Many rocks exposed to slow weathering a terrestrial desert environment end up with a greasy look after a layer of "varnish" builds up on them. Some of this, on Earth, is due to organic activity but even sterile rocks can do this.

A lot of scientists have speculated about similar processes on Mars - basically a very fine polish by being sandblasted by the micron-scale fines of Mars, whipped by dust storms and dust devils.

In a few tens or hundreds of million years, the Mars landers and rovers will probably still be shiny and polished, but starting to wear to some odd rounded shapes, and half buried by low drifts...