Cassini Archives

October 2, 2015December 23, 2015 by Paul Patton

Is There a Kraken in Kraken Mare? What Kind of Life Would We Find on Titan?

The left image shows a mosaic of images of Titan taken by the Cassini spacecraft in near infrared light. Titan’s polar seas are visible as sunlight glints off of them. The right image is a radar image of Kraken Mare. Credit: NASA Jet Propulsion Laboratory.

Could there be life on Saturn’s large moon Titan? Asking the question forces astrobiologists and chemists to think carefully and creatively about the chemistry of life, and how it might be different on other worlds than it is on Earth. In February, a team of researchers from Cornell University, including chemical engineering graduate student James Stevenson, planetary scientist Jonathan Lunine, and chemical engineer Paulette Clancy, published a pioneering study arguing that cell membranes could form under the exotic chemical conditions present on this remarkable moon.

In many ways, Titan is Earth’s twin. It’s the second largest moon in the solar system and bigger than the planet Mercury. Like Earth, it has a substantial atmosphere, with a surface atmospheric pressure a bit higher than Earth’s. Besides Earth, Titan is the only object in our solar system known to have accumulations of liquid on its surface. NASA’s Cassini space probe discovered abundant lakes and even rivers in Titan’s polar regions. The largest lake, or sea, called Kraken Mare, is larger than Earth’s Caspian Sea. Researchers know from both spacecraft observations and laboratory experiments that Titan’s atmosphere is rich in complex organic molecules, which are the building blocks of life.

All these features might make it seem as though Titan is tantalizingly suitable for life. The name ‘Kraken’, which refers to a legendary sea monster, fancifully reflects the eager hopes of astrobiologists. But, Titan is Earth’s alien twin. Being almost ten times further from the sun than Earth is, its surface temperature is a frigid -180 degrees Celsius. Liquid water is vital to life as we know it, but on Titan’s surface all water is frozen solid. Water ice takes on the role that silicon-containing rock does on Earth, making up the outer layers of the crust.

The liquid that fills Titan’s lakes and rivers is not water, but liquid methane, probably mixed with other substances like liquid ethane, all of which are gases here on Earth. If there is life in Titan’s seas, it is not life as we know it. It must be an alien form of life, with organic molecules dissolved in liquid methane instead of liquid water. Is such a thing even possible?

The Cornell team took up one key part of this challenging question by investigating whether cell membranes can exist in liquid methane. Every living cell is, essentially, a self-sustaining network of chemical reactions, contained within bounding membranes. Scientists think that cell membranes emerged very early in the history of life on Earth, and their formation might even have been the first step in the origin of life.

Here on Earth, cell membranes are as familiar as high school biology class. They are made of large molecules called phospholipids. Each phospholipid molecule has a ‘head’ and a ‘tail’. The head contains a phosphate group, with a phosphorus atom linked to several oxygen atoms. The tail consists of one or more strings of carbon atoms, typically 15 to 20 atoms long, with hydrogen atoms linked on each side. The head, due to the negative charge of its phosphate group, has an unequal distribution of electrical charge, and we say that it is polar. The tail, on the other hand, is electrically neutral.

phospholipid membrane — Here on Earth, cell membranes are composed of phospholipid molecules dissolved in liquid water. A phospholipid has a backbone of carbon atoms (gray), and also contains hydrogen (sky blue), phosphorus (yellow), oxygen (red), and nitrogen (blue). Due to the positive charge associated with the nitrogen containing choline group, and the negative charge associated with the phosphate group, the head is polar, and attracts water. It is therefore hydrophilic. The hydrocarbon tail is electrically neutral and hydrophobic. The structure of a cell membrane is due these electrical properties of phospholipids and water. The molecules form a double layer, with the hydrophilic heads facing outward, towards water, and the hydrophobic tails facing inward, towards one another. Credit: Ties van Brussel

These electrical properties determine how phospholipid molecules will behave when they are dissolved in water. Electrically speaking, water is a polar molecule. The electrons in the water molecule are more strongly attracted to its oxygen atom than to its two hydrogen atoms. So, the side of the molecule where the two hydrogen atoms are has a slight positive charge, and the oxygen side has a small negative charge. These polar properties of water cause it to attract the polar head of the phospholipid molecule, which is said to be hydrophilic, and repel its nonpolar tail, which is said to be hydrophobic.

When phospholipid molecules are dissolved in water, the electrical properties of the two substances work together to cause the phospholipid molecules to organize themselves into a membrane. The membrane closes onto itself into a little sphere called a liposome. The phospholipid molecules form a bilayer two molecules thick. The polar hydrophilic heads face outward towards the water on both the inner and outer surface of the membrane. The hydrophobic tails are sandwiched between, facing each other. While the phospholipid molecules remain fixed in their layer, with their heads facing out and their tails facing in, they can still move around with respect to each other, giving the membrane the fluid flexibility needed for life.

