Uranus is Weird

Back 1986, Voyager 2 flew by the planet Uranus and gave us our only set of closeup data so far. Recently, that flyby data has shown us the Uranus is even stranger than we thought.

The planet’s rotational axis is tipped over almost 90 degrees, and the Voyager data has revealed that its magnetic axis points about 60 degrees away from the rotational axis. The planet’s magnetosphere wobbles around, and to date, no one has come up with a reasonable explanation of for the odd offset.

Image Credit: NASA

The North Pole of Uranus

This snapshot of Uranus taken by the Hubble Space Telescope reveals a vast bright cloud cap across the north pole which is believed to be caused by the planet’s odd orientation. Unlike the other planet in the Solar System, Uranus is tipped over almost onto its side. During the planet’s summer the Sun shines almost directly onto the north pole as a result of the extreme tilt and never sets. It’s almost mid summer on Uranus, and the polar storm may have formed because of seasonal changes in atmospheric flow.

Uranus is an ice giant planet. It has no solid surface but rather mantles of hydrogen and helium surrounding a water-rich interior which is probably wrapped around a rocky core. Methane in the atmosphere absorbs red light and scattered blue-green light back into space, giving each planet its cyan hue.

Image Credit: NASA

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

To Be or Not To Be Among the Fairies

What is Hamlet doing hanging out with Oberon from A Midsummer Night’s Dream?

Since 1919, the International Astronomical Union has been charged with the task of establishing “conventional” nomenclature for planets, satellites, and surface features. Namesakes from Shakespearean works have been chosen for moons and objects around Uranus. Thus, Oberon, king of the fairies, is also Uranus’ most distant and second largest moon, and Hamlet is a tragically large and princely crater on its surface, the large dark crater to the right of center. This picture shows the known surface features of Oberon. It was constructed by the U.S. Geological Survey based on data from NASA’s Voyager 2.

Image Credit: NASA/USGS

Uranus and Four of Its Satellites

This infrared image of the planet Uranus was captured by the European Southern Observatory’s Very Large Telescope during the planet’s 2008 equinox. Every 42 years, the ring (and satellites) plane of Uranus line up with the Sun causing them to appear on edge from Earth’s point of view. A one minute exposure time was used, the maximum allowable to prevent the moving satellites from appearing as streaks. The IR filter used matches the absorption bands of the methane in the atmosphere of Uranus, making the relatively bright planet (almost) completely disappear. That permits the otherwise invisible rings and small satellites of Uranus to be detected instead of being lost in the glare of the planet. The bright spots on each side of Uranus are Miranda (~470 km diameter) and Ariel (~1100 km diameter). Two much smaller satellites can be seen just above the ring plane,to the left of the planet. Puck (~150 km diameter) is closer to the Planet than Portia (~100 km diameter).

Image Credit: ESO

The Rings of Uranus

Saturn’s rings are so prominent that they easily visible from Earth with a small telescope. All the other gas giant planets have ring as well, but they weren’t discovered until we were able to look at those planets from above the Earth’s atmosphere. Here are some pictures of the ring system around Uranus taken by the Hubble Space Telescope as our point of view shifted over several years. The next time the rings will be edge-on will be in 2049.

Image Credit: NASA / ESA / STScI

Titania

titania3_voyg2Titania’s tortured terrain is a mix of valleys and craters. Voyager 2 passed this moon of Uranus in 1986 and took this photograph. The long valleys indicate that Titania underwent some unknown tumultuous resurfacing event in its distant past. Titania is essentially a large dirty iceball composed of a roughly 50/50 mix of water ice and rock. It was discovered by William Hershel in 1787.

Image Credit: NASA

The Rings of Uranus

Uranus w ringsThese images from the Hubble Space Telescope show how the ring system around Uranus varies in appearance as viewed from Earth—culminating in the rings being seen edge-on. The edge-on rings appear as two spikes above and below the planet. The rings cannot be seen running fully across the face of the planet because the bright glare of the planet has been blocked out in the photo.  Shorter exposure color images of Uranus have been combined with the ring images to show the planet’s size and position relative to the ring plane.

From Earth, we can only see the rings’ edge every 42 years as the planet follows its 84-year orbit about the Sun. However, the last time the rings were tilted edge-on to Earth astronomers didn’t even know they existed. They were discovered in 1977.

Until Voyager 2 flew by Uranus in January, 1986, the rings were only known from the way they temporarily blocked the light of stars passing behind the planet. Hubble provided some of the first images of the ring system as viewed from Earth’s distance of approximately 2 billion miles. More recently, adaptive optics have allowed ground-based observers using large telescopes comparatively sharp views.

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

To Be or Not To Be Among the Fairies

What is Hamlet doing hanging out with Oberon from A Midsummer Night’s Dream?

Since 1919, the International Astronomical Union has been charged with the task of establishing “conventional” nomenclature for planets, satellites, and surface features. Namesakes from Shakespearean works have been chosen for moons and objects around Uranus. Thus, Oberon, king of the fairies, is also Uranus’ most distant and second largest moon, and Hamlet is a tragically large and princely crater on its surface, the large dark crater to the right of center. This picture shows the known surface features of Oberon. It was constructed by the U.S. Geological Survey based on data from NASA’s Voyager 2.

Image Credit: NASA/USGS

Mixed Mythologies

Uranus’ moon Ariel (white dot) and its shadow (black dot) were caught crossing the face of Uranus in this Hubble Space Telescope image.

Uranus is named for the Greek deity of the sky (Οὐρανός). Ariel is named for a sky spirit in Pope’s The Rape of the Lock and Shakespeare’s The Tempest. All of Uranus’s moons are named after characters from the works of William Shakespeare or Pope’s The Rape of the Lock.

Image Credit: NASA