Saturn
Naked Saturn
Here's one of the first raw images of Saturn taken by the Cassini spacecraft just after equinox, on August 12, 2009. The planet sure looks naked without its rings! But not to fear, the rings are still there; we just can't see them very well — only a thin line. That's because the sun was shining directly straight-on at the rings at Saturn's equinox, and the spacecraft was in the right place, too. Equinox occurs every half-Saturn-year which is equivalent to about 15 Earth years. The illumination geometry that accompanies equinox lowers the sun's angle to the ringplane and causes out-of-plane structures and some moons to cast long shadows across the rings. The ring shadows themselves have become a rapidly narrowing band cast onto the planet. Below, see another image with the rings visible as the spacecraft changed its angle.
Saturn's rings at equinox. Credit: NASA
Mystery Object Pierces a Saturn's Ring
August 11, 2009—A mystery object that punched through one of Saturn's thin outer rings created a glittering spray of ice crystals and pulled some material along in its wake, as seen in this rare image recently released by NASA's Cassini orbiter.
The puncture, which Cassini snapped on June 11, is among the many marvels that have been revealed in the weeks leading up to Saturn's equinox, which happens today. (Find out how the equinox has helped astronomers find new moons, how it will make Saturn temporarily "lose" its rings, and more.)
Equinoxes occur at the point in a planet's orbit when the sun shines directly on the planet's equator. Earth has two equinoxes each year, one in spring and another in autumn. (See an equinox video.)
Because of Saturn's comparatively long orbit around the sun, the ringed planet has an equinox just once every 15 Earth years.
This celestial alignment lowers the angle at which sunlight strikes Saturn, causing objects higher or lower than the ring plane to cast long shadows—and offering scientists a completely new look at the gas giant and its rings.
"We're finally seeing the rings in three dimensions," said Carolyn Porco of the Space Science Institute in Boulder, Colorado. "This is the fist time in history that we've been there watching as this happens."
The object is most likely a small satellite, or moonlet, that is zipping around Saturn on an inclined orbit, said Porco, who leads Cassini's imaging team.
The moonlet appears to be about 0.6 mile (a kilometer) wide, but a comet-like haze of ice crystals and dust surrounding it may be obscuring its true size, Porco added.
Scientists think Saturn's moonlets are fragments of larger moons that were shattered by asteroids or comets eons ago.
It's possible that the object is a small comet or asteroid barreling through the ring, said Nicole Albers, a Saturn-ring scientist at the University of Colorado at Boulder. But such occurrences are rare.
"Since we suspect that there are moonlets in the F ring," Albers said, "it's really not unlikely that it is one."
Temporary Radiation Belt Discovered at Saturn
A new, temporary radiation belt has been detected at Saturn, located about 377,000 km from the center of the planet , near the orbit of the moon Dione. The temporary radiation belt was short-lived and formed three times in 2005. It was observed as sudden increases in the intensity of high energy charged particles in the inner part of Saturn's magnetosphere, in the vicinity of the moons Dione and Tethys, and likely was caused by a change in the intensities of cosmic rays at Saturn.
"These intensifications, which could create temporary satellite atmospheres around these moons," said Dr. Elias Roussos, "occurred three times in 2005 as a response to an equal number of solar storms that hit Saturn's magnetosphere and formed a new, temporary component to Saturn’s radiation belts.”
The discovery was made possible by Cassini's five-plus year mission, allowing scientists to observe and assess changes in Saturn's radiation belts. An international team of astronomers made the discovery analyzing data from the Magnetospheric Imaging instrument (MIMI) on Cassini MIMI’s LEMMS sensor, which measures the energy and angular distribution of charged particles in the magnetic bubble that surrounds Saturn.
The new belt, which has been named “the Dione belt”, was only detected by MIMI/LEMMS for a few weeks after each of its three appearances. The team believe that newly formed charged particles in the Dione belt were gradually absorbed by Dione itself and another nearby moon, named Tethys, which lies slightly closer to Saturn at an orbit of 295 000km.
Unlike the Van Allen belts around the Earth, Saturn’s radiation belts inside the orbit of Tethys are very stable, showing negligible response to solar storm occurrences and no variability over the five years that they have been monitored by Cassini.
Interestingly, it was found that the transient Dione belt was only detected outside the orbit of Tethys. It appeared to be clearly separated from the inner belts by a permanent radiation gap all along the orbit of Tethys.
“Our observations suggest that Tethys acts as a barrier against inward transport of energetic particles and is shielding the planet’s inner radiation belts from solar wind influences. That makes the inner, ionic radiation belts of Saturn the most isolated magnetospheric structure in our solar system“, said Dr Roussos.
The radiation belts within Tethys's orbit probably arise from the interaction of the planet’s main rings and atmosphere and galactic cosmic ray particles that, unlike the solar wind, have the very high energies needed to penetrate the innermost Saturnian magnetosphere. This means that the inner radiation belts will only vary if the cosmic ray intensities at the distance of Saturn change significantly.
However, Roussos emphasized that outside the orbit of Tethys, the variability of Saturn's radiation belt might be enhanced in the coming years as solar maximum approaches. "If solar storms occur frequently in the new solar cycle, the Dione belt might become a permanent, although highly variable, component of Saturn's magnetosphere, which could affect significantly Saturn's global magnetospheric dynamics,” he said.
The new findings were presented at the European Planetary Science Congress in Potsdam, Germany.
http://www.nancyatkinson.com/
Uranus
Uranus is much smaller than Jupiter and Saturn, but with a diameter of 51,118 km, it is still over four times that of Earths. It would take 15 of our planet to equal the mass of Uranus.
