Universe Today

When the European Space Agency’s Huygens probe visited Saturn’s moon Titan last month, the probe parachuted through humid clouds. It photographed river channels and beaches and things that look like islands. Finally, descending through swirling fog, Huygens landed in mud.

To make a long story short, Titan is wet.

Christian Huygens wouldn’t have been a bit surprised. In 1698, three hundred years before the Huygens probe left Earth, the Dutch astronomer wrote these words:

“Since ’tis certain that Earth and Jupiter have their Water and Clouds, there is no reason why the other Planets should be without them. I can’t say that they are exactly of the same nature with our Water; but that they should be liquid their use requires, as their beauty does that they be clear. This Water of ours, in Jupiter or Saturn, would be frozen up instantly by reason of the vast distance of the Sun. Every Planet therefore must have its own Waters of such a temper not liable to Frost.”

Huygens discovered Titan in 1655, which is why the probe is named after him. In those days, Titan was just a pinprick of light in a telescope. Huygens could not see Titan’s clouds, pregnant with rain, or Titan’s hillsides, sculpted by rushing liquids, but he had a fine imagination.

Titan’s “water” is liquid methane, CH4, better known on Earth as natural gas. Regular Earth-water, H2O, would be frozen solid on Titan where the surface temperature is 290o F below zero. Methane, on the other hand, is a flowing liquid, of “a temper not liable to Frost.”

Jonathan Lunine, a professor at the University of Arizona, is a member of the Huygens mission science team. He and his colleagues believe that Huygens landed in the Titan-equivalent of Arizona, a mostly-dry area with brief but intense wet seasons.

“The river channels near the Huygens probe look empty now,” says Lunine, but liquids have been there recently, he believes. Little rocks strewn around the landing site are compelling: they’re smooth and round like river rocks on Earth, and “they sit in little depressions dug, apparently, by rushing fluids.”

The source of all this wetness might be rain. Titan’s atmosphere is “humid,” meaning rich in methane. No one knows how often it rains, “but when it does,” says Lunine, “the amount of vapor in the atmosphere is many times that in Earth’s atmosphere, so you could get very intense showers.”

And maybe rainbows, too. “The ingredients you need for a rainbow are sunlight and raindrops. Titan has both,” says atmospheric optics expert Les Cowley.

On Earth, rainbows form when sunlight bounces in and out of transparent water droplets. Each droplet acts like a prism, spreading light into the familiar spectrum of colors. On Titan, rainbows would form when sunlight bounces in and out of methane droplets, which, like water droplets, are transparent.

“Their beauty [requires] that they be clear….”

“A methane rainbow would be larger than a water rainbow,” notes Cowley, “with a primary radius of at least 49o for methane vs 42.5o for water. This is because the index of refraction of liquid methane (1.29) differs from that of water (1.33).” The order of colors, however, would be the same: blue on the inside and red on the outside, with an overall hint of orange caused by Titan’s orange sky.

One problem: Rainbows need direct sunlight, but Titan’s skies are very hazy. “Visible rainbows on Titan might be rare,” says Cowley. On the other hand, infrared rainbows might be common.

Atmospheric scientist Bob West of NASA’s Jet Propulsion Laboratory explains: “Titan’s atmosphere is mostly clear at infrared wavelengths. That’s why the Cassini spacecraft uses an infrared camera to photograph Titan.” Infrared sunbeams would have little trouble penetrating the murky air and making rainbows. The best way to see them: infrared “night vision” goggles.

All this talk of rain and rainbows and mud makes liquid methane sound a lot like ordinary water. It’s not. Consider the following:

The density of liquid methane is only about half the density of water. This is something, say, a boat builder on Titan would need to take into account. Boats float when they’re less dense than the liquid beneath them. A Titan-boat would need to be extra lightweight to float in a liquid methane sea. (It’s not as crazy as it sounds. Future explorers will want to visit Titan and boats could be a good way to get around.)

Liquid methane also has low viscosity (or “gooiness”) and low surface tension. See the table below. Surface tension is what gives water its rubbery skin and, on Earth, lets water bugs skitter across ponds. A water bug on Titan would promptly sink into a pond of flimsy methane. On the bright side, Titan’s low gravity, only one-seventh Earth gravity, might allow the creature climb back out again.

Back to boats: Propellers turning in methane would need to be extra-wide to “grab” enough of the thin fluid for propulsion. They’d also have to be made of special materials resistant to cracking at cryogenic temperatures.

And watch out for those waves! European scientists John Zarnecki and Nadeem Ghafoor have calculated what methane waves on Titan might be like: seven times taller than typical Earth-waves (mainly because of Titan’s low gravity) and three times slower, “giving surfers a wild ride,” says Ghafoor.

Last but not least, liquid methane is flammable. Titan doesn’t catch fire because the atmosphere contains so little oxygen–a key ingredient for combustion. If explorers visit Titan one day they’ll have to be careful with their oxygen tanks and resist the urge to douse fires with “water.”

