APOD and General Astronomy Discussion Forum
https://asterisk.apod.com/
Mercury, by virtue of being the closest planet to the sun, has been notoriously difficult to study over the centuries. Telescopes have to contend with the sun's glare, while space probes — pulled along by the sun's gravity — must burn a lot of fuel to slow down for more than just a fleeting zoom past the small planet.
In fact, only two spacecraft have ever successfully visited Mercury: NASA's Mariner 10, back in the mid-70's, and now MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) which, after three flybys since 2008, finally settled into orbit around Mercury just this March. The Messenger mission should help answer many of the vexing Mercurial mysteries, including these following puzzlers.
Why so dense?
Mercury is the second-densest planet in the solar system, just a smidge less than Earth. Scientists think Mercury must have a giant core that makes up two-thirds of its mass; on Earth, the core comprises just one-third. Collisions between rocky bodies early in the solar system's history most likely knocked off some of Mercury's less-dense outer layers, leaving just the heavy stuff behind, said Sean Solomon, director of the department of terrestrial magnetism at the Carnegie Institution of Washington, and principal investigator for the Messenger mission. Chemical analyses by Messenger of Mercury's surface — which will be released soon — should put this impactor theory to the test.
Magnetic shield
Besides Earth, Mercury is the only other rocky inner solar system planet to have a significant magnetic field (though only about 1 percent the strength of Earth's). The existence of a magnetic field is not just a planetary trivia question — ours shields organisms from damaging radiation from the sun and outside the solar system. Solomon describes the Earth's magnetic field as "our umbrella against incoming radiation," and without such a field, it would be very difficult for life to develop or persist.
Researchers believe Mercury's magnetic field is generated by the same "dynamo" process as the Earth's, driven by the roiling of electrically conductive, liquid metal in the planet's outer core. Messenger will map the geometry of the field in detail, Solomon told Life's Little Mysteries, which should help scientists pin down its origin.
Ice, ice Mercury?
Sun-blasted Mercury is hardly the place one might think to look for ice. But some craters at Mercury's poles appear to be in a permanent shadow, and the mercury (pun intended) on these crater floors could plunge to minus 280 degrees Fahrenheit. These "deep-freeze traps," as Solomon called them, could hold much more ice than deposits found on the moon. While that's still not a lot of agua, it still goes to show that in the solar system "water is everywhere, at least as a molecule," Solomon said.
Persistent atmospheric wisps
Though it's the smallest planet and therefore has little gravity, Mercury somehow has an atmosphere, albeit a very tenuous one. Even stranger is the fact that Mercury is losing this atmosphere, the gases of which contribute to the comet-like tail that trails the planet. "Somehow on Mercury, the atmosphere has to be constantly regenerated," Solomon said. Scientists think captured material from the "solar wind" — the stream of particles radiating out from the sun — contributes, as well as dust kicked up by micrometeorite impacts.
Bonus Boggler: Bringer of doomsday?
Mercury already has the most eccentric (which in astronomical terms means oval-shaped) orbit of all of the planets in our solar system. Recent computer simulations showed that, over the course of a few billion years, this orbit could become even more eccentric and Mercury stands about a 1 percent chance of colliding with Venus or the sun. More disturbingly, in tandem with the outer giant planet's gravity, Mercury's chaotic orbit could disrupt the orbits of the inner planets such that Mercury, Venus or Mars smashes into the Earth — a cataclysm of truly doomsday-esque proportions.
Although the second planet from the sun is named after the Roman goddess of love, Venus is anything but lovely, at least from a hospitality perspective. For starters, its surface temperature pushes 900 degrees Fahrenheit, making Venus the hottest planet in the solar system.
It gets worse: A thick shroud of carbon dioxide presses down with 92 times the pressure of Earth's atmosphere on a bone-dry landscape. The opaque clouds that block our view of the world's surface are laced with sulfuric acid.
As you might imagine, studying Venus has proved difficult. But bit by bit, scientists are learning more about Earth's closest planetary neighbor. Here are some of the biggest mysteries regarding the brightest object in our sky after the sun and the moon.
Climate gone to ruin
Venus is sometimes referred to as Earth's "evil twin." In terms of size, composition and orbital location, hellish Venus is actually the planet that's most similar to our own (that we know of). Early in Venus' history, scientists think, the world was probably a lot like Earth, with oceans and a much cooler climate.
But over a few billion years, a runaway greenhouse effect seems to have taken over. Venus is about a third closer to the sun than Earth, and so it receives twice the amount of sunlight. This extra heat caused greater evaporation of initial surface water. In turn, the water vapor trapped more heat, further warming the planet, triggering more evaporation, and so on, until the oceans were gone.
"This is a mechanism that makes sense to get from an early earthlike Venus to the Venus we know today," said David Grinspoon, curator of astrobiology at the Denver Museum of Nature & Science and an interdisciplinary scientist on the Venus Express mission, a spacecraft that has been orbiting Venus since 2006.
Figuring out exactly when and how Venus became a furnace will help with modeling Earth's changing climate, as well as avoiding the possibility of sharing Venus' fate.
Super-rotating atmosphere
Venus turns on its axis much more slowly than Earth — a single Venusian day lasts 243 Earth days, which is longer than Venus' year, which takes 224 Earth days. Belying this gentle pirouette, the winds at Venus' cloud tops can reach 220 miles per hour (360 kilometers per hour), or about 60 times the pace of the planet's turning. (Winds are caused in part by planetary rotation.) Proportionally, if the same gusts blew on Earth, equatorial cloud winds would reach an astonishing 6,000 miles per hour (9,650 kilometers per hour).
The driver of Venus' atmospheric super-rotation must ultimately be energy from sunlight, Grinspoon said, but the full workings of the phenomenon remain unknown.
Spinning backwards
All of the planets in the solar system orbit the sun in a counterclockwise direction when viewed from the sun's north pole, and nearly all spin in this same direction on their axes. Not so on Venus, which has retrograde rotation (Uranus does this, too). On Venus, in other words, the sun rises in the west and sets in the east.
