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A New Dark Vortex on Neptune


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An outburst of cloud activity on Neptune in 2015 led to speculation about whether the clouds were convective in nature, a wave phenomenon, or bright companions to an unseen dark vortex (similar to the Great Dark Spot studied in detail by Voyager 2). The Hubble Space Telescope (HST) finally answered this question by discovering a new dark vortex at 45 degrees south planetographic latitude, named SDS-2015 for "southern dark spot discovered in 2015." SDS-2015 is only the fifth dark vortex ever seen on Neptune. In this paper, we report on imaging of SDS-2015 using HST's Wide Field Camera 3 across four epochs: 2015 September, 2016 May, 2016 October, and 2017 October. We find that the size of SDS-2015 did not exceed 20 degrees of longitude, more than a factor of two smaller than the Voyager dark spots, but only slightly smaller than previous northern-hemisphere dark spots. A slow (1.7–2.5 deg/year) poleward drift was observed for the vortex. Properties of SDS-2015 and its surroundings suggest that the meridional wind shear may be twice as strong at the deep level of the vortex as it is at the level of cloud-tracked winds. Over the 2015–2017 period, the dark spot's contrast weakened from about to about , while companion clouds shifted from offset to centered, a similar evolution to some historical dark spots. The properties and evolution of SDS-2015 highlight the diversity of Neptune's dark spots and the need for faster cadence dark spot observations in the future.
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A New Dark Vortex on Neptune
Michael H. Wong
, Joshua Tollefson
, Andrew I. Hsu
, Imke de Pater
, Amy A. Simon
, Ricardo Hueso
Agustín Sánchez-Lavega
, Lawrence Sromovsky
, Patrick Fry
, Statia Luszcz-Cook
, Heidi Hammel
, Marc Delcroix
Katherine de Kleer
, Glenn S. Orton
, and Christoph Baranec
University of California, Berkeley, CA 94720, USA;
NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
Universidad del Pais Vasco Bilbao, Spain
University of Wisconsin, Madison, WI 53706, USA
American Museum of Natural History and Columbia University, New York NY, USA
Association of Universities for Research in Astronomy, Washington DC 20004-1752, USA
Societe Astronomique de France, France
California Institute of Technology Pasadena CA 91109, USA
Jet Propulsion Laboratory, 4800 Oak Grove Drive, California Institute of Technology, Pasadena, CA, USA
Institute for Astronomy, University of HawaiiatMānoa, Hilo, HI 96720-2700, USA
Received 2017 November 21; revised 2017 December 22; accepted 2017 December 29; published 2018 February 15
An outburst of cloud activity on Neptune in 2015 led to speculation about whether the clouds were convective in
nature, a wave phenomenon, or bright companions to an unseen dark vortex (similar to the Great Dark Spot studied
in detail by Voyager 2). The Hubble Space Telescope (HST)nally answered this question by discovering a new
dark vortex at 45 degrees south planetographic latitude, named SDS-2015 for southern dark spot discovered in
2015.SDS-2015 is only the fth dark vortex ever seen on Neptune. In this paper, we report on imaging of SDS-
2015 using HSTs Wide Field Camera 3 across four epochs: 2015 September, 2016 May, 2016 October, and 2017
October. We nd that the size of SDS-2015 did not exceed 20 degrees of longitude, more than a factor of two
smaller than the Voyager dark spots, but only slightly smaller than previous northern-hemisphere dark spots. A
slow (1.72.5 deg/year)poleward drift was observed for the vortex. Properties of SDS-2015 and its surroundings
suggest that the meridional wind shear may be twice as strong at the deep level of the vortex as it is at the level of
cloud-tracked winds. Over the 20152017 period, the dark spots contrast weakened from about
7% to about
3%, while companion clouds shifted from offset to centered, a similar evolution to some historical dark spots.
The properties and evolution of SDS-2015 highlight the diversity of Neptunes dark spots and the need for faster
cadence dark spot observations in the future.
Key words: astrometry hydrodynamics planets and satellites: atmospheres planets and satellites: gaseous
planets planets and satellites: individual (Neptune)techniques: high angular resolution
1. Introduction
Voyager 2 provided the rst high-resolution glimpse of
Neptune in 1989, observing two dark spots. We assume these
features are anticyclonic vortices based on their behavior
(Smith et al. 1989), although there has never been an actual
measurement of their internal circulation. Since Voyager, only
Hubble has discovered dark vortices: two in the 1990s, and one
in 2015 (Table 1). Ground-based facilities lack the resolution to
detect these low-contrast features at blue optical wavelengths,
while infrared observations do not detect the dark spots
themselves, only their bright companion features. This is not
quite the case for Uranus, where we have seen dark features at
red wavelengths in Hubble images (Hammel et al. 2009)and at
near-IR wavelengths in Keck H-lter images (Sromovsky
et al. 2015). The ve Neptune dark spots exhibited surprising
diversity, in terms of size, shape, companion cloud distribution,
oscillatory behavior, meridional drift rates, and meandering.
Neptunes dark vortices come and go on much shorter
timescales compared to similar anticyclones on Jupiter, which
evolve over decades (e.g., Ingersoll et al. 2007; Shetty &
Marcus 2010; Simon et al. 2014). Many questions remain as to
how dark vortices originate, what controls their drift and
oscillation, how they interact with the environment, and how
they eventually dissipate. Vortex behavior also provides insight
into the structure and dynamics of the surrounding atmosphere.
The discovery of SDS-2015 was enabled by the annual
cadence of the Outer Planet Atmospheres Legacy (OPAL)
program, which acquires maps covering two consecutive
rotations of each giant planet, annually. The Neptune
observations are part of OPALs large time-domain survey of
atmospheric evolution (Simon et al. 2015), which began in
2014 with observations of Uranus (Irwin et al. 2017).
Companion clouds are usually associated with dark vortices
and are thought to result from ambient air being diverted
upward as ow is perturbed over the vortex (Smith et al. 1989;
Stratman et al. 2001). The appearance and evolution of
companion clouds has been different for all of the dark
vortices observed to date, with SDS-2015 (Figure 1)having
persistent companions that may exhibit longitudinal oscilla-
tions in their mean position (Hueso et al. 2017). The brightness
of companion clouds may be related to the vortex top altitude; a
The Astronomical Journal, 155:117 (9pp), 2018 March
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global circulation model of the GDS (Stratman et al. 2001)
found that cloud opacity weakened as the top of the vortex
reached higher into the tropopause region. Their deepest model
vortex had the highest companion cloud opacity.
The Voyager observations revealed many different types of
oscillations and drifts in the behavior of dark vortices GDS and
DS2 (Sromovsky et al. 1993). The center of the GDS oscillated
in longitude, and also drifted slowly equatorward, with some
intermittent poleward meandering. It also exhibited a rolling
appearance, where the aspect ratio oscillated in phase with a
rotation of the ellipse axes. LeBeau & Dowling (1998)modeled
these oscillations and identied a common physical mechanism
explaining the drift of both terrestrial hurricanes, and the GDS
on Neptune. In both cases, the spinning of the vortices advects
the background atmospheric potential vorticity, creating a weak
dipole of vorticity aligned approximately northsouth. LeBeau
& Dowling (1998)postulated the destruction of these vortices,
as they drift equatorward and experience either Rossby-wave
radiation, or loss of material as they merge with regions of
similar potential vorticity. The actual origin or dissipation of a
Neptunian dark vortex has never been conclusively documen-
ted. Discovery dates are listed in Table 1because origin dates
are unknown. Likewise, the demise or dissipation of these
vortices has never been precisely determined. End dates in the
table are estimates based on extrapolated positions, the
disappearance of bright companions, and absence in subse-
quent observations.
The meridional drift of vortices acts as a probe of Neptunes
zonal ow, which is puzzling in many ways (Limaye &
Sromovsky 1991; Sromovsky et al. 1995; Hammel &
Lockwood 1997; Sromovsky et al. 2001; Fitzpatrick et al.
2014; Sánchez-Lavega et al. 2018; Tollefson et al. 2018).
Neptune has a single retrograde equatorial jet with a speed of
, and two prograde circumpolar jets with speeds
around 250ms
. Dispersions above 200ms
have been
measured between cloud features at the same latitude on
Neptune. This observed velocity dispersion may be explained
by temporal variability in the wind eld, vertical wind shear,
rapid evolution of cloud morphology, and/or longitudinal
variability. Wave propagation, rather than mass transport, may
also determine the observed motion of cloud features.
