The Icy Moon That Toppled Over
By Judith E Braffman-Miller
Saturn’s ocean-bearing, icy moon Enceladus is a small, bright world of mystery. For decades, planetary scientists did not know why Enceladus was the brightest world in our entire Solar System, or how it was linked to its parent planet’s E ring, but NASA’s highly successful Cassini mission to the ringed gas-giant planet revealed the bewitching answers to these intriguing questions. Cassini found that both the fresh snowy coating on the surface of this distant moon-world, as well as the icy material within the E ring of Saturn, originate from vents that are connected to a subsurface global saltwater ocean concealed beneath the little moon’s icy, frozen crust–and where liquid water exists, there is the possibility, though not the promise, of life as we know it to exist, as well. In May 2017, planetary scientists with the Cassini mission announced yet another fascinating feature that Enceladus has kept well hidden from the prying eyes of curious astronomers. The new findings suggest that Enceladus may have toppled over in the distant past, perhaps due to a catastrophic collision with a smaller body, such as an asteroid, long ago when our Solar System was young–and dangerous.
Before Cassini arrived at the Saturn system, curious planetary scientist-explorers only had some tantalizing hints that there might be something interesting going on at Enceladus. Images taken from the Voyager spacecraft back in the 1980s suggested that even though this bright moon-world is small–only approximately 310 miles across–its icy surface shell is extremely smooth in some areas, and a dazzling, brilliant white all over. Indeed, Enceladus is the most reflective body in our Solar System.
The Cassini scientists reported that they had discovered evidence that the bright little moon-world’s spin-axis (the line through the north and south poles) has reoriented. The planetary scientists propose that this reorientation could be the result of a catastrophic collision with a smaller body, such as a wayward asteroid. Studying the moon’s features, the team of researchers demonstrated that Enceladus appears to have tipped away from its original axis by approximately 55 degrees–more than halfway toward rolling over completely onto its side.
“We found a chain of low areas, or basins, that trace a belt across the moon’s surface that we believe are the fossil remnants of an earlier, previous equator and poles,” commented Dr. Radwan Tajeddine in a May 30, 2017 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Tajeddine is a Cassini imaging team associate at Cornell University in Ithaca, New York, and lead author of the paper. The JPL is in Pasadena, California.
The region surrounding the bright little moon-world’s current south pole is a geologically active area where long, linear fractures playfully called tiger stripes slash across the icy surface. Dr. Tajeddine and his team propose that the culprit asteroid may have blasted into this region in the past when it was closer to the equator. “The geological activity in this terrain is unlikely to have been initiated by internal processes. We think that, in order to drive such a large reorientation of the moon, it’s possible that an impact was behind the formation of this anomalous terrain,” Dr. Tajeddine added in the May 20, 2017 JPL Press Release.
Exploring Saturn’s Realm
Saturn is the smaller of the duo of gas-giant planets dwelling in the outer Solar System–the banded behemoth Jupiter is by far the larger of the two. The other two gaseous giants of our Solar System’s outer regions, Uranus and Neptune, are classified as ice-giants, and sport larger solid cores and thinner atmospheres than the gas-giants. However, with its majestic system of magnificent rings, sparkling icy moons, dancing frozen moonlets, and other icy fragments, Saturn is arguably the most beautiful major planet in our Sun’s family.
Enceladus is a dazzling, brilliant little moon-world–showing off a sparkling white surface coating of highly reflective pristine ice. It also has an extremely active geology, which is the reason why its surface is almost completely crater-free. This is because Enceladus is constantly being resurfaced by gushing fountains of ice erupting from its many geysers. The geysers of this tiny moon-world account for the fresh, bright, pristine snow that keeps the surface of Enceladus smooth and dazzling.
The geyser eruptions on Enceladus are the result of its close proximity to its huge ringed parent-planet. The geyser jets erupting from the surface of Enceladus have been compared to adjustable water hose nozzles. The nozzles are opened widest when the moon is farthest from Saturn–and almost closed when it is closest to its parent-planet. Planetary scientists believe that this is caused by the way Saturn squeezes and then releases Enceladus as a result of its powerful gravitational grip on the tiny moon.
The water ice erupting from Enceladus’ numerous geysers are laced with organic particles that are hurled out from its tiger stripes. The tiger stripes were given their colorful nickname when planetary scientists noticed an interesting similarity between the fissures slashing through the south pole of the moon-world, and the stripes that decorate a tiger’s coat. The warm vents that are responsible for the tiger stripes erupt immense plumes of water vapor, organic particles, and ice hundreds of miles into space. This provides a gift of sorts to planetary scientists because it gives them a way to study the subsurface ocean from a distance–a very great distance.
NASA’s Cassini orbiter detected the jets of water vapor and icy particles spraying out from Enceladus’ tiger stripe fissures. This provided the first substantial indication that a subsurface ocean is venting directly into space from beneath the active fractures of the south polar terrain.
