Geological mapping of Venus

Global surface of Venus

Geological Mapping on Venus characterizes geological features and units on Venus. It involves surface radar images of Venus, construction of maps, and the identification of geological units from the maps.

Surface radar provides imagery of the surface morphology by using the physical properties of wave reflection. Long wavelength microwaves are used to penetrate the thick atmosphere of Venus and reach to the surface. Different surface features will reflect waves with different strengths of signal (which will be discussed later), then the maps of Venus can be reconstructed. The principle of Venus surface image construction is similar to satellite imagery of Earth's surface.

After collection of the images of Venus surface, people started to map and identify different geologic materials and units according to distinctive surface features. Different groups of workers will have different mapping areas, schemes and interpretation of features observed. The classification of units and comparison of their mapping will be discussed later also.

Background

Before Magellan

Venusian cloud structure captured by the Pioneer Venus Orbiter using ultraviolet in 1979.

Main Article: Observations and explorations of Venus, Venera

Before the development of radar ground-based observation, the thick yellow atmosphere hid detailed Venusian surface features.^[1] In the 1920s, the first Venus ultraviolet project captured the thick atmosphere of Venus, but provided no information about the surface.

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 man made lander to make a soft landing through the atmosphere on Venus (also the first for other planetary objects). The probe only survived for at most 23 minutes before being destroyed by the Venusian atmosphere. The Venera series space probes returned images of the Venusian surfaces,^[2] shown as below with the landing locations of the probes.

Position of Venera landing sites and the radar topography obtained. Red points denote sites returning images from the surface, black central dots sites of surface sample analysis.

Magellan Mission

Synthetic Aperture Radar (showing right-looking in this diagram).

Main Article: Magellan (spacecraft), Synthetic aperture radar

The surface of Venus was mapped in detail by the Magellan Spacecraft in 1990-91. The spacecraft orbited around Venus and captured images of the adjacent surface. During three orbits, the surface image were transmitted back to the Earth. These three orbiting motions of the spacecraft are called cycle 1, 2 and 3.

During the 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%.^[3][4][5]

SAR image on Venus. This map is a mosaic of the "left-looking" data collected during cycle 1, produced by the USGS of the Magellan images.

Proposed Future InSAR Mapping

Main Article: Interferometric synthetic aperture radar

NASA InSAR satellite measuring small scale earthquakes

The use of Interferometric synthetic aperture radar (InSAR) for mapping Venus has been proposed.^[6]

Instead of surface mapping by SAR discussed above, InSAR is used to measure the terrain motions during events (such as earthquakes or tectonic movements). By preforming the radar mapping at two separated times (before and after an event) over the same area, the terrain shift will be shown as different phase due to path difference in the radar echo returned.^[6][7]

However, there are recently no new planetary missions to Venus announced. It maybe a possible future work can be done to explore more about this planet and its potential tectonic movements.

Construction of Map

Main Article: Radar mapping Synthetic aperture radar

From the Magellan mission data, 3 types of images have been produced: (1) SAR images, (2) topographic images and (3) meter scale slope image.^[7][8]

SAR Images

SAR images is the most important base map for mapping as the highest resolution data set. Microwave are used to penetrate the thick atmosphere and map the surface of Venus.

The SAR images are white and black images, which shows the surface features using the intensity of radar return (echo), either due to surface roughness or orientation.^[7]

If the surface is rougher, the radar will be scattered away and the intensity of echo will be weaker, which are represented by brighter regions in normal SAR images. On the other hand, if the surface is smoother, the wave will be reflected along the same direction, resulted as a higher intensity of echo and represented by darker regions.

Difference in wave reflection on smooth and rough surfaces.

The orientation of surface is depends on the looking direction of the radar. Unlike satellite images of Earth surface, SAR images tells no clues on the color of surface, but the reflection of wave on the surface at a particular looking direction (direction of wave propagation). For example, when there is a light source shining on the blue cap on the left (left-looking), there will be shadows on the another side of the cap where light wave 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.

Real life example on difference in looking direction in SAR images. Same object with difference looking direction shows completely different result in SAR image.

Left (left-looking) and right (right-looking) images showing the difference in SAR images on the same location on Venus. (Venus images extracted from USGS)

The USGS Branch of Astrogeology^[9] has produced a 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 has a coverage of around 92% (combination of the 2 left-looking cycles).^[3][10] The resolution of it is 75 m/pixel, which is the highest resolution of Venusian map.

Topographic Maps

Topographic images were collected using radar altimetry. Compared to the SAR images, the topographic image has a significantly lower resolution of around 3–5 km/pixel. These images show lower elevations with darker pixels with higher elevation is shown by brighter pixels. Despite of the low resolution, it is useful to study the regional feature of Venus, including initial evidence for the existence of rift zones.^[7]

Topographic Map of Venus

Topography and surface observations

There are 3 types of topography on Venus

1. Highlands with elevation greater than 2 km, covers about 10% of the surface
2. Deposition Plains with elevation around 0 to 2 km, covers more than 50% of the surface
3. Lowlands (accumulation of eroded highlands) with negative elevation, covers the rest of the surface

The surface observation includes impact craters, volcanoes and lava flow channels, which give clues of surface age estimation, possible global resurface event, tectonic activities, internal structure and surface processes.

