Intratessera basin, Intratessera plains, Circular trough

The basins of tessera areas are often filled with smooth, radar-dark material. Among them, semicircular basins stand out by their unusual shape.

Cofrade et al. (2019) noticed semicircular basins only a few years ago. They straight away counted 609 of them. Such crescent-shaped basins occur only in tessera areas, especially in the “basin-and-dome” tesserae shown on the plate. Here the basins may be regarded as the buried landform, and what we see at the surface as a plain (that is, a filled basin). Semicircular basins are on average 24 km across and 3 km wide.

There are two main explanations for their origin. In the first, they form through some local diapirism, as material rises from depth. The diapir may be driven by differences in magma composition (that is, density). Or it may be a slow, rising flow of material driven by temperature differences.

As a more exotic idea, the authors propose that these may be diapirs of salt deposits beneath young volcanic areas. The salt would have built up and dried out as seas from an earlier era of Venus evaporated. Salt diapirism is a known process on Earth. Byrne et al. (2020), however, reject this theory. In their view, compression in the tessera folded and lifted the surface. The raised layers were then eroded, and the sediment was deposited in the neighbouring trough, which folding had bent into an arc.

Tesserae also contain basins of other shapes. These are thought to be lava fills. Ivanov and Head (2015) suggest that they might serve as stratigraphic age markers, if they filled at about the same time. The lavas filling the enclosed basins inside tesserae may have erupted in separate events in each depression. By contrast, material that flooded the lower parts and valleys like “fjords” from beyond the margin probably flooded the outer edge of a tessera all at once.

Khawja et al. (2020) interpret these dark-filled basins and troughs as ancient river valleys, or perhaps wind-cut grooves. The layout of the troughs matches the patterns of river valleys on Earth. They may still preserve traces of fluvial erosion from before the runaway greenhouse era (see also: Valley network). If fluvial erosion shaped the surface of tesserae, this strong erosion would explain why impact craters are so rare on tesserae, and why the few that exist are heavily eroded.

The complex ridge-and-valley structure of tesserae can, however, also be explained without fluvial and wind erosion, or alongside them. It may record the history of layers that were cut by fractures, folded and eroded (Byrne et al. 2020).