CFHT, Current Image of the Week


February 7th, 2000

Uranus: Ground-based detection of the minor satellite Portia

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Uranus: Ground-based detection of the minor satellite Portia

Credit: Image courtesy of C. Dumas (Caltech/JPL), and J.-L. Beuzit & F. Ménard (CFHT)

Uranus, the Seventh Planet of our solar system, was discovered by William Herschel in 1781. It is located at a mean distance of 2.7 billion kilometers and orbits the sun in 84 years. Uranus is the third largest planet of the solar system, it is 15 times heavier and 4 times larger in radius than the Earth. Despite its large size, it is barely visible with the naked eye in a dark location (its V magnitude is ~5.7). Its main particularity comes from the fact that its rotation axis is highly inclined with respect other Solar System planets, lying almost in the plane of the ecliptic (~98 degrees inclination of equator to orbit).

Similarly to the other giant planets Jupiter and Saturne, Uranus is surrounded by a system of rings, at least 10 of them. But contrarily to the Saturnian rings, the rings of Uranus are very narrow and their material is probably confined in these narrow regions by small satellites orbiting at the inner and outer edges of the rings.

Most of the information we have about Uranus' minor satellites and rings come from the Voyager 2 spacecraft fly-by on January 24, 1986. The Hubble Space telescope also recorded images of the Uranus system, but thanks to Adaptive Optics, we can now routinely study the Uranian system in great details from the ground.

This week's image shows Uranus in two near-infrared colours. The left panel is an H-band image centered at 1.65 microns, in a region of the spectrum where we can study the tropospheric aerosols over the planet. The right panel shows a K-band negative image centered at 2.2 microns, in a region of the spectrum corresponding to the absorption of methane . At this particular wavelength, the amount of scattered light produced by the planet is greatly diminished, allowing high-contrast imaging of the system of faint satellites and rings. Uranus' rings are visible, in silhouette in this negative K-band image. They are better seen in this positive image, especially near the top and bottom. Three rings are clearly visible, in particular the outermost Epsilon ring.

Interestingly, the minor satellites Puck and Portia are also detected in the K-band (right) image. Portia's image is captured here unambiguously for the first time from the ground! Its diameter is only 110km. Puck is only barely larger than Portia, at 154km in diameter. Both satellites orbit Uranus more than once a day. Also, the satellites Miranda and Ariel are clearly visible. So far, 18 satellites of Uranus are known, and the confirmation for 3 more is pending (2 of them also discovered at CFHT!). In the H-band image (left side), clouds in the upper atmosphere of the planet are seen. They are the white streaks to the left of the Planet. These clouds are located around the South Pole of Uranus! Uranus's rotation axis is tilted by 90degrees, it is the only planet of our solar system to behave like this. To convince yourself, compare the orientation of the rings around Uranus, they are oriented North-South in this image, with those of Saturn or Neptune, oriented East-West, shown in previous images of the week at CFHT.

Technical description: These images were obtained with PUEO in August 1999 in the H- and K-band filter. They are single 90sec frame. North is approximately up and East to the left. The K-band image is presented in negative to better enhance the presence of Portia and Puck. The Satellite Titania was used as a reference for wave-front sensing. No image restoration technique, such as deconvolution , has been applied to the image.

next week: Observational Cosmology: A Cluster of Galaxies at High Redshift

editors: François Ménard & Jean-Charles Cuillandre
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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.