In the midst of reorganizing, I recently unearthed some deeply buried personal geologic records from past research projects.  One discovery was an image that previously adorned my wall:  a Magellan radar image of arachnoids on Venus.

Magellan radar image of arachnoids on Venus. (Image courtesy of NASA/JPL)

Magellan radar image of arachnoids on Venus. (Image courtesy of NASA/JPL)

I think this image is a perfect fit for Accretionary Wedge #32 “favorite geologic picture.” It’s not the most colorful or dramatic image, but it holds an important place in my professional experience in the geosciences.

Arachnoids are geological features on Venus. They are characterized by a combination of radar-bright, concentric rings (like a bull’s eye) and radiating lineations (line-like features) and were named “arachnoids” because of their spider and web-like appearance.

Although these features had been identified on earlier Soviet Venera mission data, the NASA Magellan mission provided sufficiently high resolution radar imagery and elevation data to investigate them more closely.

These features were one of my first forays into geology research as an undergraduate student. What were they? Did all features catalogued as arachnoids based on the radar images have similar topographic features? What caused them? Where they all the same age? Where were they located? Was there a terrestrial geology analog? I had many questions, a patient research advisor, and a short summer.

The personal outcome was clear, though: I was hooked. Geology was fascinating, no matter where in the solar system. There were so many questions, and we didn’t have all the answers!  This was a far cry from the textbook science of packaged information and rote facts to memorize. The science was alive, the questions infinite, and the data plentiful.

Whenever I see these data images, they represent all of this in one snapshot: the excitement and mystery of geology.

Old Research with Interesting Titles

Looking back, this short-lived research project has stayed with me in one unexpected way: almost 20 years later, I still get asked about it in job interviews if people see it on a publications list. Regardless of the relevance or timeliness, people always ask about it because they think arachnoid is a typo and I’ve written about spiders on Venus (not to be confused with Spiders from Mars).

What are Arachnoids

So now you want to know more about arachnoids? My research on these features didn’t extend beyond my undergraduate studies, but here is a basic summary:

Arachnoids on Venus are features visible on radar imagery and are characterized by a combination of radar-bright concentric rings and radiating lineations. The name “arachnoids” is based on their spider and web-like appearance. They are typically found in clusters along deformation belts on Venus and show a complicated geologic history. Land-surface elevation data indicate that arachnoids are circular depressions (low areas) typically less than 200 kilometers in diameter and less than l kilometer in depth. The shallow depression and lack of large-scale faulting have been interpreted as indicating gradual rather than sudden subsidence, which may be a result of the gradual evacuation of an underlying magma chamber. The bright radiating lineations are likely dikes through which the magma was evacuated.


Image courtesy of NASA JPL, online at


Dawson, C.B. and Crumpler, L.S., 1993, Arachnoids on Venus: Analysis of Recent Magellan Mission Data, in Lunar and Planetary Science Conference XXIV (1993) Proceedings. Online at

Grosfils, E.B., and Head, J.W., 1994, Emplacement of a radiating dike swarm in western Vinmara Planitia, Venus: interpretation of the regional stress field orientation and subsurface magmatic configuration: Earth, Moon, and Planets Vol. 66, No. 2, pp.153-171, DOI: 10.1007/BF00644129 Online at

Kostama, V-P., 2002, The four arachnoid groups of Venus, in Lunar and Planetary Science Conference XXXIII (2002) Proceedings. Online at

Krassilnikov, A.S., 2002, Tectonic Structure, Classification, and Evolution of Arachnoids on Venus: Preliminary Results: Solar System Research, Vol. 36, No.5 (September), pp 374-402. doi 10.1023/A:1020411404593. Online at