Comparison and Interpretation of Spectral Characteristics of the Leading and Trailing Hemispheres of Europa and Callisto

Europa’s surface water ice crystallinity: Discrepancy between observations and thermophysical and particle flux modeling

“…The small projection area-weighted factor these regions provide to the disk-integrated crystallinity suggest that high latitude processes alone are unlikely to explain the crystallinity discrepancy. ∼ 65-71% for the original Busarev et al (2018) data. However, we model a crystallinity of ∼ 80-95% when incorporating effects from thermal relaxation and ion particle bombardment.…”

“…Using full-disk ground-based observations from Grundy et al (1999), Busarev et al (2018), and the APO TripleSpec instrument, concurrently with laboratory spectra from Mastrapa et al (2008) and the ISL, we calculate Europa's leading hemisphere crystallinity percentage as ∼ 27-36% (for the Grundy et al, 1999, Busarev et al, 2018 5-point moving window, andAPO Triplespec data), and ∼ 65-71% (for the original Busarev et al, 2018 data). The full-disk crystallinity percentage as approximated by thermophysical and radiation flux modeling with ICICLE is ∼ 80-95%.…”

“…Both Grundy et al (1999) and Busarev et al (2018) acquired ground-based, full-disk, spectroscopic observations of the leading and trailing hemispheres of several icy outer Solar System satellites, including Europa. Grundy et al (1999) obtained surface ice temperatures using the strengths of notable water ice absorption bands in the 1.5 − 1.65 µm region, whereas Busarev et al (2018) investigated spectral and compositional differences between the leading and trailing hemispheres. In order to ensure that no drastic events have occured to significantly change the crystallinity by a detectable amount over time from 1998 (Grundy et al, 1999) to 2016/2017 (Busarev et al, 2018) to 2018 (this work), we obtain additional NIR reflectance spectra of Europa's leading hemisphere and supplement our observations with those of Grundy et al (1999) and Busarev et al (2018) in the crystallinity calculation outlined in §2.3.…”

“…In §2, we derive the observed crystallinity of Europa's leading hemisphere using NIR ground-based observations from Grundy et al (1999), Busarev et al (2018), and the Apache Point Observatory (APO) in Sunspot, NM, and laboratory spectra from Mastrapa et al (2008) and the Ice Spectroscopy Lab (ISL) at the Jet Propulsion Laboratory (JPL) in Pasadena, CA. In §3, using a 1D thermophysical model (Spencer et al, 1989) of Europa's surface, we calculate the expected full-disk crystallinity of Europa's leading hemisphere by incorporating the thermal relaxation of amorphous to crystalline water ice and the formation of amorphous water ice by irradiation.…”

“…Ground-based observations fromGrundy et al (1999) (dark gray triangles),Busarev et al (2018) (light gray squares),Busarev et al (2018) with the 5-point moving window (dark red solid line), and APO TripleSpec (black circles). Atmospheric water vapor absorption in the 1.35 − 1.43 µm and 1.80 − 1.92 µm regions led to either data gaps or extremely high noise and should be flagged as untrustworthy regions.…”