Plate tectonics on Europa

Plate tectonics on Europa

Europa, named after a lover of Zeus, is the fourth largest moon of Jupiter. Europa is more massive than all moons smaller than itself combined. Still, it is the smallest of the four Galilean satellites, trailing behind Ganymede, Callisto, and Io. Our moon also carries slightly more mass than Europa.

Jupiter and the Galilean satellites, Io, Europa, Ganymede, and Callisto
Montage of photographs by Voyager 1

This distant icy moon remains a fascinating place. Europa has the smoothest face of all solid body objects in the Solar System. The smoothness of Europa supports evidence that beneath the surface an ocean of liquid water flows. Intermittently, the surface cracks or rifts apart allowing liquid water to flow on the surface. The formation and later processing of these cracks has shown Europa to be the first place, other than Earth, where plate tectonics play a role in shaping surface geography.

Earth, Moon, and Europa shown to scale
Image credits: NASA/JPL

The face of Europa is smooth, but highly distinctive. Europa has character; striking and dramatic dark lines crisscross the entire globe. These lines, or lineae, measure up to 20 km (12 mi) across. Along the edges, these bands can run a dark umber, while the center of the stripes shows some clearing. The borders of these dark streaks at times blush, tending to blend into the surrounding surface features. The smoothed over, bluish regions contain predominately water ice, while the darker strikes are enriched in other minerals.

Mid-Atlantic Rift in Iceland
Credit: Debivort CC BY-SA

Detailed images returned from the Voyager and Galileo spacecraft reveal that opposite edges of these rifts have moved laterally over time. Thus, it seems that the surface of Europa is cracking and separating, driven by pressures beneath. As this is happening the emerging plates slide along next to one another. 

The tidal forces wrenching on Europa from the immensity of Jupiter account for these incipient cracks along the face. Since Europa is locked in position with Jupiter, the same side always facing the planet, it could be expected that the cracks would take on predictable patterns. However, that is not what the images show.

The reason may be that the crust of Europa appears to be rotating slightly faster than the moon's interior. It takes a long time for these effects to be noticeable. Older fractures crossing the face of Europa stray farther in their orientation than fresh fractures. Astronomers have now compared back-logged images or Europa from the Voyager spacecraft with more recent images from Galileo. They estimate that it takes at least 12,000 years for the outer crust of Europa to make a complete revolution over the interior. Additional images demonstrate evidence for subduction of surface ice. As plates cross over each other, material is forced beneath the surface for recycling.

Complex surface geology of Europa
Composite images from Galileo spacecraft 

All of this offers proof that Europa is the first world beyond our own with a surface governed by plate tectonics. Since Galileo Galilei discovered Europa in 1610, it remains a place of wonder. We now see that beneath Europa, a subsurface ocean of liquid water flows. What that ocean may hold and how it relates to possible life beyond Earth, is a big question right now. The upcoming Europa Clipper mission will hopefully provide answers.

From the ice of Saturn's rings, the moons emerge

Origins of an Ice Moon 

Part 2

Tethys, Enceladus and Mimas

NASA/Cassini, via Wikimedia Commons

In 1610, Galileo Galilei peered through the eyepiece of his telescope at the giant planet Saturn. He saw more details of the planet than any prior human in history. Galileo's telescope was the best in the world. He still could not fully resolve the rings of Saturn clearly enough to even appreciate that they were rings. Later, astronomers came along with better telescopes. Astronomers like Huygens, Hooke, and Cassini resolved the shadow of Saturn on the ring and the major divisions of the rings. Cassini also discovered a number of moons orbiting Saturn, including Tethys, Iapetus, Rhea, and Dione.


Centuries later, we are still not certain exactly how the rings formed. Modern missions of space exploration to Saturn include: Pioneer 11, Voyager 1, Voyager 2, and the current mission Cassini. Along with extraordinary amounts of data, these space probes returned beautiful images of the Saturn system.