Phospholipid bilayer membranes are the basis of all terrestrial cell membranes. Even on its own, a liposome can grow, reproduce and aid certain chemical reactions important to life, which is why some biochemists think that the formation of liposomes might have been the first step towards life. At any rate, the formation of cell membranes must surely been an early step in life’s emergence on Earth.

water and methane — At the left, water, consisting of hydrogen (H) and oxygen (O), is a polar solvent. Oxygen attracts electrons more strongly than hydrogen does, giving the hydrogen side of the molecule a net positive charge and the oxygen side a net negative charge. The delta symbol ( ) indicates that the charge is partial, that is less than a full unit of positive or negative charge. At right, methane is a non-polar solvent, due to the symmetrical distribution of hydrogen atoms (H) around a central carbon atom (C). Credit: Jynto as modified by Paul Patton.

If some form of life exists on Titan, whether sea monster or (more likely) microbe, it would almost certainly need to have a cell membrane, just like every living thing on Earth does. Could phospholipid bilayer membranes form in liquid methane on Titan? The answer is no. Unlike water, the methane molecule has an even distribution of electrical charges. It lacks water’s polar qualities, and so couldn’t attract the polar heads of phospholipid molecule. This attraction is needed for the phospholipids to form an Earth-style cell membrane.

Experiments have been conducted where phospholipids are dissolved in non-polar liquids at Earthly room temperature. Under these conditions, the phospholipids form an ‘inside-out’ two layer membrane. The polar heads of the phospholipid molecules are at the center, attracted to one another by their electrical charges. The non-polar tails face outward on each side of the inside-out membrane, facing the non-polar solvent.

membranes in polar and non-polar solvents — At left, phospholipids are dissolved in water, a polar solvent. They form a bilayer membrane, with their polar, hydrophilic heads facing outward towards water, and their hydrophobic tails facing each other. At right, when phospholipids are dissolved in a non-polar solvent at Earthly room temperature, they form an inside-out membrane, with the polar heads attracting one another, and the non-polar tails facing outwards towards the non-polar solvent. Based on figure 2 from Stevenson, Lunine, and Clancy (2015). Credit: Paul Patton

Could Titanian life have an inside out phospholipid membrane? The Cornell team concluded that this wouldn’t work, for two reasons. The first is that at the cryogenic temperatures of liquid methane, the tails of phospholipids become rigid, depriving any inside-out membrane that might form of the fluid flexibility needed for life. The second is that two key ingredients of phospholipids; phosphorus and oxygen, are probably unavailable in the methane lakes of Titan. In their search for Titanian cell membranes, the Cornell team needed to probe beyond the familiar realm of high school biology.

Although not composed of phospholipids, the scientists reasoned that any Titanian cell membrane would nevertheless be like the inside-out phospholipid membranes created in the lab. It would consist of polar molecules clinging together electrically in a solution of non-polar liquid methane. What molecules might those be? For answers the researchers looked to data from the Cassini spacecraft and from laboratory experiments that reproduced the chemistry of Titan’s atmosphere.

Titan’s atmosphere is known to have a very complex chemistry. It is made mostly of nitrogen and methane gas. When the Cassini spacecraft analyzed its composition using spectroscopy it found traces of a variety of compounds of carbon, nitrogen, and hydrogen, called nitriles and amines. Researchers have simulated the chemistry of Titan’s atmosphere in the lab by exposing mixtures of nitrogen and methane to sources of energy simulating sunlight on Titan. A stew of organic molecules called ‘tholins’ is formed. It consists of compounds of hydrogen and carbon, called hydrocarbons, as well as nitriles and amines.

The Cornell investigators saw nitriles and amines as potential candidates for their Titanian cell membranes. Both are polar molecules that might stick together to form a membrane in non-polar liquid methane due to the polarity of nitrogen containing groups found in both of them. They reasoned that candidate molecules must be much smaller than phospholipids, so that they could form fluid membranes at liquid methane temperatures. They considered nitriles and amines containing strings of between three and six carbon atoms. Nitrogen containing groups are called ‘azoto’ –groups, so the team named their hypothetical Titanian counterpart to the liposome the ‘azotosome’.

Synthesizing azotosomes for experimental study would have been difficult and expensive, because the experiments would need to be conducted at the cryogenic temperatures of liquid methane. But since the candidate molecules have been studied extensively for other reasons, the Cornell researchers felt justified in turning to the tools of computational chemistry to determine whether their candidate molecules could cohere as a flexible membrane in liquid methane. Computational models have been used successfully to study conventional phospholipid cell membranes.

acrylonitrile — Acrylonitrile has been identified as a possible basis for cell membranes in liquid methane on Titan. It is known to be present in Titan’s atmosphere at a concentration of 10 parts per million and has been produced in laboratory simulations of the effects of energy sources on Titan’s nitrogen-methane atmosphere. As a small polar molecule capable of dissolving in liquid methane, it is a candidate substance for the formation of cell membranes in an alternative biochemistry on Titan. Light blue: carbon atoms, dark blue: nitrogen atom, white: hydrogen atoms. Credit: Ben Mills as modified by Paul Patton.

acrylonitrile membrane — Polar acrylonitrile molecules align ‘head’ to ‘tail’ to form a membrane in non-polar liquid methane. Light blue: carbon atoms, dark blue: nitrogen atoms, white: hydrogen atoms. Credit: James Stevenson.

The group’s computational simulations showed that some candidate substances could be ruled out because they would not cohere as a membrane, would be too rigid, or would form a solid. Nevertheless, the simulations also showed that a number of substances would form membranes with suitable properties. One suitable substance is acrylonitrile, which Cassini showed is present in Titan’s atmosphere at 10 parts per million concentration. Despite the huge difference in temperature between cryogenic azotozomes and room temperature liposomes, the simulations showed them to exhibit strikingly similar properties of stability and response to mechanical stress. Cell membranes, then, are possible for life in liquid methane.

