Cometary Science Newsletter
- Issue
- 88
- Month
- July 2022
- Editor
- Michael S. P. Kelley (msk@astro.umd.edu)
Refereed Articles
Abstracts of articles in press or recently published. Limited to 3000 characters.
Cometary Activity beyond the Planets
- Algiers Observatory
- UCLA University
Recent observations show activity in long-period comet C/2017 K2 at heliocentric distances beyond the orbit of Uranus. With this as motivation, we constructed a simple model that takes a detailed account of gas transport modes and simulates the time-dependent sublimation of supervolatile ice from beneath a porous mantle on an incoming cometary nucleus. The model reveals a localized increase in carbon monoxide (CO) sublimation close to heliocentric distance rH = 150 au (local blackbody temperature ∼23 K), followed by a plateau and then a slow increase in activity toward smaller distances. This localized increase occurs as heat transport in the nucleus transitions between two regimes characterized by the rising temperature of the CO front at larger distances and nearly isothermal CO at smaller distances. As this transition is a general property of sublimation through a porous mantle, we predict that future observations of sufficient sensitivity will show that inbound comets (and interstellar interlopers) will exhibit activity at distances far beyond the planetary region of the solar system.
The Astrophysical Journal (Published)
Dust Evolution in the Coma of Distant, Inbound Comet C/2017 K2 (PANSTARRS)
- Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
- Department of Astronomy, University of Maryland, College Park, MD, USA
C/2017 K2 (PANSTARRS) is an Oort cloud comet previously observed to be active at heliocentric distances r>20 au on what is likely its first passage through the inner solar system. We observed the comet on 2021 March 19-20 at r=6.82 au pre-perihelion and 8.35 deg phase angle with the Hubble Space Telescope (HST), and obtained high spatial resolution photometry and polarimetry mapping the properties of dust over the coma prior to the onset of water ice sublimation activity on the nucleus. We found clear radial gradients in the color and polarization of the coma: the F475W-F775W (g'-i') reflectance slope increased from ~4.5% per 100 nm within ~10,000 km of the nucleus up to ~7% per 100 nm by ~50,000 km, while the negative polarization in F775W (i') strengthened from about -2% to -3.5% over the same range. The radial intensity profiles moreover strongly deviate from profiles simulated for stable dust grains. Near-infrared imagery obtained with the Palomar Hale Telescope on 2021 May 18 at r=6.34 au revealed a continued absence of micron-sized grains in the tail, but showed no clear spatial gradient in JHKs colors. The observed patterns collectively appear consistent with the inner coma being optically dominated by sublimating, micron-sized water ice grains, unlike the tail of more stable, millimeter-sized grains. Finally, we evaluated these results alongside other Oort cloud comets, and found in a reanalysis of HST observations of C/2012 S1 (ISON) that the near-nucleus polarimetric halo reported for that comet is likely an observational artifact.
The Planetary Science Journal (Published)
DOI: 10.3847/PSJ/ac6d58 NASA ADS: 2022PSJ.....3..135Z arXiv: 2205.02854
Hubble Space Telescope Observations of Active Asteroid P/2020 O1 (Lemmon-PANSTARRS)
- Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, Germany
- Department of Earth, Planetary and Space Sciences, UCLA, USA
- Space Telescope Science Institute, Baltimore, USA
- Johns Hopkins University Applied Physics Laboratory, Laurel, USA
We present Hubble Space Telescope observations of active asteroid P/2020 O1 taken to examine its development for a year after perihelion. We find that the mass loss peaks <~1 kg/s in 2020 August and then declines to nearly zero over four months. Long-duration mass loss (~180 days) is consistent with a sublimation origin, indicating that this object is likely an ice-bearing main-belt comet. Equilibrium sublimation of water ice from an area as small as 1580 m^2 can supply the observed mass loss. Time-series photometry shows tentative evidence for extremely rapid rotation (double-peaked period < 2 hr) of the small nucleus (effective radius ~420 m). Ejection velocities of 0.1 mm particles are comparable to the 0.3 m/s gravitational escape speed from the nucleus, while larger particles are ejected at speeds less than the escape velocity. These properties are consistent with the sublimation of near-surface ice aided by centripetal acceleration. If water ice sublimation is confirmed, P/2020 O1 would be the icy asteroid with the smallest semimajor axis (highest temperature), setting new bounds on the distribution of ice in the asteroid belt.
The Astrophysical Journal Letters (Published)
DOI: 10.3847/2041-8213/ac78de arXiv: 2206.07703