Constraints On Europa’s Water Group Torus From HST/COS Observations

By Keith Cowing
Status Report
April 18, 2023
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Constraints On Europa’s Water Group Torus From HST/COS Observations
Column density maps for H2 (top left), H (top right), and O (bottom) from the torus model (Smith et al. 2019, 2022) in a plane through Jupiter and corresponding to the viewing geometry of the first torus-centered exposure le2oa1010 (Table 1) from 2020. For O, the tori of Europa-genic (left) and Io-genic (right) material are shown separately. Jupiter (large black circle) is centered and Io (white, behind Jupiter) and Europa (black) are shown by the smaller disks to the right (west of Jupiter). The densities are higher closer to the moons (here, west of Jupiter) and the HST/COS observations scanned over the densest regions of the Europa-genic neutrals from 8.0 RJ to 12.5 RJ. (Note that the viewing geometry in Figure 1 corresponds to an 1.5-hour earlier exposure). — astro-ph.EP

In-situ plasma measurements as well as remote mapping of energetic neutral atoms around Jupiter provide indirect evidence that an enhancement of neutral gas is present near the orbit of the moon Europa.

Simulations suggest that such a neutral gas torus can be sustained by escape from Europa’s atmosphere and consists primarily of molecular hydrogen, but the neutral gas torus has not yet been measured directly through emissions or in-situ. Here we present observations by the Cosmic Origins Spectrograph of the Hubble Space Telescope (HST/COS) from 2020 and 2021, which scanned the equatorial plane between 8 and 10 planetary radii west of Jupiter.

No neutral gas emissions are detected. We derive upper limits on the emissions and compare these to modelled emissions from electron impact and resonant scattering using a Europa torus Monte Carlo model for the neutral gases. The comparison supports the previous findings that the torus is dilute and primarily consists of molecular hydrogen. A detection of sulfur ion emissions radially inward of the Europa orbit is consistent with emissions from the extended Io torus and with sulfur ion fractional abundances as previously detected.

Lorenz Roth, H. Todd Smith, Kazuo Yoshioka, Tracy M. Becker, Aljona Blöcker, Nathaniel J. Cunningham, Nickolay Ivchenko, Kurt D. Retherford, Joachim Saur, Michael Velez, Fuminori Tsuchiya

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2304.09150 [astro-ph.EP] (or arXiv:2304.09150v1 [astro-ph.EP] for this version)
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Submission history
From: Lorenz Roth
[v1] Tue, 18 Apr 2023 17:43:05 UTC (2,725 KB)

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