Observations of the Interstellar Medium Along the Historical Solar Trajectory: Temporal Evolution of the Heliosphere Over the Last 40 Million Years

Over the course of our motion through the Galaxy, the Solar System has encountered many interstellar environments of varying characteristics. ISM density variations spanning six orders of magnitude are commonly seen throughout the general Galactic environment, and a sufficiently dense cloud within this range has the potential to compress the heliosphere to within one AU. We present a reconstruction of the density profile for the clouds we have most recently passed through based on high-resolution optical spectra towards nearby stars. Observations were made of interstellar NaI and CaII absorption towards 43 bright stars along the historical path of solar motion in our orbit around the center of the Galaxy. No absorption is seen out to a distance of 120 pc (consistent with the Local Bubble), but a complex collection of absorbers (up to 10 components) is seen in stars between 130 and 610 parsecs. A possible link between our local interstellar environment, cosmic rays, and our planetary climate has long been a subject of interest. Compression of the heliosphere (one of our three cosmic ray shields) due to passage through a dense interstellar cloud could have significant effects on Earth: global cooling (atmospheric dust deposition), weather patterns (cloud nucleation), and evolution (DNA mutations). A time scale of interaction with each ISM component in this path can be constructed and ultimately compared with Earth's geologic record. While a number of assumptions go into translating the ISM features observed today, with the interstellar environment with the Sun in the past, this work at least provides a plausible temporal evolution of the heliosphere. Indeed, we now know that many, if not all, nearby stars have exoplanets. These are traversing their own unique paths through the ISM. As we plan to evaluate planet habitability, the temporal evolution of the helio/astrosphere and cosmic ray flux, as dictated by the surrounding interstellar medium will be of critical importance. Column density profile along historic solar path, determined from the sodium (left) and calcium (right) absorption profiles detected toward 43 nearby stars (horizontal lines). The distance axis is also displayed as a function of time, based on current values of the solar motion. Significant interstellar material is detect at, and beyond, the Local Bubble boundary.