Credit: Reta Beebe, Amy Simon (New Mexico State Univ.), and NASA
This Hubble Space Telescope image provides a detailed look at a cluster of three white oval-shaped storms southwest of Jupiter's Great Red Spot. The clouds, as imaged on February 13, 1995, look considerably different from their appearance only seven months earlier. The features are moving closer together as the Great Red Spot is carried westward by prevailing winds and the white ovals are swept eastward. (This change in appearance is not an effect of last July's comet Shoemaker-Levy 9 collisions with Jupiter.)
The outer two of the white storms formed in the late 1930s. They are high pressure systems where the air is rising, carrying ammonia gas upward. White ice crystals form when the upwelling gas freezes as it reaches the cloud top level where temperatures are-200 F (-130 C).
The intervening white storm center, the ropy structure to the left of the ovals, and the small brown spot have formed in low pressure cells. The white icy clouds sit above locations where gas is descending to lower, warmer regions. Melting of the ice exposes varied amounts of Jupiter's underlying brown haze. The stronger the down flow, the less ice, and the browner the region.
A scheduled series of Hubble observations will help target regions of interest for detailed scrutiny by the Galileo spacecraft, which will arrive at Jupiter in early December this year. Galileo will obtain close-up images of the structure of clouds that make up large storm systems such as the Great Red Spot and white ovals seen in this picture.
Astronomers using the Goddard High Resolution Spectrograph on the Hubble Telescope detected the spectral signature of molecular oxygen on Europa in ultraviolet light during observations made on June 2, 1994. They believe they have identified an extremely tenuous atmosphere of molecular oxygen around Jupiter's moon, Europa. "Europa's oxygen atmosphere is so tenuous that its surface pressure is barely one hundred billionth that of the Earth," said principal investigator Doyle Hall, of Johns Hopkins "It is truly amazing that the Hubble Space Telescope can detect such a wispy gas so far away."
If all the oxygen on Europa were compressed to the surface pressure of Earth's atmosphere, it would fill only about a dozen Astrodome-sized stadiums. The researchers caution that the detection should not be misinterpreted as evidence for the presence of life on the small, frigid moon. Not only is it too cold (- 230 F ;-145 C) to support life as we know it on Earth, but the oxygen detected is produced in a completely different way.
Unlike Earth, where living organisms generate and maintain a 21% oxygen atmosphere, Europa's oxygen atmosphere is produced by purely nonbiological processes. Its icy surface is exposed to sunlight and impacted by dust and charged particles trapped within Jupiter's intense magnetic field. Combined, these processes cause the frozen water ice on the surface to produce water vapor as well as gaseous fragments of water molecules.
The gas molecules undergo a series of chemical reactions that ultimately form molecular hydrogen and oxygen. The relatively lightweight hydrogen gas escapes into space, while the heavier oxygen molecules accumulate to form an atmosphere which may extend 125 miles (200 kilometers) above the surface. The oxygen gas slowly leaks into space and is replenished continuously.
Europa is approximately the size of Earth's Moon, but its appearance and composition are markedly different. The satellite has an unusually smooth and nearly craterless surface of solid water ice. Mysterious dark markings crisscross the surface, giving the moon a "cracked eggshell" appearance. Under the fragmented icy crust, tidal heating by Jupiter might heat the icy material enough to maintain a subsurface ocean of liquid water.
A mission to the Moon has been selected for funding as part of NASA's Discovery Program of frequent, low-cost solar system exploration missions. Missions to study the Sun, Venus, and a comet also have been selected for further detailed study under the Discovery effort.
The mission to the Moon, called Lunar Prospector, was judged mature enough to proceed directly to full development and construction following final technical definition. Scheduled for launch in June 1997, the $59 million orbiter mission will map the chemical composition of the lunar surface and the Moon's global magnetic and gravity fields. The mission will also look for significant quantities of water ice in permanently shadowed craters near the lunar poles that were imaged for the first time by the Clementine spacecraft in 1994. Such a discovery could be a boon to future human exploration of the Moon.
