Salt on Venus
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From: Andrew Usher on 30 May 2010 22:33 In my previous posts on the run-away greenhouse (which we know must have happened to Venus some time in its past), I have assumed that liquid water would disappear when the surface temperature reached the critical point (705 degrees F). This is not true, though, because of the presence of salts which will raise the critical point beyond any reasonable surface temperature [1] causing the last parts of the ocean to remain liquid as long as the atmosphere is near saturated with water vapor, which it will be until almost all the water is lost to space. When this occurs there should remain large deposits of salts on the surface. Now though Venus has HCl in its atmosphere, it is only a small fraction (<10^-4) of the amount that must have existed in its oceans. So where is all the salt on Venus? Obviously, it is thought that Venus underwent complete resurfacing ~700 my, which must have been later than losing its water. Therefore there need be no large salt deposits today, but it would still be interesting to now where the halogens go in the absence of water; they are still largely incompatible elements. [1] There is probably no limiting critical point, and salts and water have then a continuous critical curve, but that can never be determined in the laboratory. Not only NaCl, but I don't know any salts that have a low enough critical point for observation and do not decompose or react with water below that temperature. Andrew Usher
From: Sam Wormley on 30 May 2010 23:19 On 5/30/10 9:33 PM, Andrew Usher wrote: > In my previous posts on the run-away greenhouse (which we know must > have happened to Venus some time in its past), I have assumed that > liquid water would disappear when the surface temperature reached the > critical point (705 degrees F). This is not true, though, because of > the presence of salts which will raise the critical point beyond any > reasonable surface temperature [1] causing the last parts of the ocean > to remain liquid as long as the atmosphere is near saturated with > water vapor, which it will be until almost all the water is lost to > space. When this occurs there should remain large deposits of salts on > the surface. Now though Venus has HCl in its atmosphere, it is only a > small fraction (<10^-4) of the amount that must have existed in its > oceans. Atmosphere of Venus http://en.wikipedia.org/wiki/Atmosphere_of_Venus "The Venusian atmosphere supports opaque clouds made of sulfuric acid, making optical observation of the surface impossible". HCl is 0.1�0.6 ppm "Through studies of the present cloud structure and geology of the surface combined with the fact that the luminosity of the Sun has increased by 25% since around 3.8 billion years ago, it is thought that the atmosphere of Venus up to around 4 billion years ago was more like that of Planet Earth with liquid water on the surface. The runaway greenhouse effect may have been caused by the evaporation of the surface water and the rise of the levels of greenhouse gases that followed. Venus' atmosphere has therefore received a great deal of attention from those studying climate change on Earth." "There are no geologic forms on the planet to suggest the presence of water over the past billion years. However there is no reason to suppose that Venus was an exception to the processes that formed Earth and gave it its water during its early history, possibly from the original rocks that formed the planet or later on from comets. The common view among research scientists is that water would have existed for about 600 million years on the surface before evaporating, though some such as David Grinspoon believe that up to 2 billion years could also be plausible". Also keep in mind that The higher temperature of the early Venus may have led to higher humidity so that atmospheric water vapor produced a greenhouse effect. This may have raised temperature and humidity and improved the greenhouse effect still further. Also, as oceans shrank carbon dioxide couldn't be stored away in carbonate rocks and would have entered the atmosphere to improve the greenhouse effect.
From: YKhan on 31 May 2010 02:27 On May 31, 8:33 am, Andrew Usher <k_over_hb...(a)yahoo.com> wrote: > In my previous posts on the run-away greenhouse (which we know must > have happened to Venus some time in its past), I have assumed that > liquid water would disappear when the surface temperature reached the > critical point (705 degrees F). This is not true, though, because of > the presence of salts which will raise the critical point beyond any > reasonable surface temperature [1] causing the last parts of the ocean > to remain liquid as long as the atmosphere is near saturated with > water vapor, which it will be until almost all the water is lost to > space. When this occurs there should remain large deposits of salts on > the surface. Now though Venus has HCl in its atmosphere, it is only a > small fraction (<10^-4) of the amount that must have existed in its > oceans. Holy-free-holy! The boiling point of seawater is 2500°C¿!? I realized that salt would raise the boiling point of the water, but I was thinking along the lines of going from 100°C to maybe 110°C or at the outermost 150°C, but I never imagined 2500°C! boiling point seawater - Wolfram|Alpha "temperature | elements | boiling point: 2500 deg C (degrees Celsius) " http://www.wolframalpha.com/input/?i=boiling+point+seawater > So where is all the salt on Venus? Obviously, it is thought that Venus > underwent complete resurfacing ~700 my, which must have been later > than losing its water. Therefore there need be no large salt deposits > today, but it would still be interesting to now where the