Infrared rainbows, towering waves, seas beckoning to sailors. Huygens saw none of these things before it plopped down in the mud. Do they really exist?

“…there is no reason why the other Planets should be without them.”

Original Source: Science@NASA

Using an innovative camera on NASA’s Cassini spacecraft, scientists have captured images of a radiation belt inside the rings of Saturn and have the clearest picture yet of the planet’s giant magnetosphere, according to a mid-year report of the spacecraft published today in the journal Science.

The Cassini spacecraft entered Saturn’s orbit in July 2004, kicking off a four-year study of the sixth planet from the sun. Among the 12 science instruments on the spacecraft is the Magnetospheric Imaging Instrument (MIMI) — developed by the Johns Hopkins University Applied Physics Laboratory (APL) — which scientists are using to study the energetic charged particle environment at Saturn and obtain images of the ringed planet’s magnetosphere.

“Every time we fly a new instrument in space, it reveals new vistas of whatever object we happen to be studying,” says Dr. Stamatios (Tom) Krimigis, principal investigator for the MIMI experiment, of APL.

This time, says Krimigis, the MIMI instrument has enabled scientists to “visualize the invisible” — to “see” the plasma and radiation belts in Saturn’s environment in an image; to discover that the belts are more intense on the night-side of the planet; that there is an unexpected radiation belt inward of the “D” ring, the fourth major ring closest to the tenuous upper atmosphere of the planet; and that there is a virtual soup of ions that derive from the dissociation of water, most likely due to radiation impacting the rings.

These images were captured during Saturn orbit insertion with MIMI’s Ion and Neutral Camera (INCA), which measures the three-dimensional distribution, velocities and rough composition of magnetospheric and interplanetary ions for regions in which the energetic ion fluxes are very low. It also provides a global view of the energetic neutral emission of hot plasmas in the Saturnian magnetosphere, measuring the composition and velocities of those energetic neutrals for each image pixel.

“By detecting various energetic particles and discriminating among them according to energy and mass, the camera is able to obtain remote images of the global distribution of these particles,” says Dr. Donald Mitchell of APL, who leads the camera science team.

“Using INCA, we also discovered a radiation belt in a place where no spacecraft can go — inside the planet’s rings,” says APL’s Dr. Ed Roelof, a coinvestigator on the MIMI team. “We never knew this belt existed, but we saw it and were able to determine some of its properties and characteristics.”

The properties of the main radiation belts are perhaps among the more significant of the findings, says Dr. Doug Hamilton of the University of Maryland , College Park, who led the instrument team measuring the composition. “It’s comprised mostly of oxygen and water products,” he says. “That is most likely the result of the bombardment of the planet’s rings and the icy moons by the radiation trapped in Saturn’s magnetic field. And by this bombardment, the water is released and it becomes charged.”

According to Krimigis, the ability to visualize a planet’s magnetosphere will enable scientists to better monitor space weather. “This will benefit science and, in the case of Earth, may lead to space weather forecasts that will give advance warning of electromagnetic storms, which in the past have disrupted communications and crippled electrical power grids.”

In addition to Krimigis, Mitchell and Roelof, research team members at APL and co-authors on the Science paper, “Dynamics of Saturn’s Magnetosphere from MIMI During Cassini’s Orbital Insertion,” include Stefano Livi, Barry Mauk, Christopher Paranicas, Pontus Brandt, Andrew Cheng, Teck Choo, John Hayes, Stephen Jaskulek, Edwin Keath, Martha Kusterer, David LaVallee, Richard McEntire, Joachim Saur, Franklin Turner and Donald Williams.

The Cassini-Huygens mission is a cooperative project of NASA, ESA and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA. The MIMI team includes investigators and expertise from APL; the University of Maryland, College Park; University of Kansas, Lawrence; University of Arizona, Tucson; Bell Laboratories, Murray Hill, N.J.; the Max Planck Institute for Solar System Research, Lindau, Germany; and the Centre d’Etude Spatiale des Rayonnements in Toulouse, France.

Original Source: JHU News Release

Image credit: Hubble
Data from the Cassini-Huygens satellite showing oxygen ions in the atmosphere around Saturn’s rings suggests once again that molecular oxygen alone isn’t a reliable indicator of whether a planet can support life.

That and other data are outlined in two papers in the Feb. 25 issue of the journal Science co-authored by University of Michigan engineering professors Tamas Gombosi, J. Hunter Waite and Kenneth Hansen; and T.E. Cravens from the University of Kansas. The papers belong to a series of publications on data collected by Cassini as it passed through the rings of Saturn on July 1.

Molecular oxygen forms when two oxygen atoms bond together and is known in chemical shorthand as O2. On Earth, it is a continual byproduct of plant respiration, and animals need this oxygen for life. But in Saturn’s atmosphere, molecular oxygen was created without life present, through a chemical reaction with the sun’s radiation and icy particles that comprise Saturn’s rings.

