Why Do Jupiter's Moons Still Shine When Eclipsed?
These images show Ganymede eclipsed by Jupiter, obtained during its eclipse, by the Subaru and Hubble Space Telescopes.
Credits: NASA/Science Channel
On August 4, the much-anticipated "Wonders of the Solar System" documentary will premier on the Science Channel.
Presenter and physicist Prof. Brian Cox will show you the hidden mysteries of our interplanetary neighborhood, as well as breathtaking sights of the planets, moons and the sun. Cox will also examine some of Earth's extreme environments to see how life has adapted, perhaps helping us understand whether life can exist elsewhere in our solar system. To provide a taster of what you can expect from "Wonders," Discovery News has gathered some facts, figures and the best images of our solar system to assemble a special Wide Angle supporting this groundbreaking documentary. Let's begin, in the orbit of the solar system's biggest planet, Jupiter...
Moons of Jupiter: Io
Welcome to Jupiter, the solar system's biggest planet and host to more than 63 peculiar moons -- a steady contender for moons compared to satellite-strewn Saturn. From sulfur-spewing furnaces to ice-encrusted oceans, these Jovian satellites are anything but normal! Browse through 10 of our favorites here. If you're looking for a fiery, volcanic spectacle, head to Io, the innermost of Jupiter's "big four" moons. Jupiter's gravity pulls at Io so strongly that the land has tides of up to 300 feet (100 meters). This gravitational tug-of-war produces scorching heat and raises more volcanic activity here than anywhere else in the solar system. Io has a sulfurous surface, and its volcanoes spew silicate magma, causing the hellish moon's surface look like a pizza.
Moons of Jupiter: Metis
While it doesn't look like much in this image (the highest-resolution available!), Metis is Jupiter's closest companion -- for now. It's orbiting the gas giant at a distance of just 75,500 miles (128,000 kilometers) and moving faster than Jupiter spins. Metis is so close to Jupiter's surface, in fact, that it will gradually succumb to the planet's gravity and plunge into its churning clouds. This same principle applies to man-made satellites orbiting the Earth; if their orbit is too low, they'll eventually fall.
Moons of Jupiter: Adrastea
Like Metis, Adrastea is on its way down, eventually: Its orbit is just 1,000 kilometers (621 miles) outside of its fellow doomed moon. The two bodies probably provide lots of the material making up Jupiter's main ring, shown in this Galileo spacecraft image. Adrastea is also tiny at a wee 12 miles (20 kilometers) in diameter.
Moons of Jupiter: Ganymede
If Ganymede orbited the sun instead of Jupiter, it would be a planet of its own -- it's even bigger than Mercury. Its interior is made from layers surrounding a rocky core, and its surface is covered in water ice. Although Ganymede doesn't have much of an atmosphere, it does have some ozone gas near its surface. This gas comes from charged particles in Jupiter's magnetic field smacking into the moon's icy crust.
Credit: Courtesy of Damien Perrotin
Moons of Jupiter: Themisto
We're not entirely sure what Themisto looks like, but scientists know it one odd little satellite. Unlike most other moons, Themisto is oblong and doesn't fit into the traditional groups of Jovian moons. Because this body is so small -- just 5 miles (8 kilometers) in diameter -- astronomers in 2000 confused it for a new object when it was originally found in 1975. Shown here is a speculative illustration of how the irregular moon might be shaped.
Moons of Jupiter: Callisto
Along with Io, Ganymede, and Europa (which is yet to come in this slide show), Callisto is one of the four Galilean satellites. These are the moons of Jupiter that Galileo discovered while looking through a telescope in 1610. Callisto is about the same size as Mercury, and it's a heavily cratered moon that has almost no geological activity. Callisto's surface may also be one of the oldest landscapes in the solar system, including Earth's moon -- about 4 billion years old.
Moons of Jupiter: Thebe
Along with Metis and Adrastea, Thebe was discovered by scientists studying images from the Voyager spacecraft in 1979 and 1980. It's closer to Jupiter than Io is, but it's not so close that it's in danger of losing its altitude. The material in Jupiter's Gossamer ring probably comes from Thebe and another moon, Amalthea.
Credit: NASA/Michael Carroll
Moons of Jupiter: Amalthea
Speaking of Amalthea, shown here is an artist's rendition of the mysterious moon (inset: our best real view so far). Astronomers don't know much about it, but they do know it's the reddest body in the solar system. Unlike the other three moons that lie within Io's orbit -- Thebe, Metis and Adrastea -- Amalthea wasn't discovered by the Voyager science team. Edward Emerson Barnard discovered Amalthea it in 1892, and it had been almost 300 years since anyone had discovered a new moon orbiting Jupiter. Before that, the last person to discover a Jovian moon was Galileo.
