Caltech Prepares for the Future of Sample Return Science

The Mars Perseverance rover on the surface of the Red Planet. Credit: NASA/JPL-Caltech/MSSS

Sample-return missions are vital for advancing our understanding of the solar system and to continue the search for life beyond Earth. They are also a chance for Caltech to extend its legacy of scientific leadership in planetary exploration.

By Ker Than

In the decades ahead, fragments of distant worlds will journey millions of miles through space to reach Earth, passed between robotic couriers in carefully orchestrated feats of technological and scientific daring. 

These precious pieces of planets, moons, asteroids, and comets have the potential to unlock the secrets of our solar system’s formation, offer clues regarding Earth’s future, and help answer one of the most profound scientific questions of all: Does extraterrestrial life exist?

“Either there is life elsewhere in the solar system, now or in the past, or there isn’t,” says John Eiler, the Robert P. Sharp Professor of Geology and Geochemistry and the Ted and Ginger Jenkins Leadership Chair of the Division of Geological and Planetary Sciences (GPS). “These are historical events, and you won’t solve the question without physical evidence. You have to actually get samples that record what happened.”

Gerald Wasserburg, the late John D. MacArthur Professor of Geology and Geophysics, Emeritus, with the Lunatic I mass spectrometer in 1969. Courtesy of Caltech Archives

For Caltech, this new era of sample-return science presents an opportunity to continue its role as a leader in the analysis of extraterrestrial materials, a position it has assumed for more than half a century. Caltech’s Gerald Wasserburg, the late John D. MacArthur Professor of Geology and Geophysics, Emeritus, constructed a high-precision mass spectrometer designed specifically for interrogating Moon samples collected by the Apollo missions. The instrument, dubbed Lunatic I, was a decisive factor in NASA’s decision to entrust lunar samples to Caltech for study.

In the 1970s, Caltech’s Ed Stolper (now the Judge Shirley Hufstedler Professor of Geology) was one of the first to suggest that meteorites found on Earth, known as shergottites, originated on Mars based on the gases found inside them. This was confirmed later by the Viking landers, which analyzed gases in the thin Martian atmosphere.

From lunar samples to Martian meteorites, several high-profile missions are poised to return samples of interest to scientists in the coming years. NASA and the European Space Agency (ESA), for example, are collaborating on the Mars Sample Return (MSR) mission, set to retrieve samples collected by the Perseverance rover by the 2030s. China’s Tianwen-2, planned for 2025, will target the near-Earth asteroid Kamoʻoalewa to collect and return samples. India’s Chandrayaan-4, expected to launch in 2028, will attempt to bring back material from the Moon’s south pole. And Japan’s Martian Moons eXploration (MMX) mission, launching in 2026, aims to return samples from Phobos, one of Mars’s moons, by the early 2030s.

“There are many different tasks that will be part of the study of returned materials,” Eiler predicts. “Some of them will be necessary but not important, others will be important but not super important, and others will be potentially transformational.”

Why Sample Return?

Taken by the Viking 1 lander shortly after it touched down on Mars, this image is the first photograph ever taken from the surface of Mars. It was taken on July 20, 1976. The primary objectives of the Viking mission, which was composed of two spacecraft, were to obtain high-resolution images of the Martian surface, characterize the structure and composition of the atmosphere and surface and search for evidence of life on Mars. Credit: NASA/JPL-Caltech

The advantages of sample-return science lie in the precision and detail of analysis that can be performed on Earth, Eiler explains. “When you get these materials back to Earth, you’re playing a completely different game because you have a depth of sophistication in your capacity to observe and describe that is so many orders of magnitude beyond what can be done remotely.”

Jonathan Lunine (PhD ’85), chief scientist at NASA’s Jet Propulsion Laboratory (JPL) and a professor of planetary science at Caltech, agrees, noting that field investigations—including space missions—are only the beginning of a geological science team’s process. Once the most promising samples have been accessed and gathered, they must be brought back to laboratories for deeper analysis. In planetary science, the next stage of exploring many solar system bodies will require the return of samples to Earth so that scientists can address questions that rovers and remote sensing cannot answer.

Caltech and JPL have been pivotal in pioneering this field, contributing to several groundbreaking sample-return missions over the years. These include the Genesis mission, which returned solar wind particles to Earth in 2004, and the Stardust mission, which brought back cometary particles from Comet Wild 2 in 2006.

More recently, researchers at Caltech, including those in the lab of François Tissot, professor of geochemistry and Heritage Medical Research Institute Investigator, have analyzed samples from the JAXA (Japan Aerospace Exploration Agency) Hayabusa2 mission, using advanced isotopic techniques to study material from the asteroid Ryugu.

However, these efforts only scratch the surface of what is possible. More asteroid samples are needed to resolve lingering discrepancies between asteroid-composition data sent from remote-sensing technologies and the findings from studies of meteorites that have landed on Earth.

Furthermore, scientists currently have a skewed perspective on meteorites, as most available samples come from the asteroid belt between Mars and Jupiter, with limited representation from the inner and outer solar system.

“To overcome this bias, we need sample-return missions to places like Mercury or Venus as well as objects far from the Sun, such as Kuiper Belt objects,” says Tissot, who uses isotope analysis to study the origins of the solar system and Earth’s early environment. 

By studying asteroid samples from both the inner and outer solar system, scientists could then piece together how these materials mixed and interacted to form planets like Earth, Mars, and Venus. “To really understand the solar system’s architecture, we need to know how much of the inner and outer solar system materials contributed to the formation of these planets,” Tissot says.