Computational chemistry simulations show that acrylonitrile and some other small polar nitrogen containing organic molecules are capable of forming ‘azotosomes’ when they are dissolved on liquid methane. Azotosomes are small membrane bounded spherules like the liposomes formed by phospholipids when they are dissolved in water. The simulations show that acrylonitrile azotosomes would be both stable and flexible in cryogenically cold liquid methane, giving them the properties they need to function as cell membranes for hypothetical Titanian life, or for life on any world with liquid methane on its surface. The azotosome shown is 9 nanometers in size, about the size of a virus. Light blue: carbon atoms, dark blue: nitrogen atoms, white: hydrogen atoms. Credit: James Stevenson.

The scientists from Cornell view their findings as nothing more than a first step towards showing that life in liquid methane is possible, and towards developing the methods that future spacecraft will need to search for it on Titan. If life is possible in liquid methane, the implications ultimately extend far beyond Titan.

When seeking conditions suitable for life in the galaxy, astronomers typically search for exoplanets within a star’s habitable zone, defined as the narrow range of distances over which a planet with an Earth-like atmosphere would have a surface temperature suitable for liquid water. If methane life is possible, then stars would also have a methane habitable zone, a region where methane could exist as a liquid on a planet or moon, making methane life possible. The number of habitable worlds in the galaxy would be greatly increased. Perhaps, on some worlds, methane life evolves into complex forms that we can scarcely imagine. Maybe some of them are even a bit like sea monsters.

References and Further Reading:

N. Atkinson (2010) Alien Life on Titan? Hang on Just a Minute, Universe Today.

N. Atkinson (2010) Life on Titan Could be Smelly and Explosive, Universe Today.

M. L. Cable, S. M. Horst, R. Hodyss, P. M. Beauchamp, M. A. Smith, P. A. Willis, (2012) Titan tholins: Simulating Titan organic chemistry in the Cassini-Huygens era, Chemical Reviews, 112:1882-1909.

E. Howell (2014) Titan’s Majestic Mirror-Like Lakes Will Come Under Cassini’s Scrutiny This Week, Universe Today.

J. Major (2013) Titan’s North Pole is Loaded With Lakes, Universe Today.

C. P. McKay, H. D. Smith, (2005) Possibilities for methanogenic life in liquid methane on the surface of Titan, Icarus 178: 274-276.

J. Stevenson, J. Lunine, P. Clancy, (2015) Membrane alternatives in worlds without oxygen: Creation of an azotosome, Science Advances 1(1):e1400067.

S. Oleson (2014) Titan submarine: Exploring the depths of Kraken, NASA Glenn Research Center, Press release.

Cassini Solstice Mission, NASA Jet Propulsion Laboratory

NASA and ESA celebrate 10 years since Titan landing, NASA 2015

August 19, 2015December 23, 2015 by David Dickinson

Cassini’s Farewell Look at Dione

NASA’s Cassini spacecraft paid a visit to Saturn’s moon Dione this week, one final time.

Cassini passed just 474 kilometers (295 miles) above the surface of the icy moon on Monday, August 17^th at 2:33 PM EDT/18:33 UT. The flyby is the fifth and final pass of Cassini near Dione (pronounced dahy-OH-nee). The closest passage was 100 kilometers (60 miles) in December 2011. This final flyby of Dione will give researchers a chance to probe the tiny world’s internal structure, as Cassini flies through the gravitational influence of the moon. Cassini has only gathered gravity science data on a handful of Saturn’s 62 known moons.

“Dione has been an enigma, giving hints of active geologic processes, including a transient atmosphere and evidence of ice volcanoes. But we’ve never found the smoking gun,” said Cassini science team member Bonnie Buratti in a recent press release. “The fifth flyby of Dione will be the last chance.”

Voyager 1 gave us our very first look at Dione in 1980, and Cassini has explored the moon in breathtaking detail since its first flyby in 2005. This final pass targeted Dione’s north pole at a resolution of only a few meters. Cassini’s Infrared Spectrometer was also on the lookout for any thermal anomalies, a good sign that Dione may still be geologically active. The spacecraft’s Cosmic Dust Analyzer also carried out a search for any dust particles coming from Dione. The results of these experiments are forthcoming. In a synchronous rotation, Dione famously displays a brighter leading hemisphere, which has been pelted with E Ring deposits.

The raw images from this week’s flyby are now available on the NASA Cassini website. You can see the sequence of the approach, complete with a ‘photobomb’ of Saturn’s moon Enceladus early on. Dione then makes a majestic pass in front of Saturn’s rings and across the ochre disk of the planet itself, before snapping into dramatic focus. Here we see the enormous shattered Evander impact basin near the pole of Dione, along with Erulus crater with a prominent central peak right along the day/night terminator. Dione has obviously had a battered and troubled past, one that astro-geologists are still working out. Cassini then takes one last shot, giving humanity a fitting final look at Dione as a crescent receding off in the distance.

It’ll be a long time before we visit Dione again.

