BRF, you grade an "F" on my planets course. ;-)

Of course, that's not fair, since you haven't sat through it, and it isn't even on the web. (Note to self, must put course outline on the web).

The most important point that I (try to) drum into my students is the disparity between internal and external heating. The two are so wildly different in their effects they (almost) never overlap.

Let's start with Earth...

Solar heat in full sun is 1350 W/m^2. Averaged over 24 hrs, and over the surface of the sphere (or oblate spheroid, if you want to be picky), that's an average of 337.5 W/m^2.

Geothermal heat flow averages ~80 milliWatts/m^2

So, Solar heating is over 4,000 times stronger than geothermal heating. Except for local gothermal hot spots, where ground heating is significant, the thermal state of the surface and atmosphere of a terrestrial planet depends solely on sunshine. It follows that atmospheric temperature depends on solar input, modified by greenhouse effects, and surface temperature is the temperature of the base of the atmosphere, or the bare-rock black body temperature in the absence of an atmosphere.

Now, INTERNAL or subsurface temperatures depend on geothermal heating. Rather than heat flow, it is easier to think in terms of geothermal gradient. In sedimentary basins on Earth, this is ~ 30 K/km. geothermal gradient is proportional to heat flow (which varies between planets), and inversely proportional to conductivity, so different materials like granite, basalt, sediment, or ice have different gradients, even on the same planet.

Even if Earth, or Mars, or Venus, experienced an episode of 100 times the normal rate of heat release at the surface due to a spectacular global volcanic process, or to total disruption of the crust, then Solar energy would still be an order of magnitude more important than internal heat flow!

Even for Io - the most intensely internally heated body in the solar system, Internal heat flow is ~ 2 W/m^2, while sunlight is 12.5 W/m^2, still 6 times greater, so the ~ 115K surface temperature of Io is essentially due to sunlight, while the volcanoes are due to tidal heating.

To recap, surface temperature is given by sunshine + greenhouse effect. Then subsurface temperature increases more or less linearly from this level.

Some examples:

Earth:

Black body surface temperature ~260 K
greenhouse effect 25 K
nett surface temerature 285 K
Shallow Geothermal gradient 20-30 K/km
Temperature at 10 km: 500-600

Venus:

Black Body surface temparature ~ 310 K
greenhouse effect 435 K
nett surface temperature 745 K
Shallow Geothermal gradient ~ 20 K/Km (?)
temperature at 10 Km 950 K

Mars:

BBST ~ 205 K
GH 8 K
NST 213 K
SGG 5-7 K/km (?)
10 Km temp = 265-285 K (melting point for ice/clathrate)

These are average values. Planets are warmer at the equator and colder at the poles. Thick atmospheres (and oceans) tend to even out temperatures - Venus has ~ 3K temperature difference, globally!

Mercury is interesting:

N.B. This is a special case where sunlight actually DOES affect the lithosphere!

The planet's spin axis is almost perfectly perpendicular to its orbital plane, so the poles NEVER see direct sunlight, and the equator ALWAYS gets vertical sunlight every rotation ("day"). [planets with strong inclinations show more even heating, and for high obliquities (>50 degrees), the EQUATOR is the coldest place on the planet, on average!)]

Consequently, polar mean temperatures are ~ 90 K (which is why there can be ice in the poler regolith), while the equator averages ~ 600 K.

What are the implications for the thickness of the lithosphere? The lithosphere is the mechanical boundary between the "frozen" outer silicate skin and the deeper more plastic mantle (not the chemically differentiated crust, but the mechanical lid to the convecting mantle). Typically, this boundary occurs ~ 1600 K

At the poles of Mercury, one needs to go deep enough for temperature to increase from 90 K to 1600K, i.e. ~ 1500 K

At the equator, one only needs 1000 K increase in temperature.

Therefore, at the equator of Mercury, the lithosphere is 1.3 thinner than at the poles !

{Class dismissed}