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08/10/2026 | Press release | Distributed by Public on 08/10/2026 06:14

Mark Thiemens Receives the Magellanic Premium, America’s Oldest Medal

Published Date

August 10, 2026

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University of California San Diego Distinguished Professor of Chemistry Mark Thiemens is the 43rd recipient of the Magellanic Premium, given by the American Philosophical Society (APS). Thiemens was recognized for his groundbreaking work in mass-independent isotope fractionation, his discovery of which has transformed the way we study everything from climate change to planetary evolution. APS, created in 1743 by Benjamin Franklin, is America's oldest learned society, and the premium, first given in 1790, is America's oldest medal.

"UC San Diego has always been at the frontier of science and Mark has been a leader in the frontier, traveling to the edges of the Earth in pursuit of discovery," said Chancellor Pradeep K. Khosla. "This unique recognition from the American Philosophical Society underscores the significant impact Mark's work has had in solving real-world problems and helping us answer deeply human questions."

Thiemens received the Magellanic Premium during the American Philosophical Society's symposium at the Royal Society in London, of which Thiemens is a fellow. In a statement, APS CEO Patrick Spero said Thiemens' "groundbreaking discoveries have deepened our understanding of the natural world, including the origins of the planet itself, while also providing scientists with powerful tools to study and address some of the most urgent environmental challenges facing the planet today."

To Mars and Back Again

Each chemical element has a fixed number of protons. Isotopes are variations of an element with a differing number of neutrons (see sidebar). Thiemens likens studying isotopes to chemical forensics because they leave behind "fingerprints" that can tell us a lot about everything from climate history and the sources of greenhouse warming gases to reaction mechanisms and solar system planetary formation. Throughout his long career, studying isotopes has taken Thiemens around the world, and his measurements have even made it into space.

Until the 1980s, certain patterns found in meteorites were thought to be the result of massive explosions from dying stars (called supernovae) because scientists didn't think the patterns could be caused by a chemical process. However, Thiemens found a new way to study oxygen isotopes in meteorites through a mechanism called mass-independent isotope fractionation (MIF). MIF is a chemical or physical process that separates isotopes based on something other than their differences in mass, and thanks to Thiemens's discovery, researchers now know that these meteorite patterns can be identically created by chemical reactions. Because oxygen is so abundant on rocky planets like the Earth, this insight helps explain a major event in the formation of our solar system: that planets like Earth are not formed from supernovae explosions.

Then Thiemens wondered what other areas of science might benefit from his MIF discovery. The answer was atmospheric chemistry, specifically the ozone layer. The growing hole in the ozone layer was one of the biggest environmental issues of the '90s because the hole allowed more of the sun's harmful UV rays to make it to Earth, where it adversely affected everything from human health to agriculture and marine life.

Using high altitude balloons and rockets he built, Thiemens collected samples from the upper atmosphere, providing new insight into the chemical processes occurring there extending to the atmosphere-space boundary. In addition to UV screening, the upper atmosphere also removes greenhouse agents-a process that is now better understood because of isotopes.

Thiemens's research, along with faculty colleague Bill Trogler, identified the origins of one of the most damaging chemicals, nitrous oxide, whose levels in the atmosphere have been steadily increasing since the Industrial Revolution. Thiemens' lab was able to pinpoint adipic acid, used to manufacture nylon, as responsible for 30% of the manmade nitrous oxide in the atmosphere. The research, published in 1991, had an immediate effect when Dupont, one of the world's largest producers of nylon, announced it would stop using adipic acid in their manufacturing process. Other companies followed suit, thus keeping an amount of greenhouse gas out of the atmosphere that is nearly equal to California's yearly CO2 emissions.

When Thiemens looks back on his work, he feels optimistic: "Nitrous oxide lasts for more than 100 years, so this was a big win for society. I'm really proud of that one."

Thiemens continued to study the ozone layer. Atmospheric ozone at ground level is made from oxygen, a byproduct of photosynthesis. It is an important component in the complex oxygen processes in the atmosphere that determine how long pollutants exist and how they are removed.

Related content

The Incredible Isotope

Atoms with the same number of protons, but different numbers of neutrons are called isotopes. They share many of the same chemical properties, but the differing number of neutrons give them different masses and, therefore, different physical properties. There are stable isotopes, which do not emit radiation, and there are unstable isotopes, which do emit radiation. Stable isotopes are often used to study the history and source of water, while unstable isotopes can be used in environmental tracing.

Thiemens received the Magellanic Premium during the American Philosophical Society's symposium at the Royal Society in London, of which Thiemens is a fellow.

Trying to find greater insight, Thiemens headed to the South Pole, where he dug up several tons of ice to sample oxygen levels from the last 100 years. Later he went to the opposite end of the world to Greenland, where he drilled into ice cores that allowed him to sample oxygen levels going back three millennia. Both provide a chemical signature of the Earth's changing climate from manmade sources as well as major natural events, such as volcanic eruptions.

"We can see what the atmosphere looked like. We can see oxidation changes in the polar ice, as well as the detailed changes in the global environment," he stated. "You just have to dig in a pit in below zero weather at 10,000 feet to get the samples. It's cold and hard to breathe, but except for that, it's a lot of fun."

Still, Thiemens wondered where else isotopes could lead him, and the answer was Mars. Mars once held water, but scientists didn't know how it got there or where it went.

"Water makes sulfates. When my water glass goes dry, it leaves a white film behind. That's carbonate and sulfate," he stated. "The same thing happened on Mars when it had water which was later lost, leaving behind a record that is recorded in its meteorites as carbonate."

Thiemens was able to detail processes that occurred between the atmosphere and surface of Mars over billions of years. Water escaping from the upper atmosphere provides a likely explanation for how water left Mars and the isotopes support this, although uncertainties remain.

In the last decade, Thiemens has crisscrossed Tibet, tracking how quickly the ice is melting in the Himalayas. Forty percent of the world gets their drinking water from the Himalayas; the rapidity of the ice melt could be a humanitarian crisis for half the world's population. Thiemens hopes the incredible isotope will, once again, help find a solution.

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