Presentation #101.07 in the session Cometary Volatiles.
We report on the activity of comet C/2014 S2 (PANSTARRS). We use photometric data from PanSTARRS, ATLAS, CFHT, DECam, and amateur data, beginning at a heliocentric distance r = 5.00 au through r = 8.13 au post-perihelion. We present a model of the activity that incorporates surface water-ice sublimation and sub-surface CO2 sublimation that matches the photometry and gas production rates. Using this model, we estimate the nucleus radius to be 3.2 km (upper limit 3.7 km) assuming an albedo of 0.04. From NEOWISE observations, we calculate production rates of QCO2 = (4.28 ± 0.06) x 1027, (3.09 ± 0.04) x 1027, (2.61 ± 0.01) x 1027, and (6.10 ± 0.08) x 1027 [s-1] at r = 2.43 au, 2.13 au pre-perihelion, and r = 2.14 au and 2.67 au, post-perihelion. Keck telescope NIRSPEC observations yield QH2O = (2.13 ± 0.61) x 1028 s-1 at r = 2.17 au post-perihelion. We observe a dramatic increase in brightness at r ~ 2.56 au pre-perihelion likely from a delayed on-set of subsurface CO2 We observed three suspected CO2-driven outburst events, one pre-perihelion at r = 5.38 au, and two post-perihelion at r = 2.30 au and 5.08 au. To match the observed H2O production from Keck and Lowell observatories, we combined water sublimating from both the nucleus surface and from large (>50 μm) ice grains ejected via CO2 sublimation. These large grains, having been lifted off the surface through smaller outbursts, enter the coma and can begin sublimating, contributing to higher than expected activity for a comet of this size (effective fractional active area of 36% for water). For various grain radii and albedos, we find that there is a sufficient number of grains produced by our models to make up the difference in sublimating area and that they, with the exception of those with very large radii (~200 μm) and very high (~0.8) albedos, sublimate completely before crossing out of the observing aperture. We compute estimates for the energy per unit mass of ejecta and compare it with the specific energy of an amorphous to crystalline ice transition to compare to possible mechanisms for the outbursts