How do scientists determine that a rock on Earth came from Mars instead of another planet?

The best idea scientists have of the geochemistry of Mars comes from the two Viking robots that landed on Mars in 1976.

Findings from the robots’ weeks of readings of things like the Martian atmosphere are compared with the chemical signatures of meteorites found on Earth.

The first such object confidently identified as Martian was reported in 1983. The object, a rock eight inches in diameter that weighed 17.5 pounds, had been picked up in 1979 at the Elephant Moraine near McMurdo Sound in Antarctica.

Trapped in bits of glass in the object, designated EETA 79001, were some so-called noble gases neon, argon, krypton, and xenon, strikingly similar in abundance to those of the Martian atmosphere, as determined by the Viking missions.

There are now about two dozen meteorites thought to be from Mars.