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In a paper (PDF of full article) which has been accepted for Geophysical Research Letters, NASA researchers
created a suite of 3-dimensional climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 billion years ago, present-day Venus orbital parameters, an ocean volume consistent with current theory and measurements, and an atmospheric composition estimated for early Venus.
According to the output of the general circulation model, "Venus may have had a climate with liquid water on its surface for approximately 2 billion years." In the simulation, extensive, highly reflective, H2O clouds formed on the lit side of the planet. "A strong day-night circulation" carried heat to the dark side. These factors limited the range of temperatures, in spite of a slow rotation rate. The authors note that liquid water can be not only a sign of habitability, but a cause of it:
[...] while the possibility of surface liquid water defines the traditional habitable zone, our results suggest that a planet with a modest amount of surface liquid water is more conducive to habitability over a wide range of stellar fluxes than a planet largely or completely covered by water. The inner edge should therefore be considered a transition region in which the probability of habitability gradually decreases inward rather than a strict boundary separating completely different regimes.
Venus today has little water. The high ratio of deuterium to protium (as compared to the ratio in Earth's surface water) leads us to believe that large most of the planet's hydrogen has escaped to space.
in the popular press:
(Score: 2) by MichaelDavidCrawford on Monday August 15 2016, @10:46PM
I read this in a science fiction story. The story presented it as a commonly-accepted theory but I heard no mention of it while studying Astronomy at Caltech, or anywhere else.
Earth's volcanoes spew poisonous gasses such as sulfur dioxide into the atmosphere. But there is a very tenuous atmosphere as far out as the moon, where the moon's slingshot effect accellerates those noxious gasses into solar orbit.
The theory states that without a moon like we've got, earth would have a poisonous atmosphere like Venus has.
This seems reasonable but now I wonder whether the author of that story was just making it up.
Yes I Have No Bananas. [gofundme.com]
(Score: 0) by Anonymous Coward on Monday August 15 2016, @11:01PM
It doesn't seem reasonable to me. The same effect would also act on our atmosphere as well, not just the noxious elements, and we would end up with no atmosphere under this scenario.
(Score: 3, Informative) by rleigh on Monday August 15 2016, @11:26PM
Additionally, these gases such as S0₂ are heavier than the most common constituents of the atmosphere and so would be far less likely to escape. It's going to stick around near the ground. Plus, we also have handy biologically-driven and chemically-driven sulphur cycles which recycle it so it doesn't build up (search for "sulphur cycle").
(Score: 3, Informative) by stormwyrm on Tuesday August 16 2016, @12:10AM
Numquam ponenda est pluralitas sine necessitate.
(Score: 2) by butthurt on Tuesday August 16 2016, @12:46AM
TLDR: Sulphur dioxide would be the last to go. I think the losses are mainly due to heat, the solar wind, and gravity.
The temperature of a gas is the average kinetic energy of its atoms or molecules. The mean free path is the average distance an atom or molecule can travel without a collision; the denser the gas the shorter the mean free path. Escape velocity is the speed at which an object must travel so that another body's gravity cannot recapture it.
In a gas, there's a distribution of kinetic energies. Some atoms or molecules with high kinetic energy can attain escape velocity--but if they collide with other particles, they could slow down below escape velocity. The lighter the atom or molecule, the faster it is going, for a given kinetic energy. In a mixture of gases at a given temperature, the lighter atoms/molecules will on average be moving faster. Hydrogen and helium are lost more readily than nitrogen or oxygen. If energetic particles from the solar wind dissociate a molecule in the upper atmosphere, its atoms will be lighter than the original molecule and hence more readily lost to space. Sulphur dioxide (SO2) and carbon dioxide (CO2) are heavier than molecular oxygen (O2) or molecular nitrogen (N2). Even a sulphur atom has about the same mass as molecular oxygen.
I would guess that the main factors in removal of hydrogen sulphide and sulphur dioxide from the Earth's atmosphere are oxygen and water. Those noxious gases can both dissolve in water, and hydrogen sulphide reacts with oxygen.