Tectonics on ten solid worlds I - The inner Solar System
Necessary planetary differences:
Factors that make each world unique.
- Heat sources: Magnitude and relative contributions of different heat sources (primordial, radiogenic, tidal)
- Size: Surface/vol ratio determines rate of cooling.
- Resurfacing: Presence and rate of resurfacing processes like impacts or erosion, transport, and sedimentation that mask crustal tectonic processes.
- Rate and extent of cooling: How prevalent is volcanic activity? How thick has the lithosphere become. Does an asthenosphere exist at all?
The Inner Solar System:
Earth: Recall the distinctive features of Earth's lithosphere:
- Active plate tectonics: Rigid lithospheric plates move over asthenosphere
- Volcano distribution: Volcanic activity concentrated at plate boundaries and hot spots
- Oceanic vs. continental crust: Dichotomy between continental and oceanic crust yields bimodal hyspography. (Surface elevation is either low or high but not much in between.)
- Constant resurfacing: Plate tectonics drive processes like subduction and erosion that result in destruction of old features like impact craters, which are scarce.
The Moon from Wheat, not Oats, Dear
1. The Moon:
The manner in which the Moon formed determines much about its differentiation and character:
- Having coalesced from the vaporized mantles of planetary embryos, is is mostly made of silicates. Indeed, there was an early time in which the moon was completely molten. That enabled global differentiation to be especially thorough.
- Its core is small, (less than 20% of its radius, compared to roughly 50% for most terrestrial planets.)
- Its crust consists of the differentiated "scum" that floated to the surface of its primordial magma sea. This solidified to form the "highlands," which is rich in anorthosite, a rock consisting primarily of the mineral plagioclase. The oldest radiometrically dated highland rocks rocks are roughly 4.4 - 4.5 Ga. The far side is almost entirely highlands. Note: Based on data from JAXA's Kayuga lunar orbiter, Yamamoto et al., 2010 suggests that mantle may be exposed in some impact basins. (See panorama of Apollo 16 Descartes Highlands landing site.)
- Being small, the Moon lost its primordial heat quickly, but being mostly silicate, it contained enough radioisotopes to drive radiogenic heating and widespread volcanic activity during its earlier history. Today, the outer part of its small core is thought to be molten.
- Because it has lost so much heat, and because its low gravity generates little internal pressure, the Moon's modern lithosphere (zone of brittle deformation) extends, essentially, down to its core.
Mare Imbrium laps against lunar highlands.
Hadley Rille, a volcanic feature, snakes from bottom to top.
From Texas Tech University
- Impact cratering effects the entire lunar surface. The ancient highlands are completely saturated by impact craters. Indeed, impact cratering seems to be the only process shaping the topography of the highlands.
- Volcanism The comparatively dark lunar maria (sing. "mare") are extensive flows of basalt - the common volcanic rock of the ocean floors. The eruptions that formed them occurred in the floors of the largest and deepest impact basins. (Highlands are on average 2.75 km above maria.) Maria primarily formed 3.8 - 3.2 Ga (but some maybe as recent as 0.8 Ga). Being younger surfaces, the maria show fewer impact craters, but are, nevertheless, extensively cratered. Most maria are on the Moon's Earth-facing side. (See panorama of Apollo 12 Mare Procellarum site.) The maria contain volcanic features including:
- Rilles - collapsed lava tubes like the Hadley Rille (right).
- Wrinkle ridges - Ridges formed by the contraction of cooling lava. (E.G. wrinkle ridge photographed by Surveyor IV.
Mercury from Wikipedia
Like the Moon, Mercury shows a dichotomy of:
- crater-saturated highlands
- Oversized core = puny mantle: Whereas the Moon is mostly mantle, density data indicate that Mercury is mostly core. The relative puniness of its mantle suggests a deficit of radiogenic heating, however moment of inertia data suggest that portions of its core remain molten. Nevertheless, despite its hot surface, Mercury's interior seems to have cooled rapidly.
- No global magma sea: Because Mercury accreted gradually rather than forming, like the moon, from a giant impact, its highlands are not dominated by anorthosite. Subtle differences in color between Mercury and the Moon confirm that they are compositionally different.
- Impact basins: Like the moon, Mercury possesses giant impact basins, the largest being Caloris Planitia. Its lava flows arguably emanate from fractures associated with them. Additionally, directly opposite Caloris Planitia is the mysterious weird terrain, possibly formed by the convergence of shock waves from the Caloris impact.
- Mercury's surface is crossed by a global network of contraction features including:
- Many wrinkle-ridges.
- Rupes: Scarps that represent the surface expression of thrust faults. These "wrinkles" may indicate the physical contraction of Mercury's core and mantle as it cooled. (Note: thrust faults exist on other planetary bodies, including Earth, but for different reasons.)
