Abstract
The heavily deformed upland tesserae are some of the most ancient geologic units on Venus and, as such, record the longest history of surface evolution. Our geologic understanding of these landforms is based largely on radar images from the Magellan mission, in which gross morphology and small-scale properties can be difficult to deconvolve. Here we use Magellan radar backscatter data for ridge slope surfaces in 22 highland areas to understand whether the tesserae can be subdivided in ways that differentiate surface property variations. Significant variations occur in the mean backscatter of ridge slopes, and we divide the tesserae into two groups with echoes lower (n = 15) or higher (n = 7) than an average tessera radar scattering behavior. While both few-kilometers-scale slopes and centimeter-scale roughness can modulate the radar returns, at least seven out of 15 tesserae with lower echoes are correlated with fine-grained impact crater ejecta deposits that smooth the surface. We propose that distal ejecta deposition plays a major role in creating the observed range of tessera radar properties and obscuring aspects of their original formation and in situ weathering. Our twofold classification system provides a new way of assessing the physical characteristics of tesserae from the Magellan data. Upcoming missions must consider both their original morphology and post-emplacement processes if we are to unlock the geologic record preserved in tesserae.
INTRODUCTION
The climate of Venus was likely much different in the past (e.g., Way et al., 2016; Way and Del Genio, 2020). While most of Venus is geologically young (Schaber et al., 1992) and does not preserve evidence of these ancient climate conditions, the oldest materials, known as tesserae, might. Tesserae cover only ~8% of the surface of Venus (Ivanov and Head, 2011) but may hold the key to deciphering Venus’s ancient geologic history. While tesserae have been suggested as forming globally within a limited period, much work now suggests that they are simply the oldest materials in any given region (Basilevsky and Head, 1998; Guest and Stofan, 1999). In either case, more work is required to understand tesserae as the upcoming DAVINCI, VERITAS, and EnVision missions seek to reveal the ancient Venus climate (Garvin et al., 2022; Smrekar et al., 2022; Straume-Lindner et al., 2022).
Tesserae have a complex morphology with two or more intersecting sets of linear tectonic structures (Barsukov et al., 1986; Ivanov and Head, 1996). Owing to this complex morphology and perhaps a high degree of small-scale surface fragmentation due to folding and weathering, tesserae appear radar-bright in synthetic aperture radar (SAR) images. Several researchers have proposed regional morphologic classification systems based on the shape and density of tectonic landforms (e.g., Vorder Bruegge and Head, 1989; Bindschadler and Head, 1991; Hansen and Willis, 1996), and recently there has been an effort toward a global morphologic map of tesserae (Albach and Whitten, 2022).
Some authors postulate that tesserae comprise materials similar to those of the continents on Earth (e.g., Nikolayeva, 1990). Near-infrared data have been used to infer that at least portions of some tesserae are indeed more silicic than the low-lying plains (Hashimoto et al., 2008; Helbert et al., 2008; Mueller et al., 2008; Basilevsky et al., 2012). For example, infrared radiance anomaly measurements from the Venus Express mission VIRTIS instrument suggest that Alpha Regio has two distinct compositional regions, interpreted as a deformed, basaltic western edge and a more silicic central upland (Gilmore et al., 2015). Microwave emissivity data from the Magellan mission may also identify differences in tessera mineralogy. Brossier and Gilmore (2021) interpreted microwave emissivity behaviors with altitude to indicate varying roles of ferroelectric or semiconductor minerals. All of these interpretations must consider a wide set of contributing surface parameters such as weathering products, spatial resolution effects, and mineralogic differences in the original tessera-forming material.
In this work, we analyze Magellan SAR data to further explore variations in tesserae surface properties by quantifying the statistical distribution of radar backscatter power from ridge slopes in 22 regions. As with other methods, the tesserae pose a challenge to remote sensing, in this case due to their mix of kilometer-to tens-of-kilometers-scale deformation and surface roughness on scales well below the resolution of observations. Radar has the unique capability to characterize decimeter-scale roughness, and our goal here is to assess (1) whether significant differences in radar properties among the tesserae are discernible, and (2) the degree to which we may infer the balance of kilometer-scale slope and small-scale roughness effects. These results bear directly on the interpretation of surface properties and mineralogy from existing orbital infrared observations and those of upcoming missions.
