CHAOS, CRACKS AND RIDGES:

SURFACE EFFECTS OF THIN ICE OVER LIQUID WATER ON EUROPA.

R. Greenberg, G.V. Hoppa, B.R.Tufts, P. Geissler, and J. Riley

Lunar and Planetary Lab, Univ. of Arizona, Tucson, AZ 85721-0092 (greenberg@lpl.arizona.edu)

The surface of Europa appears to consist of two

dominant types of terrain: tectonic terrain consisting of

cracks, ridges and bands; and chaotic terrain with a

textured lumpy matrix, often around blocks of

displaced older surface. Initial geological studies of

Europa, based on the images of the surface returned by

the Galileo spacecraft, have described surface

morphology [1]. As the first data were returned,

interpretations were made for various specific features,

generally based on terrestrial or other planetary analogs

[e.g. 2,3]. Even now, only one hemisphere has been

imaged at km-scale resolution, only selected regions at

about 200m resolution, and only very small sites at

tens of m resolution. However, the coverage is now

sufficiently broad that we can identify, classify and

interpret terrains and features in a global and process-

oriented context. We identify two dominant classes of

terrain: tectonic and chaotic. Here we discuss how a

model of Europa with a very thin ice shell over liquid

water can produce the observed characteristics of the

terrains and their defining surface features.

As discussed below, our investigation of the

character and formation of these terrains shows that,

while it is evident that formation of chaos terrain can

destroy earlier densely ridged terrain, cracking and

subsequent ridge formation can in turn destroy chaos

terrain, restoring the surface to a densely ridged

appearance. Thus, chaos areas are not necessarily the

most recent, and chaos and ridge formation may both

have been diachronous over the geological age of

Europa. The two fundamental resurfacing processes

may have alternated over Europa's geological history,

with melt-through (at various places and times)

forming chaos terrain, and criss-crossing by tectonic

features (cracks, ridges, and bands) forming densely

ridged (and related tectonic) terrain.

Chaos Terrain

We have mapped and characterized chaotic terrain

as it appears in all available Galileo imaging at ~200m

resolution though orbit E15 [4]. (Extension to include

E17 and higher resolution images is in progress). The

identification of chaos areas is based on similarity in

texture and morphology to the archetype Conamara

Chaos, where blocks of previous terrain have been

moved around and are embedded in a characteristically

lumpy matrix. Chaos areas are wide-ranging in size,

location and age. Our classification includes most of

the several-km, roundish features called "lenticulae"

[1,2]. The areas mapped to date show the largest

chaos areas to be at lower latitudes, but E17 images

show extensive chaos near the south pole, so it is

premature to draw conclusions about global

distributions or latitude dependence.

In chaos areas, the lumps that give the general

texture to the matrix span a broad range of sizes, from

the limit of resolution up to several-km rafts with

recognizable old terrain on them. (The latter blocks

are not required in our definition of chaos.) There

appears to be a continuum such that even the small

lumps may simply be smaller chunks of ice from the

original crust, too small to have had or retained

recognizable segments of old terrain. Some chaos

boundaries are cliffs dropping down to the matrix. In

other cases (especially smaller, round chaos) the

surrounding terrain ramps down to the boundary and

the matrix is bulged up relative to the immediate

boundary, although there is no definitive quantitative

evidence that it is elevated relative to the surrounding

terrain. The extent and nature of chaos areas is often

influenced by preexisting ridge systems, which may

form boundaries of chaos, or survive preferentially in

chaos areas as causeways, peninsulas or chains of

minimally displaced blocks across chaos. Chaos areas

have a continuous size distribution from a few km (the

smallest recognizable size at 200m resolution) to

hundreds of km.

These characteristics are consistent with a model in

which chaos is created by melt-though of liquid water

from below, for example if local or regional areas of are

heated by concentrations of warmer water. As the crust

thins, isostatic sinking creates pits or depressions,

such those observed in many places. Also, even

without complete melt-through, surface topography

may relax forming smooth patches, and sublimate

leaving smooth darkened areas [5], such as the smooth

dark patch in E4 high resolution images or the margins

of Thrace. Where melt-through reaches the surface, a

chaos area is created. At the margins, the crust dips

isostatically, forming the characteristic ramped

shoreline. Blocks of crust may break off forming cracks

commonly seen around chaos areas, or be displaced

forming rafts and leaving behind the typical cliff

shoreline. Later, upon refreezing, the matrix bulges up

isostatically, consistent with observed topography.

Preexisting ridge topography is relatively resistant to

melt-through, explaining its observed preferential

survival.

What we classify as small chaos areas have been

designated as "lenticulae" or as "pits, spots and

domes" in other taxonomies [1,2]. Alternative models

for their formation include viscous cryovolcanic flow

onto the surface [1] and solid-state convention cells [2].

