Lifetimes and occurrence rates of dark vortices on Neptune from 25 years of Hubble Space Telescope images. (2019)
Keywords :
hydrodynamics - planets and satellites individual (Neptune) - planets and satellites: atmospheres - planets and satellites: gaseous planets - time
Abstract:We scoured the full set of blue-wavelength Hubble Space Telescope images of Neptune, finding one additional dark spot in new Hubble data beyond those discovered in 1989, 1994, 1996, and 2015. We report the complete disappearance of the SDS-2015 dark spot, using new Hubble data taken on 2018 September 9-10, as part of the Outer Planet Atmospheres Legacy (OPAL) program. Overall, dark spots in the full Hubble data set have lifetimes of at least one to two years, and no more than six years. We modeled a set of dark spots randomly distributed in time over the latitude range on Neptune that is visible from Earth, finding that the cadence of archival Hubble images would have detected about 70% of these spots if their lifetimes are only one year, or about 85%-95% of simulated spots
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We collected 256 Hubble images to search for dark spots on Neptune from
1994 to 2018 (Table 3). Images are all from 400 to 500 nm wavelength and
the years range from 1994 to 2018 (except for one WFPC2 image using the
F555W filter, analyzed as a check to confirm that a detected feature was
a previously recognized dark spot). Wavelengths in the range of 400-500 nm
(blue visible light) have the best chance of capturing a Neptune dark spot
(Sromovsky et al. 2002Icar..156...16S; Wong et al. 2018AJ....155..117W).
All of the images are full resolution. We processed the images to remove
geometric distortion, navigated them, and applied corrections for limb
darkening as described in Wong et al. (2018AJ....155..117W). Data were
analyzed from three of Hubble's cameras: WFPC2, ACS, and WFC3.
The goal of the Monte Carlo simulation is to answer the following
questions. Given the set of Hubble observations, what is the detectability
of dark vortices on Neptune as a function of vortex lifetime? Given the
number of vortices actually detected, what are the constraints on dark
spot longevity and occurrence rate? We created a set of six simulations,
each consisting of a large number of dark spots with an assumed lifetime,
{tau}_life_, in the one to six year range. By choosing 8000 simulated
dark spots per value of {tau}_life_, we ensured that there would be
~300 simulated dark spots at any given time, producing accurate detection
probabilities and permitting the results to be broken down into
latitude bins.
- B/hst : HST Archived Exposures Catalog (STScI, 2007)
- J/AJ/152/142 : Spitzer and WISE light curves of Neptune (Stauffer+, 2016)