“This will be our last chance to see Dione up close for many years to come,” said Cassini mission deputy project scientist Scott Edgington. “Cassini has provided insights into this icy moon’s mysteries, along with a rich data set and a host of new questions for scientists to ponder.”

Cassini also took a distant look at Saturn’s tiny moon Hyrrokkin (named after the Norse giantess who launched Baldur’s funeral ship) earlier this month. Though not a photogenic pass, looking at the tinier moons of Saturn helps researchers better understand and characterize their orbits. Even after more than a decade at Saturn, there are tiny moons of Saturn that Cassini has yet to see up close.

Next up for Cassini is a pass 1,036 kilometers (644 miles) from the surface of Titan on September 28^th, 2015.

Launched in 1997, Cassini has given us over a decade’s worth of exploration of the Saturnian system, including the delivery of the European Space Agency’s Huygens lander to the surface of Titan. The massive moon may be the target of a proposed mission that could one day sail the hazy atmosphere of Titan, complete with a nuclear plutonium powered MMRTG and deployable robotic quadcopters.

Cassini is set to depart the equatorial plane of Saturn late this year, for a series of maneuvers that will feature some dramatic passes through the rings before a final fiery reentry into the atmosphere of Saturn in 2017.

Astronomer Giovanni Cassini discovered Dione on March 21^st, 1684 from the Paris observatory using one of his large aerial refracting telescopes. About 1,120 kilometers in diameter, Dione is 1.5% as massive as Earth’s Moon. Dione orbits Saturn once every 2.7 days, and is in a 1:2 resonance with Enceladus, meaning Dione completes one orbit for every two orbits of Enceladus.

In a backyard telescope, Dione is easily apparent along with the major moons of Saturn as a +10.4 magnitude ‘star.’ Saturn is currently a fine telescopic target in the evening low to the south on the Libra-Scorpius border, offering prime time observers a chance to check out the ringed planet and its moons. Fare thee well, Dione… for now.

July 29, 2015December 23, 2015 by Jason Major

What Are These Strange Scarlet Streaks Spotted on Tethys?

Enhanced-color image from Cassini showing red streaks on Tethys (NASA/JPL-Caltech/Space Science Institute)

Resembling what the skin on my arms looks like after giving my cat a bath, the surface of Saturn’s moon Tethys is seen above in an extended-color composite from NASA’s Cassini spacecraft showing strange long red streaks. They stretch for long distances across the moon’s surface following the rugged terrain, continuing unbroken over hills and down into craters… and their cause isn’t yet known.

According to a NASA news release, “The origin of the features and their reddish color is currently a mystery to Cassini scientists. Possibilities being studied include ideas that the reddish material is exposed ice with chemical impurities, or the result of outgassing from inside Tethys. The streaks could also be associated with features like fractures that are below the resolution of the available images.”

The images were taken by Cassini during a flyby of the 660-mile-wide (1,062 km) Tethys on April 11, 2015 at a resolution of about 2,300 feet (700 meters) per pixel. They were acquired in visible green, infrared, and ultraviolet light wavelengths and so the composite image reveals colors our eyes can’t directly perceive. The combination of this and the solar illumination needed to image this particular area as the spacecraft passed by are why these features haven’t been seen so well until now.

“The red arcs really popped out when we saw the new images,” said Cassini participating scientist Paul Schenk of the Lunar and Planetary Institute in Houston. “It’s surprising how extensive these features are.”

Extended color mosaic of Tethys from Cassini images acquired on April 11, 2015. The region where the streaks are is outlined. Click for original hi-res version. (NASA/JPL-Caltech/SSI)

While the nature of Tethys’ streaks isn’t understood, the observations do indicate a relatively young age compared to the surrounding surface.

“The red arcs must be geologically young because they cut across older features like impact craters, but we don’t know their age in years.” said Paul Helfenstein, a Cassini imaging scientist at Cornell University in Ithaca. “If the stain is only a thin, colored veneer on the icy soil, exposure to the space environment at Tethys’ surface might erase them on relatively short time scales.”

Reprocessed Galileo image of Europa's frozen surface by Ted Stryk (NASA/JPL/Ted Stryk) — Reprocessed Galileo image of Europa’s streaked surface by Ted Stryk (NASA/JPL/Ted Stryk)

Could these arcs be signs of an underground ocean or reservoir of briny liquid, like Enceladus’ tiger stripes (aka sulcae) or the streaks that crisscross Europa’s ice? Or are they the results of infalling material from one of Saturn’s other moons? More observations with Cassini, now in its eleventh year in orbit at Saturn, are being planned to “study the streaks.”

“We are planning an even closer look at one of the Tethys red arcs in November to see if we can tease out the source and composition of these unusual markings,” said Linda Spilker, Cassini project scientist at JPL.

Source: NASA JPL

May 29, 2015December 23, 2015 by Jason Major

Cassini to Perform Its Final Flyby of Hyperion

Enhanced-color image of Hyperion from Sept. 26, 2005. (NASA/JPL/SSI)

On Sunday, May 31, the Cassini spacecraft will perform its last close pass of Hyperion, Saturn’s curiously spongelike moon. At approximately 9:36 a.m. EDT (13:36 UTC) it will zip past Hyperion at a distance of about 21,000 miles (34,000 km) – not its closest approach ever but considerably closer (by 17,500 miles/28,160 km) than it was when the image above was acquired.*

This will be Cassini’s last visit of Hyperion. It will make several flybys of other moons within Saturn’s equatorial plane over the course of 2015 before shifting to a more inclined orbit in preparation of the end phase of its mission and its operating life in 2017.

