Ken Farley

Chapter 1 · 1986 – 2012 A noble gas chemist

Ken Farley did not start with Mars. For twenty-five years he learned to make nature’s most discreet atoms speak: the noble gases trapped in the rocks and minerals of Earth.

Yale, Scripps, Caltech

He earned a bachelor’s degree in chemistry from Yale University in 1986, then, in 1991, a doctorate in Earth science from the Scripps Institution of Oceanography, University of California San Diego. In 1993 he began his professorial career at the California Institute of Technology, Caltech, where he became the W. M. Keck Foundation Professor of Geochemistry in the Division of Geological and Planetary Sciences.

Reading time in atoms

His research centers on developing and applying geochemical techniques, especially the isotopes of noble gases such as helium or argon, which react with almost nothing and so keep a memory of what has happened to them. He applies them to a wide range of questions about Earth and the solar system: dating rocks, the chemical evolution of Earth, the behavior of these gases in minerals. In 1994, for example, with Caltech colleagues, he studied the noble gases of a volcanic arc in Papua New Guinea to understand how volatile substances are recycled where one ocean plate dives beneath another.

Sources: NASA Ames Research Center, “The Mars 2020 Perseverance Mission: First step in Mars Sample Return,” lecture of February 2, 2021, speaker biography (Yale in 1986, Scripps in 1991, Caltech since 1993, Keck professorship, noble gases, geochronology, Earth’s evolution); K. Farley, B. McInnes and D. Patterson (Caltech), “Recycling of volatiles at subduction zones: Noble gas evidence from the Tabar-Lihir-Tanga-Feni arc of Papua New Guinea,” 1994 (NTRS 19950015370). The explanation of the low reactivity of noble gases is a general notion.

Chapter 2 · 2013 – 2016 Dating a rock on Mars

Until 2013, no one had ever dated a rock directly on another planet. Ken Farley and his coauthors did it with the Curiosity rover, using a sixty-year-old method.

Potassium and argon

Before Curiosity, the ages of Martian terrains were estimated by counting impact craters: the more a surface has, the older it is, and that scale had been calibrated on the Moon thanks to the rocks returned by Apollo. Farley’s team adapted a method used on Earth for sixty years: an isotope of potassium slowly turns into argon, an inert gas. Argon escapes when a rock melts, then builds up again once it has hardened. Measuring that argon gives the rock’s age.

The second rock drilled by Curiosity, nicknamed “Cumberland,” thus became the first to be dated from its minerals on another planet: between 3.86 and 4.56 billion years old. The result was announced on December 9, 2013. “The age is not surprising, but what is surprising is that this method worked using measurements performed on Mars,” Farley said. “When you’re confirming a new methodology, you don’t want the first result to be something unexpected. Our understanding of the antiquity of the Martian surface seems to be right.”

How long in the open?

The team also measured how long the rock had been within arm’s reach of the surface, where cosmic rays produce gases in its atoms that Curiosity can measure. Three different gases gave an exposure age of 60 to 100 million years: the layers that shielded it had been stripped away fairly recently, probably by wind eating away at the cliffs. “The exposure rate is surprisingly fast,” he said. “The place where you’ll find the rocks with youngest exposure age will be right next to the downwind scarps.” Those are the best places to look for organic molecules, which radiation destroys over time.

In 2016, with Paulo Vasconcelos and others, he applied the same methods to the Windjana sandstone at Mount Sharp; the ages obtained there were discordant, a sign of a more complicated history.

Sources: NASA, “NASA Rover Results Include First Age Measurement on Mars and Help for Human Exploration,” December 9, 2013 (crater counting, Apollo, 60-year-old potassium-argon method, Cumberland, 3.86 to 4.56 billion years, quotations, exposure ages of 60 to 100 million years, three gases, scarps, organics); P. M. Vasconcelos, K. A. Farley et al., “Discordant K-Ar and Young Exposure Dates for the Windjana Sandstone, Kimberley, Gale Crater, Mars,” 2016 (NTRS 20170003168).

