Two NASA Studies Trace How the Sun's Ancient History Shaped Earth's Climate

Two new NASA-funded studies trace how events in the Sun's distant past may have shaped Earth's climate. Our solar system sits inside the heliosphere, a bubble of charged solar wind that the Sun has blown outward for its entire 4.6-billion-year history as it orbits the center of the Milky Way. In a paper published Aug. 21 in the Annual Review of Astronomy and Astrophysics, researchers at NASA's SHIELD center used computer modeling to reverse-engineer the heliosphere's path through the galaxy.

The team, led by Merav Opher of Boston University, found that the Sun has passed through dense interstellar "cold clouds" at least three times in the past few million years — roughly 2 to 3 million, 6 to 7 million, and 13 to 14 million years ago — each time compressing the heliosphere to smaller than Earth's orbit and exposing the planet's atmosphere directly to interstellar gas. The simulations line up with geologic evidence: elements common in interstellar dust show up in deep-sea sediment cores, Antarctic snow and lunar samples from those same time periods, and the exposures appear linked to shifts in Earth's ancient climate, including possible ice ages.

A separate study addresses an older puzzle: 3 billion years ago, the young Sun was only 70% as bright as it is today, which should have left Earth frozen solid — yet geological evidence shows liquid water existed anyway, a contradiction known as the Faint Young Sun paradox. NASA's Vladimir Airapetian proposes that the young Sun, like other young Sun-like stars observed erupting with massive superflares by NASA's retired Kepler telescope, regularly bombarded Earth with high-energy particles.

To test the idea, Airapetian's team mixed nitrogen, ammonia, carbon dioxide and carbon monoxide in a sealed chamber to simulate early Earth's atmosphere, then fired protons into it to mimic a superflare's particle bombardment. The reaction produced nitrous oxide, a greenhouse gas roughly 300 times more potent than carbon dioxide, suggesting the young Sun's outbursts could have warmed the planet enough to keep water liquid.