It’s a space navigator’s dream! The Cassini spacecraft will perform close flybys of two of Saturn’s most enigmatic moons all within less than 48 hours, and with no maneuvers in between. Enceladus and Titan are aligned just right so that Cassini can catch glimpses of these two contrasting moons – one a geyser world and the other an analog to early Earth.
Cassini will make its closest approach to Enceladus late at night on May 17 Pacific time, which is in the early hours of May 18 UTC. The spacecraft will pass within about 435 kilometers (270 miles) of the moon’s surface.
The main scientific goal at Enceladus will be to watch the sun play peekaboo behind the water-rich plume emanating from the moon’s south polar region. Scientists using the ultraviolet imaging spectrograph will be able to use the flickering light to measure whether there is molecular nitrogen in the plume. Ammonia has already been detected in the plume and scientists know heat can decompose ammonia into nitrogen molecules. Determining the amount of molecular nitrogen in the plume will give scientists clues about thermal processing in the moon’s interior.
Then on to Titan: the closest approach will take place in the late evening May 19 Pacific time, which is in the early hours of May 20 UTC. The spacecraft will fly to within 1,400 kilometers (750 miles) of the surface.
Cassini will primarily be doing radio science during this pass to detect the subtle variations in the gravitational tug on the spacecraft by Titan, which is 25 percent larger in volume than the planet Mercury. Analyzing the data will help scientists learn whether Titan has a liquid ocean under its surface and get a better picture of its internal structure. The composite infrared spectrometer will also get its southernmost pass for thermal data to fill out its temperature map of the smoggy moon.
Cassini has made four previous double flybys and one more is planned in the years ahead.
For more information on the Enceladus flyby, dubbed “E10,” see this link.
For more information on the Titan flyby, dubbed “T68,” see this link.
There’s nothing up the sleeves of the Cassini imaging team in this image; it is real! Is the moon Titan being cut in half by Saturn’s rings? What is actually happening here is that the middle part of the rings are made dark as Saturn casts its shadow across them. Cassini was just in the right place at the right time, making it appear as though Titan is being sliced in half! The night side of the planet is to the left, out of the frame of the image. Illuminated Titan can be seen above, below and through gaps in the rings. Click the image for a larger version.
As an added benefit in this shot, Mimas (396 kilometers, 246 miles across) is near the bottom of the image, and Atlas (30 kilometers, 19 miles across) can barely be detected near the thin F ring just above the center right of the image. Lit terrain seen here is the area between the leading hemisphere and Saturn-facing side of Titan (5,150 kilometers, 3,200 miles across). This view looks toward the northern, sunlit side of the rings from just above the ringplane.
Below are a few more magical images from Cassini:
Here the moon Enceladus appears strung along a wispy ring of Saturn, likely the G ring. Look close and Enceladus’ plumes are visible, too.
Two of Saturn’s small moons appear to be sitting on Satun’s thin F ring in this image.
From the CICLOPS website:
Pandora (81 kilometers, 50 miles across) is on the left, and Epimetheus (113 kilometers, 70 miles across) is on the right. This view looks toward the northern, sunlit side of the rings from just above the ringplane. Both moons are closer to Cassini than the rings are. Pandora is slightly closer to Cassini than Epimetheus here.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Nov. 23, 2009. The view was acquired at a distance of approximately 1.3 million kilometers (808,000 miles) from Pandora and Epimetheus. Image scale is 8 kilometers (5 miles) per pixel.
Observations from two instruments on the Cassini spacecraft shows the moon Enceladus leaves a complex pattern of ripples and bubbles in its wake as it orbit Saturn. The ringed planet’s magnetosphere is filled with electrically charged particles (plasma) originating from both the planet and its moons, and as Enceladus plows through the plasma “spiky” features form that represent bubbles of low energy particles, said Sheila Kanani who led a team of scientists from University College, London who discovered the phenomenon.
Cassini has made nine flybys of the icy, geyser-filled moon Enceladus (Saturn’s sixth-largest moon) since 2005. The closest of these have taken the spacecraft’s suite of instruments just 25 km from Enceladus’s surface, which scientists believe conceals a saline ocean. Heated vents at the south pole of the moon release a plume of material, consisting mainly of icy grains and water vapour, into space.
