Slightly smaller than Earth’s moon, Jupiter’s moon ‘Europa' is the sixth closest to the gas giant and the smallest of the 67 Galilean satellites.
Europa has been an other-worldly wonder for planetary scientists, but more so for astrobiologists due to the intriguing cracks and fissures among the icy crust. These cracks or ‘lineae’, are indicative of geologic activity similar to plate tectonics, which we experience and study on Earth as the crust of our planet moves and slides beneath our feet via the lithosphere, driving what is known as ‘continental drift' amongst geoscientists.
Crack/rift in the Antarctic ice shelf of the Pine Island glacier photographed by NASA/GSFC
This image, taken by the Galileo space probe, reveal rust-colored fissures or ‘lineae’, similar to Earth’s oceanic ridges, which force fresh material upward from below the subsurface, effectively replacing/repairing these surface blemishes.
Continental drift results in subduction (see subduction zones) involving the movement of tectonic plates; in Earth’s case, these are the oceanic and continental crusts. The catalysts for this geologic activity are our distance from the sun and (Earth’s) moon. A similar relationship exists between Jupiter and Europa.
Why is this important?
Being as far (or close) to Jupiter as it is along with an accompanying eccentric (oval) orbit, vulnerable Europa is thought to experience “tidal flexing" whereby the ocean beneath its surface is pulled and pushed by the tremendous gravitational force exerted by Jupiter. This process indeed flexes the icy crust of Europa to hyperextension of its subsurface, which is composed of a 62 mile-deep outer layer of water above a silicate mantle bolstered by an assumed iron/metallic core.
Subduction, subsurface tidal flexing, shifting of organic material…just think "lava lamp"…
What does this mean?
It means further exploration for closer examination is needed to determine anything. However, at this point, here’s what we’ve come to understand:
These artist’s drawings depict two proposed models of the subsurface structure of the Jovian moon, Europa. Geologic features on the surface, imaged by the Solid State Imaging (SSI) system on NASA’s Galileo spacecraft might be explained either by the existence of a warm, convecting ice layer, located several kilometers below a cold, brittle surface ice crust (top model), or by a layer of liquid water with a possible depth of more than 100 kilometers(bottom model).
Random Sagan Fact: Carl Sagan was the first to propose the possibility of a subsurface ocean underneath Europa’s icy crust.
If a 100 kilometer (60 mile) deep ocean existed below a 15 kilometer (10 mile) thick Europan ice crust, it would be 10 times deeper than any ocean on Earth and would contain twice as much water as Earth’s oceans and rivers combined. Unlike the Earth, magnesium sulfate might be a major salt component of Europa’s water or ice, while the Earth’s oceans are salty due to sodium chloride (common salt). While data from various instruments on the Galileo spacecraft indicate that an Europan ocean might exist, no conclusive proof has yet been found. To date Earth is the only known place in the solar system where large masses of liquid water are located close to a solid surface. Other sources are especially interesting since water is a key ingredient for the development of life as we know it. [source]
But wait! There’s more…
As we continue to scrupulously pore over the data via the Galileo spacecraft’s 7.75 year mission in the Jovian neighborhood, we are discovering more about this planetary body than we (especially Galileo himself) could’ve ever predicted.
Recently observed (in images the spacecraft took in 1998) are “clay-like” minerals called phyllosilicates. The origin of these minerals are being best explained by a collision with an asteroid (likely 3,600ft in diameter) or comet (5,600ft. in diameter). Results of this study can be found here.
If this explanation sounds a bit far-fetched, think again. Asteroid/comet impacts are frequent among the solar system, as the July 1994 event of Comet Shoemaker-Levy 9 reminded us. Astronomers witnessed the comet breaking apart, colliding with Jupiter and producing a 24,000 K (as in Kelvin) fireworks display rivaling 6,000,000 megatons of TNT or 600x the world’s nuclear arsenal. Shoemaker-Levy 9 alerted us to the cosmic shooting gallery we find ourselves in, along with our under preparedness for sporadic rendezvous with these double-edged, life-dispersing/revoking crumbs from our solar system’s early formation.
Orbits of 1,000 categorized potentially hazardous asteroids (PHA’s) [source]
Thus, the importance of space exploration and specifically, the recent measure introduced by the UN to formulate an asteroid defense plan.
Recommended: BBC Animated Guide To Shoemaker-Levy 9’s Impact With Jupiter; Impact Jupiter: The Crash of Shoemaker-Levy 9 by David H. Levy, co-discoverer of the comet
Although Shoemaker-Levy 9 may seem at first glance to be an isolated incident (broken up into its 21 constituent icy chunks, mind you…) it’s far from the first or last time Jupiter has been blindsided by a collision of such magnitude.
