I just post this as a question to ponder, the answer seems to be yes. Data sent back from Magellan show that the Sulfuric Acid clouds in Venus' upper atmosphere had reduced by some 60% (off the top of my head figure) since the last recorded data from the Mariner crafts.

So, does that mean we happend upon a weird time in Venus' history?

To me it makes sense that the Sulfur would indeed rain down and combine to form various sulfides and sulfates and sulfo-salts and so the Sulfur is not a constantly replenished resource in the atmosphere, unlike H2O is for Earth, which evaporates shortly after falling to the Earth.

If that's true that would mean a another great mystery. After a few million sciencehours studying Magellans images there is rather compelling evidence that Volcanism on Venus is practically extinct and tectonics seem to be dead. So, if the sulphur rains down so quickly, what has brought it up so quickly in the very recent past?

Well, I suggested an "increase" in Volcanism, or a temporary out-pouring, and Volcanism isn't nearly dead, just stagnant Like you said, there's little tectonism and so what Volcanism there is has to be due to mantle-plumes? Which tend to be less common...but not uncommon...

Even if it isn't temporary now, would it be possible to remove the clouds artificaially by one means or another? If you remove the insane amount of cloudcover, you cut the greenhouse effect by a large amount. Venus suddenly gets a lot closer to habitability.

The problem is the CO2 content, which is extremely more concentrated than the SO2. Hmm...maybe an increase in volcanism is the reason for the decrease in sulfur clouds, as you need H2 to create H2SO4, necessary for the raining of sulfuric acid which reduces SO2 concentrations?

Are there any known methods of converting CO2 to solid form without involving life processes? The most convenient solution would involve getting mountains of graphite in exchange.

Well if there are such vast changes on a decadal scale in the sulphur acid clouds one would expect to see those changes reflected in albedo chances but I'm unaware that somebody reported that.

About Volcanism, considering that the sulphur output of Krakatau, St Helens, Pinatubo is many of orders of magnitude less than any change on Venus, I would expect that any out-pouring event on that global scale would have shown on the Magallan images, like a fresh, unscarred area.

Mantle plumes? If the coronae represent mantle plumes, which is not an unreasonable hypothesis, then Venus has (had) plenty of them. The question is if they are still active. IMHO the main reason to assume that Venus is "dead" is the decline of temperature gradient of the lithosphere with time:

http://adsbit.harvard.edu/cgi-bin/np...&filetype=.pdf

I would say that the lack of spin and the total absence of a magnetic field contribute to the idea that Venus is dead. My personal "maverick/crackpot" hypothesis predicts that the "resurfacing" was a single event, that vulcanism is (about) extinct and that it will never re-energize (contrary to the periodically resurfacing idea).

Oh, interesting you brought up the magnetosphere (lack there of) as I have a different hypothesis.

First, I don't think Venus is a geologically dead world, I think it is still extremely active and that we don't see the same occurrence of activity due to it being in a completely different condition of that of the Earth, but internally, I think it's still very hot.

After all, the Surface temperature and pressure is already that of about a mile down in the Earth...so it's not losing a lot of heat?

Anyway, more interestingly is the magnetosphere...

I think, since the world is NOT turning very much, that there is no "dynamo" (this hypothesis assumes that the dynamo theory is indeed what is occurring).

The Earth rotates about 1,000 miles per hour in the outter-circumference, along the equator, I'm not sure what it is by the inner core and outter core, but it'd be less somewhat...anyway, our turning, would create a dynamo right?

Two different densities and even phase-states, would suggest that one rotational speed will be different from the other? So the outter-core could be spinning around the inner-core, creating the dynamo.

Who is to say without a planet's rotation, that same outter-core spin around the inner-core would occurr? (just a piss poor suggestion at this time, but food for thought).

If Venus was internally still very hot then the demonstrated dramatic change in thermal gradient was not possible. Considering that the "greenhouse" effect heated the lithosphere from the outside and the internal heat should have prevented the lithosphere to cool of from the inside. But yet the Lithosphere has cooled hundreds of degrees within a few hundred million years. This would suggest that Venus as a whole has cooled considerable, and this would only have been possible if Venus had been melted in the past, then convection currents could have cooled Venus so quickly.

For magnetism, the geodynamo seems to be a useful hypothesis.

http://www.es.ucsc.edu/~glatz/geodynamo.html

moreover, if only the spinning of the planet was to generate the magnetic field, how would you explain the magnetic pole flips? But the geodynamo is based on convective heat exchange in the outer core. Since Venus has no magnetic field this suggests that there is no heat exchange in the cores, meaning that the cores have cooled.

So what is the current internal temperature in the lithosphere compared to the Earths?

Oh also, if that is the case, then what is keeping the Earth so toasty? I've often wondered if the tidal forces between the Earth and Moon may have had a beneficial factor in keeping us geothermally alive (which seems to be a necessity for life).

Read the link: http://adsbit.harvard.edu/cgi-bin/np...&filetype=.pdf

and then your guess is as good as mine. Note that for a scientific paper there are an unusual number of superlatives: "extraordinary low temperature gradient", "unusually high heat flux", etc.

If my pet ideas are right, that would be mainly friction heat due to mantle outer core friction and outer core - inner core - friction both caused by precession forces.

Surely radioactive decay is at least an order of magnitude greater than tidal effects?

Who knows? Radioactive decay, mainly 40K / 40Ar has been assumed to explain the heat. That doesn't mean that all possiblities have been exhausted. If for instance we infer that we need a certain physical mechanism to explain about all geologic unusual phenomena from the mid Pleistocene until now with peak activities about every 100,000 years, and we realize that this mechanism has as side effect a considerable transfer from kinetical ernergy to heat, not unlike Venus, then we simply have another -competing- explanation.

Since the jury is and stays out, we have to judge to all the evidence around us, which scenario is most likely.

Perhaps, if there are no major magma eruptions taking place on Venus contemporarily (again, we can't be sure), it might be that the sulphur cycle is distinct from what we would normally call volcanism...

Certainly, the vast quantities of atmospheric sulphur would have been outgassed from volcanos originally, but what if there is a surface process which recycles precipitated sulphur? Something like the geysers or aquifers on Earth: sulphur compounds might accumulate on the ground and seep down into cracks in the rocks, where a local buildup of heat would presently re-vapourise them and lead to a sudden small-scale outgassing; or a constant gradual seepage of vapour at lower altitudes, where the ground level drops below the "sulphur table"?

Events like these would be far too small to show up on satellite mapping, but could operate over most of the planet's surface and collectively suffice to keep the atmosphere acidic.