The mapping of Venus refers to the process and results of human description of the geological features of the planet Venus. It involves surface radar images of Venus, construction of geological maps, and the identification of stratigraphic units, volumes of rock with a similar age.
Satellite radar provides imagery of the surface morphology by using the physical properties of wave reflection. Long wavelength microwaves are used to penetrate the thick, cloudy atmosphere of Venus and reach to the surface. Different surface features reflect waves with different strengths of signal, producing images from which the maps are constructed.
After collection of the images of the Venusian surface, scientists started to map and identify different geologic materials and units according to distinctive surface features. Different groups of scientists analyzed different mapping areas, schemes and interpretation of features observed, to produce a classification of the units and comparison of their mapping.
Overview
Before the development of radar-based observation, the thick yellow Venusian atmosphere hid surface features.Ross, F. E. (1928). "Photographs of Venus". Astrophysical Journal. 68–92: 57 In the 1920s, the first Venus ultraviolet project captured the thick atmosphere of Venus, but provided no information about the surface. Venera program
From 1961 to 1984, the Soviet Union developed the Venera probes for surface mapping by radar. The Venera 4 (on October 18, 1967) was the first lander to make a soft landing on Venus (also the first for another planetary object). The probe operated for about 23 minutes before being destroyed by the Venusian atmosphere. The Venera 15 and 16 orbiters returned radar topography images of part of the Venusian surface.Goldstein, R. M.; Carpenter, R. L. (1963). "Rotation of Venus: Period Estimated from Radar Measurements". Science. 139 (3558): 910–911.
Magellan mission
The global surface of Venus was fully mapped by the Magellan orbiter during 1990–1991 with 50 km spatial and 100 m vertical resolution. During three orbit regimes, the surface images were transmitted back to the Earth. These three orbiting motions of the spacecraft are called mapping cycle 1, 2 and 3.
During the mapping cycle 1 (left-looking) radar surface mapping on Venus (September 15, 1990 to May 15, 1991), around 70% of the Venusian surface was mapped by synthetic aperture radar. In cycle 2 (right-looking), 54.5% of the surface was mapped, mainly the south pole regions and gaps from cycle 1 during May 15, 1991 to January 14, 1992. Combining cycle 1 and 2 results in a total coverage of 96% of Venusian surface mapped. Cycle 3 (left looking) filled remaining gaps and collected stereo imagery of approximately 21.3% of the surface, increasing the total coverage to 98%.Howington-Kraus, E., Kirk, R. L., Galuszka, D., & Redding, B. (2006). USGS Magellan stereomapping of Venus. In European Planetary Science Congress 2006 (p. 490)."Mission Information: MAGELLAN". NASA / Planetary Data System. 1994-10-12. Retrieved 2011-02-20.Grayzeck, Ed (1997-01-08). "Magellan: Mission Plan". NASA / JPL. Retrieved 2011-02-27.
Proposed future InSAR mapping
The use of Interferometric synthetic aperture radar (InSAR) for mapping Venus has been proposed.Meyer, Franz J., and David T. Sandwell. "SAR interferometry at Venus for topography and change detection." Planetary and Space Science 73.1 (2012): 130–144.
Instead of surface mapping by SAR as done by previous missions, InSAR would measure the terrain motions during events such as earthquakes or tectonic movements. By performing the radar mapping at two separated times (before and after an event) over the same area, the terrain changes could be revealed.
Mapping strategies
From the Magellan mission data, 3 types of images have been produced: (1) SAR images, (2) topographic images and (3) meter scale slope image.Kazuo, O. "Recent Trend and Advance of Synthetic Aperture Radar with Selected Topics: Remote Sensing." (2013): 716–807.Graff, Jamie R. MAPPING AND ANALYSIS OF THE TECTONO-MAGMATIC FEATURES ALONG THE HECATE CHASMA RIFT SYSTEM, VENUS. Diss. Carleton University Ottawa, 2014.
SAR imaging SAR images provide the highest resolution data set. Microwave radiation is used to penetrate the thick atmosphere and map the surface of Venus.
The SAR images are black and white images, which show the surface features using the intensity of radar return (echo), either due to surface roughness or orientation. For SAR imaging, spacecraft do not point directly downward (nadir) but slightly to the side—anywhere from about 10° to 45°. If the surface being mapped is smooth, the incident radar pulse will be reflected away from the spacecraft, resulting in a weak echo, which is represented by darker regions SAR images. On the other hand, if the surface is rougher, then more of the radar wave will be backscattered, and the intensity of echo will be stronger, which are represented by brighter areas in SAR images.
SAR images do not provide the color of surface, only the intensity of reflection of radar waves on the surface at a particular incidence angle. For example, when there is a light source shining on the blue cap on the left (left-looking), there will be shadows on the other side of the cap where light waves are blocked by the cap and no reflection occurs. If the looking direction is changed to the right, the shadowing (dark on SAR image) part will be at the opposite side.
The USGS Branch of AstrogeologyHerrick, R. R., & Sharpton, V. L. (2000). Implications from stereo-derived topography of Venusian impact craters. Journal of Geophysical Research: Planets, 105(E8), 20245–20262. has produced full resolution radar maps (also known as FMAPs) of Venus from the SAR data collected from the mission, called the Magellan F-BIDRs (Full resolution Basic Image Data Records). The maps have a coverage of around 92% (combination of the 2 left-looking cycles).Howington-Kraus, E., et al. "USGS Magellan stereomapping of Venus." European Planetary Science Congress 2006. 2006. The resolution is 75 m/pixel, which is the highest resolution Venusian map.
