Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.cosmos.ru/conf/2009elw/presentations/presentations_pdf/session5/Patterson_ELW.pdf Äàòà èçìåíåíèÿ: Mon Mar 2 19:59:52 2009 Äàòà èíäåêñèðîâàíèÿ: Mon Apr 6 23:52:59 2009 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: arp 220

Understanding Europa's Radiation Environment and How it Influences Landing Site Characterization

G.W. Patterson, L.M. Prockter, C. Paranicas
Applied Physics Laboratory


Introduction
· The surface of Europa is weathered by charged and neutral particles, micrometeroids, and photons.
­ It has been demonstrated that these weathering processes are not uniform with respect to location and depth [Cooper et al., 2001; Paranicas et al., 2007].

· We have begun an effort to characterize the variability of weathering processes with location and depth globally for Europa.
­ With this information, we can identify regions on Europa that provide greater protection against the harsh Jovian radiation environment and/or have high science value.


Background
· Criteria for characterizing potential landing sites [e.g., Figueredo et al., 2003]:
­ Relative surface age ­ Surface roughness ­ Evidence for material exchange between surface and subsurface


Background
· Criteria for characterizing potential landing sites [e.g., Figueredo et al., 2003]:
­ Relative surface age ­ Surface roughness ­ Evidence for material exchange between surface and subsurface

(Figueredo et al., 2004)


Background
· Criteria for characterizing potential landing sites [e.g., Figueredo et al., 2003]:
­ Relative surface age ­ Surface roughness ­ Evidence for material exchange between surface and subsurface

QuickTimeTM and a decompressor are needed to see this picture.

QuickTimeTM and a decompressor are needed to see this picture.

(Figueredo et al., 2004)


Background
· Criteria for characterizing potential landing sites [e.g., Figueredo et al., 2003]:
­ Relative surface age ­ Surface roughness ­ Evidence for material exchange between surface and subsurface

(Figueredo et al., 2004)


Background


Background


Background


Background


Background
· Criteria for characterizing potential landing sites [e.g., Figueredo et al., 2003]:
­ Relative surface age ­ Surface roughness ­ Evidence for material exchange between surface and subsurface


Background
· Criteria for characterizing potential landing sites [e.g., Figueredo et al., 2003]:
­ Relative surface age ­ Surface roughness ­ Evidence for material exchange between surface and subsurface ­ External Environment


Background
· Criteria for characterizing potential landing sites [e.g., Figueredo et al., 2003]:
­ Relative surface age ­ Surface roughness ­ Evidence for material exchange between surface and subsurface ­ External Environment
· Important from an engineering and science standpoint


Radiation Environment
Jupiter System


Radiation Environment
Ganymede
· Polar caps related to differences in plasma-induced brightening in polar and equatorial regions

(Khurana et al., 2007)

open/closed field line boundaries: above plasma sheet mid-plane below plasma sheet


Radiation Environment
Penetration Depths
· Charged particles primarily affect the top few cm of Europa's icy shell [Cooper et al., 2001]
­ Ions have shallow penetration depths ­ High-energy electrons can penetrate up to a meter or more [Paranicas et al., 2007] ­ The significance of electron bombardment with depth is enhanced by secondaries
· These photons have a wide range of frequencies and can add energy deep in the layer

QuickTimeTM and a decompressor are needed to see this picture.


Radiation Environment
Europa
· Electrons in the 100s of keV to 10s of MeV range, which dominate the radiation dose at Europa, preferentially get deposited into the trailing hemisphere [Paranicas et al. 2007]


Analysis
Europa
· This suggests that Europa's leading hemisphere, particularly near the apex, is effectively shielded from a significant fraction of the radiation present


Radiation Environment
Short-term variability
· Solar Wind variability
­ The magnetopause of Jupiter varies with solar wind dynamic pressure ­ Likely affects corotation and reconnection patterns within the magnetosphere ­ Will have some effect on the weathering of Europa


Radiation Environment
Short-term variability
· · Solar Wind variability Magnetic draping
­ Europa's induced field can impact the flow of cold plasma on the satellite ­ The strength of Europa's induced field varies as it passes in and out of Jupter's magnetic equator ­ We have not yet examined the effects of this source of variability in a quantitative sense
(Khurana et al., in press)


Radiation Environment
Short-term variability
· · · Solar Wind variability Magnetic draping Flux of neutrals
­ Neutrals act as a `buffer', effectively cooling energetic particles ­ Volcanic activity on Io can effect the population of neutrals around Europa
(Smyth and Marconi, 2006)


Radiation Environment
Short-term variability
· · · Solar Wind variability Magnetic draping Flux of neutrals
(Smyth and Marconi, 2006)

These sources of variability effect the radiation dose at Europa but we do not believe they greatly effect the strong asymmetry present


Results


Results


Further Considerations
· Impact gardening by micrometeorite bombardment results in vertical mixing of the surface of Europa
­ This mechanism is expected to preferentially affect the leading hemisphere [Schenk et al., 2004]. ­ Given a mean surface age for Europa of ~107 yr [Zahnle et al., 1998], gardening should extend to a depth of 1.3 m [Cooper et al., 2001]. ­ Mixing rates at Europa can be as high as 1.2 m/yr for a fresh surface while it has been suggested that the sputtering rate due to radiolytic processes is more than an order of magnitude less at ~0.02 m/yr [Cooper et al., 2001].


Further Considerations
· Modeling suggests that the decoupled outer ice shell of Europa should undergo nonsynchronous rotation with respect to its interior due to torques imposed by tidal forces [Greenberg and Weidenshilling, 1984; Ojakangas and Stevenson, 1989]
­ Comparisons of Voyager and Galileo images [Hoppa et al., 1999] suggest that this mechanism would lead to rotations of 1° in longitude over timescales >103 yr ­ Such a process would `smear' the effects of radiolysis and impact gardening in the near-term


Summary
· Electrons in the 100s of keV to 10s of MeV range, which dominate the radiation dose at Europa, preferentially get deposited into the trailing hemisphere.
­ Their bombarding fluxes systematically decrease across the remainder of the satellite as a function of longitude and latitude. ­ This is important to consider when determining where to land (i.e. total ioniziing dose (TID), single event upsets, etc.).

· · ·

Impact gardening and nonsynchronous rotation also effect the surface and will need to be characterized These processes are ongoing and interact with each other to produce a complex and global cycle of chemical alteration and surface erosion Understanding how this cycle works can provide essential information for assessing the science value and risk associated with potential landing sites


Results



Radiation Environment
Short-term variability
· · · Solar Wind variability Magnetic draping Flux of neutrals
(Mauk et al., 2004)

(Smyth and Marconi, 2006)