“That means you don’t need biology to produce an O2 atmosphere,” Waite said. “If we want indicators to use in the search for life on other planets, we need to know what to look for. But oxygen alone isn’t it.”

Because Saturn’s rings are made of water ice, one would expect to find atoms derived from water, such as atomic oxygen (one atom) rather than O2, Waite said. However, the paper, called “Oxygen Ions Observed Near Saturn’s A Ring,” suggests the formation of molecular oxygen atmospheres happens more often in the outer solar system than expected. There is earlier evidence of molecular oxygen atmospheres elsewhere in the solar system?for instance above the icy Galilean moons of Jupiter?he said.

Four U-M College of Engineering faculty members are involved in the Cassini mission to explore Saturn’s rings and some of its moons. Waite leads the team operating the ion and neutral mass spectrometer, the instrument that detected and measured the molecular oxygen ions. Other team members are J.G. Luhmann of the University of California, Berkeley; R.V. Yelle, of the University of Arizona, Tuscon; W.T. Kasprzak, of the Goddard Space Flight Center; R.L. McNutt of Johns Hopkins University; and W.H. Ip, of the National Central University, Taiwan.

A second, viewpoint paper called, “Saturn’s Variable Magnetosphere,” by Hansen and Gombosi, who is chair of the College of Engineering’s department of Atmospheric, Oceanic and Space Sciences, reviews key findings from the other Cassini teams, including new information that contradicts data gathered 25 years ago, when the space craft Voyager passed by the planet.

Original Source: UMICH News Release

A large, bright and complex convective storm that appeared in Saturn’s southern hemisphere in mid-September 2004 was the key in solving a long-standing mystery about the ringed planet.

Saturn’s atmosphere and its rings are shown here in a false color composite made from Cassini images taken in near infrared light through filters that sense different amounts of methane gas. Portions of the atmosphere with a large abundance of methane above the clouds are red, indicating clouds that are deep in the atmosphere. Grey indicates high clouds, and brown indicates clouds at intermediate altitudes. The rings are bright blue because there is no methane gas between the ring particles and the camera.

The complex feature with arms and secondary extensions just above and to the right of center is called the Dragon Storm. It lies in a region of the southern hemisphere referred to as “storm alley” by imaging scientists because of the high level of storm activity observed there by Cassini in the last year.

The Dragon Storm was a powerful source of radio emissions during July and September of 2004. The radio waves from the storm resemble the short bursts of static generated by lightning on Earth. Cassini detected the bursts only when the storm was rising over the horizon on the night side of the planet as seen from the spacecraft; the bursts stopped when the storm moved into sunlight. This on/off pattern repeated for many Saturn rotations over a period of several weeks, and it was the clock-like repeatability that indicated the storm and the radio bursts are related. Scientists have concluded that the Dragon Storm is a giant thunderstorm whose precipitation generates electricity as it does on Earth. The storm may be deriving its energy from Saturn’s deep atmosphere.

One mystery is why the radio bursts start while the Dragon Storm is below the horizon on the night side and end when the storm is on the day side, still in full view of the Cassini spacecraft. A possible explanation is that the lightning source lies to the east of the visible cloud, perhaps because it is deeper where the currents are eastward relative to those at cloud top levels. If this were the case, the lightning source would come up over the night side horizon and would sink down below the day side horizon before the visible cloud. This would explain the timing of the visible storm relative to the radio bursts.

The Dragon Storm is of great interest for another reason. In examining images taken of Saturn’s atmosphere over many months, imaging scientists found that the Dragon Storm arose in the same part of Saturn’s atmosphere that had earlier produced large bright convective storms. In other words, the Dragon Storm appears to be a long-lived storm deep in the atmosphere that periodically flares up to produce dramatic bright white plumes which subside over time. One earlier sighting, in July 2004, was also associated with strong radio bursts. And another, observed in March 2004 and captured in a movie created from images of the atmosphere (http://photojournal.jpl.nasa.gov/catalog/PIA06082 and http://photojournal.jpl.nasa.gov/catalog/PIA06083) spawned three little dark oval storms that broke off from the arms of the main storm. Two of these subsequently merged with each other; the current to the north carried the third one off to the west, and Cassini lost track of it. Small dark storms like these generally get stretched out until they merge with the opposing currents to the north and south.

These little storms are the food that sustains the larger atmospheric features, including the larger ovals and the eastward and westward currents. If the little storms come from the giant thunderstorms, then together they form a food chain that harvests the energy of the deep atmosphere and helps maintain the powerful currents.

Cassini has many more chances to observe future flare-ups of the Dragon Storm, and others like it over the course of the mission. It is likely that scientists will come to solve the mystery of the radio bursts and observe storm creation and merging in the next 2 or 3 years.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . For images visit the Cassini imaging team home page http://ciclops.org .

Original Source: NASA/JPL/SSI