Before Cassini reached the Saturn system more than a decade ago, planetary explorers had only some small, but tantalizing hints, that there were some interesting things going on beneath the surface of that distant frozen moon-world. However, it was the data derived from the magnetometer aboard the Cassini spacecraft that sung a sirens’ song to scientists, luring them into taking a closer peek at Enceladus with a targeted flyby. Something, they did not know precisely what, was pushing against Saturn’s magnetic field in the vicinity of Enceladus. This indicated that gases may have been originating from the moon’s surface or interior.
Cassini finally unveiled the dramatic truth: Enceladus is an active moon that hides a global ocean of liquid salty water under its pristine crust of highly reflective ice. In addition, jets of icy particles from that ocean, laced with a blend of simple organic chemicals and water, erupt into space constantly from this bewitching moon. The material shoots out at approximately 800 miles per hour and creates a plume that reaches hundreds of miles into space. Some of that icy blend tumbles back down onto Enceladus, and some escapes to create Saturn’s vast E ring.
The E ring is primarily composed of little drops of ice, but among them are some strange nanoparticles. Cassini spotted these passing silica nanograins, which can only be generated where rock and liquid water do a dance together at temperatures greater that 200 degrees Fahrenheit. This provided a strong indication that hydrothermal vents exist deep down beneath Enceladus’ shiny bright icy shell. The hydrothermal vents on that distant moon-world are not unlike the hydrothermal vents that exist on the ocean floor of our own planet.
“Enceladus discoveries have changed the direction of planetary science. Multiple discoveries have increased our understanding of Enceladus, including the plume venting from its south pole; hydrocarbons in the plume; a global, salty ocean and hydrothermal vents on the seafloor. They all point to the possibility of a habitable ocean world well beyond Earth’s habitable zone. Planetary scientists now have Enceladus to consider as a possible habitat for life,” commented Dr. Linda Spilker in a JPL report on the Cassini mission to the Saturn system. Dr. Spilker is a Cassini project scientist at the JPL.
After spending almost two decades in space, the Cassini spacecraft now approaches the end of its remarkable expedition of exploration that has appropriately been named its Grand Finale.
Between April and September 2017, Cassini undertakes a daring series of orbits that, in many ways, seem like an entirely new mission. After a final close flyby of Saturn’s largest moon–the misty moisty hydrocarbon-slashed, smoggy orange-shrouded Titan–Cassini will take a giant leap over Saturn’s icy rings and begin a set of 22 weekly dives between the planet and the rings.
What planetary scientists learn from these final orbits will help improve our understanding of how giant planets–as well as planetary systems beyond our own Solar System–are born and evolve.
When Cassini makes its final orbit around the planet that it has been watching for almost a generation, it will plunge down into Saturn’s atmosphere. As it takes this last, fatal dive, it will send back to Earth new and unique scientific discoveries right up to the very end of its existence. After finally losing contact with Earth, the spacecraft will burn up–becoming, in the end, a part of the planet Saturn itself.
But Cassini’s Grand Finale means so much more than just its last great dive into the planet Saturn. This dramatic end, of a highly successful mission, marks six months of remarkable exploration and scientific discovery. Even though the mission team is confident that the risks of Cassini’s final act are well understood, there could still be some surprises. This is the kind of risky adventure that can only be taken as the final curtain closes on a mission.
As Cassini makes its final dive, it will collect some extremely valuable information that was too risky to obtain earlier.
The Icy Moon That Toppled Over
Whether it was the result of a catastrophic impact or, possibly, some other process, Dr. Tajedddine and his colleaques think the disruption and formation of the tiger stripe terrain caused some of Enceladus’ mass to become redistributed. This would cause some of the moon’s rotation to become unsteady and wobbly. However, this wobbly rotation would eventually stabilize over time–although it would probably take over a million years to do this. By the time the rotation at last settled down, the north-south axis would have reoriented to pass through different points on the surface–a mechanism that scientists refer to as “true polar wander.”
The polar wander proposal helps explain why Enceladus’ current north and south poles apparently are very different. The south pole is geologically young and active, while the north pole is scarred by numerous craters and appears to be considerably older. Heavily cratered surfaces usually indicate an ancient surface, pelted by numerous crashing objects. In contrast, smooth terrain is considered to be young, and recently resurfaced. This resurfacing would effectively erase crater scars. Enceladus’ original poles would have looked more alike before the horrendous crash that caused the little moon to topple over. The event would also have relocated the disrupted tiger-stripe terrain to Enceladus’ south polar region.
Cassini zipped by Enceladus for the last time in October 2015, but astronomer-explorers will be studying the data that it sent back to Earth for years to come–all the while planning for the future when Earthlings again return to uncover still more of the secrets this tiny icy moon holds in its hidden heart.
Dr. Spilker noted in the JPL report that “As we continue to learn more about Enceladus, and compare data from different instruments, we are finding more and more evidence for a habitable ocean world. If life is eventually discovered in Enceladus’ ocean by a mission after Cassini, then our Enceladus discoveries will have been among the top discoveries for all planetary missions.”
The results of the study, proposing that little Enceladus toppled over in the past, are published in the April 30, 2017 issue of the journal Icarus.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book “Wisps, Ashes, and Smoke,” will be published soon.