Uranus and Neptune aren't the identical egg-blue twins they appear to be in natural color, according to NASA Hubble Space Telescope images released today.

Atmospheric features on Uranus and Neptune are revealed in images taken with the Space Telescope Imaging Spectrograph and the Advanced Camera for Surveys aboard NASA's Hubble Space Telescope. The top row reveals Uranus and Neptune in natural colors, showing the planets as they would appear if we could see them through a telescope. In the bottom images, astronomers used different color filters to detect features we can't see. Credit: NASA and Erich Karkoschka, University of Arizona

Erich Karkoschka of the University of Arizona Lunar and Planetary Laboratory used different color filters on Hubble's Space Telescope Imaging Spectrograph and the Advanced Camera for Surveys for the observations, taken in August 2003.

Karkoschka used red, green, and blue light filters to show Uranus and Neptune in their natural colors. He used other filters, including near-infrared, for enhanced views. Enhanced views show that Uranus and Neptune are two different worlds.

"I took extraordinary care that the natural-color images are very close to what a human would see from a spacecraft near these planets," he said. "The enhanced-color images show how an instrument with different spectral sensitivity than that of the human eye can change the view. There is more to everything than what the eye can see."

The new images show how Uranus's rotational axis is tilted almost 90 degrees to Neptune's axis. The south poles of Uranus and Neptune are both tilted slightly toward Earth. Uranus shows greater contrast between its hemispheres, which may be caused by its extreme seasons.

Bands of clouds and haze are aligned parallel to the equator on both planets. Colors in the bands show layers of clouds and haze at different altitudes and thicknesses.

Some cloud features appear bright orange or red, a color caused by methane absorption in the red part of the spectrum. Methane is the third most plentiful gas in both planets' atmospheres, second only to hydrogen and helium.

Uranus' faint rings and several of its satellites are visible in a wider view of Uranus. These include Uranus' bright moon Ariel and darker moons Desdemona, Belinda, Portia, Cressida, and Puck.

This wider view of Uranus reveals the planet's faint rings and several of its satellites. The area outside Uranus was enhanced in brightness to reveal the faint rings and satellites. The outermost ring is brighter on the lower side, where it is wider. It is made of dust and small pebbles, which create a thin, dark, and almost vertical line across the right side of Uranus. The bright satellite on the lower right corner is Ariel, which has a snowy white surface. Five small satellites with dark surfaces can be seen just outside the rings. Clockwise from the top, they are: Desdemona, Belinda, Portia, Cressida, and Puck. Credit: NASA and Erich Karkoschka, University of Arizona

Karkoschka has been studying the atmospheres of outer planets for 21 years, first as a graduate student and since as a researcher with UA's Lunar and Planetary Laboratory. He has used the Hubble Space Telescope to take images of Saturn and Titan, as well as Uranus and Neptune, to study the vertical structure of their gases and aerosols. He made spectroscopy observations at the European Southern Observatory in 1993 and 1995 for more such data.

Karkoschka is currently modeling Saturn's atmosphere based on images of that planet he took with the Hubble Space Telescope in March 2003. These images are currently featured in the cover story of Arizona Alumnus, the winter 2004 issue.

The Space Telescope Science Institute (STScI) is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).