NASA-Funded Studies Link Sun's Ancient Journey and Eruptions to Earth's Climate

The Sun does more than provide Earth light and heat. Two NASA-funded studies suggest events from the Sun's distant past may have shaped Earth's climate, from ancient temperature shifts to keeping the young planet warm enough for liquid water.

One study, from NASA's SHIELD (Solar Wind with Hydrogen Ion charge Exchange and Large-Scale Dynamics) center, examined how the heliosphere — the protective bubble the Sun's solar wind blows around the solar system — has moved through the Milky Way. Published Aug. 21 in the Annual Review of Astronomy and Astrophysics, the study was led by Merav Opher, SHIELD's principal investigator at Boston University.

Using computer simulations, Opher and colleagues found the solar system passed through cold, dense interstellar “cold clouds” at least three times in the past several million years: roughly 2 to 3, 6 to 7, and 13 to 14 million years ago. These clouds may have compressed the heliosphere enough to shrink it smaller than Earth's orbit, briefly leaving Earth outside the Sun's protective bubble. The timing matches elements linked to interstellar dust found in deep-sea sediment cores, Antarctic snow, and lunar samples from those periods.

In the simulations, exposing Earth's atmosphere to a dense, cold cloud of galactic hydrogen raised atmospheric water vapor and altered upper-atmosphere conditions that eventually reached the surface, possibly contributing to long-term climate changes, including ice ages. SHIELD, one of NASA's DRIVE (Diversify, Realize, Integrate, Venture, Educate) Science Centers, is building a “digital twin” of the heliosphere.

A second study, led by Vladimir Airapetian of NASA's Goddard Space Flight Center in Greenbelt, Maryland, examined the Faint Young Sun paradox: about three billion years ago the Sun was only 70% as bright as today, so Earth should theoretically have been frozen solid, yet geological evidence shows stable liquid water existed long before then. The findings suggest powerful solar eruptions from the young Sun may have helped generate strong greenhouse gases in Earth's early atmosphere.