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.

先前的调查证明，欧罗巴大气层中存在局部密度增加的现象，从而提出了可能起源于水羽的想法。但是，这些测量策略对总吸收或原子发射很敏感，这限制了评估水含量的能力。在这里，我们提供了凯克天文台直接搜索2016年2月至2017年5月期间欧罗巴水汽的信息。我们在红外波长下进行的全球调查导致17个日期中有16个没有被检测到，其上限低于先前估计得出的水丰度。在某一天（2016年4月26日），我们在欧罗巴领先的半球测得的水蒸气为2,095±658吨。我们建议，欧罗巴上水蒸气的除气量低于以前的估计水平，