The aptly named Titan has a handful of peers in the size department, but all orbit a different jovian planet.
Scientists peer under Titan's thick fog
 


Titan's surface as seen using a new optical system on the Canada-France-Hawaii telescope


Europa

Date of discovery: 1610
Distance from Jupiter: 415,958 miles (670,900 km)
Diameter: 1,946 miles (3,138 km)
Atmosphere: thin, tenuous layer of oxygen
Unique fact: Has a smooth surface with an almost complete absence of craters or vertical relief
Unsolved mystery: Does Europa harbor a liquid ocean below its icy crust?

May 18, 2001 Posted: 4:32 PM EDT (2032 GMT)
 CNN      By Richard Stenger     
 
(CNN) -- Taking a peek under the thick orange haze that enshrouds Titan, astronomers found a second, mysterious bright spot on the planet-sized moon.

Space and ground telescopes had earlier detected a large, vivid feature on the surface of the moon, which scientists speculated was a massive continent in an ocean of hydrocarbons.

Advanced optical techniques have allowed astronomers to take a closer look, revealing another such peculiarity, the European Space Agency announced this week.

"Another bright feature at Titan's Western limb was noticed for the first time," said Athena Coustenis, an astronomer at a Paris observatory.

The contrast between the bright spot and the darker areas around it "are compatible with a combination of organic deposits and ice extents, possibly related to topography," Coustenis added.

In other words, the finding gives a boost to the theory that Titan possesses landforms, possibly mountains, composed of frozen organic compounds.

The Saturn moon, larger in diameter than Mercury and Pluto, is thought to contain a rich soup of complex organic compounds like the primordial Earth.

What really lies underneath the opaque atmosphere could remain a mystery until an ESA robot craft plunges through the hydrocarbon smog in three years.

The Huygens probe is hitching a ride aboard NASA's Cassini spacecraft, which should arrive in orbit around Saturn in 2004 after an almost seven-year journey.

Months later, the Saturn orbiter will release Huygens, a suite of instruments packed in what resembles a large automobile hubcap, which will then parachute through the atmosphere and presumably land several hours later.

During the descent, the half-ton probe will study the organic makeup of the chemical fog, take photographs and keep an eye out for lightning. Huygens was designed to withstand such electrical outbursts, which are thought to take place on the moon.

Titan boasts an atmosphere, composed mostly of nitrogen and methane, ten times thicker than that on Earth. Scientists theorize that hydrocarbon rain and snow drizzle down to the surface.

The new Titan images were taken by the Canada-France-Hawaii observatory, located on top of Mauna Kea, a dormant volcano on the island of Hawaii.
 

Io

Date of discovery: 1610
Distance from Jupiter: 261,640 miles (422,000 km)
Diameter: 2,250 miles (3,630 km)
Atmosphere: thin, primarily sulfur dioxide
Unique fact: It's the most volcanically active body in the solar system
Unsolved mystery: How do ionized particles from Io interact with the remainder of the Jupiter system?

Source: NASA
Composite of surface images obtained by Galileo space probe

Ganymede

Date of discovery: 1610
Distance from Jupiter: 663,400 miles (1,070,000 km)
Diameter: 3,266 miles (5,268 km)
Atmosphere: May have a thin, tenuous layer of oxygen
Unique fact: Largest moon in the solar system
Unsolved mystery: How can Ganymede have its own magnetic field?

Source: NASA
Galileo space probe image

Callisto

Date of discovery: 1610
Distance from Jupiter: 1,167,460 miles (1,883,000 km)
Diameter: 2,980 (4,806 km)
Atmosphere: Thin, tenuous carbon dioxide layer
Unique fact: Has the oldest surface of any known moon or planet in the solar system
Unsolved mystery: Why is the surface of Callisto so different from other jovian moons?

Source: NASA
Galileo space probe image

Titan

Date of discovery: 1655
Distance from Saturn: 757,534 miles (1,221,830 km)
Diameter: 3,193 miles (5,150 km)
Atmosphere: Mostly nitrogen
Unique fact: Larger than the planets Mercury and Pluto; has a planet-like atmosphere
Unsolved mystery: What lies hidden beneath Titan's impenetrable layers of atmosphere and clouds?

Source: NASA
Image taken with the Keck Telescope near infrared camera