Credit: University of Hawaii/Scott Sheppard/D
Moons of Jupiter: S/2000 J11
This not-to-be-named moon marks an interesting dividing line in the wealth of satellites orbiting Jupiter. S/2000 J11, named after it was found in 2000, moves in the same direction that Jupiter spins -- as do all the moons that are closer to the planet. Almost every distant moon, however, orbits in the opposite (retrograde) direction, as this diagram shows. The only known exception is Carpo, sometimes known as Karpo, which was discovered in 2003.
Moons of Jupiter: Europa
About two-thirds of the Earth is covered in water, but Europa has about twice as much water as our home planet. Europa's might surface is covered in ice, but astronomers are almost convinced there is an ocean of liquid water underneath -- an environment shielded from Jupiter's intense radiation that might be hospitable to life. A cold, salty planet might not seem like a good place to live, but there are species on Earth that thrive in just those conditions.
Slideshow originally posted Feb. 2009.
Jupiter's largest moons don't go completely dark when the giant planet blocks their sunlight, astronomers have found.
The discovery could reveal more about Jupiter's mysterious upper atmosphere, which the researchers suspect is responsible for keeping the moons lit when they are not directly illuminated by the sun. This research could also help scientists better understand the atmospheres of alien planets, study team members said.
Jupiter, the largest planet in the solar system, has 67 known moons — more than any other planet. Jupiter's four largest moons — Io, Europa, Ganymede and Callisto — are known as the Galilean satellites, after their discoverer, famed astronomer Galileo Galilei. [Photos: The Galilean Moons of Jupiter]
The researchers made their discovery about the Galilean moons accidentally. Their original plan was to detect the diffuse light from the most distant parts of the universe. They wanted to find dark objects in space that could block this far-off light. The difference in brightness between those dark objects and the surrounding sky could then help them determine how bright the diffuse and distant light is.
The researchers assumed the Galilean satellites would be dark while immersed in Jupiter's shadow. As such, they "planned to use the Galilean satellites in eclipse as 'occulters' to block distant background emissions," said lead study author Kohji Tsumura, an astronomer at Tohoku University in Japan.
Instead, using the Subaru Telescope and Hubble Space Telescope, the researchers found an unexpected surprise: The Galilean satellites were still slightly bright even when eclipsed. This effect is especially pronounced for Ganymede and Callisto.
"This is a serendipitous discovery, which makes us surprised and excited," Tsumura told Space.com.
Making this discovery was very challenging because the Galilean satellites are extremely faint while eclipsed, and the incredibly bright face of Jupiter near them can blind attempts to see them. Furthermore, the eclipses only take place at specific times, and Jupiter and its moons are continuously in motion, which makes observations very complex, the researchers said.
All in all, when eclipsed, the luminosity of these moons was one-millionth to one-ten-millionth of their uneclipsed brightness — dim enough for the phenomenon to remain undetected until now, even though researchers have observed the Galilean moons in eclipse for centuries.
A schematic image of the model shows Jovian shadow eclipsing the Galilean satellites and illuminating the moons by scattered sunlight created in the haze of the Jovian upper atmosphereNAOJ/JAXA/Tohoku University/NASA
It remains uncertain what causes the slight brightening. However, Tsumura and his colleagues believe the upper part of Jupiter's atmosphere may be responsible.
Jupiter's clouds, which give the giant planet its striped appearance, grow from tiny particles called aerosols or hazes. Prior studies have hinted that these hazes form in the upper part of Jupiter's atmosphere. The researchers suggested that hazes in the upper atmosphere may scatter sunlight onto the Galilean satellites, illuminating them. This effect is similar to the one that causes Earth's moon to look red during a total lunar eclipse.
The new finding could help scientists analyze the hazes in Jupiter's atmosphere, which are otherwise difficult to study. By studying the spectrum of light from the eclipsed Jupiter moons, the researchers could learn about the compositions of the hazes, Tsumura said.
In addition, this new method of studying Jupiter's upper atmosphere could help researchers investigate the atmospheres of exoplanets around distant stars. Exoplanets are often seen when they pass in front of their stars, and scientists can glean details about their atmospheres when starlight passes through them.
"It is important to study sunlight transmitted through the atmospheres of the planets in our solar system for comparison with light through the atmospheres around the exoplanets," Tsumura said.
Tsumura and his colleagues detailed their findings in the July 10 issue of The Astronomical Journal.
More from SPACE.com:
Jupiter Quiz: Test Your Jovian Smarts
Touring Jupiter's Big Moons: Io, Ganymede, Europa, Callisto
Celestial Photos: Hubble Space Telescope's Latest Cosmic Views