Measurements from the Cassini Plasma Spectrometer (CAPS) and the Magnetospheric IMaging Instrument (MIMI) show that both the moon and its plume are continuously soaking up the plasma, which rushes past at around 30 kilometers per second, leaving a cavity downstream. In addition, the most energetic particles which zoom up and down Saturn’s magnetic field lines are swept up, leaving a much larger void in the high energy plasma. Material from Enceladus, both dust and gas, is also being charged and forming new plasma.
The mysterious spiky features in the CAPS data shows a complex picture of readjustment downstream from Enceladus.
“Eventually the plasma closes the gap downstream from Enceladus but our observations show that this isn’t happening in a smooth, orderly fashion. We are seeing spiky features in the plasma that last between a few tens of seconds and a minute or two. We think that these might represent bubbles of low energy particles formed as the plasma fills the gap from different directions,” said Kanani. Since Cassini arrived at Saturn, it has been building up a picture of the vital and unexpected role that Enceladus plays in Saturn’s magnetosphere.
“Enceladus is the source of most of the plasma in Saturn’s magnetosphere, with ionised water and oxygen originating from the vents forming a big torus of plasma that surrounds Saturn. We may see these spiky features in the wake of Saturn’s other moons as they interact with the plasma but, to date, we have only studied Enceladus in sufficient detail,” said Kanani.
She presented her results at the Royal Astronomical Society’s National Astronomy Meeting in Glasgow, Scotland this week.
The Cassini mission keeps churning out the hits, and here’s a collection of some of the latest stunning images released by the CICLOPS (Cassini Imaging for Central Operations) team. Above, the small moon Janus is almost hidden between the planet’s rings and the larger moon Rhea. The northern part of Janus can be seen peeking above the rings in this image of a “mutual event” where Janus (179 kilometers, 111 miles across) moved past Rhea (1,528 kilometers, 949 miles across). Mutual event observations such as this one, in which one moon passes close to or in front of another, help scientists refine their understanding of the orbits of Saturn’s moons. Click here to see a movie of the event.
Grab your 3-D glasses for this one! This 3-D view is a close-up of Saturn’s potato-shaped moon Prometheus, showing the moon’s leading hemisphere. The image was created by combining two different black and white images that were taken from slightly different viewing angles. The images are combined so that the viewer’s left and right eye, respectively and separately, see a left and right image of the black and white stereo pair when viewed through red-blue glasses.
At first glance, you might think this scene simply shows a bright chunk of Saturn, along with a crescent of the moon Enceladus at top right. But a closer look at the center of the image reveals a dramatic surprise: plumes of water ice spew out from the famed fractures known as “tiger stripes” near the south pole of the moon. And one other surprise: Although it may appear that Enceladus (504 kilometers, 313 miles across) is in the background here, the moon actually is closer to the spacecraft than Saturn is. This view looks most directly toward the side of Enceladus that faces away from Saturn. North on Enceladus is up and rotated 1 degree to the left.
Newly released images from last November’s close flyby over Saturn’s icy moon Enceladus the Cassini spacecraft reveal geyser jets spraying all along the prominent fractures, or “tiger stripes” that cross the moon’s south polar region. Additionally, a new detailed temperature map of one fracture reveals warmer temperatures than what was expected. “Enceladus continues to astound,” said Bob Pappalardo, Cassini project scientist at the Jet Propulsion Laboratory. “With each Cassini flyby, we learn more about its extreme activity and what makes this strange moon tick.”
The new images from the imaging science subsystem and the composite infrared spectrometer teams include the best 3-D image ever obtained of a tiger stripe fissure that sprays icy particles, water vapor and organic compounds. There are also views of regions not well-mapped previously on Enceladus, including a southern area with crudely circular tectonic patterns.
For Cassini’s visible-light cameras, the Nov. 21, 2009 flyby provided the last look at Enceladus’ south polar surface before that region of the moon goes into 15 years of darkness, and includes the most detailed look yet at the jets.
Scientists planned to use this flyby to look for new or smaller jets not visible in previous images. In one mosaic, scientists count more than 30 individual geysers, including more than 20 that had not been seen before. At least one jet spouting prominently in previous images now appears less powerful.
“This last flyby confirms what we suspected,” said Carolyn Porco, imaging team lead based at the Space Science Institute in Boulder, Colo. “The vigor of individual jets can vary with time, and many jets, large and small, erupt all along the tiger stripes.”
