The equatorial ridge is the tallest mountain feature on

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; ...

moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...

Iapetus. It is high, and is the third tallest mountain structure in the Solar System. It runs along most of Iapetus'

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can al ...

. It was discovered by the '' Cassini'' probe in 2004. The ridge's origin is unknown. There are bright areas on the sides of the equatorial ridge near Iapetus' bright trailing hemisphere, which were already visible in ''

Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on ...

'' images appearing like mountains and were nicknamed the "Voyager Mountains".

Discovery

Iapetus's equatorial ridge was discovered when the ''Cassini'' spacecraft imaged Iapetus on 31 December 2004. Peaks in the ridge rise more than 20 km above the surrounding plains, making them some of the

tallest mountains in the Solar System This is a list of the tallest mountains in the Solar System. This list includes peaks on all celestial bodies where significant mountains have been detected. For some celestial bodies, different peaks are given across different types of measurem ...

. The ridge forms a complex system including isolated peaks, segments of more than 200 km and sections with three near parallel ridges.

Origins

Within the bright regions there is no ridge, but there are a series of isolated 10 km peaks along the equator. The ridge system is heavily cratered, indicating that it is ancient. The prominent equatorial bulge gives Iapetus a walnut-like appearance. It is not clear how the ridge formed. One difficulty is to explain why it follows the equator almost perfectly. There are at least four current hypotheses, but none of them explains why the ridge is confined to

Cassini Regio Cassini Regio (adjective ''Cassinian'' ) is the enigmatic dark area that covers the leading half of Saturn's moon Iapetus. It is named after Giovanni Cassini, the discoverer of Iapetus; ' Regio' is a term used in planetary geology for a large ...

. * A team of scientists associated with the ''Cassini'' mission have argued that the ridge could be a remnant of the oblate shape of the young Iapetus, when it was rotating more rapidly than it does today. The height of the ridge suggests a maximum rotational period of 17 hours. If Iapetus cooled fast enough to preserve the ridge but remained plastic long enough for the

tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables can ...

s raised by Saturn to have slowed the rotation to its current tidally locked 79 days, Iapetus must have been heated by the radioactive decay of

aluminium-26 Aluminium-26 (26Al, Al-26) is a radioactive isotope of the chemical element aluminium, decaying by either positron emission or electron capture to stable magnesium-26. The half-life of 26Al is 7.17 (717,000) years. This is far too short for the i ...

. This

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass number ...

appears to have been abundant in the

solar nebula The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened int ...

from which Saturn formed, but has since all decayed. The quantities of aluminium-26 needed to heat Iapetus to the required temperature give a tentative date to its formation relative to the rest of the Solar System: Iapetus must have come together earlier than expected, only two million years after the

asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic o ...

s started to form. * The ridge could be icy material that welled up from beneath the surface and then solidified. If it had formed away from the position of the equator at the time, this hypothesis requires that the rotational axis would have been driven to its current position by the ridge. * Iapetus may have had a ring system during its formation due to its large

Hill sphere The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sp ...

, and the equatorial ridge could have then been produced by collisional accretion of this ring. * Iapetus could have been impacted by a body that created both a ring system and a

subsatellite A subsatellite, also known as a submoon, or moonmoon, is a "moon of a moon" or a hypothetical natural satellite that orbits the moon of a planet. It is inferred from the empirical study of natural satellites in the Solar System that subsatellites ...

, which would be possible due to the large Hill sphere mentioned above. The subsatellite then proceeds to quickly force the ring system down onto Iapetus, creating the ridge system, before escaping Iapetus' gravity via

tidal acceleration Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away f ...

. * The ridge and the bulge could be the result of ancient convective overturn. This hypothesis states that the bulge is in isostatic equilibrium typical for terrestrial mountains. It means that under the bulge there is material of low density (roots). The weight of the bulge is compensated by

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...

forces acting on the roots. The ridge is also built of less dense matter. Its position along the equator is probably a result of the Coriolis force acting on a liquid interior of Iapetus.

References

{{Iapetus Surface features of Iapetus (moon) Extraterrestrial ridges Extraterrestrial mountains

Discovery

Origins

See also

References