Mars has long been considered a prime target for a sample-return mission. “Understanding the origin of organic molecules detected by the last couple of Mars rovers or establishing a timeline of key Martian geological events can only be achieved by returning samples to Earth and then analyzing them,” Lunine says.

“Establishing a chronology of impacts on Mars will allow us to tie together the chronology of Mars with that of Earth,” he adds. “And that’s very, very important,” not only for placing Mars’s history within the broader context of the inner solar system’s evolution but also for understanding why Earth remains habitable while Mars is cold and dry.

A High Priority

Martian samples could also hold the key to solving another long-standing puzzle: Did life begin elsewhere in our own solar system? “If so, was it an independent form of life or the result of a transfer of material between Earth and Mars?” Lunine says.

Such questions have made MSR, a joint NASA and ESA mission designed to bring Martian soil and rock samples back to Earth for analysis, a high priority for the US National Academy of Science and NASA advisory groups. 

Shown here is an annotated representation of the 13 sample tubes containing rock-core samples that are being carried aboard NASA’s Perseverance rover as of Dec. 12, 2023, when the mission was marking its 1,000th Martian day, or sol, on the Red Planet. To the right of each sample is the associated abrasion patch that was created at the same location where the core was extracted. Credit: NASA/JPL-Caltech/ASU/MSSS

While rovers like Curiosity and Perseverance have detected organic molecules on Mars, such detections are not definitive proof of past biological activity, Eiler explains, because organic chemistry is ubiquitous in the universe, from interstellar space to planetary surfaces (organic molecules are compounds that contain carbon atoms bonded to hydrogen and often other elements like oxygen and nitrogen). In other words, organic molecules may be necessary for life, but they do not always constitute it.

“The chemistry of the universe is mostly organic,” Eiler explains. “You will bring back objects from other places in the solar system, and 100 percent of those samples will have organic molecules in them.” 

This abundance of carbon-rich material would present a significant challenge for scientists analyzing samples from Mars. Most organic matter in the solar system—whether in asteroids or as grains in space—does not exist as free-floating molecules. Instead, it exists in a solid “refractory” state in which molecules are entwined in complex webs.

“As soon as you go from a free molecule to a solid, our capacity to understand what we’re looking at dramatically diminishes,” Eiler says. 

To address this, scientists analyze the isotopic structure of organic solids. Isotopic content reveals unique details about a molecule’s origin, evolution, and processing history that cannot be gleaned from simple chemical analysis. Recent advances in mass spectrometry, such as the Orbitrap instrument Eiler helped develop (see “Signatures From the Past,” Caltech magazine, Fall 2024), have given scientists the tools to analyze isotopic structures in new ways and with unprecedented precision to draw fresh insights about a sample’s makeup.

A recent experiment demonstrates the Orbitrap’s potential: Last year, Eiler’s former graduate student Sarah Zeichner (PhD ’24), now a Resnick postdoctoral scholar at Caltech, used the instrument to analyze a tiny amount of material taken from an asteroid fragment collected by the Hayabusa2 sample-return mission. 

Zeichner developed a method to work with a mere six drops of material extracted from the asteroid that contain a concentration of molecules 1,000 times smaller than what is required for traditional isotope analysis. With the power of the Orbitrap technology, however, she was able to use the information in those six drops to discover that certain large carbon-based molecules trapped in this asteroid formed within cold molecular clouds in interstellar space, likely during or before the birth of the solar system.

The Next Wave

Looking ahead, scientists are also considering sample-return missions to the moons of planets beyond Earth. Both ESA and JAXA, for instance, have proposed sample-return missions to the Martian moon Phobos. “A sample from Phobos could help answer the question of whether Mars’s moons formed the same way as Earth’s, through impact, or if they were captured bodies,” Tissot says.

Icy moons in the outer solar system such as Europa and Enceladus are also high on scientists’ wish lists. These moons are believed to harbor subsurface oceans, and the potential for life in these environments has tantalized scientists for decades.

John Eiler in his lab. Credit: Lance Hayashida

“Most of the outer solar system is cold, and most of their surfaces are icy,” Eiler says. “The next major step for planetary science will be to explore these icy worlds in more detail.” 

However, there are significant hurdles to overcome. “We’re entering a period where many parties exploring the solar system will be reaching into icy regions, but we have never actually brought ice back to Earth in a way that was preserved and subject to study,” Eiler says. “We need to figure out how to bring ice back, how to sample it, how to handle it in the lab, and how to analyze it.”

As the new head of Caltech’s GPS division, one of Eiler’s goals is for its faculty, staff, and students to improve the accuracy of isotopic measurements and enhance scientists’ ability to study organic materials. This will be accomplished, he says, by creating an environment where both the scientific questions and the technologies needed to answer them can advance side by side.  

“We want to engineer a lab that can actually answer the question of whether life ever existed on Mars,” Eiler explains. “We’re at the cusp of making measurements that could radically improve our understanding of the solar system’s history. But we need to keep improving our instruments, making them more precise and efficient.”

Reflecting on the rapidly expanding field of planetary exploration and sample return science, Eiler notes how the increasing number of players—NASA, international space agencies like JAXA and ESA, private companies, and academic institutions—has fostered a uniquely dynamic and collaborative environment. 

“What is Caltech’s role in a setting like this?” Eiler asks. “We’ve never made our mark by being bigger than anyone, nor do we aim to be better than anybody in particular. Instead, we strive to be different—to find the spaces in fundamental science that no one else is exploring. That’s where we want to go.”