Venus is hot because its atmosphere is carbon dioxide (CO2)

• It's atmosphere is 92 times thicker than Earth's
• It's crust is thinner than the Earth's allowing the internal core heat to reach the surface

The fact that it is closer to the Sun apparently has no effect - really.

Data from NASA's Planetary Fact Sheets	Earth	Venus	Moon
Black body temperature	254.3 K	231.7 K	274.5 K
Average atmospheric temperature at the surface	288 K (15 C)	737 K (464 C)	na
Atmospheric density at the surface	1.217 kg/m3	~65 kg/m3	na
Atmosphere mean molecular weight	28.97 g/mole	43.45 g/mole	na
Diurnal temperature range	283 K to 293 K (10 to 20 C)	737 K to 737 K (change of ~0 K)	>100 K to <400 K (roughly -250 F to +250 F)
Diurnal rotational period	1 day	117 days	28 days
Wind speeds at the surface	0 to 100 m/s	0.3 to 1.0 m/s	na
Dry Adiabatic Lapse Rate	9.760 k/km	10.468 k/km	na

Notice that both Earth and Venus have surface temperatures that are hotter than their respective black body temperatures - this difference IS the Greenhouse Effect. In both cases, this is caused by the atmosphere holding heat at the surface.

Also notice that the Venusian surface temperature is near constant day and night - diurnal (daily) temperature change is about zero. Note: The NASA page is wrong - Diurnal temperature range is NOT 0 K, the daily change is 0 K ... not the range (min and max are both 737 K).

For the purposes of argument, I will use just the Semi-major axis and 3 significant figures to estimate the relative energy received from the sun. I know that more accurate values can be computed, but this is good enough to make the point.

Energy per unit area at Venus = (92.9 E6)^2 / (67.2 E6)^2 = 
  1.91 times the energy per unit area on Earth

Some people will argue that Venus has a higher albedo than Earth (0.65 vs 0.367) and, therefore, the energy absorbed per unit area is about the same. It should be noted that these values are for the reflection of visible light, not IR. While I have not found good numbers for the IR albedo (of either planet), the data I've seen indicates that most of the IR energy is either reflected back into space or absorbed in the upper atmosphere (that's right - it does NOT heat the surface).

	Venus	Earth
Surface pressure	9,200 kPa 1,334.35 psi	101.3 kPa 14.69 psi
Gravity	0.904 g	1 g

This is the same as being cold ... so you add 92 more blankets.

This is speculative - I am not sure the science is correct, but ... correcting for a weaker gravity, the Venusian atmosphere should produce

(9,200 kPa (V) / 101.3 kPa (E)/ATM E) / (0.904 g V/E) = 100.5 ATM on Earth

Most reports indicate that the atmosphere of Venus is either 90 or 92 times heavier than on Earth. Since I have not found any Raw data, I do not know if that is actual measured pressure or if it has already been corrected in some way. In this document, I have tried to consistently use the 92 ATM's value, but I suspect that the atmosphere is actually 100 times thicker on Venus.

Note: In addition to the arguments above - If the atmosphere of Venus was placed around a larger Earth it would not be as thick because it would fill a larger volume (because of the difference in size) and because the gas would be pulled closer to the surface (because of the difference in gravity). However, this is not relevant in this discussion. The only important item is that the atmosphere of Venus has about 100 times more gas than Earth between the surface and space.

For coal

C + O2 -> CO2

For more complex petroleum products, the chemistry is more complex, partly because most carbon atoms have 2 or 3 hydrogen atoms attached, and partly because the number of carbons per molecule varies. This example assumes a simple 8-carbon molecule (octane - the main component of gasoline).

 
2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O

But don't worry that this is actually causing Global Cooling - there really isn't enough fossil fuel on the planet to worry about the atmosphere getting too thin or about running out of oxygen. The most damage humans can do is to remove only about 0.04% of the total atmosphere.

When hot dry air rises, it cools at a lapse rate of 9.78 °C/km - this is known as the dry adiabatic lapse rate (DALR).

If the actual lapse rate of the atmosphere is less than the DALR, then the atmosphere is stable and convective mixing does not occur unless there is external forcing (like a hot sun).

