Transient Water Vapor at Europa’s South Pole (Roth et al.)

Europa  Smallest of the Galilean Moons with a diameter of ~3100km  Tenuous atmosphere (or exosphere) composed of mostly O  Fe or Fe-S core and Si mantle  H 2 O-rich ice crust about km in depth  ~10% water by mass (ice-regolith and subsurface ocean)  Smoothest surface—lacks mountains or craters  High surface albedo of 0.64 Basics

 Nov. & Dec. 2012: Hubble Space Telescope “imaged Europa’s ultraviolet emissions in the search for vapor plume activity”  Two ~200km high vapor plumes were modeled with HST data at the moon’s southern hemisphere on Dec  It can be inferred that Europa’s orbital and tidal phases affect plume activity Abstract:

o Same side of Europa (leading hemisphere) always faces Jupiter—this phenomenon is called tidal locking o Has an eccentric orbit o Satellite’s libration or wobbling Tidal Heating on Europa o 3.5 Earth day orbit around Jupiter Behavior and Tidal Activity b.examiner.com/sites/default /files/styles/article_large/has h/84/a4/84a410b5c118eecd ac6672e07.jpg?itok=u 1WfGqqg

Hubble Space Telescope (HST) HST Space Telescope Imaging Spectrograph (STIS) HST Advanced Camera for Surveys (ACS) 1995  HST detected exosphere “through UV detections of O emissions at 130.4nm and 135.6nm” generated by sputtering and radiolysis (confirmed by Galileo in 1997) HST images were spatially revolved and inhomogeneous emission pattern was observed at the two O multiplets, which means the moon does not have a homogeneous surface atmosphere ACS detected enhanced emission at 90°W longitude where shear stresses were predicted to occur but this remains ambiguous due to lack of data

 STIS imaging detected surpluses of Hydrogen (H) Lyman-α and Oxygen (OI) emissions at the moon’s southern hemisphere  Inverting images of Europa with a solar UV spectrum allowed for reduction in albedo  Better models can be created because of STIS’ UV observations; otherwise, surface reflection would be too bright to produce viable models Space Telescope Imaging Spectrograph

Images of Europa taken by Galileo and Voyager I and II o Maps G-I were used to model surface-reflected solar Lyman-α radiation

 HST orbited the moon 5 times over a period of ~7 hours for all observations 5 Oct  HST/STIS observes trailing hemisphere 8 Nov  HST/STIS observes trailing hemisphere 30 Dec  HST/STIS observes leading hemisphere  Dec. 2012: Water vapor detected only when Europa was at apocenter True anomalies (f ) :  Oct. 1999: ~360 degrees  Nov. 2012: ~300 degrees  Dec. 2012: ~200 degrees

 Three wavelengths of interest—H 121.6nm, OI 130.4nm, and OI 135.6nm  UV wavelength gave more precise emission spectra (due to surface albedo)  H Lyman-α line for C shows a surplus of emission  Photon flux peaks at HI 121.6nm Combined exposure spectral images and integrated spectra

Row 1  Jovian hemispheres Rows 2 & 3  Hydrogen Lyman-α emissions; surface albedo subtracted in 3 rd row Row 4  Plume highly visible left of south pole on Dec Negligible emission surpluses for OI 135.6nm because of low signal-to-noise ratio (SNR) Similarly colored images in lower left hemisphere Observed images of hemispheres and combined H & O emission STIS images

 ~500R H 121.6nm  ~25R OI 130.4nm  Plume location indicates atmospheric inhomogeneity  Non-detection at HI 121.6nm and OI 135.6nm, which indicates localized abundance of H 2 0 at limb bins 12 and 13 December orbits over ~7 hours

 1 R E (Europa radius, 1561km) and 1.25 R E (above-limb altitude of 390km)  Divided into eighteen 20° bins around Europa’s disk  Lyman-α brightnesses are 420 ± 136 Rayleigh and 604 ± 140 Rayleigh for limbs 12 and 13, respectively  These emissions exceed the average limb emission—46 R—by “2.8 and 4.0 times the propagated uncertainty (σ)  Brighter OI 130.4nm emission in bin 13—59 ± 18 R—and bin 12 was only slightly brighter at 35 ± 17 R (average for OI 130.4nm is 16 R) Limb Bins 12 and 13

Eighteen 20° bins from 1999 and November 2012 Black solid line (with error bars) are measured Rayleigh (R) brightness from Oct and Nov Red dotted line shows a model H2O atmosphere that should fit the Lyman-α emission surpluses from Dec H Lyman-α and OI 130.4nm brightnesses and their ratio Large dip in plume brightness shows when Europa is farthest away from its plasma sheet

 HI 121.6nm and OI 130.4nm showed highest emission surpluses  In accordance with previous figure Lyman-alpha emission peaked at limb bins 12 and 13, which are located left of Europa’s south polar region  12 and 13 are localized areas of interest, where water vapor plumes were ~200km in height and ~250km in width and had column densities of H 2 O/m -2  Lyman-α plume morphology and brightness decrease with altitude  O molecules are more abundant at higher altitudes Comparison of Dec observations and atmosphere plume model results

 Localized H 2 O atmosphere because of the emission surplus at HI 121.6nm and OI 130.4nm  H 2 O column density  1.5 x m -2  O 2 column density  ~5 x m -2  Excitation of H 2 O = 27 R and O 2 = 29 R  Consistent with 59 ± 18 R in limb bin 13  Densities of plumes arbitrarily centered on anti-Jovian meridian (180°W) because surface location cannot be determined by looking at images  55°S at limb 12 and 75°S at limb 13  H 2 O densities: Limb 12  4.9 × Limb 13  8.2 ×  Together these densities are one to two orders of magnitude higher than Europa’s H 2 O abundance without plumes Evidence of Water Vapor Plumes!

 Figures A, B, and C represent data from Oct. 1999, Nov. 2012, and Dec. 2012, respectively.  White circle indicates no tensile stress (compression); black indicates high tensile stress (tension)  Like Enceladus, fractures are present when satellite is at apocenter—south pole region (C)  It is believed that water vapor plumes “escaped” from fractures caused by tension  Diurnal (daytime) tides might cause the fracturing of icy surface Stereographic projection of south polar region

 Saturn’s 6 th largest moon…believed to be habitable like Europa  [Europa’s] ”high plume velocities and low number densities…are consistent with vapor emission from narrow fractures (15), as occurs at Enceladus” (Roth et al.)  There is more vapor plume activity at apocenter when satellites are farthest away from their planets Europa & Enceladus

Tidal Modeling Predictions Enceladus’ south pole has more active plumes near apocenter than at pericenter This knowledge gives more ground to the existence of vapor plumes on Europa Europa’s tidal stress would most likely cause similar linea features to those of Enceladus

High variability in plume activity on Europa caused by its orbital phases Plumes at Enceladus are more active near apocenter than pericenter—this is the same case with Europa Vapor plume variability gives more ground to the theory of tidal-flexing models HST images near Europa’s south pole supports idea that water vapor plumes exist through observed Hydrogen and Oxygen emissions Summary