Tessera

Tesserae are windows on the oldest surfaces of Venus. They are also the most tectonically deformed pieces of its surface.

In many places, strikingly radar-bright pieces of surface, densely cut by troughs and ridges, rise above the lower-lying lava plains. From their pattern they were first called parquet (Barsukov 1986). Later the word tessera was used: Greek for “four”, and from this also a mosaic tile (Basilevsky and Head 1998).

In tessera areas at least two directions of deformation can be identified. The deformation may be compressional (ridges) or extensional (troughs, fractures). It may also be ductile folding or brittle fracture. All of these tell us about the thermal state and the stress field of the surface when a given structure formed.

Since the 1990s researchers have defined a dozen morphological types. Different authors separate them by different criteria: how far apart the ridges and grooves are, what pattern the overlapping deformations make, and what their “shorelines” look like.

Ribbon tessera (see the plate) (Bindschadler et al. 1990) has a wavy graben-and-horst structure. Ribbons of narrow troughs and ridges repeat at regular intervals. They run at right angles to the broad ridges that deform them and make the ribbons wavy. These may be the oldest preserved landforms on crustal plateaus.

S-C tessera has wider troughs and narrower ridges at right angles to them (Hansen and Willis 1996). The centres of crustal plateaus hold the surfaces with the most complex deformation history. There, curved ridges form a honeycomb pattern. In basin-and-dome tessera (see plate 2), dark (smooth) material fills the lower-lying basins (Ivanov and Head 1996, Hansen et al. 2000).

Most tesserae lie on crustal plateaus, so their histories are linked. Their raised margins are clearly flooded by the surrounding lava plains. This allows two models of their origin. 1) These complexly deformed surfaces were once global and wrapped Venus like the layers of an onion, but later lava flows flooded the low-lying parts. 2) Tesserae formed only in raised positions, on the crustal plateaus. The raised position raises another question. Do tesserae still form in these “sheltered” places today, or was this typical only of an earlier period of Venus?

Only one planned future mission includes a probe that will descend to the surface: the American DAVINCI mission. It would make a hard landing in a tessera area and take high-resolution optical images on the way down.