- Domes: Nevertheless, some parts of Mercury show signs of expansion, as if magma had been injected beneath the solidified surfaces of existing lava flows, causing them to dome upward and fracture. Parallel arrays of such fractures - "normal faults" form graben valleys - stretch-marks - in the crust. E.G. the Caloris Planitia (one of the largest impact structures in the Solar System) is domed so that its center achieves a higher elevation than its rim (See link above).
Martian topography from Wikiedia
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
- It's not moving: On Earth, rising mantle plumes cause volcanic hot spots like the Hawaii hot spot. When lithospheric plates move over a hot spot, a chain of extinct volcanoes results. Each volcano is of a finite size because it only has limited time in which to grow before it is move away from the hot spot. On Mars, there are no chains, only extremely large volcanoes, suggesting that individual volcanoes sit on top of their hot spots forever and are not moved aside by lithospheric motion.
- It's thick: On Earth, the weight of the relatively modest volcano Mauna Kea pressing into the lithosphere creates a measurable dimple in the ductile mantle. The giant Olympus Mons and its companions do not. Mars' lithosphere must be considerably thicker than Earth's. Not surprising considering that Mars is so much smaller and consequently has a higher SA/V ratio.
Martian paleomagnetism from Adam Maloof - Princeton University
- The large Hellas and Argyre impact basins (and most of the northern plains) have no remnant magnetism. They evidently formed after Mars' geodynamo had shut down.
- The Tharsis Bulge is not magnetized. It must also postdate the magnetic field.
- But (and this is so cool) the ancient rocks of Mars' highlands show parallel stripes of alternating polarity.
Sound familiar? Arguably when it was very young and hot, for a brief interval, Mars had something like Earth style sea floor spreading. The fact that this ancient surface is saturated with impact craters dating back to the Late Heavy Bombardment indicates that any tectonics had ended by 3.8 Ga. Indeed, the orientation of the Vallis Marineris conforms with that of the stripes. Could it represent some last hurrah of Martian lithospheric tectonics?
Adding color to this argument:
- The geochemical comparison of Mars surface (observed by rovers) and deep (sent to Earth as Mars meteorites) rocks by Tuff et al., 2013 suggesting active subduction during Mars' first 0.5 gy.
- The discovery by Sautter et al., 2015 of crustal rocks whose chemistry (diorite and granodiorite) is typical of continental crust, observed by the Curiosity rover in Gale Crater.
On Earth, plate tectonics involve:
- The subduction of cold lithospheric plates at convergent boundaries.
- Subducting slabs are "lubricated" by partial melting of adjecent mantle rocks as a result of the infusion of water from the subducting slab.
- Q: What happens if the surface is too hot for oceans to exist?
A: No melting occurs near subducting slab, so slab is not lubricated and can't move.
- Q: What happens if the lithosphere stays very hot because of surface conditions?
A: Lithosphere doesn't subduct because it is not relatively cool.
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
- Surprisingly few impact craters. We don't expect little ones because small impactors burn up in the dense atmosphere. Big ones, however, like the twenty mile wide Dickinson are also rare.
- The ones there are uniformly distributed, not clustered in older regions.
- And they haven't been deformed by tectonic processes.
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
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:
- Sea floor formation
- Subduction zone volcanism.
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:
- Anorthosite/basalt dichotomy
- Highland/Mare basin dichotomy
- Oversized core
- Normal faults and graben valleys
- Mostly covered with basalt and its derivatives
- Giant volcanoes indicate thick, immobile lithosphere
- Evidence for early plate tectonics including:
- Ancient magnetic reversals
- Rocks with continental crust chemistr
- Plate vs. "blob" tectonics - tectonics without subduction.
- Impact crater enigma - relatively young surface
- Volcanoes everywhere
- Venus' internal heat flow model
- Lithospheric overturn
- V. Sautter, M. J. Toplis, R. C. Wiens, A. Cousin, C. Fabre, O. Gasnault, S. Maurice, O. Forni, J. Lasue, A. Ollila, J. C. Bridges, N. Mangold, S. Le MouŽlic, M. Fisk, P.-Y. Meslin, P. Beck, P. Pinet, L. Le Deit, W. Rapin, E. M. Stolper, H. Newsom, D. Dyar, N. Lanza, D. Vaniman, S. Clegg. 2015. In situ evidence for continental crust on early Mars. Nature Geoscience 8, 605-609
- Satoru Yamamoto, Ryosuke Nakamura, Tsuneo Matsunaga, Yoshiko Ogawa, Yoshiaki Ishihara, Tomokatsu Morota, Naru Hirata, Makiko Ohtake, Takahiro Hiroi, Yasuhiro Yokota, and Junichi Haruyama. 2010. Possible mantle origin of olivine around lunar impact basins detected by SELENE. Nature Geoscience 3, 533 - 536