When Earth was a mass of newly minted rock some 4.5 billion years ago, the solar system was a cold place. Physicists predict our young sun put out some 15% to 25% less energy than it does today—enough to freeze over Earth’s oceans and make Mars even colder. Yet ancient rocks suggest water flowed across both planets, posing a perplexing puzzle.
For years, climate modelers solved this so-called “faint young sun” paradox by proposing that ancient atmospheres on both planets had the right composition of greenhouse gases to insulate them and keep them above freezing. But if the young sun reached its current weight only after a diet—shedding perhaps 5% of its early mass in a stellar wind of escaping particles—it would have burned brighter in its past than predicted, resolving the paradox. The only problem with that hypothesis? Scientists have had no way of knowing whether this stellar slim-down happened.
Now, astronomers say they have come up with a potential “fingerprint” of the sun’s ancient mass—climate cycles preserved in bands of martian rocks. To find their marker, Christopher Spalding, a planetary astronomer at Yale University, geobiologist Woodward Fischer at the California Institute of Technology in Pasadena, and astronomer Gregory Laughlin of Yale started with an orbital cycle that both Earth and Mars experience. As the solar system’s planets revolve around the sun, their own gravity tweaks each other’s orbits.
One of many such interactions pulls the orbits of Earth and Mars back and forth between a more circular path and a more elliptical one. This pattern, a relative of the cycles responsible for Earth’s ice ages, repeats every 405,000 years. According to the team’s simulations, that cycle has kept dependable time over the entire history of the solar system.
Spalding’s team proposes that, as their changing orbits took them closer and farther from the sun, the climates on Earth and Mars shifted, leaving cyclical striping patterns in sedimentary rock, like the layered bands on the walls of Scandinavian fjords. When the orbits of the early planets took them closer to the sun, for example, already wet areas would receive more heat, more rainfall, or snow, and thus more erosion. Layers of sediment would be relatively thicker at these times than in colder parts of the cycle.
And that means it could be used to track the mass of the sun. If the sun were 5% heavier a few billion years ago, it would have tugged the planets harder, increasing the cycle’s frequency by a matching 5%, to roughly once per 386,000 years.
Earth, unfortunately, preserves little of its ancient rock, because of the churn of plate tectonics. But Mars does. Spalding suggests a future rover there, armed with dating equipment, could do the trick, he reports in a paper accepted to The Astrophysical Journal Letters. “You’ll have to do it as a side project,” he says, “because everyone wants to find life more than they want to find 400,000-year banding.”
In 2006, another team laid the groundwork for Spalding’s hypothesis, pointing out the linear relationship between the sun’s mass and the larger family of interplanetary orbital cycles. But they stopped at that point because they felt “the climate record, or the geological record, does not have enough resolution,” says Renu Malhotra, a planetary scientist at the University of Arizona in Tucson who led the earlier study. She has similar reservations about Spalding’s approach, she says.
Meanwhile, Dawn Sumner, a geobiologist at the University of California, Davis, and a member of NASA’s Curiosity rover team says modern Mars rovers could do at least part of the work Spalding’s team has suggested. Curiosity has already measured the thicknesses of sedimentary layers on exposed slopes, and the newly selected landing site for the 2020 rover seems to have steep cliffs, which may reveal similar stripes. “If we found the right spot, this is something people will do,” she says.
But Sumner is less sanguine about dating the various layers, crucial to reveal minute changes in orbital cycles. On Earth, she says, that kind of precision dating requires lots of fieldwork to find the best samples and cart them back to the lab. A rover, by contrast, would be hard-pressed to do it all on site. Facing that obstacle, she says, “It’s probably impossible to test it in the next few decades on Mars.”