Tectonics on ten solid worlds I - The inner Solar System

Necessary planetary differences:

Factors that make each world unique.

The Inner Solar System:

Earth: Recall the distinctive features of Earth's lithosphere:


The Moon from Wheat, not Oats, Dear

1. The Moon:

The manner in which the Moon formed determines much about its differentiation and character:



Mare Imbrium laps against lunar highlands.
Hadley Rille, a volcanic feature, snakes from bottom to top.
From Texas Tech University
Looking at the Moon's surface, we see the effects of two processes:

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Mercury from Wikipedia

2. Mercury:

Like the Moon, Mercury shows a dichotomy of:

However it is different in significant ways:

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Martian topography from AstroEffectZ

3. Mars:

Differentiation: Mars is intermediate between the Moon and Earth - roughly 1/9 as massive as Earth and nine times as massive as the Moon. Although it is larger than Mercury, its lower density gives them similar surface gravities. The proportions of its core and mantle are similar to those of Earth and Venus, although its crust is thicker.

Surface composition: Today Mars shows global dichotomy between ancient (4.5 - 3.5 Ga) crater-saturated highlands in the southern hemisphere (E.G.Gusev crater)low-lying and flat (E.G. Viking II site) younger (3.8 Ga - 10 Ma.) northern hemisphere plains. Whereas the Moon's surface shows a compositional dichotomy between anorthosite highlands and basalt maria, most Mars rocks seem to be made of basalt or sedimentary rocks made of basalt derivatives. Compositionally, highlands and northern plains seem similar. Note: highly weathered basalt has a reddish color, and the dust that blankets Mars is red for that reason. Scratch the surface almost anywhere, however, and fresh black basalt is exposed, including dunes of basalt sand. (Compare with rare black sand beaches on Earth.)

Impact basins: Mars resembles the Moon this much - its giant impact basins Hellas and Argyre are definitely filled with basalt flows, as are the northern plains. Recent research suggests that the northern plains may, themselves, be a giant impact basin, the result of the impact of a Pluto-sized Planetary embryo. To date, this is the only plausible proposal to explain Mars' global dichotomy. If true, it would make them the largest impact basin in the Solar System.


The Tharsis Plateau from Lunar and Planetary Institute
No current plate tectonics: The giant volcanoes of the Tharsis Bulge tell us much about Mars' lithosphere:


Martian paleomagnetism from Adam Maloof - Princeton University
Ancient plate tectonics? Today, Mars has no global magnetic field - just regional relicts of an ancient one. Surveys of its remnant paleomagnetism, however, reveals the picture at right - the paleomagnetic signature of an ancient global magnetic field. Note:

Adding color to this argument:

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4. Venus:

On Earth, plate tectonics involve:

But consider: That sums up the situation on Venus, where there seems to be lithospheric movement, but no subduction zones or clear plate boundaries.

Subductions zones and their volcanic arcs are the "refineries" at which continental and oceanic crust are differentiated. Lacking them, Venus lacks the global dichotomy (maybe) of continents and ocean basins that characterize Earth, even though it has continent-like elevated regions. (Compare this image of Earth surface elevations to this one of Venus.)


Dickinson crater from Washington State University - Astronomy Resources
Crater enigmas: Venus has: So Venus' surface is all roughly the same age and, assuming reasonably constant impact rates, it appears that most of Venus' surface formed around 500 Ma.

How does an entire planet get resurfaced all at once?? An ongoing puzzle.

What we know:


Corona and pancake domes from Washington State University - Astronomy Resources
Volcano enigma: That surface formed through volcanic activity. Venus is covered with expressions of volcanic activity including Earth-like volcanoes, broad lava domes, and uniquely Venusian forms like Aine Corona (right). These coronae might represent the surface expression of Venusian hot-spots. There is some evidence for current Venusian volcanic activity, but nothing like the volcano-party of 500 Ma.

What we speculate:

Internal heat enigma: On Earth, heat brought from great depths by convection comes to the surface through the processes of plate tectonics including:

By this means, the lithosphere becomes an extension of mantle convection.

What happens in a planet like Venus where the mantle convects but the lithosphere doesn't. Any heat that makes it through the lithosphere must do so by conduction. The result is the accumulation of heat beneath the lithosphere. Over time, the upper mantle heats to a threshold where widespread melting occurs and the mechanical instability of a solid lithosphere resting on a molten asthenosphere causes the two regions to "overturn" in a paroxysm of subduction over a period of roughly 100 Ma, a period of intense volcanic activity during which heat is transported to the surface by advection. Venus' 500 Ma surface seems mostly to record the last turnover pulse, although evidence does point to some contemporary hot-spot style volcanism.

In effect, Venus could have brief temporary episodes of rapid plate tectonics separated by long periods of quiescence.

Of course, this erases any record of Venus' earlier history. Determining whether Venus ever had Earth-style plate tectonics will be a major priority of future exploration. The identification of distinct continental crust would be a clincher.


Key concepts and vocabulary:
Additional reading: