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Robert Pappalardo (Ph.D. Arizona State Univ. '94, B.A.
Cornell University '86) researches processes that have shaped the surfaces of
icy outer planet satellites. Past research focused on Miranda, an icy
satellite of Uranus; examination of ridge and trough sets on Miranda led to
the conclusion that large upwellings within the satellite shaped its surface.
More recent research focuses on the Galilean satellites of Jupiter,
especially Europa and Ganymede. Ganymede research involves the nature,
origin, and evolution of its bright grooved terrain, specifically the style
of tectonism and implications for the satellite's geological history. Europa
research includes the possibility that solid-state convection has played an
important role in the satellite's history, investigation of regions of
separation and spreading of the satellite's icy lithosphere, and implications
of the surface geology for lithospheric properties. Overall, Europa research
relates to the question of whether the satellite has a liquid water ocean
beneath its icy surface which might possibly be a habitat for life.
Nimmo, F., and R. T. Pappalardo. Diapir-Induced Reorientation of Saturn’Äôs
moon Enceladus. Nature, 441, 614-616, 2006.
Enceladus is a small icy
satellite of Saturn. Its south polar region consists of young, tectonically
deformed terrain and has an anomalously high heat flux. This heat flux is
probably due to localized tidal dissipation within either the ice shell3 or the underlying silicate
core. The surface deformation is
plausibly due to upwelling of low-density material (diapirism5) as a result of this tidal heating. Here we
show that the current polar location of the hotspot can be explained by
reorientation of the satellite’Äôs rotation axis because of the presence of a
low-density diapir. If the diapir is in the ice shell, then the shell must be
relatively thick and maintain significant rigidity (elastic thickness greater
than ~0.5 km); if the diapir is in the silicate core, then Enceladus cannot
possess a global subsurface ocean, because the core must be coupled to the
overlying ice for reorientation to occur. The reorientation generates large
(~10MPa) tectonic stress patterns that are compatible with the observed
deformation of the south polar region2.
We predict that the distribution of impact craters on the surface will not
show the usual leading hemisphere’Äìtrailing hemisphere asymmetry. A
low-density diapir also yields a potentially observable negative gravity
anomaly.
Patterson, G. W., J. W. Head, and R. T. Pappalardo. Plate motion on Europa
and nonrigid behavior of the icy lithosphere: The Castalia Macula Region. J.
Struct. Geol., in press.
We have developed a generalized quantitative technique for determining the
finite pole of rotation between two rigid plates and use it to critically
examine differing reconstructions of a region surrounding a prominent dark
spot on Europa, Castalia Macula. This region is located near the equator of
Europa's trailing hemisphere and has been suggested as a site where crustal
convergence may have occurred. Previous reconstructions of the region have
indicated that a ridge set and/or band-like complex that define a collection
of tectonic plates in the region accommodated surface contraction. However, a
critical examination of the differences between these reconstructions has
been complicated by the lack of a finite pole of rotation for the plates
involved in either reconstruction. We have applied our modeling technique,
coupled with a detailed examination of the morphology and cross-cutting
relationships involving this ridge set and band-like complex, to determine if
a unique reconstruction exists for a number of tectonic plates in this
region. The cross-cutting relationships involving the ridge set also allow us
to test the general assumption that plates behave rigidly on Europa. Assuming
rigid behavior our results suggest that a unique reconstruction does exist
indicating the ridge set accommodated surface contraction. However, analysis
performed to test the assumption of plate rigidity indicates that one or more
of the plates in the region did not behave rigidly. This does not rule out
surface contraction along the ridge set but does indicate that a component of
nonrigid behavior must be taken into account.
Barr, A. C. and R. T. Pappalardo. Onset of Convection in the Icy Galilean
Satellites: Influence of Rheology. J. Geophys. Res., 110, E12005,
doi:10.1029/2004JE002371, 2005.
Ice I exhibits a complex rheology at temperature and pressure conditions
appropriate for the interiors of the ice I shells of Europa, Ganymede, and
Callisto. We use numerical methods and existing parameterizations of the
critical Rayleigh number to determine the conditions required to trigger
convection in an ice I shell with the stress-, temperature- and grain size-
dependent rheology measured in laboratory experiments by Goldsby and Kohlstedt
[2001]. The critical Rayleigh number depends on the ice grain size and the
amplitude and wavelength of temperature perturbation issued to an initially
conductive ice I shell. If the shells have an assumed uniform grain size 0.4
mm, deformation during initial plume growth is accommodated by Newtonian
volume diffusion. If the ice grain size is between 0.4 mm and 3 cm,
deformation during plume growth is accommodated by weakly non-Newtonian grain
boundary sliding, where the critical ice shell thickness for convection
depends on the amplitude of temperature perturbation to the -0.5 power. If
the ice grain size exceeds 2 cm, convection cannot occur in the ice I shells
of the Galilean satellites regardless of the amplitude or wavelength of
temperature perturbation. If the grain size in a convecting ice I shell
evolves to effective values greater than 2 cm, convection will cease. If the
ice shell has a grain size large enough to permit flow by dislocation creep,
the ice is too stiff to permit convection, even in the thickest possible ice
I shell. Consideration of the composite rheology implies that estimates of
the grain size in the satellites and knowledge of their initial thermal
states are required when judging the convective instability of their ice I shells.
Stempel, M. M., A. C. Barr, and R. T. Pappalardo, Model constraints on the
opening rates of bands on Europa. Icarus, 177, 297-304, 2005.
A mid-ocean-ridge spreading analog is used to constrain the opening rates and
brittle-ductile transition depths for two axisymmetric ridged bands on
Europa. Estimates of brittle-ductile transition depth based on the
morphologies of Yelland and Ino Lineae are combined with a conductive cooling
model based on a mid-ocean ridge analog to estimate the opening rates and
active lifetimes of the bands. This model limits local strain rates to
~10^-15 to 10^-12 s^-1, opening rates to 0.2 to 40 mm yr^-1, and active
lifetimes of the bands to 0.1 to 30 Myr. If the observed structures in the
outer portions of ridged bands are indeed normal faults, the estimated range
for the tensile strength of ice on Europa is 0.4 to 2 MPa, consistent with
nonsynchronous rotation as the dominant driving mechanism for band opening.
Pappalardo, R. T., and G. C. Collins, Extensional tectonics on Ganymede as
recorded by strained craters. J. Struct. Geol., 27, 827-838, 2005.
High-resolution images of Jupiter's satellite Ganymede obtained by the
Galileo spacecraft reveal several elliptical craters that are transected by
sets of subparallel ridges and troughs oriented roughly orthogonal to the
long axis of the crater, implying that these craters have been extensionally
strained. We analyze five such craters, four in the ancient dark terrain and
one in bright grooved terrain, inferring their strain histories by assuming
each crater was originally circular. These strained craters have been
elongated by amounts ranging from ~5 to ~50%. All are deformed by extension
near-perpendicular to subparallel ridges and troughs, and some are also
deformed by distributed simple shear parallel to these tectonic structures.
The rift zones that deform these craters show extensions of ~5 up to ~180%
for a well-defined fault zone that rifts Saltu crater within dark terrain of
Nicholson Regio. Extensional fault zones consisting of domino-style normal
faults and representing strains of several tens of percent may be common on
Ganymede, and strains of this order can alter pre-existing surface features
beyond recognition through the process of tectonic resurfacing.
Prockter, L. M., R. T. Pappalardo, and F. Nimmo. A Shear heating origin for
ridges on Triton. Geophys. Res. Lett., 32, L14202, doi:10.1029/2005GL022832,
2005.
Triton and Europa each display a variety of distinctive curvilinear ridges
and associated troughs. Here we propose that ridges on Triton may have formed
by diurnal tidal stresses, in a manner similar to that proposed for ridge
formation on Europa. The greater width of ridges on Triton is likely a
consequence of the lower surface temperature and greater brittle-ductile
transition depth on this body compared to Europa. The magnitudes of the
stresses and heat fluxes required to generate ridges of the correct scale are
comparable to likely values generated during the latter part of Triton's
orbital evolution from an initial highly eccentric state.
