NASA researchers have mapped how the Sun's interactions with the galaxy shaped Earth's climate across billions of years, according to two agency-funded studies.
In a study published in the Annual Review of Astronomy and Astrophysics, scientists at NASA’s SHIELD center modeled the path of the heliosphere through the Milky Way. The heliosphere is a protective bubble generated by solar wind. Boston University researcher Merav Opher and her team found that the solar system crossed dense interstellar gas and dust clouds at least three times over the past several million years: roughly 2 to 3 million, 6 to 7 million, and 13 to 14 million years ago.
These encounters compressed the heliosphere inside Earth's orbital path, leaving the planet exposed to interstellar radiation and cold hydrogen clouds. NASA stated that the influx of hydrogen altered upper-atmospheric dynamics and increased water vapor, driving surface cooling and potential ice ages. Interstellar dust signatures from deep-sea sediment cores, Antarctic snow, and lunar rock samples match the timing of the modeled compressions.
Faint Young Sun warming
A separate study led by Vladimir Airapetian at NASA’s Goddard Space Flight Center examined how Earth maintained liquid water three billion years ago, when the Sun was 70 percent as bright as it is today.
Airapetian used observations from NASA's retired Kepler space telescope, which documented daily superflares on young Sun-like stars. To test the impact of those solar storms, researchers mixed molecular nitrogen, ammonia, carbon dioxide, and carbon monoxide inside a sealed chamber and bombarded the gases with protons.
The proton collisions triggered the formation of nitrous oxide, a greenhouse gas 300 times more potent than carbon dioxide. Computer models published in Astrophysical Journal Letters showed that even if ultraviolet light destroyed 90 percent of the compound, the remaining nitrous oxide would keep equatorial surface temperatures at approximately 41 degrees Fahrenheit (5 degrees Celsius). That temperature range accelerates the formation of complex amino acid chains necessary for prebiotic synthesis.