Different Venus' unit classification and their mapping schemes (Global mapping)

There are many groups of workers mapped the surface of Venus after the Magellan Mission. Different groups mapped different cartographic quadrangles for 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 of the Magellan Science Team (1994),^[11] Vicki L. Hansen (2005)^[12] and Mikhail A. Ivano and James W. Head (2011).^[13] The possible matching of the above units are in accordant to their radar back scatter and surface features.

Mapping Groups	Magellan Science Team (1994)	Mikhail A. Ivanov and James W. Head (2011)	Vicki L. Hansen (2005)
Mapping Scheme	Global-scale geologic mapping scheme (defined by difference in radar back scatter, 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 Classification	Stratigraphic 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 1996[14])
		2. Mountain belts (mb)
	1. Lineated plains	3. Densely lineated plains (pdl)	2. Flow material from different origin locally
	2. Reticulate plains	4. Ridged plains (pr)
	/	5. Regional plains (rp, upper and lower units)
	3. Bright plains	/
	4. Dark plains	6. Smooth plains (ps)
	5. Mottled plains	7. Shield plains (psh)
		8. Shield clusters (sc)
	6. Digitate plains (lava flow fields)	9. Lobate plains (pl)
	Geomorphic Units:	Structural features
	1. Complex ridged terrain (CRT or tesserae)	(Tesserae is a geological material instead of structural features)
	2. Ridged and fractured terrain
	3. Ridge belts (including mountain belts)	1. Groove belt (gb)	1. Secondary Structures
	4. Fracture belts
	/	2. Rift zones (rz)
	Deposits: - Associated with impact events	Impact Crater Forming Materials
	1. Crater material	1. Crater materials (c)	1. Crater material
	2. Bright diffuse deposits	2. Impact crater flow material (cf)	2. Flooded crater material
	3. Dark diffuse areas

The details of the above mapping scheme and units will be discussed one by one below.

Global-scale geologic mapping scheme by the Magellan Science Team (1994)

It was an very early mapping done after the Magellan Mission (1990-1991). Instead of identifying different geological materials, it basically grouped the global surface units with different radar back-scatter (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 is classified as 6 types of plains:

Stratigraphic Units
Units	Radar Back-scatter	Surface features	Interpreted Geological materials
Lineated plains	Moderate and homogeneous	Abundant fractures, forming grids or orthogonal patterns	/
Reticulate plains	Intermediate and homogeneous	Abundant and low sinuous ridges	/
Dark plains	Homogeneous, dark local areas	Smooth	Lava flow
Bright plains	Homogeneous, bright local areas	/	Lava flooding with extension and rifting
Mottled plains	Extensive areas with both bright and dark materials	Mottled textures with abundant small shields and despoites	/
Digitate plains	Bright and dark deposites	In digitate patterns	Lava 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 Units
Units	Complex ridged terrain (CRT or tesserae)	Ridged and fractured terrain	Ridge belts	Fracture belts
Surface features	Ridges and fractures with deformations	Same as CRT, but with a single direction of deformations dominate	Linear, which parallel to the nearby ridges	Dense parallel linear fractures, mainly around equatorial and southern regions
Topography	Regional highland areas	Regional highland areas	Elevated rudges	/
Images	Aphrodite Terra Complexly deformed	Lineated terrain Alpha Regio	Northern part of the Akna Montes (mountains) of Venus with a ridge belt.	Lineated Plains on Venus

Deposits

The deposits are mainly the impact crater materials and its deposits:

Deposits
Units	Radar Back-scatter	Surface Features	Image
Crater material	Radar-bright impact crate ejected materials	/	Mapping of Venus Crater based on 3 units: (1) crater material (2) bright diffuse deposits and (3) dark diffuse deposits.
Bright diffuse deposits	Radar-bright materials	Forming "wispy patterns"
Dark diffuse deposits	Radar-dark materials	Parabola in shape

The stratigraphic classification scheme by Mikhail A. Ivano and James W. Head (2011)

One way to do mapping on Venus and characterization on the geological units on Venus is by the stratigraphic classification scheme.^[15] Mikhail A. Ivano and James W. Head (2011) mapped the area of geotraverses at 30N^[16] and 0N. They traced and discussed the global spatial distribution of rock-stratigraphic units and structure, and suggested their time correlation and geological history.^[13]

Stratigraphic units

This mapping scheme suggested that there are around 12 global units on Venus by the work of Mikhail A. Ivano and James W. Head (2011), and these units are used throughout their mapping on different quadrangles on Venus. The stratigraphic units and landforms mapped by Mikhail A. Ivano and James W. Head (2011)^[13] are listed below in terms of mechanism from the oldest to the youngest.

