Guide to Magellan Image Interpretation

 

Chapter 6. Surface Modification Processes

Catherine M. Weitz

 

Introduction

[57] Wind-related features provide direct evidence for the interaction of an atmosphere with a surface, and the dominant process currently affecting the surface of Venus appears to be wind. At least four kinds of eolian features have been identified on Venus: wind streaks, dunes, yardangs, and extended crater deposits [Greeley et al., 1992; Arvidson et al., 1991]. The orientation of these features provides clues to the wind direction at the time of their formation. In addition to eolian activity, there is evidence for degradation of lava flows on the plains. Arvidson et al. [1992] found that young volcanic flows in Sedna Planitia have higher backscatter cross-sections than older flows. Volcanoes and tessera units at high elevations generally have high backscatter cross sections, high reflectivity, and low emissivity values. There is a strong possibility that at these high elevations a chemical reaction causes the minerals in the rocks and soils to weather to materials with higher dielectric constants [Pettengill et al., 1988; Klose et al., 1992]. Finally, mass movements, including rock slumps, rock and/or block slides, rock avalanches, debris avalanches, and possibly debris flows have been identified on Venus [Malin et al., 1992].

 

Eolian Features

Large areas on Earth and Mars are covered by eolian features [Cutts and Smith, 1973; Breed et al., 1979]; these features were predicted for Venus [Greeley et al., 1984; Saunders et al., 1990]. Images of the surface and measurements of near-surface winds made by the Russian Venera landers suggest local modification of the surface by wind.

The Venera 9 and 10 landers measured wind velocities between 0.5 and 1.0 m/s at I m above the Venusian surface [Florensky et al., 1977]; these speeds are believed to be capable of transporting particles on the surface.

 

Wind Streaks

Most wind streaks on Earth and Mars are associated with topographic obstacles, and the streaks form in response to turbulence around the obstacles. Wind streaks on Earth are visible in radar images typically when roughness differences (such as those between bedrock and sediment) result in contrasts in the backscatter cross sections [Greeley et al., 1989; Saunders et al., 1990]. Wind streaks occur in several forms and in a variety of settings on Earth. Field observations of a wind streak at the Amboy lava flow in California indicate that the streak is dark at visible wavelengths because the basalt surfaces are swept free of sand. In radar images, the streak appears bright because the basalt surface has no mantling of sand to smooth the surface [Greeley et al., 1989; Saunders et al., 1990]. In Seasat and Shuttle radar images, streaks are usually visible where deposits of windblown sand appear radar dark against the surrounding radar-bright exposures of rougher alluvium or bedrock [Greeley et al., 1983 and 1989; Saunders et al., 1990]. Similarly, most wind streaks seen in Magellan images are visible because of differences in roughness between the streak and the surrounding terrain [Greeley et al., 1992]. The majority of streaks on Venus appear to be associated with impact craters, suggesting that the streaks represent debris produced during the impact and later modified by surface winds.

Wind streaks on Venus can be classified by their radar appearance and plan-view shape; five different shapes are [58] identified: fan, linear, transverse-ragged, transverse-smooth, and wispy [Greeley et al., 1992]. The fan streaks shown in Figure 6-1 are radar bright, but several have radar-dark halos. These halos could represent accumulation of debris around the edge of the erosional core. The orientation of the fan streaks indicates a wind from the north. The rougher, heavily fractured underlying terrain appears bright on the image compared to the adjacent smooth terrain. Because the underlying fractured terrain is visible only in the streaks, the streaks are probably areas of nondeposition or scouring.

Linear streaks are typically more than 20 times longer than their widths and generally occur in sets. Dozens of radar-dark and radar-bright streaks are visible in Figure 6-2. In this image, it is difficult to determine whether these are dark streaks on a bright background or bright streaks on a dark background, which also makes determination of wind direction difficult. However, several radar-bright fan streaks associated with small volcanic domes to the south of the linear streaks indicate a wind flow from the northwest. The linear streaks are located near Mead, the largest impact crater on Venus (see Chapter 7).

Transverse streaks form downwind of topographic barriers; these barriers, such as fractures, ridges, and hills, are perpendicular to the wind direction. Examples of transverse-ragged streaks are illustrated in Figure 6-3. These radar-dark streaks occur on radar-dark lava flows and indicate a wind flow from the southeast. "Ragged" refers to the serrated margin on the downwind side of the streak sets. The streaks are depositional in character because they obscure the underlying terrain, and their low backscatter indicates a composition of fine material that has smoothed the underlying terrain. Figure 6-4 shows transverse-ragged radar-dark streaks associated with tessera terrain in northwestern Aphrodite Terra. The region slopes to the west, yet the wind streaks indicate a wind from the west; thus, the streaks indicate a wind flowing upslope. The source of the debris composing the streaks may be the tessera terrain, which appears heavily fractured in this region.

