Tesserae on Venus may preserve evidence of fluvial erosion

Venus, often called Earth's sister planet due to similarities in size, mass, and composition, presents a stark contrast in its current state. Its surface temperature of approximately 450°C and dense CO2 atmosphere preclude the presence of liquid water and, consequently, fluvial erosion. This view is reinforced by the extensive, seemingly uneroded volcanic flows observed across the planet, primarily from the Magellan mission data. However, recent global circulation models propose that Venus may have experienced Earth-like conditions earlier in its history, before a runaway greenhouse effect drastically altered its climate. This possibility necessitates a reassessment of existing geological interpretations, especially for the oldest and most complex Venusian terrains: the tesserae. These elevated, tectonically complex regions comprise approximately 8% of Venus' surface and, based on superposition relationships, are considered the planet's oldest features. The nature of the pre-deformation terrain, the processes involved in their formation, and the role of subsequent erosion remain open questions. The central research question of this paper is whether the morphological features within the tesserae regions are consistent with the effects of fluvial erosion, which would lend significant support to models proposing a habitable period in Venus's early history. The importance of this study lies in its potential to fundamentally reshape our understanding of Venus's geological and climatic evolution and to inform future exploration strategies for this enigmatic planet. The results would have significant implications for our understanding of planetary evolution in general, and for the habitability of Earth-like exoplanets.

Existing literature strongly supports the absence of extensive erosion on Venus, given its present-day extreme conditions. Studies focusing on younger lobate flows show simple superposition relationships and radar backscatter properties consistent with primary flow morphology, supporting this conclusion. Features such as wrinkle ridges, graben-fissure systems, and lava channels exhibit topographic signatures that would be significantly altered or removed by erosion; however, the mostly pristine state of impact craters within younger volcanic plains further indicates a lack of substantial erosional activity. This prevailing viewpoint contrasts with recent climate modeling which suggests a period of Earth-like conditions early in Venus’ history. This hypothesis challenges the existing assumptions about Venus' geological evolution, motivating the authors to investigate the possibility of fluvial erosion in the oldest terrains.

To overcome limitations in the resolution of available topographic data, the authors employ an indirect technique to identify paleo-valleys within the tesserae. Instead of relying directly on the often-ambiguous topographic data from Magellan's altimetry (50-100m vertical, 10-20km horizontal resolution), they focus on areas where younger lava flows have partially flooded the margins of the tesserae. The lava flows serve as a relatively horizontal datum against which variations in elevation can be more readily identified. The patterns created by these lava flows within the tesserae, forming embayments, are used as a proxy to identify paleo-valleys, even if subsequent localized uplift has occurred. This approach draws a conceptual analogy to terrestrial features like flooded fjords, nunataks, and yardangs, where water, ice, and sand, respectively, provide similar reference surfaces. Eight tesserae regions are selected for study. Magellan SAR images, along with altimetry and (in some cases) higher-resolution stereo topographic data, are used to map the patterns of lava flooding. The resulting valley patterns are then compared with known terrestrial stream drainage patterns categorized by Howard (1967) and Bridge and Demicco (2008). Three-dimensional oblique views, generated by draping SAR images onto Magellan altimetry and stereo-topography data using ArcScene software, provide visual support for the interpretations. The vertical exaggeration used in the 3D models is acknowledged as potentially distorting the true dip angles of observed geological features.

The analysis of lava-flooded tesserae reveals valley patterns that strongly resemble several terrestrial stream drainage patterns, including trellis, radial, parallel, rectangular, and potentially dendritic systems. These similarities suggest that fluvial erosion, in addition to tectonic activity, has played a significant role in shaping the tesserae topography. The authors specifically highlight the example shown in Figure 1, interpreting it as a NNE-SSW trending plunging syncline that has been significantly widened by erosion. The basin-parallel lineament sets observed appear inconsistent with purely tectonic interpretations, further supporting the combined tectonic and fluvial model. Figure 3 provides a detailed presentation of eight case studies, each showing lava flooding, an interpreted valley network, and a comparison to corresponding terrestrial drainage patterns. The authors also present examples of kipukas within lava-flooded valleys, whose tear-drop shapes suggest the direction of fluid flow. The match between Venusian valley patterns and terrestrial stream drainage patterns is considered compelling evidence supporting the hypothesis of past fluvial activity. The variations in drainage patterns across different tesserae are linked to differences in underlying geology and slope, consistent with terrestrial analogues.

The findings of this study provide compelling evidence for the possibility of significant fluvial erosion shaping the oldest Venusian terrains. This interpretation directly challenges existing paradigms of Venusian geology that emphasize the lack of erosion given the planet's current extreme surface conditions. The remarkable similarity between the observed valley patterns and terrestrial stream drainage patterns provides robust support for a model incorporating fluvial processes alongside tectonic activity. The observed lineaments in the tesserae can be reinterpreted to reflect both tectonic features and differentially eroded volcano-sedimentary stratigraphy. The existence of intra-tessera plains could also be explained as erosional remnants of larger volcanic events. This interpretation offers a resolution to the age conundrum of tesserae, as continuous erosion could account for the apparent younger age indicated by impact crater density. The study strongly supports the viability of models that propose Earth-like conditions during a significant portion of early Venusian history, highlighting the need for higher-resolution data from future missions to definitively test the hypothesis. This new model of early Venus opens avenues for further geological interpretations and shapes future exploration strategies.

This paper presents a novel interpretation of Venusian tesserae, suggesting that fluvial erosion has played a significant role in shaping their topography. The use of lava flooding as a proxy for identifying paleo-valleys overcomes limitations of existing topographic data. The striking similarity between Venusian valley patterns and known terrestrial stream drainage patterns provides compelling evidence supporting past fluvial activity. This finding provides strong support for climate models suggesting a cool and wet early Venus, drastically altering our understanding of Venusian geological and climatic history. Future missions with improved resolution data should focus on verifying and expanding this research. Further exploration of the relationship between drainage patterns and underlying geology could unlock detailed insights into the composition and structure of the tesserae.

The primary limitation of this study is the reliance on Magellan data, which has limited resolution. While the indirect method employed mitigates some of these issues, finer-scale topographic data from future missions would allow for more precise mapping of the valley networks and a more definitive assessment of their origin. The interpretation of valley patterns is also inherently subjective, although the comparison to known terrestrial patterns provides a valuable framework. The absence of direct evidence of past liquid water remains a caveat, though the presence of the valley patterns strongly suggests its former existence.