Equatorial ridge

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Equatorial ridges of Saturn's moons

An equatorial comb is in the astronomy a ridge of a celestial body which the equatorial exactly follows. Equatorial ridges are a feature of at least three moons of Saturn : the large moon Iapetus and the small moons Atlas and Pan . They appear to be unique to the Saturn system, but it is uncertain whether the events are related or random. All three equatorial ridges were discovered by the Cassini probe in 2005 . Daphnisalso seems to have such a crest. The ridge on Iapetus is almost 20 km wide, 13 km high and 1,300 km long. The ridge on Atlas is even more remarkable in terms of size, as the moon is much smaller and therefore has a disk-like shape. Images of Pan show a structure similar to Atlas. Titan may also have had an equatorial ridge in the past.

Emergence

It is not certain how the equatorial ridges were formed or whether there is any connection between them. Since Atlas and Pan orbit Saturn in its rings, a likely explanation for their equatorial ridges is that during their orbit they stir up ring particles that build up around their equators. This theory is less applicable to Iapetus, whose orbit is outside of Saturn's rings. There is no ridge in the bright regions of Iapetus, but a series of isolated 10 km high peaks along the equator. The ridge system is very crater-like, which indicates that it is ancient. The distinctive equatorial bulge gives Iapetus a walnut-like appearance. There are at least four current hypotheses, but none explain why the ridge is limited to the Cassini Regio :

Flight over the equatorial ridge of Saturn's moon Iapetus
  • A team of scientists associated with the Cassini mission has argued that the crest may be a holdover of the flattened shape of the young Iapetus when it was rotating at a higher speed. The height of the ridge indicates a maximum rotation time of 17 hours. If Iapetus cooled fast enough to preserve the ridge but remained plastic long enough for the tides raised by Saturn to slow the rotation to the current 79 days, then Iapetus must have been heated by the radioactive decay of aluminum-26 . This isotope appears to have been abundant in the solar nebula from which Saturn was formed, but has since decayed completely. The amounts of aluminum-26 needed to heat Iapetus to the required temperature give a tentative date for its formation compared to the rest of the solar system: Iapetus must have formed earlier than expected, just two million years after the asteroids formed had formed.
  • The ridge could be icy material that oozed out from beneath the surface and then solidified. If it had formed away from the position of the equator at this point, this hypothesis requires that the axis of rotation would have been driven from the ridge to its current position.
  • The equatorial ridges are the result of accretion . Iapetus may have had its own ring system. The debris lost energy through collisions inside the ring, until it finally rained at a shallow angle on the equator, piling up the ridge.
  • The crest and bulge could be the result of an old convective overturn . This hypothesis states that the bulge is in the isostatic balance typical of terrestrial mountains . This means that there is low density material under the bulge. The weight of the bulge is balanced by the buoyancy forces acting on it. The comb is also made of less dense matter. Its position along the equator is likely a result of the Coriolis force acting on a fluid interior of Iapetus.

literature

  • Thomas Bührke, Roland Wengenmayr: Mysterious Cosmos: Astrophysics and Cosmology in the 21st Century . John Wiley & Sons, Weinheim 2012, ISBN 9783527661473 , pp. 126 & 127.
  • Joshua Colwell: The Ringed Planet, Second Edition: Cassini's Voyage of Discovery at Saturn . Morgan & Claypool Publishers, San Rafael 2019 (2nd Edition), ISBN 9781643277141 , pp. 4-26-4-28.

Individual evidence

  1. ↑ a b heise online: A moon like a walnut. Retrieved January 7, 2021 .
  2. ^ Richard A. Kerr: How Saturn's Icy Moons Get a (Geologic) Life . In: Science . 311, No. 5757, January 6, 2006.
  3. ↑ W.-H Ip: On a ring origin of the equatorial ridge of Iapetus . In: Geophysical Research Letters . 33, No. 16, 2006. bibcode : 2006GeoRL..3316203I .
  4. ↑ L. Czechowski, Leliwa-Kopystynski: Isostasy on Iapetus: the myth of fossil bulge . In: EPSC Abstracts . 7, September 25, 2012, p. 834.
  5. ^ L. Czechowski, Leliwa-Kopystynski: Remarks on the Iapetus' bulge and ridge . In: Earth, Planets and Space . 65, No. 8, September 25, 2013, pp. 929-934. bibcode : 2013EP & S ... 65..929C .