Europa 445
probably the result of radiolytic disruption of the regular
crystalline structure. This disruption can be counteracted by
thermal annealing and recrystallization, but these processes
are impeded by Europa’s cold surface temperatures.
Sputtering andthermal desorptionact to remove wa-
ter from water ice and hydrated minerals. These water
molecules may either escape to space, but more typically,
recondense elsewhere on the satellite as frost. This deposi-
tion will vary depending on temperature and surface albedo,
so the frost will be more likely to be deposited at high lati-
tudes and on bright surfaces than at the warmer equatorial
latitudes on darker materials. Frost deposition may be re-
sponsible for the brightening and whitening of Europa’s
dark reddish surface features over time. In addition, radi-
olysis itself may cause chemical changes that brighten the
surface.
Mass wasting, which is movement of material downs-
lope under the influence of gravity, is less significant on Eu-
ropa than on the other Galilean satellites because the sur-
face is so young. Mass wasted material is commonly dark and
is likely the non-ice debris that remains after the surround-
ing ice has been removed by sputtering and sublimation.
This lag material may be salts and impactor contaminants.
Sublimation (Fig. 20) has played a significant role in
shaping and muting the topography of Callisto and, to a
lesser extent, Ganymede, and has occurred only in darker
warmer regions on Europa, potentially including where
warm material has been in close proximity to the surface.
Sublimation lags have been suggested to result as water
molecules are driven off by the intrusion of warm water or
ice along ridges and chaos regions. This process has been
suggested as the origin of low albedo spots along triple bands
and of dark material along the flanks of ridges. Some craters
have dark material in their floors, which is consistent with a
thermal lag produced during the impact cratering process,
when very hot material from the impact would have rapidly
sublimated any water ice off the surface, and by the downs-
lope movement of dark lag material onto the crater floor.
7. Surface Age and Evolution
7.1 Surface Age
Europa’s surface age can be coarsely estimated from the
number of large impact craters on its surface, if accurate
estimates of the impactor flux can be made. Modeling of the
dynamics of small solar system bodies suggests that the im-
pactor population at Jupiter’s orbit is dominated by comets,
specifically,Jupiter-family comets. From the paucity of
large (>10 km diameter) craters on Europa, this model
implies a surface age of∼60 Ma (million years), with un-
certainties of about a factor of 3.
Another way to estimate Europa’s age is to use esti-
mates of ice sputtering, which occurs when high-energy
FIGURE 20 Sublimation of water molecules by sunlight results
in a dark lag deposit, which can move down slopes to collect at
their bases. This process appears to have occurred at the cliff in
the center of this high-resolutionGalileoimage. The water
molecules may be “cold-trapped” on brighter, icier surfaces,
forming frost deposits. (Image: NASA/JPL.)particles swept along with Jupiter’s magnetic field impact
Europa’s surface, causing ice particles to be dislodged, most
of which then escape to space. This process has a number
of uncertainties, but measurements fromGalileo’s Ener-
getic Particle Detector (EPD) have lead to estimates that
a couple of centimeters to over half a meter of ice may
be removed every million years. High-resolution imaging
has shown numerous examples of topography on vertical
scales of tens of meters; this observation is consistent with
an age similar to that predicted by the comet impactor
model.
The dearth of impact craters on Europa makes it an ex-
cellent place to study the ratio of primary to secondary
craters, something that is very difficult to accomplish
on heavily cratered bodies like the Moon and Mercury.
Although high-resolution imaging of Europa’s surface is