Chapter 3 · 2014 – 2020 Choosing Jezero, preparing the tubes

By 2014, Ken Farley was project scientist of Mars 2020, the Perseverance rover mission. For the first time, a rover was to collect Martian samples meant to return to Earth one day.

A site long out of reach

On July 31, 2014, he took part, as Mars 2020 project scientist, in the presentation of the future rover’s instruments. On November 19, 2018, NASA announced the site: Jezero crater, an ancient lake with its delta. “The Mars community has long coveted the scientific value of sites such as Jezero Crater, and a previous mission contemplated going there, but the challenges with safely landing were considered prohibitive,” he explained. “But what was once out of reach is now conceivable, thanks to the 2020 engineering team and advances in Mars entry, descent and landing technologies.” Jezero was chosen, he would say, because it is such a promising location for finding organic molecules and other potential signs of microbial life.

Earth’s oldest fossils

In August 2019, scientists from Mars 2020 and the European ExoMars mission traveled to the Pilbara, in Australia. “The Pilbara Outback is home to the oldest confirmed fossilized lifeforms on Earth, called stromatolites,” Farley explained. “If we can better understand how these fossils came to be here — and the nearby geological signposts that help point the way to them — we’ll be that much more prepared when hunting for signs of life on Mars.”

Cleaner than clean tubes

The samples would be sealed in tubes that had to return to Earth uncontaminated by terrestrial life. “Compared to Mars, Earth is filled with evidence of the life that covers our planet,” he said. “We needed to remove those signs so thoroughly that any scant evidence remaining can be confidently detected and differentiated when these first samples are returned.”

Sources: NASA, “NASA Hosts 3 p.m. EDT Teleconference with Mars 2020 Principal Investigators,” July 31, 2014 (Ken Farley, Mars 2020 project scientist); NASA, “NASA Announces Landing Site for Mars 2020 Rover,” November 19, 2018 (quotation); NASA/JPL, “7 Things to Know About the Mars 2020 Perseverance Rover Mission,” July 8, 2020 (Jezero chosen as a promising location for organic molecules); NASA, “Scientists Explore Outback as Testbed for Mars,” August 23, 2019 (quotation); NASA, “A Martian Roundtrip: NASA’s Perseverance Rover Sample Tubes,” December 22, 2020 (quotation). The July 2014 teleconference is the earliest source consulted in which he appears in that role; we do not know the exact date of his appointment.

Chapter 4 · 2021 – 2024 Tube 266 and Cheyava Falls

Perseverance landed in Jezero on February 18, 2021. For Ken Farley, the work he had been chosen for began: deciding, rock after rock, what Earth would one day receive from Mars.

The first sample

On September 6, 2021, NASA announced that Perseverance had filled its first tube, number 266, with a rock core. “Getting the first sample under our belt is a huge milestone,” Farley said. “When we get these samples back on Earth, they are going to tell us a great deal about some of the earliest chapters in the evolution of Mars.” A few days later he summed up: “It looks like our first rocks reveal a potentially habitable sustained environment. It’s a big deal that the water was there a long time.” Further on lay “South Séítah,” a series of ridges covered by sand dunes, boulders and rock shards that he likened to “broken dinner plates.” He also wanted a sample of the atmosphere: the composition of Martian air is among the questions sample return is meant to answer.

“I was beginning to despair”

One puzzle held out: where did the crater-floor rocks come from? In December 2021, the PIXL instrument examined an abraded rock in South Séítah. “I was beginning to despair we would never find the answer,” Farley admitted. “But then our PIXL instrument got a good look at the abraded patch of a rock… and it all became clear: The crystals within the rock provided the smoking gun.” Those crystals had formed by settling slowly in a cooling magma, and the rock had then been altered by water several times: “a treasure trove” for dating events in Jezero. In August 2022 he was lead author of one of the Science papers presenting these results: these igneous rocks will tell when the lake existed, and when Mars’s climate turned cold and dry.

