Elsevier

Icarus

Volume 172, Issue 2, December 2004, Pages 548-554
Icarus

Evidence for temporal change at Uranus' south pole

https://doi.org/10.1016/j.icarus.2004.07.009Get rights and content

Abstract

Analysis of Hubble Space Telescope images of Uranus taken between 1994 and 2002 shows evidence for temporal changes in zonal brightness patterns in the south polar region. Between 1994 and 2002, a relatively bright ring developed near 70° S. The pole itself, which was the brightest area of the southern hemisphere in 1994, has become relatively dark. The polar collar at 45° S has also become brighter relative to the rest of the southern polar region. Comparison of images through different filters suggests that the change is occurring at pressures of 2–4 bars in the atmosphere. A change at this depth is consistent with radio measurements which indicate seasonal variability in Uranus' deep atmosphere. Disk-integrated photometry at visible wavelengths also exhibits variability on seasonal (∼ decades) timescales. The observed changes are not predicted by existing dynamical models of Uranus' atmosphere.

Introduction

The atmosphere of Uranus provides an important extremum in outer planet atmospheric modeling. The uranian rotational pole lies only 8° from the plane of the planet's orbit. This anomalous obliquity causes radically variable seasonal insolation when compared with other planets. (Obliquity is thought to play an important role in climate change, e.g., for Earth, see Zachos et al., 2001; for Mars, see Nakamura and Tajika, 2002, Nakamura and Tajika, 2003.)

When Voyager 2 encountered Uranus in 1986, the planet's south pole was pointed almost directly at the Sun. Voyager images Smith et al., 1986 and their subsequent radiative transfer analysis Rages et al., 1991 revealed the existence of a southern “polar cap” of relatively thick clouds, thought to be composed of methane ice, extending southward from about 45° S latitude. Vigorous image processing revealed further latitudinal structure in the polar cap: a very dark region at the pole itself in violet, a broad relatively bright region in all filters around 50° S, and a brightness minimum near 65° S Smith et al., 1986.

Beginning in 1994, Hubble Space Telescope (HST) observations of Uranus have shown both the bright southern polar cap and additional banded structure at low latitudes, most easily seen at near-infrared wavelengths (700–1000 nm) which were beyond the range of Voyager's vidicon camera. But the polar cap itself still appeared featureless to casual inspection. That is now changing.

Section snippets

HST observations

Beginning in 2000, we used HST's Wide Field and Planetary Camera 2 (WFPC2) to obtain a consistent long-term series of Uranus images to study seasonal change as the planet approached equinox. As we had hoped, these observations reveal short-term (∼1 terrestrial year) changes in reflectivity, ruling out a hypothesis that the planet was static and that observed changes were due to differences in viewing geometry Karkoschka, 2001.

Figure 1 shows the latitudinal structure of Uranus' southern polar

Vertical structure implications

The most likely cause of the overall darkening of Uranus' south polar regions over the past decade is a decrease in the opacity of the condensed methane cloud at ∼1.3 bar, which had an optical depth in excess of unity when Voyager flew past in 1986 Rages et al., 1991. The narrow band developing at 70° S cannot be located at this altitude, however, since there is little or no trace of it in the 619-nm methane band filter, which is quite sensitive to variations near the 1-bar pressure level (Fig.

Implications for dynamics

A reduction in the opacity of the condensed methane cloud in Uranus' south polar region has not been predicted by any existing seasonal or dynamical model. The dynamical model of Friedson and Ingersoll (1987) predicted a convective pulse of internal heat should be released following the solstices, but only in the dark (northern) hemisphere, and in any case the extra convection would be expected to promote rather than reduce the overall cloud opacity.

Hofstadter and Butler (2003) suggest a

Acknowledgment

Support for this publication was provided by NASA through Proposal number GO-08680.03 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, under NASA contract NAS5-26555.

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Based in part on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.

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