At 255 x 163 x 137 miles (410 x 262 x 220 km) in diameter, Hyperion is the largest of Saturn’s irregularly-shaped moons. Researchers suspect it’s the remnant of a larger body that was blown apart by an impact. Hyperion’s craters appear to have a “punched-in” look rather than having been excavated, and have no visible ejecta or secondary craters nearby.

Impactors tend to make craters by compressing the surface material, rather than blasting it out. (NASA/JPL/SSI. Edit by J. Major.) — Impacts on Hyperion tend to “punch in” the surface material, rather than blasting it out. (NASA/JPL/SSI. Edit by J. Major.)

Hyperion orbits Saturn in an eccentric orbit at a distance of over 920,000 miles (1.48 million km)…that’s almost four times the distance our Moon is from us! This distance – as well as constant gravitational nudges from Titan – prevents Hyperion from becoming tidally locked with Saturn like nearly all of its other moons are. In fact its rotation is more of haphazard tumble than a stately spin, making targeted observations of any particular regions on its surface virtually impossible.

Images from the May 31 flyby are expected to arrive on Earth 24 to 48 hours later.

As small as it is Hyperion is Saturn’s eighth-largest moon, although it appears to be very porous and has a density half that of water. Read more about Hyperion here and see more images of it from Cassini here and here.

Source: NASA

*Cassini did come within 310 miles (500 km) of Hyperion on Sept. 26, 2005, but the images to make up the view above were acquired during approach.

UPDATE June 1, 2015: the raw images from Cassini’s flyby have arrived on Earth, check out a few below. (Looks like Cassini ended up with the same side of Hyperion again!)

Hyperion on May 31, 2015. Credit: NASA/JPL-Caltech/SSI. (Minor editing by J. Major.)

Hyperion on May 31, 2015. Credit: NASA/JPL-Caltech/SSI.

January 9, 2015December 23, 2015 by Elizabeth Howell

How NASA Is Saving Fuel On Its Outer Solar System Missions

While Saturn is far away from us, scientists have just found a way to make the journey there easier. A new technique pinpointed the position of the ringed gas giant to within just two miles (four kilometers).

It’s an impressive technological feat that will improve spacecraft navigation and also help us better understand the orbits of the outer planets, the Jet Propulsion Laboratory (JPL) said.

It’s remarkable how much there is to learn about Saturn’s position given that the ancients discovered it, and it’s easily visible with the naked eye. That said, the new measurements with the Cassini spacecraft and the Very Long Baseline Array radio telescope array are 50 times more precise than previous measurements with telescopes on the ground.

“This work is a great step toward tying together our understanding of the orbits of the outer planets of our solar system and those of the inner planets,” stated study leader Dayton Jones of JPL.

Saturn and its rings, as seen from above the planet by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute. Assembled by Gordan Ugarkovic. — Saturn and its rings, as seen from above the planet by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute/Gordan Ugarkovic

What’s even more interesting is scientists have been using the better information as it comes in. Cassini began using the improved method in 2013 to improve its precision when it fires its engines.

This, in the long term, leads to fuel savings — allowing the spacecraft a better chance of surviving through the end of its latest mission extension, which currently is 2017. (It’s been orbiting Saturn since 2004.)

The technique is so successful that NASA plans to use the same method for the Juno spacecraft, which is en route to Jupiter for a 2016 arrival.

Juno will repeatedly dive between the planet and its intense belts of charged particle radiation, coming only 5,000 kilometers (about 3,000 miles) from the cloud tops at closest approach. (NASA/JPL-Caltech)

Scientists are excited about Cassini’s mission right now because it is allowing them to observe the planet and its moons as it reaches the summer solstice of its 29-year orbit.

This could, for example, provide information on how the climate of the moon Titan changes — particularly with regard to its atmosphere and ethane/methane-riddled seas, both believed to be huge influencers for the moon’s temperature.

Beyond the practical applications, the improved measurements of Saturn and Cassini’s position are also giving scientists more insight into Albert Einstein’s theory of general relatively, JPL stated. They are taking the same techniques and applying them to observing quasars — black-hole powered galaxies — when Saturn passes in front of them from the viewpoint of Earth.

Source: Jet Propulsion Laboratory

December 30, 2014December 23, 2015 by Elizabeth Howell

Where Did Europa’s Water Geyser Go? Hubble Double-Checking Its Work

Rendering showing the location and size of water vapor plumes coming from Europa's south pole. Credit: NASA/ESA/L. Roth/SWRI/University of Cologne

It was about this time last year that Europa really began to excite us again. Following a sci-fi movie about the Jupiter moon, astronomers using the Hubble Space Telescope announced they had found possible water vapor near the icy moon — maybe from geysers erupting from its icy surface. (That is, if the finding was not due to signal noise, which researchers acknowledged at the time.)

As NASA ramped up (distant) plans to get close to Europa again, scientists began plumbing data from the Cassini spacecraft to see if its glance at the moon circa 2001 revealed anything. Turns out that the spacecraft didn’t see any sign of a plume. Which leads to the greater question, what is happening?

