Speaker
Description
In its travel through the Milky Way, the Sun traverses a variety of Galactic environments, including dense interstellar clouds. Astronomical effects on Earth’s past climate have been limited to 10,000-year scales variations in Earth’s orbital parameters while our recent studies suggest that longer-term climate shifts that occur every few million year may be linked to compression of the heliosphere (the “cocoon” formed by the solar wind) when the Sun crosses dense clouds as it travels through the Milky Way. During such periods Earth was exposed to increased radiation and large amounts of hydrogen, potentially altering its climate. These events are consistent with independent $^{60}$Fe records indicating nearby astrophysical encounters at ~2–3 and ~6–7 million years ago (Ma), as well as $^{10}$Be anomalies near ~10 Ma that may reflect prolonged exposure to enhanced radiation during a cold cloud crossing. A convergence of recent advances across astronomy, space physics, and paleoclimate creates an unprecedented opportunity to rigorously test this hypothesis. We now have high-precision astrometry from the Gaia mission that allows one to reconstruct the Sun’s trajectory through the Galaxy and to identify, with remarkable accuracy, the interstellar structures it has encountered over the past ~10 Ma. Major theoretical and modeling advances now enable quantitative predictions of how the heliosphere evolved during these encounters. I this talk I will discuss our recent work that show that during such periods, Earth was exposed to increased radiation and large amounts of hydrogen. I will discuss our preliminary results that show that the increase in hydrogen augmented mesospheric water vapor, leading to increased formation of both polar mesospheric clouds and polar stratospheric clouds. The amount of radiation that Earth experiences from such events depends on the duration of the crossing and the amount of compression of the heliosphere, with implications for Earth’s climate. I will discuss our results as well that indicate that high temporal 10Be signal in ocean records and ice cores can distinguish between alternative scenarios such as supernova explosions and cold cloud crossings. Finally, I will comment on the consequences of these studies for understanding long-term climate variability and assessing planetary habitability across the galaxy.