Nasa Cassini: Origin of the Equatorial Ridge on Iapetus

Raw image from Cassini space probe of the equatorial ridge on Saturn's moon Iapetus. 

Image: NASA

A combined team of researchers from Brown University in Rhode Island and the Lunar and Planetary Institute in Texas is suggesting in a paper they've uploaded to the preprint server arXiv, that an equatorial mountainous ridge on Iapetus, one of Saturn's moons, has an exogenic origin.

They are basing their theory on 3D models of the moon they've created and an analysis of the types of peaks present.

Iapetus, the 3rd largest of Saturn's approximately 60 moons, is distinct for two reasons.

One is its odd two-tone colouring; the other is the back-bone looking mountain range straddling part of its equator.

Scientists have been puzzled by the origin of the mountain range as the moon doesn't have other geologic qualities that could have given rise to it, such as shifting plates or volcanic activity.

Thus, some have suggested that the mountains came from above, rather than below, or in other words, they have an exogenic origin, meaning they came from somewhere else.

To gain a better understanding of the mountain range, the research team built a 3D model of it on a computer in their lab, faithfully replicating the 12 mile high by 12 mile wide by 800 miles long range in miniature, using data from the Cassini space probe.

Once created, the team set to work measuring the shape of the peaks, which they believed should offer clues as to their origin.

They found that the majority of the peaks sat in what is known as their angle of repose, which is the maximum angle at which material can rest on a peak without falling down to its base.

Normal geologic activity tends to create peaks that are shallower and have less uniformity.

This suggests, the researchers claim, that the mountains did not form due to geologic activity but more likely are part of a ring of material that once circled the moon and was pulled down to the surface.

A ring around the moon would most likely have come about due to a collision, either between another body and the moon, or two other bodies nearby.

The resulting material would have formed a ring around the equator which over time, would have been pulled to the surface by gravity.

Such a theory, the team notes, would also explain Iapetus's asymmetrical orbit and also why it orbits with the same face pointing at Saturn all of the time.

More information: Topographic Constraints on the Origin of the Equatorial Ridge on Iapetus, arXiv:1404.2337 [astro-ph.EP] arxiv.org/abs/1404.2337

NASA Cassinni: Iapetus, Saturn's 'Yin and Yang' Moon

The Cassini spacecraft snapped this image of Saturn's "yin and yang" moon Iapetus on Aug. 30, 2013. 

Credit: NASA/JPL-Caltech/Space Science Institute

Yin and yang appear to meet on one of Saturn's moons in a photo taken by a spacecraft exploring the ringed planet and its many moons.

The new Cassini photo of Saturn's moon Iapetus shows the satellite's dark and light sides as comma-shaped features that bear a striking resemblance to the yin and yang symbol in Chinese philosophy.

The image was released in December, but actually taken by the Cassini spacecraft in August.

"This view looks toward the Saturn-facing hemisphere of Iapetus," NASA officials wrote in an image description.

"North on Iapetus is up and rotated 30 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Aug. 30, 2013."

Saturn's Moon Iapetus: Dichotomy

These two global images of Iapetus show the extreme brightness dichotomy on the surface of this peculiar Saturnian moon. 

The left-hand panel shows the moon's leading hemisphere and the right-hand panel shows the moon's trailing side. Image published Dec. 10, 2009. 

CREDIT: NASA/JPL/Space Science Institute

The light and dark faces of Saturn's moon Iapetus create a contrast that helped to hide the satellite for years, despite its distant orbit.

The moon, which keeps the same face perpetually turned toward Saturn, remained shrouded in shadows, with its bright side only visible from Earth when it traveled on the west side of its primary.

Discovery and naming

Iapetus is one of four moons discovered by Giovanni Cassini in 1672. But the icy moon challenged the Italian astronomer.

He first spotted it on the west side of the planet, but when he attempted to observe it 39 days later on the eastern side, no trace of the satellite could be found.

The pattern repeated itself over the course of the moon's 79-day orbit, leading Cassini to surmise that the moon was tidally locked, with one face darker than the other.

Cassini named the four moons that he discovered (Iapetus, Rhea, Dione, and Tethys) the Sidera Lodoicea, or the Stars of Louis, after King Louis XIV.

For years, the moons were referred to numerically, based on their distance from Saturn. Distant Iapetus started out as Saturn V, but the discovery of Mimas, Enceladus, and Hyperion bumped it down to Saturn VIII.