This clockwise spin is probably the result of a cosmic collision early in Venus' history. Many large bodies hurtled about the young solar system then, and such an impact to Earth is thought to have gouged out the material that formed the moon. Further understanding of the structure and composition of Venus with data from future lander probes should reveal what it was that sent the planet into its backward revolution.
Flash, boom?
It's still an open question if lightning indeed zaps from the Venusian clouds. Although the Venus Express spacecraft has "heard" the electromagnetic static that lightning characteristically produces on Earth, cameras have yet to capture a clear optical flash coinciding with these readings, Grinspoon said.
How this lightning might form is also mysterious. On Earth, a key role is played by ice crystals inside clouds, an ingredient that is in short supply in the hyper-arid atmosphere of Venus.
Bonus boggler: Alien life hot spot?
Although it's a long shot, Grinspoon said, there is a plausible argument for Venusian life — not on the planet's superheated surface, but in the clouds. Some 30 miles up, there should be a habitable niche where pressure and temperature are earthlike. For energy, floating creatures resembling bacteria could use ample sunshine or chemicals in the clouds. Of course, these beings would have to tolerate sulfuric acid, but so-called extremophiles on Earth have shown that life can thrive in even the harshest environments.
"It's worth exploring the clouds for many reasons," Grinspoon said, "and one of them is the possibility of some kind of exotic life."
Although it is the closest celestial body to us, the moon still harbors secrets aplenty. "Closest," of course, is a relative term: The great gray and white orb in our sky never veers much nearer than 225,000 miles (362,000 kilometers), and getting there is no easy feat, especially in the case of manned missions. No human has left boot prints in the lunar regolith since 1972.
Yet over that nearly four-decade absence, nations around the world have dispatched a number of probes and conducted gobs of research on our only natural satellite. Meanwhile, lunar rocks originally ferried back by the Apollo program decades ago are still offering up vital clues about the history of the moon.
Future missions by both robots and people should help solve some of the key puzzles, which are:
How did the moon get there?
Cultures worldwide have long offered up myths to explain the moon's existence. Nowadays scientists have other ideas of what really happened.
Many lines of evidence — including the moon's smallish core, its complement of certain elements, and computer simulations rewinding the Earth-moon orbital dance over eons — point to the moon being spawned in a giant impact. According to this theory, about 4.5 billion years ago a Mars-size body slammed into a young, molten Earth, and that collision gouged out the material that would coalesce into our lunar neighbor.
This picture has problems, however. The theoretical impactor, dubbed Theia, should have left residue with distinctive characteristics, but they have not been detected. And the amount of certain substances in the moon — too much water (frozen), for example — does not readily mesh with a hot, cataclysmic origin scenario.
"Blue" moon
Indeed, it is the presence of a great deal more water in and on the moon than expected that has really thrown researchers for a loop. "The more astronomers look, the more we find water in different places and depths," said Neil Comins, a professor of physics at the University of Maine.
Water ice has turned up in craters near the poles, particularly in a plume kicked up by the deliberate impact of NASA's LCROSS probe in 2009. Studies have suggested the interior of the moon is far wetter than ever supposed (though still hyper-arid compared with modern-day Earth). Recent re-examinations of the rock samples brought back to Earth by astronauts have even yielded signs of agua.
Icy comets likely delivered a substantial portion of this water when they smashed into the moon, but scientists are still scratching their heads. "It's really an open question about the origin and distribution of water on the moon," Comins said.
Why two-faced?
The moon is "tidally locked" to Earth, meaning only one hemisphere faces us. We know that side well, with its dark regions called maria, or "seas," of cooled magma.
Oddly, however, these maria are virtually absent from the back side of the moon, as has been revealed to us by probes (and seen in person by Apollo 8 astronauts). The proverbial "dark side of the moon" also is much more pockmarked by craters.
The starkly different hemispheres have been partly explained by the far side having a thicker crust — perhaps by 9 miles (15 km) or so — than the near side. "This made it easier for the crust on our side to have cracked" under the onslaught of meteorites, which released maria-forming magma from deeper in the moon, said Comins. But that crustal asymmetry is an enigma itself.
The extra cratering, meanwhile, could stem from greater exposure to space on the far side than on the Earth-shielded near side. A greater modeling of the moon's interior and a better understanding of the damage wrought by impacting bodies might help explain this strange two-facedness.
Bonus boggler: Key to our existence?
At a quarter of the diameter and more than 1 percent of Earth's mass, our moon is a hefty one: the fifth largest natural satellite in the solar system, and the biggest compared with its host body.
With the moon's considerable mass, its gravity stabilizes the "wobble" in Earth's axis, moderating our seasonal shifts. Plus, the moon causes marine tides on Earth that might have helped "stir the primordial soup," as Comins told Life's Little Mysteries — maybe getting the chemistry of life into gear more than 3 billion years ago.
In short, astronomers have wondered if Earthlike worlds need large moons like ours in order for life to develop.
An answer might be waiting for us next door, so to speak — on Mars. The Red Planet sports two measly, little moons, thought to be captured asteroids. Should Martian life ever be found — unlikely, but not out of the question — that would aid in squashing the moon-assisted habitability debate.
"When we go to Mars and determine if and how far life evolved there," Comins said, "that will help us better understand how life could have formed here without the moon."
Mars, the fourth planet from the Sun, gets its name from the Roman god of war on account of its blood-rust color. From an exploratory point of view, the moniker is fitting: Mars has fought off most of our scientific advances. Over half of the more than 40 spacecraft sent to study the Red Planet have failed; some have been lost in space and others have crashed onto the planet's surface.
Despite these setbacks, our curiosity regarding Mars has never abated. Several missions are in the works — one in fact involving a rover named Curiosity — that will help answer the major mysteries about this world, which include:
Abode of life?
You can't talk about Mars without raising the question of life. The Red Planet beckons because many scientists consider it the likeliest place in our solar system for extraterrestrial life to have once developed – or even still persist.