Tectonic units

Tectonic units are formations formed due to large-scale crustal processes. In this mapping scheme, these surface units are grouped into possible same set of geological materials, shown by similar surface features.

Tessera (t)

Outline of tessera terrain imposed on "GIS Map of Venus" (GIS Map of Venus source: USGS Astrogeology Science Center)

Maxwell Montes's tessera (t) terrain seen in appearing as white in SAR image

Tesserae are heavily deformed highland regions (greater than 2 km in elevation) on Venus. This unit is believed to be the oldest material on Venusian surface with highest level of tectonic deformation.^[17][18] It is of high topography and seen in white on the SAR images with high radar backscatter.^[19] The materials composed tessa terrain, which was named as unit Tt in the mapping of V-17(Basilevsky, A. T.,1996).^[20]

The intersecting of material and tectonic structures are the defined characteristic of tessera, but the sets are not always seen in the images.^[13] Due to the heavily tectonic deformed, it contains both contractional features of ridges and extensional features of graben and fractures.^[13]

The boundaries of tessera shows embayment by other materials of other units. By this cross-cutting relationship, it provides evidence of tessera being the oldest unit within the strata.^[13]

Densely lineated plains (pdl)

Densely lineated plains (pdl) on Venus

This unit is defined by the dense and parallel lineaments packed on the unit.^[13] The made up a small area on the Venus global surface of around 7.2 x 10⁸ km².^[13] The lineament is the pattern of deformation, which make it a typical structural–material unit.^[13]

There are evidence showing the embayment of tessera by pdl's materials in some tessera margins. Thus, it is possible that this unit is younger than the tessera unit.^[13]

In SAR images, it also shows a high backscatter imagery, but lighter than that of tessera.

Ridged plains (pr)

The ridge belt that forms in the western part of the elevated smooth plateau of Lakshmi plain.

This unit are lava plains deformed by ridges. It has a smooth surface with relatively higher elevation than the surroundings.^[13] The ridges are usually symmetrical in cross-section and collected into prominent belts.^[21][22]

There are evidence in places showing that pr unit is embaying the t and pdl unit. Also, the deformation of pr took place after the formation of t and pdl units. Thus, pr unit is possible younger than both unit t and pdl.^[13] As most of the deformation features on pr is far away from that on t and pdl units, it is difficult to tell the age relationship of deformation directly.^[23][24][25] However, there are some tessera-like deformation additional to the ridge belts, it may suggest there are some possible overlapping of formation time in unit t and pr.^[13]

In SAR images, pr units have noticeably higher radar backscatter than surrounding regional plains, but lower than tessera (t) and densely lineated plains (pdl) units. Ridges planes are having older ages compare to surrounding regional plains (pr) due to the difference in radar albedo and embayment relationship suggested by McGill and Campbell (2006).^[26]

The major occurrence of this unit is located among Vinmara, Atalanta, Ganiki, and Vellamo Planitiae, shich in a broad fan-shape,^{[27][28][29][30]} and also appears between Ovda and Thetis Regiones and in the southern hemisphere within Lavinia Planitis.^[31][32]

Some researchers mapped ridges of the pr unit as deformed structure instead of a unit.^{[33][34][35][36][37]}

Mountain belts (mb)

Perspective View of Ishtar Terra showing the mountain belt (mb) units with Akna Montes and Freyja Montes along the western and northwestern edge of Lakshmi Planum, and Maxwell Montes along its eastern edge.

This unit is the only real mountain range on Venus in the area surrounding Lakshmi Planum, which covers only 1.3 x 10⁶ km² of the Venusian global surface,^{[27][38][39][40][41]} while involves structural deformation of different materials in their formation.^[13] There are in total four major mountain belts mapped on Venus, including the belts of Danu Montes, Akna Montes, Freyje Montes and Maxwell Montes (the highest mountain on Venus with elevation of around 12 km).^[13]

In looking at the cross-cutting relationship, the inner ridges of the belts seems to be embayed by the material of regional plains (pr), which covered the plateau surface. There are deformation afterwards in terms of tilting towards the belts and wrinkle ridges parallel to the belt. It suggested formation formed right before the deposition of regional plains and later deformation of the belts.^[13]

Shield plains (psh)

Formation of Shield Domes.

Accruva Formation. Evaluation Diagram of Shield plain formation from Shield Domes over time.

This units refers to plains with volcanic edifices of shield-like features.^[42][43][44] In most of the psh regions. the plains are concentrated and forms a group. It is the first oldest unit in the strata showing no widespread deformation, which only few tectonic deformation is observed, such as ridges and fractures.^[13] Compare to the above units, this units seems to cover high proportion of the Venusian surface of around 79.3 x 10⁶ km². Although the distribution of psh is widely spread and homogeneous, however there are also some regions with no psh units, including the Lakshmi Planum and some lowland of regional plains,^[13] The shield plains are formed from shield domes over time and suggested that psh may be associated as volcanic plains with small sources of volcanic materials and mildly deformed by tectonics.^[13]