Persistent, well-separated features at different latitudes have
been observed to move at a common zonal rate, invisibly
linked (Sromovsky et al. 2001; Martin et al. 2012). LeBeau &
Dowling (1998)found that the rate of northsouth drift of the
vortex is very sensitive to the background potential vorticity
gradient (i.e., the horizontal wind shear). Thus, measuring
meridional drift may reveal ne structure in the zonal wind
prole that cannot be derived by cloud-tracking, while
oscillation may reveal meandering in the background jet
(Karkoschka 2011).
Here, we report the size, shape, motion, and contrast of SDS-
2015, based on observations spanning 20152017.
2. Observations
Observations reported here were taken with the UVIS
detector of the WFC3 instrument aboard the Hubble Space
Telescope (HST). Spectral responses of WFC3/UVIS lters
listed here are given in Dressel (2016).
2.1. Cadence
Table 2lists four epochs of observations of the dark vortex.
In Figure 1, we show images and maps of Neptune at each
epoch. The original discovery of SDS-2015 in OPAL data led
to a supplementary Midcycle program, focusing on the blue
wavelengths where the vortex itself is most evident. Only
16 days before the OPAL 2015 observations, WFC3 imaging
of Neptune taken in support of VLA observations (GO-14044)
barely missed the dark vortex. Companion clouds were seen,
but the exposures were acquired some 2030 minutes before
SDS-2015 rotated onto the disk.
2.2. Detector Readout
We used 512×512 subarrays to minimize read time and
instrument memory transfer overhead (buffer dumps), except
for exposures using the methane-band quad lters (FQ727N,
FQ619N), which generate 2K×2K subarray frames, like all
quad-lter exposures. Rather than pairing FQ727N methane-
band observations with the FQ750N continuum lter, we used
F763M. This lters wider bandpass includes both continuum
and CH
-band contributions, preserving information content
(and vertical resolution)while still permitting the advantageous
512×512 subarray readout. Although visual comparison of
FQ727N and FQ750N images is more straightforward, using
F763M enables more time-on-target during an HST orbit.
2.3. Photometric Calibration
For the FQ727N frames, we applied corrections to remove
photometric errors from fringing, an effect expected to be in the
2%5% range based on analysis of at eld data (Wong 2010).
The actual magnitude of the correction across the regions of the
Table 1
Characteristics of Neptunes Dark Vortices
Name Discovery Demise Latitude
Lon.×Lat. Meridional Drift References
GDS 1989 1990 -20 38°×15°+0°.11/day equatorward Smith et al. (1989)
DS2 1989 <1994 -55 39°×6°±2°.4 oscillation Sromovsky et al. (1993)
NDS-1994 1994 19982000 +32 35°×10°0°/day Hammel et al. (1995), Sromovsky et al. (2001)
NDS-1996 1996 19971998 +15 22°×12°0°/day Sromovsky et al. (2001)
SDS-2015 2015 >2017 -49 15°×5°-2.5/year poleward Wong et al. (2016)
Note. Latitude, size, and meridional drift rates are representative values. Actual values were known to vary in time, except for meridional drift of NDS-1994 and NDS-
1996, which were stable in latitude. The actual demise of a dark vortex has never been observed, so values listed here are estimates (see the text). The northern spots
have been renamed in this paper to establish a consistent nomenclature for post-Voyager dark spots.
Planetographic latitude is used throughout this work.
The Astronomical Journal, 155:117 (9pp), 2018 March Wong et al.
detector where Neptune was observed was 0.5%1.5%, a
smaller effect than predicted. This is because part of the overall
2%5% fringing amplitude in this lter bandpass is corrected
by the pipeline at elds (obtained using a continuum light
source), leaving just the error due to the difference in spectral
energy distribution between the calibration source and
Neptunes scattered solar spectrum. We also minimized the
fringing amplitude by placing Neptune on an area of the
detector where fringing is relatively smooth over the 50 pixel
size scale of Neptunes disk. In some areas, fringing maxima/
minima are separated by only 10 pixels.
In the 20152017 time range, the WFC3 instrument team at
Space Telescope Science Institute implemented a set of
signicant revisions to their photometric calibration pipeline,
collectively termed UVIS 2.0.The revisions included new at
elds and normalization procedures (Mack et al. 2016), and new
sensitivity values for each lter (Deustua et al. 2016,2017).All
Neptune data presented here, although acquired by HST prior to
Figure 1. Gallery of SDS-2015 images and maps. [A, B, C, D]Red/green/blue (RGB)composite images at four epochs (see Table 2), co-added from multiple frames
using the frydrizzle rotational dithering approach described in Section 2.5.[E, F, G, H]RGB cylindrical maps of the dark vortex at each epoch. [I, J, K, L]Blue
(F467M)lter images show the dark vortex itself most clearly. The brightness scale is the same for all epochs. The F763M data (red channel in color maps)span I/F
0.08 to 0.35 on a square root stretch. The F547M data (green channel in color maps)span I/F 0.48 to 0.60 on a linear stretch. The F467M data (blue channel in color
maps, greyscale in panels IL)span I/F 0.63 to 0.76 on a linear stretch (see colorbar).
The Astronomical Journal, 155:117 (9pp), 2018 March Wong et al.
these revisions, have been recalibrated using the UVIS 2.0
photometry. The UVIS 2.0 improvements have not yet been
implemented for WFC3/UVIS quad lters.
2.4. Navigation and High-level Science Products (HLSPs)
We navigated the images by aligning synthetic limb-
darkened planetary disks to the data (Lii et al. 2010), giving
a navigation precision of 0.0825 pixels. We estimate a
navigation accuracy of about 0.18 pixels for the F467M lter
images, leading to disk-center latitude/longitude uncertainties
of about 0°. 3. Global maps of Neptune acquired by the OPAL
program are available as HLSPs on the MAST archive,
described in Simon et al. (2015,2016).
Navigated single-frame image data for all programs listed in
Table 2can also be obtained from MAST
as HLSPs ending in
the extension nav.ts.Each le is a multi-extension FITS
le, similar to HST drz.tsles but with additional
processing and data. The processing consists of corrections
for cosmic ray hits using the LA Cosmic method (van
Dokkum 2001)and for fringing in long-wavelength narrow-
band lters (Wong 2011). The additional data includes FITS
keywords containing target ephemeris data provided by JPL
and four FITS extensions giving the planeto-
graphic latitude, west longitude, emission angle, and solar
incidence angle for each pixel on the planets disk. In addition
to the STScI-supplied PHOTFLAM inverse sensitivity key-
word that converts image data units from e
, we include a PHOTIF keyword that
converts e
to planetary reectivity units of I/F, and an
associated SIG_PHOT keyword giving the photometric
uncertainty in the I/Fcalibration.
2.5. Rotational Dithering
Relative to OPAL program observations, the midcycle
program was intended to improve image quality and short-
wavelength coverage, specically focusing on the dark vortex.
Short-wavelength coverage was improved by adding blue
lters and dropping two longer wavelength lters, and image
quality was improved by taking a larger number of frames in
the blue lters. We then co-add the map data, accounting for
the planets rotation and the longitudinal drift of the dark spot.
Astronomical images of static targets can be improved by
combining individual frames with drizzle methods (resampling
and co-adding), which has the added benet of being able to
reject cosmic ray events and other image defects (e.g., Fruchter
& Hook 2002). We would also like to co-add Neptune images,
to improve sampling of the HST/WFC3 PSF and to average
over pixels with nonuniform response, but, each exposure in
our program differs due to Neptunes rotation as well as
advection by atmospheric winds. In order to combine frames,
additional steps must be taken, following an algorithm known
informally as frydrizzle. The frydrizzle algorithm is fully
described in Section 3.1 of Fry et al. (2012). The principal steps
of our slightly modied procedure (we add limb-darkening
treatment that was not necessary at the wavelengths used in Fry
et al. 2012)are:
1. Navigate each frame to associate each element of the
image data with the appropriate latitude, longitude, and
emission/incidence angles.
2. Apply a limb-darkening correction to normalize reec-
tances to a common viewing geometry.
3. If images separated by a signicant amount of time, shift
(or advect)each latitude using an a-priori zonal wind
eld, to correct for differential rotation.
