Detection of Phosphates Originating from Enceladus’ Ocean by Cassini’s Cosmic Dust Analyzer

Introduction: Enceladus’s subsurface global ocean (Thomas et al. 2016) can be probed by sampling the gaseous and icy material the moon expels into its cryovolcanic plume and - even further out - into Saturn’s E ring (Postberg et al. 2009, 2011, Hsu et al. 2015, Waite et al. 2017). The observed high heat fluxes (Spencer et al. 2006) caused by tidal heating (Choblet et al. 2017), together with rich and complex organic chemistry (Postberg et al. 2018a, Khawaja et al. 2019) mean the moon appears to be one of the prime candidate locations for the emergence of extraterrestrial life in our solar system. Among the critical elements C, H, N, O, P and S that are considered to be essential for life, all except phosphorous have either been identified (Waite et al. 2017, Postberg et al. 2018a, Khawaja et al. 2019.) or - in the case of sulfur - tentatively detected (Postberg et al. 2018b), in emissions from Enceladus by Cassini instruments. Recent geochemical modelling claims that P will be severely depleted in ocean worlds and thus P could be a bottle neck for the emergence of life in subsurface oceans (Lingam & Loeb, 2018.).

Here we present results from a major re-analysis of mass spectrometric data from Cassini’s Cosmic Dust Analyzer (CDA), that shows proof of sodium-phosphate salts in E ring ice grains originating from Enceladus’s subsurface ocean.

Results: Most of the mass spectra of ice grains emitted by Enceladus that CDA recorded over the course of Cassini’s 13-year mission at Saturn, were detected in Saturn’s E ring. The spectra have been classified into a small number of compositional archetypes (Postberg et al. 2018b), of which the so-called Type 3 is defined by its high concentration of sodium and potassium salts (chlorides and carbonates). Type 3 ice grains probably originate from frozen aerosolized droplets of Enceladus’s salty subsurface ocean and are therefore believed to reflect the ocean’s composition close to its surface (Postberg et al. 2009, 2011).

We analyzed approximately 1000 mass spectra of individual Type 3 grains. Within this dataset we found a small number of ice grains whose spectra clearly indicate the presence of at least two sodium orthophosphates, at levels well above CDA’s detection limit: Na₃PO₄ and Na₂HPO₄. These CDA spectra have been subsequently reproduced in the laboratory using the proven method of Laser Induced Liquid Beam Ion Desorption (LILBID), which enables the quantitative evaluation of CDA spectra (Wiederschein et al. 2015, Klenner et al. 2019). The inferred salt concentrations in each of the small number of ice grains with detectable amounts of phosphates range from 0.1 M – 0.5 M. However, due to the small fraction of P-rich grains the P-concentration averaged over the entire dataset of salt rich grains, thought to reflect the near-surface ocean composition n, is of the order of a mM.

Discussion

The finding of phosphate-rich ice grains that originating from Enceladus’s subsurface ocean further increases the astrobiological potential of the moon. In addition to water, heat, and a complex organic chemistry, the presence of phosphates means that Enceladus’s subsurface waters now seem to provide all conditions considered astrobiologically essential. If our preliminary results indicating an phosphate concentration of the order of 1 mM are confirmed by ongoing analysis, P in fact seems to be a very abundant element in Enceladus’s ocean.

References:

Thomas et al. 2016, Icarus 264: 37-47

Postberg et al. 2009, Nature 474: 620-622.

Postberg et al. 2011, Nature 459: 1098-1101.

Hsu et al. 2015, Nature, 519: 207-210.

Waite et al. 2017, Science 356: 155-159.

Postberg et al. 2018a, Nature 558: 564 -568.

Khawaja et al. 2019, MNRAS 489: 5231-5243.

Spencer et al. 2006, Science 311: 1401 – 1405.

Choblet et al. 2017, Nat. Astron. 1: 841-847.

Postberg et al., 2018b, in Enceladus and the Icy Moons of Saturn (P.M. Schenk et al., eds.), 129 - 162.

Lingam & Loeb, 2018, Astron. J. 156: 151.

Wiederschein et al. 2015, Phys. Chem. Chem. Phys. 17, 6858 – 6864.

Klenner et al. 2019, Rapid Commun. Mass Spectrom. 33, 1751-1760.