Lunar and Planetary Science XLVIII (2017)
1955.pdf
BREAK THE WORLD'S SHELL: AN IMPACT ON ENCELADUS: BRINGING THE OCEAN TO THE
SURFACE. J. H. Roberts1 and A. M. Stickle1, 1Johns Hopkins University Applied Physics Laboratory, 11100
Johns Hopkins Rd., Laurel, MD 20723, [email protected]
Introduction: The south polar region on Enceladus
is characterized by plume activity, in which jets of
vapor and ice emanate from four long fractures called
the tiger stripes [1,2]. Salts observed in these plumes
suggest that a subsurface ocean is in direct communication with the surface [3]. A substantial thermal
anomaly of ~10 GW is associated with the south polar
region [4] and strongly correlates with the location of
the tiger stripes. Tides control these fractures and are
the probable energy source for the thermal anomaly.
However, tidal heating is symmetric about the equator,
and no corresponding thermal anomaly or activity is
observed in the north.
The presence of significant lateral variations in the
mechanical properties of the ice shell can break this
symmetry. A south polar sea [5] may be more stable
against total freezing than a global ocean [6] and is
consistent with gravity measurements made by Cassini
[7]. However, libration observations indicate that the
ice shell is mechanically decoupled from the silicate
core [8], requiring a global ocean that may approach
the surface in the south polar region.
However, a physical mechanism to break the tidal
symmetry and promote activity in only one hemisphere
has not been identified. Here we examine the effects of
a large impact on both the initial meltwater production
and on softening of the surrounding ice by shock heating and fracturing of the crater floor. We model the
uplift of the crater and draining of the meltwater in the
days following the impact, and the longer term tidal
dissipation and thermal evolution of the ice shell.
Models: We use the CTH hydrocode [9] in 2D to
simulate a vertical impact into an ice shell over an
ocean and use the 5-phase ANEOS for ice [10] to
compute the temperature change and melt production.
We model thermal evolution of the ice shell using Citcom in 2D-axisymmetric geometry [11]. We compute
the tidal heating using the propagator-matrix code TiRADE [12]. To do this, we read in the impact heating
and initial tidal heating into Citcom, and allow the
temperature to evolve, periodically updating the tidal
heating based on the evolving viscosity structure.
Results: In Figure 1, we show the temperature increase resulting from the vertical impact of a 5-km
diameter icy projectile into a 40-km thick ice shell over
an ocean at 20 km/s, scaled to create a crater the approximate size of the south polar terrain (SPT) [1].
Unless such an impact occurred in the very recent past,
the thermal effects would not be observable today.
Even though an ice shell this thin does not convect, the
Figure 1: Temperature profile predicted from CTH
simulations in and below the ice shell, 30 s after an
SPT-forming impact.
impact heat diffuses away in ~1 My. This result is
broadly similar to that of earlier studies of a very slow
collision with a co-orbital object [13]. Local softening
of the ice by impact heating only sustains this thermal
anomaly for a few My.
More significantly, such a projectile excavates
2×105 km3 of material. After ~10 minutes, the transient
cavity reaches a depth of > 30 km deep, approaching
the ocean in the example above (Figure 2 top), or penetrating entirely through a somewhat thinner ice shell.
Approximately 5000 km3 of meltwater is retained,
enough for a 5 km melt pond at the bottom of the
crater. The impact's effects extend far beyond the transient cavity however. The shock pressure far exceeds
the yield strength of ice nearly everywhere in the region, and the floor of the crater is extensively fractured.
The transient cavity collapses over the next ~1
hour. Following existing scaling relations for impacts
in ice [14], the final crater in this example will be ~150
km wide and 11 km deep (Figure 2, center). Although
volume is conserved in the collapse, the surface area
triples, and the fracturing is even more pervasive.
Based on scaling derived for melt drainage from impacts on icy satellites [15], we expect over 3×104 fractures. The melt pond spreads out and is < 2 km deep
after collapse. However, due to the extensive fracturing
of the crater floor the melt can drain through the ice
into the ocean below. The timescale for this is controlled by the permeability of the ice. This is not well
known, but assuming that it resembles coarse gravel,
Lunar and Planetary Science XLVIII (2017)
the melt would take days to years to finish draining.
During this time period, the top few meters of the melt
pond may freeze. This resulting ice sheet would collapse as the underlying melt drains away and form an
icy regolith on the crater floor.
While the melt drains, the surface uplifts until the
region is isostatic (Figure 2, bottom). This leaves a ~1
km topographic depression at the surface.
Discussion: Our scenario results in an ice shell that
is thinner in the impact site. We note that the impact
need not have occured at the present-day south pole.
Figure 2: Sketch of the ice shell (gray) and ocean
(blue) of Enceladus beneath the impact site, shown
after excavation (top, t ~10 min.); collapse (center,
t ~1 hr.); and draining and uplift (bottom, t ~3
weeks), Stippled area indicates extensive fracturing. The top of the silicate core is shown in brown.
1955.pdf
The impact feature is a negative mass anomaly that
would induce reorientation of the ice shell, bringing
the impact to the pole from whereever it occured [16].
Here we have started with a 40-km thick ice shell,
which gets reduced by ~9 km under the impact site.
However, this amount of thinning is relatively insensitive to the initial shell thickeness. One caveat to this is
if the impact were to penetrate entirely through the ice
shell, which may occur for certain combinations of
initial shell thickness and projectile size and velocity.
We are currently running hydrocode models to explore
this possibility.
Although we have illustrated a mechanism for
breaking the tidal symmetry about the equator, and to
enable communication between the ocean and the surface, more work is needed to understand how we get
from a highly fractured crater floor to only four huge
fractions. The smaller fractures will freeze more quickly. Based on the scalings in [15], a 5-cm fracture
would freeze in only a few minutes at surface temperatues, but larger fractures could remain open for
weeks. Warm ice at depth would extend these timescales dramatically, though warm ice is susceptible to
ductile flow which could also close fractures [17]. Extensive fracture modeling [18] is needed to quantify
how the four tiger stripes could be sustained over geologic time scales. Finally, we would like to be able to
test the hypotheses we have proposed here. The current
south polar terrian is not the original impact structure;
that would have been heavily modified by geologic
activity. Possible evidence for such an impact may
inlcude ejecta (or secondary craters) far from the impact site, or potentially degradation in crater morphology with latitude.
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