- 3820 - SATURN - a planet you can see with binoculars? Of all the planets visible in the night sky, either with the naked eye or the aid of a powerful telescope, none is more recognizable or iconic than Saturn. With its giant system of rings, Saturn’s appearance is immediately discernible, setting it apart from all the other known planets.
--------- 3820 - SATURN - a planet you can see with binoculars?
- Saturn's rings have been observed since the
invention of the telescope back in 1609.
Saturn's rings were a wholly unique feature within our Solar System.
While the other giant planets have since been discovered to have rings, they're
faint and unimpressive compared to Saturn's. Despite all we've learned about
our Solar System, the origin of Saturn's rings have remained an unsolved
puzzle.
-
- First observed as “ears” by Galileo in
1609, a sharper view reveals that Saturn doesn’t have a shape like an
amphibian’s eyes, but rather an expansive set of rings, detached and separated
from the planet it surrounds. Over time, gaps, moons, moonlets, and a plethora
of other features have been found above, below, inside, outside, and even
within Saturn’s rings.
-
- None of the rocky planets, asteroids, or
Kuiper belt objects have rings. Jupiter, Uranus, and Neptune possess them, but
they’re all much fainter, sparser, smaller, and less massive than
Saturn’s. Saturn’s rings are tilted, are
made almost exclusively of water-ice, and are in the process of evaporating.
-
- Once thought to have been a mainstay of the
Solar System, we now believe that Saturn’s rings formed in the cosmic blink of
an eye approximately 100 million years ago, and should be gone within less than
another 100 million.
-
- How did Saturn’s rings form? A single violent event, just 150 million
years ago, could explain not only Saturn’s rings, but a series of bizarre
properties found only in the Saturnian system.
-
- From its unique vantage point in the shadow
of Saturn, the atmosphere, the main rings, and even the outer E-ring are all
visible, along with the visible ring gaps of the Saturnian system in eclipse.
Saturn’s rings and inner 23 moons all orbit in roughly the same plane.
-
- Whenever a giant planet, like Jupiter or
Saturn, forms in a stellar system like our own, we can expect a number of steps
to occur. From an initial, central protostar with a protoplanetary disk
surrounding it.
-
- Rock-and-metal cores will develop around
large, growing instabilities within the disk, those cores will begin attracting
surrounding material and growing rapidly, and upon reaching a critical size,
will begin hanging onto volatile compounds and elements, forming gas giant
worlds with circum-planetary disks around them.
-
- Where those disks rapidly develop
instabilities and form moons of varying sizes and compositions, with volatiles
existing in the solid, liquid, and / or gas phases depending on the
temperatures of those moons and their distance from the parent star.
-
- Jupiter and Saturn have some remarkable
differences between them: more striking than their differing masses, sizes,
colors, and compositions. Although they rotate with similar periods (9.9 hours
to 10.5 hours), Saturn has a much greater axial tilt: 26.73° to 31.3°.
-
- Saturn’s ring system is far more expansive
over a thousand times and perhaps as much as 100 million times as massive as
Jupiter’s. And while all of Jupiter’s very massive moons orbit within <1° of
Jupiter’s rotation axis, Saturn has exceptions, with Iapetus, its second
most-massive moon, orbiting more than 15° out of its rotational plane.
-
- Saturn’s axis also precesses with a period
of about 1.83 million years, similar to
Neptune’s orbital plane shifting with a period of 1.87 million years.
-
- A few of the findings from Cassini’s direct
sampling is the discovery that complex organics rain down from Saturn’s rings
onto its equator; inner-ring particles take on electric charges and travel
along magnetic-field lines toward high latitudes; they follow and interplay
with a complex electric-current system and a radiation belt; and we discovered
that Saturn possesses a magnetic-field with a near-zero tilt.
-
- Saturn’s highly reflective rings, composed
mostly of water-ice are in the process of disappearing. Saturn is devouring its own rings rapidly
through a combination of two related processes: ionized ring rain and dusty/icy
equatorial infall.
-
- Ultraviolet light from the Sun strikes the
water-ice rings, as do plasma clouds from meteoroid strikes. These excite the
molecules and atoms in the rings, creating ions. Then, Saturn’s electrically
charged ionosphere interacts with those ions, funneling them toward high
northern and southern latitudes: giving rise to the ring rain.
-
- As Cassini passed between the rings and
the planet, it discovered that the inner ring particles are onfalling onto the
planets equatorial region. Combining these two effects, equatorial infall and
high-latitude ring rain, allows us to measure the rate of mass loss within the
ring system, and constrain the age and lifetime of Saturn’s rings.