A transverse-smooth streak is shown in Figure 6-5. The radar-bright streak has formed on the downwind (south) side of a hill. "Smooth" refers to the downwind margin of the streak. The streak obscures the underlying terrain and is brighter than the surrounding terrain, which suggest that the streak covers the surrounding terrain with rougher material.

Wispy streaks are probably composed of fine debris because they are always radar dark. They consist of wavy, meandering patterns usually associated with ridges and impact craters. It is often difficult to determine the source of the streak and the wind direction because of this meandering character. In Figure 6-6, however, the wispy streaks clearly originate from a dark splotch (Chapter 7) that is probably debris from an object that was pulverized as it traveled through the dense Venusian atmosphere [Schaber et al., 1992; Schultz, 1992]. Because the origin of the streaks is clear, a wind from the northeast can be inferred.

 

Dunes

Dunes on Earth result from the accumulation of saltated particles. Radar echoes from sand dunes are highly directional and sensitive to the imaging geometry of the radar system. Studies of look-angle effects of terrestrial dunes in airborne, Seasat, and shuttle radar images indicate that dunes are bright in small-look-angle radar images because of quasispecular reflections from smooth dune faces that are near normal to the radar beam (i.e., the local incidence angle at the dune face is zero) [Blom and Elachi, 1981 and 1987]. Strong radar backscatter from dune slip faces is possible only at look angles less than the angle of repose for windblown sand, which is 33 deg on Earth. At look angles > 33 deg, radar scattering from terrestrial dune faces is not returned to the antenna, and a black image tone appears in the radar images [Blom and Elachi, 1981 and 1987].

Two possible dune fields have been identified on Venus [Weitz et al., 1992; Greeley et al., 1992]. The first, centered at 25°S, 340°E, is located in an outflow deposit about 100 hen north of the 65-km-diameter impact crater Aglaonice (Figure 6-7). It appears that the wind has reworked the outflow deposits to form wind streaks and dunes. The dunes range in length from 0.5 to 5 km and the nearby wind streaks indicate a westward wind flow, with the dunes oriented transverse to the wind. The radar look angle at this latitude is approximately 35 deg, so assuming a similar slope geometry for Venusian dunes, the dune slip faces may be oriented nearly perpendicular to the radar illumination, producing quasispecular reflections as on Earth.

The Fortuna-Meshkenet dune field (Figure 6-8) is in a valley between Ishtar Terra and Meshkenet Tessera. Approximately 40 radar-bright fan streaks occur within the field. The wind streaks indicate that the dunes are oriented transverse to a prevailing wind from the east-southeast. The streaks appear to originate from small, radar-bright cones (Figure 6-9) and probably consist of the same high radar reflectivity material as the cones. The Cycle 1 image (Figure 6-8) and the Cycle 2 image (Figure 6-10) were both obtained at a 25-deg look angle, but from opposite sides. There appears to be no movement of the dunes or streaks between the Cycle 1 and 2 images. Because the illumination directions are opposite and because there are dune faces in the southern....

 

[59]

Figure 6-1.Radar-bright fan steaks, some of which have a radar-dark halo.

Figure 6-1.Radar-bright fan steaks, some of which have a radar-dark halo. The streaks probably represent zones of scouring or nondeposition. The wind streaks indicate a southward wind flow. Radar illumination is from the left at an incidence angle of 44 deg.

[60]

Figure 6-2. Linear radar-bright and -dark wind streaks centered at 15°N, 60.43°E.

Figure 6-2. Linear radar-bright and -dark wind streaks centered at 15°N, 60.43°E. It is difficult to determine if the streaks represent radar-bright streaks on a dark background or radar-dark streaks on a bright background. Some radar-bright fan streaks near the bottom of the image indicate a wind direction from the northwest. Radar illumination is from the left at an incidence angle of 46 deg.

 

....part of the dune field that are parallel to the radar illumination, there must be other scattering effects, such as a change in roughness or dielectric constant, to account for the high backscatter for these features in both images.

 

Yardangs

Yardangs are streamlined hills oriented parallel to the prevailing winds and produced by wind erosion of rock or soft sediments. They are visible in radar images because of their linear shape, sharp boundaries, and topographic relief. Some possible yardangs have been identified in one location on Venus, centered at 9°N, 60.5°E [Greeley et al., 19921. The yardangs shown in Figure 6-1 1 are sets of slightly sinuous, parallel ridges and grooves. The features are believed to be yardangs rather than wind streaks because they have well-defined boundaries and they do not originate from topographic features. The yardangs occur in two sets with both sets indicating a wind from the northeast. Because the yardangs are about 300 km southeast of the crater Mead, they are interpreted as deposits from the formation of Mead that have been eroded by later winds.

