As NASA's Mars Perseverance rover explores Jezero Crater on the red planet, scientists back at at Caltech are eagerly launching into what is quite possibly humanity's best chance for finding ancient signs of extraterrestrial life to date.
In an effort to ensure Perseverance landed safely, engineers at JPL, which Caltech manages for NASA, directed the rover to land some 5 kilometers' drive away from what many believe to be its most tantalizing destination: the remnant of a delta where, about 3.7 billion years ago, an ancient river terminated in a lake during a time when liquid water still flowed on Mars' surface. Setting speed records across the crater floor, the rover reached the delta in April 2022 and is now investigating and collecting samples.
If you are looking for signs that life once existed on Mars, scientists say that delta is a great place to search, which is why the Perseverance team targeted it in the first place. Thus far, Perseverance has been trundling over regolith (ground-up Martian rock that exists in a fine dust all over the surface) and igneous rocks.
"In terms of the major mission goal of looking for signs of ancient life, the delta is the highest priority target," says Ken Farley, the W. M. Keck Foundation Professor of Geochemistry and Perseverance project scientist. That is because the geological characteristics inherent to delta formation make it a likely location to have sealed away organic evidence of life in fine-grained sediments that do not have a lot of pore space for fluids to later enter and destroy. "More broadly, a lake is a good place to live," Farley says,
That is not to say the rover has not already encountered interesting terrain. Months after the rover landed in February 2021, it was discovered that much of the base of Jezero Crater is made up of igneous rocks, as opposed to sedimentary rocks, which came as a surprise to some.
Igneous rocks (such as the basalt that makes up island chains like Hawaii) are cooled flows of magma that have solidified underground or emerged during volcanic eruptions from the interior of whatever planet they formed on. Sedimentary rocks, on the other hand, are the result of rocks that eroded into fine grains and then cemented together into a single rock through heat, pressure, and chemical processes.
If you have ever noticed layers upon layers of rock on a cliffside near a beach or in a hillside that has been bulldozed for a freeway, it was likely sedimentary rock. The layers each represent a different era of deposition, usually from a river, lake, or ocean.
"The igneous rocks at the center of Jezero Crater were a huge surprise for us, and it took us a long time to figure out that's what we were seeing," says Kelsey Moore, postdoctoral scholar research associate in planetary science. The rocks' presence is a clue that sheds light on the formation of the crater and its history, which could prove crucial if signs of life are in fact discovered nearby.
"The finding of igneous rocks in the floor of the crater says that Jezero is more complex than the model of a lake basin that is filling with sediments over time with the delta being the last landform," says Bethany Ehlmann, professor of planetary science and associate director of the Keck Institute for Space Studies. "It says that the region has had a rich geological history that has had both igneous and sedimentary processes."
What is more, the igneous rocks that Perseverance has sampled so far show signs of mineral alteration by water, as well as organic compounds, as noted by Eva Scheller (PhD '22) and colleagues at the Lunar and Planetary Science Conference in March 2022. This finding supports previous studies indicating that Jezero Crater could have been habitable billions of years ago.
It should be noted that, despite their name, organic compounds—chemical compounds with carbon—hydrogen bonds—are not direct evidence of life in and of themselves. They can be created through abiological processes.
"A rock formation isn't a single snapshot in time, but rather the aggregate result of eons of evolution," Moore says. "The igneous rocks could indicate that water in the crater was periodic." The aqueous alteration of igneous rocks even raises the possibility of a lake with hydrothermal springs or vents, like the ones that support microbial life in the deep ocean on Earth, Ehlmann says.
That said, Perseverance has had set its sights set on the delta since the beginning. By their nature, river deltas—for an example closer to home, the delta where the Mississippi River meets the Gulf of Mexico—are sedimentary. These landforms are created when a swiftly flowing river terminates in a deeper, slower-moving body of water, like a lake or ocean. When this occurs, the water that was flowing in the river fans out and abruptly slows down, and rocks and sediments carried by the river fall to the bottom of the water. The ability of a river to carry rocks and other objects is largely based on its speed: the faster the water moves, the bigger the rocks it can carry. The deposition creates aggregations of clays and sand that eventually become sedimentary rocks.
"On Earth, the deltas are where fine materials can be transported down by the river system to accumulate in the quiet water where a delta forms," says John Grotzinger, the Harold Brown Professor of Geology and the Ted and Ginger Jenkins Leadership Chair of the Division of Geological and Planetary Sciences (GPS). "When the river flow intrudes into a body of standing water, it expands laterally, and slows down, and all of the fine particles like clay and organics settle out. If there were microorganisms in the lake, or on the lake floor, they can be trapped and preserved. Deltas are a great place to explore for carbon and possibly signs of life." This tantalizing potential for discovery is why Perseverance hoofed it at an unprecedented pace to the delta.