"What everyone wants to know is: has the planet ever harbored life?" said Steve Squyres, a professor of astronomy at Cornell University. Squyres is the principle investigator for the Mars Exploration Rover mission that put the rovers Spirit and Opportunity on the Red Planet in early 2004. (Spirit stopped working last year, but Opportunity is still chugging along.)
Today, and for most of its history, Mars has been a "cold, dry, desolate world," said Squyres. But multiple lines of evidence point to Mars as having been warm, wet and far more Earthlike about four billion years ago, early in its planethood.
To make life, you (most likely) need water. And guess what? Signs point to Mars once being absolutely drenched. Minerals on the surface, such as sulfates and clays, could have formed only in the presence of water. Many geologic features suggest great torrents of the stuff flowing overland. A huge amount of water still exists on Mars, but it is frozen away in the polar ice caps, as permafrost and in recently discovered, giant underground glaciers.
Hints of Martian microbial life have come in many guises: a contested Viking lander experiment in the 1970s; a famous "Martian meteorite," recovered in Antarctica with odd structures that some researchers construe as tiny fossils preserved before the rock was blasted off of Mars; and whiffs of methane in the thin atmosphere that could have a biological origin.
Warm and wet to cold and dry
The next biggest mystery concerning Mars is, "What happened?" as Squyres put it.
"Mars was a warm, wet interesting place for just 500 million to one billion years," Squyres said, "and then the fun was over."
Future Mars exploration missions will carry more sensitive equipment to help answer the interrelated questions about life and the stark change in conditions on the Red Planet.
NASA's Mars Science Laboratory and its car-sized, six-wheeled rover Curiosity will land next summer and begin analyzing rocks and sending pictures back to Earth for study. In addition, the European Space Agency has its first rover, ExoMars, gearing up for a 2018 launch. A sample return mission of Martian soil and rocks has long been considered but is not yet scheduled, Squyres said.
Bit by bit, scientists hope to piece together whether Mars ever had a thicker atmosphere, as well as how geologic activity and volcanism have influenced the world over the eons. After all, Mars is home to the Valles Marineris, one of the longest canyon systems known, and Olympus Mons, the biggest volcano in the solar system.
A tale of two hemispheres
Mars is very different from north to south: Smooth, younger lightly cratered lowlands predominate in the north, while ancient, heavily cratered highlands characterize the southern hemisphere. The northern hemisphere is also on average three miles (five kilometers) lower than the south. What gives?
The best bet for this so-called hemispheric dichotomy is a giant impact to the north from a Pluto-sized body four billion years ago or so. If the theory, which Squyres proposed in 1984, is correct, it would mean that the northern 40 percent of Mars is actually an impact crater. That would give the Red Planet another geological superlative — for having the largest impact crater in the solar system.
Bonus boggler: Funky, lumpy moons
Mars has two small, potato-shaped moons called Phobos and Deimos.
In many respects, including size, shape, color and apparent composition, the moons look to be wayward asteroids captured by Mars' gravity. But the fact that Phobos and Deimos have circular orbits above Mars' equator challenges this capture concept.
Two asteroids flying past would be unlikely to both have such a trajectory, and subsequent history after capture, to settle them into such an orbital arrangement. "How [Phobos and Deimos] really got there is hard to say," Squyres said.
Instead, the moons could have formed from material blasted out of Mars by an impact, just like our moon, and retained a lumpy, uneven shape because they lack the mass and corresponding gravity to become spherical.
At any rate, the speculation in the 1950s and 1960s that Phobos and Deimos might be artificial in nature has long been debunked, along with all the other wild sci-fi rumors about irrigation "canals," human faces carved in rocks and little green men with ray guns.
Beyond the orbit of Mars, but not as far as Jupiter, lurk the many hundreds of thousands of rocky bodies collectively known as the asteroid belt.
Many solar systems are thought to contain such belts, and science fiction movies and television shows often present these bands as rock-clogged expanses that would challenge any celestial navigator. It may be so, in other systems, but in our asteroid belt, the rocky bodies are actually quite far apart from each other.
Humankind will soon be getting an inside look at this often overlooked bit of celestial real estate, courtesy of NASA's Dawn mission. On Saturday, July 16, after a four-year journey, the Dawn spacecraft will reach Vesta, the second-largest body in the belt. [What's the Difference Between an Asteroid and a Comet?]
From there, Dawn will go on to orbit the belt's biggest object, Ceres, in 2015. Ceres accounts for nearly a third of the asteroid belt's mass, and is the largest "dwarf planet" in the solar system, outranking Pluto.
Dawn will be the first spacecraft to orbit one body, let alone two, in the asteroid belt. In so doing, Dawn will further characterize two distinct, major objects in the belt, shedding light on some of its greatest mysteries, which are:
Origin of the scattered stones
A major planet never formed where the asteroid belt lies, scientists think, because nearby of disturbances caused by Jupiter's gravitational tug. The giant planet's gravity accelerated the growing agglomerations of dust in the region of the belt, interfering with the slow, step-wise buildup to larger bodies, and booting some objects out entirely.
"The asteroid belt suffered from having this really bad neighbor next door," said Christopher Russell, a professor of geophysics and space physics at the University of California, Los Angeles, and principal investigator for the Dawn mission.
Learning more about the locations of asteroid belts in other solar systems will help confirm the theory that our belt's sparse rocks are a result of the gravitational meddling of giant planets.
Dry to wet
Although Vesta and Ceres are relatively close to each other (Vesta's orbit is about 2.4 times the Earth-Sun distance and Ceres' is 2.8 times that distance), the two objects are strikingly different. Essentially, Vesta is "dry" while Ceres is "wet."
"Vesta is very much like the Moon and Earth," Russell said. "It's a rocky body with an iron core." Ceres, for its part, "is more like rock and water," he told Life's Little Mysteries.