Voyager 2 image of Saturn, the rings and four moons
NASA (1981), via Wikimedia Commons

From the ice of Saturn's rings: moons emerge

We now know that the rings of Saturn consist primarily of ice. Around 90 to 95 percent of the mass of Saturn's rings is water ice. What remains may be in essence pollution from relentless pummeling of the rings by meteors and space debris. The formation of Saturn's rings is a mystery, but we are getting closer to an answer.

Over generations, astronomers developed countless theories on how the rings formed. One theory suggested that Saturn's rings formed when a small moon collided and broke apart. Another suggested that a passing comet shattered under the tidal pull of the giant planet.

In 2010, astrophysicist Robin Canup, writing in the Journal Nature presented numerical models looking at development of the rings of Saturn. She points out that old theories failed to explain why there is so much ice and relatively little rock.

Dr Canup may have found an explanation for not only why the rings of Saturn are so rich in ice. Her mathematical simulation also described the process leading to the birth of icy moons orbiting near the planet. Additional information from Cassini suggests that at least one of these icy moons, Enceladus, could support life.

Daphnis, an inner moon of Saturn, disturbing the rings
NASA/JPL/SSI, via Wikimedia Commons

The lost moon of Saturn

Saturn likely had another moon in the remote past, around the size of Titan. Titan is a giant among moons. Even larger than the planet Mercury, Titan is the second largest moon in the Solar System. The fate of a lost sibling moon of Titan may hold the secret of the rings. At some point, tidal forces of Saturn stripped off the lost moon's thick outer crust of ice. On some level, perhaps, Galileo's joke about Saturn devouring his children may have been true.

As the ice was stripped away, the moon become nothing more than naked rock. This core material, under the pull of gravity, tumbled into oblivion beneath the shroud of Saturn's atmosphere. For a while, the ice stripped away remained spread diffusely across space around Saturn, one vast homogeneous ring.

With time, ripples in the rings developed through small collisions. Larger accumulations of ice formed and continued to collide. Eventually gaps between rings emerged and the Cassini Division came into being. Dr Canup's model predicts that the amount of material in the rings decreases as these events unfold. Meanwhile, the chunks of debris come together and moons are born. Her predictions fit the masses observed of Tethys, Enceladus, and other Ice Moons of Saturn.

Ice Moon Station will publish more about Saturn in future posts.

Subscribe by Email, or follow us on Facebook and Twitter for updates.

Icy Tethys: the Second-brightest Moon of Saturn

Tethys

Tethys seen 900,000 miles from Saturn

Credit: NASA/JPL/SSI

The Cassini spacecraft sent home images of Tethys starting in 2004, that made this mid-sized moon of Saturn an international celebrity. Odysseus, the largest crater on Tethys, bears striking resemblance the to the laser array on Darth Vader's malevolent Death Star station. (Another ice moon of Saturn, Mimas, looks even more convincing). The resemblance is coincidental, as high definition images were not obtained until after George Lucas and his team were already at work on the Star Wars film.




Italian astronomer Giovanni Cassini discovered Tethys in 1684, along with three other moons of Saturn: Iapetus, Rhea, and Dione. The Cassini spacecraft, named in his honor, sent back phenomenal images of Tethys, along with other detailed information. Prior to the Cassini flyby, our modern understanding of Tethys came primarily from three earlier space probe missions to the outer Solar System.

Tethys, seen by Cassini in 2005
Credit: NASA/JPL/SSI

On September 1, 1979,  Pioneer 11 made its closest approach to Tethys, at a distance of 330,000 km. Next, Voyager 1 flew past in 1980, sending back detailed images including that of a giant rift valley, the Ithaca Chasm.  The chasm reaches three-quarters of the way around Tethys, making it one of the longest rifts in the Solar System.  Tethys welcomed another visitor the following year when Voyager 2 flew past. The Voyager 2 spacecraft sent back images of Tethys from a distance of 594,000 km.  These images revealed the surface of Tethys as a contrasting terrain of densely cratered ancient surfaces among fresher, more recent plains. Tethys measures 1,060 km (660 mi) in diameter, orbiting deep within the magnetosphere of Saturn.