 

Crater Extended Deposits

Large radar-dark parabolic deposits associated with impact craters seem unique to Venus. These features are also related to eolian activity [Arvidson et al., 1991; Campbell et al., 1992] (Figure 6-12). The majority of the 57 parabolic-shaped features identified by Campbell et al. [1992] on data.....

 

[61]

Figure 6-3. Radar-dark transverse-ragged streaks on radar-dark lava flows.

Figure 6-3. Radar-dark transverse-ragged streaks on radar-dark lava flows. The streaks are composed of fine debris and indicate a wind direction from the southeast. Radar illumination is from the left at an incidence angle of 45 deg.

 

.....covering 92% of the surface are oriented east-west with the apex at the east end and the impact crater located west of the apex. Nine extended circular features associated with impact craters were also identified. Many parabolic features also have signatures in the emissivity and reflectivity data; lower emissivities and higher reflectivities imply higher dielectric constants [Plaut and Arvidson, 1992]. The parabolic deposits are believed to represent some of the youngest stratigraphic features on Venus. These deposits are apparently fine ejecta lofted to high altitudes where strong prevailing zonal east-to-west winds carry the particles many hundreds of kilometers before deposition [Campbell et al., 1992]. Because the material is fine, it has lower backscatter than the rougher surrounding terrain it mantles. Wind streaks are often associated with the parabolic features [Schultz, 1992].

 

Surficial Modification of Lava Flows

Backscatter signatures of the lava flows of different ages in Sedna Planitia were studied by Arvidson et al. [1992] to determine the degradation of lava flows with time. Young flows were found to have backscatter values similar to those

 

[62]

Figure 6-4. Radar-dark transverse-ragged streaks in Ovda Regio indicate that the wind is flowing upslope from the west to the east.

Figure 6-4. Radar-dark transverse-ragged streaks in Ovda Regio indicate that the wind is flowing upslope from the west to the east.

 

The streaks may represent debris eroded from the tesserae. Radar illumination is from the left at an incidence angle of 46 deg. of fresh terrestrial basalt flows. Older flows have backscatter signatures similar to those of degraded terrestrial basalt flows. Arvidson et al. concluded that in situ mechanical and chemical weathering were the dominant mechanisms that degraded the Venusian plains. Mechanical weathering would tend to fill any depressions in the lava flows with debris and rubble. Once the depressions were filled, the smoother surface would yield decreased backscatter cross-sections for these older flows. Chemical weathering would cause volume changes that would also smooth the surface and decrease backscatter.

 

Surficial Modification at High Elevations

Analyses of Pioneer Venus radar images revealed that the highest mountain peaks have very high backscatter cross sections and reflectivities [Pettengill et al., 1988; Garvin et al., 1985]. Magellan data a]so show strong correlations between high elevation, high backscatter cross section, high reflectivity, and low emissivity. Figure 6-13 shows central and northern parts of Ovda Regio. The complex structures indicate several tectonic events over a long history. The youngest event is the flooding of low-lying basins by smooth....

 

[63]

Figure 6-5. Transverse-smooth streak located at 26.2°N, 331.4°E.

Figure 6-5. Transverse-smooth streak located at 26.2°N, 331.4°E. The radar-bright streak, which may represent a deposit of rough material, indicates a wind from the northeast. Radar illumination is from the left at an incidence angle of 43 deg.

 

....(radar-dark) lava flows; the ridges and fractures are rougher and consequently appear brighter. The brightest terrain in the image is at the highest elevations.

The most likely explanation for the very high backscatter cross sections at high elevations is chemical alteration [Pettengill et al., 1988; Klose et al., 1992]. Klose et al. [1992] studied the relationship between altitude and emissivity for different highland regions and noted that above a critical altitude emissivity undergoes an abrupt and steep decrease that must denote a difference in surface mineralogy between the elevations. This suggests that the material at the higher elevations has been weathered to material with bulk dielectric constants higher than those of the surface material at the lower elevations, because the higher bulk dielectric constant causes a stronger radar return. The mineral most likely responsible for the low emissivities on mountaintops appears to be the electrical semiconductor pyrite (FeS2) [Pettengill et al., 1988; Klose et al., 1992].

One exception to this behavior is Maat Mons, a volcano high enough to produce a low emissivity zone near its summit. The lack of a low emissivity zone may mean that volcanic activity at Maat Mons is relatively recent, and the volcanic rocks at the highest altitudes have not had time to weather sign)ficantly [Klose et al., 1992].

