Earth’s fossil record suggests that by 3.5 billion years ago, life had found a footing on our planet. Yet the very processes that would shape the further evolution of that life — such as plate tectonics, erosion and weathering — also destroyed or muddled the crucial first records of life’s emergence, presenting a significant challenge for researchers trying to understand how life arose.
Mars, however, is seemingly inhospitable to life now but may not have always been so. And with nearly half of its surface rocks over 3.7 billion years old, Mars may have retained the records to show it. In short, 4 billion years ago, Earth had oceans and land, while Mars had wet climates and standing water, at least episodically. While one world went on to teem with life, the other may yet hold the key to understanding how life starts.
“On Mars, we have a high-fidelity record of what happened between 3.5 and 4 billion years ago, when the planet looked a lot like Earth,” says John Grotzinger, a professor of geology and geobiology and division chair for geological and planetary sciences at the California Institute of Technology and co-director of the Simons Collaboration on the Origins of Life (SCOL).
Formed in 2013 and now comprising 25 investigators and eight postdoctoral fellows working in geology, chemistry and biology, SCOL seeks to elucidate the origins of life, both on Earth and on other potentially habitable planets. Several collaboration members are working closely with NASA’s upcoming Mars 2020 mission on its goal to determine if Mars ever supported life. “The focus of the Mars missions has gone from the search for water to the search for habitability, and now to the search for life,” says Grotzinger, who was project scientist for the Curiosity rover from 2007 to 2015.
SCOL investigator David Catling, a professor of earth and space sciences at the University of Washington, made a case for where the new rover — recently named Perseverance — should land for the Mars 2020 mission. At NASA’s final landing-site selection workshop in October 2018, he suggested landing in Jezero Crater, arguing that it would be the best place to look for signs of prebiotic chemistry. The NASA Science Mission Directorate later chose that location from a list that had started with 60 possible sites. Several other SCOL investigators contributed to the presentation, including Tanja Bosak, Roger Summons and Joel Hurowitz.
Jezero Crater is about 50 kilometers in diameter and contains a 3.8-billion-year-old delta deposit, indicating it was once a lake. The crater may have trapped within its clays and other minerals the vestiges of ancient prebiotic chemistry: the interactions between molecules that directly preceded life. While actual fossils are rare on Earth and could well be even rarer on the exposed surface of Mars, the planet could be a graveyard of materials and chemistries that record the preamble to the emergence of self-replicating organisms, says Grotzinger. If Perseverance discovers such molecular fossils and their origins are determined to be biological or from meteorites, it could reshape our understanding of life’s start on Earth. “If Mars’ early environment reached a stage of prebiotic chemistry, those chemicals that may have survived can give us a glimpse of a chemistry long erased on Earth,” says Catling.
The central challenge faced by those who will interpret the Mars samples will be how to distinguish prebiotic signatures from organic matter not associated with life, such as that found in meteorites. “We’ll look for molecules that reflect a non-randomness in their chemical structures,” says Summons, the Schlumberger Professor of Geobiology at the Massachusetts Institute of Technology. For example, organic molecules in meteorites often show evidence of being randomly built from additions of single carbon atoms. In contrast, in biology, large and complex molecules are constructed from small sets of common building blocks. In lipids, for instance, carbon atoms are added in twos or fives. However, molecules like ferrocyanides and cell membranes, which are inherently in a reduced state, will require special circumstances to be preserved in Mars’ oxidizing environment. “We’ll need to look for the magic minerals, like silica, clays and carbonates, that can entomb these molecules and lock them away from oxidation and destruction by ultraviolet light,” says Grotzinger.
In her lab, Bosak, a professor of geobiology at MIT, is working on experimental fossilization of microbes in a Mars-like environment. The rocks on Mars are basalt-based, with more magnesium and iron and less aluminum and silica than most rocks on Earth. “We’ll see chemical reactions uncommon on Earth, and this will have consequences for sedimentary features and the kinds of microbes preserved,” says Bosak. One early finding from her lab showed the generation of hydrogen gas from fine-grained basalt and other mixtures of minerals, analogous to those expected in Jezero Crater sediments, when they were mixed into carbonated water. In addition to hydrogen bubbles, surface features like ridges formed along with the precipitation of new minerals. These are the sorts of features Perseverance will be able to capture on camera, says Bosak. Because several kinds of microbes consume hydrogen gas, the gas-related features could be a good place to look for microbial biosignatures.
As a returned sample scientist and a member of the project science team for the mission, Bosak will guide the selection of samples that NASA will send to Earth on a later mission, aiming to optimize the chances of bringing back rocks with prebiotic molecules as well as microbial fossils. Using the rover’s imaging of rock formations and laser spectroscopy that can tell investigators which elements are present, Bosak will help decide where the rover will drill for 20 or more samples, each of which will be the size of a stick of blackboard chalk. Collecting samples with a known geological context will provide a revolutionary opportunity to explore early life on Mars, says Bosak.
For the first time on a Mars mission, chemical information tied to the texture of the rock will be provided by an X-Ray fluorescence instrument called PIXL (Planetary Instrument for X-Ray Lithochemistry), which will be mounted on the rover’s arm. SCOL investigator Joel Hurowitz serves as deputy principal investigator for the instrument. Hurowitz, an assistant professor of geosciences at Stony Brook University, has worked on Mars missions since 2004 and hopes the 2020 mission will result in a set of measurements that allow the reconstruction of the ancient environment at Jezero Crater. The identity and composition of the rocks — information provided by PIXL — will be the crucial starting point for experiments. “Then we can go into the lab and try to figure out the range of chemical conditions — pH, temperature, redox state, salinity — that can make those minerals,” says Hurowitz. Once his lab has done the astrobiological forensics needed to paint a full chemical picture of the lake 3.8 billion years ago, the researchers can begin to understand what kinds of prebiotic chemical reactions may have occurred there.
Hurowitz’s lab is working now to experimentally precipitate minerals similar to those found in ancient sedimentary rocks on Mars and Earth. For example, spectroscopic analysis of Jezero Crater from orbit shows a predominance of magnesium carbonates. The lab is working to understand what conditions would precipitate magnesium carbonate and what this implies for salinity. The carbonates in other experiments are being used to generate calibration data that can ultimately aid in determining the temperature of the water in Jezero Crater’s long-gone lake using a technique that relies on the tendency of heavier carbon isotopes to clump together at lower temperatures.
Mars 2020 researchers credit SCOL with bringing together a large interdisciplinary team to assist with one of science’s greatest unsolved challenges. Getting at fundamental questions about life’s origins would not be possible without this multidisciplinary group, says Summons.
If the mission finds life on Mars, says Catling, the questions then will be: How different is it from ours? Is there a universal biochemistry? But even if only prebiotic precursors are found rather than biological remnants, scientists will nonetheless reap the reward of being able to refine and possibly expand their prebiotic schemes. “One of the most exciting parts of this work is using a particular planet as a test case for other planets when we consider the emergence of life on Earth,” says Hurowitz. And Bosak says about the 2020 mission, “This is a once-in-a-lifetime opportunity.”