On Jan 4, 3:03 pm, Tom Adams <[email protected]> wrote: > On Jan 3, 6:47 pm, Alastair <[email protected]> wrote: > > > > > On Jan 3, 9:57 pm, Tom Adams <[email protected]> wrote: > > > > Really? Do you think Earth ran around like a short stop and caught > > > them all? How the heck could little ole Earth's cross-section > > > (relative to the vastness of its obital cross secton) protect Venus? > > > Earth would not act as a backstop to regular comets coming from the > > the Oort and Kuiper belts, but if the Main (asteroid) Belt Comets, > > with circular orbit gradually became more elliptical they would crash > > with Earth first, and not become elliptical enough to crash with > > Venus. These are asteroids that formed beyond the snowline. Of course > > icy asteroids forming further from the snowline would be more likely > > to be disturbed by Jupiter and crash into > > Earth.http://www2.ess.ucla.edu/~jewitt/papers/2006/F06.pdf > > > Here is a news release which is a little more > > explicit:http://www.hawaii.edu/news/article.php?aId=1379 > > Note, a couple of telling points. Earth formed within the snowline so > > it should have been dry. The H/D ratio of Earth's water does not match > > that of comets, or of meteoric water from the Earth's mantle. The late > > heavy bombardment has not yet been explained, but could have been > > caused by the adjustment of the orbits Jupiter and Saturn disrupting > > the orbits of the outer asteroids which were beyond the snowline. See > > also:http://www.ifa.hawaii.edu/~meech/NAIJC/papers/gaidos_jc2.pdf > > > Here's a list of Jewitt's publications, which will be useful if you > > want to follow this up.http://www2.ess.ucla.edu/~jewitt/bib.html > > > Not sure if that will convince you, but it makes sense to me. > > > Cheers, Alastair. > > I guess that might make it plausible. > > But the references you cite say nothing about Venus being protected. > > Hansen cites hydrogen isotope evidence from the current atmosphere of > Venus that it once had lots of water: > > http://www.pas.rochester.edu/~blackman/ast104/vatmosphere.html
Well, he would say that wouldn't he? :-) http://en.wikipedia.org/wiki/Mandy_Rice_Davies#.22Well_he_would.2C_wouldn.27t_he.3F.22 I presume you are referring to this: ======================================= Absence of Water Vapor The clouds contain little water vapor, and there is little evidence for water in any form on Venus. It is speculated that the absence of water is because most water that may have initially been on Venus made its way to the upper atmosphere where it was broken down by sunlight and interactions with cosmic rays and the solar wind into oxygen and hydrogen, which was then lost to interplanetary space. There are several reasons to believe this. First, there is evidence that there was a lot of water on Venus at one time. This comes form Pioneer spacecraft observations which showed 150 times more DEUTERIUM (heavy isotope of hydrogen) per hydrogen atom than on Earth. This deuterium naturally forms from the dissociation of water (H20) into its respective components hydrogen and oxygen. Because deuterium is a heavier isotope of hydrogen (having 1 neutron in the nucleus in addition to just the proton) it rises more slowly in the atmopshere. Thus the lighter isotope of hydrogen largely escaped but the heavier isotope remained. This provides evidence that there was more water on Venus. Enough to make a surface covering ocean about 25 meters deep. The reason that the UV radiation could penetrate the atmosphere is because the oxygen was not able to form ozone but instead mainly formed oxides in the soil. ====================================== The evidence of higher levels of HDO on Venus than Earth cuts both ways. The level of HDO on earth is similar to that on Mars, in carbon chondrite meteorites, and so presumably in icy asteroids. It is higher than Kuiper belt comets, and even higher in Oort belt comets. So it would seem that the high HDO level on Venus is due to its proximity to the sun, where the gravitational effects raised the levels of the heavier HDO molecules. The values on the Earth would have been similar but for the input from the Main Belt Comets. The idea that the ozone layer would have protected water vapour on Earth from being disassociated is not convincing. The ozone layer only formed after the atmosphere became oxidized which was half way through its life at around 2200 Ma. Moreover, the ozone layer resides in the stratosphere and disassociation happens above the mesosphere, so the ozone layer could not protect the water vapour there. However, the stratosphere (ozone layer) does trap the water vapour below it, so without an ozone layer it might be possible for the water vapour to convect up to mesospheric altitudes. However I doubt even that, since CO2 also absorbs solar radiation and so it would form an inversion layer trapping the water vapour below the region of dissociation. Occam's Razor suggests that the Earth's water comes from icy MLB comets since it is likely that their HDO ratios match. But that's enough from me, Cheers, Alastair. -- You received this message because you are subscribed to the Google Groups Global Change ("globalchange") newsgroup. Global Change is a public, moderated venue for discussion of science, technology, economics and policy dimensions of global environmental change. Posts will be admitted to the list if and only if any moderator finds the submission to be constructive and/or interesting, on topic, and not gratuitously rude. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/globalchange