Scientists' best guess as to the reason behind these contrasting compositions has to do with when the bodies formed. Both Vesta and Ceres are in the ballpark of 4.6 billion years old, coming together when the rest of the solar system's major bodies took shape. "But exactly when they were made back then, if differing by a few million years, is important," said Russell.
Our solar system emerged from the collapse of a massive cloud of gas and dust. An explosion of a nearby star in a supernova seeded this cloud with heavy elements, including short-lived radioactive ones such as aluminum-26.
Those bodies that accreted first contained more short-lived elements, which then decayed and heated the surrounding matter. "The body gets to the boiling point, so then water starts to boil off and that starts to dry the material," explained Russell.
The thinking is that Vesta formed just a few million years before Ceres, and as such became hot, molten and dried out. Ceres, instead, chilled out.
Not much Vesta there, but plenty here
If Vesta did indeed form before Ceres, that might also explain the mystery of why there are so few "V-type," or Vesta-like asteroids observed in the belt. Most of those known appear to have come from Vesta itself, having been blasted out by a collision long ago.
That blast apparently sent some Vesta fragments Earth's way, too. About one out of 20 meteorites — space rocks that survive passage through Earth's atmosphere all the way to the ground — appear to have come from Vesta, Russell said.
More puzzingly, none of the meteorites that have ever been recovered appear to have originated from Ceres. Russell said this is probably because the icy chunks that have been knocked off of Ceres sublimate — that is, turn to gas — when subjected to sunlight or the heat of entry into Earth's atmosphere, and so they never reach terra firma.
The Dawn probe will study Ceres' surface to gauge this hypothesis. Alternatively, Jupiter's gravity might again play a role, pumping much more of Vesta's shrapnel our way compared to Ceres'.
Bonus boggler: Bringers of life and death?
While planning the Dawn mission, some scientists voiced concerns about sending the probe to Ceres. "They said Ceres is an object of astrobiology interest," Russell said. "If it's got water and a good temperature out there under its surface, we don’t want [the Dawn mission] contaminating it."
Russell said his team will certainly aim to prevent Dawn from accidentally crashing into Ceres. A future mission could someday assess the dwarf planet's habitability.
That Ceres or other objects in the asteroid belt might harbor life, or its ingredients, speaks to the "panspermia" theory of life's origins here on Earth. The panspermia theory suggest that life did not begin here, but rather that biological entities developed elsewhere, and then a meteorite delivered them to Earth. Perhaps that rock chunk chipped off Ceres, or another icy asteroid, and somehow made it to Earth.
Overall, asteroids certainly appear to have had quite an impact, literally and figuratively, on life on Earth. A minimum six-mile-wide asteroid helped doom the dinosaurs when it crashed here 65 million years ago.
Yet bombardment from icy asteroids early in Earth's history possibly brought huge amounts of water and carbon-containing compounds to the planet, both of which are critical for creating and supporting life.
"You're looking at two scenarios, where alternatively life was negatively affected by asteroids and other times positively affected by asteroids," Russell said. "Asteroids are neither bad nor good."
The planet Jupiter rightly gets its name from the king of the gods in Roman mythology: As the biggest planet, Jupiter lords over the rest of our solar system. The bloated world is so big, in fact, that if all the other objects in the solar system (excluding the Sun) were mashed together, they would all fit inside the sphere of Jupiter.
Matching Jupiter's gargantuan size are its many huge scientific mysteries. In early August, NASA will launch the next major Jupiter mission, called Juno, a spacecraft that will rendezvous with Jupiter in 2016 and help shed light on the planet's greatest mysteries, which include:
Cloud bands and storms
Jupiter looks like a carefully dyed Easter egg. Lighter-hued bands, called zones, and darker ones, called belts, gird the massive world. Just how deep these features go, however, is entirely uncertain.
"We don't know if the beautiful zones and belts are just a surface feature, and on the inside Jupiter's rotating like a solid body," said Scott Bolton, principal investigator for the Juno mission and director of the space science and engineering division at the Southwest Research Institute in San Antonio, Texas. Alternatively, Jupiter could be "a series of concentric cylinders and you're seeing the tops of those pop out as zones and belts," Bolton told Life's Little Mysteries.
Whole stripes have been known to disappear without a trace; one vanished in May 2010 that was twice as wide as Earth. Why bands stay separate and come and go is not well explained, nor even how the zones and belts get their distinctive colors.
Great vortices swirl in Jupiter's atmosphere, but these too are not well understood. The Great Red Spot is the most-recognized of these tempests, having been observed for more than 300 years. "We don't know what's powering that thing," Bolton said.
Juno will gather temperature measurements from all over the gas giant to help inform our patchy atmospheric models.
Where's the water?
Along with Saturn, and the vast majority of exoplanets detected thus far, Jupiter is classified as a gas giant. That's mostly what the world is: A great ball of hydrogen and helium gas, the two most common elements in stars and the universe. Most of the leftovers from the sun and the solar system's formation ended up in Jupiter.
Yet the amounts of heavier elements, such as carbon, nitrogen and sulfur, also floating amongst Jupiter's clouds strangely exceed those found in the Sun. Scientists think that water in Jupiter's atmosphere might have helped enrich the planet with these elements. As water freezes, it captures stray materials, and Jupiter could have gobbled up lots of these element-enriched chunks.
But the trouble is, water has not been found in the expected concentrations, Bolton said. Juno will look for water's signature to help explain why Jupiter has its distinctive composition.
Learning that, in turn, will speak to how the solar system's smaller bits came to be.
"Juno's primary goal is to understand the fundamentals of how the solar system formed and how the planets were made," Bolton said. "We're trying to figure out the recipe for making planets, and we're back at the ingredients list.
A "core" issue
Researchers think Jupiter might have a core of sorts, perhaps made of superheated rock under high pressure, but the jury is decidedly out.
"The models don't constrain [the core's mass] at all," said Bolton. "It could be zero, it could be twenty earth masses, and that's because of a lack of data."
Juno will help fill in the blanks by taking gravity field measurements, which speak to the distribution of mass deep inside Jupiter.
