The Legacy of Spirit and Opportunity

The findings from the Mars Exploration Rovers allowed the Mars science community to develop our strategy for Mars exploration beyond “follow the water” to the more complicated question of whether these watery environments were ever habitable. Very loosely defined, a habitable environment is one that has the 2 other essential requirements in addition to liquid water that are needed to support life as we know it: a source of carbon and a source of energy. The Mars Science Laboratory mission’s Curiosity rover, which landed on Mars in 2012, carried a larger and more complicated payload than the Mars Exploration Rovers. Curiosity is capable of finding evidence of all 3 of these requirements. In fact, it has succeeded: within its landing site at Gale crater, Curiosity found ancient river and lake deposits that preserved carbon-containing compounds as well as evidence for water chemistry that could power microbial metabolism. Today, we not only know that Mars was once wet—it was also habitable.

Almost 2 decades after Spirit and Opportunity and a decade after Curiosity landed, NASA’s Mars 2020 rover and ESA’s ExoMars rover are now poised to address the most challenging question in our search for life on Mars when it lands in February 2021: did it exist? The designs of these missions and the selections of their landing sites rest on what we learned from Spirit and Opportunity.

At the Thai-food-fueled, end-of-mission celebration, people were in a reflective mood. They told stories about their experiences during mission development and reminisced about the escapades that had occurred over 14 years of operations. For all this looking back, there was just as much anticipation of the road ahead: “What are you working on now?” “Can you tell me more about your new mission concept?”

Spirit and Opportunity took humanity on a great adventure and set the bar extremely high for what we can accomplish with robotic space exploration. Although the rovers leave big wheel prints to fill, they have also opened the door to dreams of even greater voyages. We’ve demonstrated the power of mobility on the ground. Now, we are looking to the skies of other worlds to travel farther and faster. A helicopter will ride along with the Mars 2020 rover, and NASA recently announced that it will send a drone called Dragonfly to soar above the sand dunes and lakes of Titan in the 2030s.

Spirit and Opportunity showed us just a small taste of the richness of Mars’ past. I believe we have only just begun to understand the full complexity of Martian history and to piece together what Mars’ 3-to-4-billion-year-old rocks can tell us about how planets inside and outside our solar system can evolve into habitable worlds. As the mission’s principal investigator Steve Squyres said at the end of the final science team meeting, “The biggest stuff is still ahead, and I think [enabling] that in large measure may be the greatest legacy that this project has.”

(For those wondering, the Thai food I had ordered for the celebration exceeded mission-success criteria, like Spirit and Opportunity. We ended up with abundant leftovers that were consumed by eager Caltech grad students the next day.)