James E. Richardson, Jr.

Planetary Scientist

Abstract and Figures from:

Titan's Surface and Rotation: New Results from Voyager 1 Images

James Richardson, Ralph Lorenz, & Alfred McEwen

Published in Icarus, July 2004, Vol. 170/1, pp. 113-124

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For many years, the conventional wisdom has been that Voyager 1 was unable to detect the surface of Saturn's moon Titan during the spacecraft's November 1980 close flyby, due to the moon's thick, hazy atmosphere. Recent re-processing of these images has shown that the Vidicon imaging systems onboard Voyager 1 did indeed detect the surface of Titan, although faintly and at low resolution. At left is a 'classic' full-color view of Titan's hazy atmosphere, produced from Voyager 2 images (courtesy SEDS), while at right is a new (as of this work) orange filter (600 nm) view of Titan's surface albedo features, produced from Voyager 1 images. The finding of Titan's surface features in these orange filter images from Voyager 1 has positive implications for the Cassini mission, indicating a large range of possible wavelengths over which Cassini's superior imaging systems may be used to study the cloaked surface of this large moon.



Contents



Abstract

We present an analysis of images of Saturn's moon Titan, obtained by the Voyager 1 spacecraft on November 8-12, 1980. Orange filter (590-640 nm) images were photometrically corrected and a longitudinal average removed from them, leaving residual images with up to 5% contrast, and dominated by surface reflectivity. The resultant map shows the same regions observed at 673 nm by the Hubble Space Telescope (HST). Many of the same albedo features are present in both datasets, despite the short wavelength (~600 nm) of the Voyager 1 images. A very small apparent longitudinal offset over the 14 year observation interval places tight constraints on Titan's rotation, which appears essentially synchronous at 15.9458 +/- 0.0016 days (orbital period = 15.945421 +/- 0.000005 days). The detectability of the surface at such short wavelengths puts constraints on the optical depth, which may be overestimated by some fractal models.



Fig. 1: Voyager/HST filters vs. Titan Albedo Spectrum

Relative response of the Voyager 1 orange filters (NA and WA cameras) and the HST filters used to make the maps shown in this paper, compared with an albedo spectrum from Karkoschka (1995} for reference. Also shown is the single scattering albedo, $\omega$, for tholin particles computed by Mckay et al. (1989} -- despite the relatively short wavelength, the single scattering albedo in the Voyager 1 orange filter range is still quite high (~0.88) and hence, most light is scattered rather than absorbed. It is likely that Titan's surface can be detected at all wavelengths longward of 600 nm except in methane absorption bands, not just in the prominent window at 950 nm.


Fig. 2: Voyager Images Processing Procedure

An example of the six basic stages used to extract the `surface component' from the 14 selected Voyager 1 orange filter images: (a) pre-processed image, calibrated and with Reseau marks removed, (b) synthetic image used to properly register the true image, (c) `normalized' image, orthographically projected to an equatorial view at a center resolution of 5 pixels per degree, (d) photometrically corrected image, showing the north/south albedo dichotomy of the Titan atmosphere and a north polar hood, (e) the longitude-smeared average of all 14 images, used to represent the `atmospheric component,' and (f) the final stretched and trimmed image, showing the ratio of the selected image to the averaged image and displaying about 5% maximum contrast. This image shows the `spit' region and the two dark regions north and south of the equator west of 180 deg longitude.


Fig. 3: Regional Views of Titan from Voyager 1 and HST

(left column) Three views of Titan's surface from the VG1 ORANGE filter (~600 nm), displayed using a `red-temperature' color gradient scale, as compared to map projections from (center column) the HST F673N filter (673 nm) and (right column) the HST F850LP filter (940 nm). From top to bottom, these regional views show: (top row) an average of the three first images in the series, centered on 120 deg W Longitude and showing the Titan bright region at low sunlight incidence and emission angles, (center row) an average of the three middle images in the series, centered on 150 deg W Longitude and showing the large `sickle' shaped dark lane just to the west of the bright region, and (bottom row) an average of the three final images in the series, centered on 180 deg W Longitude and showing the two very dark regions to the west of the Titan `spit' region. Arrows track the rotation of two prominent portions of the Titan bright region (areas A2 and A3 in the map drawing below) along their high contrast border with the dark 'sickle' region.


