Uranus and Neptune Albedos May Need Revision

We analyzed all available images of Uranus and Neptune taken by Voyager 1's narrow-angle camera while travelling many AU from either planet in the 1980s. Analyses performed in the 1980s (by the first author) used imperfect techniques. Those poorly-calibrated results had been used with other data to calculate Bond albedos or to check such calculations, because these data provide full-disk reflectivity at solar phase angles not available from Voyager 2 or Earth.

All images taken by Voyager spacecraft were stored as "quick-look engineering data records" (QEDRs) in JPL's MIPL facility, without human-readable headers or labels. Since 2018, we processed all 26,834 QEDRs and added VICAR headers to them. PDS4 labels are being developed and the bundle will be submitted to the PDS later this year.

We knew the time each Voyager 1 Uranus or Neptune image was acquired in 13 epochs (thus 13 phase angles) through 1987. Two epochs could not be found in the QEDRs. Information in binary headers found us two epochs in 1988 and 1990. All images recovered from 13 epochs were analyzed. Images from repeated downlinks were "bested" to form a PDS4 Raw data collection. Artifacts were removed. Then, dark-current images acquired during these epochs and calibration curves were used to generate PDS4 Calibrated images.

Total brightness in the planet's disk, a dot or streak in each Calibrated image, was calculated. This shows the planet's albedo at that solar phase angle. Accurately determining phase integrals and Bond albedos in the future requires analysis of these data along with all Voyager 2 images, albedo channel data from Voyager 2's IRIS instrument, and a variety of data from Earth based instruments. However, we already have some new results.

After we correct old and new results for geometry (less illumination at high phase angles, going from disk to crescent), we plot reflectivity vs phase angle to compare our new results with the old. Uranus's reflectivity declines slower from 25° to 75° than previously thought and Neptune's declines faster from 15° to 45°. This might imply that Uranus's Bond albedo needs to be revised upward and Neptune's downward.