Now scientists are scurrying for a second look. Hubble is in the midst of a six-month search of the moon (from afar) to see if any more of the plumes are showing up. Now the theory is that the plumes, if they do exist, would be intermittent — at least, that’s according to the team looking at data from Cassini’s ultraviolet imaging spectograph (UVIS).

Europa (bottom left) in orbit around its planet, Jupiter, as spotted from the Cassini spacecraft in 2000. Credit: NASA/JPL/University of Arizona

“It is certainly still possible that plume activity occurs, but that it is infrequent or the plumes are smaller than we see at Enceladus,” stated co-author Amanda Hendrix, a Cassini UVIS team member with the Planetary Science Institute in Pasadena. “If eruptive activity was occurring at the time of Cassini’s flyby, it was at a level too low to be detectable by UVIS.”

This finding was part of a greater set of observations showing that it’s not really Europa that is contributing plasma (superheated gas) to space — it’s the ultra-volcanic moon Io. And Europa itself is sending out 40 times less oxygen than previously believed to the area surrounding the moon.

“A downward revision in the amount of oxygen Europa pumps into the environment around Jupiter would make it less likely that the moon is regularly venting plumes of water vapor high into orbit, especially at the time the data was acquired,” NASA stated. This would stand in contrast to, say, Saturn’s Enceladus — which Cassini has seen sending plumes high above the moon’s surface.

The findings were presented at the American Geophysical Union meeting earlier this month and also published in the Astrophysical Journal. The research was led by Don Shemansky, a Cassini UVIS team member with Space Environment Technologies.

Source: Jet Propulsion Laboratory

December 30, 2014December 23, 2015 by Elizabeth Howell

Best Space Photos Of 2014 Bring You Across The Solar System

A raw shot from the front hazcam of NASA's Opportunity rover taken on Sol 3757, on Aug. 19, 2014. Credit: NASA/JPL-Caltech

Feel like visiting a dwarf planet today? How about a comet or the planet Mars? Luckily for us, there are sentinels across the Solar System bringing us incredible images, allowing us to browse the photos and follow in the footsteps of these machines. And yes, there are even a few lucky humans taking pictures above Earth as well.

Below — not necessarily in any order — are some of the best space photos of 2014. You’ll catch glimpses of Pluto and Ceres (big destinations of 2015) and of course Comet 67P/Churyumov–Gerasimenko (for a mission that began close-up operations in 2014 and will continue next year.) Enjoy!

The Philae that could! The lander photographed during its descent by Rosetta. Credit: ESA/Rosetta/MPS for Rosetta Team/

The Aurora Borealis seen from the International Space Station on June 28, 2014, taken by astronaut Reid Wiseman. Credit: Reid Wiseman/NASA.

NASA's Mars Curiosity Rover captures a selfie to mark a full Martian year -- 687 Earth days -- spent exploring the Red Planet. Curiosity Self-Portrait was taken at the 'Windjana' Drilling Site in April and May 2014 using the Mars Hand Lens Imager (MAHLI) camera at the end of the roboic arm. Credit: NASA/JPL-Caltech/MSSS — NASA’s Mars Curiosity Rover captures a selfie to mark a full Martian year — 687 Earth days — spent exploring the Red Planet. Curiosity Self-Portrait was taken at the ‘Windjana’ Drilling Site in April and May 2014 using the Mars Hand Lens Imager (MAHLI) camera at the end of the roboic arm. Credit: NASA/JPL-Caltech/MSSS

This global map of Dione, a moon of Saturn, shows dark red in the trailing hemisphere, which is due to radiation and charged particles from Saturn's intense magnetic environment. Credit: NASA/JPL/Space Science Institute — This global map of Dione, a moon of Saturn, shows dark red in the trailing hemisphere, which is due to radiation and charged particles from Saturn’s intense magnetic environment. Credit: NASA/JPL/Space Science Institute

Comet Siding Spring shines in ultraviolet in this image obtained by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Credit: Laboratory for Atmospheric and Space Physics/University of Colorado; NASA

This "movie" of Pluto and its largest moon, Charon b yNASA's New Horizons spacecraft taken in July 2014 clearly shows that the barycenter -center of mass of the two bodies - resides outside (between) both bodies. The 12 images that make up the movie were taken by the spacecraft’s best telescopic camera – the Long Range Reconnaissance Imager (LORRI) – at distances ranging from about 267 million to 262 million miles (429 million to 422 million kilometers). Charon is orbiting approximately 11,200 miles (about 18,000 kilometers) above Pluto's surface. (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute) — This “movie” of Pluto and its largest moon, Charon b yNASA’s New Horizons spacecraft taken in July 2014 clearly shows that the barycenter -center of mass of the two bodies – resides outside (between) both bodies. The 12 images that make up the movie were taken by the spacecraft’s best telescopic camera – the Long Range Reconnaissance Imager (LORRI) – at distances ranging from about 267 million to 262 million miles (429 million to 422 million kilometers). Charon is orbiting approximately 11,200 miles (about 18,000 kilometers) above Pluto’s surface. (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