Bonus boggler: Heckuva light show
Jupiter possesses the strongest magnetic field in the solar system, excepting the sun's. Researchers think the field is generated by a highly compressed layer of hydrogen, which develops liquid metallic properties, deep in Jupiter.
The structure that this magnetic field forms — called a magnetosphere — as the "solar wind" of charged particles from the sun stream past is truly titanic.
"Jupiter's magnetosphere is arguably the largest structure in the entire solar system," said Bolton, "other than the heliosphere" — the solar wind-blown bubble surrounding the Sun. "[Jupiter's magnetosphere] gets dragged out windsock-fashion all the way out to Saturn's orbit."
Auroras — like our Northern and Southern Lights, though immensely more powerful and with notably different characteristics — glow at Jupiter's poles. Juno, which will circle the planet in a polar orbit, will have a great view, helping tease out the mechanisms that drive Jupiter's magnetic maelstroms.
The biggest planet in the solar system, Jupiter, also boasts the most moons, with 64 currently cataloged. Most of these moons are tiny, lumpy rocks — apparently asteroids captured by Jupiter's gravity — and they swarm about the giant planet like so many bees around a hive.
Four of Jupiter's moons, however, are quite substantial — so much so that they can be seen through a rudimentary telescope. The inventor of just that instrument, Italian astronomer Galileo Galilei, first saw the thusly named "Galilean moons" in 1610: Io, Europa, Ganymede and Callisto.
Together, these four moons comprise more than 99.9 percent of the mass of Jupiter's satellites. Each of them has a distinctive character, and they all present vexing scientific puzzles. Here is a rundown of the top mysteries regarding Jupiter's primary four moons.
Io, the hyperactive pizza moon
Io is the closest of the Galilean moons to Jupiter. This proximity is thought to help explain the moon's uniquely hellish, sulfur-yellow, red-splotched and pockmarked appearance.
Those pocks, in fact, are volcanoes. Io sports 400 or so active volcanoes, as well as soaring mountains formed by tectonics. Overall, the moon is the most geologically active object in our solar system.
The energy powering this activity comes largely from a gravitational tug-of-war between Jupiter and the other three Galilean moons with Io caught in the middle. The constant stretching and compressing that this tug exerts on Io heats its interior, prompting the moon to often ooze out lava and spew sulfur and ash into space.
Such tidal forces, however, might not account for all this oomph. The history of variances in the gravitational flexing of Io also remains murky.
"I don’t think we know enough about the exact frequency of these things to adequately assess the whole mechanism," said Scott Bolton, principal investigator for NASA's Juno spacecraft mission, which launches this year to study Jupiter.
Given how interesting the moon is, "Io could be the focus of an entire mission," added Bolton, who, in addition to his Juno post, is also director of the space science and engineering division at the Southwest Research Institute in San Antonio, Texas.
Europa, a smart bet for extraterrestrial life?
The moon of Jupiter that's definitely highest on the list for someday getting its own dedicated mission is Europa. This icy-white object with brownish streaks on its surface stands as one of the best candidates for hosting extraterrestrial life in our solar system.
Under an icy crust anywhere from a couple to perhaps 20 miles (three to 32 kilometers) thick, Europa probably harbors a saltwater ocean. Depending on the assumptions and models used, this ocean could have twice the volume of all those on Earth.
Understandably, astronomers are bubbling over with questions about this subterranean (sub-Europian?) sea. The chief query: "Might it allow for life development in any way?" asked Bolton.
The idea is not so far-fetched. Tidal flexing from Jupiter could keep the interior of Europa warm. This energy could, in turn, support microbial life analogous to that found around hydrothermal vents in Earth's oceans. Cosmic rays from space striking the crustal ice could even free up oxygen to power bigger life forms, such as fish.
Ganymede, big and oddly magnetic
Jupiter's largest moon, Ganymede, reigns as the biggest moon in the solar system. In face, it's even larger than the planet Mercury.
Another distinction for Ganymede: it's the only moon with its own magnetosphere, which is a region surrounding the world where charged particles from the sun are deflected by a magnetic field.
"How that [magnetosphere] gets created is very fascinating," said Bolton. "We don’t know of another small body that has that."
Ganymede's magnetosphere is most likely made in a manner much like Earth's, due to convection in the moon's liquid iron core. Learning how it's generated would help with better understanding our own planet's magnetic field.
To boot, Ganymede might also a hidden ocean sloshing under its gray, rocky and icy crust.
Battered Callisto
The Galilean moon with the farthest orbit from Jupiter is Callisto. Unlike Io and Europa (and even Ganymede to an extent), where geologic activity has erased many craters, Callisto bears the scars of eons' worth of meteorite impacts. The geologically dead moon is considered the most heavily cratered object in the solar system.
Callisto's landscape is therefore among the oldest on record, aged some four billion years. Analyzing its surface materials would be like opening a time warp back to the early solar system.
Callisto might be full of surprises on the inside, too — an underground ocean could lurk here as well, yet another possible abode for alien life out in Jupiter's neighborhood.
Bonus boggler: Ringed remnants of a destroyed moon
Since its discovery in 2000, a tiny moon just 2.5 miles (four kilometers) in diameter and given the designation S/2000 J 11 has gone missing. Astronomers think the moonlet has actually smashed into Himalia, Jupiter's fifth most massive moon after the four Galileans.
That possible impact appears to have created a streak of material, observed in 2006, that might even be a whole new ring around Jupiter. The planet's faint rings naturally do not get the fanfare of Saturn's resplendent rings, but as with Saturn, moons play a key role in supplying the particles that make up the giant disks.
After Earth and perhaps Mars, Saturn might be the most-recognizable world in our solar system, courtesy of its unique and resplendent ring system.
These rings, however, are but the tip of the iceberg when it comes to the strangeness and wonder of this planet. Since 2004, NASA's Cassini spacecraft has observed the Saturn, its rings and its moons in great detail. The mission is helping to solve some of the major scientific mysteries about Saturn, which are:
Where do you get those rings?
