Plumes on Mars?

¹Department of Geosciences, Virginia Tec, 4044 Derring Hall (0420, Blacksburg, VA 24061

²Department of Earth and Atmospheric Sciences, 550 Stadium Mall Drive, Purdue University, West Lafayette, IN 47907-2051¹sdk@vt.edu , ²redmondh@purdue.edu

Tharsis rise is one of the most prominent features on the surface of Mars and dominates the global topography and areoid on Mars (Figure 1). The broad topographic swell, more than 6,500 kilometers wide, includes the shield volcanoes Arsia Mons, Pavonis Mons, Ascraeus Mons, Alba Patera and Olympus Mons, the largest known shield volcano in the solar system, towering 21.3 km above the Martian surface. To get a sense of the size of Olympus Mons one should compare this height to Mauna Loa, Hawaii, which rises 8.9 km above the ocean floor. The high-resolution Mars Orbiting Laser Altimeter (MOLA) topography (Figure 1) provides a striking view of the relationship between the Tharsis province and the shield volcanoes (Smith et al., 1999). Of particular note, Olympus Mons sits off the western edge of the Tharsis rise rather than sitting on the flank, as in previous topographic models (c.f., Wu, USGS Map, 1991; Smith et al., 1999). Even so, there remains an assumed link between Olympus Mons and Tharsis rise in many researchers minds because of the spatial proximity of these features.

Figure 1: Martian topography taken from the Mars Orbiting Laser Altimeter (MOLA). This image was produced using an Initial Experiment Gridded Data Record on the Planetary Data System (PDS) geoscience node. The resolution is 16 pixels per degree.

The MOLA topography provides a detailed view of Tharsis rise, clearly showing that the Tharsis region consists of two broad rises; the southern rise (from 50°S to 20°N) which includes the Tharsis Montes (Arsia, Pavonis, and Ascraeus Mons) and the more northern rise (from ~20°N to 60°N) which is dominated by Alba Patera. The southern rise contains heavily cratered regions of Noachian age (4400–3600 Ma). The broad, elevated, ancient terrains are consistent with the view that some of the uplift of Tharsis is tectonic and may not entirely be volcanic construction (c.f., Banerdt et al., 1982; Solomon & Head, 1982; Phillips et al., 1990; Tanaka et al., 1991; Schultz & Tanaka, 1994).

Martian Geological Periods
Noachian	4.6-3.5 Ga
Hesperian	3.5-1.8 Ga
Amazonian	1.8 Ga - present

Given the popularity of the Plume Hypothesis (Wilson, 1963; Morgan, 1971) as an explanation for swells and intraplate volcanism on Earth, it is not surprising that a plume has been proposed as the formation mechanism of Tharsis rise (c.f., Zuber, 2001; Schubert et al., 2001, and references therein). A recent modification of the plume model for Tharsis is that multiple plumes feed (or once fed) each volcano (Kiefer, 2003). Because the natural convection pattern in a spherical shell forms cylindrical, plume-like upwellings, it seems reasonable to assume that mantle plumes exist, or once existed, on other terrestrial planets (Schubert et al., 2001). However, a number of lines of evidence are leading to the conclusion that Tharsis is not underlain by a plume now (e.g., Lowry & Zhong, 2003; Zhong & Roberts, 2003) and others go further to speculate that it may never have been underline by a plume (e.g., Stegman et al., 2003).

Two lines of evidence are generally used to question the plume explanation of Tharsis topography: heatflow and mantle energetics; and topography and areoid (i.e., the Martian geoid). Note that there is a variety of opinions ranging through:

Mars has a larger surface area to volume ratio than Earth because the radius of Mars is about half that of Earth. This could allow for more rapid heat loss and cooling of the Martian interior as compared with Earth. The complicating factor is that Earth has mobile plates at the surface that significantly aid cooling, but Mars has a stagnant lid (c.f., Gurnis, 1989; Solomatov & Moresi, 2000). If Mars ever had plate tectonics, it has now ceased (Sleep, 1994). In addition, crustal differentiation on Mars appears to have been more efficient (Spohn, 1991; Schubert et al., 1992) and a significant fraction of the incompatible, heat-producing elements are thought to be near the surface of Mars where heat loss would be dominated by conduction (Schubert et al., 2001).