Fig. 4: Map Views of Titan: Voyager 1 and HST

Three cylindrically projected map views of the Titan surface features (displayed using a `red-temperature' color gradient scale), as imaged by (top) the VG1 ORANGE filters at 590-640 nm, (center) the HST F673N filter at 673 nm (Smith et al. 1996), and (bottom) the HST F850LP filter using the methane window at 940 nm (Smith et al. 1996). These maps all show good general agreement, especially the top two, which were imaged at similar wavelengths.


Fig. 5: Voyager Based Map Drawing of Titan Surface Albedo Features

Two map drawings produced from the VG1 ORANGE images of the Titan surface albedo features, covering about 24% of the Titan surface, and shown using a six-level, gray gradient scale. Bright regions are shown with white labels while dark regions are shown with black labels. The identified features are:
     (A1) the Titan bright region eastern extension,
     (A2) the Titan bright region north point,
     (A3) the Titan bright region southwestern area,
     (B) the widest portion of the dark lane called the `sickle,'
     (C) the center of the `spit' region, with north, east, and west brighter areas,
     (D1) the northern very dark region, west of the `spit,' and
     (D2) the southern very dark region, west of the `spit.'


Fig. 6: Equatorial Profiles of Surface Albedo: Voyager 1 and HST

(solid line) A equatorial contrast profile through the VG1 ORANGE mosaic of Titan shown in the previous map figures, plotting the average of the pixel values within $\pm6^{\circ}$ latitude of the equator, and taken at intervals of 0.5 deg longitude. The HST F850LP filter profile (dotted line) and HST F673N filter profile (dashed line) plot the average of the pixel values within +/- 7.5 deg latitude of the equator, and taken at intervals of 3.0 deg longitude. These profiles show general agreement in both shape and contrast levels, especially the VG1 ORANGE and HST F673N profiles, which were imaged at similar wavelengths.


Fig. 7: Surface Feature cross-correlations: Rotation Period Determination

The cross-correlation of a resolution degraded VG1 ORANGE filter image map with (1) the HST F673N filter map (solid line), and (2) the HST F850LP filter map (dashed line) of Titan's surface. The cross-correlation was calculated over a range of West longitude offsets for the VG1 ORANGE filter map. The maximum correlation between the two map sets occur when the VG1 ORANGE filter mosaic is shifted by (1) 2.0 deg W +/- 6.4 deg for the HST F673N map, and (2) 3.2 deg W +/- 9.9 deg for the HST F850LP map (FWHM error above r = 0.50). This gives an average surface feature longitude offset between the VG1 and HST observations (1980 to 1994) of 2.6 deg W +/- 5.9 deg (essentially 0 deg offset, within accuracy limits). This very small apparent longitudinal offset over the 14 year observation interval places tight constraints on Titan's rotation, which appears essentially synchronous at 15.9458 +/- 0.0016 days (orbital period = 15.9454 days).


Fig. 8: Cassini ISS filters vs. Titan Albedo Spectrum

Relative response of selected Cassini ISS filters compared with a Titan albedo spectrum from Karkoschka (1995) for reference (upper curve). Shown are the responce curves for the RED filter (dotted), infrared filters IR1 through IR4 (alternating dashed and dot-dashed), and continuum band filters CB1 through CB3 (solid). Note that these ISS continuum band filters were designed for methane windows and should enable better surface imaging than the HST filters. Furthermore, ISS has IR polarizer filters that will screen out up to 50% of the haze at phase angles near 90 degrees (West & Smith, 1991).