A new map that combines heat data with visible-light images shows a 40-kilometer (25-mile) segment of the longest tiger stripe, known as Baghdad Sulcus. The map illustrates the correlation, at the highest resolution yet seen, between the geologically youthful surface fractures and the anomalously warm temperatures that have been recorded in the south polar region. The broad swaths of heat previously detected by the infrared spectrometer appear to be confined to a narrow, intense region no more than a kilometer (half a mile) wide along the fracture.
In these measurements, peak temperatures along Baghdad Sulcus exceed 180 Kelvin ( – 92 C, -135 F), and may be higher than 200 Kelvin (- 73 C, -100 F). These warm temperatures probably result from heating of the fracture flanks by the warm, upwelling water vapor that propels the ice-particle jets seen by Cassini’s cameras. Cassini scientists will be testing this idea by investigating how well the hot spots correspond with the jet sources.
“The fractures are chilly by Earth standards, but they’re a cozy oasis compared to the numbing 50 Kelvin (-223 C, -370 F) of their surroundings,” said John Spencer, a composite infrared spectrometer team member based at Southwest Research Institute in Boulder, Colo. “The huge amount of heat pouring out of the tiger stripe fractures may be enough to melt the ice underground. Results like this make Enceladus one of the most exciting places we’ve found in the solar system.”
Some of Cassini’s scientists infer that the warmer the temperatures are at the surface, the greater the likelihood that jets erupt from liquid. “And if true, this makes Enceladus’ organic-rich, liquid sub-surface environment the most accessible extraterrestrial watery zone known in the solar system,” Porco said.
The Nov. 21 flyby was the eighth targeted encounter with Enceladus. It took the spacecraft to within about 1,600 kilometers (1,000 miles) of the moon’s surface, at around 82 degrees south latitude.
Caption: Geysers on Enceladus. Credit: NASA, JPL, Space Science Institute
One of the most exciting but unexpected discoveries of the Cassini mission is seeing the activity taking place on Saturn’s small moon Enceladus. Between the active geysers, the unusual “tiger stripes” and the surprisingly young surface near the moon’s south pole, Enceladus has surprised scientists with almost all the images and data the gathered by the spacecraft. But is the moon always active, or are we just in the right place at the right time, lucky to be catching it during an active phase? A recent paper outlines a model in which the kind of geologic eruptions now visible on Enceladus only occur every billion years or so.
“Cassini appears to have caught Enceladus in the middle of a burp,” said Francis Nimmo, a planetary scientist at the University of California Santa Cruz. “These tumultuous periods are rare and Cassini happens to have been watching the moon during one of these special epochs.”
Nimmo and co-author Craig O’Neill of Macquarie University in Sydney, Australia propose that blobs of warm ice that periodically rise to the surface and churn the icy crust on Saturn’s moon Enceladus explain the quirky heat behavior and intriguing surface of the moon’s south polar region.
The most interesting features by far in the south polar region of Enceladus are the fissures known as “tiger stripes” that spray water vapor and other particles out from the moon. While Nimmo and O’Neill’s model doesn’t link the churning and resurfacing directly to the formation of fissures and jets, it does fill in some of the blanks in the region’s history.
“This episodic model helps to solve one of the most perplexing mysteries of Enceladus,” said Bob Pappalardo, Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., of the research done by his colleagues. “Why is the south polar surface so young? How could this amount of heat be pumped out at the moon’s south pole? This idea assembles the pieces of the puzzle.”
But not everyone is convinced this model answers all the questions about Enceladus. Carolyn Porco, who leads the imaging team for Cassini said via Twitter regarding this paper, “Beware! Several different models out there say different things.”
About four years ago, Cassini’s composite infrared spectrometer instrument detected a heat flow in the south polar region of at least 6 gigawatts, the equivalent of at least a dozen electric power plants. This is at least three times as much heat as an average region of Earth of similar area would produce, despite Enceladus’ small size. The region was also later found by Cassini’s ion and neutral mass spectrometer instrument to be swiftly expelling argon, which comes from rocks decaying radioactively and has a well-known rate of decay.
Calculations told scientists it would be impossible for Enceladus to have continually produced heat and gas at this rate. Tidal movement – the pull and push from Saturn as Enceladus moves around the planet – cannot explain the release of so much energy.
The surface ages of different regions of Enceladus also show great diversity. Heavily cratered plains in the northern part of the moon appear to be as old as 4.2 billion years, while a region near the equator known as Sarandib Planitia is between 170 million and 3.7 billion years old. The south polar area, however, appears to be less than 100 million years old, possibly as young as 500,000 years.
