Observations of Sedimentary Features near Select Tesserae on Venus

Observations and Mapping of Sedimentary Features near Select Tesserae on Venus

Petersen, S. K.¹ and Carter, L. M.² (¹Department of Planetary Sciences, ²Lunar and Planetary Laboratory)

Abstract

From spring 2024 to spring 2025, I worked with Professor Lynn Carter at the Lunar and Planetary Laboratory on a project to determine how tesserae on Venus interact with the planet’s sedimentary cycle and associated processes. We produced a map of sedimentary features that is on this ReDATA repository and associated with an abstract and poster presentation for the 56th Lunar and Planetary Science Conference (LPSC). We also adapted a dune crestline mapping algorithm from Telfer et al. (2015) to apply to dune fields on Earth to attempt to use the dune fields as analogs for microdunes on Venus. We found that sedimentary features near tesserae were somewhat rare. Nonetheless, we mapped the location of wind streaks, possible mass wasting, low emissivity terrain, and newly classified “outlines” in the immediate vicinity of several tesserae. We also discovered two microdune fields in Tellus tessera and Husbishag tessera. While we ran out of time to use our crestline mapping algorithm to study analogs of Venus microdunes, we were able to qualitatively determine that dune morphology does affect their appearance in SAR imagery depending on the direction they are imaged from.

Note: The following is an abridged version of reports/ScottPetersen_SeniorThesis.pdf. Please see that document for important details.

1. Introduction

Most of the data in this project came from NASA’s Magellan mission’s SAR imagery, though we also used the mission’s radiometry data. Magellan acquired S-band HH SAR images that were either westward looking or eastward looking (Ford et al., 1993). These image sets are called “left look” and “right look” images, respectively. We also used Sentinel-1 SAR imagery for the Earth analog sites. Sentinel-1 uses a C-band radar imager (European Space Agency, 2025a).

This work was done to increase understanding of tesserae by providing a map of sedimentary features in the vicinity of several tesserae. The map is intended primarily to aid in targeting the VenSAR instrument aboard the EnVision spacecraft. VenSAR is expected to have a maximum resolution an order of magnitude greater than Magellan’s maximum resolution (European Space Agency, 2025b). As VenSAR will be mapping pre-selected targets on Venus’s surface, helping to determine the most useful sediment/tessera-related sites will aid in maximizing the gain in understanding of tesserae’s relationship to sediments.

2. Methods

2.1 Mapping

Mapping of Venus was done in ArcGIS Pro (Esri, 2025). Magellan imagery was acquired from the online USGS Astropedia tool (USGS…, n.d.). Section bounds were determined by using Ivanov and Head (2011)’s geologic map of Venus, which allowed us to acquire imagery of our target tesserae.

Mapping was conducted by manually scanning across an image section at 1:400,000 scale while adding encountered sedimentary features to corresponding ArcGIS feature classes. Features were identified manually based on comparison to past works, especially Greeley et al. (1995) and Malin (1992). As mapping progressed, a greater diversity of features was sampled and classified. As a result, not all mapped tesserae contain each class of feature; however, this does not mean that certain features are absent near specific tesserae.

2.2 Microdune Analysis

The morphology of microdunes is impossible to determine without higher resolution imagery. We hoped to indirectly infer some of their properties by determining how the morphology of dune fields on Earth affect their appearance in SAR images, as Blom and Elachi (1981) have previously found SAR images of dune fields to be sensitive to look direction and incidence angle. We used Sentinel-1 SAR images acquired from ESA’s online Copernicus system (European Space Agency, 2025c). Due to time constraints, we only investigated the dune field at White Sands National Park. We then adapted an automatic dune crestline mapping algorithm from Telfer et al. (2015) to attempt to quantitatively characterize the dunes using the USGS’s DEMs.

3. Results

3.1 Description of Mapping Data

Our mapping campaign covered approximately 7% of Venus’ surface. The following sections describe each class in detail, each named after the corresponding .shp file.

3.1.1 MassWasting_LowConfidence

Mass Wasting, Low Confidence features are suspected instances of mass wasting but are too small to be confidently interpreted. They were identified by the presence of lobe shapes, a pattern of dark-bright-dark on slopes facing away from the radar beam, or visual similarity to talus slopes as shown in Malin (1992) and Carter (2023). These features occur frequently in graben near tesserae but are rare within or on the margins of tesserae. We believe the dark-bright-dark pattern is the result of roughening on the scale of Magellan’s radar wavelength occurring at the base of a slope due to the presence of talus deposits.

3.1.2 MassWasting_HighConfidence

These features are also suspected instances of mass wasting but are large enough to allow a highly confident interpretation as mass wasting. Most instances of this class of feature are more related to coronae or chasmata than tesserae, which is supported by Jesina et al. (2025)’s discussion of mass wasting as a tracer for seismic activity.

3.1.3 Pits

This class of feature refers to any chain of pits observed. There would often be instances of mass wasting, low confidence on large pit’s walls. We interpreted these mostly as volcanic dikes breaching the surface or as incipient graben. They occurred frequently within and outside of tesserae.