Voyager 2 image of Tethys 

Tethys is the least dense of all major moons in the Solar System. In simple terms, Tethys is not very heavy for its size. We now know from spectroscopic studies that water ice is the main constituent of the Tethysian surface. Water ice is generally less dense than other materials. That is why ice tends to float. Only a small portion of Tethys is rock, along with some as yet unidentified darker material. Tethys reflects back a high proportion of the light striking its surface. A rain of ice from the E ring of Saturn may explain why the surface of Tethys has such high albedo. Interestingly, the ice deposits of the E Ring may be a product of famous ice geysers from Enceladus.  Enceladus is even brighter than Tethys: the brightest moon in the Solar System.

Ice Moon Station will publish more on Tethys in future posts.  
Subscribe by Email, or follow us on Facebook and Twitter for updates.

Cassini spacecraft finds warmth beneath Enceladus' south pole

More evidence for subsurface ocean on Enceladus

Cassini photograph of Enceladus: October 28, 2015
(NASA/JPL)

Enceladus continues to offer up exciting new information. This icy moon of Saturn remains one of the most likely exotic places in our Solar System to support life. Since its discovery in 1789 by William Herschel, it has captivated astronomers. Over the last 40 years, space missions have targeted Saturn an its local system. Enceladus, along with other ice moons near the planet are some of the most fascinating and important. Currently, the Cassini probe is orbiting Saturn, returning compelling new information on a regular basis.  



Yesterday, March 13, 2017 and international team of astrophysicists published exciting new findings from Cassini in the journal Nature Astronomy. Using microwave RADAR technology, Cassini targeted an area close to one of the active cracks near the south pole of Enceladus. They found a warm region beneath the surface, that had never before been detected.  What this suggests is that a large ocean of warm water may be churning beneath an icy crust. It appears that this is happening within a few kilometers of the surface.

Enceladus discoveries have changed the direction of planetary science.
- Linda Spilker, Cassini Project Scientist

Enceladus may support life

Enceladus, with Titan in the background
(NASA/JPL-Caltech/SSI)

Since 2005, we have known that Enceladus is an active moon. That year Cassini returned images showing geyser-like plumes of debris escaping from cracks neat the south pole. Cassini witnessed over 100 geysers, spraying out over 440 pounds of material per second. Jets of water vapor, ice, and debris spew with torrential force from deep beneath the frozen surface of this remote outpost. Sophisticated analysis of the ice plumes of Enceladus, including spectroscopic studies, revealed evidence of organic molecules. In the presence of heat, water, and these molecules, we suspect that life could exist on Enceladus.

Icy jets from Enceladus
(NASA/JPL), via Wikimedia Commons

Much of this ejected matter falls back to the surface -- a rocky, salty, icy snow. This fresh dusting keeps Enceladus one of the most reflective surfaces in the Solar System. The rest of the debris escapes the gravity of the planet, joining up with material from other recent and ancient eruptions, to form the E ring around Saturn. From there, some of it may get deposited on other inner moons of Saturn, including Tethys. Where else it may go is uncertain, but conceivably some of it could piggyback on an asteroid or comet, disseminating life beyond. This process is described as panspermia. Extrapolating past this may seem the realm of science fiction. With future missions, including the upcoming Europa Clipper mission to a moon of Jupiter, this is an amazing time for ice moon expeditions.


Ice Moon Station will publish more on Enceladus and other Ice Moon discoveries.
Subscribe by Email, or follow us on Facebook and Twitter for updates.

Where did the ice for Saturn's inner moons come from?