There are embayment relationship showing that this unit is youngest than the above highly tectonized units (t and pdl) at global scale. However, the absence of unit in some regions makes this unit difficult to fit in to the strata, especially between the highly tectonized units mentioned above and regional plains will be mentioned in the next section.^[13]

In SAR images, the psh unit shows a higher radar backscatter compare to surrounding overlaying regional plains, still lower than units of t, pdl and pr.^[13]

Regional Plains (rp)

This unit is the most widely spread unit in Venusian surface of about 182.8 x 10⁶ km².^[13] It is defined as smooth and homogeneous plains. which are deformed into networks of linear subparallel or intersecting ridges.^[45] This unit is interpreted orgined with volcanic origin with deformarion of wrinkle ridges. However, the source of volcanism is not obvious in the Magellan data.^[13]

Regional Plains are divided into abundant lower unit (rp₁, Rusalka Formation) with smooth surface and relatively low radar backscatter and upper unit (rp₂, Ituana Formation) with also smooth surface, but higher radar albedo. Wrinkle ridges are heavily deforming the lower unit while moderately deforming the upper unit. The lower unit is heavily tectonized and embayed by lava plains and flows. The younger upper unit is lacking in large heavily tectonized tessera regions.^[13]

In SAR image, it shows an intermediate radar backscatter imagery around the Venusian global.

Shield clusters (sc)

Volcanic Domes on Venus

This unit is similar to shield plains, but tectonically undeformed. Based on the analysis by Crumpler and Aubele (2000),^[46] there are 10% of this unit shows evidence younger than regional plains (rp).^[47] Some of the small shield clusters are founded embaying the regional plains of both lower and upper layers, while in some regions, this unit are founded on top of the rp unit and deformed together by wrinked ridges.^[13]

Smooth plains (ps)

Adivar crater. Dark (Smooth plain) deposition around the crater.

This unit belongs to Gunda Formation, which is smooth and featureless surface without tectonic marks. It only make up for about 10.3 x 10⁶ km² of Venusian surface. These plains are usually with impact craters, which is tectonic undeformed.^[13] These plains are rarely with low domes. These suggested three type of setting for this unit:

(1) Many fields of smooth plains are near to the region with young volcanism (such as Bell Regio) with the lobate plains (pl). However the relationship of smooth and lobate plains are uncertain.

(2) Some of the unit is located as deposition around the impact crater, possibly associated with the impact events.^[48][49]

(3) small ps units are inside the tessera regions (such as Ovda Regio), which may associate with a volcanic origin,

Due to the usual higher elevation of smooth plains, it may be possible that the volcanic material of smooth plains is of younger unit.^[13]

Lobate plains (pl)

This unit is smooth surface being crossed with some extension features associated to rift zones. These feature is making up to around 37.8 x 10⁶ km², which is significant. The origin of lobate plains are believed to be associated with large volcanoes, which sometimes appears with large dome-shaped rises.^[13] One possible origin of this unit is from massive and multiple eruptions from large and localized volcanoes with little later extensional deformation.^[13]

By cross-cutting relationship, the plains is embaying the wrinkle ridges containing regional plains, which suggested that lobate plains are younger.^[13] However, as the lobate plains, smooth plains, shield cluster and rift zoons are often seems as small fractures, it is difficult to tell their time relationship.

In SAR image, they shows uneven radar back-scatter flow-like pattern.

Structural Units

Structural units are formed due to deformation. The resulted properties depends one the stress applied to the formation and the stain of the rocks.

Tessera-forming structures (ridges and grooves)

Ridges structures are mainly discussed in the ridged plains (pr) part above.

For groove belt (gb), it is belongs to the Agrona Formation, which refers to the dense extensional structures. This unit appears to be sets of subparallel lineaments of fractures or graben.^[13] This deformation unit makes up to around 37.1 x 10⁶ km² of the Venusian surface. These fractures are the most obvious and very abundance on the surface of Venus, which is cross different units on the surface. It appears to be a younger units on the surface. However, some vast plains unit are found embaying the grooves in some areas. It may suggested the formation of gb unit before the formation of plains.^[13]

The major difference between grooves unit and dense lineated plains are the former is belt-like and the latter is patch-like.^[13]

It is very important to map this fractures, as sometimes the rock unit may be too deformed and are not recognizable, which it can be mapped as "fractured plains materials" according to the guidelines of Wilhelms (1990).^[50]

In SAR image, this fracture are of high radar albedo as the tessera unit.^[13]

Rift zones (rz)

This unit is belongs to the Devana Formation, which it also made up of dense extensional structures with defined numbers of fissures and trough containing flat-floors.^[13]

It is found that the rift zone are usually related to the lobate plains, which may indicate that the rifting are related to the young volcanism and also young volcanic plains formed.^[13]

Impact Crater Forming Materials

Dickinson Crater on Venus. The structure can be clearly seen.