METHODOLOGY
The Magellan left-look SAR data, with a spatial resolution of ~150 m/pixel (Saunders et al., 1992), were used to derive the surface backscatter coefficient (σo), a measure of the amount of microwave energy scattered back to the spacecraft (Texts S1 and S2 and Fig. S2 in the Supplemental Material1). Changes in Magellan SAR backscatter are predominantly controlled by few-kilometers-scale slopes and centimeter-scale surface roughness (Campbell, 1994). Radar-dark signatures are associated with slopes that face away from the radar or with processes that smooth the surface, while radar-bright signatures are associated with radar-facing slopes or rougher surfaces. Impact cratering, volcanism, weathering, and aeolian transport can modify surfaces and their radar echo behaviors (Text S3). Derived σo values were filtered to avoid echoes due to anomalously low (<0.7) microwave emissivity values associated with high dielectric materials (Klose et al., 1992).
In this work, the statistical distribution of σo values for ridge faces that slope away from a vector toward the spacecraft were determined for 22 tesserae selected based on their size and latitudinal and longitudinal extent: Alpha, Sudenitsa, Tellus, Dolya, Nedolya, Fortuna, Cocomama, Pasom-mana, Clotho, unnamed tesserae around Cline crater, Athena, Ananke, Nemesis, Lahevhev, Hyndla, Mamitu, Ustrecha, Zirka, a portion of Thetis (near Whiting crater), Vako-nana, Virilis, and Husbishag (Fig. 1A). We calculate a mean and standard deviation of the σo values of all the sampled slope areas in each tessera and plot these against the average Magellan boresight angle, θM, with respect to a flat surface at the same latitude (Fig. S1). These echo values reflect a combination of the centimeter-scale and few-kilometers-scale topography, and we discuss additional regional observations that suggest a potentially dominant role for small-scale surface roughness changes due to impact ejecta.
TESSERA BACKSCATTER COEFFICIENT VALUES
Spatially coherent patterns of σo variations occur across and within the 22 measured tesserae, suggesting different physical properties (Fig. 2; Fig. S2; Table S2). The σo values were averaged in 2° bins of the Magellan nominal incidence angle, θM (Fig. 2A). There is a clear decline in power with θM, consistent with a generally similar, few-kilometers-scale slope distribution across the tesserae being imaged at progressively greater local incidence angles. Figure 2A also shows the Muhleman angular scattering law implemented to scale Magellan images (Saunders et al., 1992); that function captures the behavior of a low-roughness, plains-like surface (Text S2).
The tessera σo values generally fall above those of the Muhleman law, more so if that function is shifted to account for surfaces with east-facing slopes (i.e., tessera backslopes) (Fig. 2A). This is consistent with the general sense of a tessera surface being much rougher at the centimeter scale than the typical plains. The backscatter values do remain physically reasonable, in that highly diffuse-scattering (minimally angle-dependent) terrestrial a‘a flows reach σo values of about –8 dB (Campbell, 2009). To make a comparison among the 22 study sites, we fit an average scattering function to the data (Fig. 2). This is only a “true” backscatter function if the few-kilometers-scale slope distribution among the tesserae is similar, but we use it to make first-order inferences.
From these data, we note that backslopes in Thetis are offset from the best-fit line by the largest positive values (Fig. 2; Fig. S2). Portions of Vako-nana have the largest negative offset, followed by portions of Cline and Ustrecha. Overall, the σo differences between tesserae are subtle (e.g., ~2 dB between Fortuna and Alpha), but Figure 1A shows that these differences reflect spatially coherent behaviors as opposed to simply random sampling effects. The 2–3 dB standard deviation of each central value is due to averaging of individual pixels from all the backslope areas; if these values are averaged locally to reduce speckle effects, the typical standard deviation is 1–2 dB.
DISCUSSION
These tessera σo behaviors imply real differences between few-kilometers-scale slopes and/or centimeter-scale roughness (Fig. 2; Fig. S2B; Table S1), consistent with previous analyses of Thetis, Tellus, and Beta Regiones using Pioneer Venus Radar Mapper and Arecibo telescope data (Ford and Senske, 1990). The average σo values of the 22 tesserae are offset to different degrees from a best-fit “scattering law,” and two tesserae, Thetis and Husbishag, have higher standard deviations than is typical of the rest (Fig. 2B). The simplest approach to these data is to explore a twofold classification of tesserae that fall above or below the best-fit line. In this approach, 15 tesserae have below-average σo and seven have above-average σo (Fig. 3).
