Presented by: Prof. Merav Opher

Affiliation: Boston University

 Location: 3853 Slichter Hall

 Abstract:  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 60Fe records indicating nearby astrophysical 
encounters at ~2–3 and ~6–7 million years ago (Ma), as well as 10Be 
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. In 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.