Observations Relevant to Cometary Hyperactivity (The Year of 46P/Wirtanen)

University Of Maryland, College Park, College Park MD

Investigators

Abstract

During the fall of 2018, the very bright, very active comet 46P/Wirtanen will share the sky with two other bright comets, 21P/Giacobini-Zinner and 64P/Swift-Gehrels, as they reach their closest approach to the sun. Comet 46P/Wirtanen will be especially bright. The investigators will collect images and spectra of these three small comets whose orbits are controlled by Jupiter's presence. The observations during their passages near the sun this year will provide future studies with significant data to quantify compositions and physical characteristics of gases and dust around the comet, investigate the presence of icy grains, and determine common aspects or key differences among the comets. These observations are time critical as these good observing conditions will not present themselves again for over a decade for comet 21P, and more than two decades for comets 46P and 64P. The investigators will share their interpretations of the measured cometary activity, as well as the expected stunning and informative images of these comets, with the broader community through various public talks. Unlike the hyperactive comet 46P, most Jupiter-family comets display a level of activity that can be accounted for by sublimation from a small fraction of their surface area. Comet 103P/Hartley 2 (the target of the NASA space probe Deep Impact), on the other hand, is a representative member of a hyperactive population that, paradoxically, exhibit active fractions > 100%. During its flyby of Hartley 2, Deep Impact observed a large carbon dioxide jet, suffused with small grains of water ice, originating from one lobe of the comet. The carbon dioxide escaping from the nucleus likely dragged subsurface water-ice grains into the coma. Once in the coma, the distributed ice grains dramatically increased the surface area for water vapor sublimation, explaining Hartley 2's hyperactivity. This model suggests that hyperactive comets are governed by subsurface volatiles with low sublimation temperatures that entrain ice grains to produce anomalously high-water production rates. Since the Deep Impact measurements, however, no other hyperactive comet has been sufficiently observed to confirm the applicability of the Hartley 2 model; this will change with observations of 46P. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.