After much anticipation, NASA has finally released a shortlist of landing sites for its Mars rover mission slated for July 2020. The three finalists are Northeast Syrtis, which may have been warmed by volcanic activity; the Jezero Crater, which could be the remnants of a Martian lake; and Columbia Hills, which NASA's Spirit lander explored in early-to-mid 2000s. Though Jezero is reportedly the favourite among NASA scientists, in this case, it's the underdog that could be our best shot at finding hints at past or present life on the Red Planet — which is exactly what the 2020 mission seeks to do.
One reason some of the scientists working on the 2020 rover aren't enthusiastic about Columbia Hills is because it's already been scoped out (hey, Spirit). But Steve Ruff, a planetary scientist at Arizona State University, says this isn't a valid reason to rule out Columbia Hills as a landing site. In fact, he thinks we're not truly examining the evidence right under our noses.
"I wouldn't be pushing it if I didn't think there was a case to be made [for Columbia Hills]," Ruff told Gizmodo. "The bottom line comes down to what we observed with Spirit, which is so compelling. We've published observations that we found potential biosignatures — features in the rocks there that could have been formed by a combination of biology and geology."
Ruff is referring to the fact that in 2007, Spirit found opaline silica deposits near the "Home Plate" feature within Columbia Hills. These deposits, which are made from a mineral called amorphous opal, are also found on Earth, usually around hot springs or areas where volcanic activity has occurred. In fact, in certain Chilean hot springs, opaline silica precipitates out of water in the presence of microorganisms. Ruff believes opaline silica deposits on Mars could have formed in a similar way. He has studied these deposits on Mars for years, even comparing them to ones found on Earth.
"Oftentimes, the microbes influence the shape of the rocks as they're being formed," Ruff said. "Hot spring fluids are mineral rich, and those little channels of hot spring fluids have microbial communities. The minerals precipitate, and if there are microbes on the rocks as they're forming from that precipitation, those microbes effect the shape of the rock. We think that's what we see next to 'Home Plate' in the Columbia Hills."
Spirit drove to this baseball plate-looking plateau back in February 2007, looking for evidence of an ancient lake in Gusev Crater. But Spirit unexpectedly found opaline silica instead, which was likely accumulated over time billions of years ago due to hot spring activity. In some ways, this was a much more exciting find, considering how important hot springs are biologically — they may, in fact, be where life first emerged on Earth.
"Basically all hot springs on Earth are known to harbour microbial life," Ruff explained. "Bugs love these hot spring environments because they have the right combination of hot water [and] the chemicals in the water."
Since we know hot springs are habitable environments on Earth — and that hot spring deposits can capture and preserve microbes — it's not impossible that Columbia Hills might be hiding some fossilised Martian microbes in its opaline silica deposits.
"I think [Columbia Hills] offers the best opportunity, because of how much we understand about these places on Earth that are similar to what we found on Mars," Ruff said. "Hot springs have been a holy grail of Martian exploration for decades, and now we think we found one."
At this point, scientists involved with the rover mission seem underwhelmed at the idea of revisiting Columbia Hills. Some feel that such missions are too rare — and expensive — to revisit Spirit's old stomping grounds. That said, old ground may wind up being our best shot at finally resolving the longstanding question of life on Mars.
"It's very clear why there is this sentiment in the community [that] we should put the precious rover somewhere else," Ruff said. "But the fact is the observations we have from a previous rover are so compelling in terms of searching for life on Mars that it does make sense to use this precious opportunity to go back. To me, this place — this opportunity — is the best one we have among the candidates."