-
- Saturn’s rings. haven’t been around for all
4.5 billion years of the Solar System’s history: rather, they were likely
created no more than a scant 100 million years ago, and will be almost
completely gone within the next 100 million years.
-
- Where did Saturn’s rings come from? How were
they created? Although we are only getting a snapshot of the Saturnian system
as it exists today, there are some clues that are encoded in a variety of the
surviving objects.
-
- Clue #1: Mimas. Although there are numerous moons and
moonlets located within Saturn’s main rings, Mimas, Saturn’s 7th largest moon
overall, is the first moon located outside of the ring system. Mimas is
spheroidal despite a mean diameter of only 400 kilometers, making it the
smallest moon in the Solar System to be pulled into a spheroidal shape.
-
- However, Mimas also possesses an enormous
impact crater (named Herschel), which itself is approximately one-third of the
diameter of the entire moon itself. The impact that formed this crater must
have nearly shattered the entire world, as substantial fractures can be found
focused on the exact opposite side of Mimas from Herschel itself: at the
antipodes.
-
- Although Herschel was estimated to have
formed approximately 4.1 billion years ago, an indication that Mimas may have
been an original moon of Saturn, it’s a stark reminder that worlds can be
destroyed entirely by large enough impacts. (Tethys, Saturn’s 5th largest moon,
has a similarly large impact crater on it, an indication that Mimas is not
unique.)
-
- The next major moon of Saturn, moving
outward from Mimas, is Enceladus: larger and more massive than Mimas, but also
far more active in a puzzling way. Enceladus, despite experiencing much smaller
tidal forces from Saturn than Mimas, experiences large eruptions coming from
its south pole, where plumes chemically composed of saltwater, sand, ammonia,
and organic molecules routinely extend more than 300 kilometers above the icy
surface of the world itself. These materials don’t all fall back onto
Enceladus, but rather get stretched out to form a diffuse ring made of
primarily water-ice coincident with Enceladus’s orbit: Saturn’s E-ring.
-
- Because Enceladus is losing mass so
rapidly, and also appears to have a substantial subsurface ocean. How old is Enceladus? Was it formed from the
primeval Saturnian nebula that created Mimas and many of the other moons? Or
did it arise much later, formed out of the debris from the remnants of a
previously destroyed satellite?
-
- Enceladus may be relatively young compared
to the other large moons orbiting Saturn, with two recent estimates putting
Enceladus’s age at 100 million years and 1.0 billion years. It’s a sober
reminder that things, as we see them today, may not be a reflection of how they
were a relatively short (amount of cosmic) time ago.
-
- Saturn has a substantial axial tilt just
like Earth does: of 26.7 degrees, leading to its seasons. Where as Earth’s
seasons last roughly 3 months apiece, seasons on Saturn last 7 years each.
-
- If you were to look at these two clues, you
might imagine a very reasonable possibility for the origin of Saturn’s rings:
perhaps a previously-existing Moon, orbiting within the inner regions of
Saturn, was struck by a large, fast-moving object, and was shattered
completely. This material would then re-form into some new Moons, Enceladus and
the innermost ones within the rings, and the rings themselves. This sort of
scenario could explain Saturn’s young, ice-rich rings, as well as the bizarre
properties of Enceladus, without messing up the properties of the other
Saturnian moons.
-
- This explanation hasn’t been ruled
out, but there are other properties that
it fails to explain. It cannot explain why Saturn has such a large axial tilt,
and why all of the moons (interior to Iapetus) as well as the rings have the
same tiny orbital inclination with respect to Saturn’s rotation.
-
- Why would such a collision create new rings
and new moons in the same plane as all of the old rings and moons? And why is
Saturn (and why are its rings and moons) so severely tilted relative to, say,
Jupiter and its rings and moons?
-
- Clue #3: Iapetus. Often noted to be the most bizarre moon in
the Solar System, Iapetus has three very rare features that set it apart from
most other large moons. All of Saturn’s
other major moons, including every moon and moonlet located interior to
Iapetus, orbit Saturn to within 1.6° of Saturn’s rotational axis. But not
Iapetus, which is inclined at 15.5° relative to all of the other Saturnian
satellites.
-
- Iapetus, at its equator, has an enormous
equatorial ridge. It spans 1,300 kilometers across: nearly the full diameter of
the world. The ridge has a width of 20 kilometers and rises to a height of 13
kilometers, following the equator almost perfectly, yet with multiple
disconnected segments and isolated peaks.
