From: IN%"Amanda.Hendrix@lasp.Colorado.EDU" 23-OCT-1998 20:33:40.54 To: IN%"kent.tobiska@jpl.nasa.gov" "W. Kent Tobiska" CC: IN%"barth@pisces.colorado.edu" "Charles Barth", IN%"hord@pisces.colorado.edu" "Charlie Hord", IN%"jajello@jpluvs.jpl.nasa.gov" "Joe Ajello", IN%"simmons@pisces.colorado.edu" "Karen Simmons", IN%"ktobiska@mail1.jpl.nasa.gov" "Kent Tobiska" Subj: RE: UVS/EUV prime mission discoveries Return-path: Received: from lasp.Colorado.EDU by pisces.colorado.edu (PMDF V4.2-13 #26137) id <01J3BN3W71B4HSJAXG@pisces.colorado.edu>; Fri, 23 Oct 1998 20:31:41 GMT Received: from [128.138.137.147] by lasp.Colorado.EDU (SMI-8.6/SMI-SVR4) id OAA04795; Fri, 23 Oct 1998 14:32:40 -0600 Date: Fri, 23 Oct 1998 14:32:45 -0600 From: Amanda.Hendrix@lasp.Colorado.EDU Subject: RE: UVS/EUV prime mission discoveries To: "W. Kent Tobiska" Cc: Charles Barth , Charlie Hord , Joe Ajello , Karen Simmons , Kent Tobiska , Wayne Pryor , "Ian A.F. Stewart" , raw@west.jpl.nasa.gov, Arthur.L.Lane@mail1.jpl.nasa.gov Message-id: MIME-version: 1.0 Content-type: text/plain; charset="us-ascii" Content-transfer-encoding: 7BIT X-Sender: hendrix@lasp Kent, here are some SWG bullets for Jim Erickson. Feel free to edit if they're too long. IO: Near-UV observations show variations in amount of SO2 frost as well coverage by and thickness of patchy SO2 gas regions. The SO2 gas varies in thickness depending on SO2 frost sublimation and volcanic plume/hotspot activity. (FYI - we have a paper in review for JGR about this.) EUROPA: In near-UV low-resolution observations, an "SO2-like" absorption feature becomes stronger closer to trailing hemisphere apex, where the corotating magnetospheric plasma sweeps by Europa. Higher resolution observations from the primary mission and continuing into the GEM are showing that the absorption feature is also related to certain terrain types. The absorption feature may be due to a combination of implantation/sputtering by the magnetospheric plasma and endogenic processes at the surface. (FYI - I will give a talk on this at AGU. We also have a paper in the most recent Icarus issue) GANYMEDE: (besides the atomic hydrogen you mentioned) In the near-UV an ozone-like absorption feature has been detected, likely due to bombardment by charged particles on Ganymede's trailing hemisphere. The absorber appers to be destroyed by sunlight (converted into molecular oxygen) but survives in the low-sun regions of the poles and near the terminator. (FYI - we have a paper in review for JGR about this.) CALLISTO: Callisto also has a tenuous atmosphere of about three times less hydrogen as Ganymede. Ganymede's escaping hydrogen is likely due to a combination of 1) charged particles impacting the water ice surface, ejecting hydrogen atoms, and 2) ultraviolet photodesorption causing ejection of hydrogen atoms from the surface water ice. In contrast, Callisto's escaping hydrogen is likely due mostly to ultraviolet photodesorption, as there are less charged particles at Callisto's orbit to break up the water ice than at Ganymede's orbit; there is also less exposed water ice at Callisto's surface than on Ganymede's surface to serve as a source of escaping hydrogen.