Origins of an Ice Moon

Part 1

Tethys: Ice Moon among Saturn's Rings
Credit: NASA/JPL-Caltech/SSI, via Wikimedia Commons

Saturn has a lot going on.  The breath-taking rings steal most of the attention.  Every kid at some point has a T-shirt, a sticker, or a coloring book with Saturn's Rings proudly displayed. For most, this is enough: Saturn is the planet with the rings.  Others may wonder about the size of Saturn. They may sort out if it is closer or further off than Jupiter.  They may ponder what exists beneath the impenetrable atmosphere of Saturn. Fewer still might hang around long enough to notice the enormous moon, fittingly named Titan. 

The lucky ones hang around even longer. The Bright Red Spot on Jupiter may beckon, but they hang on. Is there something more for Saturn aficionados to savor, they ask? The Ice Moons are their reward. Ice Moons like Enceladus, Tethys, Mimas, and Dione are bit players in the grand drama of the Saturnian System. Yet their roles may be ultimately the most amazing. Emerging evidence suggest some, like Enceladus, might support life. So how did they come into being? To understand that, let us go back to the famous rings.

The rings of Saturn: how did they happen?

Saturn seen by the Cassini Orbiter in 2004
Credit: By NASA/JPL/SSI, via Wikimedia Commons

Saturn's rings, though easily one of the most recognized features in the Solar System, in many ways remain a great mystery. It has been over 400 years since their discovery, yet it seems that the question of how they formed is still out there.

Let's start with the discovery of the rings. Early humans looked up in antiquity to witness the wandering planets. In time, humans developed technology capable of probing deeper into the nature of these mysterious bodies. With the invention of the telescope, finer details emerged.

Galileo Galilei sighted the giant planet Saturn with his telescope in 1610. At the time, his telescope was the cutting edge: a pinnacle of human achievement. Even so, initially Galileo could not make out the presence of the rings. What he saw was certainly not the perfect orb he expected. Uncertain what to make of the images, he considered that there were three separate planets: the largest in the middle, Saturn.

"Has Saturn swallowed his children?"

Early drawing of Saturn, summer 1666

Credit: Robert Hooke, via Wikimedia Commons

Two years later, the orientation of Saturn's rings shifted, angled such that they were no longer visible from Earth. Galileo, and the world astronomy community were at a loss.

In mythology, prophecy held that Saturn would be overthrown by his own children. To prevent this from playing out, he devoured them. When Galileo looked up to see that the planet Saturn now stood alone, he perhaps jokingly wondered if this had played out in the sky above. Galileo pondered, "has Saturn swallowed his children?" When the rings returned to view some time later, Galileo still had no answer.

After another generation of technological advances, astronomers were finally able to clearly see the rings. In the late 1600s, a new group of astronomers came along, better equipped. Christiaan Huygens described the rings in finer detail. Robert Hooke resolved the shadow of the planet upon the rings. Giovanni Domenico Cassini discovered a wide gap, since known as the Cassini Division.

 These were the first of many discoveries to bring us closer to an understanding of the rings of Saturn. We were learning more about what they are, yet it remained a mystery as to how they formed. 

Ice Moon Station will publish more Origins of an Ice Moon in future posts.
Subscribe by Email, or follow us on Facebook and Twitter for updates.

Enigmatic Face of an Ice Moon: Europa

Europa: Ice Moon of Jupiter

Jupiter and the Galilean Satellites

Top to bottom: Io, Europa, Ganymede, Callisto
Credit: NASA/JPL/DLR (Public Domain)

Discovery of Europa and the Galilean Moons

Europa, an ice moon of the giant planet Jupiter, looms large in the Solar System. Only five other moons in our Solar System outsize Europa; three of them join her in orbit around Jupiter. Italian astronomer Galileo Galilei first witnessed Europa over 400 hundred years ago on January 8, 1610. Unleashing one of the great controversies of the period, Galileo famously discovered a Jovian system of orbiting moons, including Europa. Coincidentally, this occurred within a very short time of Europa's discovery by another famous astronomer, Simon Marius. Subsequently, each astronomer held firm in a debate over who first cast their sight upon the moon. Ultimately, Galileo first published his discovery. Galileo named these moons "Cosimo's stars", as an homage to Cosimo II de' Medici, Grand Duke of Tuscany.