4. Co-add data in latitude/longitude space rather than image
5. If creating a co-added disk image on the sky (rather than
co-added map in longitude-latitude space), then restore
limb-darkening effects and re-map reectances onto the
3. Results
3.1. Morphology
Measurements of the size and location of SDS-2015 are
based on albedo contrasts inside and outside the vortex, which
appears as a 5%-level contrast feature at blue wavelengths. We
estimate the boundary of the vortex to be located where the
contrast reaches its half-power point, and accurate measure-
ment of this location relies on determining the effect of limb
darkening, to distinguish between the intrinsic albedo contrast
of the vortex itself, and apparent contrasts due to variation in
emission angle across the span of the dark spot.
Table 2
HST/WFC3 Observations of Neptune in the SDS-2015 era
Times Program Filters
(UTC)ID (number of frames)
(2015 Sep 18 16:48 to 2015 Sep 19 06:19)
OPAL GO-13937
(PI Simon)
F467M (4), F547M (4), F657N (4), F763M (4),
F845M (8), FQ727N (4), FQ619N (4)
(2016 May 16 03:47 to 04:16)
Midcycle GO-14492
(PI Wong)
F336W (1), F390M (1), F410M (1), F467M (6),
F547M (6), F763M (3), FQ727N (2)
(2016 Oct 03 11:04 to 2016 Oct 04 04:59)
OPAL GO-14334
(PI Simon)
F467M (4), F547M (4), F657N (4), F763M (4),
F845M (8), FQ727N (4), FQ619N (4)
(2017 Oct 06 05:49 to 2017 Oct 07 03:15)
OPAL GO-14756
(PI Simon)
F467M (8), F547M (8), F657N (8), F763M (8),
F845M (16), FQ727N (8), FQ619N (8)
Note.Observing programs listed here mapped Neptune over at least one full rotation, but times listed apply only to exposures in which SDS-2015 is on the face of the
disk. Additional data from program GO-14044 (PI de Pater), 2015.67, are not listed but were used for background atmosphere analyses (e.g., Table 3).
OPAL archive page:,10.17909/T9G593.
Dark vortex archive page:,10.17909/
JPL Horizons server:
The Astronomical Journal, 155:117 (9pp), 2018 March Wong et al.
We use the Minnaert equation to convert observed
obs. to
limb-darkening corrected
IF IF ,1
corr. obs. 0
is the cosine of the incidence angle and μis the
cosine of the emission angle.
corr. can also be called the
disk-center I/F, or the Minnaert albedo. This limb-darkening
function was originally used to describe lunar reectivity
(Minnaert 1941). Although simple functions of this type do not
perfectly capture limb-darkening behavior of Neptunes
vertically and horizontally inhomogeneous atmosphere, we
still use Equation (1)to achieve a rst-order brightness
correction, especially at emission/incidence angles less than
75°. For each wavelength, we derived the appropriate value of
kby tting to data at longitudes excluding the dark vortex. Best
t parameters based on four data sets (two in 2015 and two in
2016)are listed in Table 3.
Maps in Figure 1show reectivity with this rst-order limb-
darkening correction applied. Eastwest cuts through these
maps are shown in Figure 2. These photometric proles, from
co-added F467M maps, were used to estimate widths and
positions of SDS-2015 at the four observational epochs, with
results listed in Table 4.
The largest source of uncertainty in the size and position
estimates of Table 4comes from the inuence of the
omnipresent companion clouds. Comparison of the RGB and
F467M maps in Figure 1consistently show reduced contrast
within the vortex at locations where companion cloud opacity
is high. Thus, we have dened error bars that encompass the
directly observed vortex bounds (half-power points in contrast
proles like those in Figure 2)and the potentially larger size of
the vortex including a qualitative compensation for vortex area
masked by the companion cloud.
3.2. Contrast
The contrast of the vortex with respect to the surroundings
was determined for wavelengths where the dark spot is visible,
and reported in Figure 3. The contrast of the vortex in F467M
has been decreasing since its discovery, with a very low
contrast detected in the 2017 observations.
Contrasts were determined using the darkest location in the
vortex (not necessarily the center, where companion cloud
reectivity was sometimes signicant). At wavelengths longer
than 547 nm, the dark spot is actually brighter than its
surroundings, because the contrast is dominated by aerosols
associated with the companion cloud. The relative background
at the darkest location was found by linear interpolation, tted
to background levels outside the vortex. As with Sromovsky
et al. (2002),wend maximum contrast in the F467M
bandpass, although the SDS-2015 contrasts are consistently
larger than most contrast measurements of NDS-1994 and
NDS-1996 reported by Sromovsky et al. (2002).
3.3. Drift Rates
Over the time that SDS-2015 has been observed, it drifted
polewards until it reached
49 S(Figure 4). There is no large
statistical difference between two models of its meridional
drift: (1)a linear poleward drift of
/year in 20152016,
with no change between 2016.76 and 2017.76; or (2)a
poleward drift of
/year over the full period of the
observations. Model 1 (3-point t)gives a drift rate very close
to the change in companion cloud position of ~
2.4/year based
on all 2015 measurements in Hueso et al. (2017), specically
their Figure 19.
Longitudinal drift rates for SDS-2015 and its companion
clouds are listed in Table 5. Data are derived from Hueso et al.
(2017)for the entire 2015B period (2015.512015.95), and
from OPAL data for the remaining three epochs. The 2016.37
epoch covered only a single Neptune rotation, so the temporal
baseline was not long enough to measure longitudinal drifts
with any certainty.
Zonal mean velocities in Table 5, from the 6th-order
polynomial t of Sromovsky et al. (1993), were measured by
tracking discrete cloud features, presumably composed of CH
located at pressures of 1.4 bar or less (de Pater et al. 2014).
Dark vortex companion clouds are also thought to be CH
Table 3
Limb Darkening Parameters Used for Image Analysis
HST WFC3/UVIS Filter k(global)k(35°55°S)
F336W 0.804±0.011 0.804±0.012
F390M 0.825±0.005 0.807±0.009
F410M 0.836±0.007 0.813±0.017
F467M 0.867±0.016 0.823±0.026
F547M 0.798±0.041 0.765±0.074
Note. Limb darkening is modeled using the Minnaert equation (Equation (1)).
Values of kwere found by linearizing image data where emission and
incidence angles <75°, and uncertainties reect scatter from instrumental noise
and from actual spatial/temporal variation. Data from epochs 2015.67,
2015.71, 2016.37, and 2016.76 were used to derive limb-darkening
Figure 2. Photometric cuts through SDS-2015 used for size, position, and
contrast measurements. Top panel: eastwest cuts are shown in the highest-
contrast lter (F467M)at the four epochs. Vortex center positions are as listed
in Table 4. Bottom panel: eastwest cuts for multiple lters are shown, for the
2016.37 epoch. The latitude of these cuts is slightly to the north of the
estimated dark spot center (compare F467M trace with that in top panel, and
refer to map in Figure 1(J). This was done to measure the maximum contrast at
other wavelengths where the companion cloud contribution diminishes contrast
at the center of the vortex. Limb darkening corrections have been applied.
The Astronomical Journal, 155:117 (9pp), 2018 March Wong et al.
clouds (Smith et al. 1989). Since the companion clouds are
within the same altitude range of clouds used to derive zonal
wind proles, it might be expected that they would drift at the
mean zonal speed, but Table 5shows that they do not. This can
be explained by their dynamical connection to the dark vortex,
which itself had a 2030ms
velocity offset from the
zonal mean.
Measurement of companion cloud drift rates was done by
manually tracking the center of light in individual images and
dividing by image offset times to obtain velocities. For the
2016 and 2017 dark spot data, drift rates were determined by
shifting longitudinal photometric proles (as in Figure 2)using
different rates, until the half-power points in the proles were
best aligned by eye. This method could not be used for the
2015.71 data, because SDS-2015 was positioned so close to the
limb in some data frames that part of the vortex became
indistinguishable from the surrounding atmosphere. To com-
pensate for this effect, we manually determined the vortex
position in each frame in the 2015.71 epoch, using WinJUPOS
For all measurements using the HST data, we
estimate an uncertainty of about 10ms
, including error
contributions from image navigation uncertainty, noise in the
image, and feature centroiding.