O’Neill had originally developed the model for the convection of Earth’s crust. For the model of Enceladus, which has a surface completely covered in cold ice that is fractured by the tug of Saturn’s gravitational pull, the scientists stiffened up the crust. They picked a strength somewhere between that of the malleable tectonic plates on Earth and the rigid plates of Venus, which are so strong, it appears they never get sucked down into the interior.
Their model showed that heat building up from the interior of Enceladus could be released in episodic bubbles of warm, light ice rising to the surface, akin to the rising blobs of heated wax in a lava lamp. The rise of the warm bubbles would send cold, heavier ice down into the interior. (Warm is, of course, relative. Nimmo said the bubbles are probably just below freezing, which is 273 degrees Kelvin or 32 degrees Fahrenheit, whereas the surface is a frigid 80 degrees Kelvin or -316 degrees Fahrenheit.)
The model fits the activity on Enceladus when the churning and resurfacing periods are assumed to last about 10 million years, and the quiet periods, when the surface ice is undisturbed, last about 100 million to two billion years. Their model suggests the active periods have occurred only 1 to 10 percent of the time that Enceladus has existed and have recycled 10 to 40 percent of the surface. The active area around Enceladus’s south pole is about 10 percent of its surface.
The ongoing search for the existence of life that doesn’t call the Earth ‘home’ could potentially find that life right here in our own Solar System. There is considerable debate about whether evidence for that life has already been found on Mars, but astronomers might do well to look at other, more exotic locations in our neighborhood.
At the recent meeting of the American Geophysical Union in San Fransisco, Francis Nimmo, who is a professor of Earth and planetary sciences at UC Santa Cruz, said that the conditions on Saturn’s moon Enceladus, and Jupiter’s moon Europa may be just right to harbor life.
Nimmo said, “Liquid water is the one requirement for life that everyone can agree on.” The water underneath the icy crusts of Enceladus and Europa may just be teeming with alien fish and algae, or more basic forms of life such as bacteria.
Nimmo is one of a long list of scientists speculating on the existence of life on these watery moons. A discovery of any life form originating from a planet other than the Earth “would be the scientific discovery of the millennium,” Nimmo said. And even saying that is an understatement.
If life were able to exist in the watery oceans of the moons around Saturn and Jupiter, Nimmo said, it would mean that the ‘habitable zone’ around a star would extend much further out than previously thought, to moons that orbit large gas giants in other systems around faraway stars.
The possible ocean under the surface of Enceladus may receives its heat from the tidal forces of Saturn. That is, if there is an ocean under the surface of Enceladus, as that topic is still somewhat debated among astronomers. The constant tug of Saturn’s gravitational pull may stretch the interior of the planet enough to heat the water below the crust of ice, which is estimated to vary in thickness between 25km to 45km. Geysers of frozen water forced out of crack on Enceladus’ surface have been observed by the Cassini mission, and the craft has even flown through the plume of one of these jets.
Here’s a video of Carolyn Porco, who leads the imaging team on the Cassini mission, talking about the potential for life inside the moon, and some of the discoveries made by Cassini so far:
Evidence for the ocean under Europa’s icy skin comes from the Galileo mission, which passed by the moon in 2000 and took measurements of the moon’s magnetic field. Variations in the magnetic field have led astronomers to believe there is a vast ocean of water under the surface, leading to natural suppositions about the potential of its habitability.
Europa’s ocean is heated much in the same way as that of Enceladus: both moons have an eccentric orbit around their much more massive planets, and this orbit causes a shift in the way the planet tugs on their interiors, causing friction in the cores which in turn heats them up.
The core and surface of these moons both are possible sources of chemicals that are necessary for life to form. Impacts from comets can leave molecules on the surface, and light from the Sun breaks down compounds as well. Organic molecules and minerals may originate in the cores of the moons, streaming out into the watery ‘mantle’. Such nutrients could potentially support small communities of exotic bacteria like those seen around hydrothermal vents here on Earth.
Of course, just because these moons are habitable doesn’t mean that life exists there, as Nimmo and other planetary scientists are quick to point out. Cassini may still provide evidence of life on Enceladus, as the data from this last flyby of the plumes is still being analyzed. Future missions to Europa, such as the proposed ‘interplanetary submarine‘, may also give us an answer to the question of life’s existence elsewhere, and of course the quest continues for a mission to Mars that will finally give us some idea of its habitability now or in the past.