Barr, A. C., S. Zhong, and R. T. Pappalardo. Convective instability in ice I
with non-Newtonian rheology: Application to the Galilean satellites. J.
Geophys. Res., 109, E12008, doi:10.1029/2004JE002296, 2004.
Convective instability in ice I with non-Newtonian rheology: Application to
the Galilean satellites. J. Geophys. Res., 109, E12008,
doi:10.1029/2004JE002296, 2004.
At the temperatures and stresses associated with the onset of convection in
an ice I shell of the Galilean satellites, ice behaves as a non-Newtonian
fluid with a viscosity that depends on both temperature and strain rate. The
convective stability of a non-Newtonian ice shell can be judged by comparing
the Rayleigh number of the shell to a critical value. Previous studies
suggest that the critical Rayleigh number for a non-Newtonian fluid depends
on the initial conditions in the fluid layer, in addition to the thermal,
rheological, and physical properties of the fluid. We seek to extend the existing
definition of the critical Rayleigh number for a non-Newtonian, basally
heated fluid by quantifying the conditions required to initiate convection in
an ice I layer initially in conductive equilibrium. We find that the critical
Rayleigh number for the onset of convection in ice I varies as a power (-0.6
to -0.5) of the amplitude of the initial temperature perturbation issued to
the layer, when the amplitude of perturbation is less than the rheological
temperature scale. For larger-amplitude perturbations, the critical Rayleigh
number achieves a constant value. We characterize the critical Rayleigh
number as a function of surface temperature of the satellite, melting
temperature of ice, and rheological parameters so that our results may be
extrapolated for use with other rheologies and for a generic large icy
satellite. The values of critical Rayleigh number imply that triggering
convection from a conductive equilibrium in a pure ice shell less than 100 km
thick in Europa, Ganymede, or Callisto requires a large, localized
temperature perturbation of a few kelvins to tens of kelvins to soften the
ice and therefore may require tidal dissipation in the ice shell.
Schenk, P.M. and R.T. Pappalardo. Topographic variations in chaos on Europa:
Implications for diapiric formation. Geophys. Res. Lett., 31, L16703,
doi:10.1029/2004GL019978, 2004.
Disrupted terrain, or chaos, on Europa, might have formed through melting of
a floating ice shell from a subsurface ocean [Carr et al., 1998; Greenberg et
al., 1999], or breakup by diapirs rising from the warm lower portion of the
ice shell [Head and Pappalardo, 1999; Collins et al., 2000]. Each model makes
specific and testable predictions for topographic expression within chaos and
relative to surrounding terrains on local and regional scales. Highresolution
stereo-controlled photoclinometric topography indicates that chaos
topography, including the archetypal Conamara Chaos region, is uneven and
commonly higher than surrounding plains by up to 250 m. Elevated and undulating
topography is more consistent with diapiric uplift of deep material in a
relatively thick ice shell, rather than melt-through and refreezing of
regionally or globally thin ice by a subsurface ocean. Vertical and
horizontal scales of topographic doming in Conamara Chaos are consistent with
a total ice shell thickness >15 km. Contact between Europa's ocean and
surface may most likely be indirectly via diapirism or convection.
Pappalardo, R. T., and A.C. Barr, The origin of domes on Europa: The role of
thermally induced compositional diapirism, Geophys. Res. Lett., 31, L01701,
doi:10.1029/2003GL019202, 2004.
The surface of Jupiter's moon Europa is peppered by topographic domes,
interpreted as sites of intrusion and extrusion. Diapirism is consistent with
dome morphology, but thermal buoyancy alone cannot produce sufficient driving
pressures to create the observed dome elevations. Instead, diapirs may
initiate by thermal convection that induces compositional segregation.
Exclusion of impurities from warm upwellings allows sufficient buoyancy for
icy plumes to create the observed surface topography, provided the ice shell
has a small effective elastic thickness (~0.2 to 0.5 km) and contains
low-eutectic point impurities at the few percent level. This model suggests
that the ice shell may be depleted in impurities over time.
Head, J. W., R. Pappalardo, G. Collins, M. J. S. Belton, B. Giese, R. Wagner,
H. Breneman, N. Spaun, B. Nixon, G. Neukum, and J. Moore. Evidence for
Europa-like resurfacing styles on Ganymede. Geophys. Res. Lett.,
10.1029/2002GL015961, 2002.
Very high-resolution imaging and stereo topographic data obtained during the
Galileo G28 encounter with Ganymede show 1) evidence for Europa-like, crustal
spreading and resurfacing to form portions of the bright terrain, and 2)
bright terrain that appears smooth at Voyager resolution (and thus a strong
candidate for cryovolcanism) but instead is tectonically deformed and lacks
embayment relationships when viewed at high resolution. In contrast to
previous views, these new data show that tectonism has been the dominant
process in shaping some very smooth areas and that Ganymede appears to have
experienced Europa-like crustal spreading during its previous history.
Jones, K. B., J. W. Head, R. T. Pappalardo, and J. M. Moore, Morphology and
origin of palimpsests on Ganymede from Galileo observations, Icarus, 164,
197-212, 2003.
Palimpsests are large, circular, low-relief impact scars on Ganymede and
Callisto. These structures were poorly understood based on Voyager-era
analysis, but high-resolution Galileo images allow more detailed inspection.
We analyze images of four Ganymedean palimpsests targeted by Galileo: Memphis
and Buto Faculae, Epigeus, and Zakar. Ganymedean craters and Europan ring
structures are used as tools to help better understand palimpsests, based on
morphologic similarities. From analysis of Galileo images, palimpsests
consist of four surface units: central plains, an unoriented massif facies, a
concentric massif facies, and outer deposits. Using as a tie point the
location in these structures where secondary craters begin to appear, outer
deposits of palimpsests are analogous to the outer ejecta facies of craters;
the concentric massif facies of palimpsests are analogous to the pedestal
facies of craters; and the unoriented massif facies and central plains are
analogous to crater interiors. These analogies are supported by the presence
of buried preexisting structure beneath the outer two and absence of buried
structure beneath the inner two units. Our observations indicate that
palimpsest deposits represent fluidized impact ejecta, rather than
cryovolcanic deposits or ancient crater interiors.
Nimmo, F., R. T. Pappalardo, and B. Giese. On the origins of band topography,
Europa. Icarus, 166, 21-32.
We use stereo-derived topography of extensional bands on Europa to show that
these features can be elevated by 100-150 m with respect to the surroundings,
and that the positive topography sometimes extends beyond the band margins.
Lateral variations in shell thickness cannot maintain the observed topography
for timescales greater than ~0.1 Myr. Lateral density variations can maintain
the observed topography indefinitely; mean density contrasts of 5 and 50
kgm^3 are required for shell thicknesses of 20 and 2 km, respectively.
Density variations caused by temperature contrasts require either present-day
heating or that bands are young features (< 1 Myr old). Stratigraphic
analyses suggest that these mechanisms are unlikely. The observation that
bands form from ridges may be explained by an episode of shear-heating on
ridges weakening the ridge area, and leading to strain localization during
extension. Fracture porosity is likely to persist over Myr timescales in the
top one-third to one-quarter of the conductive part of the ice shell. Lateral
variations in this porosity (of order 20%) are the most likely mechanism for
producing band topography if the ice shell is thin (_ 2 km); porosity
variations of 2% or less are required if the shell is thicker (_ 20 km). If
the ice shell is thick, lateral variations in salt content are a more likely
mechanism. Warm ice will tend to lose dense, low-melting temperature phases
and be buoyant relative to colder, salt-rich ice. Thus, lateral density
variations will arise naturally if bands have been the sites of either
localized heating or upwelling of warm ice during extension.
Nimmo, F., B. Giese, and R. T. Pappalardo, Estimates of Europa's ice shell
thickness from elastically-supported topography. Geophys. Res. Lett., 30(5),
10.1029/2002GL016660, 2003.
