What do you do when you find the fourth artifact on lunar colony?

When you have the geiger counter, it will lead you to the Purple Artifact, just a little bit below the Rock lab on the map. Use the winch to pull off the rocks, and it will rise up out of the ground. Follow the light beam to the right where it intersects another beam (connecting the Yellow and Red artifacts). Get out and look around. Salerno will show up with a shovel to help.