Its primary mission was slated for just over 90 days. Opportunity's twin, Spirit, was supposed to be another 90-day wonder, but it soldiered on for six years before falling silent.
The last decade's worth of missions to Mars have been framed in three words: "follow the water" – the search for past or present evidence of a fluid deemed essential for organic life. But the effort also has contributed to a more sophisticated grasp of the birth and evolution, not only of Mars, but of all the planets in the solar system over its 4.6 billion-year history.
Initially, the notion was that the terrestrial or inner planets "all started out with the same brownie mix," says Darby Dyar, a planetary geologist at Mount Holyoke College in South Hadley, Mass., and a member of Curiosity's science team.
"It went into the oven, and it came out differently as a function of how far [planets] were from the sun. The last 10 years has challenged that assumption and shown us that there are specific and fundamental differences among the terrestrial planets, even though [they are] really close together."
The geological processes that shape these planets "are fundamentally the same," she says. "But the way they play out in detail, which we can see with the exploration program we've had over the last 10 years, is very different."
It turns out, for instance, that Mars' crust has much higher levels of sulfur than Earth's crust. While the distinction might seem esoteric, it suggests a far different environment early in the planet's history than Earth's – one that would have a direct influence on any life that might have emerged then, Dr. Dyar says.
Efforts to follow the water have revealed a remarkable planet whose prospects for serving as a habitat – even if it ultimately didn't host life of any sort – seem to increase with each new mission.