Just like the impact crater on Earth and other terrestrial planetary bodies, impact craters on Venus includes central peak, rim, floor, walls, ejected deposits and outflows from the craters. There are 2 groups of materials, including undivided crater materials (c) and impact crater flow material (cf).^[51]

The study of impact crater on Venus is important for discovering its geological history. In testing the model of catastrophic and equilibrium Model (another hypotheses instead of global stratigraphy^[52]) on Venus, it is found that the older regional plains (rp) are embaying only around 3% of the impact craters and the younger lobate plains (pl) are embaying around 33% of the impact crater on Venus. It suggested that it is likely to have at least two geological period on Venus:

(1) Earlier global volcanic regime stage (Formation of older regional plains), when the high rate of volcanic activities overwrote the marks of impact cratering

(2) Later network-rifting and volcanic regime stage (Formation of younger lobate plains), when the intensity of volcanism is reduced and allowed more impact cratering left on the surface.

Thus, the studying of crater distribution and randomness may give clues for Venusian geological history.^[53]

Global stratigraphy

A simple Venus global strata base on Ivanov and Head's model (2011). (Note that it is just a simple global strata, not every region on Venus is having this strata.)

Under the Global stratigraphic Classification Scheme, by correlating the units mentioned above (Mikhail A. Ivano and James W. Head, 2011),^[13] the researchers suggested three phase of Venusian geological history:

(1) The earliest period, Fortunian Period, involved intensive formation of tessera (t) (building of thick crust at the same time).

(2) Then, it came to Guinevere Period, which at the first place, there are formation of Atropos (dense lineated plains, pdl), Lavinia (Ridged plains, pr), Akna (Mountain belts, mb), and Agrona (groove belt, gb). In the later time, there are global emplacement of Accruva (shied plains, psh), Rusalka (lower regional plains, rp1), and Ituana (upper regional plains, rp2) Formations. There are events of wrinkle ridges formed around the global. Mostly of the surface of Venus are resurfaced in this period

(3) In the Altlian period, there are limited formations of smooth plains (ps), Gunda Formation, and shield clusters (sc), Boala Formation, possible due to Atlian volcanism. There are significant reduction in the rate of volcanism and tectonism.^[13] However, these proposed events and formation of units are not yet fully explained by a complete Venus geological model, such as resurfacing of Venus or heat-pipe hypothesis.

The mapping scheme by Vicki L. Hansen (2005)

The mapping scheme applied by Vicki L. Hansen is mainly under regional based, instead of under global stratigraphy as Mikhail A. Ivano and James W. Head did. This mapping scheme focus on regional origin of geological materials.

Tectonic units

There are only two major units classified under this group. These two units are further classified as below:

Tessera Terrain

Tessera terrain is seen as locally the oldest unit on Venus.

It can be further classified into 8 groups according to the features of deformation:^[14]

1. Fold Terrain
2. “Lava Flow” Terrain
3. S-C Terrain
4. Extended Fold Terrain
5. Folded Ribbon Terrain
6. Basin-and-Dome Terrain
7. “Star” Terrain
8. Tessera Inliers

Some of the terrains are having multiple deformation, but it is not a must for it to have a complex deformation.

Flow materials from different origin

The relatively low-lying plains are mapped as flows from different origins locally. These material are believed to be thick young sediments deposited rapidly. In SAR images, the flows material can be both radar-dark or bright.

Structural features

Structural deformations is treated as features instead of units.

There are some common features mapped, such as linear fractures, ridge and wrinkled ridge identified in many regions, and other local features only founded in some regions, such as dome, belt fractures, ribbon, graben, etc.

Impact Crater Forming Materials

The classification of impact crater forming materials are (1) crater materials and (2) flooded crater materials,^[12] which is similar to the Stratigraphic Classification Scheme.

Controversial difference between different mapping scheme

Here are difference on terminology and classification of units:

(1) The term "Complex ridged terrain (CRT or tesserae)"

(2) Treating tessera terrain as a global stratigraphic unit

(3) Terminology and classification of "plains"

The terminology of "Complex ridged terrain (CRT or tesserae)"

Hansen (2005) suggested that the tessera terrain should not be named as "complex ridged terrain (CRT)". For the term "complex ridged terrain (CRT)" used by the Magellan Science Team (1994),^[11] it carries confusions.^[12] Ridges can be also understood as fold. Fold is a contractional features. However, not all tessera deformation are due to contraction.

Treating tessera terrain as a global stratigraphic unit

For treating tessera terrain as the oldest global unit in the Stratigraphic Classification Scheme, it is questioned under Hansen (2005)'s mapping scheme.^[12] Although it is commonly the oldest unit mapped in different Venusian areas, it may not be the case for everywhere. The assumption of all the tessera are formed at the same time and the oldest around the global are remained untested.

Terminology and classification of "plains"

There are a major difference in terminology between the Stratigraphic Classification Scheme and Hansen (2005)'s mapping scheme, which Hansen (2005) suggested that "fold material" should be used instead of "plains with different surface features". It can be explained by three reasons:^[12]

1. "Plains" is not used to describe geological material, but surface physical features.
2. Also, according to fundamental geological mapping principles, secondary structure (such as lineated,ridged and wrinkled) should not be used to define geological units.
3. There is no evidence that the Venusian plains are volcanic products resulted from extensive flood lava

Thus, in Hansen's mapping scheme (2005), plains are defined as flow from different local origins in regional mapping.