Although the other three gas giants in our solar system — Jupiter, Uranus and Neptune — also have rings, none are as dense, thick and just plain eye-popping as Saturn's.
These bands of mostly icy particles start about 4,000 miles (6,437 kilometers) above Saturn's equator and extend out some 75,000 miles (120,700 kilometers) into space. Numerous gaps in the rings exist, carved out by tiny moons or from gravitational interactions with more distant orbiting bodies. [Why Does Saturn Have Rings Around It?]
Interestingly, the rings could be young, relatively speaking, at just a few hundred million years old. Or not: they might date back to Saturn's birth more than four billion years ago.
"We don't know how the rings formed or how long they last," said Carolyn Porco, leader of the Cassini Imaging Science team and the director of the Cassini Imaging Central Laboratory for Operations (CICLOPS) at the Space Science Institute in Boulder, Colorado.
Scientists believe that the material for the rings has one of two origins: The destruction of a moon, ripped apart by Saturn's gravity or blasted apart by an impacting comet, or the ancient leftovers from Saturn's formation.
Understanding the processes that create the rings' structure will shed light on disk formation all over the universe. "Here's a place where we have the nearest analog to us in our solar system to all the disk systems in the cosmos," said Porco. "That includes other solar systems that we know are forming and also reaching a trillion times in size to the pinwheels of stars of gas and dust we call spiral galaxies."
Storms a' ragin'
Compared to the other gas giants, and especially Jupiter, Saturn's nearest neighbor in size, the ringed world has a subdued complexion of beige cloud bands. Usually, that is; last December, a huge white storm erupted in the planet's northern hemisphere.
Astronomers have observed the biggest of these storms flare up every 30 years or so going back more than 180 years, Porco said. A Saturn year is equal to about 30 Earth years, suggesting some sort of seasonal connection with the storms. But if that were simply the case, the big storm raging now should not have been showed up until sometime in 2020. [Will We Really Find Alien Life in 20 Years?]
The ultimate origin and power source for these massive storms are unknown. "These storms have a lot of energy in them," Porco said. "They could say something fundamental about the difference between the way giant planet atmospheres work and, say, the atmospheres of Earth or Venus."
Puzzling polar hexagon
In the early 1980s, the passing Voyager spacecraft spotted a surprising six-sided cloud pattern above Saturn's north pole. Cassini has followed up in recent years with close-ups of this strangely shaped weather phenomenon, which could hold four Earths within its boundaries.
Researchers have simulated hexagonal and other polygonal shapes by whirling liquid inside a tank at varying speeds, suggesting Saturn's "hexagon might be an oddity of fluid mechanics on a rotating body," said Porco.
Nevertheless, the remarkable longevity and stability of this jet stream will have scientists scratching their heads for years to come.
Bonus boggler: Nailing down the day length
Gauging the length of the day on Saturn — or any other gas giant — is tricky. Unlike a world with solid ground and landmarks, the cloud patterns on a gas giant do not necessarily represent the internal spinning of the interior and the core.
To compensate, scientists record a planet's rhythm of naturally generated radio emissions. Such a technique worked well for Jupiter, and seemed to for Saturn based on Voyager data. Yet Cassini's measurements in 2004 indicated a day mysteriously longer by about six minutes.
Later work showed that Saturn's magnetic field, which produces the radio signals, does not stay in sync with the rotation of the planet. (For Jupiter, the fact that the two phenomena are clearly not hitched makes for easier day-length calculations.)
Overall, scientists have had to average data from the Pioneer, Voyager and Cassini probes to best-guesstimate: Saturn's days last 10 hours, 32 minutes, and 35 seconds (give or take 13 seconds).
That's pretty precise. Over a Saturn year, however, that margin of error can add up to about four Saturn days' worth of extra or subtracted calendar space, which could definitely mess up a meticulous-kept day planner.
The space around Saturn is a busy place. In addition to the planet's giant rings, more than 60 known moons zip about the world. Some moons actually orbit within the rings, which themselves are composed of countless grainy bits and boulder-sized "moonlets."
Overall, Saturn's brood of satellites is an intriguing, diverse bunch. Yet a few of these objects stand out from the pack. NASA's Cassini spacecraft, in orbit around Saturn since 2004, buzzes these moons from time to time, and is helping solve some of their key mysteries, including:
A hot spot for alien life?
Despite being among the smallest spherical bodies in the solar system, Enceladus knows how to stir up a ruckus. In 2008, Cassini confirmed that geysers spew from weirdly warm cracks near the moon's south pole, creating streaks dubbed "tiger stripes." These watery jets suggest a subsurface reservoir of salty liquid. Cassini detected organic (carbon-containing) compounds, the building blocks of life, in the jets' plumes.
"Enceladus has the trifecta – water, organic materials and excess energy," said Carolyn Porco, leader of the Cassini Imaging Science team and the director of the Cassini Imaging Central Laboratory for Operations (CICLOPS) at the Space Science Institute in Boulder, Colorado. "It's the go-to place for astrobiology."
A future lander mission could take a closer look for life without having to drill through miles and miles of surface material, as it would on Jupiter's moon Europa. From an exploratory viewpoint, "we think [Enceladus] is the most accessible habitable zone we have in our solar system," Porco told Life's Little Mysteries.
Scientists do not yet know exactly what powers Enceladus' geysers. But they do have a good idea where the sprays end up – the spaceborne particles form Saturn's diffuse and distant E ring.
Unlocking Titan's secrets
Saturn's most famous moon is Titan, the second biggest moon in the solar system after Jupiter's Ganymede. At the outset, Titan has a couple key distinctions. First, it's the only moon that possesses a thick atmosphere, and second, it's the only place beyond Earth to have stable bodies of liquid on its surface. [The 'Goldilocks Zone' Explained]
Rather than water, ethane and methane – which exists as a gas on Earth, but is liquid in Titan's -290 Fahrenheit (-179 degrees Celsius) environment – fill Titan's lakes and seas. Methane rains on Titan much like water does on our planet, eroding the moon's mountains and carving river-like channels. Volcanoes have also blasted the icy liquid forth on Titan, and might still.