When the sun heats the ground, it is possible to heat the air near the ground enough that it becomes lighter than the air a few hundred feet above the ground. When that happens, the hotter air begins to rise. Based on the temperature difference, it is possible to compute how high it should rise before the rising air reduces its temperature (through adiabatic expansion) to that of the surrounding air. Since the surrounding air also decreases in temperature with height, the following simple equation can be used.

Tf = Ts - L * H

Tf       Final Temperature at height H
Ts       Temperature at start - either atmosphere or hot surface
L        Lapse rate
L(DALR)  9.78 °C/km - Earth, 10.4 °C/km - Venus
H        Height (km) of the lower final temperature
delta T  The temperature difference between heated and unheated air near the ground

Tf = Ts(surface) - L(DALR) * H = Ts(atm) - L(atm) * H

Ts(surface) - Ts(atm) = delta T = [L(DALR) * H] - [L(atm) * H]

delta T = [L(DALR) - L(atm)] * H

Without getting into a lot of graphs, the following table shows the atmospheric thickness and the ground temperature offset necessary to move heat above the insulating atmosphere to a zone where it can radiate into space (ie, above most of the water vapor and CO2). Note that the Earth values are *typical* and will vary considerably from hour to hour. Even the sign of the lapse rate will vary between morning and afternoon. The Venus lapse rate and surface temperature appear to be constant day or night (by itself, this implies that the Sun has little effect on the planet's temperature). (This data is open to disagreement - my sources are good but not as rigorous as they should be.)

The point of this is that to cool the Earth, hot air only needs to rise about 2 km (an easy task) to get above most of the green house gases - on Venus, it has to rise 55 km. In my opinion, this is the reason Venus is so hot - in fact, if the Venusian atmosphere had the same composition as Earth, it would be even hotter (because water vapor is a much better green house gas than CO2).

The fact that the Venusian surface temperature is the same after 58 days of sun light and 58 days of darkness is really the main reason I claim that the Sun does not heat the surface. (Venus rotates with respect to the Sun once every 116.75 Earth days.)

On Earth, the minimum expected difference would be 50°C for a 2,800 hour day, on Venus, no difference is reported.

The "official" explanation for Venus having a constant surface temperature is strong winds - I am not convinced and I have not seen any evidence to support that position.

From NASA - Venusian wind speeds: 0.3 to 1.0 m/s (surface)

It is frequently stated that because of the high albedo and thick atmosphere, almost no solar energy reaches the surface of Venus. If this is true, then how come the surface temperature is given as over 900F? Obviously, all this heat is coming from the planet itself, and not the Sun.

Coupled with the thicker atmosphere, and the fact that the surface temperature is near constant day and night, this is why Venus is hot.

So, maybe the crust thickness is irrelevant - just the atmospheric thickness is important.

Venus is hot because its atmosphere is mostly CO2 - therefore, if we add more CO2 to the Earth's atmosphere the Earth will get as hot as Venus.

For instance, in the NOVA program "What's Up With the Weather?", Dr. Pieter Tans of the National Oceanic and Atmospheric Administration (NOAA) says

Venus has an extremely high concentration of CO2 in its atmosphere, and it's blazing, boiling hot at the surface of Venus.

• No Surface wind
• Constant surface temperature
• Extremely slow planetary rotation

• The Sun has no effect on the surface weather
• The temperature of the surface is driven by internal heating
• Because the atmosphere has only a single green house gas, the surface temperature is much cooler than would otherwise be the case

The NASA Planetary Fact Sheets are the primary source for a lot of data.

This Wikipedia article on Venus is an alternate reference for physical properties - Wikipedia also provides additional data on the atmosphere of Venus.

The Soviet Exploration of Venus provides a very interesting chronicle of the history of Venus exploration - it was a long and expensive road to collect the data I've presented above. I highly recommend this page.

The planetary fourier spectrometer (PFS) onboard the European Venus Express mission is basically a proposal for future research - but it has lots of graphs showing what is known from previous probes. Even though the satellite has been on station around Venus for over one year (since April 2006), I can not find any current data.

I have spent many hours searching for raw data from Venus probes - I can not find any online. I am specifically frustrated that there appears to be none from the Venus Express probe that has been on station for almost one year (at this writing). I presume that, when the data becomes available (Last Update: 13 Nov 2006), it will unambiguously support or refute the speculative claims I have made above.

URL: http:// questionable-science.com / Global_Warming / Venus.html