; it's entirely possible Earth will be tidelocked >> long before Sol goes Big Red. >> >> Mark L. Fergerson- Hide quoted text - >> >> - Show quoted text - > > Neglecting the expansion of the sun, the earth will become tidelocked > to the moon, so each body keeps one face toward the other. (Pluto and > Charon are already in this configuration). The sun will continue to > produce tidal friction, so the whole earth-moon system will lose > angular momentum. The moon will therefore spiral inward toward the > earth, and the rotations will speed up. Eventually, the moon will get > so close to the earth that it will be broken up, forming a ring. The > ring will spread. Material from its outer edge will be lost to space, > and material from the inner edge will fall into the earth. This will > add more angular momentum to the earth, which will then be spinning > very fast. When the ring has disappeared, tidal friction from the sun > will slow the earth's spin, until it is synchronized with its orbital > motion. So, over a period of many billions of years, the earth's > rotation will slow down, then speed up, then slow down again. > > dow Due to the influence of the Moon in orbit around the Earth, the Earth will never be tide-locked to the Sun. No matter what the Moon has to go around the Earth, and it has a greater influence on the Earth than the Sun does. Yousuf Khan
From: Sam Wormley on 7 Sep 2009 20:03
Yousuf Khan wrote: > dow wrote: >> On Sep 6, 7:22 pm, alien8er <alien8...(a)gmail.com> wrote: >>> AIUI the timescale for the Earth becoming tidelocked is not nailed >>> down all that well; it's entirely possible Earth will be tidelocked >>> long before Sol goes Big Red. >>> >>> Mark L. Fergerson- Hide quoted text - >>> >>> - Show quoted text - >> >> Neglecting the expansion of the sun, the earth will become tidelocked >> to the moon, so each body keeps one face toward the other. (Pluto and >> Charon are already in this configuration). The sun will continue to >> produce tidal friction, so the whole earth-moon system will lose >> angular momentum. The moon will therefore spiral inward toward the >> earth, and the rotations will speed up. Eventually, the moon will get >> so close to the earth that it will be broken up, forming a ring. The >> ring will spread. Material from its outer edge will be lost to space, >> and material from the inner edge will fall into the earth. This will >> add more angular momentum to the earth, which will then be spinning >> very fast. When the ring has disappeared, tidal friction from the sun >> will slow the earth's spin, until it is synchronized with its orbital >> motion. So, over a period of many billions of years, the earth's >> rotation will slow down, then speed up, then slow down again. >> >> dow > > Due to the influence of the Moon in orbit around the Earth, the Earth > will never be tide-locked to the Sun. No matter what the Moon has to go > around the Earth, and it has a greater influence on the Earth than the > Sun does. > > Yousuf Khan "Never" is a word to be avoided sometimes. The moon is walking away from the earth and at some point would have less influence than the sun, but I doubt that the earth (nor the moon) will be around long enough for that to happen. |