On 18 February, NASA’s Mars 2020 Perseverance rover successfully landed on Mars at Jezero Crater. The $2.7 billion space probe will look for evidence of past life, launch a helicopter that will hover over the surface for up to 90 seconds at a time, and drill samples for collection by later missions. The hope is that a sample return mission could launch as early as 2026, with Mars rocks delivered to Earth by 2031.
Physics Today spoke to Kenneth Williford and Kathryn Stack Morgan, both deputy project scientists for the rover mission, about the role of Perseverance in NASA’s decades-long exploration of the Red Planet. Williford created and runs the Astrobiogeochemistry Laboratory at NASA’s Jet Propulsion Laboratory. Stack Morgan started as a graduate student working on the Curiosity rover and eventually led one of the geoscience teams.
PT: The 1976 Viking probes, the first to successfully land on Mars, looked specifically for existing life. Today we’re looking for records of ancient life. How has our perspective changed over the past 45 years?
WILLIFORD: In my view, Mars 2020 is the first NASA mission since Viking to be so directly concerned with looking for evidence of life on another planet. At the time of Viking, our knowledge of Mars was quite limited. We had only recently gotten the first pictures of Mars from the Mariner missions. The first time you’re landing on another planet, it’s a natural question to ask: Is this planet inhabited? There was not at that time the fairly complex understanding of Mars and its geology that we have today.
After Viking, a lot of planetary geologists studied data from orbit but with better images and spectrometers that could analyze the composition of Mars’s surface. The data started to suggest that large amounts of liquid water had flowed across the surface in the past. Geologists debated whether these features they saw on the surface represented flowing water or maybe lava. The Mars exploration rovers of the early 2000s, Spirit and Opportunity, landed in a couple of these places, and they were both very successful in showing good evidence from the surface of liquid water.
Then Curiosity landed. The rover was larger and more complex, with some very capable science instruments. The mission was to look for evidence of a habitable environment in a more complex way than Spirit or Opportunity. How long was the water there? Did it provide a stable environment for life to evolve and thrive? Were there energy sources for metabolism? And early in the mission, the rover documented an ancient habitable environment there. [See the article by Ashwin Vasavada, Physics Today, March 2017, page 34.]
PT: So where does Perseverance fit in?
WILLIFORD: Mars 2020 follows on the footsteps of all these past missions. We’re going to look for evidence of liquid water. We’re going to look for habitable environments, taking an approach very similar to Curiosity’s. But then we’re going to directly seek evidence of ancient life in the environments that we explore.
The thing that really makes our mission distinct is that it’s our job to select and collect core samples of rock and then to seal them in titanium tubes placed on the surface of Mars, so future missions that are now being designed in cooperation between NASA and the European Space Agency could go collect our samples, get them into orbit around Mars, and then rendezvous with an orbiter and fly them back to Earth.
PT: Perseverance has a suite of instruments superior to those of its predecessor, Curiosity. So why is it so important to take the samples back to Earth rather than analyzing them in situ?
WILLIFORD: It can be quite difficult to get scientific consensus regarding the oldest evidence for life on Earth. Anything we find will need deeper study back on Earth with much more powerful instruments to better understand what we’re seeing with the instruments on Mars 2020. And that’s just from the astrobiology side. We also want to know how Mars evolved as a planetary system. Getting samples back offers this tremendous opportunity to understand very basic questions about the formation of the solar system and the formation and evolution of terrestrial planets. There’s a lot of big science that can be done.
We have much better instruments on Earth than we can fly to Mars. Probably the best example is a technique we commonly use to study the most ancient record of life on Earth: synchrotron radiation. There’s really no way at present to make a synchrotron source small enough to fly to Mars.
There’s also the sample preparation aspects. Synchrotron and other similar techniques commonly require a sample to be sliced extremely thin—in some cases, nanometers. So you’re doing a sort of nanofabrication of the sample just to enable the analysis. The machine that’s required to do the sample preparation is as big as Perseverance. Even simpler techniques, such as geological thin section photography—where we cut a sample, mount it to a glass microscope slide, grind it until it’s thinner than a sheet of paper, and put on a very fine polish—is quite difficult to do with a robot on Mars.
PT: If Earth and Mars were once habitable, what made their friendliness to life diverge?
STACK MORGAN: The general high-level idea is that because Mars is a smaller planet, it had more difficulty retaining the heat internally that would have kept the core molten. A molten core helps to generate things like electric fields and a magnetic field. So with the solidification of the core, it could no longer support and create those dynamic fields. And then once that happened, it kicked off a series of events that helped put Mars down this very different path from Earth’s.
PT: Some of your team went to Western Australia to look at rocks that contain evidence for some of the oldest life on Earth. What lessons might there be for Perseverance’s search on Mars?
WILLIFORD: I think that life could have emerged on both planets very early in their histories, sometime around 4 billion years ago, and potentially reached some similar microbial state. So if there had been anything living in Jezero, we expect it could have left a record like the roughly 3.5-billion-year-old fossilized microbial mats called stromatolites that we see preserved in Western Australia. Microscopic organisms join together in groups and form layers upon layers with sediment trapped in between. They eventually build up into a macroscopic pile of pond scum that would be big enough to be picked up by instruments in situ.
Still we believe that almost no matter what we see, we really need to get those samples back to Earth to look inside at much finer spatial resolution, just to be sure that the things that we’re seeing were indeed produced by life. There are many geologic and chemical processes that don’t require biology yet can still form interesting lifelike shapes and even lifelike compositions. We need to make sure we’re not being fooled when we find interesting features on Mars.
PT: How did Jezero Crater emerge as the preferred landing site?
STACK MORGAN: I joined the mission in 2017, when the team had just narrowed the selection of landing sites down to the final four. At that point, the science team hadn’t yet really dug into the geology of the landing sites. So we ran a series of team exercises and group activities to really understand what was present at each site.
There are certain aspects of Jezero that allow us to have a leg up on understanding the geologic context versus some other places on Mars. We have from orbit a very clear view of sedimentological features such as this beautiful river delta with features preserved in the rocks. These are the kinds of things that you would expect to see in an outcrop here on Earth, but the features are so well-preserved in Jezero that you can actually see them from an orbiting satellite.
I’ve been staring at orbiter images of Jezero for years now, trying to pull out every little bit of interpretation that I can. We relied heavily on images from the HiRISE camera aboard the Mars Reconnaissance Orbiter, and on thermal imaging and spectroscopic data from the satellite’s CRISM instrument, which is no longer operational, to tell us what minerals are present at the surface at Jezero. But they don’t tell you everything.
When we see Jezero from the ground with our own eyes through the rover, we’ll just have this whole other richness of understanding about what we’re looking at. So I’m excited to see those first images because I think we’ll answer a lot of questions—such as, Are the rocks sedimentary or volcanic?—very early on.
PT: Considering the complexity of this latest-generation rover, are there worries about the effects of Martian dust on all the moving parts?
STACK MORGAN: Dust is an interesting thing. It’s hard to predict how the rover will behave and how mechanisms will respond to all the electrostatic forces caused by dust. But we’ve done tests in the laboratory and in the simulated Mars environment here on Earth, and the engineers are confident that it will work.
PT: Do you have any new techniques for analyzing the data when you start collecting it?
STACK MORGAN: I have a postdoc who is working on novel ways to analyze the spectroscopic data that we get from instruments like CRISM. We, of course, look to our experience with past rovers like Curiosity, but there’s also plenty of room to advance with new ways of looking at data. As new graduate students and technicians come along, people are always finding new ways to see the data and to pull science information out of the data.
