* Astronomy

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RE: Mercury

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Mercury's Orbit Eccentricity Precession, Last Million Years

[youtube=http://youtube.com/watch?v=3Fxs7cvJZmc]

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Mercury's Orbit Inclination Precession, Last Million Years

[youtube=http://youtube.com/watch?v=PHIIc21uRPs]

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Chefs have long used a simple trick to differentiate between a raw and hard-boiled egg. By spinning an egg and watching how it behaves when the spin is disrupted, it's easy to tell whether its interior is solid or liquid.
Applying a similar test to the planet Mercury, astronomers have found strong evidence that the planet closest to the sun has a fluid core. The research, led by Jean-Luc Margot, assistant professor of astronomy at Cornell, appears this week on the Web site of the journal Science.
Margot and collaborators conducted a series of observations over five years using a novel technique to detect tiny twists in Mercury's spin as it orbits the sun. The twists, called longitudinal librations, occur as the sun's gravity exerts alternating torques on the planet's slightly asymmetrical shape.
They found that the magnitude of the librations was double what would be expected for a completely solid body -- but explainable for an object whose core is molten and not forced to rotate along with its shell.
Mercury is thought to consist of a silicate mantle surrounding an iron core, but because small planets like Mercury cool off rapidly, the core should have frozen long ago. Maintaining a molten core over billions of years requires that it also contain a lighter element, such as sulphur, to lower the melting temperature of the core material. The presence of sulphur supports the idea that radial mixing, or the combining of elements both close to the sun and farther away, was involved in Mercury's formation process.
A molten core also gives weight to the idea that Mercury's magnetic field, which is about 1 percent as strong as Earth's, is caused by an electromagnetic dynamo.
The researchers used three telescopes -- the NASA/JPL 70-meter antenna at Goldstone, California; and the National Science Foundation's Arecibo Observatory in Puerto Rico and Robert C. Byrd Green Bank Telescope in West Virginia -- to measure slight changes in Mercury's spin. The system involved sending a powerful radar signal at the planet, then receiving the signal's echo, which appeared as a unique pattern of speckles reflecting the roughness of the planet's surface, at two locations separated by about 2,000 miles.
Measuring how long it took for a particular speckle pattern to reproduce at the two locations (about 10 seconds) allowed Margot to calculate Mercury's spin rate with an accuracy of one part in 100,000.
The experiment included 21 such measurements, very carefully timed since Mercury and Earth are only in the necessary alignment for periods of 20 seconds at a time.

"Everything has to happen within that 20-second time window" - Jean-Luc Margot.

Mercury's spin is a subject the paper's second author, University of California-Santa Barbara physics professor emeritus Stan Peale, first studied as a graduate student at Cornell decades ago.
In the years since, Peale showed that Mercury's interior could be characterised in detail if four properties could be determined: the libration amplitude; the planet's obliquity, or the inclination angle of its rotational axis in relation to its orbital plane; and two values called gravitational harmonic coefficients. Peale's formula also required that Mercury be in a Cassini state, a stable orbital configuration that characterises the end of tidal evolution.
Finding those properties from Earth was long thought to be impossible. But this unusual radar technique (the method was first proposed to determine the spin rate in the 1960s; its use for finding the spin orientation was proposed by co-author I.V. Holin about two decades later) allowed the researchers to measure the planet's librations and obliquity -- and to show that Mercury is almost certainly in the required Cassini state.
Mercury still has its share of mysteries. Some may be solved with the NASA spacecraft Messenger, though, launched in 2004 and expected to make its first Mercury flyby in 2008. The spacecraft will begin orbiting the planet in 2011.

"It is our hope that Messenger will address the remaining questions that we cannot address from the ground" - Jean-Luc Margot.

The paper's other authors are R.F. Jurgens and M.A. Slade of NASA's Jet Propulsion Laboratory.

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A UNC-Chapel Hill astronomer captured rare photographs of Mercury on Friday. He did it by aiming a telescope 4,599 miles away in Chile at a planet 91 million miles from Earth.

"There are only a few objects left unseen in the solar system. One is that hemisphere of Mercury" - astrophysicist Gerald Cecil.

The best photographs of Mercury, the planet closest to the sun, are 32 years old and incomplete. Snapped by an unmanned spacecraft in 1974, they missed 55 percent of the planet because it rotates very slowly.
Cecil pulled more of Mercury out of the shadows by seizing an unusual opportunity. Half of Mercury's missed hemisphere points Earth's way this month. And the planet, normally hard to see because it's close to the sun, is popping above Earth's horizon briefly before dawn, making it visible to a telescope.

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A German scientist believes he has resolved a three-decade mystery about why Mercury has such a weak magnetic field.
Mercury is believed to generate its magnetism by a dynamo, caused by the rotation of molten iron at its core. If this theory is right, its magnetism should be 30 times stronger than it is.
Ulrich Christensen believes that its core's outer layers are "stably stratified" - they are largely insulated from the heat of the inner core. As a result, only the inner core rotates effectively to generate the magnetic field.
This effect is important because Mercury has a very slow rotation, which also affects the dynamo's power.

Source: The Star

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New computer simulations of Mercury’s formation show the fate of material blasted out into space when a large proto-planet collided with a giant asteroid 4.5 billion years ago. The simulations, which track the material over several million years, shed light on why Mercury is denser than expected and show that some of the ejected material would have found its way to the Earth and Venus.

“Mercury is an unusually dense planet, which suggests that it contains far more metal than would be expected for a planet of its size. We think that Mercury was created from a larger parent body that was involved in a catastrophic collision, but until these simulations we were not sure why so little of the planet’s outer layers were reaccreted following the impact” - Dr Jonti Horner, who is presenting results at the Royal Astronomical Society’s National Astronomy Meeting on 5th April.

To solve this problem, Dr Horner and his colleagues from the University of Bern ran two sets of large-scale computer simulations. The first examined the behaviour of the material in both the proto-planet and the incoming projectile; these simulations were among the most detailed to date, following a huge number of particles and realistically modelling the behaviour of different materials inside the two bodies. At the end of the first simulations, a dense Mercury-like body remained along with a large swathe of rapidly escaping debris. The trajectories of the ejected particles were then fed in to a second set of simulations that followed the motion of the debris for several million years. Ejected particles were tracked until either they landed on a planet, were thrown into interstellar space, or fell into the Sun. The results allowed the group to work out how much material would have fallen back onto Mercury and investigate other ways in which debris is cleared up in the Solar System.
The group found that the fate of the debris depended on whereabouts Mercury was hit, both in terms of its orbital position and in terms of the angle of the collision.

Whilst purely gravitational theory suggested that a large fraction of the debris would eventually fall back onto Mercury, the simulations showed that it would take up to 4 million years for 50% of the particles to land back on the planet and in this time many would be carried away by solar radiation. This explains why Mercury retained a much smaller proportion than expected of the material in its outer layers.
The simulations also showed that some of the ejected material made its way to Venus and the Earth. While this is only a small fraction, it illustrates that material can be transferred between the inner planets relatively easily. Given the amount of material that would have been ejected in such a catastrophe, it is likely that there is a reasonable amount (possibly as much as 16 million billion tonnes) of proto-Mercury in the Earth.

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