 |
| Titel |
What does Cassini ENA observations tell us about gas around Europa? |
| VerfasserIn |
Pontus Brandt, Barry Mauk, Joseph Westlake, Todd Smith, Donald Mitchell |
| Konferenz |
EGU General Assembly 2015
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107614
|
| Publikation (Nr.) |
 EGU/EGU2015-7322.pdf |
|
|
|
|
|
| Zusammenfassung |
| From about December 2000 to January 2001 the Ion and Neutral Camera (INCA) imaged
Jupiter in Energetic Neutral Atoms (ENA) from a distance of about 137-250 Jovian planetary
radii (RJ) over an energy range from about 10 to 300 keV. A forward model is employed to
derive column densities and assumes a neutral gas-plasma model and an energetic ion
distribution based on Galileo in-situ measurements. We demonstrate that Jupiter observations
by INCA are consistent with a column density peaking around Europa’s orbit in the range
from 2x1012 cm-2 to 7x1012 cm-2, assuming H2, and are consistent with the upper
limits reported from the Cassini/UVIS observations. Most of the INCA observations
are consistent with a roughly azimuthally symmetric gas distribution, but some
appear consistent with an asymmetric gas distribution centred on Europa, which
would directly imply that Europa is the source of the gas. Although our neutral
gas model assumes a Europa source, we explore other explanations of the INCA
observations including: (1) ENAs are produced by charge exchange between energetic
ions and neutral hydrogen originating from charge-exchanged protons in the Io
plasma torus. However, estimated densities by Cheng (1986) are about one order of
magnitude too low to explain the INCA observations; (2) ENAs are produced by charge
exchange between energetic ions and plasma ions such as O+ and S+ originating
from Io. However, that would require O+ plasma densities higher than expected to
compensate for the low charge-exchange cross section between protons and O+; (3) We
re-examine the INCA Point-Spread Function (PSF) to determine if the ENA emissions in
the vicinity of Europa’s orbit could be explained by internal scattering of ENAs
originating from Jupiter’s high-latitude upper atmosphere. However, the PSF was well
constrained by using Jupiter from distances where it could be considered a point source. |
|
|
|
|
|
|