Enceladus (moon)

Enceladus is the brightest of the satellites located within the rings of Saturn. It has much in common with what scientists expected to find on comets, especially water ice. However, it appears to have been formed more than four billion years ago as a spinning mass of soft material.

Overview

Enceladus was discovered on August 28, 1789, by William Herschel. Its orbit around Saturn has a semimajor axis of 238,037 kilometers and an eccentricity of 0.0047, with a period of 118,386.82 seconds (nearly 33 hours). The orbit is inclined at 0.019° to Saturn’s equator and located inside the E ring around Saturn. This places Enceladus at roughly 4 Saturn radii, located between the orbits of the moons Mimas and Tethys, which are about one Saturn radius on either side. Its rotation is synchronous and has no axis tilt, so the same hemisphere always faces Saturn. It is nearly spherical, with a mean diameter of 504.2 kilometers, being a slightly flattened ellipsoid with dimensions of 513.2 kilometers along the orbit radius pointed at Saturn, 502.8 kilometers along the orbit path, and 496.6 kilometers between the north and south poles.

It is the sixth-largest moon of Saturn. It has a mass of 1.08022 10²⁰ kilograms and a mean density of approximately 1,610 kilograms per cubic meter. The value of acceleration due to gravity at the surface of the equator is 0.111 meters/second². The escape velocity at the surface is 0.238 kilometers/second, neglecting atmospheric drag. Enceladus has a very high Bond albedo of 0.99 and geometric albedo of 1.375, the highest among the satellites embedded in the Saturnian rings, indicating strong reflection. Its apparent magnitude from Earth is 11.7.

Enceladus is tidally locked in a synchronous orbit around Saturn, meaning that the hemisphere with the higher density always faces Saturn. Looking into the sky from the Saturn-facing side, the planet would occupy roughly 30° of the sky and appear to be spinning in roughly the same position at all times. From the side hidden from Saturn, the Sun would appear very small, rising and setting in roughly seventeen hours as Enceladus completed half an orbit around Saturn.

The orbital eccentricity of Enceladus is attributed to a resonance with the satellite Dione, with Enceladus completing two orbits for each orbit by Dione. This resonance may also drive the tidal heating of Enceladus. The shape of Enceladus is very close to that of an equilibrium-flattened ellipsoid, hydrostatically balanced by gravity and spin, which is the shape that an object would have in space if it were composed of homogeneous and fluid material. However, simulations of orbital evolution and tidal locking suggest some variation in its internal density. The higher average density than that of water indicates denser material, possibly silicates, inside Enceladus.

Most of the data about Enceladus's surface and orbital environment come from close flyby observations by the Voyager and Cassini spacecrafts. The Voyager mission revealed evidence of a complex thermal history of Enceladus and showed several provinces with distinct geographical features. Short periods of intense heating and geological activity appear to have been separated by long periods of inactivity. Surface features include long, narrow depressions (fossae), ridges with cliffs of several hundred meters (dorsa), plains (plantia), long, parallel canyons (sulci), and craters. The terrain of the northern latitudes appears to be more than 4.2 billion years old, with more than one crater every five square kilometers, most of them bowl-shaped. Some craters are as large as thirty kilometers in diameter, while most are very small, less than a meter in diameter. The equatorial plains, named the Sarandib Plantia, show striations and folding, with about one crater every seventy square kilometers. Craters in these younger regions show viscous relaxation, indicating mechanisms for melting or distortion of the surface.

The ridged and grooved plains of the Samarkand Sulci, at 55° to 65° south latitude, are 100,000 to 500,000 years old, and the fractured regions south of that show few craters. The Cassini orbiter approached the south polar region of Enceladus to within 168 kilometers on July 14, 2005, taking images with a resolution of four meters per pixel. House-sized boulders believed to be made of ice littered the polar landscape, but craters were nearly absent. A set of parallel “tiger-stripe” fractures—roughly 500 meters deep, two kilometers wide, and 130 kilometers long—are flanked by 100-meter high ridges. South of 55° south latitude, a chain of fractures and ridges circumscribes the moon. Some fractures intersect and overlay others. Scientists associate these fractures and ridges with the flattening and extension due to gravitational interactions with Saturn and other moons.