If the heatflow from the core is small, as some thermal history models suggest (e.g. Hauck & Phillips, 2002), then it may be difficult for a plume to exist at present and the plume mode of convection may have shut off early in Mars history. The evidence for a magnetic field – and apparently early shutoff of the magnetic field about 4 Gya – has led to the idea that the core dynamo may have shut off early in Mars history (Acuna et al., 1999). The argument is that the core dynamo shut off as the core cooled. The problem with this argument is that the dynamo on Earth is most likely powered by compositional as opposed to thermal, buoyancy (Braginsky & Roberts, 1995; Buffett et al., 1996; Schubert et al., 2001). So it isn't clear that the magnetic field and core heatflow are directly related.

The lack of seismic instrumentation on Mars is a significant limitation. The exact size of the core, the presence or absence of an inner core, whether the core is completely solid or partially molten, the velocity structure of the mantle and the structure of the Moho, all would be potentially resolvable with seismic instruments on Mars. Unfortunately, seismometers always seem to be cut from Mars missions.

Zhong & Roberts (2003) and Lowry & Zhong (2003) have shown that a significant amount of the support of the Tharsis topographic anomaly can be explained by a volcanic structure modifying the lithosphere. Approximately 75% of the areoid and topographic profiles over Tharsis can be explained by crustal thickness and surface volcanic construction (c.f., Solomon & Head, 1982; Zuber, 2001; Zhong, 2002; Lowry & Zhong, 2003; Zhong & Roberts, 2003). Given the crustal thickness anomalies, some Mars researchers question whether a mantle plume could be active on Mars presently and speculate that recent volcanism on Tharsis may have formed by a non-plume mechanism (e.g., King & Ritsema, 2000). Redmond & King (in press) show that it is possible to produce a mantle plume that has a small enough topography and geoid anomaly to explain the residual topography and areoid (the part not explained by the 75% from the Zhong & Roberts (2003) study). Because gravity and dynamic topography models are non-unique, it seems likely that gravity and topography will not be able to definitively rule out a present-day plume without additional information (such as the seismic structure of the Tharsis swell).

One first-order observation of the long-wavelength areoid and topography is that there is a striking spherical-harmonic degree-2 order-2 pattern (i.e., two equatorial highs almost 180 degrees apart) (Figure 2). Phillips et al. (2001) have shown that this can be modeled considering elastic stresses in the lithosphere from a single topographic load at Tharsis rise. While it has not been demonstrated, we strongly suspect that this is independent of the source of the load, that is dynamic uplift from a buoyant mantle source would also lead to the same degree 2-2 pattern in the areoid.

Figure 2: Martian Areoid from model JOD75E60. Taken from the Planetary Data System (PDS) geoscience node. Black line is the zero contour of the topography from Figure 1. This approximates the location of the crustal dichotomy.

Even though Mars is smaller than Earth and the Martian mantle is likely to be convecting less vigorously than Earth's mantle, convection calculations in 3D spherical geometry with the appropriate parameters are time-dependent (Schubert et al., 1990). In these calculations plumes migrate relative to each other with time (Schubert et al., 2001), particularly early in Mars' thermal evolution. Schubert et al. (1990) point out that during the later stages of the calculations, the mantle settles into a stable, two-plume planform. However, earlier in the same calculations they observe six or more plumes. These calculations are difficult to reconcile with the observation that there has been one dominant volcanic feature on Mars that was active from at least 3 Gyr before present until about 40 Mya and that this volcanic feature has remained stationary with respect to the crust. The volcanic activity at Tharsis and Elysium could possible indicate two plumes (following the plume hypothesis). However, the early stages of all Martian convection calculations have more than two plumes. In these calculations, plumes are always scattered, and not clustered as they would have to be to explain Tharsis. Why the additional plumes that appear early in the convection calculations never developed significant volcanic features remains unanswered.