Marius was a German astronomer with highly respected skills of astronomical observation. He we went on to describe the orbits of these moons with more precision that Galileo. Marius discussed his discovery of the Jupiter moons with another famous astronomer of the time, Johannes Kepler. They decided to name these four moons after mythological lovers of Zeus (the Greek equivalent of Jupiter). The world astronomy community accepted these names, despite protests from Galileo and the names have held ever since. In order from smallest to largest, the Galilean satellites are: Europa, Io, Callisto, and Ganymede.

Europa, still an enigma.
Pioneer 10 in 1973
Credit: NASA (Public Domain), via Wikimedia Commons

Detailed Views of Europa Emerge

The familiar variegated geography of our Earth's one moon presents broad open flat-lands, along with harsh ridges, steep valleys, and dramatic craters. We know intimate details of these features thanks to intrepid missions of the Apollo era: from the placid station at the Sea of Tranquility, to the debris laden field of Fra Mauro, and the tumultuous complex features of Hadley–Apennine. Europa, meanwhile, reveals a visage smoother than any other solid object in the Solar System. Centuries of Earthbound telescopic observation had not prepared astronomers for the striking image of Europa. Modern space probe flybys revealed a smooth faced, ice-encrusted moon.

Europa: Galileo spacecraft 1996
Credit: NASA/JPL/DLR (Public Domain), via Wikimedia Commons

The Pioneer 10 spacecraft, launched in 1972, started things off. Pioneer 10 offered up a tantalizing image of Europa, sorely lacking in detail. Astronomers glimpsed a highly reflective surface, with an albedo similar to that of ocean ice. Europa, in fact, shines among the brightest of moons in our Solar System. (The brightest moon, Enceladus, the ice moon of Saturn, reflects nearly one hundred percent of incident sunlight.) 

Pioneer 10 produced a treasure trove of information about Jupiter and its satellites.  However, Europa was not the primary target. Most of what we now know about Europa derives from the accomplishments of an expedition launched in 1989, the Gallileo mission.  Meanwhile, the Hubble Space Telescope detected stunning evidence of water vapor plumes. We have seen other hints of a complex, teeming subsurface world of Europa. More than some far fetched fantasy, there is a reasonable suspicion that Europa might harbor simpler forms of extraterrestrial life. Future missions to Europa are in the works, including the Europa Clipper. This mission, set to launch around 2022, will include multiple flybys, and even more exciting, a landing craft.

Ice Moon Station will publish more on Europa, Pioneer 10, the Gallileo mission, and upcoming missions in future posts.  Subscribe by Email, or follow us on Facebook and  Twitter for updates. 

Dune Buggies on the Moon, Part 3

Driving the Moon Buggy

Apollo 17 Lunar Roving Vehicle
Image: NASA (Public domain)

The Lunar Rover moved about under power from four drive motors and two steering motors.  The mission commander controlled the motors and brakes while sitting in the left hand driver's seat.  Moving the T-shaped controller forward would cause the buggy to accelerate, while pulling back would trigger the brakes.  The commander moved the controller left or right to steer and he could push a switch on the controller to back it up.  When parked, the brake could be set by pulling the controller all the way back.  Operational updates on motor temperatures, velocity, heading, and energy reserve were provided by gauges mounted nearby.  Fail-safe navigational aids with more advanced instruments as well as Sun tracking devices provided additional security.  The buggy was never driven so far from the landing module that the astronauts could not walk back without exhausting their life support systems.

Read more here: 
Dune Buggies on the Moon, Part 1 

Dune Buggies on the Moon, Part 2

Ice Moon Station will publish more on the Apollo missions in future posts.  
Subscribe by Email, or follow us on Facebook and Twitter for updates.