The Mars Reconnaissance Orbiter took this image of a "circular feature" estimated to be 1.2 miles (2 kilometers) in diameter. Picture released in December 2014. Credit: NASA/JPL-Caltech/University of Arizona — The Mars Reconnaissance Orbiter took this image of a “circular feature” estimated to be 1.2 miles (2 kilometers) in diameter. Picture released in December 2014. Credit: NASA/JPL-Caltech/University of Arizona

Jets of gas and dust are seen escaping comet 67P/C-G on September 26 in this four-image mosaic. Click to enlarge. Credit: ESA/Rosetta/NAVCAM

Ceres as seen from the Earth-based Hubble Space Telescope in 2004 (left) and with the Dawn spacecraft in 2014 as it approached the dwarf planet. Hubble Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), L. McFadden (University of Maryland, College Park), and M. Mutchler and Z. Levay (STScI). Dawn Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Photo Combination: Elizabeth Howell

December 29, 2014December 23, 2015 by Elizabeth Howell

Incredible Towering Structures Cast Shadows Across Saturn’s Rings

Vertical structures cause shadows on Saturn's B ring in this August 2009 picture from the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute

From a distance, Saturn’s rings look like a sheer sheet, but peer up close and you can see that impression is a mistake. Shadows from rubble believed to be two miles (3.2 kilometers) high are throwing shadows upon the planet’s B ring in this image from the Cassini spacecraft.

While the picture is from 2009, it caught the eye of the lead of the Cassini imaging team, who wrote eloquently about it in a blog post recently celebrating the link between wonder and the holidays.

“I have often thought: What a surreal sight this would be if you were flying low across the rings in a shuttle craft. To your eyes, the rings would seem like a gleaming white, scored, gravelly sheet below you, extending nearly to infinity,” wrote Carolyn Porco, the lead imager for the mission at the Cassini Imaging Central Laboratory for Operations (CICLOPS).

“And as you flew, you would see in the distance a wall of rubble that, eventually, as it neared, you would come to realize towered two miles above your head. There isn’t another sight like it in the Solar System!”

A 2007 artist impression of the aggregates of icy particles that form the 'solid' portions of Saturn's rings. These elongated clumps are continually forming and dispersing. The largest particles are a few metres across.They clump together to form elongated, curved aggregates, continually forming and dispersing. Credit: NASA/JPL/Univ. of Colorado — A 2007 artist impression of the aggregates of icy particles that form the ‘solid’ portions of Saturn’s rings. These elongated clumps are continually forming and dispersing. The largest particles are a few metres across.They clump together to form elongated, curved aggregates, continually forming and dispersing. Credit: NASA/JPL/Univ. of Colorado

Besides the inherent beauty and delicacy of this picture, another notable feature is how hard it is to capture. According to CICLOPS, one can only take this photo during Saturn’s equinox — once every 15 years in Earth time! That’s because the angle of the Sun’s light reaches the plane of the rings, allowing shadows to fall. The area itself is likely filled with moonlets of a kilometer (0.62 miles) in size.

“It is possible that these bodies significantly affect the ring material streaming past them and force the particles upward, in a ‘splashing’ manner,” the CICLOPS website notes.

We’ve included more pictures of Saturn’s rings below, all taken from the Cassini spacecraft. The machine is healthy and working hard after about 10.5 years working at the planet. One of its major tasks now is to observe changes in the planet and particularly its large moon, Titan, as the system nears the solstice.

Saturn's rings. Credit: NASA/JPL/Space Science Institute. — Saturn’s rings. Credit: NASA/JPL/Space Science Institute.

Enceladus and Tethys hang below Saturn's rings in this image from the Cassini spacecraft. Credit: NASA/JPL-Caltech/SS — Enceladus and Tethys hang below Saturn’s rings in this image from the Cassini spacecraft. Credit: NASA/JPL-Caltech/SS

Raw Cassini image of Titan and Enceladus backdropped by Saturn's rings. Image Credit: NASA/JPL/Space Science Institute — Raw Cassini image of Titan and Enceladus backdropped by Saturn’s rings. Image Credit: NASA/JPL/Space Science Institute

A close look at Enceladus, with Saturn's rings in the background. Credit: NASA/JPL/Space Science Institute — A close look at Enceladus, with Saturn’s rings in the background. Credit: NASA/JPL/Space Science Institute

The Cassini spacecraft looks close at Saturn to frame a view encompassing the entire C ring. Image credit: NASA/JPL/SSI

Raw image of Saturn's rings. Credit: NASA/JPL/Space Science Institute — Raw image of Saturn’s rings. Credit: NASA/JPL/Space Science Institute

Rhea poses with Saturn's rings; Janus and Prometheus are off in the distance. Credit: NASA/JPL/Space Science Institute. Click for larger version — Rhea poses with Saturn’s rings; Janus and Prometheus are off in the distance. Credit: NASA/JPL/Space Science Institute. Click for larger version

Spokes visible in Saturn's B ring. Credit: NASA/JPL/Space Science Institute — Spokes visible in Saturn’s B ring. Credit: NASA/JPL/Space Science Institute

Looming vertical structures, seen here for the first time and created by Saturn's moon Daphnis, rise above the planet's otherwise flat, thin disk of rings to cast long shadows in this Cassini image. Credit: CICLOPS — Looming vertical structures, seen here for the first time and created by Saturn’s moon Daphnis, rise above the planet’s otherwise flat, thin disk of rings to cast long shadows in this Cassini image. Credit: CICLOPS

December 17, 2014December 23, 2015 by Elizabeth Howell

Gallery: Saturn Moons Show How Not To Be Seen In Cassini Images

Tethys is mostly obscured behind Rhea as the moons orbit Saturn. The picture was captured by the Cassini spacecraft in April 2012 and highlighted in December 2014. Credit: NASA/JPL-Caltech/Space Science Institute

Peekaboo! Tethys makes a (mostly in vain) attempt to hide behind Rhea in this picture taken by the Cassini spacecraft a couple of years ago, but highlighted by NASA in a recent picture essay. Besides the neat view of the orbital dance, one thing that is clearly visible between the two moons is the different colors — a product of their different surfaces. It turns out that Tethys’ bright surface is due to geysers from another moon.