Harder & Christensen (1996) and Harder (1998) show that a perovskite-magnesiowustite layer at the base of the Martian mantle will suppress short-wavelength instabilities from the bottom thermal boundary layer, making it is possible to produce a single upwelling plume. It requires more than 4 Ga of evolution of the calculation before the convective flow settles into a one-plume planform, however, which begs the question of why the early volcanic activity appears to adopt the single plume planform. Harder (1998) estimates the depth of the perovskite phase transition on Mars to be roughly 1900 km, which requires the core to be smaller than 45% of the planetary radius. Without the stabilizing effect of the perovskite layer at the base of the mantle, Schubert et al. (1990) demonstrate that a planform with six to twelve upwelling plumes is likely. As the planet cools, a planform with two stable upwellings can be achieved without the deep perovskite layer (e.g., Schubert et al., 2001).

The crustal dichotomy may be one of the oldest features on Mars. The southern hemisphere of Mars is highly cratered and approximately 4.5 km higher in mean elevation than the smooth and relatively-low-cratered surface northern hemisphere. A larger number of buried craters that were not visible in previous spacecraft images have been identified in the MOLA topography (Frey et al., 2002). Thus, while the crater age of the southern hemisphere has always been known to be old, it is now clear that the northern hemisphere is also old with the bulk of the older, Martian crust emplaced during the Noachian (Schubert et al., 2001). The heavily cratered, older crust of the northern hemisphere has since been overlaid by lava flows dating back to the late Hesperian up through the late Amazonian (Schubert et al., 2001) forming the smooth, northern plains observed today. Logically it follows that the crustal dichotomy is also an old feature. Tharsis rise straddles the crustal dichotomy of Mars and this leads to the question of whether the dichotomy played a role in the formation and evolution of Tharsis.

The elevation difference between the northern and southern hemispheres requires a thinner crust in the north. The lack of correlation between the gravity field (Figure 2) and the dichotomy boundary (Figure 1) is consistent with the hypothesis that the crustal dichotomy is an ancient feature (see Kaula (1967) for the discussion of the lack of correlation between the gravity field and continents on Earth). Hence a relatively sharp discontinuity in crustal thickness must have been maintained thoughout much of Martian history. Thus, mass variations in the Martian crust are assumed to be due to changes in crustal thickness of a nearly uniform density crust (Zuber et al., 2000). The analogy of passive margins on Earth might lead one to think that there is a compositional difference. However, there is no indication that there has been silicic volcanism on Mars. The variation of crustal thickness over a short length-scale, as suggested by the sharp dichotomy boundary, is not unlike that at a passive margin or craton boundary. Edge-driven convection may play a role in some hotspots on Earth (King & Anderson, 1998; 1998; King & Ritsema, 2000) and the dichotomy boundary may be an ideal situation for edge-driven convection.

In a lab experiment performed by Wenzel et al. (2004), it was shown that if a thick insulating layer is placed over half of a tank containing a two-layered mantle-like system, an upwelling will develop early under the insulating layer and persist for billions of years. Although the upwelling forms beneath the layer and not at the edge, where Tharsis lies, it is still strongly suggestive that the crustal dichotomy may have played a role in the formation of the Tharsis rise.

At present it is difficult to unequivocally favor or rule out any mechanism for the formation of Tharsis rise and the shield volcanoes of Mars. However, several things are clear:

There are some aspects of Tharsis rise that are generally consistent with a plume model, and some that are not. Knowing the seismic velocity structure of Tharsis rise and the Martian mantle below it would be very useful. However, it is difficult to envision the kind of seismic experiment that has been performed at a number of hotspots on Earth being possible on Mars in the near future. Even first-order unknowns, such as the radius of the core, whether there is an inner core, and a 1D velocity model for the Martian mantle would be extraordinarily helpful.