The Cassini flyby of Enceladus in 2005 revealed a water-rich plume ejecting as narrow jets from vents in the tiger-stripe sulci of the south polar region. Since then, Cassini completed several flybys, many close to the satellite. The orbiter has recorded and analyzed the material spewing from the jets, and scientists have determined that in addition to water and organic material, the jets contain salt with a salinity level equal to that of Earth's. The fine sizes of the particles in the plume, which freeze to ice or sublimate soon after ejection, suggest to scientists that the plume originates in a subsurface body of liquid water, which makes Enceladus possible to support life. Some of the water jets reach exit speeds of 600 meters per second, well above the 238 meters per second needed to escape the gravity of Enceladus. The mass flow rate of the plume is 100 kilograms per second, comparable to that of air through a modern supersonic jet fighter engine. Much of this mass may initially escape Enceladus into the ring around Saturn and may be the origin of the mass in the E ring of Saturn. Enceladus also captures mass as snow falling to the surface from the E ring.

Cassini approached within fifty-two kilometers of the southern middle latitudes on March 12, 2008, in its E3 close approach, then dropped behind Enceladus in its orbit. It grazed the edge of the plume, which trails the satellite like the tail of a comet. The density of these portions of the plume had been predetermined from Earth by observing the dimming of starlight as the plume crossed in front of a star with an ultraviolet imaging spectrograph (UVIS). Roughly seventy seconds after the closest approach, the craft was 250 kilometers above the surface when the onboard ion and neutral mass spectrometer (INMS) encountered a peak particle density of nearly ten million particles per cubic centimeter. This was still on the outer edge of the plume. The sharply defined plume edge provides further strong evidence that the plume comes out as a supersonic jet, as opposed to a diffuse subsonic plume caused by friction heating of ice due to the tidal stresses at the fractures. Measured gas density was twenty times higher than predicted based on thermal expansion.

Knowledge Gained

Initial data from the INMS suggested that the plume contains, beyond nearly pure water (ice), significant amounts (ranging from 1 to 10 percent) of carbon dioxide, methane, and other organic molecules, both simple and complex. There was a strong signal from something of molecular weight near 28, but there is debate over whether this substance is nitrogen (per the INMS data) or carbon monoxide (per the Cassini plasma spectrometer data). The values were similar to those predicted for many comet tails. The organic molecules detected include acetylene (C²H²), ethane (C²H⁶), hydrogen cyanide HCN, formaldehyde (H²CO), propyne (C₃ H₄), propane (C₃ H₈), and acetonitrile (C₂ H₃ N). The craft's cosmic dust analyzer instrument failed to capture the particle sizes. The Cassini plasma spectrometer (CAPS), which measures ions, also detected much larger particles on the order of nanograms. If these were ice particles, they may have been as large as 0.01 to 0.1 millimeters in diameter, comparable to the particle sizes calculated from surface observation data of the south-polar terrain. CAPS also detected positively and negatively charged ions, segregated in different plume regions.

Enceladus has no measurable internal magnetic field, but it significantly influences Saturn's magnetosphere, strongly deflecting magnetic lines. This is attributed to the water plume. Ions accelerated to energy levels of twenty kilo-electron volts by Saturn’s strong magnetic field collide with the molecules in the plume, breaking them up into atoms and ionizing them. The magnetic field again accelerates these fresh ions, and, in turn, Enceladus substantially deflects the magnetic field lines of Saturn. Scientists also associate clouds of oxygen and hydrogen observed around Saturn with the atoms and ions generated when the water molecules in the plume from Enceladus collide with high-energy ions in the E ring.

In late 2008, presentations at the American Geophysical Union meeting in San Francisco revealed that data from Cassini’s observations of Enceladus strongly indicate that the satellite’s surface displays action similar to that of Earth’s ocean floor, where new crustal material emerges from slits in the crust. Cassini imaging team leader Carolyn Porco proposed that liquid water was present on Enceladus’s surface, and the surface splits and spreads apart. On Earth, a molten rock rising from deep in the planet causes the crustal spreading, whereas on Enceladus, the surface spreading originates with the upwelling of liquid, presumably water. Evidence suggests that the tiger stripes formations near the satellite’s south pole resemble the mid-ocean ridges on Earth’s sea floor. Close flybys of that region have resulted in more data on water eruptions through vents in the tiger stripes formation. Combined with evidence of crustal spreading, these data revealed Enceladus as a surprisingly active world.

