J/AJ/157/152 Dark spots on Neptune from 25 years of HST images (Hsu+, 2019)
Lifetimes and occurrence rates of dark vortices on Neptune from 25 years
of Hubble Space Telescope images.
Hsu A.I., Wong M.H., Simon A.A.
<Astron. J., 157, 152 (2019)>
=2019AJ....157..152H 2019AJ....157..152H (SIMBAD/NED BibCode)
ADC_Keywords: Solar system ; Planets ; Photometry, HST ; Models, evolutionary
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
with lifetimes of two or more years. Based on the Hubble data set, we
conclude that dark spots have average occurrence rates of one dark spot
every four to six years. Many numerical models to date have simulated much
shorter vortex lifetimes, so our findings provide constraints that may
lead to improved understanding of Neptune's wind field, stratification,
and humidity.
Description:
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 2002Icar..156...16S; Wong et al. 2018AJ....155..117W 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 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,
τlife, in the one to six year range. By choosing 8000 simulated
dark spots per value of τlife, 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.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 88 256 Observation catalog
table4.dat 31 8000 Monte Carlo simulation for 1-year lifetime
table5.dat 31 8000 Monte Carlo simulation for 2-year lifetime
table6.dat 31 8000 Monte Carlo simulation for 3-year lifetime
table7.dat 31 8000 Monte Carlo simulation for 4-year lifetime
table8.dat 31 8000 Monte Carlo simulation for 5-year lifetime
table9.dat 31 8000 Monte Carlo simulation for 6-year lifetime
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See also:
B/hst : HST Archived Exposures Catalog (STScI, 2007)
J/AJ/152/142 : Spitzer and WISE light curves of Neptune (Stauffer+, 2016)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Inst Instrument on Hubble
11- 19 A9 --- Dataset Dataset identifier
21- 30 A10 "date" Date Start date
32- 36 A5 "h:m:s" Time Start time (hh:mm)
38- 40 I3 s Exp [8/130] Total exposure duration
42- 52 A11 --- Filt Filter/Grating
54- 59 F6.2 deg Ob-Lon [5.69/356.17] Sub-observer longitude
61- 67 F7.2 deg Ob-Lat [-261/26.1] Sub-observer latitude
69- 73 I5 --- PID [5221/15262] Proposal ID
75- 84 A10 --- PI Principal Investigator last name
86- 88 A3 --- DSpot [Yes/No] Whether dark spot is detected
in the image
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Byte-by-byte Description of file: table[456789].dat
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Bytes Format Units Label Explanations
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1- 4 I4 --- ID [1/8000] Dark spot identifier
6 I1 yr LS [1/6] Lifespan of this simulated dark spot
8- 12 F5.1 deg Lat [-90/50] Latitude of dark spot
14- 20 F7.2 yr Birth [1988.51/2018.49] Time in years the dark spot
is born
22- 28 F7.2 m/s Drift [-448.97/289.44] Drift rate added to Sromovsky
zonal wind
30- 31 I2 --- Ndet [0/81] Number of detections
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History:
From electronic version of the journal
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 12-Jul-2019