Moonraker: Enceladus Multiple Flyby Mission

Abstract

Enceladus, an icy moon of Saturn, possesses an internal water ocean and jets expelling ocean material into space. Cassini investigations indicated that the subsurface ocean could be a habitable environment having a complex interaction with the rocky core. Further investigation of the composition of the plume formed by the jets is necessary to fully understand the ocean, its potential habitability, and what it tells us about Enceladus’s origin. Moonraker has been proposed as an ESA M-class mission designed to orbit Saturn and perform multiple flybys of Enceladus, focusing on traversals of the plume. The proposed Moonraker mission consists of an ESA-provided platform with strong heritage from JUICE and Mars Sample Return and carrying a suite of instruments dedicated to plume and surface analysis. The nominal Moonraker mission has a duration of ∼13.5 yr. It includes a 23-flyby segment with 189 days allocated for the science phase and can be expanded with additional segments if resources allow. The mission concept consists of investigating (i) the habitability conditions of present-day Enceladus and its internal ocean, (ii) the mechanisms at play for the communication between the internal ocean and the surface of the South Polar Terrain, and (iii) the formation conditions of the moon. Moonraker, thanks to state-of-the-art instruments representing a significant improvement over Cassini's payload, would quantify the abundance of key species in the plume, isotopic ratios, and the physical parameters of the plume and the surface. Such a mission would pave the way for a possible future landed mission.

How to identify cell material in a single ice grain emitted from Enceladus or Europa

“…The total number of ions arriving at SUDA's multiplier will be in the order of 600,000 to 1,000,000 (~100 to 160 fC) for a typical ice grain, similar to the total number of ions represented in a LILBID spectrum that is co-added from typically 200 to 500 individual spectra (see the "LILBID ion number calculations" section in Materials and Methods). However, the SNR of all signals would be even higher in mass spectra generated by future SUDA-type mass spectrometers considering the higher sensitivity of these instruments (7,10,11) compared to our laboratory mass spectrometer (41,55). While approximately 100 ions of a single species are sufficient to generate a detectable signal with SUDA, 200 to 700 ions are needed to generate a signal with SNR = 2 using LILBID in the laboratory (see the "LILBID ion number calculations" section in Materials and Methods).…”

“…Future SUDA-type instruments will be capable of analyzing 10,000 to 100,000 single ice grains during one plume flythrough (depending on altitude and speed). With at least 10 or more flybys during a mission (8,10,11), this would enable the detection of the biosignatures of a fraction of a cell that may be present in just a handful of ice grains among the 100,000s sampled during such a mission. A scenario in which 0.01% of a cell is present in only 1 out of 10,000 or 1 out of 100,000 plume grains resembles a cell density of 2 × 10 3 or 2 × 10 2 cells/ml, respectively, if integrated over the entire icy material in the plume (see the "Preparation of S. alaskensis cell samples" section in Materials and Methods).…”

“…Several mission concepts and techniques have been proposed to look for prebiotic chemistry or even evidence of life on Enceladus or Europa (8,10,11,(23)(24)(25)(26)(27)(28). Of the techniques discussed, to our knowledge, only impact ionization mass spectrometry can analyze the unique compositions of individual plume ice grains, typically only a few micrometers in diameter.…”

“…While CDA was able to analyze the compositions between 30 and 300 ice grains during a single passage through Enceladus's plume, future instruments, such as SUDA (7), ENIA (10), or HIFI (11), would be able to sample 10,000 to 100,000 individual ice grains in a diameter range from 0.5 to 50 μm per plume flythrough. The total ice grain emission rate of Enceladus's plume is 15 to 65 kg/s, of which ~10% escape the moon's gravity and enter Saturn's E ring (39,40).…”

“…For Enceladus, evidence from multiple Cassini measurements indicates that the plume is sourced from its liquid water ocean and not a near-surface reservoir ( 5 ). The compositions of single ice grains can be sampled in situ during spacecraft flybys by impact ionization mass spectrometers, such as the Cosmic Dust Analyzer (CDA) ( 6 ) onboard the past Cassini mission, the SUrface Dust Analyzer (SUDA) ( 7 ) onboard NASA’s upcoming Europa Clipper mission ( 8 ), or even more capable instruments proposed for future Enceladus missions, such as the ENceladus Ice Analyzer (ENIA) ( 9 , 10 ) or the High Ice Flux Instrument (HIFI) ( 11 ). Other capable impact ionization mass spectrometers include the Interstellar Dust Experiment (IDEX) onboard NASA’s upcoming Interstellar Mapping and Acceleration Probe (IMAP) ( 12 ) and the Destiny + Dust Analyzer onboard JAXA’s upcoming DESTINY + mission ( 13 ).…”