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A measurement of water vapour amid a largely quiescent environment on Europa


Previous investigations proved the existence of local density enhancements in Europa’s atmosphere, advancing the idea of a possible origination from water plumes. These measurement strategies, however, were sensitive either to total absorption or atomic emissions, which limited the ability to assess the water content. Here we present direct searches for water vapour on Europa spanning dates from February 2016 to May 2017 with the Keck Observatory. Our global survey at infrared wavelengths resulted in non-detections on 16 out of 17 dates, with upper limits below the water abundances inferred from previous estimates. On one date (26 April 2016) we measured 2,095 ± 658 tonnes of water vapour at Europa’s leading hemisphere. We suggest that the outgassing of water vapour on Europa occurs at lower levels than previously estimated, with only rare localized events of stronger activity.

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Fig. 1: Spectra of co-added water lines in April 2016.
Fig. 2: Most sensitive estimates of water versus longitudinal coverage.

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Data availability

All data are publicly available at the Keck Observatory Archive. Any other details of this work are available from the corresponding author on reasonable request.


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We thank the staff of the W. M. Keck Observatory for their support throughout our long Europa observation programme. L.P. acknowledges support from NASA’s Keck PI Awards (grant numbers RSA 1541943, 1466335), Solar System Observations (grant number NNX17AI85G) and Solar System Workings (grant number 80NSSC19K0811). L.R. is supported by the Swedish Research Council (2017-04897). The authors recognize and acknowledge the very important cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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Authors and Affiliations



L.P. and L.R. planned and performed the astronomical observations and strategy. L.P., G.L.V., A.M.M. and T.A.H. contributed to data analysis. All authors contributed to the interpretation of results.

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Correspondence to L. Paganini.

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Peer review information Nature Astronomy thanks Melissa McGrath and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary text, Table 1 and Figs. 1–11

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Paganini, L., Villanueva, G.L., Roth, L. et al. A measurement of water vapour amid a largely quiescent environment on Europa. Nat Astron 4, 266–272 (2020).

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