Quadrangles' Mapping of Venusian Geological Units (Regional Mapping)

The quadrangles mapping and classification of geological units by different groups of researchers are mainly based on regional units mapped locally. Different groups have their own grouping of units, which are not fully coherent with others work and the proposed global stratigraphy. Also, there are some regional features being classified regionally.

Cartography

Quadrangles on Venus

The United States Geological Survey defines sixty two cartographic quadrangles for the surface of Venus,^[54] with V-1 as the north pole region and V-62 as the south pole region. Base on the FMAPs, different groups of Venus researchers are mapping different quadrangles for the surface of Venus, resulting in different type of units defined.

Here are some examples of quadrangle mapping and their ways of classifying and grouping observed geological units. Some of them are having a similar time sequence as the global stratigraphy mentioned above and will be highlighted below.

Examples of Quadrangle Mapping Unit classification

Here are the list of examples comparing the mapping schemes and units in quadrangles (regional mapping):

Quadrangles	Mapping Groups, Year of Publishing	Mapping Scheme	Stratigraphic Units Identified	Structural Units Mapped	Other Information
V-5 Barrymore Quadrangle Mapping[55]	Elizabeth Rosenberg and George E. McGill, 2001	Similar to the global stratigraphic mapping scheme with the oldest tessera, followed by dense lineated materials, up to other younger plains materials.	Plains materials (p, local plains and regional plains of different features) Flow materials (f) Corona materials (co) Linear belts (bl) Dense lineated materials (ld) Terrera materials (t) Impact crater materials (c)	Linear belts Wrinkle ridges Radar-bright Linear features Coronae Corona-like features	/
V-13 Nemesis Tesserae Quadrangle Mapping[51]	Mikhail A. Ivanov and James E. Head, 2005	Global Stratigraphy Units Classification	Plains materials of dense lineated plains (pdl), ridged and grooved plains (prg), shield plains (psh), wrinkle ridged plains (pwr) and smooth plains (ps) Tessera material of tessera unit (t) Crater material of undivided crater materials (c) and impact crater flow material (cf)	/	/
V-35 Ovda Regio Quadrangle Mapping[56] Ovda Regio in V-35.	Leslie F. Bleamaster, III, and Vicki L. Hansen, 2005	Mapping by grouping local formations and deformations, instead of global stratigraphic	Western regions (mainly flow materials with different origin and Ovda Regio tessera terrain ) Northeastern region (mainly flow materials with different origin and tessera terrainof Thetis Regio and undivided part) South-central to southeastern region (mainly flow materials with different origin and Boszorkany Dorsa basal material) Widespread units (Chasmata flow material, crater material, flooded crater material and tessera inlier terrain)	/	Regionally, the tessera unit are also the oldest unit with younger flows embaying it.
V-48 Artemis Chasma Quadrangle Mapping[57]	Roger A.Bannister and Vicki L.Hansen, 2010	Mapping by grouping local formations and deformations, instead of global stratigraphic	Crater materials (c) Flow and shield materials (f) Tectonic and fracture terrain units (fr, t) Radar unit (rf)	/	Tectonic and fracture terrain units is the oldest unit as it is mainly the tessera unit. All crater material and radar unit forms throughout the whole time period. The radar units are the high backscatter radar facies marked by penetratively developed, and it does not represent single geological units,

Examples of Regional Geological Mapping

Here is an example of geological map in quadrangle V-20. The units are classified as (1) tessera material, (2) plains materials, (3) materials of coronae and (4) materials of domes and miscalleneous flows, with structures like ridges, wrinkle ridge and lineations.

Ｇeological Map of V-20	Original SAR Image of V-20
V-20 Venus Geologic Map.	Irnini Mons on Venus (V-20)