3.1.4 Dark_LowEpsilon

These features were characterized by the presence of radar dark terrain and a low dielectric constant. We used Magellan’s global emissivity map (Magellan Team, 1993) to derive a global map of the surface dielectric constant using the methodology in Ford et al. (1993). The globally averaged surface dielectric constant of Venus is approximately 4 (Pettengill et al., 1992), while the emissivity-derived dielectric constant in Dark Low Epsilon features was generally between 2 and 2.5.

3.1.5 Wind Streaks

Wind streaks were abundant, though it was very rare for any to be emanating from a tessera. Wind streaks were subdivided into five classes based on Greeley et al. (1995)’s classification scheme for wind streaks. Several streaks had also been previously noted by Greeley et al. (1995), but we were unable to verify exactly which ones due to difficulty precisely georeferencing their maps.

Streaks_Linear_Sparse

Greeley et al. (1995) had a single category for “linear streaks,” however we decided to split this category into “sparse” and “dense” subclasses. The sparse linear streaks were characterized by having a length to width ratio significantly greater than the topographic feature they were attached to and not being directly attached to other wind streaks.

Streaks_Linear_Dense

Dense linear streaks are streaks that occur in a very closely packed group but are still resolvable as individual streaks with high length to width ratios. As noted in Greeley et al. (1995), these streaks can be so densely packed that it becomes difficult to determine if the streaks are radar bright and the underlying terrain is radar dark, or vice versa.

Streaks_Transverse

Streaks of this class were identified based on criteria identical to the transverse streaks in Greeley et al. (1995). They always occurred immediately adjacent to a linear topographic feature. While they could cover the same area as a dense linear streak, they were not composed of many individual streaks and instead were a continuous covering of radar bright or dark material.

Streaks_Wispy

Wispy streaks were also classified the same way as in Greeley et al. (1995). These streaks had meandering, linear forms and could approach lengths of 100 km. They often seemed to be approximately perpendicular to nearby streaks, though it seems their directionality was mainly controlled by the presence of topographic features, as was also the case in Greeley et al. (1995).

Streaks_Fan

Fan streaks were any streaks that occurred adjacent to shield volcanoes. While Greeley et al. (1995) had a broader characterization of fan streaks as any streak that occurred in isolation with a length to width ratio smaller than 20:1, we only qualitatively noted the length to width ratio of all streaks. The streaks we classified as fan streaks had distinctively small length to width ratios.

3.1.6 Outlines

Outlines are features we define as a diffuse radar bright or radar dark patch of terrain on the margins of a topographical feature. While they share some similarities with transverse streaks such as their diffuse form and appearance next to topography, they do not seem to have any directionality. They could be intermittent or continuous. We divided them into two subclasses based on their radar brightness relative to the surrounding terrain.

Outlines_Bright

Bright outlines were often near instances of Mass Wasting, Low Confidence. They occasionally had nearly identical characteristics to Mass Wasting, Low Confidence, but were still classified as outlines due to the qualitatively larger distance the radar bright region extended. There were also several instances where the outlines only featured the diffuse brightness around a topographic margin.

Outlines_Dark

Dark outlines sometimes occurred in near wind streaks, so such instances of dark outlines may just be an unusual expression of streaks. Dark outlines were mainly found on tessera margins. Dark outlines also sometimes share Dark Low Epsilon features’ low dielectric constant but were not classified as such because they were not as dark.

3.2 Microdunes

3.2.1 Two New Microdune Fields

Two new microdune fields were discovered during our mapping campaign. The first was in Tellus tessera. The difference of the absolute value is less than 5° between Cycle 3 left-look and right-look images (Ford et al., 1993), which means changes in brightness are likely not attributable to the change in incidence angle and thus must be from some asymmetric feature on the surface: microdunes.

We also discovered a microdune field in Husbishag tessera. While the difference in absolute value of incidence angle between left- and right-looking images is greater than 10° at this latitude (Ford et al., 1993), we still believe that this feature is a microdune field because dense linear streak-like features in the Husbishag microdunes only appear in right-looking images.

3.2.2 Analog Comparison using White Sands National Park

We were unable to analyze the dune field characteristics or apply the mapping routine adapted from Telfer et al. (2015) to dune fields outside of White Sands, and as such were not able to determine the morphological properties that most affect changes in dune fields’ radar backscatter. However, our crestline mapping routine is close to being usable.

4. Discussion and Conclusion

This project led to the creation of a map of sedimentary features near tesserae on Venus, the discovery of two new microdune fields on Venus, and extensive development of an automated dune field crestline mapping routine. We hope that this work will help support the VenSAR and EnVision teams by facilitating targeting of sedimentary features, and possibly enabling future work on aeolian Earth analogs for Venus.

5. Data and Code Availability

The code for the crestline mapping routine and map of sedimentary features are available on this repository and on the following GitHub repository: https://github.com/ScottyP123/Observations-and-Mapping-of-Sedimentary-Features-near-Select-Tesserae-on-Venus.

6. Acknowledgements

Scott Petersen would like to thank Professor Carter for her support and mentorship throughout this project.

Travel to LPSC and parts of this work were funded by NASA grant 80NSSC23K0158 to L. Carter as part of the VenSAR Science Team.

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