Abstract
We analyzed Cassini Imaging Science Subsystem (ISS) images of the plume of Enceladus to derive particle number densities for the purpose of comparing our results with those obtained from other Cassini instrument investigations. Initial discrepancies in the results from different instruments, as large as factors of 10–20, can be reduced to ∼2 to 3 by accounting for the different times and geometries at which measurements were taken. We estimate the average daily ice production rate, between 2006 and 2010, to be 29 ± 7 kg/s, and a solid-to-vapor ratio, S/V > 0.06. At 50 km altitude, the plume's peak optical depth during the same time period was τ ∼ 10−3; by 2015, it was ∼10−4. Our inferred differential size distribution at 50 km altitude has an exponent q = 3. We estimate the average geothermal flux into the sea beneath Enceladus' south polar terrain to be comparable to that of the average Atlantic, of order 0.1 W/m2. Should microbes be present on Enceladus, concentrations at hydrothermal vents on Enceladus could be comparable to those on Earth, ∼105 cells/mL. We suggest the well-known process of bubble scrubbing as a means by which oceanic organic matter and microbes may be found in the plume in significantly enhanced concentrations: for the latter, as high as 107 cells/mL, yielding as many as 103 cells on a 0.04 m2 collector in a single 50 km altitude transect of the plume. Mission design can increase these numbers considerably. A lander mission, for example, catching falling plume particles on the same collector, could net, over 100 Enceladus days without bubble scrubbing, at least 105 cells; and, if bubble scrubbing is at work, up to 108 cells. Key Words: Enceladus—Microbe—Organic matter—Life detection. Astrobiology 17, 876–901.
Highlights
Cassini’s exploration of the small saturnian moon Enceladus by both remote sensing and in situ investigations has demonstrated its status as a prime candidate for astrobiological study and the search for evidence of life
mean anomaly (MA): Mean anomaly of Enceladus at time the image was taken, where MA = 0° means that Enceladus is at its closest distance to Saturn and MA = 180° means that Enceladus is at its farthest distance from Saturn
We have found that the apparent discrepancies among the particle number densities and/or mass production rates from the different Cassini instruments and data analyses, initially on the order of a factor of 10–20, have been reduced to factors of 2–3 by accounting for the differing times and geometries of the observations and the natural, known variabilities in the plume
Summary
Cassini’s exploration of the small saturnian moon Enceladus by both remote sensing and in situ investigations has demonstrated its status as a prime candidate for astrobiological study and the search for evidence of life. It is home to a sub-ice-shell global ocean (e.g., Thomas et al, 2016) of salty water (Postberg et al, 2009), that is likely long-lived (e.g., Fuller et al, 2016; Lainey et al, 2017) and vents through four long, prominent fractures in the south polar terrain (SPT) ice shell in fissure-type eruptions that take the form of more than 100 discrete geysers of vapor and icy particles, with faint sheets of material in between (Porco et al, 2014; Teolis et al, 2017, in this issue). H2 has been detected in the vapor (Waite et al, 2017), and ENCELADUS’ PLUME: IMPLICATIONS FOR FUTURE MISSIONS evidence has been found in the plume ice particles of seafloor hydrothermal activity with alkaline pH values and temperatures (Hsu et al, 2015), similar to those of Lost City, a mid-ocean-ridge flanking, ‘‘low temperature,’’ hydrothermal zone in the mid-Atlantic
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