Tyre Macula is a large, dark circular feature on Europa, identified first by Lucchita and Soderblom  as a possible impact feature. While acknowledging some similarities to palimpsests on Ganymede (believed to be ancient relaxed impact craters), they concluded that there was insufficient evidence to definitely confirm an exogenic origin.
Here, I describe and investigate several features of Tyre Macula that allude to an endogenic origin. In particular, I investigate the proposal  that Tyre Macula could actually be a large Ice Cauldron - a feature created through localised ice subsidence as a result of subsurface geothermal heating.
Tyre Macula (Figure 1) is centred at 31.7°N 147°W near the junction of several lineaments. It is overlain by two wide north-south striking semi-cuspoid ridges (A andB), and another east-west striking lineament (C) interpreted here as a normal fault. When Lineament (C) passes over the Macula itself its albedo increases dramatically, although it apparently disappears at Tyre's western edge. Tyre is therefore interpreted here to be younger than Ridges (A) and (B), but older than Lineament (C).
Tyre Macula is a dark circular feature approximately 150 km in diameter. Concentric structures have been identified  and are visible and labelled in Figure 1.
Figure 1: Tyre Macula.
A circular structure (D, Figure 1) is visible in the centre of Tyre and is interpreted here (based on brightness variations within it) to be a bowl-shaped depression approximately 25 km in diameter. This bowl is situated within a slightly brighter central region 75 km in diameter, which itself is surrounded by a more sharply defined darker ring out to a diameter of 150 km that describes the accepted external circumference of Tyre. Finally, a tenuous fuzzy halo that darkens the underlying terrain extends beyond this to a diameter of approximately 240 km.
|Figure 2a||Figure 2b (labelled)|
Voyager image c2065211 (Figure 2a) shows Tyre under low-angle illumination and reveals some internal topographic detail. Some features are highlighted in Figure 2b - ridges are coloured red, concentric features are coloured blue, and the bright lineament is shown in black - the bowl-shaped central depression is not outlined here for the sake of clarity. The concentric features within Tyre are largely visible due to the topography associated with them, particularly around the outer edge of the Macula.
Two of the ridges are particularly of note here - the easternmost of the ridges (1a) that interact with Tyre appears to be older than the Macula, as it disappears underneath it and continues to the northwest (1b). To the southwest of this ridge is another lineament (2a) that disappears beneath Tyre in a similar manner, but reappears quite a distance beyond the Macula to the northwest (2b). These further serve to limit the age of Tyre Macula, though note that the identification of these features is based entirely on low resolution images (approx. 5 km) of the region that can be interpreted in a number of other ways.
The bright lineament (3 in Figure 2b) cutting across the Macula from east to west is interpreted as a steep slope facing towards the sun, thus resulting in its brighter appearance. As such (and based on the sequence of events described in the Macula formation proposed later), I believe it to be the exposed face of a normal (extensional) fault that faces a southerly direction.
A circular feature to the north of Tyre (4 in Figure 2b), situated between three dark lineaments, is also highlighted and is visible on other images of this area. This may be an albedo feature, and appears to be at least partly surrounded by a dark halo.
At first glance, Tyre Macula is similar in appearance to some of the palimpsests on Ganymede. The most widely accepted explanation for ganymedian palimpsests is that they are ancient relaxed craters . However, the morphology of Tyre Macula is distinctly unlike that of these features. While Lucchita and Soderblom  concluded that there are too many differences between Tyre and a typical ganymedian palimpsest to state that they formed via the same (exogenic) processes, they presented this as the best explanation at the time.
Assuming ganymedian palimpsests are ancient impact features (though it should be noted that endogenic origins have been recently proposed by Croft ), is Tyre similar in nature to these structures? I believe that evidence from the Voyager images indicates that it is not - a comparison of some of the more common features of palimpsests and those associated with Tyre Macula is given in Table 1 below.
|Multiple concentric structures within feature?||No||Yes|
|Brighter central region?||Yes||Yes|
These differences lead me to the conclusion that Tyre Macula is not a palimpsest, while the features that are seen and described above lead me to propose that it was formed by some hitherto unrecognised endogenic process.
|Figure 3a||Figure 3b|
The appearance of Tyre Macula is similar to that of Ice Cauldrons found in terrestrial domains that are both volcanic and ice-covered (such as Iceland), except on a much larger scale. On Earth, Ice Cauldrons vary in size from several hundred metres to a few (< 10) km across and are up to 200 metres deep. While no definite correlations between diameter, depth and thickness of ice sheet have been noted so far, the larger ones do appear to be deeper and are situated on thicker glaciers.
Ice Cauldrons are large circular features found on some glaciers that overlie volcanic terrain. They vary in appearance, but most consist of a central unfractured depression surrounded by an extensive ring of concentric and eccentric fractures, the separation of which increases radially with distance from the centre . At least some Ice Cauldrons on Earth are temporary features, disappearing as a result of ice creep and snow cover.
On Earth, Ice Cauldrons such as the ones shown in Figures 3a and 3b are believed to form through subsidence caused by subglacial heating: Ice at the base of the glacier is melted by localised geothermal heating to form a dome-shaped reservoir of meltwater. After a few years, the water pressure exceeds the ice overburden pressure (usually due to the extra volume required by the intruding lava) and the water bursts forth from under the glacier, causing a sudden glacial flood, or jökulhlaup. The glacier then rapidly sags into the void left by the water, creating concentric fractures in the ice through brittle deformation . No examples have been noted on Earth where the water has been extruded via these fractures - it always escapes from elsewhere along the base of the glacier - although in some cases a small lake forms in the centre of the structure (as shown in Figure 3a) if the sinking glacier intersects the water table.