Neither model can be ruled out, although the former

must contend with the lower density of ice than liquid

water by introducing ad hoc materials, and the latter

still requires consideration of the dynamics of the

process of convection where it reaches the surface. An

appeal of the melt-through model is that it invokes

conditions that can also explain the tectonic terrain of

Europa.

Cracks and Ridges

Sequences and orientations of regional scale

lineaments (mostly ridge systems, but the youngest are

cracks) correlate with stress patterns expected from tidal

distortion of an elastic shell over a thick fluid layer if

(and only if) effects of assumed non-synchronous

rotation are included [6]. Strike-slip displacement on

Europa correlates with a process of "walking" driven by

diurnal tides [7]. The mammoth strike-slip feature

Astypalaea Linea has ridges lining its shear zones [8].

Thus ridge systems are known to develop along

cracks. Ridge formation can be explained by diurnal

tidal working of cracks pumping liquid from below [6],

squeezing or spattering [9] slush to the surface, with

some distortion of the lip of the crack [10] by the daily

pounding, all of which builds double (Class 1) ridge

pairs lining the crack. An alternative ridge model

based on linear diapirism tilting the crust upward

along the sides of cracks [11] is less successful at

explaining the observed properties of ridges for several

reasons, for example: it does not explain the uniformity

along great lengths of ridges; preexisting terrain

features are not found on ridge flanks; there is no

convincing evidence of central ridges of upwelled

material. In our model [6], as cracks dilate, they form

wide (Class 2) ridges if pushed open by internal debris,

or bands if dilated by regional extension [12]. Ridge

complexes (Class 3) result from lateral cracking due to

downwarping under the load of ridges, with formation

of additional ridges. All of these processes are readily

explained in a global model [6] context similar to that

invoked in our model of chaos formation.

Cracks and subsequent ridges form across

previously existing features. Thus they can readily

modify or overprint previously existing terrain,

including preexisting chaos areas. Several examples of

partial erasure of chaos areas have been identified. The

impression that chaos is in general a more recent

phenomenon than ridge formation [2,13] may be an

artifact of the difficulty of recognizing terrains that have

been cut up by cracks, ridges and bands. Old craters

have been similarly difficult to find, but we know they

represent a diachronous process. Similarly, we have

no reason to believe that chaos formation has not been

on-going throughout Europa's geological history.

Discussion

Chaos ubiquity suggests Europan geology has been

dominated by the effects of having liquid water under a

very thin ice shell, with chaos regions being

widespread examples of essentially zero shell

thickness. Features ranging in size from small

lenticulae up to regional chaos areas have been

identified as having common characteristics and fit this

universal model of formation. Smooth patches may be

part of a genetic continuum with chaos areas as well:

in some cases revealing the characteristic matrix texture

wherever resolution is good enough to show it, or in

other cases having been smoothed by near surface

heating, but not melt-through. The two major terrain-

forming processes on Europa are melt-through creating

chaos, and tectonic processes of cracking followed by

ridge and band formation. These two major terrain-

forming processes have continually destroyed pre-

existing terrain, depending on whether local or regional

heat concentration was adequate for large-scale melt-

through, or small enough for re-freezing and

continuation of tectonism. As a concurrent,

diachronous process, melt-through may have provided

the area to accommodate crustal expansion observed

widely in the tectonic record.

This model depends on the long-term existence of

a broad and continuous layer of liquid water. It does

not constitute proof of the existence of a global ocean

or that chaos formed the way we propose. However,

because most of the properties of chaos areas, and

indeed of observed tectonic terrain, follow from our

general model, without invoking a variety of

specialized processes to explain each variation, our

broad picture provides a plausible hypothesis for the

dominant processes driving the surface geology of

Europa.

References: [1] Greeley et al. 1998, Icarus 135, 4;

[2] Pappalardo et al. 1998, Nature 391, 365; [3]

Sullivan et al. 1998, Nature 391, 371; [4] Greenberg et

al. 1999, submitted to Icarus; [5] Fagents et al. 1999,

LPSC XXX, this volume; [6] Greenberg et al. 1998,

Icarus 135, 64; [7] Hoppa et al. 1999, submitted to

Icarus; [8] Tufts et al. 1999, LPSC XXX, this volume;

[9] Kadel et al. 1998 Eos Trans. AGU 79, S203; [10]

Turtle et al. 1998 Eos Trans. AGU 79, S203; [11]

Head et al. 1999, submitted to JGR Planets; [12] Tufts

et al. 1999, submitted to Icarus; [13] Head et al. 1998,

Eos Trans. AGU 79, F534.

CHAOS, CRACKS AND RIDGES: R. Greenberg et al.

Formation of chaotic terrain by melt-through from below, and tectonic terrain (cracks, ridges, and bands) by tides have probably been the two dominant diachronous resurfacing processes over Europa's geological history.