A New Dark Vortex on Neptune

Abstract

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.

Formation of a New Great Dark Spot on Neptune in 2018

“…The longitude discrepancy could be the result of an error as small as 0.8% in the drift rate measured from the 5 and 6 November data or a small change in drift rate over the~56-day interval between the observations, which is common among giant vortices (Smith et al, 1989;Wong et al, 2018). This drift rate extrapolates to a location of 308°W on 10 September, while the dark feature is observed at 339°W on that date.…”

“…The maximum contrast of SDS-2015 was about 6.7% in the F467M filter, and contrast decreased by about 1.7% per year over the 2015-2017 time period (Wong et al, 2018). Although data sampled at an annual cadence are not sufficient to test this hypothesis, it is still clear that over these years, discrete regions of cloud activity were present on Neptune at the same latitude as NDS-2018, while all other longitudes within this band of activity lacked bright cloud features.…”

“…The most recent OPAL Neptune observations in 2018 reveal a new, distinct, dark spot with bright companion clouds, centered on 23°N planetographic latitude. These features are small with relatively low contrast, and they are visible only at blue/green wavelengths (e.g., Hammel et al, 1995, Wong et al, 2018. NDS-2018 is the sixth of a series of dark vortices discovered by Voyager and the Hubble Space Telescope (HST).…”

“…The demise of a dark vortex has only been documented for SDS-2015 in HST OPAL data (Wong et al, 2018) and possibly NDS-1994 during a period of intensive HST Neptune observations following the Voyager encounter . No internal velocity field has been measured for any of Neptune's dark spots.…”

“…-2015 was discovered at 46°S in 2015, drifting to 49°S in 2017, with companion clouds offset to the north by 1-4°(Wong et al, 2018). Prior to its discovery, ground-based observations showed bright cloud features in 2014-2015 at latitudes of 35-45°S, while features in 2013 were spread over a wider range of 35-52°S(Hueso et al, 2017).…”