Until the data comes back from these missions, though, we’ll still have to wait and speculate.
Raw images are already being returned from Cassini’s Nov. 21 “E-8” or eighth flyby of the tiger-striped moon Enceladus. Visible in this raw image are several plumes from fissures in the south polar region of the moon. These fissures spew jets of water vapor and other particles hundreds of kilometers from the surface. This flyby included a very different geometry to the flyby trajectory – and a different look at the plumes — approaching within 1,606 kilometers (997.9 miles) of the surface, buzzing over 82 degrees south latitude. This is the last look we’ll have for several years at this intriguing area of Enceladus before winter darkness blankets the area. See below for looks at Baghdad Sulcus, the “tiger stripe” that scientists were focusing on.
While Cassini was taking these high-resolution images of the southern part of the Saturn-facing hemisphere, the Composite Infrared Spectrograph (CIRS) instrument was collecting data to create a contiguous thermal map of Baghdad Sulcus. This image was taken approximately 1,858 kilometers away.
Here’s a look at Baghdad Sulcus from 3,556 kilometers away. And below is a 3-D version, created by Stu Atkinson. Stay tuned for more details on the data gathered from the flyby!
Carolyn Porco, the lead for Cassini’s imaging team, warned on Twitter that the flyby of Saturn’s moon Enceladus performed by the spacecraft on Nov. 2 wasn’t really an “imaging” flyby, and that we might have to wait until the Nov. 21 flyby for really good images. But just take a look the images returned so far, with stunning looks at the jets shooting from the moon! Another image takes a close look at the surface. These are raw, unprocessed images, but what images they are! This is the second image from today’s flyby returned by the spacecraft. See below for more.
Cassini came within about 100 kilometers (62 miles) of the surface. The spacecraft has gone closer during a previous flyby (25 kilometers or 15 miles). This is the third image sent back so far from this flyby, showing the surface of the tiger-striped, geyser-spewing moon. According to the CICLOPS website, this image was taken in visible green light with the Cassini spacecraft narrow-angle camera at a distance of approximately 14,000 kilometers (8,700 miles) from Enceladus. The plan was for the spacecraft to go deep into the heart of the plume from the geysers on the tiger-striped moon; as of yet no images from the plume have been released. The objective of this flyby was to analyze the particles in the plume with instruments that can detect the size, mass, charge, speed and composition. The spacecraft spent only about a minute in the plume.
Here’s a view from farther away, with the plumes visible against the backlit moon.
Scientists for the Cassini mission called their flyby of Saturn’s small moon Enceladus on August 11 a “skeet shoot,” partially in honor of the current Olympic games underway, but mostly because the spacecraft would be trying to shoot rapidly at the moon with its array of cameras and scientific instruments. As the images begin to stream back, the scientists are definitely excited about what they’re seeing.
“What a dazzling success!” said Carolyn Porco, the Cassini Imaging Team Leader. “There doesn’t even appear to be any smear.” Scientists compared Cassini’s fast flyby of Enceladus to trying to capture a sharp, unsmeared picture of a roadside billboard about a mile away with a 2,000 mm telephoto lens held out the window of a car moving at 50 mph. The imaging team is still poring over the pictures to see if they were successful in “shooting” their target: the active vent regions on the tiger stripe-like features on the moon’s south pole that create the geysers on Enceladus. But the amazingly clear images show a fractured surface littered with boulders and what Porco said could possibly be ice blocks.
Cassini flew over the surface of Enceladus at tremendous speed; about 18 km/sec (about 40,000 mph), which makes taking clear images very difficult. The imaging team devised a technique of turning the spacecraft while taking pictures in rapid succession, shooting at seven, very high priority surface targets. The suite of images ranged in resolution from 8 to 28 meters/pixel, using exposure times that were long enough to see the surface in the twilight near the terminator yet short enough to avoid smear.
The tiger stripes, officially called sulci, have been identified by the imaging cameras on earlier flybys of Enceladus as the sources of the jets, and also as the “hot spots” or warmer areas on the moon identified by the Cassini’s Composite Infrared Spectrograph.
Porco said the team still has much work to do to decipher all the information in the images and data from the other instruments. “In this painstaking work, we proceed, step by step, to lay bare those things which hold the greatest promise of comprehension, the greatest significance for piecing together the story of the origins of the bodies in our solar system, our Earth, and indeed ourselves,” she wrote in her blog.
We’ll provide further updates on the flyby images as information becomes available.