Nearly 200 years later, John Herschel suggested that the moons around the ringed planet be named for the Titans, the siblings of the Roman god Cronus, who was Saturn to the Greeks.

In Greek mythology, Iapetus is a son of Uranus and Gaia and the father to Atlas and Prometheus.

His relationship to Prometheus, the Titan who gave fire to humans, led early Greeks to regard Iapetus as the father of the human race.

Features on Iapetus take their names from the French epic poem "The Song of Roland."

Composition and orbit

Iapetus has a low density of only 1.083 grams per cubic centimeter, just a little more than liquid water, implying that it composed primarily of water, with rock making up less than a quarter of its composition.

The walnut-shaped moon is not spherical, but has a bulging equator with squashed poles. The squashed shape of the moon resembles a satellite that rotates once every 10 hours, rather than the 79 days it takes Iapetus to make a full revolution.

The angular structure of the moon has led to conspiracy-like suggestions that it is not a natural satellite but could have been built or modified by another civilization.

However, its odd shape could easily have a natural explanation. Early in its life, when the moon still spun rapidly, a thick crust could have frozen the moon into its odd shape.

Over time, the satellite's spin would have slowed until it eventually became tidally locked. Iapetus is the third-largest moon orbiting Saturn, with a diameter of 914 miles (1,471.2 km).

Although its radius is about two-fifths that of Earth's moon, its icy composition means that it is only about 2 percent as massive.

Saturn's Icy Moon, Iapetus struck by 50-Mile Wide Landslides

A giant landslide on Iapetus reaches halfway across a 75-mile (120 kilometer) impact crater.

CREDIT: NASA/JPL/Space Science Institute

Long landslides spotted on Saturn's moon, Iapetus, could help provide clues to similar movements of material on Earth.

Scientists studying the icy satellite have determined that flash heating could cause falling ice to travel 10 to 15 times farther than previously expected on Iapetus.

Extended landslides can be found on Mars and Earth, but are more likely to be composed of rock than ice.

Despite the differences in materials, scientists believe there could be a link between the long-tumbling debris on all three bodies.

"We think there's more likely a common mechanism for all of this, and we want to be able to explain all of the observations," lead scientist Kelsi Singer of Washington University, Dept of Earth and Planetary Sciences, reported.

Cassini Set to Do Retinal Scan of Saturnian Eyeball Mimas

NASA's Cassini spacecraft made its closest examination yet of Mimas, an eyeball-shaped moon of Saturn that has also been likened to the Death Star of "Star Wars."

The spacecraft returned the highest-resolution images yet of this battered satellite.

Mimas bears the mark of a violent, giant impact from the past - the 140-kilometer-wide (88-mile-wide) Herschel Crater - and scientists hope the encounter will help them explain why the moon was not blown to smithereens when the impact happened. They will also be trying to count smaller dings inside the basin of Herschel Crater so they can better estimate its age.

In addition, Cassini's composite infrared spectrometer was used to determine the thermal signature of the moon, and other instruments made measurements to learn more about the surface composition.

The Mimas flyby involved a significant amount of skill because the spacecraft passed through a dusty region to get there. Mission managers have planned for the Cassini spacecraft to lead with its high-gain antenna to provide a barrier of protection.

At closest approach, the spacecraft came within 9,500 kilometers (5,900 miles) of the moon. Cassini began taking images and measurements shortly after closest approach.

Mimas is an inner moon of Saturn that averages 396 kilometers (246 miles) in diameter. The diameter of Herschel Crater is about one-third that of the entire moon. The walls of the crater are about 5 kilometers (3 miles) high, and parts of the floor are approximately 10 kilometers (6 miles) deep.

Saturn's Moon Iapetus covered in foreign dust

New views of Saturn's moon Iapetus accompany papers that detail how reddish dust swept up on the moon's orbit around Saturn and migrating ice can explain the bizarre, yin-yang-patterned surface.

The papers, led by Cassini scientists Tilmann Denk and John Spencer, appeared online in the journal Science on Dec. 10, 2009.

The new image in the left-hand panel of PIA11690 shows the most nearly complete view to date of Iapetus' charcoal-dark leading hemisphere.

The right-hand panel, which had been released previously, shows the trailing hemisphere, where wide swaths are covered by bright ice. The new three-panel image PIA11689 uses false-color views in increasing levels of contrast to reveal the reddish dust that overlays the bright-dark pattern.

Minimal enhancement was applied to the left panel, with increasing contrast added to the middle and right-hand images.

Credit: Southwest Research Institute (SwRI) News.