The fate of the Earth?
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From: dow on 5 Sep 2009 11:35 > It's possible it's just a coincidence, but there are a lot of > astronomical coincidences that we're experiencing right now. Not only > the Venus thing, but also the fact that the Moon's disk is so close in > size to the apparent disk size of the Sun. Etc. > > Yousuf Khan If there were no accidental approximate coincidences, then that in itself would be very interesting. By chance, these things do happen. If Venus's rotation were not approximately synchronized with the motions of the earth in the Venusian sky, then it might be synchronized with the motions of Mercury, or Mars, or Jupiter. There are enough possibilities that it is quite likely that one of them will be true, just by chance. The angular diameters of the sun and moon in the sky vary considerably becaue the orbits of the earth and moon are not circular. The ranges of the two diameters overlap. Sometimes the moon is bigger than the sun in the sky, so a total solar eclipse can occur. Sometimes the sun is bigger than the moon, so an annular eclipse can occur.. It's not really much of a coincidence. dow
From: Odysseus on 5 Sep 2009 18:49 In article <51d4dffc-f7a9-4199-af16-6f164f58b602(a)g23g2000vbr.googlegroups.com>, dow <williamsdavid65(a)gmail.com> wrote: <snip> > The angular diameters of the sun and moon in the sky vary considerably > becaue the orbits of the earth and moon are not circular. The ranges > of the two diameters overlap. Sometimes the moon is bigger than the > sun in the sky, so a total solar eclipse can occur. Sometimes the sun > is bigger than the moon, so an annular eclipse can occur.. It's not > really much of a coincidence. But in the distant past no annular eclipses were possible, the Moon being near enough to the Earth to make all central solar eclipses total, while in the distant future, when the Moon will be too far out for its disc to completeky cover the Sun's, the converse will be the case. -- Odysseus
From: alien8er on 6 Sep 2009 01:54 On Sep 3, 9:00 am, YKhan <yjk...(a)gmail.com> wrote: > On Sep 3, 2:39 am, alien8er <alien8...(a)gmail.com> wrote: > > > On Sep 2, 10:55 pm, Yousuf Khan <bbb...(a)yahoo.com> wrote: > > > No, I don't think it's good theory. The Venus greenhouse was caused by a > > > lack of magnetic field. The lack of magnetic field was caused by Venus' > > > lack of rotation. Venus' lack of rotation was caused by god-knows-what. > > > 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 Earths 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." > 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. > > > I don't think there is any good theory as to what caused Venus to stop > > > spinning, although the favourite theory seems to suggest a resonance > > > with Jupiter caused it. But that doesn't explain why Earth and Mars > > > weren't affected, but why Mercury and Venus might have been. > > > 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. 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: http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1802.pdf (More will be known when MESSENGER goes into Mercury orbit in 2011, hopefully.) Bottom line, you're right that there's no good theory to explain Venus' tidelocking, and lots of good reasons it shouldn't be. > 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. > > OTOH Venus' orbit is the most nearly perfectly circular of all the > > planets. Since there are two glaring anomalies there, no spin and a > > nearly perfectly circular orbit, it wold seem there's a connection, > > but it sure isn't obvious. > > Here's a size comparison of what the Sun looks like at aphelion vs. > perihelion from Earth. > > Astrophotography by Anthony Ayiomamitishttp://www.perseus.gr/Astro-Solar-Scenes-Aph-Perihelion.htm > > > > Regarding Earth's greenhouse fate in the future. It's been suggested > > > that within 1 billion years, the Sun will be 10% hotter than it is > > > today, rendering habitation on Earth impossible. That ignores the fact > > > that the Sun was 10% cooler, 1 billion years ago too. The Earth adjusted > > > to the cooler Sun then, and it may very well adjust to the warmer Sun.. > > > Certain theories suggest that lifeforms on Earth itself alter the > > > atmosphere enough to keep the planet habitable. > > > But then the Earth is also slowly fleeing the sun as its orbital > > energy radiates away as (theoretical) gravitons. > > 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. > But losses due to gravitational friction don't happen until the two > bodies are significantly closer to each other. And those energy losses > would result in the Earth getting closer not further away. > > > Different timescales for all these processes though, and life can > > only do so much if the sun gets bright enough to compensate for the > > increasing distance. > > Life has already completely changed the entire composition of the > Earth's atmosphere. For example, prior to Life, there was no oxygen > molecules on Earth. Then the Great Oxidation Event made the Earth rich > in oxygen nearly 2.5 billion years ago. > > The Rise of Oxygen > "Oxygen has not always been as abundant as it is today. Most > scientists believe that for half of Earth's 4.6-billion-year history, > the atmosphere contained almost no oxygen."http://www.astrobio.net/exclusive/541/the-rise-of-oxygen > > It is expected that Life will begin to pull greenhouse gases out of > the atmosphere and bury it into the sea or land, as the Sun heats up. > It's already happening now. Despite the mass hysteria about carbon > dioxide and global warming, the long term trend is actually that CO2 > has been going down, not up. Yeah, let's not get into the whole AGW thing just now- I don't agree with it either. 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. Mark L. Fergerson
From: dow on 6 Sep 2009 10:59 On Sep 5, 6:49 pm, Odysseus <odysseus1479...(a)yahoo-dot.ca> wrote: > In article > <51d4dffc-f7a9-4199-af16-6f164f58b...(a)g23g2000vbr.googlegroups.com>, > > dow <williamsdavi...(a)gmail.com> wrote: > > <snip> > > > The angular diameters of the sun and moon in the sky vary considerably > > becaue the orbits of the earth and moon are not circular. The ranges > > of the two diameters overlap. Sometimes the moon is bigger than the > > sun in the sky, so a total solar eclipse can occur. Sometimes the sun > > is bigger than the moon, so an annular eclipse can occur.. It's not > > really much of a coincidence. > > But in the distant past no annular eclipses were possible, the Moon > being near enough to the Earth to make all central solar eclipses total, > while in the distant future, when the Moon will be too far out for its > disc to completeky cover the Sun's, the converse will be the case. > > -- > Odysseus True. We just happen to be living in the period when the ranges of the apparent angular diameters of the sun and moon overlap. Similarly, we are living in the period when the rotation speed of Venus is very close to being synchronized with the motion of the earth in the Venusian sky. Neither coincidence was true in the distant past, and neither will be true in the distant future. Last Friday (the most recent business day), the price index of the Toronto Stock Exchange and the Dow Jones index in New York both rose by 96 points. Usually, they change by substantially different amounts. Sometimes they move in opposite directions. But on Friday their changes were almost identical, to within one percent. It was just accidental coincidence, of course. Accidental coincidences do happen, both in business and in astronomy. dow
From: dow on 6 Sep 2009 11:23
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. 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. dow |