Venus's surface is hot. If we dig and cover ourselves with regolith, will the temperature increase or decrease as we dig? If decrease, how far would we have to dig in order to have reasonable (say sub 100C) temperatures to put a habitat?
5$\begingroup$ As a far as I am aware the sub surface temperatures of Venus have not been measured. Given the conditions on Venus, this is not surprising. Venus is thought to be an actively volcanic planet. Earth is a volcanically active planet. Because of this, it has a geothermal gradient, where temperatures increase with depth. I suspect Venus, like Earth, would have a sub surface thermal gradient that increased with depth. I do not envisage sub surface temperatures on Venus being less than 100 C. $\endgroup$– FredApr 29, 2021 at 18:25
1$\begingroup$ Further to my comment above. The reason why near surface temperatures on Earth can be low is because the surface of the Earth is cooled by the atmosphere. With an atmospheric temperature of 460 C, the surface of Venus does not have the same degree of "cooling" as Earth, hence sub surface temperatures on Venus will be high. $\endgroup$– FredApr 29, 2021 at 18:35
1$\begingroup$ The subsurface temperature profile of Mercury might be more interesting. $\endgroup$– CornelisApr 30, 2021 at 10:26
Due to thermodynamics, the temperature must increase.
Heat flows from hot to cold, and can not go the other direction. If there is a cold pocket between the (hot) core and the (hot) atmosphere of Venus, heat will flow into it. For it to remain cold, this heat would have to be dumped elsewhere, but since there's no colder place nearby for it to leak heat, it will heat up until it reaches an equilibrium with the core and the atmosphere.
Therefore, you can not dig down on Venus to find a layer with habitable temperatures.
While there is still much research to be done on the geology of Venus, one estimate I could find models the geothermal gradient of Venus as 25 K/km, that is, increasing as you go down.
To find 100ºC temperatures on Venus, you would instead have to go up in the atmosphere, where heat can leak into space.
$\begingroup$ The only loophole in this answer is that if Venus's climate had warmed quite recently, the subsurface temperatures might be cooler because the ground had not warmed up yet. But Venus's temperature has been super-hot for hundreds of millions of years, which is far too long for subterranean coolness to persist. $\endgroup$ Apr 30, 2021 at 12:55
$\begingroup$ So you could (theoretically) operate a heat pump between a few meters down and somewhere up in the atmosphere. (?) Your reference doesn't seem to show the estimated temperature curve from subsurface to surface to atmospheric heights. $\endgroup$ Apr 30, 2021 at 13:22
$\begingroup$ @CarlWitthoft Yes, you could do that, even on Earth. The fact that this is not used for electricity production should hint at the efficiency :) The atmosphere of Venus is much better known than its lithosphere, atmosphere of Venus should give decent coverage. Temperature gradients in atmospheres are additionally complicated by heat convection. $\endgroup$ Apr 30, 2021 at 13:34
The geothermal gradient on Venus may be no higher than 14K/km, that it get's warmer (not cooler) with increasing crustal depth. Locally crustal units are not closed systems but in exchange with adjacent parts, heat from below and atmosphere from above, so the gradient may be 0 or even negative, for instance when fluids take heat away or in subduction zones (on earth). The subducting crustal parts are naturally much colder than the surrounding.
But this style of tectonics does not exist on Venus, though it is assumed that plume activity creates an environment where short lived subduction around a plume that penetrated the crust can exist, that is, where crustal units at shallow depth could be colder than the overall gradient suggests.
But since the surface temperature on Venus is so high and subduction like features are situated in the planet's lower regions there is not really a chance that anywhere in the Venusian crust pockets might exist with temperatures in the suggested range of <100°C.