Table 4
Morphology of SDS-2015
Epoch Longitudinal Width (a
)Latitudinal Width (b
)Central Longitude (
0)Central Latitude (f0)Aspect Ratio (a/b)
2015.71 16°.8±4°.0 (5120 ±1230 km)5°.5±0°.6 (2300 ±270 km)9°.1±1°.0 45°.6±0°. 2 S 2.2±0.6
2016.37 19°.5±1°.5 (5770 ±440 km)6°.4±3°.8 (2690 ±1580 km)140°.3±3°.3 46°.3±1°. 0 S 2.1±1.3
2016.76 11°.3±2°.7 (3190 ±770 km)5°.8±3°.1 (2450 ±1310 km)142°.5±1°.1 48°.9±0°. 9 S 1.3±0.8
2017.76 13°.0±2°.7 (3720 ±770 km)3°.8±0°.5 (1580 ±210 km)134°.8±0°.9 49°.0±0°. 2 S 2.4±
Note. Centers and widths are determined by measuring half-power points in brightness proles (Figure 2top)in the F467M data. Multiple images are combined per
epoch to achieve higher signal to noise; this requires combining data after correction for limb darkening (using Equation (1)and values in Table 3). Aspect ratios are
calculated using linear dimensions (km)rather than longitude/latitude. Uncertainties include the potential obscuration of part of the dark spot by the bright companion
clouds (see last paragraph of Section 3.1).
Figure 3. Spectral contrast between SDS-2015 and its surroundings. The
contrast Cis dened as, =-()(
DS surr. surr. , where
FDS is
dened at
0,f0(see Table 4), and
surr. is a linear t to data at longitudes
well separated from the spot, at the same latitude of the vortex. Error bars
include random noise, systematic photometric uncertainty, and spatial variation
across the dark spot. Data points are averages of multiple frames (Table 2).
Data points for F467M and F547M data are horizontally offset for clarity, but
in all cases the horizontal error bars give the lter widths taken from Table 6.2
of Dressel (2016).
Figure 4. Poleward drift of SDS-2015. A linear drift rate of -2.5/year ts
SDS-2015 positions in the 20152016 time period (with no change in latitude
between 2016.76 and 2017.76). A drift rate of -1.7/year ts SDS-2015
positions over the full 20152017 period. Neither t is strongly favored over
the other statistically: cc==
() ()3 points 1.5; 4 points 1.6
Table 5
Eastward Drift Rates
Eastward Velocity (ms
Date Lat. (deg)Zonal Mean SDS-2015 Companions
2015B 40.1 142 L94
2015.71 45.6 69 90 L
41.0 131 L95
2016.76 48.9 22 40 L
46.5 56 L42
2017.76 49.0 20 39 L
47.5 42 L35
Note. The rst epoch of measurement, 2015B, gives the drift rate of the
companion cloud determined from ground-based and HST observations
spanning the 2015.512015.95 time range from Hueso et al. (2017). Other
epochs are based on two consecutive Neptune rotations, as described in
Table 2. The formal uncertainty of the 2015B drift rate is only 0.1ms
, due
to the long temporal baseline and high density of measurements. Uncertainties
in HST-derived drift rates are about 10ms
. Zonal mean velocities are
calculated using the sixth order polynomial prole from Sromovsky
et al. (1993).
WinJUPOS is available from
The Astronomical Journal, 155:117 (9pp), 2018 March Wong et al.
4. Discussion
4.1. Drifts and Oscillations
Dark vortices on Neptune have exhibited a range of
meridional motions (Table 1), but SDS-2015 is the only one
seen to drift poleward. Drift rates of SDS-2015 (Figure 4)are
about an order of magnitude smaller than the equatorward drift
rate of the GDS seen by Voyager 2, and two orders of
magnitude smaller than the oscillatory drift of DS2 (Hammel
et al. 1989; Sromovsky et al. 1993). Our observations cannot
distinguish between several descriptions of the meridional
motion of SDS-2015: poleward drift over the full 20152017
period, poleward drift from 2015 to 2016 followed by rest at
49°S, meandering, or poorly sampled oscillation. Latitudinal
oscillation in the dark spot, if any, is not reected in the
behavior of bright companion clouds (Hueso et al. 2017),to
within a precision of 1°. 5, over the 2015 JulyDecember period.
The latitudinal stability of the companion clouds is robust,
whether measurements are limited to large telescopes (HST,
Keck, Lick, Calar Alto, Palomar)or combined with more
frequently obtained amateur data. Most anticyclones in the
vortex simulations of LeBeau & Dowling (1998)drifted
equatorward, although some remained at a constant latitude
and some experienced periods of poleward drift.
Longitudinal drift rates were measured separately for SDS-
2015 and for its companion clouds (Table 5). At all epochs, the
vortex system moved at a different rate compared to the zonal
mean, and the companion clouds shared a common long-
itudinal drift rate with the dark spot.
Oscillations in shape were one of the most remarkable
ndings regarding the GDS (Smith et al. 1989).Voyager
imaging sequences showed the GDS aspect ratio to vary by
about a factor of 3 over a 290 hr period (assuming sinusoidal
behavior). Table 4lists aspect ratios for the shape of SDS-2015.
The aspect ratio values lack the precision or temporal
resolution that would be needed to demonstrate any oscillation
in shape on such timescales. One signicant limitation to
determining the shape of the spot is set by the companion
cloud, which locally reduces the photometric contrast needed to
establish the dark spot boundary (Figure 1). A constant aspect
ratio of 2 is consistent with the 20152017 measurements.
4.2. Implications for the Background Wind Field
As discussed in Section 1, Neptunes wind eld may have
complexities beyond the smooth Voyager prole of Sromovsky
et al. (1993). The behavior and properties of SDS-2015 and its
surroundings can provide insights into these complexities.
In the meridional direction, SDS-2015 originated in a dark
band centered near
45 latitude, then drifted southward
toward a bright band centered near
55 . No relationship
between these bands and the zonal wind structure has ever been
established. Albedo patterns are variable on decadal timescales
at these latitudes. In 1989, Voyager found a broad dark band
from 45°to 65°S(Smith et al. 1989). In 2003, HST/STIS
observations instead found a narrower dark band centered at
60°S(Karkoschka & Tomasko 2011); the 45°55°S range had
grown bright. Twin dark bands are present in the 20152017
epoch, centered at 45°S and 60°S, with an intervening bright
band at 51°S(Figure 1). To summarize, the dynamic area on
Neptune is bounded near 65°S and 40°S in all three epochs,
although the patterns of bright and dark between these bounds
varied over time.
Jupiter has many alternating bright and dark bands, along
with alternating eastward and westward jets. The jets are
largely stable in position and magnitude, while coloration is
variable and sometimes reverses (e.g., Rogers 1995; Fletcher
et al. 2011; Tollefson et al. 2017b). Future research may
determine whether any common processes operate on Neptune
and Jupiter, where horizontal bands may remain stable in
position, but be variable in color/brightness. Zonal winds on
Jupiter seem to set up xed boundaries for light and dark
bands, although the winds cannot be the only control on
coloration because band colors sometimes change even while
winds remain stable. In Neptunes case, the smooth Voyager
zonal wind prole provides no dynamical boundaries to
constrain the dark bands evident in Figure 1, but there may
be ne structure not currently resolved by the observations.
Vertical wind shear is expected to be present on Neptune,
given the vertical and latitudinal variation in temperatures
derived from Voyager data (Conrath et al. 1991; Fletcher
et al. 2014). Observationally, the dispersion among zonal
speeds of individual cloud tracers in ground-based imaging
data (Fitzpatrick et al. 2014)may be a result of vertical wind
shear. Dark vortices lie deeper in the atmosphere than other
cloud tracers (Tollefson et al. 2017a), so their behavior may
reveal differences between the deeper wind eld and the zonal
wind eld based on Voyager observations of bright cloud
Although ow within the vortex cannot be directly
measured, we can estimate the tangential velocity Vof the
vortex, based on assumptions regarding its vorticity. The spot
resides in a latitude with anticyclonic vorticity. The relative
vorticity z==-´
du dy 3.4 10
at the latitude of
SDS-2015. The Coriolis parameter =- ´ -
f1.6 10 4s
the absolute vorticity z+=- ´ -
f1.9 10 4s
. From Jupiter
and Saturn we know that typically the intrinsic vorticity z0of
vortices is halfway between ζand f(Sromovsky et al. 1993;
Legarreta and Sánchez-Lavega 2005; García-Melendo et al.
