[25] In 2014, NASA reported that Cassini found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km (6 mi). 5 With the planning for the Europa Clipper and the Europa Lander in advanced stages and a powerful member of Congress supporting the twin missions, re-tasking the probes to Enceladus is likely not in the cards. [151] Therefore, several robotic missions have been proposed to further explore Enceladus and assess its habitability; some of the proposed missions are: Journey to Enceladus and Titan (JET), Enceladus Explorer (En-Ex), Enceladus Life Finder (ELF), Life Investigation For Enceladus (LIFE), and Enceladus Life Signatures and Habitability (ELSAH). Much of Enceladus's surface is covered with craters at various densities and levels of degradation. [38], Like most of Saturn's larger satellites, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. [136], Enceladus is losing mass at a rate of 200 kg/second. In November 2017, Russian billionaire Yuri Milner expressed interest in funding a "low-cost, privately funded mission to Enceladus which can be launched relatively soon. [41], Mathematical models show that the E ring is unstable, with a lifespan between 10,000 and 1,000,000 years; therefore, particles composing it must be constantly replenished. On April 13, 2017, NASA announced the discovery of possible hydrothermal activity on Enceladus's sub-surface ocean floor. Castillo et al. Enceladus: a cradle of life of the Solar System? [1] The name, like the names of each of the first seven satellites of Saturn to be discovered, was suggested by William Herschel's son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope. more radiogenic heating or a more eccentric orbit in the past). [51] Based on the connection between Enceladus and the E ring, scientists suspected that Enceladus was the source of material in the E ring, perhaps through venting of water vapor. [12][63][120], The observed power output of 4.7 gigawatts is challenging to explain from tidal heating alone, so the main source of heat remains a mystery. [84][82] Subsequent radioactive and tidal heating would raise the temperature of the core to 1,000 K, enough to melt the inner mantle. [95], The Cassini spacecraft flew through the southern plumes on several occasions to sample and analyze its composition. Mass estimates from the Voyager program missions suggested that Enceladus was composed almost entirely of water ice. [6] This libration could have provided Enceladus with an additional heat source. [42] Enceladus is orbiting inside the ring, at its narrowest but highest density point. [122] A more recent analysis claimed that "a model of the tiger stripes as tidally flexed slots that puncture the ice shell can simultaneously explain the persistence of the eruptions through the tidal cycle, the phase lag, and the total power output of the tiger stripe terrain, while suggesting that eruptions are maintained over geological timescales. [153] It has been speculated that such activity could be a potential oasis of habitability. [65], The boundary of the south polar region is marked by a pattern of parallel, Y- and V-shaped ridges and valleys. [104], A model suggests that Enceladus's salty ocean (-Na, -Cl, -CO3) has an alkaline pH of 11 to 12. However, the Falcon Heavy has two distinct advantages over the Space Launch System. [51] Results from Cassini suggest that tectonics is the dominant mode of deformation on Enceladus, including rifts, one of the more dramatic types of tectonic features that were noted. The "salty" particles are heavier and mostly fall back to the surface, whereas the fast "fresh" particles escape to the E ring, explaining its salt-poor composition of 0.5–2% of sodium salts by mass. However, flybys by Cassini provided information for models of Enceladus's interior, including a better determination of the mass and shape, high-resolution observations of the surface, and new insights on the interior.[79][80]. "[68] Previous models suggest that resonant perturbations of Dione could provide the necessary periodic eccentricity changes to maintain the subsurface ocean of Enceladus, if the ocean contains a substantial amount of ammonia. [6] Heating from long-lived radioactive isotopes uranium-238, uranium-235, thorium-232 and potassium-40 inside Enceladus would add 0.3 gigawatts to the observed heat flux. One-kilometer high domes have also been observed. 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