Gas Giants Consistently Larger Than Their Moons
Boulder CO (SPX) Jun 15, 2006
Each of the solar system's outer gaseous planets hosts a system of multiple satellites, and these objects include Jupiter's volcanic Io and Europa with what is thought to be a sub-surface ocean of water, as well as Titan with its dense and organic-rich atmosphere at Saturn.
The properties of the individual moons vary greatly, but the planetary systems all share a striking similarity: the ratio of the total mass of each satellite system compared to the mass of its host planet is very nearly constant: roughly 1:10,000.
Reporting in the June 15 issue of Nature, researchers at the Southwest Research Institute have proposed an explanation of why the nearby gas-giant planets display this consistency, and why the satellites of gas planets are so much smaller compared to their planet than the principal satellites of solid planets.
Jupiter's four Galilean satellites each are roughly similar in size, while Saturn has one large satellite together with numerous much smaller satellites. Even so, the total mass in both satellite systems is about a hundredth of one percent (0.0001) of their respective parent planet's mass.
The Uranian satellite system structure is similar Jupiter's, and it also exhibits the same mass ratio.
In contrast, the large satellites of solid planets contain much larger fractions of their planet's masses, with the Moon containing 1 percent of Earth's mass, and Pluto's satellite, Charon, containing more than 10 percent of its mass.
Why do the gas planets, with unique formation histories of their own, harbor satellite systems containing a consistent fraction of each planet's mass, and why is this fraction so small compared to solid planet satellites?
Robin Canup and William Ward of the SwRI Space Studies Department propose it was the presence of gas, primarily hydrogen, during the formation of these satellites that limited their growth and selected for a common satellite system mass fraction.
As the gas planets formed, they accumulated hydrogen gas and solids such as rock and ice. The final stage of a gas planet's formation is thought to involve an inflow of both gas and solids from solar orbit into planetary orbit, producing a disk of gas and solids orbiting the planet in its equatorial plane. Within that disk, the satellites are thought to have formed.
Canup and Ward considered that a growing satellite's gravity induces spiral waves in a surrounding gas disk, and gravitational interactions between these waves and the satellite cause the satellite's orbit to contract.
This effect becomes stronger as a satellite grows, so the bigger a satellite gets, the faster its orbit spirals inward toward the planet.
The team proposed that the balance of two processes - the ongoing inflow of material to the satellites during their growth and the loss of satellites to collision with the planet - implies a maximum size for a gas planet satellite consistent with observations.
Using both numerical simulations and analytical estimates of the growth and loss of satellites, the team argues that multiple generations of satellites were likely, with the existing satellites being the last surviving generation that formed as the planet's growth ceased and the gas disk dissipated.
The research demonstrates that during multiple cycles of satellite growth and loss, the fraction of the planet's mass contained in its satellites at any given time maintains a value not very different from 0.0001 across a wide range of model parameter choices.
The team's direct simulations are also the first to produce satellite systems similar to those of Jupiter, Saturn and Uranus in terms of number of satellites, their largest masses and the spacing of the large satellite orbits.
"We believe our results present a strong case that the satellite systems of Jupiter and Saturn formed within disks produced as the planet itself was in its final growth stages," Canup said. "However, the origin of the Uranian satellite system remains more uncertain, and the likelihood of our results being applicable to that planet depends on how Uranus achieved its nearly 98-degree axial tilt, which is a topic of active study."
For extrasolar systems, Canup and Ward suggest, the largest satellites of a Jupiter-mass planet would be Moon-to-Mars sized, so that Jovian-sized exoplanets would not be expected to host satellites as large as Earth. This is relevant to the potential habitability of satellites in extrasolar systems.
NASA planetary photojournal
Southwest Research Institute
Two Great Jovian Storms Converging
Greenbelt MD (SPX) Jun 12, 2006
The two biggest storms in the solar system are about to bump into each other in plain view of backyard telescopes. Storm 1 is the well-known Great Red Spot, twice as wide as Earth itself, with winds blowing at 350 miles per hour - faster than a Category 5 tornado.
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