Though Uranus is the next planet out from Saturn, it is over twice Saturn's distance from the Sun. Being 28.69 x 108 km from the Sun, Uranus is a cold world with an average temperature of -221 C above cloud tops. And clouds are all we have ssen of this distant cold world. Even the Voyager spacecraft could not see beyond those clouds.
Uranus was not known to the ancient astronomers who knew only those planets out to Saturn. Uranus was too distant to be seen with the unaided eye. This planet had to wait until 1781 when the astronomer William Herschel discovered it while hunting for comets.
Neptune
Neptune is a big world, about 49,530 km in diameter, but despite being sometimes referred to as a lesser gas-giant, it’s not a colossus like Jupiter. It’s not even mostly gas. The sphere we see from outside is only the top of a deep, thick atmosphere. About 20% of Neptune’s apparent diameter is air, mostly hydrogen and helium, but with a fair tincture of methane and ammonia, and some more complex organics at trace levels. As one dives into Neptune the outer air starts off very cold. The planet is 30 AU from the Sun, after all. But inside Neptune the heat rises with depth… along with pressure. At a depth where the air is about as dense as what you’re breathing, the temperature is about -200° C, or ~70 Kelvins… but by the time things warm up to room temperature – around 300 Kelvins – you’d be crushed by over 50 atmospheres of pressure. One of those deep-diving suits from the old movies would be required to survive (along with oxygen tanks, of course).
Below that, things get weird. I said there’s no ocean on Neptune, but there is something like an ocean. As pressure climbs during your dive into Neptune, the atmosphere would gradually thicken into something very much like a liquid. True liquid water can’t exist anywhere inside Neptune, because there is too much hydrogen. The hydrogen molecules shove their way in amongst individual water molecules so much that water can’t fully condense into a true liquid. But at the crushing pressures of Neptune’s interior the thick, hot H2O, H2 and He gas compresses to water-like densities… except it’s not liquid water, and the temperature is over 2000° C. Those temperatures are hot enough to melt iron, and are far too hot for any organic compounds to survive. It is decidedly not an abode for life (as we know it, I must add with geeky circumspection).
But what if all that dense quasi-fluid were cooler? Neptune generates 2.6 times more energy in its core than it receives from the Sun, mostly from radioactive decay in its rocky heart. Really, “core” is an odd word to use, because this rocky kernel is more massive than four Earths. For all we know it might have an adorable little metallic iron core all its own, about the size of ours. In any event, heat from below combined with the thin solar light at 30 AU keep Neptune’s water from condensing as rain. But… if Neptune were colder at its cloud tops, and cooler within, its water could drop out to form a massive ocean realm.
What would such a realm be like? Half the planet’s apparent diameter is currently a hot, ionic quasi-liquid right now, but cooled that material would condense into an ocean thousands of kilometers deep. As a comparison, the Mariana Trench is the deepest part of our ocean, at 10 km. Neptune’s ocean would be hundreds of times deeper, and much stranger. If Neptune had a water ocean, its floor wouldn’t be rock, it would be solid ice. Heavy ice… ice VII, or X, or XI, or some other weird crystalline water-lattice that can survive at ten million atmospheres of pressure. Heat would rise from below, from the imprisoned mass of searing rock in Neptune’s center. Volcanic heat from the core mass would bubble up, melting the icy oceanic crust, and inseminating the deep ocean with precious minerals. Could life exist in such an alien environment?
Maybe. If Neptune could cool enough that it supports a liquid ocean, it might also give birth to life. Only when its massive, world-girdling ocean cools to become normal water, instead of a seething ionic mantle, will the conditions be even remotely similar to what conventional life requires. The big question, of course, is… can that happen?
The answer appears to be a qualified yes. Eventually, when its interior furnaces wind down a bit, and its atmosphere can cool enough, seas become possible on Neptune. The only problem… and it’s a fairly big problem, from our perspective… is that before that can happen the Sun has to go away. Specifically, the Sun must live out its long, long life and ultimately wither into a fading white dwarf. The post-solar white dwarf must, in turn, fade from blazing incandescence to a dim sputter. At that point, tens of billions of years from now, Neptune will cool enough to bear oceans. But not until.
There are two ways of looking at this news. From a purely selfish point of view, I’m quite cross that I’ll never be able to see what titans of the deep Neptune’s olympian waters might birth. But from a larger perspective I’m encouraged. Even when the Sun goes dark, it will be possible for life to spawn… or transplant itself and survive… in our solar system. The details will differ from what we’re familiar with today, on Earth, but water in the dim astronomical future will still require fins, and streamlining, to move through, and the animals of any Neptunian ocean of 100 billion years from now will still need to be sleek and smooth and tapered. I don’t have to see them for myself, because I can use chemistry, thermodynamics, geology, and physics to construct a probabilistic prognostication. Or two, or ten… There are many ways the future of Neptune may work out, but among them are highly realistic ways that lead to life, and complexity, and possibly thought… in a future so deep our own biosphere will be an impossibly ancient memory from a time older than the Big Bang is to us.
Yes, and a strange question that is. But actually not an unrealistic question. Neptune, now the farthest true planet from the Sun (after Pluto’s demotion), is namesake to the ancient god of the oceans. Neptune is shrouded in layers of icy, dense air laced with enough methane to color the world azure blue… hence the world’s maritime label. But despite its pelagic associations, Neptune has no oceans… though it might have had, with a slightly different composition. And it’s very possible that Neptune will one day be covered in a deep, wine-black sea of cool, liquid water.
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