A stew of organic chemicals exists in Titan's atmosphere, on the ground and in the lakes as well. Astrobiologists have suggested that alien life in the marine areas could feed on these compounds. [What Are the Ingredients of Life?]
In many ways, Titan resembles a deep-frozen version of an early Earth before life took hold. Scientists hope to unravel how Titan's climate and geology work, and in turn learn how our own planet operates.
"We have been able to see details on the surface of Titan that have shown that in its geological complexity and geographical diversity the surface environment is rivaled only by Earth itself," Porco said.
Yin-yang "walnut" moon
Another oddity orbiting Saturn: Iapetus. The moon has differently colored hemispheres – one light, the other dark – and a mountain chain some six miles (10 kilometers) high rises from much of Iapetus' equator.
The darkened hemisphere, scientists think, is probably a result of a couple of things. First, a farther-out moon called Phoebe shed some of its own dust. "Iapetus plows through this material, which darkens its leading side," said Porco. Then, once this hemisphere became slightly darkened, it soaked up more heat from the Sun. In turn, ice there evaporated, warming that side further still and making it even darker. Over time, this runaway thermal effect led to a two-toned moon.
As for the ridge, the best bet is that it’s a remnant of when Iapetus spun faster in its early days, then rapidly cooled, and in doing so locked in the equatorial bulge. But other researchers have argued that a small moon once orbited around Iapetus, broke up and formed a ring, and then collapsed onto the surface, could explain the weird midline bump.
Bonus boggler: A moon with rings?
Such a moon-ring might have precedent at Saturn's moon Rhea.
The detection of dust, plus a drop in the number of electrons on either side of the moon during a 2008 Cassini flyby, strongly suggested the presence of three thin rings. A line of deposited material near Rhea's equator also pointed to de-orbiting debris from rings.
Yet subsequent observations by Cassini have not turned up any visual signs of a ring system, Porco said. "The original data might mean there are little bits of material in orbit around Rhea, but not rings," said Porco. "We don’t know the answer."
Bonus bonus: Golf Saturn's moons!
To learn more about Saturn's moons, Porco's group created an online gold video game called Golf Sector 6. Check out the game via the link.
Nerd bonus: The "Death Star" moon
Fans of "Star Wars" might start mouthing Obi-Wan Kenobi's line "That's no moon. It's a space station" when they get an eyeful of Saturn's moon Mimas. With its slate-gray color and a giant crater that resembles the dimple where the Death Star fires its superlaser, Mimas certainly looks like the planet-whacker.
More than a billion and a half miles away from Earth looms a huge, cyan-colored world pegged with a perilous name: Uranus. (For the record, modern astronomers tend to pronounce the planet's name as "YUR-inn-us" rather than the giggle-inducing alternative.)
Along with Neptune, Uranus is considered an "ice giant," a class of planets distinct from the much-larger gas giants Jupiter and Saturn. Although hydrogen and helium gas make up much of Uranus, significant quantities of water, methane and ammonia "ices" give the planet a different color and chemistry. Size-wise, Uranus' radius is four times that of Earth's, and about 16 Earths could fit inside the ice giant's sphere.
Humankind hasn't had a close look at Uranus since the Voyager 2 probe scoped it out back in 1986, and for now, a return mission is not in the offing. Until we get back out there, some major mysteries will continue to vex, including:
Why the sideways spin?
In terms of their rotation, the planets and the Sun can be thought of as spinning tops placed on a table – they all whirl about on an axis more or less in the same plane.
Except for Uranus. It has an axial tilt of about 98 degrees, meaning its "north" and "south" poles are instead found where Earth's equator runs. The planet looks, quite simply, as though it has been knocked over onto its side. [Amazing Views of Uranus Thrill Skywatchers]
What could have done this? Barring any likelier alternatives, scientists wager that an Earth-size body collided with Uranus early in the solar system's history and toppled the world.
"An impact is the only mechanism we can think of to do that," said Mark Hofstadter, a senior scientist at NASA's Jet Propulsion Laboratory in Calif.
The fact that Uranus' 13 rings and couple dozen-plus moons are upended as well, encircling the planet like circles in a bullseye from our perspective, lends credence to this theory. "Perhaps before the satellites formed or finished forming, everything got tilted over," said Hofstadter.
Learning more about Uranus' interior, which unlike other planets does not fit any simple models, and comparing it to its sister world Neptune would help. "There might be some compositional evidence or just interior structure evidence that tells us that, 'Okay, this thing suffered a giant impact,'" Hofstadter told Life's Little Mysteries.
Uranus keeps its cool
Puzzlingly, Uranus radiates little or no heat into space, another thing that makes it unique among our solar system's planets. Planets are expected to have heat leftover inside them from their formation process; Earth's interior, for example, remains hotly molten. [How Hot Is Hell?]
That same planetary punch that sent Uranus sideways could also explain its apparent lack of internal heat. If something giant hit Uranus, that impact could have stirred up its interior, Hofstader said. "That helped bring hot material that was deep down near to the surface, and so helped Uranus cool more rapidly."
A second idea is that normal heat flow from a warm interior to a cooler surface, called convection, is not working correctly. "We hope if we learn more about Uranus' interior structure we'll see a region where convection is inhibited," said Hofstadter. "Or, if we can tell the interior is really hot, we'll know that energy is trapped in there and not making it out."
Where was Uranus born?
Recent models of how the solar system's outer planets formed and have since evolved suggest that Saturn and the two ice giants were once scrunched in much closer to Jupiter. [What If Solar System Formed Closer to the Milky Way's Edge?]
Not long after the solar system formed, the cumulative gravitational interactions of small planetesimals whizzing around began moving Saturn, Uranus and Neptune farther away – dramatically so in the ice giants' case. "They might have doubled or tripled their distance from the Sun," said Hofstadter.
