Why is the atmospheric pressure on Venus so high as compared to Earth and Mars. (Well, Mars is smaller and "lighter" compared to Earth and hence maybe there is an explanation for Mars's atmospheric depletion).

The parametric ratios between Earth and Venus are close to 1 and yet we have very different climatic conditions on Venus.

Why is this?

The answer is actually quite simple: runaway greenhouse. It is theorized that Venus was once a warm, wet world. The problem was that it became too warm. Because of a high amount of volcanism, the atmosphere became saturated with high concentrations of greenhouse gasses, including carbon dioxide, methane and water vapour. This (and being closer to the Sun) raised the surface temperature beyond the boiling point of water.

Without the ability to form a liquid, water simply stayed in the atmosphere as water vapour, which is in itself a greenhouse gas. High concentrations of water vapour in the air, water molecules would reach much higher concentrations in the ionoshere than they do on Earth. The radiation from the Sun would split the molecules into hydrogen and oxygen ions. The hydrogen ions are light enough to reach escape velocity, and over millions of years, nearly all of the hydrogen was stripped from the atmosphere. That left a dense atmosphere thick with carbon dioxide and free oxygen. However, the oxygen wasn't being replenished by a biosphere like here on Earth; it was reacting to everything it could, including all of the free carbon to produce even more carbon dioxide. Over time, the oxygen was depleted, leaving Venus as we know it today: with a very thick atmosphere made up of 96% CO2.

The Earth could have an atmosphere that thick, but it doesn't because we have a liquid ocean. In addition to being the pimary reserve for this planet's hydrogen and a good chunk of its oxygen, the water leaches CO2 out of the atmoshere, where organisms use it to produce oxygen and calcium carbonate, the former giving us the air we breathe and the latter producing sea shells and eventually limestone, where most of this world's carbon reserves lie.

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If E = MC², why do I have less energy the more mass my body acquires?
That is all.

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OK, so the density of planetary atmosphere does not have a linear relationship to mass--other factors come into play, just as Azpod wrote. After all, the atomospheric pressure at the surface of Titan, moon of Saturn, is estimated to be 60% greater than the surface pressure on Earth despite the fact that Titan is only 40% of Earth's diameter. And of course, Titan is one whale of a lot colder than Earth, whilst Venus is one whale of a lot hotter. Go figure!

--Don Stahl

venus express is unlikely to happen, though.
http://www.space.com/scienceastronom...ss_020410.html

Which then leads me to a related point. Now I may be mistaken, but I think I remeber reading years ago about a mathematical relationship between a planet's gravitational strength, its distance to the sun, and its ability to retain an atmosphere of a given gas of a particular molecular weight. That is, the closer a planet is to the sun, the more energy the molecules of the upper atmosphere receive, so if there is not enough gravity, it will all eventually bleed off into space. And if the planet is too small, even if it is cool, certain types of gases would also eventually escape into space. That put limits on what planets could be terraformed because they simply couldn't retain N2/O2 atmospheres. Did I just misunderstand what I was reading those decades ago (highly likely), or does this sound at all familiar?

Sounds reasonable to me - even the Earth has problems hanging on to hydrogen and helium.

I am also given to understand that any terraforming of Mars would be unstable because of its light gravity - the atmosphere would eventually slip away.

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When researching a subject I'd seen on NOVA concerning a debate of Venus thick/thin lithosphere, the statement was made that Venus is dead geologically with an apparent young surface. There is low volcanism and the planet continues to radiate a lot of heat. The thin lithosphere folks said that Venus was able to shed internal heat dirctly through a thin lithosphere contributing greatly to the greenhouse effect the complete lack of moisture in surface rocks makes them more tensilely strong so that the mountains wouldn't slump giving the appearance of a geologically young surface. The thick lithosphere folks contended that Venus build up internal heat until the entire surface sublimates and the meteor impact record starts over.

The upper atmosphere is sulfuric acid if I remember correctly. This makes the place excedingly hard on surface landers.

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<font size=-1>[ This Message was edited by: Valiant Dancer on 2002-04-11 10:59 ]</font>

I thought our large moon was also a factor in the stripping away of molecules in the upper atmosphere. Would the lack of a large Venusian moon be part of the reason its atmosphere is so dense?

So, will man ever set foot on Venus??