References

1. Ross, F. E. (1928). "Photographs of Venus". Astrophysical Journal. 68–92: 57
2. Goldstein, R. M.; Carpenter, R. L. (1963). "Rotation of Venus: Period Estimated from Radar Measurements". Science. 139 (3558): 910–911.
3. Howington-Kraus, E., Kirk, R. L., Galuszka, D., & Redding, B. (2006). “USGS Magellan stereomapping of Venus”. In European Planetary Science Congress 2006 (p. 490).
4. "Mission Information: MAGELLAN". NASA / Planetary Data System. 1994-10-12. Retrieved 2011-02-20.
5. Grayzeck, Ed (1997-01-08). "Magellan: Mission Plan". NASA / JPL. Retrieved 2011-02-27.
6. 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.
7. Kazuo, O. "Recent Trend and Advance of Synthetic Aperture Radar with Selected Topics: Remote Sensing." (2013): 716-807.
8. Graff, Jamie R. MAPPING AND ANALYSIS OF THE TECTONO-MAGMATIC FEATURES ALONG THE HECATE CHASMA RIFT SYSTEM, VENUS. Diss. Carleton University Ottawa, 2014.
9. Herrick, R. R., & Sharpton, V. L. (2000). Implications from stereo‐derived topography of Venusian impact craters. Journal of Geophysical Research: Planets, 105(E8), 20245-20262.
10. Howington-Kraus, E., et al. "USGS Magellan stereomapping of Venus." European Planetary Science Congress 2006. 2006.
11. 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).
12. Hansen, V. L. (2005). Venus's shield terrain. Geological Society of America Bulletin, 117(5-6), 808-822.
13. Ivanov, Mikhail A., and James W. Head. "Global geological map of Venus." Planetary and Space Science 59.13 (2011): 1559-1600.
14. Hansen, V. L., & Willis, J. A. (1996). Structural analysis of a sampling of tesserae: Implications for Venus geodynamics. Icarus, 123(2), 296-312.
15. Basilevsky, Alexander T., and James W. Head. "The geologic history of Venus: A stratigraphic view." Journal of Geophysical Research: Planets 103.E4 (1998): 8531-8544.
16. Ivanov, Mikhail A., and James W. Head. "Geology of Venus: Mapping of a global geotraverse at 30 N latitude." Journal of Geophysical Research: Planets 106.E8 (2001): 17515-17566.
17. Ivers, Carol; McGill, George. "Kinematics of a Tessera Block in the Vellamo Planitia Quadrangle". Lunar and Planetary Science. 29
18. Hansen, Vicki; Willis, James (1998). "Ribbon Terrain Formation, Southwestern Fortuna Tessera, Venus: Implications for Lithosphere Evolution". Icarus. 132 (2): 321–343.
19. Bindschadler, Duane; Head, James (1991). "Tessera Terrain, Venus: Characterization and Models for Origin and Evolution". Journal of Geophysical Research. 96 (B4): 5889–5907.
20. Basilevsky, A. T. "Geologic mapping of V17 Beta Regio quadrangle: Preliminary results." Lunar and Planetary Science Conference. Vol. 27. 1996.
21. Frank, S.L., Head, J.W., 1990. Ridge belts on Venus: morphology and origin. Earth Moon Planets 50/51, 421–470
22. Kryuchkov, V.P., 1992. Ridge belts on plains. In: Barsukov, V.L., Basilevsky, A.T., Volkov, V.P., Zharkov, V.N. (Eds.), Venus Geology, Geochemistry, and Geophysics (Research Results from the USSR). University Arizona Press, Tucson, London, pp. 96–112
23. Gilmore, M.S., Head, J.W., 2000. Sequential deformation of plains at the margins of Alpha Regio, Venus: implications for tessera formation. Meteoritics Planet. Sci. 35, 667–687.
24. Ivanov, M.A., Head, J.W., 2001b. Geology of Venus: mapping of a global geotraverse at 30N latitude. J. Geophys. Res. 106, 17515–17566.
25. Basilevsky, A.T., 2008. Geologic map of the Beta Regio quadrangle (V-17), Venus. U.S. Geological Survey Sci. Inv. Map 3023
26. McGill, G.E., Campbell, B.A., 2006. Radar properties as clues to relative ages of ridge belts and plains on Venus. J. Geophys. Res. 111, E12006. doi:10.1029/ 2006JE002705.
27. Barsukov, V.L., Basilevsky, A.T., Burba, G.A., et al. (24 others), 1986. The geology and geomorphology of the Venus surface as revealed by the radar images obtained by Venera 15 and 16. J. Geophys. Res. 91, D399–D411.
28. Frank, S.L., Head, J.W., 1990. Ridge belts on Venus: morphology and origin. Earth Moon Planets 50/51, 421–470.
29. Kryuchkov, V.P., 1990. Ridge belts: are they compressional or extensional structures? Earth Moon Planets 50/51, 471–491.
30. Kryuchkov, V.P., 1992. Ridge belts on plains. In: Barsukov, V.L., Basilevsky, A.T., Volkov, V.P., Zharkov, V.N. (Eds.), Venus Geology, Geochemistry, and Geophysics (Research Results from the USSR). University Arizona Press, Tucson, London, pp. 96–112.
31. Squyres, S.W., Jankowski, D.G., Simons, M., Solomon, S.C., Hager, B.H., McGill, G.E., 1992. Plains tectonism on Venus: the deformation belts of Lavinia Planitia. J. Geophys. Res. 97, 13579–13599