2007), so a rough estimate is z=- -
.Dening for an
elliptical circuit z=VL
0, where L
is the length of the
ellipse (p=--(())
ae Oe21 4
),eis the eccentricity,
and p=
ab is the area. Then,
=-=- -
() ()Vb
e21 4 68 m s , 2
using b=1100 km and =- =eba10.87
22 as typical
values from Table 4.
Some time ago it was proposed that Neptunes dark spots
could be Kida type vortices (Polvani et al. 1990). A stationary
Kida vortex obeys the relationship
zz l
+() ()
is the reciprocal aspect ratio. Taking
=0.5 as
a representative value from Table 4,zz =0.67
0for a Kida
vortex, which is greater than the value of zz =0.34
above from the zonal ow and assumed intrinsic vorticity
The disagreement between these two estimates of
carry information regarding the deeper wind eld to which the
dark vortex may be sensitive. Specically, our rst estimate of
0relied on the value of
du dy at the level of the cloud-
tracked winds. But if SDS-2015 resides at a deep level where
The Astronomical Journal, 155:117 (9pp), 2018 March Wong et al.
∣∣du dy du dy2
deep Voy., then the two methods for
0would be exactly consistent.
4.3. The Demise of Dark Spots
An early goal of our observing program had been to check
for equatorward drift of the vortex, in the hope of capturing
widespread equatorial cloud activity resulting from the
destruction of the vortex (LeBeau & Dowling 1998). The
equatorward drift of the GDS may have ended in such an event,
but there was no capability in 1990 to conduct observations
with the sensitivity and resolution to detect it. The evolution of
Bergon Uranus may have been an example of this mode of
vortex demise (de Pater et al. 2011). If not for the annual
cadence of OPAL observations, it might be thought that the
eruption of widespread equatorial cloud activity in JuneJuly
2017 (Molter et al. 2017)could potentially be related to the
violent demise of SDS-2015.
But other dark spots did not seem to drift equatorward
(Table 1). DS2 oscillated in latitude but had no known overall
meridional drift; the end of this vortex was completely
unconstrained. The northern vortices NDS-1994 and NDS-
1996 appeared to have zero meridional drift (Sromovsky
et al. 2001). These features eventually disappeared several
years after discovery, but NDS-1994 curiously became
invisible, while companion clouds at the same latitude
remained for some time. The OPAL 2017 data show a
persistent compact cloud at
47 . 5 latitude, nearly centered
over a barely discernible dark spot with a much lower contrast
than at the earlier times.
It seems that three out of the ve known dark vortices (DS2,
NDS-1994, and SDS-2015)are ending in the same way: with
centered companion clouds. Companion clouds can make it
harder to see the underlying vortex at blue wavelengths at HST
resolution. At high resolution, Voyager images showed that
unlike the GDS, DS2ʼs companion clouds did not form at the
periphery of the spot, but over the center. It would be
interesting to model these data by degrading the Voyager
images to HST resolution and assessing the effect of the
centered companion clouds on the measured contrast of DS2.
The endings of NDS-1994 and NDS-1996 were not
conclusively witnessed. Persistent cloud features remained at
the latitude of NDS-1994, even after the dark vortex itself had
ceased being directly visible. SDS-2017 may be following a
similar evolutionary path. The companion clouds are centered,
and the contrast of the dark spot is reduced to the point of being
barely detectable.
4.4. Cadences for Planetary Observations
The importance of the time domain in observations of the
planets in our solar system is now widely accepted. We are no
longer in the era where the results of a yby mission, or a
single telescopic observation at a new wavelength (or with a
higher spectral or spatial resolution)will automatically lead to
major discoveries. But, discoveries remain to be made.
The OPAL program (Simon et al. 2015)represents a step
into this new era. Approved by the STScI director as a
discretionary program, this effort showed a commitment by a
major astrophysical observatory to contribute to time-domain
solar system science. The discovery of SDS-2015 is a direct
result of that commitment. The discovery of dark vortices on
Neptune is one example of science that has a timescale
perfectly suited to OPALs annual cadence of global map pairs.
Other observatories have contributed time-domain observations
of the atmospheres of Neptune and other giant planets. Kepler K2
observations produced long-duration, short-cadence photometry
(Simon et al. 2016). The resulting light curve data are valuable for
comparison with brown dwarf and exoplanet variability, because
the K2 Neptune observations were tied to resolved imaging data
to interpret the simple photometric variation. A similar effort
was done using Spitzer photometry (Stauffer et al. 2016).Twilight
adaptive-optics imaging programs are building up a new
campaign of frequent snapshot observations at Lick and Keck
observatories (Molter et al. 2017), providing a higher-resolution
complement to ground-based amateur coverage (Hueso et al.
Hypothetically, if OPAL 2017 observations had been too late
to detect a small companion cloud centered over a very low-
contrast dark spot, there would be a major open question: is the
massive equatorial cloud activity in 2017 (Molter et al. 2017)
related to the demise of SDS-2015? Proving such a link would
require dense temporal sampling over throughout 2017, with
1000 km resolution, at visible blue wavelengths. Obtaining
such coverage with the oversubscribed HST is very difcult,
and a dedicated solar system space observatory is not yet
available (Bell et al. 2015). Imaging with robotic visible-light
adaptive optics, Robo-AO (Baranec et al. 2014), can fulll the
monitoring cadence and angular resolutions (~0.
)required to
measure the dynamics of major vortex and cloud activity on
Neptune. A more capable Robo-AO system is being prepared
for deployment on the UH 2.2 m telescope at Maunakea, HI,
and part of its mission is to enable planetary science not
feasible elsewhere.
Clearly, there is much room in the discovery space of solar
system time domain science. There is room in this discovery
space for exploration by a dedicated solar system space
telescope, a network of ground facilities, and cadence programs
at astrophysical observatories with advanced capabilities.
5. Summary
A new dark spot was discovered on Neptune in 2015 and
observed through 2017. We draw the following conclusions
and inferences:
1. The vortex drifted poleward, with a drift rate in the range
of 1°.72°.5/year (Figure 4).
2. The vortex faded over time (Figure 3), with a 467 nm
contrast of
6.6% 0.8% in 2015.71 and
.4% in 2017.76. Bright companion clouds also grew
increasingly centered over the dark spot with each
successive observation (Figure 1). These evolutionary
changes may be part of a vortex dissipation sequence,
because aspects of these changes were seen in previous
dark spots: DS2 had centered companion clouds during
the Voyager 2 encounter and was never seen again, and
companion clouds of NDS-1994 persisted for some time
after the dark spot itself faded from view.
3. If we make reasonable (but poorly constrained)assump-
tions about the vorticity of SDS-2015 and its surround-
ings (described in Section 4.2), then we nd that the
horizontal wind shear
at the deep altitude of the
vortex must be about twice the value measured by
Voyager cloud-tracked winds.
The Astronomical Journal, 155:117 (9pp), 2018 March Wong et al.
4. Companion clouds create errors in measurements of the
dark spots boundaries, leading to uncertainties in the
features size, shape, drift, and position that are large
compared to other sources of error. Stated uncertainties in
Table 4include the effect of companion clouds on
measurements of dark spot morphology.
5. Additional details of dark vortex behavior and evolution may
be gleaned in the future, when faster cadence observations
(with high spatial resolution at optical wavelengths)enable
exploration of the time-domain discovery space.
Based on observations associated with programs GO-13937,
GO-14044, GO-14334, GO-14492, and GO-14756, with
support provided by NASA through grants from the Space
Telescope Science Institute (operated by the Association of
Universities for Research in Astronomy, Inc., under NASA
contract NAS 5-26555). Data were obtained from the Data
Archive at the Space Telescope Science Institute. Support was
provided by the National Science Foundation through grant
AST-1615004 and by the NASA Earth and Space Science
Fellowship through grant NNX16AP12H to I.dP. and J.T., and
by the Alfred P. Sloan Foundation and the National Science
Foundation through grant AST-1712014 to C.B. We acknowl-
edge support from grants AYA2015-65041-P (MINECO/
FEDER, UE), Grupos Gobierno Vasco IT-765-13, and UPV/
EHU UFI11/55 to A.S.L. and R.H.
We thank an anonymous reviewer for their quick and
constructive review.
Facility: HST(WFC3).
Software: Astroconda, IDL, L.A.Cosmic, WinJUPOS, JPL
Horizons Ephemerides.
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The Astronomical Journal, 155:117 (9pp), 2018 March Wong et al.