In 2009, a comet/asteroid/meteor the size of the Pacific Ocean struck Jupiter. Dubbed the ‘Wesley impact’ (due to the amateur astronomer who discovered it), the object was estimated to be about 200-500 meters in diameter. To put this in perspective: if this object would have collided with Earth, the devastation would be cataclysmic and crippling human civilization. [source]
If the allure of organic material-providing collisions isn’t enough of a driver toward an imperative mission to Europa, the most recent discovery of water vapor being ejected 200 kilometers (125 miles) above the surface via potential plumes should be a call to (scientific) arms.
"If those plumes are connected with the subsurface water ocean we are confident exists under Europa’s crust, then this means that future investigations can directly investigate the chemical makeup of Europa’s potentially habitable environment without drilling through layers of ice. And that is tremendously exciting,"
- Lorenz Roth of Southwest Research Institute in San Antonio and lead author of the Hubble Space Telescope discovery said in a NASA press release
The plumes were detected via “old faithful” herself, NASA’s Hubble Space Telescope. While viewing a “hot spot” in ultraviolet light on Europa’s south pole, the colors (wavelengths) of light revealed copious amounts of hydrogen and oxygen.
"The idea is that water erupting from Europa is exposed to space (Europa has no atmosphere). Jupiter has a ridiculously intense magnetic field, and electrons caught in that field are accelerated to high speed. These electrons slam into the water molecules from Europa, breaking them up into individual atoms of hydrogen and oxygen, which then reveal their presence by glowing in the ultraviolet.
Interestingly, earlier observations showed no trace of this light, and that actually supports the idea that this light is from a geyser. Why? Those older observations were taken when Europa was close to Jupiter, but the new observations were taken when Europa was farther away. This is critical: When the moon is close to Jupiter, the squeezing from the planet’s gravity is maximized, and when it’s farther away the squeezing is lowered. This means that any deep cracks in the surface are squeezed closed when Europa is near Jupiter and relaxed, opened up, when it’s farther away. If water from the subsurface ocean were to escape through cracks, it would be when they’re open. So these observations precisely fit the idea that this is what we’re seeing.”
- Phil Plait, Slate magazine
This artist concept illustrates two possible cut-away views through Europa’s ice shell. In both, heat escapes, possibly volcanically, from Europa’s rocky mantle and is carried upward by buoyant oceanic currents. [source]
How much water are we talking?
It’s being estimated that 7 tons of water are erupting every second at over 1,500 miles per hour. "Three times faster than a passenger jet," exclaims astronomer/science blogger Phil Plait. He continues in his recent article 'Europa Erupts! Possible Geyser of Water Seen on Jupiter’s Moon'…
"As it happens, we know of another moon with geysers: Saturn’s moon Enceladus. Europa is much larger than Enceladus (3,100 km versus 500 km) and so has much stronger gravity. That means that for a given speed for the water, the plume won’t stretch as high on Europa as it would on Enceladus.
Still, it’s worth comparing. On Enceladus, the plumes are higher, reaching 500 km (about 310 miles) off the surface, move more slowly at 300–500 meters per second (700–1,100 mph), and only out pump about 200 kilograms (about 450 pounds) of water per second. Because of Enceladus’ lower gravity, some of that material escapes from the moon into space. For Europa, with its stronger gravity, the material falls back to the surface where it freezes.
Dramatic plumes, both large and small, spray water ice out from many locations along the famed “tiger stripes” near the south pole of Saturn’s moon Enceladus. [source]
The geysers on Enceladus are also at the moon’s south pole—stress from Saturn’s gravity is strongest there, just as stress from Jupiter is strongest at Europa’s poles—and dozens have been found along long cracks colloquially called tiger stripes. The Cassini spacecraft has been orbiting Saturn for nearly a decade, and we have fantastic high-resolution images of Enceladus, allowing us to identify the regions in the cracks where the geysers originate.”
Do underground oceans vent through the tiger stripes on Saturn’s moon Enceladus? Long features dubbed tiger stripes are known to be spewing ice from the moon’s icy interior into space, creating a cloud of fine ice particles over the moon’s South Pole and creating Saturn’s mysterious E-ring. Evidence for this has come from the robot Cassini spacecraft now orbiting Saturn. Pictured above, a high resolution image of Enceladus is shown from a close flyby. [source]
To further demonstrate the importance of these plumes and what this means for planetary science/astrobiology, I invite you on a trip to Saturn, courtesy of Cassini Imaging Team lead and planetary scientist Carolyn Porco, as she delivers a Carl Sagan-esque TED talk on the visual wonders recovered by the Cassini spacecraft, along with one of the most intriguing moons in the solar system, Enceladus.
There will be so much more to come as this develops. For now, keep looking up and stay curious! The moons of Jupiter are a beautiful sight to view through a telescope; even more so now, as we continue to learn more about our neighboring planets and their accompanying satellites…