June 14th 06, 11:44 PM
http://www.swri.org/9what/releases/2006/Canup.htm
SwRI researchers offer first explanation for the near constant scale
of the gas planet satellite systems
Southwest Research Institute (SwRI) News
Boulder, Colo. -- June 14, 2006 -- Each of our Solar System's outer
gaseous planets hosts a system of multiple satellites, and these
objects
include Jupiter's volcanic Io and Europa with its believed subsurface
ocean, as well as Titan with its dense and organic-rich atmosphere at
Saturn. While individual satellite properties vary, the systems all
share a striking similarity: the total mass of each satellite system
compared to the mass of its host planet is very nearly a constant
ratio,
roughly 1:10,000.
Research by scientists at Southwest Research Institute, published in
the
June 15 issue of Nature, proposes an explanation as to why the gaseous
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 are each 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 the respective planet's mass. The
Uranian satellite system structure is similar to that of Jupiter, 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 (0.01) of the Earth's mass, and
Pluto's satellite, Charon, containing more than 10 percent (0.1) of its
mass.
Why do the gas planets, each with unique formation histories of their
own, have satellite systems containing a consistent fraction of each
planet's mass, and why is this fraction so small compared to solid
planet satellites? Dr. Robin Canup and Dr. William Ward of the SwRI
Space Studies Department propose that 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
believed
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. It is within that disk that the satellites are
believed to have formed.
Canup and Ward considered that a growing satellite's gravity induces
spiral waves in a surrounding gas disk, and that gravitational
interactions between these waves and the satellite cause the
satellite's
orbit to contract. This effect becomes stronger as a satellite grows,
so
that the bigger a satellite gets, the faster its orbit spirals inward
toward the planet. The team proposes 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 shows that multiple generations of
satellites were likely, with today's satellites being the last
surviving
generation that formed as the planet's growth ceased and the gas disk
dissipated. Canup and Ward demonstrate 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 spacings 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," says Canup. "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, this research suggests that 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 the Earth. This is relevant to the potential habitability of
satellites in extrasolar systems.
The NASA Planetary Geology and Geophysics and Outer Planets Research
programs funded this research. The article, "A common mass scaling for
satellite systems of gaseous planets," by Canup and Ward, appears in
the
June 15 issue of Nature.
Editors: An image to accompany this story is available at
http://www.swri.edu/press/2006/canupfigure.htm .
For more information, contact Maria Martinez
), Communications Department,
(210) 522-3305, Southwest Research Institute,
PO Drawer 28510, San Antonio, TX 78228-0510.
SwRI researchers offer first explanation for the near constant scale
of the gas planet satellite systems
Southwest Research Institute (SwRI) News
Boulder, Colo. -- June 14, 2006 -- Each of our Solar System's outer
gaseous planets hosts a system of multiple satellites, and these
objects
include Jupiter's volcanic Io and Europa with its believed subsurface
ocean, as well as Titan with its dense and organic-rich atmosphere at
Saturn. While individual satellite properties vary, the systems all
share a striking similarity: the total mass of each satellite system
compared to the mass of its host planet is very nearly a constant
ratio,
roughly 1:10,000.
Research by scientists at Southwest Research Institute, published in
the
June 15 issue of Nature, proposes an explanation as to why the gaseous
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 are each 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 the respective planet's mass. The
Uranian satellite system structure is similar to that of Jupiter, 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 (0.01) of the Earth's mass, and
Pluto's satellite, Charon, containing more than 10 percent (0.1) of its
mass.
Why do the gas planets, each with unique formation histories of their
own, have satellite systems containing a consistent fraction of each
planet's mass, and why is this fraction so small compared to solid
planet satellites? Dr. Robin Canup and Dr. William Ward of the SwRI
Space Studies Department propose that 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
believed
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. It is within that disk that the satellites are
believed to have formed.
Canup and Ward considered that a growing satellite's gravity induces
spiral waves in a surrounding gas disk, and that gravitational
interactions between these waves and the satellite cause the
satellite's
orbit to contract. This effect becomes stronger as a satellite grows,
so
that the bigger a satellite gets, the faster its orbit spirals inward
toward the planet. The team proposes 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 shows that multiple generations of
satellites were likely, with today's satellites being the last
surviving
generation that formed as the planet's growth ceased and the gas disk
dissipated. Canup and Ward demonstrate 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 spacings 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," says Canup. "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, this research suggests that 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 the Earth. This is relevant to the potential habitability of
satellites in extrasolar systems.
The NASA Planetary Geology and Geophysics and Outer Planets Research
programs funded this research. The article, "A common mass scaling for
satellite systems of gaseous planets," by Canup and Ward, appears in
the
June 15 issue of Nature.
Editors: An image to accompany this story is available at
http://www.swri.edu/press/2006/canupfigure.htm .
For more information, contact Maria Martinez
), Communications Department,
(210) 522-3305, Southwest Research Institute,
PO Drawer 28510, San Antonio, TX 78228-0510.