The fortuitous (and forward-thinking) large concentration of geochemists at Caltech preparing to analyze Martian sediments mirrors a similar situation from two generations earlier, when Caltech was home to geologists, geophysicists, and geochemists who were among those at the front of the line to analyze lunar samples brought home by the Apollo astronauts. Among the more than 100 scientists selected by NASA to study the lunar material in 1969 were six Caltech faculty members: geologist and geophysicist Gerald Wasserburg, geologist Leon Silver (PhD '55), geochemist Samuel Epstein, geochemist Clair Patterson, nuclear geochemist Donald Burnett, and geologist Hugh Taylor Jr. (BS '54, PhD '59). Wasserburg notably led the creation of Lunatic I, a mass spectrometer for making high-precision measurements of lunar samples obtained by the Apollo missions; while Silver instructed Apollo astronauts on geology and lunar sample selection.
"Planetary geology was essentially invented at Caltech. The close proximity to JPL, and Caltech's small size allowed close collaborations between scientists in the GPS division and the PMA [Physics, Mathematics and Astronomy] division with the engineers at JPL," Grotzinger says. The new Caltech Center for Comparative Planetary Evolution seeks to stimulate current and future collaborations of the type needed by the Perseverance rover team to identify attractive targets that may contain biosignatures.
The delta is a big place, however. So, where should the rover—which has a limited operational lifespan and a finite number of samples it can collect—go?
Oak Kanine, graduate student in geology, has been analyzing the delta from overhead (via images from the Mars Reconnaissance Orbiter, or MRO) to help chart Perseverance's path by learning more about the delta's overall structure, which in turn helps identify potentially interesting targets.
"The rover gives us absolutely incredible, detailed, high-resolution data. But from the vantage point we're at, even a small portion of the delta looks like a hill that encompasses our field of view," Kanine says. "I'm trying to get a broader, larger-scale picture of the delta architecture."
Using stereo imaging—that is, two images taken of the same location from orbit just a short distance apart—Kanine and their colleagues are able to gather data about elevation changes, and in turn use that to build a 3-D model of the crater and delta.
"The main question right now is where should we sample that is most likely to preserve biosignatures if they are present," Kanine says. "That means looking for fine-grained sediments, which are more likely to bury and preserve potential microbes and create biosignatures."
Kanine analyzes the dip, or steepness, of rock layers to extract information about depositional environment and to assess where fine-grained material is most likely to be located. As flow from river channels in deltas enters bodies of standing water such as lakes, the speed of the water rapidly decreases. Coarser sediment that the river can no longer carry begins to avalanche at this point, forming inclined beds, while smaller grains (like clays) are lofted farther out into the lake, settling to form nearly-horizontal layers.
"We're looking for those transitions into flat beds where we can say, 'OK, this is more likely to have what we're looking for,'" Kanine says. "From orbit, we can ID a few candidates, and based on these we can help steer the rover to promising targets for potential sampling."
However, whether those prioritized locations contain any signs of ancient life remains to be seen. Moore says people hoping for signs of life should not expect a smoking gun.
"Keep in mind we're doing science on Mars. It's an incredible feat," she says. "I try to temper my expectations. On Earth, when we find a fossil like a dinosaur bone, it's very obviously a fossil. But when you're looking for signs of microbes that may have existed billions of years ago on another planet, it's likely to be less obvious."
That evidence is likely to be an aggregation of data collected from numerous samples, and any positive declaration of signs of life may well have to wait for analysis of samples in terrestrial laboratories after those missions.
"If we find evidence of a rock that has biosignature preservation potential, I don't think it's going to be something where we can immediately say it contains signs of life," Moore says. "More likely what's going to happen is we'll see some things that are exciting and tantalizing, but it'll need further study. There's such a high burden of proof, we're going to need to get samples home and really put them through a rigorous round of laboratory testing."
Perseverance comes equipped with numerous instruments aimed at probing the delta sediments including SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals), which uses cameras, spectrometers, and a laser to search for organic material and minerals that have been altered by watery environments; Mastcam-Z, a mast-mounted camera that takes high-definition video, panoramic color and 3-D images; PIXL (Planetary Instrument for X-ray Lithochemistry), which measures the chemical makeup of rocks; and SuperCam, a camera equipped with a laser for clearing away dust and spectrometers for analyzing samples.
"The benefit of SHERLOC and PIXL is that they take data at 100 microns per spot or less," Farley says. "We will be looking at what the minerals are, what the cements are, which gives us clues to what the pH [the acidity] of the waters were." Meanwhile, Mastcam-Z's powerful camera will help scientists understand the history of the flow of water in the crater. Analysis of grain size and layering will offer clues to the depth of water in a given location and how fast the waters were flowing.
Even if no signs of past life are discovered, the analysis of Jezero Crater will be a firm step toward learning more about the evolution of our closest planetary neighbor—and there may still be connections to the origin of life on Earth. While life may not have emerged on Mars, it is possible that there may be a record of key precursor steps that did not quite make it to the stage of self-replication. The returned samples can be examined for complex organic molecules that represent those initial steps, and these may guide us in filling in the "missing links" in how life may have gotten started on earth.