The thickness of Europa's solid ice shell is uncertain, and has important
implications for Europa's habitability and thermal history, and the design of
future spacecraft missions. Here we obtain an estimate of the ice shell
thickness from observations of a plateau SW of Cilix impact crater. Stereo
topographic profiles suggest that the plateau is flexurally supported, with
an effective elastic thickness t_e of 6 (+5/-2) km. For a conductive
temperature profile this t_e value implies a solid ice shell thickness of 15
(+20/-9) km; if the shell is convecting, this estimate is a lower bound.
Combined with independent estimates, we infer a probable shell thickness of
_25 km. The shell thickness is likely to be uniform over the entire
satellite.
Pappalardo, R.T. and R. Greeley. A review of the origins of subparallel
ridges and troughs: Generalized morphological predictions from terrestrial
models. J. Geophys. Res., 100, 18985-19007, 1995.
The morphologies of ridges and troughs on Earth and the processes involved in
their formation are reviewed for application to sets of subparallel ridges
and troughs on outer planet satellites. We consider: tension fracturing,
normal faulting, thrust faulting, buckle folding, strike-slip tectonism,
fissure eruption, wall diapirism, surface folding of a flow, and near-surface
(laccolithic) intrusion. The landform characteristics and geologic
associations predicted of each process are generalized to be independent of
planet-specific parameters. We find that the combinations of landforms
derived from each process are unique, making each distinguishable in the
idealized case. Morphological parameters most diagnostic of formational
process are ridge and trough cross-sectional shape, termination style, and
incipient character. The characteristics described are useful in constraining
the origin of ridge and trough terrains observed in Voyager images and will
aid in analysis of high-resolution images from the Galileo and Cassini
spacecraft.
Pappalardo, R.T. and R.J. Sullivan. Evidence for separation across a gray
band on Europa. Icarus, 123, 557-567, 1996.
Thynia Linea, a gray band on Europa, is found to be a ~25 km wide and >900
km long region of lithospheric separation which has been infilled by
relatively dark material. Roughly a dozen older features and the cuspate
segments that form its outline appear to have been displaced across its
width. Displacement azimuths indicate a best fit pole of opening near 79_S,
200_W. However, displacement magnitude decreases toward either end of the
gray band, indicating that it is more akin to a "tear" in a
nonrigid europan lithosphere. Opening was in response to NW-SE directed
tensile stress, in accord with the stress predictions of nonsynchronous
rotation. Observations of Thynia Linea are consistent with a laterally mobile
brittle lithosphere, decoupled from ductile or liquid material below, as
previously suggested to account for opening of wedge-shaped bands in Europa's
antijovian region. Lithospheric separation and contemporaneous emplacement of
new material offers a possible volcano-tectonic scenario for resurfacing
Europa. If this process is ongoing, resurfacing can be accomplished on a time
scale consistent with the satellite's surface age if one gray band or zone of
wedge-shaped bands forms every ~10^3-10^4 yr and becomes unrecognizable with
age. This would imply that features on Europa brighten with age, as through
continuous deposition of frost onto the surface.
Pappalardo, R.T., S.J. Reynolds, and R. Greeley. Extensional tilt blocks on
Miranda: Evidence for an upwelling origin of Arden Corona. J. Geophys. Res.,
102, 13369-13379, 1997.
Subparallel ridges and troughs in the outer belt of Arden Corona, on the
Uranian satellite Miranda, are interpreted as tilt blocks formed by extension
and normal faulting. Fault scarps generally face outward from the corona,
exposing dark material in the subsurface. Reconstruction of faults along a
deep rift zone bounding the corona suggests initial dips of ~50_. Local
extension reaches ~70%, extremely high in comparison to previous estimates of
strain on icy satellites. A rise adjacent to the rift zone is modeled as
flexural and indicates an effective elastic lithospheric thickness of ~2 km
at the time of flexure. The assumption that faulting has significantly
weakened the lithosphere suggests a mechanical lithosphere thickness of ~5 to
10 km. Corresponding thermal gradients in a frictionally controlled ice
lithosphere are ~8 to 20 K km^-1, and lithospheric tensional strength is ~0.4
to 1.8 MPa. Normal faulting in Arden Corona indicates that internal upwelling
likely formed the corona, and the outward facing direction of faults is
consistent with such a model. An upwelling origin of Miranda's coronae
eliminates the need to invoke catastrophic breakup and reaccretion of the
satellite as an explanation for its surface geology.