Topographic mapping
Topographic images were collected using radar altimetry. Compared to the SAR images, the topographic images have a significantly lower resolution of around 3–5 km/pixel. These images show lower elevations with darker pixels with higher elevations are shown with brighter pixels. Despite the low resolution, it is useful to study the regional feature of Venus, including initial evidence for the existence of rift zones.
There are three types of topography on Venus Highlands with elevation greater than 3 km, cover about 10% of the surface Deposition plains with elevations around 0 to 2 km, cover more than 50% of the surface Lowlands (accumulation of eroded highlands) with negative elevation, cover the rest of the surface The surface observation includes impact craters, volcanoes, and lava flow channels, which give clues to estimate the surface age, possible global resurfacing events, tectonic activities, internal structure and surface processes.
Unit classification and mapping schemes Different missions have mapped different cartographic quadrangles of the surface of Venus. They applied different mapping schemes and came up with different classifications of Venusian units.
Here is a table comparing the different mapping scheme and unit identification by the Magellan science team (1994),Senske, D. A., Saunders, R. S., & Stofan, E. R. (1994, March). The global geology of Venus: Classification of landforms and geologic history. In Lunar and Planetary Science Conference (Vol. 25, p. 1245). Vicki L. Hansen (2005)Hansen, V. L. (2005). Venus's shield terrain. Geological Society of America Bulletin, 117(5–6), 808–822. and Mikhail A. Ivano and James W. Head (2011). The possible matching of the above units are in accordance with their radar backscatter and surface features. Mapping GroupsMagellan Science Team (1994)Mikhail A. Ivanov and James W. Head (2011)Vicki L. Hansen (2005)Mapping SchemeGlobal-scale geologic mapping scheme (defined by difference in radar backscatter, surface texture and topographic)Stratigraphic Classification Scheme (defined to global stratigraphy with a division of geologic time)(defined by local formations and deformations, instead of global stratigraphic)Unit ClassificationStratigraphic Units:Stratigraphic Unit(Tessera is not in this classification)1. Tessera (t)1. Tessera terrain (Further classification into 7 types, according to the features in Hansen and Willis' paper in 1996Hansen, V. L., & Willis, J. A. (1996). Structural analysis of a sampling of tesserae: Implications for Venus geodynamics. Icarus, 123(2), 296–312.)2. Mountain belts (mb)1. Lineated plains3. Densely lineated plains (pdl)2. Flow material from different origin locally2. Reticulate plains4. Ridged plains (pr)/5. Regional plains (rp, upper and lower units)3. Bright plains /4. Dark plains6. Smooth plains (ps)5. Mottled plains7. Shield plains (psh)8. Shield clusters (sc)6. Digitate plains (lava flow fields)9. Lobate plains (pl)Geomorphic units:Structural features1. Complex ridged terrain (CRT or tesserae)(Tesserae is a geological material instead of structural features)2. Ridged and fractured terrain3. Ridge belts (including mountain belts)1. Groove belt (gb)1. Secondary Structures4. Fracture belts/2. Rift zones (rz)Deposits: - Associated with impact eventsImpact Crater Forming Materials1. Crater material1. Crater materials (c)1. Crater material2. Bright diffuse deposits2. Impact crater flow material (cf)2. Flooded crater material3. Dark diffuse areas The details of the above mapping scheme and units will be discussed one by one below.
Geologic mapping scheme by the Magellan The global-scale geologic mapping scheme by the Magellan science team was a very early mapping done by the Magellan mission (1990–1991). Instead of identifying different geological materials, it basically grouped the global surface units with different radar backscatter (white and dark in SAR images), topography and surface texture.
The mapped units and their characteristics are listed below.
Stratigraphic units The stratigraphic units in this mapping scheme are classified as 6 types of plains: Stratigraphic unitsUnitsRadar backscatterSurface featuresInterpreted geological materialsLineated plainsModerate and homogeneousAbundant fractures, forming grids or orthogonal patterns/Reticulate plainsIntermediate and homogeneousAbundant and low sinuous ridges/Dark plainsHomogeneous, dark local areasSmoothLava flowBright plainsHomogeneous, bright local areas/Lava flooding with extension and riftingMottled plainsExtensive areas with both bright and dark materialsMottled textures with abundant small shields and despoites/Digitate plains Bright and dark depositesIn digitate patternsLava flow fields associated with coronae
Geomorphic units The units are defined by groups of structural features of commonly higher elevation area with ridges and deformations: Geomorphic unitsUnitsComplex ridged terrain (CRT or tesserae)Ridged and fractured terrainRidge beltsFracture beltsSurface featuresRidges and fractures with deformationsSame as CRT, but with a single direction of deformations dominateLinear, which parallel to the nearby ridgesDense parallel linear fractures, mainly around equatorial and southern regionsTopographyRegional highland areasRegional highland areasElevated ridges/Images
Deposits The deposits are mainly the impact crater materials and its deposits: DepositsUnitsRadar backscatterSurface featuresImageCrater materialImpact ejecta (bright)/Bright diffuse depositsRadar-bright materialsForming "wispy patterns"Dark diffuse depositsRadar-dark materialsParabola in shape
Stratigraphic classification scheme