One of the great successes of the Curiosity mission to Mars is that the rover landed at what turned out to be a goldmine of a location.
The mission has once and for all determined that the planet was habitable at least during its early days, that it contains the organic building blocks of life, and that liquid water ran and formed lakes. And this leaves out the more basic Mars science that some day will some day produce new headline results.
The process of anointing a successor destination for NASA’s 2020 rover mission to Mars has been going on for several years now, and the field was narrowed to three possibilities earlier this year.
Because some of the primary goals of the 2020 mission differ from those of the Curiosity mission, the potential landing sites are unlike Gale Crater and all share certain features that are, not surprising, promising in terms of the new goals. What’s new is the requirement that the 2020 mission will search for biosignatures of life in the ancient rocks and to identify, pick up and store rocks samples for later return to Earth.
Given those (and other) science goals, the leaders of the Mars 2020 mission — and the large community of scientists eager to become a formal or informal part of the mission — have been looking for sites where water was clearly present in the distant past and where conditions seem best for actually preserving fossil microbial biosignatures that may have been present.
This is quite a dramatic change, and will be the first NASA mission sent to look for life — albeit fossilized and ancient life — since the Viking missions of four decades ago.
“What we’re down to now is three sites featuring different kinds of ancient water settings,” said Kenneth Williford of NASA’s Jet Propulsion Lab. He’s deputy project scientist for the 2020 mission and a specialist in identifying fossil remnants of lifeforms in ancient Earth rocks.
“On the list we have a site that was clearly a river delta, one that had a large concentration of subsurface water, and another that may be the site of a possible hot spring. All offer great possibilities, and we have a year to decide which is most promising.”
As assessed by both the Mars 2020 team and the associated community of scientists, the two favorites are Jezero Crater and Northeast Syrtis Major. The third, Columbia Hills, is where the rover Spirit spent five years exploring. Its inclusion in the final three is somewhat controversial since the nature and even presence of the fossil hot spring that would make it a desirable landing site remains a matter of some dispute.
But Jezero Crater is well understood — from the perspective, that is, of what can be determined from orbiting satellites — and offers many scientific riches.
Several times the home to substantial lakes, with rivers both coming in to the area and going out, from above Jezero looks very much like a river delta on Earth.
“The delta was clearly part of a standing body of water,” Williford told me during the recent Astrobiology Science Conference. “Life on Earth loves shallow water environments, and the fine grain sediments are good at preserving ancient life.”
“But there are also many interesting niches for would be good for ancient microbial mats and stromatolites,” he said, referring to the mounds and columns that can be found in shallow water on Earth and that were originally formed by the growth of layer upon layer of cyanobacteria.
“There are signs of carbonates in the shallow edges of what would have been the lake. That possibility really excites me.”
Jezero has some features similar to those found in the Yellowknife Bay section of Gale Crater. A series of lakes existed there some 3.5 billion to 4 billion years ago, and conditions on the ground were determined through geochemistry to have been sufficiently benign to support life (if it ever began.)
Many layers of sedimentary rock are also visible, with some seemingly exposed in a manner similar to those in Yellowknife Bay. Most likely in past, finer grains of sediments further out in the delta were scoured and swept away by winds, and substantial walls of firmer sedimentary rocks were left for geologists to examine, interpret and delight.
Jezero is relatively close to the northern plains where some scientists infer that there was — indeed, had to be — a large ocean. While searching for signs of that ocean is not a core objective of the Mars 2020 mission, Williford said that it was entirely possible that landing at Jezero could provide information that would make a northern ocean more or less plausible.
And in terms of sample return, Williford said that Jezero offered a lot. The delta sediments, he said, concentrate extremely ancient igneous materials from the large watershed that includes Jezero and the larger Northeast Syrtis region.
The Northeast Syrtis Major region lies on the eastern edge of Syrtis Major, a huge shield volcano, and near the northwestern rim of Isidis Planitia, a giant impact basin. This region exposes early Noachian era bedrock, more than 4 billion years old, and contains many minerals that can only be formed in water.
