Wednesday, 24 April 2019

Solar System Temperatures - Not

A nice bit of sleight of hand by NASA Science:

This graphic shows the average temperatures of various destinations in our solar system. (Planets not to scale):

In general, the surface temperatures decreases with increasing distance from the sun.

Venus is an exception because its dense atmosphere acts as a greenhouse and heats the surface to above the melting point of lead, about 880 degrees Fahrenheit (471 degrees Celsius).

Mercury rotates slowly and has a thin atmosphere, and consequently, the night-side temperature can be more than 1,000 degrees Fahrenheit lower than the day-side temperature shown on the diagram. It can be as cold as -290 degrees Fahrenheit (-179 degrees Celsius) on Mercury at night.

The temperature they show on the graphic for Mercury is the day-side peak temperature, the average is 'only' 167C.

Their explanation why Venus is the exception is completely wrong, which they admit in the footnote:

Temperatures for the gas and ice giants (Jupiter, Saturn, Uranus, and Neptune) are taken from a level in the atmosphere equal in pressure to sea level on Earth.

The average temperature given for Earth is also at sea-level pressure, so there are two outliers. There is practically no atmosphere on Mercury (it could be correspondingly hotter if it had one), and Venus' surface has atmospheric pressure 90 times that of earth.

And what is the average temperature of Venus' atmosphere fairly high up, where pressure is equal to sea level pressure on Earth..?

About 65-70C of course, which is exactly what you'd expect if you extrapolate from the others.

They also don't make it explicit that the composition of the atmospheres of all these planets are wildly different, which means that it is pretty irrelevant which gases are in them, CO2 (like Venus), N (like Earth), O2, H2O vapour, whatever.


Dr Evil said...

It is surprising that a science and engineering-based organisation can come out with charts like this designed to bamboozle those without much knowledge or thinking ability regarding climate change. What it does show is what the main driver of the planetary climates is.......The Sun.

Mark Wadsworth said...

Dr E, actually the main driver is thickness* of atmosphere, plus a bit for the sun. The centres of gas giants are thousands of degrees.

* The thickness only, the type of gases in it don't seem to matter.

Mark Wadsworth said...

... but as the thickness of earth's atmosphere is fixed, the main variable in practice is minor fluctuations in what the sun does, orbital angle and so on.

Dinero said...

What about Mercury Pluto and Mars, the chart does not have a pressure criteria for them either. To put Venus on the correct place on the scale with the pressure taken into account would require that the the scale took pressure into account for the other planets but it does not do that consistently. So what can you make of it. But yes, the chart looks wrong, trivial and simplistic, a mish mash of data.

Mark Wadsworth said...

D, in fairness, those three don't have much of an atmosphere, so well never know.

Dinero said...

"In general, the SURFACE temperature decreases with increasing distance from the sun."
Some do not actually have a surface, then some have a surface and an atmosphere and others have a surface and no atmosphere, and so they then start using pressure as a substitute for surface and not doing that consistently either, and so it is not comparing like for like.

In order to ascertain the heat residing at each planet, it would be more coherent to estimate the thermal energy of the entire planetary object, including atmosphere when applicable, by adding up the thermal energy in each unit of volume of the planet solid and atmosphere to give a total figure for thermal energy for each planet.

Mark Wadsworth said...

Din, agreed. Another tweak to the basic model is to remember that Jupiter, Saturn and Uranus actually emit heat from their centres, independent of what little warmth they reflect from the sun.

Dinero said...

Yes,taking the temperature does not conclude what contributes to it.