Styles of Cratering on Europa

1
Styles of Cratering on Europa
Abstract 2005, 33rd LPSC, March 2002
Clark R. Chapman, Beau Bierhaus, and William J.
Merline
Southwest Research Institute, Boulder CO
Secondary Craters on Europa Bierhaus and his
colleagues cf. 4 have embarked on a
multifaceted approach of studying secondary
craters on Europa, which are potentially the
chief alternative to primary craters for the
smaller craters seen on Europa's surface. He has
found that (a) smaller craters are concentrated
in a Pwyll ray in the vicinity of Conamara Chaos
and decrease away from the ray (b) statistical
tests show that small craters are non-randomly
distributed on typical Galileo frames and (c)
that simple modelling (based on mass arguments)
of the number of secondary fragments from the few
large primary craters on Europa suggests that as
many as all of the smaller craters could be
secondaries. Of course, there must be some
smaller primaries, and the issue is to determine
what fraction are primaries and, if possible, to
determine which particular ones are the primaries
so that they can be used to date units. Most
recently, clustering algorithms have been used to
determine that as many as 80 of small craters on
typical Europan units (not proximate to big
primaries) are spatially clustered (but showing
no correlation with geological unit boundaries),
implying that they are secondaries 5. In this
poster we show counts of small craters around
Tyre, one of the largest ring structures on
Europa, as well as in another region, near a
strike-slip fault, far from a large primary
crater. Each dot indicates the location of a
small crater. It can be seen that the
distribution of small craters is asymmetric in
the near-field of Tyre as before, far-field
craters are distinctly clustered. The
accompanying R-plots of frequency as a function
of size show the steep slopes (-4 to -5.5
differential), another characteristic of
secondary cratering.
Introduction Since Voyager imaging, Europa has
been known to have a relatively youthful surface.
What were then thought to be poorly resolved
impact craters 10 km in diameter are now
understood to be "pits", part of a suite of
endogenic features generally known as lenticulae.
From the 12-20 large (gt20 km diameter), primary
impact craters on Europa and the (imperfectly)
known flux of small bodies (chiefly Jupiter
family comets), the average age of Europa's
surface is 50 Myr, within factors of a few 1.
During this comparatively recent epoch, the style
of resurfacing of Europa's surface has undergone
a dramatic change 2, whether it be a single
change from ridged plains to lenticulae/chaos or
only the final stage of a cyclical
phenomenon. In order to study the duration of
phases of this evolving style, and to determine
whether or not it happened contemporaneously over
the entire globe or evolved region-by-region
(e.g. perhaps more rapidly at the poles and more
slowly at the equator), it would be useful to
determine -- from crater counts -- the relative
ages of smaller geological units (or at least
regions) on Europa. However, the numbers of
smaller craters are too few to permit
statistically significant counts in smaller
units. For example, craters 1 km in diameter are
down nearly 3 orders of magnitude from empirical
saturation, with only a couple showing up in a
typical medium resolution Galileo frame that is
a distribution far too coarse to permit relative
age-dating by crater counts. While craters ltlt1
km diameter become quite numerous on Europa, at
least in certain locales, their spatial densities
and size distributions are markedly non-uniform,
raising questions about whether they are primary
craters. Indeed, Chapman 3 suggested, from
preliminary Galileo observations of small crater
densities on several Galilean satellites, that
the numbers of primary craters might be
unexpectedly small, suggesting a depletion
(relative to expectations of a collisional size
distribution) for small comets.
This locale on Europa (E17 STRSLP01) illustrates
the kind of clustering of small craters evident
in most high-resolution frames. While the
craters are somewhat easier to detect on the
smoother terrains, biases in sampling are small.
The middle panel shows the center positions (not
sizes) of all identified craters, showing the
strikingly non-random distribution. The
righthand panel represents the results of a run
of one clustering algorithm, which replaces the
center dot with a cluster sequence number (0
not in any cluster). It is evident that a rather
small proportion of craters are outside of
identified clusters. We believe that clustering
is a strong indicator of secondary cratering.
Another indicator is the steep slope of the size
distribution of the identified craters, shown in
an R-plot to the right (data for small craters,
believed to be subject to incompleteness, are
omitted from the plot.
Take-Away Message Secondary cratering
dominates the small-crater populations on Europa,
even far from the few large primaries. If this
is true on other bodies as well (where crowding
renders clustering of distant secondaries less
apparent), secondary cratering could be a more
important process than has been believed,
especially in the outer solar system, where small
comets may be few.
Discussion It is beginning to appear that only
a very small fraction of smaller craters on
Europa can be primaries. Depending on the
numbers of very distant secondaries, plus any
possible Jupiter orbital debris that impacts
Europa, the more randomly distributed subset of
small craters could be primaries. Allowing for
possible leading/trailing side differences, the
minimum crater densities may eventually be
identified permitting us to estimate the primary
contribution. Of course, at least crudely,
secondary craters may also be used to determine
relative ages of units, since some secondary
crater fields are undoubtedly older than
others. The very youthful surface of Europa is
probably the best place to study the physics of
secondary cratering. Because of the few
primaries, secondary crater fields should not
generally overlap on Europa, yet ejecta are
clearly emplaced far from primaries in order to
account for clusters of small craters seen in all
but one of many dozens of frames examined. If
similar numbers of secondaries are generated on
other icy and rocky bodies, they could be more
important contributors to small-crater
populations on other bodies than had been
thoughtespecially in the outer solar system,
where comets should dominate the primary impactor
population, but even in the inner solar system,
as well.
Although details are difficult to grasp from just
looking at this mosaic of Tyre, plots of crater
center positions reveal a striking pattern of
secondaries. Although there is a general
decrease in secondary crater density with
distance from the center of Tyre, significant
asymmetries are evident. The R-plot above
represents the size-frequency relation for the
identified craters. The slope is very steep for
sizes down to 1 km diameter. The two data points
to the left roll off into incompleteness,
although it is possible that second point is
valid and that there is a true preference for
near-field secondaries about 1 km in diameter.
References 1 Zahnle K. (2001) LPSC XXXII,
1699. 2 Spaun N.A. (2001) PhD. thesis, Brown
Univ. 3 Chapman C.R. (1997) MAPS, 32, A27.
4 Bierhaus E.B. et al. (2001) Icarus, 153,
264-276. 5 Bierhaus E.B. et al. (2001) Bull.
Am. Astron. Soc., 33, 1106-1107.