... Furthermore, since Voyager did not have the capability to provide more than filter-imaging observation of the reflected-sunlight spectra of this vortex its vertical structure remained largely unknown. However, since the discovery of the GDS, several more short-lived dark spots in Neptune's atmosphere have been detected in Hubble Space Telescope (HST) filter-imaging observations with the Wide Field and Planetary Camera 2 and Wide Field Camera 3 (WFC3) instruments [6][7][8], in both the northern and southern hemispheres. The most recent example is a northern hemisphere dark spot, discovered in 2018 at 23 • N (NDS-2018) [9]. ...
... This spot had a similar size to the GDS and was then seen to drift equatorwards [10], before apparently disappearing in late 2022 [11]. Neptunian dark spots are characterised by low reflectance at short wavelengths (λ < 700 nm), but have not been detected at longer wavelengths [4,7]. ...
... While the GALACSI adaptive optics system achieves a spatial resolution of ∼0.06" at 800 nm in Narrow-Field Mode, this performance deteriorates to ∼0.2" at shorter wavelengths ( Supplementary Fig. 9). Since the dark spot contrast increases at shorter wavelengths [4,6,7], but spatial resolution decreases, we found that wavelengths near 551 nm were optimal for dark spot detection. However, even at these wavelengths, we were unable to clearly distinguish the small, faint NDS-2018 from its surroundings in the raw data (Fig. 1a). ...
Full-text available
Previous observations of dark vortices in Neptune's atmosphere, such as Voyager-2's Great Dark Spot, have been made in only a few, broad-wavelength channels, which has hampered efforts to pinpoint their pressure level and what makes them dark. Here, we present Very Large Telescope (Chile) MUSE spectrometer observations of Hubble Space Telescope's NDS-2018 dark spot, made in 2019. These medium-resolution 475 - 933 nm reflection spectra allow us to show that dark spots are caused by a darkening at short wavelengths (< 700 nm) of a deep ~5-bar aerosol layer, which we suggest is the H$_2$S condensation layer. A deep bright spot, named DBS-2019, is also visible on the edge of NDS-2018, whose spectral signature is consistent with a brightening of the same 5-bar layer at longer wavelengths (> 700 nm). This bright feature is much deeper than previously studied dark spot companion clouds and may be connected with the circulation that generates and sustains such spots.
... The diffraction limit of HST at 0.6 µm is ∼ 0.055 arcseconds. HST images were taken on 2-4 days each year as part of the OPAL program 2 (Simon et al. 2015) and as part of mid-cycles intended to observe the dark spot NGDS2018 (Simon et al. 2019;Wong et al. 2022). Each day of observation produced a dense sampling of images, with each frame taken minutes to hours apart during these days of observation. ...
... HST images were taken by the UVIS detector of the Wide Field Camera 3 instrument and were reduced using standard HST reduction tools in the same manner as described in Wong et al. (2018). This included converting the data to units of I/F, as defined by Hammel et al. (1989a): ...
... I/F obs is the observed reflectivity, µ 0 is the cosine of the solar incidence angle, and µ is the cosine of the emission angle. The constant k is empirically defined; we used k = 0.867 (Wong et al. 2018). All Keck, Lick, and HST images were projected onto a flat latitude-longitude map (referred to as a "projected image") using the Python package "nirc2 reduce" (Molter 2022;Molter et al. 2019, see Figure 3 for an example of image projection) to facilitate assigning latitude/longitude coordinates to features on Neptune's disk. ...
Full-text available
Using near-infrared observations of Neptune from the Keck and Lick Observatories, and the Hubble Space Telescope in combination with amateur datasets, we calculated the drift rates of prominent infrared-bright cloud features on Neptune between 2018 and 2021. These features had lifespans of $\sim 1$ day to $\geq$1 month and were located at mid-latitudes and near the south pole. Our observations permitted determination of drift rates via feature tracking. These drift rates were compared to three zonal wind profiles describing Neptune's atmosphere determined from features tracked in H band (1.6 $\mu m$), K' band (2.1 $\mu m$), and Voyager 2 data at visible wavelengths. Features near $-70 \deg$ measured in the F845M filter (845nm) were particularly consistent with the K' wind profile. The southern mid-latitudes hosted multiple features whose lifespans were $\geq$1 month, providing evidence that these latitudes are a region of high stability in Neptune's atmosphere. We also used HST F467M (467nm) data to analyze a dark, circumpolar wave at $- 60 \deg$ latitude observed on Neptune since the Voyager 2 era. Its drift rate in recent years (2019-2021) is $4.866 \pm 0.009 \deg $/day. This is consistent with previous measurements by Karkoschka (2011), which predict a $4.858 \pm 0.022 \deg$/day drift rate during these years. It also gained a complementary bright band just to the north.
... Furthermore, since Voyager 2 could not provide more than filter-imaging observation of the reflected-sunlight spectra of this vortex, its vertical structure remained ill-defined. However, since the discovery of the GDS, several more short-lived dark spots in Neptune's atmosphere have been detected in Hubble Space Telescope (HST) filter-imaging observations with the Wide Field and Planetary Camera 2 and Wide Field Camera 3 (WFC3) instruments [6][7][8] , in both the northern and southern hemispheres. The most recent example is a northern hemisphere dark spot, discovered in 2018 at 23 ∘ N, designated NDS-2018 9 . ...
... This spot had a similar size to the GDS and was then seen to drift equatorward 10 before apparently disappearing in late 2022 11 . Neptunian dark spots are characterized by low reflectance at short wavelengths (λ < 700 nm) but are undetected at longer wavelengths 4,7 . North is at top right. ...
Full-text available
Previous observations of dark vortices in Neptune’s atmosphere, such as Voyager 2’s Great Dark Spot (1989), have been made in only a few broad-wavelength channels, hampering efforts to determine these vortices’ pressure levels and darkening processes. We analyse spectroscopic observations of a dark spot on Neptune identified by the Hubble Space Telescope as NDS-2018; the spectral observations were made in 2019 by the Multi Unit Spectroscopic Explorer (MUSE) of the Very Large Telescope (Chile). The MUSE medium-resolution 475–933 nm reflection spectra allow us to show that dark spots are caused by darkening at short wavelengths (<700 nm) of a deep ~5 bar aerosol layer, which we suggest is the H2S condensation layer. A deep bright spot, named DBS-2019, is also visible on the edge of NDS-2018, with a spectral signature consistent with a brightening of the same 5 bar layer at longer wavelengths (>700 nm). This bright feature is much deeper than previously studied dark-spot companion clouds and may be connected with the circulation that generates and sustains such spots.
... A few noteworthy observations have been reported since 2007: In 2015, a bright storm was seen in near-IR Keck AO data, which was identified as a companion cloud to a new dark spot on the planet (Hueso et al. 2017;Wong et al. 2018), quite similar to the companion clouds seen near the Voyager GDS and dark spots in the 1990s. A second new dark spot was detected in the north in 2018 ; this spot was exceptionally large and long-lived (Wong et al. 2022). ...
... HST data were reduced and calibrated in the manner described by Wong et al. (2018Wong et al. ( , 2022, and were converted to units of I/F using Equation 1. The HST data from 2020 were deconvolved using the method described in Fry & Sromovsky (2023). ...
... The GDS was most visible at a wavelength of 0.48 μm, but was undetectable longward of 0.7 μm. Since the Voyager flyby several new dark spots have been seen in Neptune's atmosphere from Hubble Space Telescope observations in 1994, 1996(Hammel et al., 1995Hsu et al., 2019;Sromovsky et al., 2001;Wong et al., 2018). These have all had similar spectral characteristics to those noted for the GDS, namely that they are visible near 0.5 μm, but not visible longward of 0.7 μm. ...
... Direct measurements at relevant pressure levels are only available for a single time and place: the Galileo Probe entry site at Jupiter (Magalhães et al., 2002;Seiff et al., 1998). Given the dependence of vortex vertical/horizontal aspect ratio on the difference between internal and environmental stratification (Hassanzadeh et al., 2012), and the roughly comparable horizontal dimensions of vortices on Jupiter and Neptune (Li et al., 2004;Wong et al., 2018), the thicker vertical dimensions of Jupiter's vortices may indicate that the environmental stratification is weaker on Jupiter compared to Neptune. However, this tightly constrained thickness scenario runs counter to the observation that such dark spot disturbances are often accompanied by what appear to be orographic clouds near the tropopause, for example, the "scooter" cloud that accompanied Voyager-2's GDS. ...
