  Geological activity is thought to be important for the origin of life and for
maintaining planetary habitability. We show that transient sulfate aerosols
could be a signature of exoplanet volcanism, and therefore a geologically
active world. A detection of transient aerosols, if linked to volcanism, could
thus aid in habitability evaluations of the exoplanet. On Earth,
subduction-induced explosive eruptions inject SO2 directly into the
stratosphere, leading to the formation of sulfate aerosols with lifetimes of
months to years. We demonstrate that the rapid increase and gradual decrease in
sulfate aerosol loading associated with these eruptions may be detectable in
transit transmission spectra with future large-aperture telescopes, such as the
James Webb Space Telescope (JWST) and European Extremely-Large Telescope
(E-ELT) for a planetary system at a distance of 10 pc, assuming an Earth-like
atmosphere, bulk composition, and size. Specifically, we find that a S/N of
12.1 and 7.1 could be achieved with E-ELT (assuming photon-limited noise) for
an Earth-analog orbiting a Sun-like star and M5V star, respectively, even
without multiple transits binned together. We propose that the detection of
this transient signal would strongly suggest an exoplanet volcanic eruption, if
potential false positives such as dust storms or bolide impacts can be ruled
out. Furthermore, because scenarios exist in which O2 can form abiotically in
the absence of volcanic activity, a detection of transient aerosols that can be
linked to volcanism, along with a detection of O2, would be a more robust
biosignature than O2 alone.