But unlike Jezero, the rock appears to be igneous rather than sedimentary, and the water present for long periods was a subsurface acquifer.
“Almost everything we know of the history of life on Earth comes from sedimentary rocks, but there are no clear sedimentary deposits at Syrtris,” Williford said. “So we’ll work to understand another exploration model. We’ll go to find fracture networks in the rocks and find chemical gradients where microbes could make a living.”
The region has major appeal because it is known to have features from that early Noachian era of Mars alongside features from the later Hesperian times — making it possible to study the transformations that occurred during that transition period. There is also evidence that the area had deep subsurface water as well as later running water, creating layers of clays from very different times. This is considered a great opportunity to both understand Martian history and to search for biosignatures.
Northeast Syrtis Major also had longtime volcanic activity that once warmed the region. Underground heat sources make hot springs flow and surface ice melt. Microbes could have flourished there in liquid water that was in contact with minerals. The layered terrain offers a detailed record of the interactions that occurred between water and minerals — essential to provide the molecular building blocks of life — over successive periods of early Mars history.
The remaining signatures of an ancient magnetic field at Northeast Syrtis Major is especially intriguing to scientists working to understand the fundamentals of Martian history. The planet had a much more extensive magnetic field 4 billion years ago, but the internal dynamo that powered it is inferred to have failed and left Mars with less and less protection from solar winds and cosmic rays. The result was the loss of much of the planet’s atmosphere and gradually a cooling and desiccating of the surface.
Both the Jerezo and Northeast Syrtis Major sites have substantial deposits of carbonates, which Mars scientists find especially important and interesting. These carbon compounds can both preserve ancient fossils and can potentially provide insight into the early Martian world, when conditions were warmer and wetter. Climate scientists look to greenhouse gases such as carbon dioxide and methane to warm the planet, yet the carbonates that would have been formed by these gases in later stages have not been nearly as ubiquitous as expected.
The third short-listed site, Columbia Hills, comes with some controversy. It was not given particularly active support by the scientists meeting in February for the third Mars 2020 site selection, but the Mars 2020 steering committee and mission science leaders decided to include it.
The Mars rover Spirit explored the area from early 2004 until late 2009, when it got stuck in the sand. During its time on Mars, Spirit send back images of silica deposits that had distinctive shapes, sometimes called “cauliflowers.” Two Mars scientists, Steven Ruff and James Farmer of Arizona State University, proposed that those rocks detected near the Home Plate section of Gusev Crater in Columbia Hills could have been formed inside hot springs.
They pointed to similarly shaped rocks found near the El Tatio geyser in Atacama Desert in Chile, in Yellowstone National Park in Wyoming and in the Taupo Volcanic Zone in New Zealand. The rocks from Wyoming and New Zealand have been determined to have been shaped by microbes, but those in the desiccated Atacama have not so far, although the two scientists suspect that they will be.
Clearly, to land nearby an ancient hot springs areas with rock formations potentially identical to microbial-formed rocks found on Earth has substantial appeal. And as Williford explained it, during the February meeting there was long debate about whether the shapes on Mars were or were not sculpted by life, and whether they are remnants of an ancient hot sprins — which is an ideal breeding grounds for life on Earth.
“In terms of sample return, there is a clear appeal to focusing on these features that just might be like those at El Tatio,” Williford said.” The advocates “argued in a compelling way, and there are other volcanic features and igneous rocks that would be a real interest at Columbia Hills.”
But he also said that some members of the Spirit team disagreed with the interpretation of the “cauliflowers” and the roughly 200 community scientists meeting the site selection meeting did not rank Columbia Hills especially high. Nonetheless, the committee decided to keep it in the running, pending further study.
The landing site decision will ultimately be made by the head of the NASA science mission directorate, Thomas Zurbuchen, based on the results of the site selection process. So far the engineering teams have determined all the three sites to be safe for landing.
The final decision is expected to come within one year, leaving two years before launch. This time is set aside, Williford said, to give the Mars 2020 team time to focus on detailed scientific mission planning and mapping efforts on the selected site. But the two years also leave time to switch to a different landing site if important new discoveries or problems come to light.