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We present a reanalysis (using the Minnaert limb‐darkening approximation) of visible/near‐infrared (0.3–2.5 μm) observations of Uranus and Neptune made by several instruments. We find a common model of the vertical aerosol distribution i.e., consistent with the observed reflectivity spectra of both planets, consisting of: (a) a deep aerosol layer with a base pressure >5–7 bar, assumed to be composed of a mixture of H2S ice and photochemical haze; (b) a layer of photochemical haze/ice, coincident with a layer of high static stability at the methane condensation level at 1–2 bar; and (c) an extended layer of photochemical haze, likely mostly of the same composition as the 1–2‐bar layer, extending from this level up through to the stratosphere, where the photochemical haze particles are thought to be produced. For Neptune, we find that we also need to add a thin layer of micron‐sized methane ice particles at ∼0.2 bar to explain the enhanced reflection at longer methane‐absorbing wavelengths. We suggest that methane condensing onto the haze particles at the base of the 1–2‐bar aerosol layer forms ice/haze particles that grow very quickly to large size and immediately “snow out” (as predicted by Carlson et al. (1988),<2066:CMOTGP>2.0.CO;2), re‐evaporating at deeper levels to release their core haze particles to act as condensation nuclei for H2S ice formation. In addition, we find that the spectral characteristics of “dark spots”, such as the Voyager‐2/ISS Great Dark Spot and the HST/WFC3 NDS‐2018, are well modelled by a darkening or possibly clearing of the deep aerosol layer only.
... The atmospheres of the giant planets exhibit significant variation at visible and nearinfrared wavelengths, where we observe sunlight scattered and/or absorbed by gases, clouds, and hazes [342,[358][359][360][361][362][363][364][365][366][367][368][369][370][371][372] (see Simon et al. [373] for a review). Corresponding variations in atmospheric temperatures and chemistry may naturally be expected. ...
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The mid-infrared spectral region provides a unique window into the atmospheric temperature, chemistry, and dynamics of the giant planets. From more than a century of mid-infrared remote sensing, progressively clearer pictures of the composition and thermal structure of these atmospheres have emerged, along with a greater insight into the processes that shape them. Our knowledge of Jupiter and Saturn has benefitted from their proximity and relatively warm temperatures, while the details of colder and more distant Uranus and Neptune are limited as these planets remain challenging targets. As the timeline of observations continues to grow, an understanding of the temporal and seasonal variability of the giant planets is beginning to develop with promising new observations on the horizon.
... The atmospheres of the giant planets exhibit significant variation at visible and nearinfrared wavelengths, where we observe sunlight scattered and/or absorbed by gases, clouds, and hazes [342,[358][359][360][361][362][363][364][365][366][367][368][369][370][371][372] (see Simon et al. [373] for a review). Corresponding variations in atmospheric temperatures and chemistry may naturally be expected. ...
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The mid-infrared spectral region provides a unique window into the atmospheric temperature, chemistry, and dynamics of the giant planets. From more than a century of mid-infrared remote sensing, progressively clearer pictures of the composition and thermal structure of these atmospheres have emerged, along with a greater insight into the processes that shape them. Our knowledge of Jupiter and Saturn has benefitted from their proximity and relatively warm temperatures, while the details of colder and more distant Uranus and Neptune are limited, as these planets remain challenging targets. As the timeline of observations continues to grow, an understanding of the temporal and seasonal variability of the giant planets is beginning to develop, with promising new observations on the horizon.
... Neptune's atmosphere continues to be highly active to date including formations of a new dark spot [102], an equatorial storm [103] and other smaller features [104][105][106]. Mid-infrared observations of the planet also reveal that stratospheric temperatures exhibit changes in timescales shorter than a season, which may be driven by cumulus events in the troposphere [107]. ...
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The outer planets of our Solar System display a myriad of interesting cloud features, of different colors and sizes. The differences between the types of observed clouds suggest a complex interplay between the dynamics and chemistry at play in these atmospheres. Particularly, the stark difference between the banded structures of Jupiter and Saturn vs. the sporadic clouds on the ice giants highlights the varieties in dynamic, chemical and thermal processes that shape these atmospheres. Since the early explorations of these planets by spacecrafts, such as Voyager and Voyager 2, there are many outstanding questions about the long-term stability of the observed features. One hypothesis is that the internal heat generated during the formation of these planets is transported to the upper atmosphere through latent heat release from convecting clouds (i.e., moist convection). In this review, we present evidence of moist convective activity in the gas giant atmospheres of our Solar System from remote sensing data, both from ground- and space-based observations. We detail the processes that drive moist convective activity, both in terms of the dynamics as well as the microphysical processes that shape the resulting clouds. Finally, we also discuss the effects of moist convection on shaping the large-scale dynamics (such as jet structures on these planets).
Dark spots on Neptune observed by Voyager and the Hubble Space Telescope are thought to be anticyclones with lifetimes of a few years, in contrast with very long-lived anticyclones in Jupiter and Saturn. The full life cycle of any Neptune dark spot has not been captured due to limited temporal coverage, but our Hubble observations of a recent feature, NDS-2018, provide the most complete long-term observational history of any dark vortex on Neptune. Past observations suggest some dark spots meet their demise by fading and dissipating without migrating meridionally. On the other hand, simulations predict a second pathway with equatorward migration and disruption. Our HST observations suggest NDS-2018 is following the second pathway. Some of the HST observations reveal transient dark features with widths of about 4000 to 9000 km, at latitudes between NDS-2018 and the equator. The secondary dark features appeared before changes in the meridional migration of NDS-2018 were seen. These features have somewhat smaller size and much smaller contrast compared to the main dark spot. Discrete secondary dark features of this scale have never been seen near previous dark spots, but global-scale dark bands are associated with several previous dark spots in addition to NDS-2018. The absolute photometric contrast of NDS-2018 (as large as 19%) is greater than previous dark spots, including the Great Dark Spot seen by Voyager. New simulations suggest that vortex internal circulation is weak relative to the background vorticity, presenting a clearly different case from stronger anticyclones observed on Jupiter and Saturn.
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Observations of Uranus were made on the 8/9th November with HST/WFC3 at a time when a huge cloud complex was present at 30 - 40N. We imaged Uranus in seven filters spanning 467 - 924 nm, and analysed these observations with the NEMESIS radiative-transfer and retrieval code. The same system was also observed in the H-band with VLT/SINFONI ON 31st October and 11th November (Irwin et al., 2016). To constrain the background cloud particle sizes and scattering properties we conducted a limb-darkening analysis of the background cloud structure at 30-40N by simultaneously fitting: a) these HST/OPAL observations at a range of zenith angles; b) the VLT/SINFONI observations at a range of zenith angles; and c) IRTF/SpeX observations made in 2009 (Irwin et al., 2015). We find that the observations are well modelled with a three-component cloud comprised of: 1) a vertically thin, but optically thick 'deep' tropospheric cloud at a pressure of ~2 bars; 2) a methane-ice cloud at the methane-condensation level with variable vertical extent; and 3) a vertically extended tropospheric haze. We find that the particles sizes in both haze and tropospheric cloud have an effective radius of ~0.1 micron, although we cannot rule out larger particle sizes in the tropospheric cloud. We find that the particles in both the tropospheric cloud and haze are more scattering at short wavelengths, giving them a blue colour, but are more absorbing at longer wavelengths, especially for the tropospheric haze. For the particles in the storm clouds, which we assume to be composed of methane ice particles, we find that their mean radii must be ~0.5 micron. We find that the high clouds have low integrated opacity, and that "streamers" reminiscent of thunderstorm anvils are confined to levels deeper than 1 bar. These results argue against vigorous moist convective origins for the cloud features.
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Observations of Neptune with the Kepler Space Telescope yield a 49-day light curve with 98% coverage at a 1-minute cadence. A significant signature in the light curve comes from discrete cloud features. We compare results extracted from the light curve data with contemporaneous disk-resolved imaging of Neptune from the Keck 10-meter telescope at 1.65 microns and Hubble Space Telescope visible imaging acquired 9 months later. This direct comparison validates the feature latitudes assigned to the K2 light curve periods based on Neptune's zonal wind profile, and confirms observed cloud feature variability. Although Neptune's clouds vary in location and intensity on short and long time scales, a single large discrete storm seen in Keck imaging dominates the K2 and Hubble light curves; smaller or fainter clouds likely contribute to short-term brightness variability. The K2 Neptune light curve, in conjunction with our imaging data, provides context for the interpretation of current and future brown dwarf and extrasolar planet variability measurements. In particular we suggest that the balance between large, relatively stable, atmospheric features and smaller, more transient, clouds controls the character of substellar atmospheric variability. Atmospheres dominated by a few large spots may show inherently greater light curve stability than those which exhibit a greater number of smaller features.