“Scientists believe that Tethys’ surprisingly high albedo is due to the water ice jets emerging from its neighbor, Enceladus,” NASA stated. “The fresh water ice becomes the E ring [of Saturn] and can eventually arrive at Tethys, giving it a fresh surface layer of clean ice.”

Saturn has an astounding number of moons — 62 moons discovered so far, and 53 of them named, if you don’t count the spectacular ring that surrounds the planet. The collection of celestial bodies includes Titan, the second-biggest moon in the Solar System. It’s so big that it includes a thick atmosphere. (Ganymede, around Jupiter, is the biggest.)

Below are some other pictures of moons dancing around Saturn — some harder to spot than others. All images were taken by the Cassini spacecraft since it arrived at the planet in 2004.

Titan peeks from behind two of Saturn's rings. Another small moon Epimetheus, appears just above the rings. Credit: NASA/JPL/Space Science Institute — Titan peeks from behind two of Saturn’s rings. Another small moon Epimetheus, appears just above the rings. Credit: NASA/JPL/Space Science Institute

Saturn's moons Dione and Rhea appear conjoined in this optical illusion-like image taken by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute — Saturn’s moons Dione and Rhea appear conjoined in this optical illusion-like image taken by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute

Saturn's rings, made dark in part as the planet casts its shadow across them, cut a striking figure before Saturn's largest moon, Titan. Credit: NASA/JPL/Space Science Institute — Saturn’s rings, made dark in part as the planet casts its shadow across them, cut a striking figure before Saturn’s largest moon, Titan. Credit: NASA/JPL/Space Science Institute

Three of Saturn's moons bunch together in this image by Cassini. Credit: NASA/JPL/Space Science Institute. Click for larger image. — Three of Saturn’s moons bunch together in this image by Cassini. Credit: NASA/JPL/Space Science Institute. Click for larger image.

Saturns rings with Saturns moon Mimas in the foreground (credit: NASA) — Saturn’s rings with Saturn’s moon Mimas in the foreground (credit: NASA)

Titan and Tethys line up for a portrait of 'sibling' moons. Credit: NASA/JPL/Space Science Institute — Titan and Tethys line up for a portrait of ‘sibling’ moons. Credit: NASA/JPL/Space Science Institute

December 12, 2014December 23, 2015 by Elizabeth Howell

Freak Fast Winds Created Titan’s Massive, Mysterious Dunes

Titan's surface is almost completely hidden from view by its thick orange "smog" (NASA/JPL-Caltech/SSI. Composite by J. Major)

Titan is Saturn’s largest moon and is constantly surprising scientists as the Cassini spacecraft probes under its thick atmosphere. Take its dunes, for example, which are huge and pointed the wrong way.

Why are they pointing opposite to the prevailing east-west winds? It happens during two rare wind reversals during a single Saturn year (30 Earth years), investigators suggest.

Investigators repurposed an old NASA wind tunnel to simulate how Titan is at the surface, watching how the wind affects sand grains. (They aren’t sure what kind of sand is on Titan, so they tried 23 different kinds to best simulate what they think it is, which is small hydrocarbon particles that are about 1/3 the density of what you find on Earth.)

After two years of work with the model — not to mention six years of refurbishing the tunnel — the team determined that the wind must blow 50% faster than believed to get the sand moving.

Dunes on Titan seen in Cassini's radar (top) that are similar to Namibian sand dunes on Earth. The features that appear to be clouds in the top picture are actually topographic features. Credit: NASA — Dunes on Titan seen in Cassini’s radar (top) that are similar to Namibian sand dunes on Earth. The features that appear to be clouds in the top picture are actually topographic features. Credit: NASA

“It was surprising that Titan had particles the size of grains of sand—we still don’t understand their source—and that it had winds strong enough to move them,” stated Devon Burr, an associate professor at the University of Tennessee Knoxville’s earth and planetary science department, who led the research. “Before seeing the images, we thought that the winds were likely too light to accomplish this movement.”

The winds reverse when the Sun moves over the equator, affecting Titan’s dense atmosphere. And the effects are powerful indeed, creating dunes that are hundreds of yards (or meters) high and stretch across hundreds of miles (or kilometers).

To accomplish this, the winds would need to blow no slower than 3.2 miles per hour (1.4 meters per second), which sounds slow until you consider how dense Titan’s atmosphere is — about 12 times thicker surface pressure than what you would find on Earth. More information on the research is available in the journal Nature.

Sources: Arizona State University and the University of Tennessee, Knoxville.