In the 60s and 70s, America spent £67 billion on getting rocks from the Moon back to Earth*. In 2012, America spent a couple of billions on moving an Earth's rock to Mars**. Moving rocks is a lot of fun!
Yes, there's a piece of America on Mars now, people. Of course, you can argue that all the rovers and landers on the surface of Mars are (processed) pieces of America. They are United States' territories. Roaming territories, like the Nimitz. But this is different. This is an actual piece of soil. This is the, err, motherland. Basaltic rock über alles. Heck, settlers built farms and ranches and towns on this piece of rock—a basaltic rock from lava flow found near Socorro, New Mexico! This is the land of the free on the surface of another planet. Those damn immigrant Martian bacterias better not get anywhere near this piece of country. I wonder if any bacteria born on that rock will have US passports.
But I digress. Why did NASA put this rock on Mars? Easy: it "serves as a calibration target carried on the front of NASA's Mars rover Curiosity for the rover's Canadian-made Alpha Particle X-Ray Spectrometer (APXS) instrument."
The APXS instrument is designed to detect chemical elements in Martian rocks and soil. It uses curium to emit energetic alpha particles and X-rays. When exposed to radiation, the soil and rock samples emit different kinds of X-rays based on their composition, which get detected by the instrument, giving NASA a precise list of ingredients. [NASA]
* Among many other things.
** Among many other things.