32. Ivanov, M.A., Head, J.W., 2001a. Geologic map of the Lavinia Planitia Quadrangle (V-55), Venus. U.S. Geological Survey Sci. Inv. Map I-2684.
33. Johnson, J.R., Komatsu, G., Baker, V.R., 1999. Geologic map of the Barrymore Quadrangle (V-59), Venus. U.S. Geological Survey Sci. Inv. Map I-2610
34. Campbell, B.A., Campbell, P.G., 2002. Geologic map of the Bell Regio Quadrangle (V-9), Venus. U.S. Geological Survey Sci. Inv. Map I-2743.
35. Hansen, V.L., DeShon, H.R., 2002. Geologic map of the Diana Chasma Quadrangle (V-37), Venus. U.S. Geological Survey Sci. Inv. Map I-2752
36. McGill, G.E., 2004. Geologic map of the Bereghinya Planitia Quadrangle (V-8), Venus. U.S. Geological Survey Sci. Inv. Map I-2794.
37. Campbell, B.A., Clark, D.A., 2006. Geologic map of the Mead Quadrangle (V-21), Venus. U.S. Geological Survey Sci. Inv. Map 2897.
38. Pettengill, G.H., Eliason, E., Ford, P.G., Loriot, G.B., Masursky, H., McGill, G.E., 1980. Pioneer Venus radar results: altimetry and surface properties. J. Geophys. Res. 85, 8261–8270
39. Masursky, H., Eliason, E., Ford, P.G., McGill, G.E., Pettengill, G.H., Schaber, G.G., Schubert, G., 1980. Pioneer-Venus radar results: geology from the images and altimetry. J. Geophys. Res. 85, 8232–8260.
40. Head, J.W., 1990. Formation of mountain belts on Venus: evidence for large-scale convergence, underthrusting, and crustal imbrication in Freya Montes, Ishtar Terra. Geology 18, 99–102.
41. Pronin, A.A., 1992. The Lakshmi phenomenon. In: Barsukov, V.L., Basilevsky, A.T., Volkov, V.P., Zharkov, V.N. (Eds.), Venus Geology, Geochemistry, and Geophysics (Research Results from the USSR). University Arizona Press, Tucson, London, pp. 68–81.
42. Aubele, J.C., Slyuta, E.N., 1990. Small domes on Venus: characteristics and origin. Earth Moon Planets 50/51, 493–532.
43. Head, J.W., Crumpler, L.S., Aubele, J.C., Guest, J.E., Saunders, R.S., 1992. Venus volcanism: classification of volcanic features and structures, associations, and global distribution from Magellan data. J. Geophys. Res. 97, 13153–13197.
44. Guest, J.E., Bulmer, M.H., Aubele, J., Beratan, K., Greeley, R., Head, J.W., Michaels, G., Weitz, C., Wiles, C., 1992. Small volcanic edifices and volcanism in the plains of Venus. J. Geophys. Res. 97, 15949–15966.
45. Bilotti, F., Suppe, J., 1999. The global distribution of wrinkle ridges on Venus. Icarus 139, 137–157
46. Crumpler, L.S., Aubele, J., 2000. Volcanismon Venus. In: Sigurdson, H., Houghton, B., Rymer, H., Stix, J., McNutt, S. (Eds.), Encyclopedia of Volcanoes. Academic Press, San Diego, San Francisco, New York, Boston, London, Sydney, Toronto, pp. 727–770
47. Ivanov, M.A., Head, J.W., 2004b. Stratigraphy of small shield volcanoes on Venus: criteria for determining stratigraphic relationships and assessment of relative age and temporal abundance. J. Geophys. Res. 109, NE10001. doi:10.1029/ 2004JE002252
48. Campbell, D.B., Stacy, N.J.S., Newman, W.I., Arvidson, R.E., Jones, E.M., Musser, G.S., Roper, A.Y., Schaller, C., 1992. Magellan observations of extended impact crater related features on the surface of Venus. J. Geophys. Res. 97, 16249–16278
49. Izenberg, N.R., Arvidson, R.E., Phillips, R.J., 1994. Impact crater degradation on Venusian plains. Geophys. Res. Lett. 21, 289–292.
50. Wilhelms, D.E., 1990. Geologic mapping. In: Greeley, R., Batson, R.M. (Eds.), Planetary Mapping. Cambridge University Press, pp. 208–260.
51. Ivanov, Mikhail A., and James W. Head. Geologic Map of the Nemesis Tesserae Quadrangle, V-13, Venus. US Department of the Interior, US Geological Survey, 2005.
52. Hansen, Vicki L. "Venus's shield terrain." Geological Society of America Bulletin 117.5-6 (2005): 808-822.
53. Ivanov, M. A., and J. W. Head. "Volcanically embayed craters on Venus: testing the catastrophic and equilibrium resurfacing models." Planetary and Space Science 106 (2015): 116-121.
54. Tanaka, Kenneth L., et al. The Venus geologic mappers' handbook. No. 93-516. US Geological Survey, 1993.
55. Rosenberg, Elizabeth, and George E. McGill. Geologic map of the Pandrosos Dorsa quadrangle (V-5), Venus. US Department of the Interior, US Geological Survey, 2001.
56. Geological Survey (US), Leslie F. Bleamaster, III, and Vicki L. Hansen. Geologic map of the Ovda Regio Quadrangle (V-35), Venus. US Department of the Interior, US Geological Survey, 2005.
57. Bannister, Roger A., and Vicki L. Hansen. Geologic map of the Artemis Chasma Quadrangle (V-48), Venus. US Department, of the Interior, US Geological Survey, 2010.