I haven't heard anything about the Earth's Moon stripping away molecules in the upper atmosphere. The Moon's tides would force gasses higher in the atmosphere than they would be if there were no Moon, so I could see how it could contribute to a lower atmospheric pressure, but I'm not certain how much of an effect that would have.

But yes, the gravity of a world and its proximity to the Sun does have a direct correlation on a world's ability to hold onto gasses of various molecular and atomic weights. Worlds the mass of the Earth have a hard time holding onto hydrogen and helium. Fortunately, our planet's low temperatures keep much of our water in liquid form, so very little of it reaches the ionosphere, where atoms are stripped by radiation from molecules and the hydrogen can escape.

Our planet is loosing water that way, however. I've seen estimates ranging from 100 million to 2 billion years before the Earth's hyrdosphere is gone, and the Earth becomes an arid and eventually lifeless world. With no oceans, the Earth's tetonic plates will likely lock, preventing a recycling of gasses locked in the rock. Eventually, the calcium carbonate will outgas carbon dioxide, especially as the Sun gets more luminous. Both the brighter Sun and the greater concentration of greenhouse gasses would eventually cause a runaway greenhouse situation on Earth, and Earth would resemble Venus.

Worlds with a mass less than the Earth have more problems holding onto gasses. The Moon and Mercury could be terraformed by placing a large satellite at the L1 point between them and the Sun to reflect sunlight to a second reflector in orbit. The second satellite would spead the light out enough to cover the surface of the world, giving it light and heat in a roughly 24 hour period, but only just enough to give it Earth-like temperatures.

But on both worlds, we would have to import an atmosphere from scratch, and that atmosphere would simply bleed away rather quickly. We may be able to moderate that effect somewhat by reducing the amount of ionizing radiation makes it through the satellite system. But constructing a satellite to reflect visible light is much easier than also making it shield the planet from X and gamma rays.

Mars is more realistic, given that it may have much of its atmosphere locked under its surface, and the fact that it is larger and farther from the Sun than Mercury or the Moon. But we would likely still need to use comets to construct any sort of ocean on Mars. Also, in addition to hydrogen (which makes keeping an ocean hard), Mars also bleeds out nitrogen-- something that the Earth and Venus don't do. We would have to import a significant amount of nitrogen, as well and replenish the amount that is there on a regular basis. While often overlooked next to hydrogen, carbon and oxygen, nitrogen is essential for any form of life that we know.

Terraforming Venus is certainly a possibility. It would require the two-satellite system I described above, with almost all incoming sunlight blocked for many years. This would cause Venus' atmoshere and surface to freeze over. But then, the really expensive part of the equasion begins. We can't wait the millions of years it would take to create an ocean, seed it with life and wait for the beds of calcium carbonate to form and leach out the carbon dioxide from the atmosphere. We would have to actually land on Venus, cut large chunks of dry ice off of its frozen surface, allow more sunlight in to reheat it, then import an ocean and create a biosphere. We couldn't build a space elevator like we could on Mars or the Earth to lower the costs of launch from the Venusian surface, either. Venus' nearly complete lack of rotation makes that impossible. But, Venus would hold onto any atmosphere that we give it as well as the Earth would. Also, terraforming Venus poses fewer ethical problems than terraforming Mars does, because we know for certain that there are no local lifeforms on Venus. We don't know that for certain on Mars.

Clearly, terraforming any of the worlds in the inner solar system, or preventing the loss of the Earth's biosphere as the solar system ages, are daunting tasks. The fact that none of these worlds, including the Earth, can hold onto hydrogen, and the fact that only the Earth and Venus can hold onto nitrogen means that any world that we do terraform will need constant maintenance in order to keep its biosphere over time, too.

Fortunately, when we do decide to begin to terraform the worlds of the inner solar system, we will not be doing it with 21st century technology. Most likely, we will colonize the Moon and possibly Mars, and the colonists there would be the ones pushing for terraformation. Only after we solve the issues involved in terraforming the Moon and Mars would we likely look to terraforming Venus and Mercury as well.

By then, for all I know, we may have explorers shaking hands with aliens in the Andromeda Galaxy! Or we could still be stuck here trying to figure out how to send a probe to Alpha Centauri.

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If E = MC², why do I have less energy the more mass my body acquires?
That is all.

--Azpod... Formerly known as James Justin

Perhaps an asbestos covered foot! The environment is so hostile there it's hard to fathom any reason for humans to visit. Maybe if we could terraform Venus it would be an attractive destination, but that's a big IF.