In turn, this shift in the solar system's mass cleared out most of the remaining debris from the solar system's genesis. A good many icy bodies were probably hurled in toward Earth and the inner planets during this "Late Heavy Bombardment," starting 4.1 billion years ago. Water and organic material was deposited on our planet, perhaps critical for setting the stage for the development of life.
Better computer simulations with more data should help nail down this "Nice model," named after the city in France. Uncovering Uranus' history and how it has influenced our planet speaks to the possibility of life in other solar systems: According to early data from NASA's planet-hunting Kepler spacecraft, ice giants might be the most common kind of planet in the galaxy, Hofstadter said. [Uranus, Seventh Planet in Earth's Solar System, Was First Discovered Planet]
Bonus boggler: Miranda – a cliff diver's dream
Compared to the variety of moons circling Jupiter and Saturn, Uranus' 27-strong complement of satellites is less exotic. But one moon called Miranda stands out for possessing one of the gnarliest surfaces of any known astronomical body. This small moon has deep canyons, scrapes, terraced layers and a cliff some 12.4 miles (20 kilometers) deep – the deepest known in the solar system.
One theory behind Miranda's geological mess suggests that flowing ices in the moon's interior, perhaps heated by gravitational squeezing from Uranus and other moons, pushed through onto the surface. Another holds that the moon was shattered several times and came back together, creating its jagged and mottled features.
Although the former theory is more en vogue currently, "I think that both have to be on the table at this point," Hofstadter said.
Back in 1846, when European astronomers quarreled over what to call a newly found eighth planet, they eventually settled on Neptune, after the Roman god of the sea. The name turned out to be spot-on, for Neptune, as we know in far better detail now, is colored a deep oceanic blue, with white flecks and deeper blues playing across its clouds.
Along with Uranus, astronomers classify Neptune as an "ice giant" – a large world, four times Earth's diameter, with a thick atmosphere of mostly hydrogen and helium along with some water, ammonia and other substances.
If Uranus seems far away at 1.76 billion miles from the sun, Neptune is another billion miles away – some 30 times as far away from the Sun as the Earth. Studying Neptune is, as one might imagine, very difficult. [How Far Is It to the Edge of the Solar System?]
"Neptune is at the edge of our ability to detect with ground-based telescopes, and also [the] Hubble [Space Telescope]," said Heidi Hammel, executive vice president of the Association of Universities for Research in Astronomy (AURA), a non-profit organization based in Washington, D.C.
The only up-close look we have ever had of Neptune came back in 1989, courtesy of a Voyager 2 flyby. The spacecraft's investigation brought to light many enduring mysteries, which include:
A hyperactive atmosphere
Astronomers had expected Neptune to look rather boring – a weatherless, featureless world in deep freeze. Instead, Voyager revealed a turbulent atmosphere with lighter cloud ripples and raging storms, including one dubbed the Great Dark Spot. Surprisingly, the fastest winds ever recorded in the solar system whirl on Neptune, up around 1,300 miles (about 2,100 kilometers) per hour.
Driving this meteorological activity appears to be Neptune's internal heat, which measures possibly hotter than Uranus. "As you go further from the sun, Jupiter, Saturn and Uranus are each colder in their upper atmosphere," Hammel said. "But when you get to Neptune, it's just as warm as Uranus." (Relatively speaking, of course – both planets chill in the range of -355 degrees Fahrenheit (-215 degrees Celsius).) [How Hot Is Hell?]
Typical planetary heat sources, including leftover internal heat from formation and the decay of radioactive elements, could possibly account for Neptune's temperature. Maybe Neptune is normal and Uranus is the weirdo. "It could be that Uranus is unusually cold," Hammel said.
Clumpy rings
Neptune, like its giant planet brethren of Jupiter, Saturn and Uranus, has a ring system. But rather than distinct hula hoop-like structures, Neptune's rings are perplexing chunky, with gobs of material forming arcs in the outer ring. "These clumps are places where a whole lot of ring particles are stuck together," said Hammel.
Gravitational influences from small moons might cause the regular gumming-up of the rings. But observations by Hammel and colleagues in recent years show that this mechanism appears too tidy. "The locations of the arcs relative to one another have changed in ways we really don’t understand," Hammel told Life's Little Mysteries.
Off-kilter magnetic field
When Voyager 2 detected an odd magnetic field at Uranus, scientists figured whatever collision had knocked that planet on its side had similarly scrambled its magnetic field production. Yet when Voyager 2 measured Neptune's field, it also originated from a region away from the world's heart, and nor did it align with planetary rotation as other described magnetic fields do.
"No one expected these magnetic fields offset from the center of the planet and tilted at these crazy angles," said Hammel.
The best theory, Hammel said, is that the magnetic field is generated not in the core of Neptune as it is in the Earth, Jupiter and other planets. Rather, the field emanates from an electrically conductive layer between the core and surface – a "mantle of briny water," Hammel speculated, under extreme pressure and unlike any water here on Earth.
Bonus boggler: A captured, feisty moon?
Of Neptune's 13 moons, Triton is by far the biggest and the only one massive enough to be spheroidal. Weirdly, Triton has a "retrograde" orbit, revolving in the opposite direction of the planet and other moons. Plus, the orbit is at an angle rather than in the plane around the equator like typical satellites.
These traits suggest Triton did not form around Neptune. Instead, the planet's gravity probably captured wayward Triton, a passing icy and rocky body from the Kuiper Belt, a band of bodies including Pluto beyond Neptune's realm. "The leading theory is this capture hypothesis," said Candice Hansen, a senior scientist at the Planetary Science Institute in Tucson, Ariz.
When Voyager 2 zoomed past Neptune, Hansen was on hand to see the first images, including those of Triton, which turned out to have geyser-like eruptions on its surface. "We were astounded to see those active plumes," Hansen told Life's Little Mysteries.
What powers those plumes isn’t Triton's only mystery. Its young surface is not as thoroughly hammered by craters as one would expect, pointing to geological activity that erased early craters. Triton also has intriguing and unique terrain textured like a cantaloupe.