 

Mass Movements

Mass movements, including rock slides, rock and debris avalanches, and possible debris flows, have been identified in areas of high relief and steep slopes in the Magellan images [Malin, 1992]. Figure 6-14 shows at least three landslides along a S.S-km-high scarp north of Dali Chasma. The landslides extend as much as 30 km from the base of the scarp. The hummocky surfaces of the landslides cause them to appear brighter than the adjacent smoother material.

Many steep-sided volcanic domes have rock slides associated with them [Guest et al., 1992; Malin, 1992]. Figure 6- 15 shows a rock-slide avalanche on the eastern flank of a volcanic dome. The dome is very bright and foreshortened on the flank facing the radar, indicating that the dome is steep sided; the steep, unstable margins of the dome are the most likely cause for the collapse that formed the rock slide. The very rough, hummocky surface of the landslide is radar bright compared with the smoother, radar-dark plains surrounding it.

 

[64]

Figure 6-6. Wispy streaks originating from a radar-dark splotch.

Figure 6-6. Wispy streaks originating from a radar-dark splotch. The streaks and the splotch represent fine debris that has mantled the underlying, rougher surface. Radar illumination is from the left at an incidence angle of 40 deg.

[65]

Figure 6-7. Radar-bright outflow deposits from the crater Aglaonice have been reworked by the wind to produce wind streaks and dunes (25°S, 339°E).

Figure 6-7. Radar-bright outflow deposits from the crater Aglaonice have been reworked by the wind to produce wind streaks and dunes (25°S, 339°E). The wind, which is flowing from the east to the west, has produced linear radar-bright and -dark streaks and dunes that are transverse to the streaks. The bright appearance of the dunes results from quasi-specular returns from dune faces oriented normal to the radar illumination. Radar illumination is from the left at an incidence angle of 35 deg.

[66]

Figure 6-8. Approximately 40 radar-bright streaks are located within a dune field centered 67.7°N, 90.5°E.

Figure 6-8. Approximately 40 radar-bright streaks are located within a dune field centered 67.7°N, 90.5°E. The wind streaks indicate that the dunes are oriented transverse to the prevailing winds. Radar illumination is from the left at an incidence angle of 25 deg.

[67]

Figure 6-9. Enlargement of a center portion of Figure 6-8 showing dunes and radar-bright wind streaks.

Figure 6-9. Enlargement of a center portion of Figure 6-8 showing dunes and radar-bright wind streaks. The streaks appear to originate from small volcanic domes in the dune field.

[68]

Figure 6-10. Cycle 2 right-looking image of the same area shown in Figure 6-8.

Figure 6-10. Cycle 2 right-looking image of the same area shown in Figure 6-8. There appears to have been no movement of the dunes or streaks between the two cycles. Radar illumination is from the right at an incidence angle of 25 deg.

[69]

Figure 6-11. The radar-dark linear features in this figure may represent yardangs.

Figure 6-11. The radar-dark linear features in this figure may represent yardangs. The eroded portions of the yardangs are darker than the surrounding terrain possibly because they contain smoother deposits of debris. Radar illumination is from the left at an incidence angle of 46 deg.

[70]

Figure 6-12. The crater Stuart (30.8°S, 20.5°E) has a radar-dark parabola associated with it.

Figure 6-12. The crater Stuart (30.8°S, 20.5°E) has a radar-dark parabola associated with it. The crater as a radar-bright floor and low emissivity, as do many of the craters with parabolic features. The parabola extends over 1000 km to the west and it covers all the underlying terrain. Radar illumination is from the left at an incidence angle of 33 deg.

[71]

Figure 6-13. Chemical alteration at high elevations may cause the very high backscatter cross-sections seen in this image of Ovda Regio. The regions of high backscatter also have a very high reflectivities and low emissivities. Radar illumination is from the left at an incidence angle of 45 deg.

Figure 6-14. At least three landslides extending from a steep scarp can be identified in this figure. The lobate landslides are radar bright because they are composed of rough blocks that produce a hummockry surface and overlie the finer deposits that show a smoother, radar-dark surface. Radar illumination is from the left at an incidence angle of 41 deg.

[72]

Figure 6-15. A debris apron has formed after the collapse of the eastern flank of a volcanic dome.

Figure 6-15. A debris apron has formed after the collapse of the eastern flank of a volcanic dome. The very rough, hummocky surface of the landslide is radar bright compared with the smoother plains surrounding it. Radar illumination is from the left at an incidence angle of 30 deg.

 

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