The delta, then the hoped-for rock

The rover then climbed the delta. In January 2023 it completed the “Three Forks” depot, a reserve of tubes left on the ground for the future return mission. The rocks at the base of the delta, Farley explained, were laid down in a lake; those above, by a river flowing into it. In 2024 a rock from the crater’s “margin,” made almost entirely of minerals formed in water, was sampled: “To put it simply, this is the kind of rock we had hoped to find when we decided to investigate Jezero Crater.”

In July 2024, in the ancient Neretva valley, Perseverance examined a rock named “Cheyava Falls.” “Cheyava Falls is the most puzzling, complex, and potentially important rock yet investigated by Perseverance,” Farley said: organic matter, colorful spots that could indicate chemical reactions usable by microbes, evidence that water passed through, but an origin still uncertain. “Scientifically, Perseverance has nothing more to give.” To understand it, the sample would have to be brought back to Earth.

Sources: NASA, “NASA’s Perseverance Rover Collects First Mars Rock Sample,” September 6, 2021 (tube 266, quotation); NASA, “NASA’s Perseverance Rover Collects Puzzle Pieces of Mars History,” September 10, 2021 (quotation, South Séítah, “broken dinner plates”); NASA, “NASA’s Perseverance Plans Next Sample Attempt,” August 26, 2021 (atmospheric sample); NASA/JPL, “NASA’s Perseverance Mars Rover Makes Surprising Discoveries,” December 15, 2021 (quotations, “treasure trove”); NASA, “NASA’s Perseverance Makes New Discoveries in Mars’ Jezero Crater,” August 25, 2022 (lead author); NASA, “NASA’s Perseverance Rover Completes Mars Sample Depot,” January 30, 2023 (Three Forks, lake and river); NASA, “Rock Sampled by NASA’s Perseverance Embodies Why Rover Came to Mars,” 2024 (quotation); NASA, “NASA’s Perseverance Rover Scientists Find Intriguing Mars Rock,” July 25, 2024 (Neretva Vallis, quotations). The landing date comes from NASA, “NASA’s Perseverance Rover Gives High-Definition Panoramic View of Landing Site,” February 24, 2021.

Chapter 5 · 2024 – 2026 Beyond the crater, the Mars that came before

In December 2024, Perseverance reached the top of Jezero’s rim. It left the rocks that had filled the crater to look at those that were there before it.

Pieces of the early crust

“The Northern Rim campaign brings us completely new scientific riches,” Ken Farley said on December 12, 2024. The rover was moving from rocks that partially filled Jezero, after the massive impact that dug it about 3.9 billion years ago, to rocks from deep inside Mars thrown upward to form the rim. “These rocks represent pieces of early Martian crust and are among the oldest rocks found anywhere in the solar system. Investigating them could help us understand what Mars — and our own planet — may have looked like in the beginning.”

A new role

In May 2025, NASA presented him as Perseverance’s deputy project scientist, with Katie Stack Morgan as acting project scientist. “The last five months have been a geologic whirlwind,” he said. The rover then took on “Krokodillen,” rocks formed even before Jezero, during the Noachian, the oldest period of Mars. A potential biosignature found there, he explained, would date from a far earlier epoch than that of Cheyava Falls.

In July 2026 he summed up what Perseverance was now reading in layers thrown out by ancient impacts: “On Earth, our earliest geologic history has been fundamentally broken up, deformed, and erased by plate tectonics. Because Mars lacks plate tectonics to recycle its crust, this ancient record remains intact, giving us a rare glimpse into a geological time period that doesn’t exist on our own planet.”

Sources: NASA, “NASA’s Perseverance Rover Reaches Top of Jezero Crater Rim,” December 12, 2024 (quotations, 3.9 billion years); NASA, “NASA’s Perseverance Mars Rover to Take Bite Out of ‘Krokodillen,’” May 19, 2025 (deputy project scientist, Katie Stack Morgan acting, quotation, Noachian, Cheyava Falls); NASA, “NASA’s Perseverance Rover Reads Record of Ancient Mars Impacts,” July 15, 2026 (quotation).