Terrestrial Analogues for Polygonal Terrain on Venus

Polygons of various sizes cover more than 5% of the surface of Venus, almost as much of the surface as that covered by tesserae, and yet they have been largely ignored. The most recent study [1] identified 204 polygonal terrain locations covering approximately 8 Mm², using an automated algorithm that resulted in a northern hemisphere bias. Nonetheless, they found that 65% of the identified polygonal terrain is associated with small volcanoes, 25% with coronae, 18% with tesserae, and 20% with wrinkle ridges. Polygonal terrain is currently attributed to thermal contraction by cooling, whether of lava flows, or following heating by an intrusion [2], or in response to climate change [3].

Our mapping of an additional 16 Mm² (Figure 1) reveals 6 types of polygonal terrain (plus unclassified), broadly divided into irregular (55% by area) and rectilinear patterns (37% by area). There appear to be two distinct size ranges of smaller cells close to the resolution limit (~100 m) and larger cells several km across, sometimes superposed. A thermal contraction origin by cooling is difficult to reconcile with the variety, shapes and sizes of polygons observed.

A range of processes in addition to thermal contraction can generate polygonal patterns at varying scales on Earth and Mars including lava lakes, columnar joints, karst, diagenesis, ice wedge polygons (periglaciation), desiccation (mud cracks), evaporation (salt pans), and polygonal fault systems (PFS). The first four generate polygons on the metre scale, smaller than can be resolved in Magellan imagery. The next three can generate polygons from metres up to a few hundred metres across and may therefore have generated the smaller polygons observed on Venus. PFS have so far only been identified in some terrestrial sedimentary basins [4], but they do generate polygons up to several kilometres across, similar to the larger polygons observed on Venus.

PFS form networks of small‐displacement normal faults forming laterally extensive layer‑bound polygonal patterns. They occur in fine-grained, smectite-rich sediments, usually within epicontinental marine basins [5]. Their initiation appears to depend on syneresis of smectite-rich clay gels during early diagenesis [6], while hydraulic fracturing by over-pressured pore fluids [5], aided by the low shearing resistance of clays [7], enables them to propagate and grow into larger polygonal shear faults several kilometres across. Initiation and propagation are confined within sediments buried at <1500 m depth. At greater depths, propagating fractures sense the regional stress field and generate more orthogonal patterns – a possible explanation for the rectilinear polygons observed on Venus.

In rapidly deposited, underconsolidated submarine muds, PFS can propagate to the sea floor [8]. The Grenada Basin in the Caribbean Sea has a large area of sea floor polygons, forming regular patterns in the north but downslope creep of the smectite-rich clays causes more elongate patterns in the south [9], offering an alternative mechanism for the formation of rectilinear polygons on Venus.

Since the presence of large polygonally-fractured terrain seems to require a marine sedimentary origin, we conclude that Venus must once have had oceans. Even so, extensive periglacial conditions appear unlikely and given the current surface conditions, those oceans must have evaporated under runaway greenhouse conditions. The smaller polygonal fractures (e.g. Type 1 Mesh Polygons) most like represent a combination of desiccated clay-rich marine sediments and evaporites. While terrestrial desiccation cracks most commonly occur as small-scale mud cracks, polygons up to 350 m across have been identified in desiccated lake sediments on Mars [10], while salt polygons up to ~250 m occur on Earth (Figure 2)

While not all polygons will have originated in the same way, a likely formation scenario for many is that they initiated as polygonal fault systems within a rapidly accumulating sediment pile in shallow seas. These PFS propagated to the sea floor as those seas evaporated under a runaway greenhouse. Salts would have precipitated from the increasingly briny seawater. Once the seas were gone, the wet saline sediments desiccated, forming smaller polygons within the larger PFS structures. Rising temperatures first lithified and then metamorphosed the clay-rich sediments into micaceous shales, baking in the polygonal patterns we observe today.

References

1. S. E. Smrekar, P. Moreels, and B. J. Franklin, J. Geophys. Res. Planets 107, 8 (2002)

2. C. L. Johnson and D. T. Sandwell, J. Geophys. Res. 97, 13601 (1992)

3. F. S. Anderson and S. E. Smrekar, J. Geophys. Res. 104, 30743 (1999)

4. J. A. Cartwright, Mar. Pet. Geol. 11, 587 (1994)

5. J. A. Cartwright and L. Lonergan, Basin Research 8, 183 (1996)

6. D. N. Dewhurst, J. A. Cartwright, and L. Lonergan, Mar. Pet. Geol. 16, 793 (1999)

7. J. Cartwright, Mar. Pet. Geol. 28, 1593 (2011)

8. A. Gay, M. Lopez, P. Cochonat, and G. Sermondadaz, Basin Research 16, 101 (2004)

9. A. Gay, C. Padron, S. Meyer, D. Beaufort, E. Oliot, S. Lallemand, B. Marcaillou, M. Philippon, J. J. Cornée, F. Audemard, J. F. Lebrun, F. Klingelhoefer, B. Mercier de Lepinay, P. Münch, C. Garrocq, M. Boucard, and L. Schenini, Geochemistry, Geophysics, Geosystems 22, e2021GC009809 (2021)

10. M. R. El Maarry, W. J. Markiewicz, M. T. Mellon, W. Goetz, J. M. Dohm, and A. Pack, J. Geophys. Res. Planets 115, (2010)

Figures

Figure 1. Type examples for six classes of polygon, all shown at the same comparative scale. Indistinct or otherwise unclassifiable polygons are grouped into a seventh class for mapping purposes but there is no type example. The scale bar is 10 km in each case.

Figure 2 Salt polygons in Lake Natron, Africa. Left Sentinel-2 optical image. Right Sentinel-1 SAR image. Polygons are usually obscured by a thin water layer in radar images. Scale bar is 1 km.