In 2017, a study published in the journal Nature Astronomy revealed that the south-polar region of Enceladus was warmer than expected, a few feet below its icy surface, meaning that the moon's liquid water might be closer to the surface than previously thought. Scientists believe that despite Enceladus's frozen surface temperatures, it is potentially the most habitable body beyond Earth in the solar system. A 2023 exploration on Cassini found phosphorus, an element essential for sustaining life, on Enceladus.

Context

The presence of water ice in a low-gravity body within seven years of travel time of Earth excited planners of deep space missions since water is an excellent future propellant. The discovery of high-speed water jets from the south polar region provides strong evidence of liquid water below the surface and continuing tectonic processes. Complex organic molecules, including carbon and nitrogen, discovered in the jet plume fuel speculation about precursors of life on the satellite. Enceladus is one of three known planetary bodies in the solar system (besides Earth and the Jovian satellite Io) with an internal heat visible by remote sensing. The relations between Enceladus and the E ring, the magnetosphere, and clouds of oxygen and hydrogen around Saturn are subjects of intense study.

The similarity between Enceladus and comets has raised questions about the origin and evolution of the solar system. Comets were thought to have originated far outside the orbit of Pluto, independently of the planets, while the planetary satellites formed from the same cloud as the Sun and planets. However, as predicted for comets, Enceladus appears to have nearly pure water ice. An unanswered question is how such a large and nearly spherical cometary body could have been captured in an orbit so close to Saturn.

Bibliography

Benna, M., and W. Kasprzak. “Modeling of the Interaction of Enceladus with the Magnetosphere of Saturn.” Lunar and Planetary Science, vol. 38, 2007, p. 1759.

"Cassini Sees Heat Below the Icy Surface of Enceladus." NASA, 13 Mar. 2017, www.nasa.gov/feature/jpl/cassini-sees-heat-below-the-icy-surface-of-enceladus. Accessed 20 Sept. 2023.

"Enceladus a Lifezone Hotspot—90 Geyser Jets Spewing Water Vapor and Organics." The Daily Galaxy. Galaxy Media, LLC, 28 Mar. 2012. solarsystem.nasa.gov/news/12901/enceladus-jets-surprises-in-starlight. Accessed 20 Sept. 2023.

"Enceladus Plume is a New Kind of Plasma Laboratory." NASA, 31 May 2012. www.nasa.gov/mission‗pages/cassini/whycassini/cassini20120531.html. Accessed 20 Sept. 2023.

Khurana, K. K., M. K. Dougherty, C. T. Russell, and J. S. Leisner. “Mass Loading of Saturn’s Magnetosphere Near Enceladus.” Journal of Geophysical Research, vol. 112, no. A08203, 2007, pp. 1–12. doi:10.1029/2006JA012110.

Kuthunur, Sharmila. "Saturn's Moon Enceladus has all the Ingredients for Life in its Icy Oceans. But is Life There?" Space, 14 June 2023, www.space.com/saturn-moon-enceladus-phosphorus-found. Accessed 20 Sept. 2023.

Porto, C. C., et al. “Cassini Observes the Active South Pole of Enceladus.” Science, vol. 311, no. 5766, 10 Mar. 2006, pp. 1393–1401.

Shanahan, Jesse. "Saturn's Moon, Enceladus, Is Our Closest Great Hope For Life Beyond Earth." Forbes, 4 Apr. 2017, www.forbes.com/sites/startswithabang/2017/04/04/saturns-moon-enceladus-is-our-closest-great-hope-for-life-beyond-earth. Accessed 1 May. 2017.

Schneider, Stephen E. Pathways to Astronomy. 6th ed. McGraw-Hill, 2020.

Verbiscer, A.., R. French, M. Showalter, and Paul Helfenstein. “Enceladus: Cosmic Graffiti Artist Caught in the Act.” Science, vol. 315, no. 5813, 9 Feb. 2007, p. 815.