Venus Geology Notes and References

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Regional Geologic Maps

Scale: 1 pixel = 15 km.  The grid interval is 10 degrees.

The projection is Lambert Azimuthal Equal Area, which can actually portray the entire sphere, although distortion becomes extreme toward the antipode. The maps are cut off 150 degrees from the map center, so objects that look foreshortened are actually well beyond the lunar limb and each map covers considerably more than a hemisphere.

Small Scale Geologic Maps

Scale: 1 pixel = 10 km. The grid interval is 10 degrees. Projections are Mercator, Lambert Conformal Conic, Oblique Mercator and Azimuthal Equal Area depending on latitude.

Notes on Venus Geology

Geologic mapping of Venus is a long way from completion. As of 2014, only 27 of the 62 1:5,000,000 map quadrangles have been mapped - a process that began in 1999. Many things complicate the process:

On the geologic maps shown here, yellow areas are 1 km and more above the mean elevation of Venus. These areas are generally large volcanic centers. There are two large "continents" on Venus that contain large areas of tectonically uplifted and deformed crust. Blue areas are below the mean radius of Venus and comprise most of the Planitia areas on Venus. These areas are mostly lava plains with no to moderate amounts of deformation. These maps might better be termed tectonic maps.

Structual features are shown as follows:

On waterless bodies like Venus, zero elevation is usually taken as the average radius of the planet. Venus has less relief overall than Earth. The average radius of Earth is about 2.5 km below sea level, and a map of Earth colored the same as Venus would be dominated by yellow and orange for the continents and the very deepest blues for the ocean basins, with comparatively little near zero elevation. Venus has most of its surface within a couple of kilometers of the mean radius. We do not see the sharp dichotomy between continents and oceans we see on Earth.

And of course, there's no plate tectonics. That was a huge disappointment. There are no ridges, no subduction zones, no transform faults. The only things that resemble plate tectonics are the coronae, which may be analogous to hot spots. The deformation around the margins of many coronae may be the result of thrusting as crust spreads out from the centers.

Place Names

Because of its mythological role, place names on Venus are reserved exclusively for female characters. The only exceptions were named before that naming convention was established, and are features identified by radar mapping from Earth. They include Maxwell Montes, honoring James Clerk Maxwell for his laws of electromagnetism that make radar and radio possible, and two reflective regions: Alpha and Beta Regio.

The two largest elevated regions on Venus are Aphrodite Terra, about the size of South America, and Ishtar Terra, about the size of Australia. Aphrodite and Ishtar are the Greek and Babylonian equivalents of Venus, respectively. Ishtar Terra was not named for the infamous Warren Beatty-Dustin Hoffman film, which many reviewers now consider was not nearly as bad as once believed.

There are two ways you can react to place names on Venus. You can be happy there's an entire planet devoted to honoring female figures, or disappointed that the only planet devoted to female names is completely hidden by clouds.

Geology

The low and uniform crater density on Venus suggests that the surface is all pretty much the same age. However, some features are older than others. The oldest areas seem to be tesserae, which often occur as elevated areas surrounded by lava plains. Some researchers speculate that there is a global layer of fractured tessera crust, mostly buried by lava plains.

Running across some plains are erosional channels, up to 6000 km long, longer than the Nile or Amazon, and the longest known channels in the Solar System. These are extremely unlikely to have been cut by water, and that's tossed in mostly because it's considered bad form in science to say anything is flatly impossible. But it's the next best thing. So that poses (not "begs," which is illiterate) the question what did carve the channels. The obvious candidate is lava, but it would have to be a very fluid lava with a very low melting point. Even the runniest basalts, at 1000 C, would cool long before flowing 6000 km in Venus' 350 C environment. The most obvious candidates are carbonatite lavas of some sort.

Venus has a 4.5 billion year history, just like the earth, and the natural question is, what happened before the most recent resurfacing? What fractured the tesserae? What created the highland and lowland areas that were either flooded or surrounded by lava plains? Were there previous resurfacing events? How many, and how often? What deformed the present lava plains? Are there marker units that can be used to define stratigraphy and age periods? Are there enough places where older, buried rocks have been exposed to make it possible to construct a history?

Gravity

Gravity is likely to be of limited use in studying the hidden geology of Venus. On Earth, we can trace a buried 1100 m.y. old rift valley from Lake Superor to Kansas using gravity data. But that's because the rift is filled by dense basalt, within light granitic crust and flanked by low density sedimentary rocks. Would such a feature even be detectable on Venus if the adjoining rocks are all basalt, too? Is there enough density contrast in Venusian rocks to make hidden structures apparent?

The gravity of Venus was mapped by tracking slight changes in Magellan's velocity as it orbited Venus. This method isn't as sensitive as the tandem satellite methods used for the Earth and Moon. Departures of gravity measurements from the overall gravity of the planet are termed anomalies. There's nothing wrong with "anomalies;" they're prized because of the information they furnish about hidden variations within a planet. Typically, the effects of the overall mass of the planet, variations in its shape, and effects of rotation and topography are subtracted from the raw gravity measurement. Additional factors may be subtracted depending on the features being studied.

Free Air Gravity Anomalies are anomalies remaining after effects of topography are removed. Since the mass of the topography is still there, high altitude areas tend to be positive. Bouguer Anomalies are the anomalies remaining after the effects of topographic mass have been subtracted out. Bouguer Anomaly maps are the most commonly published maps. However, they tend to have a surprising pattern. Instead of removing most of the effects of gravity and reducing anomalies to near zero, Bouguer anomaly maps display large negative values in areas of high elevation. This is generally considered to be the result of isostasy, where mass excesses on the surface are supported by the buoyancy of mass deficits below the surface.

