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Published March 2018 | Published
Journal Article Open

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 -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 Neptune's dark spots and the need for faster cadence dark spot observations in the future.

Additional Information

© 2018. The American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2017 November 21; revised 2017 December 22; accepted 2017 December 29; published 2018 February 15. 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 acknowledge 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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Created:
August 21, 2023
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October 20, 2023