halogens go > in the absence of water; they are still largely incompatible elements. Now, even if you can't reach the 2500°C in the atmosphere of Venus, there is one place on Venus you can reach those temps -- underground. On Earth, we know that vast quantities of seawater get swallowed up when ocean tectonic plates slide under continental plates. The seawater in turn helps to lubricate the sliding process. The temperatures rise above 2500°C, and the water vaporizes, and eventually comes back out through volcanoes, salt-free. Once this water reemerges into the atmosphere, it's got a normal water-only boiling point. I'm sure that is the point where it can escape from the atmosphere of Venus completely. This would also explain Venus' periodic volcanic resurfacing. It no longer has water to lubricate plate tectonics, and now the only way to relieve heat pressure is through massive volcanism only. Earth's rotation probably also fuels its plate tectonics activity, which is missing on Venus. Several factors against plate tectonics on Venus. Venus isn't really a model for what could happen on greenhouse Earth, as no matter how hot it gets here, Earth will still have seawater, and daily rotation. > [1] There is probably no limiting critical point, and salts and water > have then a continuous critical curve, but that can never be > determined in the laboratory. Not only NaCl, but I don't know any > salts that have a low enough critical point for observation and do not > decompose or react with water below that temperature. > > Andrew Usher Yousuf Khan
From: Andrew Usher on 31 May 2010 08:44 YKhan wrote: > Holy-free-holy! The boiling point of seawater is 2500°C¿!? I realized > that salt would raise the boiling point of the water, but I was > thinking along the lines of going from 100°C to maybe 110°C or at the > outermost 150°C, but I never imagined 2500°C! > > boiling point seawater - Wolfram|Alpha > "temperature | elements | boiling point: 2500 deg C (degrees Celsius) > " > http://www.wolframalpha.com/input/?i=boiling+point+seawater Obviously this is not accurate - the boiling point of seawater is about 101 C, when that of pure water is 100. A saturated solution of NaCl boils at 109 C at 1 atm, so that boiling seawater will rise to about this temperature when nearly dry. Other salts can of course raise it higher, or even produce a continuous solubility curve at 1 atm (sodium and potassium hydroxides, ammonium nitrate, most organic salts), thus giving no boiling point of a saturated solution. I was referring, though, to behavior at the critical point, and not 1 atm. > > So where is all the salt on Venus? Obviously, it is thought that Venus > > underwent complete resurfacing ~700 my, which must have been later > > than losing its water. Therefore there need be no large salt deposits > > today, but it would still be interesting to now where the halogens go > > in the absence of water; they are still largely incompatible elements. > > Now, even if you can't reach the 2500°C in the atmosphere of Venus, > there is one place on Venus you can reach those temps -- underground. > On Earth, we know that vast quantities of seawater get swallowed up > when ocean tectonic plates slide under continental plates. The > seawater in turn helps to lubricate the sliding process. The > temperatures rise above 2500°C, and the water vaporizes, and > eventually comes back out through volcanoes, salt-free. Temperatures in the upper mantle are less than 2500 C and no great temperatures are required to get it to vaporise from volcanoes. And much of the chloride is liberated at volcanoes in the form of HCl, and would be also on Venus. The acid is rapidly fixed by water on Earth, but on Venus must react with solid minerals. > Once this > water reemerges into the atmosphere, it's got a normal water-only > boiling point. I'm sure that is the point where it can escape from the > atmosphere of Venus completely. Atmospheric escape depends only on conditions at the top of the atmosphere. > This would also explain Venus' periodic volcanic resurfacing. We don't know that it's happened more than once, or whether it will happen again. > Several factors against plate tectonics on Venus. > Venus isn't really a model for what could happen on greenhouse Earth, > as no matter how hot it gets here, Earth will still have seawater, and > daily rotation. No, not so. The seas will largely boil off in the run-away greenhouse event, and I don't believe the rotation matters much if at all. Andrew Usher
From: YKhan on 2 Jun 2010 07:03
On May 31, 6:44 pm, Andrew Usher <k_over_hb...(a)yahoo.com> wrote: > YKhan wrote: > > Holy-free-holy! The boiling point of seawater is 2500°C¿!? I realized > > that salt would raise the boiling point of the water, but I was > > thinking along the lines of going from 100°C to maybe 110°C or at the > > outermost 150°C, but I never imagined 2500°C! > > > boiling point seawater - Wolfram|Alpha > > "temperature | elements | boiling point: 2500 deg C (degrees Celsius) > > " > >http://www.wolframalpha.com/input/?i=boiling+point+seawater > > Obviously this is not accurate - the boiling point of seawater is > about 101 C, when that of pure water is 100. A saturated solution of > NaCl boils at 109 C at 1 atm, so that boiling seawater will rise to > about this temperature when nearly dry. Other salts can of course > raise it higher, or even produce a continuous solubility curve at 1 > atm (sodium and potassium hydroxides, ammonium nitrate, most organic > salts), thus giving no boiling point of a saturated solution. I was > referring, though, to behavior at the critical point, and not 1 atm. So what in the world is Wolfram Alpha displaying? They won't even give you an answer on something unless they have some data about it. Yousuf Khan |