Europa is believed to have suffered extensive tidal heating as a result of the orbital evolution of the Galilean satellites (e.g. Greenberg ). Most current (pre-Galileo) models assume the presence of a thick ice layer on the surface overlying a silicate crust , with the possibility of a transient or local liquid water ocean maintained at the boundary by tidal heating. Here however, I assume that no subglacial ocean was present at the time Tyre Macula formed, so that the ice and silicates would be in direct contact in this region.
Based on these assumptions and the features described earlier, I can now propose a possible sequence of events to form Tyre Macula: A meltwater reservoir was created at the base of the ice layer by geothermal heating in the silicate crust. This reservoir may be dome shaped, but in view of conjectured eruptive processes on Europa  it is possible that an elongate sill was intruded at the ice/silicate interface and initiated melting. When the meltwater pressure exceeded the ice overburden pressure, a fracture propagated upwards to form a vent above the reservoir and allowed the water (mixed with silicate fragments to give it a characteristic 'dirty' appearance) to break through to the surface. Water flowed radially outwards from the vent until the supply was exhausted, whereupon the diameter covered by the flow was approximately 150 km. Assuming a flow thickness of 30 m, this requires a volume 530 km3 of erupted water, corresponding to a minimum reservoir radius (assuming a hemispherical reservoir) of just over 6 km. The central region then collapsed to create the central bowl.
The reservoir is similar in scale (within an order of magnitude) to the observed radius of what I interpret to be the central bowl-shaped depression; transmission of the stresses through a thick ice sheet could explain the difference in size between the reservoir and the bowl. However, I suggest that the attendant wreath of fractures only extends to the edge of the bright central region at a radius of approximately 40 km, since fracturing would expose brighter ice below the 'dirty water' veneer and thus increase the local albedo (by this interpretation, the bright lineament (C) shown in Figure 1 must be a fracture as it exposes brighter ice below the flow, and is therefore younger than the Macula). Finally, the water froze as it flowed over the surface, and the resulting ice frost settled around the exterior to form the fuzzy halo around the entire structure.
The question of whether the maculae of Europa are exogenic or endogenic in origin has been the subject of much debate over the years. The morphology of the five named europan maculae (Tyre, Thrace, Thera, Cyclades, and Boeotia)  and others visible in lower-resolution images differs widely, but falls into two main classes - circular maculae (i.e. Tyre and an unnamed macula imaged on the E4 orbit by Galileo and described by Moore et al. ) and lobate maculae (including all the other named maculae). Wilson et al.  propose that Thrace Macula - a long lobate structure in the southern hemisphere - is an eruptive, and therefore endogenic, feature. It seems likely from examination of other lobate maculae that these are also eruptive features.
The circular maculae are however more problematic; an eruptive explanation is not as straightforward as for the lobate maculae, and in fact the circular maculae are subject to various alternative interpretations. Before the Galileo Orbiter arrived at the Jovian system, the best images available of this type of macula were of Tyre, at resolutions of approx. 5 km. However, another dark circular feature was identified in global images of the trailing hemisphere. While no images have been returned by Galileo of Tyre Macula itself, the unnamed macula on the trailing hemisphere has recently been imaged at high resolution and its morphology is described elsewhere by Moore et al. . They conclude that both exogenic and endogenic origins for that feature remain viable, and put forward a diapiric model for its endogenic formation similar to that proposed for coronae on Venus. In this scenario, they note that mechanical uplift of the ice surface occurs due to a rising ductile ice diapir.
Indeed, ice diapirism may be a viable alternative origin for Tyre - extensional fractures could form as a result of uplift, followed by subsidence as the heat source below died out - however, there is insufficient evidence from Voyager images to say for certain.
In conclusion, I believe the argument that circular maculae form through endogenic processes is much stronger than that of exogenic origins. However, whether both of the known circular maculae formed through the same endogenic processes on the other hand is a different matter entirely. Certainly, in the case of Tyre (for which no high-resolution images are available yet), one cannot conclusively identify which specific process is responsible for its origin. Therefore, I suggest that Tyre Macula formed through an endogenic mechanism, either by ice-subsidence (the Ice Cauldron model) or ice-diapirism (the Venus corona model).
 Lucchita B. K. & Soderblom L.A., The Geology of Europa, in Satellites of Jupiter, 521-555, 1982.
 Thomas C., Is Tyre Macula An Ice Cauldron?, LPSC XVIII (Abstract), 1997.
 Schenk P., Origin of Palimpsests and Impact Basins on Ganymede, LPSC XXVII, 1137-1138, 1996.
 Croft S. K., Palimpsests on Ganymede: an endogenic origin?, LPSC XXV, 297-298, 1994.
 Wood C.A., Possible terrestrial analogs of Valhalla and other ripple-ring basins, Multi-Ring Basins, Proc. Lunar. Planet. Sci., 173-180, 1981.
 Greenberg R., Galilean Satellites; Evolutionary Paths in Deep Resonance, Icarus, 70: 334-347, 1987.
 Malin M. C. & Pieri D. C., Europa, in Satellites, 689-717, 1986.
 Wilson L., pers. comm., 1997.
 USGS Europan Nomenclature website (http://planetarynames.wr.usgs.gov/jupiter/euroTOC.html).
 Moore et al., Europan Macula: Possible Origins, LPSC XXVIII, 1997.
 Wilson L. et al., Thrace Macula: A Long Lava Flow on Europa?, J. geophys. Res., 1997 (in press).