Two Views of Uranus

natural and false colorThese two pictures of Uranus—one in true color (left) and the other in false color—were compiled from images returned in 1986 from the narrow-angle camera of Voyager 2. The spacecraft was 9.1 million kilometers from the planet, several days from closest approach.

The picture on the left has been processed to show Uranus as human eyes would see it. It was assembled from images taken through blue, green, and orange filters. The darker shadings at the upper right of the disk correspond to the day-night boundary on the planet. The night side of the planet northern hemisphere of Uranus. (“Hold it,” I hear the Gentle Reader cry. “Shouldn’t the half of the northern hemisphere be in daylight.” No. Not on Uranus. The planet’s axis is tilted almost 90°.) The blue-green color results from the absorption of red light by methane gas in the planets deep, cold, and remarkably clear atmosphere.

The picture on the right uses false color and extreme contrast enhancement to bring out subtle details in the south polar region of Uranus. It uses images take through ultraviolet, violet, and orange filters shifted to the same blue, green, and red colors used to produce the picture at left. The very slight contrasts visible in true color are greatly exaggerated. The false color image reveals a dark polar hood surrounded by a series of progressively lighter concentric bands. One possible explanation is that smog, concentrated over the pole, is arranged into bands by zonal winds of the upper atmosphere. The bright orange and yellow strip at the edge of the planet’s limb is an artifact of the image enhancement.

Image Credit: NASA

Internet Astronomy: Uranus and Moons

This video of Uranus and some of its moons was taken in infrared light using the ESO’s Very Large Telescope during the planet’s 2008 equinox. Every 42 years, the ring (and satellites) plane of Uranus crosses the Sun, giving observers on Earth the opportunity to observe the rings edge on. Ring plane crossing also allow us to observe the rings from their dark side (that is, while the Sun is illuminating them from the opposite side), aiding the search for faint satellites, faint rings, or faint ring structures. A ring plane crossing is also an opportunity to observe mutual interactions between satellites such as eclipses or occultation phenomena.

The movie shows the Uranus system of satellites over a two hour period. It’s easy to see the impact of fluctuating seeing conditions on the image quality. Under good seeing, the small satellites Puck and Portia become clearly visible, but the images start to blur when the seeing degrades.

Video Credit: ESO/C. Dumas, B. Sicardy, and J.-E. Arlot

This Isn’t Saturn

uranus_irThis is a near-infrared view of Uranus with its rings and some of its moons. It was made by the 8.2-m VLT ANTU telescope at the ESO Paranal Observatory in Chile. The contrast between the rings and the planet is strongly enhanced at the wavelength at which this picture was made; the incoming sunlight is almost completely absorbed by gaseous methane present in the planetary atmosphere causing Uranus to appear unsually dark, but the icy material in the rings reflects the sunlight and appears relatively bright.

Uranus is unique among the planets of the solar system in having its axis of rotation tilted almost 90°. When Voyager 2 flew by in 1986, the south pole was pointed toward the Earth. In this picture made 16 years later (and about 20 percent of the away around the planet’s orbit), the Uranian ring system were seen at an angle comparable to Saturn’s when its ring system is most open.

Seven moons of Uranus are in the picture Titania and Oberon are the brightest. The much smaller and fainter Puck and Portia have visual magnitude about 21 and are barely visible in the photo.

Image Credit: ESO

Uranus and Ariel

Uranus_ArielUranus’ moon Ariel (white dot) and its shadow (black dot) were caught crossing the face of Uranus in this Hubble Space Telescope image. Note that the cloud bands which are aligned with the planet’s rotation are nearly vertical in the picture. Uranus is the giant planet whose equator is nearly at right angles to its orbit. A collision with an Earth-sized object several billion years ago is the likely cause of Uranus’ tilt. Nearly a twin to Neptune, Uranus has more methane in its mainly hydrogen and helium atmosphere than Jupiter or Saturn. Methane gives Uranus its blue tint.

Image Credit: NASA

Infrared Uranus

infrared_uranusThe Solar System’s third largest planet usually looks quite plain. Uranus typically appears as a featureless small spot in a small telescope or a featureless large orb in a large telescope. Back in the late ’90s, the Hubble Space Telescope photographed Uranus in infrared light. In this image the distant planet’s unusual clouds, rings, and moons are visible. Analysis indicates that clouds seen here in orange appear to circle Uranus at speeds in excess of 500 km/h.

Image Credit: NASA