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Jupiter's Great Red Spot (GRS) is one of its most distinct and enduring features. Since the advent of modern telescopes, keen observers have noted its appearance and documented a change in shape from very oblong to oval, confirmed in measurements from spacecraft data. It currently spans the smallest latitude and longitude size ever recorded. Here we show that this change has been accompanied by an increase in cloud/haze reflectance as sensed in methane gas absorption bands, increased absorption at wavelengths shorter than 500 nm, and increased spectral slope between 500 and 630 nm. These changes occurred between 2012 and 2014, without a significant change in internal tangential wind speeds; the decreased size results in a 3.2 day horizontal cloud circulation period, shorter than previously observed. As the GRS has narrowed in latitude, it interacts less with the jets flanking its north and south edges, perhaps allowing for less cloud mixing and longer UV irradiation of cloud and aerosol particles. Given its long life and observational record, we expect that future modeling of the GRS's changes, in concert with laboratory flow experiments, will drive our understanding of vortex evolution and stability in a confined flow field crucial for comparison with other planetary atmospheres.
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The Hubble 2020: Outer Planet Atmospheres Legacy program is generating new yearly global maps for each of the outer planets. This report focuses on Jupiter results from the first year of the campaign. The zonal wind profile was measured and is in the same family as the Voyager and Cassini era profiles, showing some variation in mid- to high-latitude wind jet magnitudes, particularly at +40° and -35° planetographic latitude. The Great Red Spot continues to maintain an intense orange coloration, but also shows new internal structures, including a reduced core and new filamentary features. Finally, a wave that was not previously seen in Hubble images was also observed and is interpreted as a baroclinic instability with associated cyclone formation near 16° N latitude. A similar feature was observed faintly in Voyager 2 images, and is consistent with the Hubble feature in location and scale. © 2015. The American Astronomical Society. All rights reserved..
Since 2013, observations of Neptune with small telescopes have resulted in several detections of long-lived bright atmospheric features that have also been observed by large telescopes such as Keck II or Hubble. The combination of both types of images allows the study of the long term evolution of major cloud systems in the planet. In 2013 and 2014 two bright features were present on the planet at southern mid latitudes. These may have merged in late 2014, possibly leading to the formation of a single bright feature observed during 2015 at the same latitude. This cloud system was first observed in January 2015 and nearly continuously from July to December 2015 in observations with telescopes in the 2 to 10 meter class and in images from amateur astronomers. These images show the bright spot as a compact feature at 40.1 deg South planetographic latitude well resolved from a nearby bright zonal band that extended from 42 deg South to 20 deg South. Tracking its motion from July to November 2015 suggests a longitudinal oscillation of 16 deg in amplitude with a 90 day period, typical of dark spots on Neptune and similar to the Great Red Spot oscillation in Jupiter. The limited time covered by high-resolution observations only covers one full oscillation and other interpretations of the changing motions could be possible. HST images in September 2015 show the presence of a dark spot at short wavelengths in the southern flank of the bright cloud observed throughout 2015.
We present five epochs of WFC3 HST Jupiter observations taken between 2009–2016 and extract global zonal wind profiles for each epoch. Jupiter’s zonal wind field is globally stable throughout these years, but significant variations in certain latitude regions persist. We find that the largest uncertainties in the wind field are due to vortices or hot-spots, and show residual maps which identify the strongest vortex flows. The strongest year-to-year variation in the zonal wind profiles is the 24°N jet peak. Numerous plume outbreaks have been observed in the Northern Temperate Belt and are associated with decreases in the zonal velocity and brightness. We show that the 24°N jet peak velocity and brightness decreased in 2012 and again in late 2016, following outbreaks during these years. Our February 2016 zonal wind profile was the last highly spatially resolved measurement prior to Junos first science observations. The final 2016 data were taken in conjunction with Juno’s perijove 3 pass on 11 December, 2016, and show the zonal wind profile following the plume outbreak at 24°N in October 2016.
We have used the Spitzer Space Telescope in February 2016 to obtain high cadence, high signal-to-noise, 17-hour duration light curves of Neptune at 3.6 and 4.5 $\mu$m. The light curve duration was chosen to correspond to the rotation period of Neptune. Both light curves are slowly varying with time, with full amplitudes of 1.1 mag at 3.6 $\mu$m and 0.6 mag at 4.5 $\mu$m. We have also extracted sparsely sampled 18-hour light curves of Neptune at W1 (3.4 $\mu$m) and W2 (4.6 $\mu$m) from the WISE/NEOWISE archive at six epochs in 2010-2015. These light curves all show similar shapes and amplitudes compared to the Spitzer light curves but with considerable variation from epoch to epoch. These amplitudes are much larger than those observed with Kepler/K2 in the visible (amplitude $\sim$0.02 mag) or at 845 nm with the Hubble Space Telescope in 2015 and at 763 nm in 2016 (amplitude $\sim$ 0.2 mag). We interpret the Spitzer and WISE light curves as arising entirely from reflected solar photons, from higher levels in Neptune's atmosphere than for K2. Methane gas is the dominant opacity source in Neptune's atmosphere, and methane absorption bands are present in the HST 763, and 845 nm, WISE W1, and Spitzer 3.6 $\mu$m filters.
We imaged Uranus in the near infrared from 2012 into 2014, using the Keck/NIRC2 camera and Gemini/NIRI camera, both with adaptive optics. We obtained exceptional signal to noise ratios by averaging 8-16 individual exposures in a planet-fixed coordinate system. noise-reduced images revealed many low-contrast discrete features and large scale cloud patterns not seen before, including scalloped waveforms just south of the equator. In all three years numerous small (600-700 km wide) and mainly bright discrete features were seen within the north polar region (north of about 55\deg N). Over 850 wind measurements were made, the vast majority of which were in the northern hemisphere. These revealed an extended region of solid body rotation between 62\deg N and at least 83\deg N, at a rate of 4.08$\pm0.015$\deg/h westward relative to the planet's interior (radio) rotation of 20.88\deg/h westward. Near-equatorial speeds measured with high accuracy give different results for waves and small discrete features, with eastward drift rates of 0.4\deg/h and 0.1\deg/h respectively. The region of polar solid body rotation is a close match to the region of small-scale polar cloud features, suggesting a dynamical relationship. While winds at high southern latitudes (50\deg S - 90\deg S) are unconstrained by groundbased observations, a recent reanalysis of 1986 Voyager 2 observations by Karkoschka (2015, Icarus 250, 294-307) has revealed an extremely large north-south asymmetry in this region, which might be seasonal. Greatly increased activity was seen in 2014, including the brightest ever feature seen in K' images (de Pater et al. 2015, Icarus 252, 121-128). Over the 2012-2014 period we identified six persistent discrete features. Three were tracked for more than two years, two more for more than one year, and one for at least 5 months and continuing.
As new large-scale astronomical surveys greatly increase the number of objects targeted and discoveries made, the requirement for efficient follow-up observations is crucial. Adaptive optics imaging, which compensates for the image-blurring effects of Earth's turbulent atmosphere, is essential for these surveys, but the scarcity, complexity and high demand of current systems limit their availability for following up large numbers of targets. To address this need, we have engineered and implemented Robo-AO, a fully autonomous laser adaptive optics and imaging system that routinely images over 200 objects per night with an acuity 10 times sharper at visible wavelengths than typically possible from the ground. By greatly improving the angular resolution, sensitivity, and efficiency of 1-3 m class telescopes, we have eliminated a major obstacle in the follow-up of the discoveries from current and future large astronomical surveys.
We observed Neptune between June and October 2003 at near- and mid-infrared wavelengths with the 10-m W.M. Keck II and I telescopes, respectively; and at radio wavelengths with the Very Large Array. Images were obtained at near-infrared wavelengths with NIRC2 coupled to the adaptive optics system in both broad- and narrow-band filters between 1.2 and . In the mid-infrared we imaged Neptune at wavelengths between 8 and , and obtained slit-resolved spectra at and . At radio wavelengths we mapped the planet in discrete filters between 0.7 and 6 cm.