[shibumi]

Many seem to be assuming that these formed from the top down, by an impact of some sort.

When I look at the photo I'm am reminded more of the residue of a froth, as if the surface had been liquid (molten) and has had the equivalent of a carbonated effervescence that bubbled off leaving a pitted surface.

I think I've seen similar patterns in a milk shake or perhaps cake or pancake batter.

[Talisker]

Excellent theory. Really good.

[SunkenCiv]

Good suggestion, plus, a tasty treat for the first visitors.

[Carthego delenda est]

Looks more that way to me as well, per my botfly hatch theory. I think yours is a bit more plausible though.