2
$\begingroup$

The open access paper Formation of a New Great Dark Spot on Neptune in 2018 (cited in Gizmodo's Formation of Dark Vortex on Neptune Captured For the Very First Time and Phys.org's Hubble captures birth of giant storm on Neptune) shows images of Neptune taken by the Hubble Space Telescope.

The projections are called "global cylindrical maps" and those usually result in weirdly stretched features near the poles but do not result in the spiky hairdo or Cheela-like appearance in the Hubble images below.

enter image description here

Source

What causes this effect seen in these projections?

enter image description here

Individual maps from Hubble images in 2017 through 2018, in the same filters and processing as Figure 1. These maps span ±90° of latitude and longitude, and tick marks indicate 30° latitude and longitude intervals.

enter image description here

False‐color global cylindrical maps from Hubble, spanning 180° of latitude and 360° of longitude, from images at 845 (red), 547 (green), and 467 nm (blue), and all use the same contrast scaling with a light unsharp mask applied. The maps are centered on 180°W longitude, except 2015, which was centered on 0° longitude. Dotted lines are shown at ~10° and 35°N to indicate the active region prior to the discovery of NDS‐2018.


Reference Cheela, from here:

enter image description here

$\endgroup$
5
  • $\begingroup$ moved to here from here. $\endgroup$
    – uhoh
    Mar 27 '19 at 5:43
  • 1
    $\begingroup$ Wong et al. 2018 section 2.5 mentions an algorithm called "frydrizzle." $\endgroup$
    – Mike G
    Mar 27 '19 at 18:35
  • $\begingroup$ Thanks! ...which points to section 3.1 in Fry 2012 which describes an implementation that unfortunately doesn't extend to the limbs where the artifact occurs. $\endgroup$
    – uhoh
    Mar 27 '19 at 21:22
  • 1
    $\begingroup$ Here's that map wrapped onto a globe: maptoglobe.com/r105OKBEd $\endgroup$
    – PM 2Ring
    Mar 22 at 3:05
  • $\begingroup$ @PM2Ring sweet! $\endgroup$
    – uhoh
    Mar 22 at 10:35
2
$\begingroup$

Neptune has an axial tilt of 28.3 degrees and an orbital period of 164.8 Earth years; currently, the north pole is tilted away from Earth.

The maps in your question were each constructed from twelve exposures of Neptune: four each in 845 (red), 547 (green), and 467 nm (blue) wavelengths. Each of the exposures was then converted from an effectively-orthographic view of a hemisphere of Neptune to the equirectangular projection used in the maps. The precise technique is cited to https://iopscience.iop.org/article/10.3847/1538-3881/aaa6d6 and https://iopscience.iop.org/article/10.1088/0004-637X/812/1/55.

The bumpy appearance comes from using four discrete observations rather than a single continuous one; the black area is portions of Neptune that were never visible. Each of those spikes is the result of projecting a single color of a single pixel on the edge of Neptune's visible disk onto an equirectangular map.

$\endgroup$
1
  • $\begingroup$ Thank you for your answer! Yes this makes total sense; each "spike" is a single color, so it originates from a single pixel of data, and the different colors will not overlap at the last pixel. $\endgroup$
    – uhoh
    Mar 19 at 4:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.