-
- Iapetus has a two-toned color to it, with
one portion covered in darker material and the other, lighter portion covered
in ices. The last such feature is
explained by Saturn’s moon Phoebe: itself likely a captured Kuiper belt object.
But Iapetus’s inclination and equatorial ridge, which is more continuous on the
Saturn-facing side, remain mysterious.
-
- Unlike the innermost 21 moons and moonlets
of Neptune, the next three, Titan, Hyperion, and Iapetus, all have larger
eccentricities to their orbits, and no one is certain as to why. Triton’s south polar terrain, as photographed
by the Voyager 2 spacecraft and mapped to a spheroid of the appropriate shape
and size. About 50 dark plumes mark what are thought to be cryovolcanoes, with
those trails being caused by the phenomenon colloquially called ‘black
smokers.’ Triton is a captured Kuiper belt object, having most certainly
cleared out almost all of Neptune’s original moons.
-
- Clue #4: Triton. Neptune, if you look at its innermost
satellites, has 7 of them that all orbit in practically the same planet that
Neptune rotates in. The largest, Proteus, is about the size of Mimas; the most
inclined, Naiad, has an orbital inclination of 4.7°. And then, moving outward
by one more moon, you encounter Triton: the largest, most massive moon in the
Neptunian system by far: nearly 1,000 times as massive as Proteus.
------------- It
orbits at a severe angle to all the other moons,in the opposite
(retrograde) direction, with a
composition that makes it similar to Kuiper belt objects, not other Neptunian moons.
-
- Out beyond the orbit of Triton, which
orbits Neptune in a little under 6 days, the other Neptunian moons have their
orbital periods measured in years, and appear at a wide variety of angles and
with large eccentricities. Triton, at some point, came into the Neptunian
system, disrupted and/or cleared out the outermore moons, and settled into its
current orbit.
-
- Only Nereid, and even that has a big “maybe”
attached to it, might persist from among Neptune’s outermore original moons, teaching
us that large masses can easily “clear out” a planetary system: something that
clearly hasn’t happened for the inner 3.5 million kilometers around Saturn.
(Whereas Saturn’s main rings only extend for under 150,000 km.)
-
- The orbit of Iapetus extends for more than
twice the diameter of any of the other major Saturnian moons. Both a top-down
and side view show the extent of Iapetus’s orbit relative to the other moons,
while only the side view illustrates Iapetus’s orbital tilt around Saturn’s equator.
-
- Interior to Iapetus, in order, are Hyperion,
Titan, Rhea, followed by many other moons. Only interior to Enceladus and then
Mimas can the modern main rings be found.
-
- Instead of the rings, the moons within and
interior to them, and Enceladus, there was previously a large, massive moon
orbiting between Titan and Iapetus: a body named Chrysalis. Chrysalis would
have had to have been comparable in mass to Iapetus, but completing a
revolution around Saturn in around 45 days. With an additional mass present in
that location:
------------------- Saturn’s moon Titan would have been driven
outward,leading to increased eccentricities for Titan, Hyperion, and Iapetus,
as well as potentially a substantial inclination for Iapetus,
-
------------------ while Saturn acquires a large axial tilt
through a spin-orbit precession resonance with Neptune,
-
----------------- Saturn’s hypothetical Chrysalis would have
been driven inward by these interactions.
-
- Eventually, Chrysalis would reach the limit
of its ability to hold itself together: where tidal gravitational interactions
from Saturn and Titan would tear it apart, creating the debris which would
eventually re-coalesce into the modern ring system along with an additional of
inner moons.
-
-
- If Chrysalis formed early on in Saturn’s
history, it could have driven all of these processes over billions of years,
leading to not only the orbital tilt of Saturn, but the relative positions,
eccentricities, and obliquities of major moons Titan, Hyperion, and Iapetus.
-
- If Chrysalis were then torn apart
approximately 160 million years ago, it could have given rise to the inner ring
system as well as numerous moons, perhaps including Enceladus.
-
January 10, 2022 SATURN - a
planet you can see with binoculars? 3820
----------------------------------------------------------------------------------------
-----
Comments appreciated and Pass it on to whomever is interested. ---
--- Some
reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email
feedback, corrections, request for copies or Index of all reviews
--- to: ------
jamesdetrick@comcast.net
------ “Jim Detrick” -----------
--------------------- --- Wednesday, January 11, 2023 ---------------------------
-
-
No comments:
Post a Comment