Pappalardo, R.T., J.W. Head, G.C. Collins, R.L. Kirk, G. Neukum, J. Oberst,
B. Giese, R. Greeley, C.R. Chapman, P. Helfenstein, J.M. Moore, A. McEwen,
B.R. Tufts, D.A. Senske, H.H. Breneman, and K. Klaasen. Grooved terrain on
Ganymede: First results from Galileo high-resolution imaging. Icarus, 135,
276-302, 1998.
High-resolution Galileo imaging has provided important insight into the
origin and evolution of grooved terrain on Ganymede. The Uruk Sulcus target
site was the first imaged at high resolution, and considerations of
resolution, viewing geometry, low image compression, and complementary stereo
imaging make this region extremely informative. Contrast variations in these
low-incidence angle images are extreme and give the visual impression of
topographic shading. However, photometric analysis shows that the scene must
owe its character to albedo variations. A close correlation of albedo
variations to topography is demonstrated by limited stereo coverage, allowing
extrapolation of the observed brightness and topographic relationships to the
rest of the imaged area. Distinct geological units are apparent across the
region, and ridges and grooves are ubiquitous within these units. The
stratigraphically lowest and most heavily cratered units ("lineated
grooved terrain") generally show morphologies indicative of
horst-and-graben style normal faulting. The stratigraphically highest groove
lanes ("parallel ridged terrain") exhibit ridges of roughly
triangular cross-section, suggesting that tilt-block style normal faulting
has shaped them. These extensional-tectonic models are supported by cross-cutting
relationships at the margins of groove lanes. Thus, a change in tectonic
style with time is suggested in the Uruk Sulcus region, varying from horst
and graben faulting for the oldest grooved terrain units to tilt block normal
faulting for the latest units. The morphologies and geometries of some
stratigraphically high units indicate that a strike-slip component of
deformation has played an important role in shaping this region of grooved
terrain. The most recent tectonic episode is interpreted as right-lateral
transtension, with its tectonic pattern of two contemporaneous structural
orientations superimposed on older units of grooved terrain. There is little
direct evidence for cryovolcanic resurfacing in the Uruk Sulcus region;
instead tectonism appears to be the dominant geological process that has
shaped the terrain. A broad wavelength of deformation is indicated,
corresponding to the Voyager-observed topography, and may be the result of
ductile necking of the lithosphere, while a finer scale of deformation
probably reflects faulting of the brittle near-surface. The results here form
a basis against which other Galileo grooved terrain observations can be
compared.
Pappalardo, R.T., M.J.S. Belton, H.H. Breneman, M.H. Carr, C.R. Chapman, G.C.
Collins, T. Denk, S. Fagents, P.E. Geissler, B. Giese, R. Greeley, R.
Greenberg, J.W. Head, P. Helfenstein, G. Hoppa, S.D. Kadel, K.P. Klaasen,
J.E. Klemaszewski, K. Magee, A.S. McEwen, J.M. Moore, W.B. Moore, G. Neukum,
C.B. Phillips, L.M. Prockter, G. Schubert, D.A. Senske, R.J. Sullivan, B.R.
Tufts, E.P. Turtle, R. Wagner, and K.K. Williams. Does Europa have a
subsurface ocean? Evaluation of the geological evidence. J. Geophys. Res.,
104, 24015-24055, 1999.
It has been proposed that Jupiter's satellite Europa currently possesses a
global subsurface ocean of liquid water. Galileo gravity data verify that the
satellite is differentiated into an outer H2O shell about 100 km thick but
cannot determine the current physical state of this shell (liquid or solid).
