What are the little ridges on the early space vehicles?

The Mercury and Gemini capsules had that skin.

Google says it was corrugated for strength, like a cardboard box, but without the smooth skin.

Apollo and Russian spacecraft have the smooth cover like a cardboard box, but they have
strong corrugated or honeycomb structure underneath.

Here is an interesting article on the design of early space craft. Part of the design was to disrupt air flow to act as a barrier to heat.

I watched the movie Hidden Figures which is a true story of NASA’s black female scientists. In the movie it addresses this and is supposed to have been figured out by one of the women. I highly recommend the movie.

Was it perhaps to also increase the surface area helping dissipate the heat?