Planetary tectonics versus Earth tectonics: a comparative approach of working principles

Many scientists who study the tectonic inventory of planetary bodies were initially trained as Earth-based structural geologists. In this context, a comparative approach of methodology in planetary and terrestrial tectonics is helpful with regards to what works and what does not. The methodological approaches are subdivided into (i) nature, (ii) experiment, (iii) modeling.

(i) Acquisition of data in the field, which provides the ground truth for the Earth geologist, is still largely impossible on planetary bodies, at least nowadays, or limited to small regions with the help of rovers. Likewise, microstructural analysis – an important branch in structural geology - is not possible, or is limited to meteorites and the few mission return samples. Those deficits are compensated by remote sensing data. Their quality, spatial resolution and coverage varies greatly, but is steadily improving, and sometimes reaches decimeter resolution (Mars). Most data are sufficient for tectonic work, and sometimes allow the measurement of strike and dip of layers and faults and even enable the construction of cross-sections. The outcrop conditions are usually better on planetary surfaces and the context between geomorphology and tectonics is apparent and similar to neotectonics on Earth due to lower resurfacing rates. Determination of surface ages using crater size-frequency-distributions also allows dating of tectonic processes, although this approach is much less sensitive than Earth-based methods. The exploration of the subsurface by drilling and geophysical surveying is strongly limited in planetary tectonics (e.g., GPR). Detailed seismic surveys cannot be performed yet. However, geophysical measurements (gravity and magnetic field) are often available, which at least allow to decipher crustal-scale processes.

(ii) Rock-mechanical experiments are key for determining the rheology of crustal rocks in planetary and terrestrial tectonics. However, some of the physical boundary conditions to be considered in planetary tectonics are less well constrained and cover a larger range of temperatures. In planetary tectonics, basalts and various types of ices play a central role, which receive little attention in terrestrial structural geology. In tectonic analogue modeling, the parameter gravity poses a challenge. Gravity affects the scaling relationships of faults (displacement–length–width) but gravity can only be modified in centrifuges, space, or parabola flights.

(iii) The mathematical simulation of deformation processes on planetary bodies works in the same way as for terrestrial processes by discretization of the continuum. It is easily adaptable but the systems to be modeled are sometimes underdetermined with regard to the parameter space.

To conclude the methodological tools in planetary tectonics are somewhat limited compared to those applied in terrestrial structural geology. Analogue field studies in specific terrestrial environments (e.g., Svalbard, Iceland) are aimed to compensate the missing field acquisition in planetary tectonics. Despite these limitations, planetary tectonics is a fascinating endeavor that allows us to better understand the dynamic geological processes and narrow down the physical boundary conditions of planetary bodies. With the ever improving remote sensing data by recent and upcoming missions (e.g., BepiColombo, EnVision, Veritas, Juice) the field of planetary tectonics will continue to gain importance.