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Most detailed views ever of weather on Uranus

A combination of novel techniques, equipment, and stable atmospheric conditions have revealed new features of atmospheric conditions on Uranus.
18 December 2012, SPIE Newsroom. DOI: 10.1117/2.1201212.004620

The secrets of weather on Uranus have been well preserved by its great distance from the sun and the apparent absence of discrete cloud features in its atmosphere. Even in 1986, when the Voyager 2 spacecraft arrived at Uranus after a journey of eight-and-a-half years, its close-up images were bland, revealing only eight discrete cloud features in hundreds of images.1 Tracking these few features established a crude profile of wind speed as a function of latitude, but only in the southern hemisphere, because the northern hemisphere was almost entirely in the dark. Eleven years later, images from the Hubble Space Telescope of Uranus at near-IR wavelengths revealed many more discrete cloud features and extended wind measurements into the northern hemisphere.2 More prolific still were ground-based observations from the Keck II telescope, which combined a large aperture, near-IR wavelengths, and adaptive optics to reap a bounty of cloud features far beyond what Hubble was able to produce.3–6 However, there remained gaps in the latitudinal coverage of wind measurements because some regions lacked good tracers of motion. To fill these gaps, we tried to detect low-contrast cloud features using special observing and processing methods to achieve very high signal to noise ratios.7 The results provided a new window into weather on Uranus.

The Keck II telescope on the 14,000ft summit of Mauna Kea has a mirror 10m in diameter, which in space would produce an angular resolution of 0.043 ′′ (arc seconds) at 1.6μm, the wavelength of maximum cloud feature contrast. But even under good conditions, atmospheric turbulence limits angular resolution on the ground to about 0.5′′, which is about one-seventh the angular diameter of Uranus. Instead of plane waves entering the telescope, the turbulence-induced density variations distort the incoming light waves so that their angles of entry vary across the telescope aperture, leading to smeared images. Fortunately, Keck II has an excellent adaptive optics (AO) system that can largely correct for angular deviations in the wavefront.8 The AO correction is obtained by splitting the aperture into separate regions, measuring the wavefront tilt in each region, and then using that information to control a deformable mirror that corrects these angular deviations. The resulting improvement in angular resolution is spectacular, reaching close to 0.06′′ in the corrected image shown in Figure 1.

Figure 1. Effect of the Keck II Adaptive Optics (AO) system on Uranus image quality (left is AO off, right is AO on). Both images were taken with a filter centered at 1.6μm. Images courtesy of I. de Pater and H. Hammel.

Many cloud features on Uranus cannot be seen in a single image because they have very low contrast, even at these high-contrast, near-IR wavelengths. Higher signal-to-noise ratios are needed to detect these features. Normally, taking a much longer exposure solves the problem because for these images the signal-to-noise ratio increases approximately as the square root of the exposure time. However, Uranus rotates during the exposure (360° in 17.24 hours), and so we cannot expose for much longer than 1–2 minutes without smearing the feature longitudinally. The solution is to take short exposures, remap each image to a planet-fixed coordinate system, average the images in that system, and finally remap them to the original view. For cloud-tracking measurements, we average sets of eight exposures, each of 2 minutes. To obtain each of the exceptional images in Figure 2, we averaged about 100 exposures. These averages require a longer time interval, and so we also had to correct for the displacement caused by atmospheric winds, which reach speeds up to 250m/s (560mph). Finally, to see the subtle features, we subtract a smoothed image from the original image, a process that is essentially a high-pass filter. We can then boost the contrast by a factor of 25 so that the low-contrast features become visible without being overwhelmed by the large-scale brightness variations.

Figure 2. Keck II de-rotated, averaged, and high-pass filtered images of Uranus from (a) 25 July and (b) 26 July, 2012. Credit: L. Sromovsky, P. Fry, I. de Pater, and H. Hammel.

Starting with the north pole, which is at about the 4 o'clock position in Figure 2, and reaching down to ∼55°N, there is what looks like a field of cumulus convective features. However, these do not seem to be very dense clouds and they are relatively large (typically ∼450km across). This finding was unexpected because such features had never been seen in the south polar region of the planet. The last time we imaged the south pole it was in its early southern fall (1997–2003). Uranus is now in late northern spring—the last equinox was in 2007—and so this seems to be a seasonal effect whereby a long winter of cooling promotes polar convection and a long summer of heating suppresses it. Such a situation makes sense if the cooling and heating occur above the region that is ‘convective’. We are expecting that this convective activity might continue for some time as the northern hemisphere moves into summer, but expect it to eventually dissipate as summer heating intensifies.

The streaky bands in the image from ∼55–45°N have been commonly seen, although not with as much detail as obtained here. We do not know why these bands exist, nor why they are streaky. No obvious feature in the zonal wind profile is apparent that correlates with these structures. Another feature of note is the small dark spot with bright companion clouds, near the bottom of Figure 2a. The dark spot is thought to be a vortex, and is probably an anti-cyclone because the surrounding wind shear is anti-cyclonic (rotating opposite to the direction of planet rotation).

Just south of the equator we can see a previously unobserved (on Uranus) scalloped wave feature, which is similar to what is seen in regions of high horizontal wind shear (that is, rapid change of wind speed with latitude). However, the lack of cloud targets has prevented us from measuring near-equatorial wind shear, and so we are unable to confirm this hypothesis. Resolving this issue is one of the motivations to improve image quality.

The composition of the clouds is constrained by indirect measurements. Spectral observations show that the main condensable material in the upper troposphere is methane (the main component of natural gas). Given the pressures and temperatures of the brighter clouds (∼1bar and 77K), it seems certain that they mainly comprise frozen methane particles. The composition of the deeper clouds—at pressures near 1.6bar—are less certain. Hydrogen sulfide and ammonia (for the even deeper layers) might be components of these clouds.

In addition to the July observations discussed here, we obtained similar observations in August and November 2012, the latter in collaboration with Imke de Pater at the University of California at Berkeley. With these high-quality datasets in hand, and more to come, we are now working to make significant improvements in characterizing the circulation and weather features on Uranus, as well as their anticipated seasonal variations.

The authors thank NASA for supporting the observations and the staff at the W. M. Keck Observatory, which receives generous financial support from the W. M. Keck Foundation. We also thank those of Hawaiian ancestry on whose sacred mountain we are privileged guests. Without their generous hospitality none of our ground-based observations would have been possible.

Lawrence Sromovsky, Patrick Fry
Space Science and Engineering Center
University of Wisconsin–Madison
Madison, WI

Lawrence Sromovsky is a senior scientist. His research on outer planet atmospheres began with the 1981 Voyager 2 encounter with Saturn.

Patrick Fry is a researcher, specializing in outer planet imaging, spectroscopy, and photometry.

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