Here we summarize the geological evidence regarding an extant subsurface
ocean, concentrating on Galileo imaging data. We describe and assess nine
pertinent lines of geological evidence: 1) impact morphologies; 2)
lenticulae; 3) cryovolcanic features; 4) pull-apart bands; 5) chaos; 6)
ridges; 7) surface frosts; 8) topography; and 9) global tectonics. An
internal ocean would be a simple and comprehensive explanation for a broad
range of observations; however, we cannot rule out the possibility that all
of the surface morphologies could be due to processes in warm, soft ice with
only localized or partial melting. Two different models of impact flux imply
very different surface ages for Europa; the model favored here indicates an
average age of ~50 Myr. Searches for evidence of current geological activity
on Europa, such as gas/dust plumes or surface changes, have yielded negative
results to date. The current existence of a global subsurface ocean, while
attractive in explaining the observations, remains inconclusive. Future
geophysical measurements are essential to determine conclusively whether or
not there is a liquid water ocean within Europa today.
Pappalardo, R.T., J.W. Head, R. Greeley, R.J. Sullivan, C. Pilcher, G.
Schubert, W. Moore, M.H. Carr, J.M. Moore, M.J.S. Belton, and D.L. Goldsby.
Geological evidence for solid-state convection in Europa's ice shell. Nature,
391, 365-368, 1998.
Models of the interior of Jupiter's satellite Europa indicate that tidal
interactions with Jupiter might produce enough heat to maintain a subsurface
liquid water layer1; moreover, recent spacecraft images show Europa to be
sparsely cratered, suggesting that its ice-rich surface is geologically
youthful and that the moon might be active today5. Thus, critical issues
regarding the geology and physical state of Europa include its style of
resurfacing, its mechanism of heat loss, and whether it has had in the past
or has today a liquid water ocean. Here we report on the morphology and
geological interpretation of pits, domes, and spots discovered in new Galileo
high-resolution images of Europa's surface. The morphology and structure of
these features suggest vertical deformation of the surface, localized
heating, and possibly magmatism. We interpret the features as the surface manifestation
of diapirs, relatively warm ice masses that have risen buoyantly through the
subsurface. A candidate formational process is thermally induced solid-state
convection, predicted to occur within an ice shell on Europa if it overlies a
liquid water layer
Patel, J.G., R.T. Pappalardo, J.W. Head, G.C. Collins, H. Hiesinger, and J.
Sun. Topographic Wavelengths of Ganymede Groove Lanes from Fourier Analysis
of Galileo Images. J. Geophys. Res., 104, 24057-24074, 1999.
Galileo images have shown that grooved terrain on Ganymede consists of
pervasive ridges and grooves at a variety of spatial scales, which
complicates visual interpretation. We use Fourier analysis to separate
complex surface deformation into its component dominant wavelengths (closely correlated
to topographic wavelengths) to determine spatial relationships within and
among grooved terrain units. We analyze groove lanes in four Galileo target
sites (Uruk Sulcus, Byblus Sulcus, Tiamat Sulcus, and Nicholson Regio),
spanning a range of resolutions and lighting geometries, and we find multiple
dominant wavelengths in each. Fourier analysis of the complexly deformed Uruk
Sulcus shows both similarities and differences in wavelength distribution
among its tectono-stratigraphic subunits (a range of 0.5 to 6 km, with a
concentration at 1.2 km); favorable comparison is made to a stereo-derived
topographic model. Of the dominant wavelengths displayed by Byblus Sulcus
(~1, 3.3, and 10 km), the longest wavelength is revealed by profiles across
both high- and low-resolution images with very different lighting geometries.
Tiamat Sulcus displays different dominant wavelengths north (5 to 10 km) and
south (3 to 5 km) of the orthogonally trending Kishar Sulcus. Groove lanes in
Nicholson Regio are significantly different from the other sites because they
are isolated within dark terrain. Fourier analysis of these dark terrain
groove lanes shows dominant wavelengths (~2.1, 3.2, and 8.0 km) that are
similar to those in lanes of more typical grooved terrain. This suggests that
the tectonic style and lithospheric characteristics in this portion of
Ganymede's dark terrain were similar to those in bright grooved terrain at
the time of deformation. Our results support the hypothesis that longer
topographic wavelengths in Ganymede's groove lanes formed by means of
extensional necking of the lithosphere, while multiple shorter wavelengths
formed by normal faulting of the brittle lithosphere, in both bright and dark
terrains. The similar wavelengths of deformation seen in several groove lanes
in both bright and dark terrain suggest similarity in lithospheric thickness,
composition, and mechanical structure at these disparate sites. A global
process (such as differentiation) could be responsible for creating a similar
planet-wide strain and thermal regime during the time of grooved terrain
formation.
