Enceladus facts:
- Enceladus' orbit is at ~3.95 Rs, between Tethys (at 4.89 Rs)
and Mimas (at ~3.08 Rs). Main rings lie inside ~2.27 Rs.
- Enceladus' Radius is ~250 km
- Its Rotation is locked to its orbital period of ~1.4 days
- Local Keplerian and corotation speeds are ~13 km/s and ~39 km/s respectively
- Escape velocity from Enceladus is ~0.2 km/s
- Local magnetic field stength is ~312 nT
- Enceladus has the brightest surface in the solar system, and imaging
suggests much of the icy surface is young (e.g. possibly due to internal melting and
cryovolcanism)
Considerations for INMS measurements:
- Enceladus has been associated with Saturn's E-ring (suggested
sources of the thought-to-be-young E-ring are cryovolcanic droplets,
or collisional fragments from small icy bodies in the same orbit,
possibly left over from an earlier impact...HST imaging evidence of a transient arc of
particles exists (Roddier et al., 1998))
- Johnson et al. (1989) have modeled
sputtered neutral and ionized water molecule
tori from the icy satellites, including Enceladus.
Modeled neutral densities along the Rev 61 flyby
are below the INMS sensitivity limit,
but the ion densities are well within the INMS detection range.
Note this model is azimuthally symmetric and does not allow
for enhancements near Enceladus.
- Hanel et al. (1982) and Spencer et al. (1998) suggest Enceladus
may have an O2 atmosphere with a peak neutral density as high as 4.3e11 mol/cm3,
and a scale height of ~225 km. Other neutrals expected in lesser quantities
are CO2, CO, H2, NH3, and possibly O3.
- Shemansky et al. (1993) discovered a neutral OH torus
with peak densities ~500 cm-3 near Enceladus' orbit. Jurac et al. (2001)
modeled this OH torus, concluding Enceladus itself, rather than the
E-ring, is the major source- producing ~2e25 molecules/s.
Jurac et al. modeled the OH torus,
and shows contours of neutral densities in his GRL,
linked here.
- Sittler et al. (2003, accepted for publication in JGR) recently
modeled pickup ions from Enceladus expected to be observed by CAPS.
They assume a neutral water molecule density of ~1.e5 at the surface.
A picked-up H2O+ ion gyroradius is ~20 km (compared to the Enceladus
radius of ~250 km). These authors calculate pickup ion densities near
the surface of ~2.2 ions/cm3 for H2O+, 0.15 cm-3 for O2+, less for
H+, O+, H2+, OH+, H3O+ (CAPS ion detection limit is ~1e-3 cm-3 for these)
References
Johnson, R.E. et al., Icarus, 77, 311, 1989.
Roddier, C. et al., Icarus, 136, 50, 1998.
Shemansky et al., Nature, 363, 329, 1993.
Spencer, J., Icarus, 136, 349, 1998.
submitted by Janet Luhmann and Steve Ledvina 9/29/03
