"A useful rule of thumb is, all other things being equal, thermal gradient scales as for gravity (it's a mathematical function of included mass, divided by surface area).

So, if the Moon had the same composition of the earth, it would have 1/6 the thermal gradient.

Instead of ~30 K/km, it would be 5 K/km, and the temperature at 8 km (5 miles - get with the metric system), would be 40 K hotter than at the surface.

You can do the maths on other objects but beware the composition assumption. We KNOW it isn't true for our own Moon, due to it's very small core.

The Moon is depleted by about 50% compared to earth in K, and by about 10% in U and Th, the major heat-producing radiogenic elements, therefore it should be even cooler than the simple assumption... "

Interesting, I was mainly thinking of the idea that the surface of the Moon is supposed to be significantly fractured. And that a body without active plate tectonics may have this characteristic. Therefore to some extent Mars- though Mars has much more water- and Mercury could also be fractured similar to the Moon.

"Moreover, the seismic data indicated that the Moon, like Earth, is seismically active. The lunar seismometers detected numerous small moonquakes emanating both from the upper mantle and from deep within the lunar interior. Analysis also revealed that an intense meteoric bombardment had shattered and fractured the lunar crust to a depth of a 20 or 30 kilometers early in lunar history. Subsequently, the impacts continued to pulverize the upper crust and mix it to depths of at least two kilometers and perhaps as great as 10 or even 20 kilometers. Data suggested that below about 25 kilometers fractures in the crust are self-annealing and that deeper rocks may remain largely intact. "
http://www.ig.utexas.edu/about/history/planetary.htm

But if I leave that aside, and just focus on about 5 K per km on the Moon due to weaker gravity.
50 km is 250 K warmer allowing similar conditions as on earth which are about 2 km in depth and perhaps 100 km below the surface of Moon would be near the limit of depth as similar limit one would have on Earth.

And on the Moon one could expect that there could be considerable variation- instead 100 km being a global limit, depending upon the region it could be as little as 10 km and as much as 150 km or more. Perhaps.