Prockter, L.M. and R.T. Pappalardo. Folds on Europa: Implications for Crustal
Cycling and Accommodation of Extension. Science, 289, 941-943, 2000.
Regional-scale undulations with associated small-scale secondary structures
are inferred to be folds on Jupiter's moon Europa. Formation is consistent
with stresses from tidal deformation, potentially triggering compressional
instability of a region of locally high thermal gradient. Folds may
compensate for extension elsewhere on Europa and then relax away over time.
Prockter, L. M., J. W. Head III, R. T. Pappalardo, J. G. Patel, R. J.
Sullivan, A. E. Clifton, B. Giese, R. Wagner, and G. Neukum, Morphology of
Europan bands at high resolution: A mid-ocean ridge-type rift mechanism. J.
Geophys. Res., 107(E5), 10.1029/2000JE001458, 2002.
We utilize imaging data from the Galileo spacecraft to investigate band
formation on one of Jupiter's moons, Europa. Bands are polygonal features
first observed in Voyager data close to Europa's anti-Jovian point and
represent areas where preexisting terrain has been pulled apart, allowing new
material to move up into the gap. We examine the detailed morphology of
several bands imaged at different resolutions and lighting geometries. We
identify several distinct morphological characteristics, including central
troughs, hummocky textures, and ridge and trough terrains, some of which are
common among the bands studied. In many cases, bands have initiated along
segments of one or more preexisting double ridges, ubiquitous within Europa's
ridged plains. Distinctive morphological features and high standing
topography imply that the bands formed from compositionally or thermally
buoyant ice, rather than liquid water. Comparisons between Europan band
morphologies and features found on terrestrial mid-ocean ridges reveal
several similarities, including axial troughs, subcircular hummocks, normal
faults, and indications of symmetrical spreading. We conclude that
terrestrial mid-ocean ridge rifting is a good analogy for Europan band
formation. If a terrestrial seafloor-spreading model is applicable to Europan
bands, we speculate that band morphologies might be related to the relative
rate of spreading of each band. Bands may have contributed significantly to
the resurfacing of Europa. Europan bands we examine predate (but do not
postdate) lenticulae and related features, implying that the style of
resurfacing on Europa has changed over recent geological time in these
regions.
Spaun, N. A., R. T. Pappalardo, and J. W. Head. Evidence for shear failure in
forming near-equatorial lineae on Europa, J. Geophys. Res., 108(E6),
10.1029/2001JE001499, 2003.
Global stress models for Europa are unable to readily explain the
orientations and intersection angles of lineae in the equatorial region of
Europa's trailing hemisphere if lineae originate a tension cracks. Our
analysis and mapping of two equatorial, trailing regions reveals that lineae
are predominantly oriented NE and NW, while EW lineae are relatively rare;
this is contrary to predictions of existing stress and formation models. The
measured orientations are consistent with an origin by shear failure. The
studied regions of Europa are located near the point of maximum differential
stress and minimum surface tensile stresses, indicating that shear faulting
may dominate over tension fracturing. Several distinct types of lineae are
recognized and their relative abundance is inferred to have changed with
time, consistent with linea formation models that suggest an evolutionary
sequence from simple troughs to complex ridges. The opening of crevasse-like
tensile fractures is not required for the generation of all lineae; the
observations are consistent with ridge formation models where troughs (formed
in tension or shear) experience shear heating due to tidal deformation,
allowing warm ice to buoyantly uplift, creating ridges. The stratigraphic
relationships indicate the following: ridged plains formed first, followed by
continued formation of a wide range of lineae, and lastly emplacement of
lenticulae and continued linea formation. This sequence is consistent with an
early thin, brittle lithosphere that thickened with time and was subject to
diurnal tides during ~30_-90_ of nonsynchronous rotation. Ultimately the
thickened shell underwent thermally induced solid-state convection, producing
lenticulae. The young surface age of Europa implies that this entire
stratigraphic sequence was emplaced in the geologically recent past.
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