Elsevier

Icarus

Volume 180, Issue 2, February 2006, Pages 442-452
Icarus

Photometric variability of Uranus and Neptune, 1950–2004

https://doi.org/10.1016/j.icarus.2005.09.009Get rights and content

Abstract

Photoelectric intermediate-band b and y photometry of Uranus and Neptune obtained at each apparition since 1972, combined with broadband B and V photometry from 1950 to 1966, provide a record of planetary variability covering 2/3 of Uranus' 84-year orbital period and 1/3 of Neptune's 165-year orbital period. Almost all of the data were obtained with a dedicated 21-inch photometric telescope at Lowell Observatory. The data are quite homogeneous, with yearly uncertainties typically smaller than 0.01 mag (1%). The lightcurve of Uranus is sinusoidal with peaks at the solstices. The b amplitude slightly exceeds the expected 0.025 mag purely geometrical variation caused by oblateness as the planetary aspect changes, seen from Earth. The y amplitude is several times larger, indicating a strong equator-to-pole albedo gradient. The lightcurve is asymmetrical with respect to southern solstice, evidence of a temporal albedo variation. Neptune's post-1972 lightcurve exhibits a generally rising trend since 1972 interpreted by Sromovsky et al. [Sromovsky, L.A., Fry, P.M., Limaye, S.S., Baines, K.H., 2003. Icarus 163, 256–261] as a lagged sinusoidal seasonal variation. However, the 1950–1966 lightcurve segments are much fainter than expected, missing the proposed seasonal sinusoid by 0.1–0.2 mag. A major unknown component is therefore needed to explain Neptune's long-term variation. The apparent relationship between Neptune's brightness variation and the 11-year solar cycle seen in cycles 21–22 (1972–1996) has apparently now faded away. Further interpretation of the data in this paper will be found in a companion paper by Hammel and Lockwood [Hammel, H.B., Lockwood, G.W., 2005. Icarus. Submitted for publication].

Introduction

This paper combines three distinct sets of annual observations, all obtained at Lowell Observatory, that together provide over a half century of photometric coverage of Uranus and Neptune with only a relatively short hiatus from 1966 to 1972. Broadband B and V measurements on the UBV photometric system cover the interval from 1950–1966. Intermediate-band b and y filters of the Strömgren photometric system extend the series from 1972–2004. The B, V data come mainly from two Lowell Observatory Bulletins (Serkowski, 1961, hereafter S61; Jerzykiewicz and Serkowski, 1966, JS66).

New information in this paper, including our basic interpretation of the lightcurves, builds on previously published material (Lockwood and Thompson, 1999, hereafter LT99; and 2002, LT02), adding eight seasons of additional b, y magnitudes for Uranus and four for Neptune plus Lowell V data not previously included in the planetary variability record. A companion paper by Hammel and Lockwood (2005) presents additional interpretations of the planetary lightcurves.

Section snippets

Solar and planetary variability at Lowell Observatory

The motivation for this program a half-century ago was the desire, on behalf of weather and climate scientists, to assess possible solar variability by monitoring sunlight reflected from planets. Postwar improvements in photomultiplier tubes made this effort technically feasible, offering at least the possibility of obtaining definitive evidence for solar variations. We now know from spacecraft measurements that the Sun's variations are very small, less than 0.1% over the 11-year sunspot cycle (

Observations

As in our earlier papers (LT99 and LT02), we combine data from S61 and JS66 with subsequent b, y photometry, but now include V photometry from 1954–1966. From the beginning, the solar variations program focused on differential B measurements, mainly to speed up the observational cadence. A few separate planetary B, V observations were included in the much broader program of comparison star measurements to obtain seasonal B–V color indices. These, never before published outside the Lowell

Magnitude transformations

To make conjoined lightcurves we must accurately transform Bb and Vy, applying constant offset values determined from observations made in all four filters in 1973 and 1975. The offsets differ slightly for each planet since their spectra are different, and may change slightly over time if the planetary colors change significantly (possibly important for Uranus but not for Neptune). A second approach, subject to considerable uncertainty and therefore used here only for confirmation, involves

Lightcurve of Uranus

Uranus exhibits a sinusoidal seasonal 0.025 mag variation caused by its changing aspect viewed from the Earth. This “geometrical lightcurve” variation is Δm=2.5log(1ɛsin2icos2θ), where Δm is the variation in magnitude units, i is the 97.9° inclination to the ecliptic, ɛ is the 0.023 oblateness, and θ is the sub-Earth latitude on the planetary disk. Fig. 2 shows the observed variability of Uranus with the geometric variation indicated by a dotted line. The B magnitude series that began in 1950

Conclusion

We have assembled a time series of photometric measurements of Uranus and Neptune covering more than a half-century, a substantial portion of a full orbit around the Sun for each planet. These provide data for the models by Hammel and Lockwood (2005) and indicate that the seasonal Neptune model proposed by Sromovsky et al. (2003) lacks an additional (unknown) major component. A seasonal model for Uranus that incorporates pole-to-pole latitudinal contrasts using the latest Hubble Space Telescope

Acknowledgements

We thank long-time colleague Heidi Hammel and the two referees for many suggestions, Lawrence Wasserman for ephemeris calculations, and Don Thompson for sharing observation and data reduction duties from 1976 until 1997. We acknowledge the critical role played by the late John S. Hall in keeping this program alive during his tenure as Director of Lowell Observatory from 1958 to 1975. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. Currently it is

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