Three additional Discovery missions will continue detailed study development until fall 1995, when one will be chosen for flight. The Stardust Mission would fly through the extended coma of the active comet P/Wild 2, taking images and returning a sample of its cometary dust to Earth. The Venus Multiprobe Mission would drop 16 small probes into the thick venusian atmosphere to study its unusual atmospheric circulation and model it in three-dimensions. The Suess-Urey mission would collect samples of solar wind streaming outward from the Sun and return it to Earth for laboratory study.
"I am absolutely thrilled with the potential of these missions, and with the universally high quality of the 28 proposals submitted to us," said NASA Administrator Daniel S. Goldin. "The university and aerospace industry communities should be proud of their efforts, which represent a model of how to pursue scientifically first-rate space exploration using small, advanced spacecraft."
The Lunar Prospector will be built and launched on a Lockheed Launch Vehicle by Lockheed Missiles and Space Company under the direction of Principal Investigator Dr. Alan Binder of Lockheed. NASA's Ames Research Center will be responsible for one of the spacecraft's instruments and technical support.
The Suess-Urey team is led by Principal Investigator Dr. Donald Burnett of the California Institute of Technology Pasadena with Martin Marietta Astronautics as the contractor. The Venus Multiprobe Mission team is led by Principal Investigator Dr. Richard Goody of Harvard University with Hughes Space and Communications Group as the industry contractor. The Stardust team is led by Principal Investigator Dr. Donald Brownlee of the University of Washington with Martin Marietta as the contractor. NASA's Jet Propulsion Laboratory will provide project management for these three missions.
Stardust would be launched on a Med-Lite in February 1999 for a total cost to NASA of $208 million. The Venus Multiprobe Mission would be launched on a Delta II launch vehicle in June 1999 for a total cost to NASA of $202 million. Suess-Urey would be launched on a NASA Med-Lite launch vehicle in August 1999 for a total mission cost to NASA of $214 million.
NASA officials hope to release Announcements of Opportunity for new Discovery investigations on the average of every 18 months. The actual release dates depend on future approved NASA budgets and the size of previously selected missions.
The distant, blue-green planet Neptune has developed a new great dark spot in the cloudy planet's northern hemisphere, revealed in recent images from the Hubble Space Telescope. Only last June, Hubble images revealed that a great dark spot in the southern hemisphere, discovered by Voyager 2 in 1989, had mysteriously disappeared.
The new dark spot is a near mirror image of the previous feature mapped by Voyager 2. The new, northern dark spot is accompanied by bright, high-altitude clouds. As atmospheric gases flow up over the spot, they cool to form these methane-ice crystal clouds. "Hubble is showing us that Neptune has changed radically since 1989," said Heidi Hammel of the Massachusetts Institute of Technology. "New features like this indicate that with Neptune's extraordinary dynamics, the planet can look completely different in just a few weeks."
Like its predecessor, the new spot might be a hole in Neptune's methane cloud tops that gives a peek to lower levels of the atmosphere. "We weren't surprised the other spot disappeared," said Hammel. "It was kind of 'floppy' because it changed shape as atmospheric circulation carried it around the planet." (By contrast, Jupiter's Great Red Spot, which is similar to Neptune's original spot in relative size and position, has remained stable in appearance for at least 300 years.)
Hammel points out that studying the dynamics of Neptune's immense atmosphere might lead to a better understanding of Earth's atmosphere. "Neptune's unusual behavior is showing us that though we can make great models of planetary atmospheric circulation, there may be key pieces missing."
Energy from the Sun drives Earth's weather system. However, the mechanism must be very different on Neptune, because the planet radiates two times more energy than it receives from the distant Sun. The dynamics of Neptune's atmosphere may be caused by a strong internal heat source that warms the cloud layers from below. A slight change in the temperature differential from cloud bottom to top might trigger rapid, large-scale changes in atmospheric circulation. Astronomers don't know how long the new feature will last, but Hubble will allow them to track Neptune's atmospheric changes over at least a decade.