Similar to rocks found by Spirit rover, meteorite is a clue to early Mars.
No space probe has ever returned a sample from Mars. However, we have had a few Martian rocks delivered to Earth in the form of meteorites, which were dislodged by impacts and set adrift between worlds before crashing here. While studying Mars rocks in situ would be idea, the type of equipment we have on Earth is often impractical to ship aboard spacecraft and land on another planet. In that sense, meteorites provide a complementary way to study Mars, providing data that may be otherwise difficult to obtain.
The latest bit of Mars to undergo study on Earth is meteorite Northwest Africa (NWA) 7533. The chemical abundances and mineral types indicate that it originated on Mars, as it is comparable to rocks analyzed by the Spirit rover. What makes this specimen exciting is its age: radioactive analysis of zircons (silicon compounds containing uranium) revealed that the meteorite is about 4.4 billion years old. That means it formed roughly 100 million years after Mars was born, making it the oldest sample of the red planet yet collected, representative of the earliest period in the planet's history.
Of the thousands of known meteorites on Earth, scientists have confirmed that over 100 originated on Mars. This determination came from a variety of methods, including gasses trapped in the meteorites that have a composition similar to the Martian atmosphere. Additionally, radioactive dating determined that these rocks were much younger than meteorites that originated from asteroids.
Most meteorites are chondrites, planetary framents left over from when the Solar System formed roughly 4.5 billion years ago. Unlike rocks on Earth and other terrestrial worlds, which had molten surfaces after formation, these meteorites were not melted, so they provide relatively pristine data on the earliest period in the Solar System's history. In particular, chondrite meteorites and asteroids are considered to be compositionally akin to the planetesimals that built the rocky planets.
By contrast, meteorites originating from Mars or the Moon are typically regolith breccias, similar to many rocks collected by Apollo astronauts. The regolith is the powdery surface of a world created when meteorite bombardment pulverized the rock; a regolith breccia is a type of compound rock formed when the powder is fused under high temperature and pressure—during an impact, for example. Analysis of the specific composition further identifies the origin of the meteorite, since each world has a slightly different chemical make-up.
The meteorite NWA 7533 is a regolith breccia with volcanic origins. The chemical analysis showed that it is similar in composition to rocks analyzed in Gusev crater by the Spirit rover, though with less sulfur, chlorine, and zinc. But NWA 7533 also contains zircon crystals, silicon compounds hard enough to be spared pulverization in impacts. M. Humayun and colleagues measured the radioactive decay of uranium in the zircons and determined that they formed about 4.4 billion years ago, with a later disturbance roughly 1.7 billion years ago.
Confusingly, earlier measurements on another meteorite fragment from the same impact—NWA 7034—found an age of 2.1 billion years. This dating used rubidium rather than uranium, so it's possible that the inconsistency arose from two different rocks combined into a single breccia. As Harry McSween wrote in a comment accompanying the Nature article, these meteorites are "fiendishly complicated," so it may be some time before we know the full story.
The Solar System formed about 4.5 billion years ago, and the surfaces of Earth and the Moon solidified about 100 million years later. That places NWA 7533 at the same period of time when Mars must also have solidified. The existence of exposed ancient rock that is chemically distinct from younger rocks provides hope that rovers and other missions can uncover the history of Mars, providing some hints of the state of its water and atmosphere in early times.