On Venus, the free-air anomalies, as expected, are positive over areas of high elevation. Elevations more than 1 km above the mean radius of Venus are shown outlined by hachured lines, with the hachures pointing downhill. The Bouguer anomalies show a very strong correlation with altitude. Given Venus' presumably basaltic crust, one might wonder how isostasy might be a factor on Venus, but the 2.9 gm/cc of basalt (used in calculating the Bouguer anomalies) is still significantly greater than the probable 3.3 gm/cc of ultramafic mantle rocks.

It's not clear why the free-air maps are of such poor quality compared to the Bouguer maps, since both are presumably based on the same gravity and topographic data. One interesting possibility is that the free-air maps are essentially raw gravity maps, corrected only for altitude above the mean radius of Venus. On Earth, free-air maps are often based on surface measurements and have to be corrected for the elevation of the individual station (surprisingly large: about 0.3 mgal/meter). But satellite-based free-air maps don't require any topographic corrections. So the Venus free-air maps reflect the true, coarse nature of the gravity data. The Bouguer corrections, on the other hand, do require a topographic correction. For basalt (2.9 gm/cm3) it amounts to about 0.13 mgal/meter. Creating a Bouguer map entails subtracting the Bouguer correction from the free-air gravity values, and thus introducing a lot of detail derived from the topographic maps. In a sense, the Bouguer gravity contains a lot of "spurious" detail, in that the detail comes from the processing and not from the satellite data.

Data Sources

Topography

Maps were created using Global Mapper v. 14.0.

Topography was derived from NASA Magellan radar mapping, available as .img files. Now back when we were all using punch cards, and floppy disks with 300 kb cost a couple of dollars, it was probably a flash of genius to condense gridded data into an image file. Now that we can buy terabyte hard drives for $100, it's as obsolete as 8-track tape decks and running boards. There is, frankly, no excuse for the data not to be downloadable as ASCII gridded files. So to convert .img data to readable form, you convert it to ASCII. There are freeware utilities that can do this, and also an option buried in Global Mapper. Once converted, there are gaps in the data. These can be filled in from coarser Pioneer-level data with one-degree resolution. This data, perversely, is available as an ASCII file (.dat).

And despite its name and pretensions, the USGS Map-a-Planet site merely regurgitates the NASA data. At the end of 2014 it changed from mostly useless to entirely useless by requiring user accounts (because, taxpayer, you have no right to open access to the data you paid for).

Gravity data are taken from the NASA PDS Geosciences Node (Magellan Spherical Harmonics, Topography, and Gravity Data).

Published Map Quadrangles

V-3 Meskhent; James Head, Milkhail Ivanov (2008) I-3018
V-4 Atalanta Planitia; James Head, Mikhail Ivanov (2004) I-2792
V-5 Pandrosos Dorsa; George McGill, Elizabeth Rosenberg (2001) I-2721
V-7 Lakshmi Planum; James Head, Mikhail Ivanov (2010) I-3116
V-8 Bereghinya Planitia; George McGill (2004) I-2794
V-9 Bell Regio; Bruce Campbell, Patricia Campbell (2002) I-2743
V-13 Nemesis Tesserae; James Head, Mikhail Ivanov (2005) I-2870
V-14 Ganiki Planitia; Eric Grosfils (2011) I-3121
V-17 Beta Regio A.T. Basilevsky (2008) I-3023
V-20 Sappho Patera; George McGill (2000) I-2637
V-21 Mead; Bruce Campbell, David Clark (2006) I-2897
V-23 Niobe Planitia; Vicki Hansen (2009) I-3025
V-24 Greenaway; Vicki Hansen,Nick Lang (2010) I-3089
V-25 Rusalka Planitia; Vicki Hansen,Duncan Young (2003) I-2783
V-31 Sif Mons; John Guest, Duncan Copp (2007) I-2898
V-35 Ovda Regio; Vicki Hansen, Leslie Bleamaster (2005) I-2808
V-37 Diana Chasma; Vicki Hansen, Heather DeShon (2002) I-2752
V-39 Taussig; Ellen Stofan, Antony Brian (2005) I-2813
V-40 Galindo; Mary Chapman (1999) I-2613
V-43 Carson; Kelly Bender, David Senske, Ronald Greeley (2000) I-2620
V-44 Kaiwan Fluctus; George McGill, Nathan Bridges (2002) I-2747
V-46 Aino Planitia; Ellen Stofan, John Guest (2003) I-2779
V-48 Artemis Chasma; Vicki Hansen, Roger Bannister (2010) I-3099
V-52 Helen Planitia; Vicki Hansen, Ivan Lopez (2008) I-3026
V-55 Lavinia Planitia; Mikhail A. Ivanov, James Head, III (2001) I-2684
V-59 Barrymore; Jeff Johnson, Goro Komatsu, Victor Baker (1999) I-2610
V-61 Mylitta Fluctus; James Head, Mikhail Ivanov (2006) I-2920  

These maps define units mostly in terms of radar signatures and degree of deformation. Even so, there are substantial differences in unit definition. Map I-2794, in particular, doesn't distinguish degrees of deformation in plains units as much as adjacent sheets.

The maps presented on this site are intended to provide a rough overview of Venus' geology in the absence of detailed geologic mapping.


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Created 11 April 2014, Last Update 16 February 2015
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