As noted above, we do not know how few-kilometers-scale slope distributions vary among the tesserae owing to topography data limitations, so our backslope sample sites within each region could be biased toward higher or lower true local incidence angles. While the centimeter-scale roughness appears to always be greater than that of plains (Fig. 2A), this characteristic would vary with the degree of fragmentation and subsequent formation of a regolith or emplacement of crater ejecta. One rationale for expecting a greater role for centimeter-scale surface roughness is that rougher surfaces have less variability in their angular scattering behavior (Campbell, 2009), such that a few-degrees shift in the mean of the tessera slope distribution may have little effect on their observed echoes.
Some of our tessera with σo values below the mean scatter law can be definitively associated with impact crater ejecta (Campbell et al., 1992; Schaber et al., 1992; Vervack and Melosh, 1992; Schaller and Melosh, 1998) (e.g., Figs. 1B and 1C). In these regions, radar-dark crater ejecta patterns are visible on adjacent plains and extend into tesserae. Previous studies show the association between lower σo and impact crater ejecta at Alpha, Tellus, Husbishag, Virilis, and Zirka Tesserae (Campbell et al., 2015; Whitten and Campbell, 2016). In general, a meter-thick, extensive “parabolic” radar-dark ejecta deposit is emplaced west of a crater due to the direction of the prevailing wind, but quite thick deposits occur to the east and near the crater rim (Vervack and Melosh, 1992; Schaller and Melosh, 1998). Here we identify ejecta in Nemesis Tesserae and the predicted extent of parabolic ejecta for each relevant nearby crater (Fig. 3). The modeled distributions suggest that as much as 70% of the area of some tesserae might be mantled by fine-grained ejecta (Table S3). Ejecta deposits occur in some tesserae with higher average σo, such as Sudenitsa (Whitten and Campbell, 2016), and we propose that these ejecta are thinner or more weathered such that the roughness variations are averaged out when assessing whole-tessera statistics.
Husbishag Tesserae display a wide diversity of σo behavior due to ejecta from Boulanger crater (~74 km in diameter) (Figs. 1B and 1C; Table S3). The majority of Husbishag Tesserae occur east of Boulanger, i.e., within the thickest region of ejecta (Vervack and Melosh, 1992; Schaller and Melosh, 1998). Husbishag σo values in areas with overlapping ejecta are 2.4 dB lower than those outside the ejecta pattern, illustrating the effect of mantling of the surface with fine material. Portions of Husbishag also have some of the highest σo values measured in this study, but the total range in σo is similar between tessera backslopes within and outside the parabolic ejecta (Fig. 1C). Thus, while ejecta lead to a decrease in backscatter strength, the “background” structure of Husbishag must vary significantly from that of other tesserae. It is the presence of ejecta across Husbishag that has changed the backscatter to a degree comparable to the range among all other sampled tesserae (Fig. 2). When classifying tesserae or remotely measuring their rock type(s), it is thus critical to acknowledge that impact deposits may have an outsized influence on their radiophysical properties.
Of the 15 tessera regions that fall below the best-fit line, seven have a firm connection with smoothing by impact crater ejecta (Fig. 3). The other eight (Hyndla, Nedolya, Cline, Pasom-mana, Vako-nana, Mamitu, Ananke, and Athena) below the line are not associated with obvious ejecta but do fall within the outlines of model-predicted parabola extents for nearby craters, for example, Cochran crater (~100 km in diameter; 51.9°N, 143.4°E) at Ananke Tessera and Gautier crater (~60 km in diameter; 26.3°N, 42.8°E) near Vako-nana. Our interpretation of the entire data set is that tesserae exhibit a range of large-scale morphology (slope distribution), centimeter-scale fragmentation, and regolith development inherent to their formation process and age (Text S3), but their radar signatures are then strongly biased by the emplacement of impact crater ejecta deposits.
CONCLUSIONS
The radar echoes from slopes within 22 tesserae across Venus have differences that suggest a strong correlation with smoothing by impact crater ejecta. Based on an example from Husbishag Tesserae, we propose that this process of distal ejecta deposition may play a major role in creating the current observed range of tessera backscatter properties, including obscuring aspects of their original formation and in situ weathering. Our twofold classification system provides a new way of assessing the physical characteristics of tesserae from the Magellan data. Upcoming Venus missions must consider both original tessera morphology and post-emplacement processes if we are to unlock the geologic record preserved in tesserae.
ACKNOWLEDGMENTS
We thank J.W. Head and two anonymous reviewers for their thorough comments that significantly improved this manuscript. All data used in this study are publicly available on the Planetary Data System Geosciences Node (https://pds-geosciences.wustl.edu/missions/magellan/index.htm). This research was not funded by a specific grant.