I'd put my efforts into getting humans to Mars, where at least they could function with minimal protective covering.

I think this theory had some currency back in the 50's and 60's, but is now discounted. Larry Niven wrote some SF stories in which this was a plot element, but later had to admit that those parts of the plot were no longer feasible.

Thanks - you can tell I'm still living in the sixties. I didn't think a satellite with 1 or 2% of the mass of its planet would have much influence, but I'm a big Niven fan!

Does the atmosphere experience tides?

did space say Venus Express is unlikely, perhaps they got their reports wrong

All seems to be going well, ESA's Mars Express has collected some fantastic data and the project for Venus is in the final stages
Express in Solar Simulator
During the test, the Sun will be switched on at full power, representative of the Venus environment (2600 Wm-2). The test started on 20 March 2005 and will run up to early April.

http://sci.esa.int/science-e/www/obj...objectid=36908

it is beyond development.and is in the final testing stages

Venus Express will study the Venusian atmosphere and clouds in unprecedented detail and accuracy.
http://www.badastronomy.com/phpBB/vi...ighlight=&amp;

The first post by Azpod was very detailed and accurate I think.

I'll also add that since the temperatures never reached below the boiling point of water, no water vapor could condense. In addition to allowing huge amounts of water vapor to remain suspended in the atmosphere (a powerful greenhouse gas in itself), the carbon dioxide never had the opportunity to be removed from the atmosphere. So not only one greenhouse gas stayed in Venus' atmosphere, both did.

Over four billion years ago, when the Earth reached the point where water vapor could condense, clouds formed and torrential rains dissolved much of the carbon dioxide in the atmosphere, forming carbonic acid. On Earth, 50 times the amount of carbon dioxide is in the oceans, as a dissolved gas. Even more is in carbonate sediments that are under the sea.

Earth initially had similar amounts of carbon dioxide and water vapor as Venus. But Earth's water vapor is in the oceans, ice caps, rivers, etc, and most of the carbon dioxide was dissolved in the oceans or locked away in carbonate sediments. It's startling to see how much even one degree can affect the rest of a planet's life. If Earth had been even one, or two, or who knows how many degrees warmer, we wouldn't be very different from Venus today.

Hope that helps.

That would be a little difficult with the Moon, as its orbit around the Earth means it does not remain directly behind the L1 point; you could do it, but you would need a reflector/sunshade 700 thousand kilometers wide, or one which could be shifted reasonably rapidly.I wonder how quickly? I expect that even the Moon would keep an Earth-like atmosphere for a million years or so, longer if we kept topping it up. If we put a worldhouse roof on part or all of it, we could inhabit the Moon at least as long as the Earth.

It took Mars several hundred million years to lose its atmosphere, so terraforming would be worth doing there; volatile loss would be steady but relatively slow and could be topped up by water and nitrogen from the outer solar system. If we terraformed Mars and maintained its atmosphere successfully, but did not maintain the Earth's atmosphere artificially, Earth would become uninhabitable before Mars.

If we inhabit Earth for many more millions or tens of millions of years, we will probably need to learn planetary engineering skills to keep it a pleasant world to live on; these skills can be aquired experimenting on the Moon, Mars and Venus. And, perhaps Mercury too, as you point out.

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To answer the original post:
Because Venus's gravity is only slightly less than Earth's, most of its atmospheric pressure can be chalked up to the much higher total mass of its atmosphere, which is something like a hundred times greater than Earth's (IIRC).

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The data from the Russian probes to Venus in the 70's was recently reworked using modern technology:

http://news.bbc.co.uk/1/hi/sci/tech/3387895.stm

http://www.mentallandscape.com/V_DigitalImages.htm

Nick

Yes, but eventually geologically and eventually on a human scale do not match. The atmosphere on Mars would last as long as human beings stayed human.

Who wants to step in 900 Degrees Fahrenheit?

I've seen estimates like that too, but don't trust them. We've had that hydrosphere for 4 billion years. What has suddenly changed that we'd lose it in the next 0.1 billion years?

Yes indeed.

Mars is 45 hundred million years old. How did you arrive at several hundred million? Did you just take a SWAG at it?

That doesn't answer the question why the atmosphere of Venus 90 times greater than Earths. The planets are too similar to attribute it to initial composition. The run-away greehouse effect spoken of by previous posters would seem to be a reasonable explanation.

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