Mars Makes Two

Mars is Earth’s next-door neighbor, yet the Red Planet is utterly alien—frozen, arid, and roiled by massive dust storms. However, this was not always so. Data sent back from Mars, by JPL’s robotic explorers, paint the picture of two planets that once may have been much more similar. Woody Fischer, Caltech professor of geobiology, is helping to decode the history hidden in Mars’ rocky terrain.

This work in comparative planetary evolution—a funding priority of Break Through: The Caltech Campaign—engages some of humankind’s biggest questions: How did we get here? Are we alone? The new knowledge that Fischer and other scientists create is rewriting what we know about planets in general, our own home world, and even life itself.

“The idea is that N now equals two,” he says, invoking shorthand for the size of the population in a research project. “We’ve been studying Earth history for hundreds of years, but how much of that is idiosyncratic? Well, now we have two planets we can play off one another.”

One point of comparison arises from the ongoing search for signs of past life and habitability on Mars. Scientists began by divining whether water ever flowed there. The unexpected answer: Mars was wetter and warmer billions of years ago. Fischer has used Curiosity rover data to help assemble pieces of the puzzle, including a study he coauthored, which reveals an ancient Martian river delta.

“That’s one example from a set of discoveries—now dozens of papers—indicating that Mars has large packages of sedimentary rock,” he says. “It has this geological record of sediment moved around by rivers, of lakes, and of something we maybe could call an ocean. We’re learning more and more about Mars all the time. Maybe we can figure out why Mars and Earth didn’t evolve along the same paths.”

Tinier, Earlier

In his Earthbound studies, Fischer gazes back into prehistory. He took inspiration from his grandfather, a paleontologist, but chose to look both closer—down even to the nanoscale—and further back—to a time before the emergence of multicellular life.

Microbes generally do not leave behind fossils. But they do deposit subtle clues in layers of rock—chemical imprints of processes that keep them alive. Fischer and his research group trace back these reactions by applying new techniques and tools such as electron microscopes and mass spectrometers. Bit by bit, the researchers unravel genetic and molecular details that make up the intertwined stories of life and Earth.

The chance to turn the same approach toward Mars is tantalizing—especially as revelations from the planet throw established ideas about our own world into disarray.

“Mars has given us this opportunity to shake ourselves free of some core concepts about planetary evolution and planetary habitability. It’s causing us to question even the most basic ideas.”
- Woody Fischer

Breathe In, Breathe Out

As surprises from Mars hit home for Fischer, new avenues of investigation emerge.

For example, his research group peers back about 2.3 billion years to explore a pair of supremely important, intimately related revolutions on Earth: the evolution of photosynthesis and the resulting emergence of oxygen in the atmosphere. In a 2017 study, Fischer’s team plotted the family tree of cyanobacteria, likely the first life form to produce oxygen from sunlight, carbon dioxide, and water.

“You may say, ‘OK, we know you need photosynthesis to make oxygen,’” he says. “That’s very reasonable. That’s what Earth did.”

But something different appears to have happened on ancient Mars, strikingly Earth-like but with no clear signs (as of yet) that it hosted life. Although oxygen is scarce there today, Martian rock from billions of years ago bears chemical fingerprints matching those in Earth’s geologic record after the gas became plentiful. That news opens previously undreamt opportunities for Fischer’s future studies.

“Now Mars has a complicated oxygen cycle that we don’t understand at all,” he says. “So maybe planets have ways of developing oxygen in the absence of photosynthesis. You can start to consider those kinds of things when N equals two.”

To support research in comparative planetary evolution, contact Ellen Jampol, senior director of development for the Division of Geological and Planetary Sciences, at (626) 395-4374 or at

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