23 August 2014

Triton: Addenda and Errata

Today’s blog features a correction and some additional details on the new Triton map and movie  blog posted a few days ago  >  stereomoons.blogspot.com/2014/08/triton-at-25.html.  <

First, a correction.  The surface compositions of Triton and Pluto are indeed similar but not quite identical.  Triton has nitrogen, methane, carbon dioxide and carbon monoxide on its surface, and probably some water ice, but of those ices Pluto does not have carbon dioxide or water ice that we can measure from Earth.  What those differences may mean for geologic and atmospheric history no one can say as yet with confidence, but all the more reason for going to Pluto and someday back to Triton.

As a matter of personal opinion, I am sometimes asked which planets I’d like to see explored next.  Europa is first on the list, but after that we have the ice giant planets Uranus and Neptune and their strange families of icy moons (including Miranda, Ariel, and Triton to name a few).  These large bodies are distinct and different from the gas giants Jupiter and Saturn but have been visited only once, by Voyager 2 with instruments designed in the 1970s.  What we could learn by going back has been amply demonstrated by the innumerable discoveries of Cassini at Saturn.  

Triton, whose surface may be younger than a few million years and may be geologically active today, is one of the most fascinating bodies on the Solar System.  Its maximum surface temperature is only 35 degrees above absolute zero, and yet volcanoes and geysers have remade its surface, possibly within the lifetime of the human species.  Even the Voyager scientists, who had become accustomed to surprises after the discoveries on Io, Ganymede, Titan, Miranda and the rest, were left almost speechless as Voyager made its final planetary visit.  As Larry Soderblom exclaimed at the press briefing when he showed the first Triton images, “What a way to leave the Solar System!”

T-shirt printed up to during the Neptune encounter 1989.  The t-shirt and owner are now 25 years older.
Neptune was fabulous too with its strange and dynamic cloud patterns and its odd, incomplete ring system.  One of my first efforts in serious image processing was to reconstruct the Neptune ring high-phase-angle observations.  These were the best images of the rings we got, but the long exposures saturated Neptune itself and created bright haloes that were difficult to suppress.   Normally exposed Neptune crescent images were substituted but the heavy filtering required for the bright haloes also enhanced noise in the images.   The end result was a montage showing a crescent Neptune and the entire ring system.   This was done back in 1992 or 93, so I’m sure I or someone else could do a better job now.  It is a composite of 5 (or 6?) different exposures taken at different times and distances from Neptune, but all the data are real.  
Crescent mosaic of Neptune from Voyager 2 on departure, August, 1989.
Triton Map: Enhancement and 'Color'. The enhancement applied to the Triton map in the August 21 post was a modest contrast-stretch only; no differential color enhancement was applied.   Surface brightness contrasts on Triton exist but are not as strong as on Pluto.  The color does have a greenish cast in equatorial areas.  This seems to be real, but there are ‘concerns’ with Triton’s color.  First, the color images were sometimes smeared or noisy, due to long exposures under very low solar lighting intensity for which the cameras were not designed.  This explains some of the splotchy color mottling that is apparent in a few areas.  Secondly, there are some uncertainties in the photometric properties of Triton.  Earth-based spectra of Triton obtained in the 1970’s and 80’s differ in the inferred visual color of Triton and it was not possible to get an exact color ‘calibration’ on Triton.  We did our best, but the colors may not be only approximate, given the slightly different color sensitivity of the Voyager 2 camera.    

The Triton map is suitable to drop into Google Earth or similar programs!  You can now zoom and spin on Triton in any way you like.    

Neptune in the Movie.  Several have asked why Neptune doesn’t appear in our movie.  Several reasons, the most important of which is that we ran out of time for the August 25 anniversary.  The second is that we compress almost 10 days of the encounter into 1 minute.  Neptune would probably appear in 2, maybe 3 of those frames.  We are looking into it.  We know that Neptune and Triton do appear together in the sky about a day out from Triton, and again 6 days later, but do not appear in proximity to each other on the way in, apparently.  We may attempt to add Neptune back in for a final version later this year.

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