My Mars research focuses on several themes. (1) I am developing numerical simulations of the interior of Mars, focusing on the thermal structure and convective flow in mantle plumes, which are the likely cause of the youngest volcanic activity on Mars. (2) I am examining the long-term thermal evolution of Mars, including the effects of water loss from the mantle and its effects of the vigor of mantle convection, the rate of magma production and crustal formation over time, and the processes that caused the the magnetic field of Mars to turn off about 4 billion years ago. (3) I participate in laboratory studies of the melting of martian rocks, which provide important constraints on computer models of crustal production and mantle evolution. (4) I study the structure of magma chambers beneath large volcanos using gravity and topography data.
Mantle Plume Volcanism on Mars
Example mantle plume simulation for Mars, showing a hot plume originating at the core-mantle boundary and rising toward the surface in the center of the image. The Rayleigh number (the degree of convective vigor) and the amount of radioactive heating of the mantle and crust are appropriate for present-day Mars. The color scale shows the super-adiabatic temperature of the mantle, with red being hot and blue being cold. The hot plume (vertical red structure) is about 120 oC hotter than the surrounding average mantle (orange). In this image, the region in white is above the solidus temperature for the martian mantle. Image is based on Figure 1 of Li and Kiefer (Geophys. Res. Lett., 2007).
One aspect of my current convection modeling concerns the nature of mantle convection on Mars. The Tharsis plateau is the most obvious manifestation of mantle convection on Mars. The plateau is about 4500 kilometers across and up to 10 kilometers high. It contains numerous large shield volcanos, including Olympus Mons, the largest volcano in the solar system. Volcanic activity in Tharsis has occurred during most of the history of Mars and extends to essentially the present day. Radiometric age dating of some martian meteorites indicate that some martian volcanism is no more that 180 million year old. Some lava flows imaged at very high resolution virtually lack impact craters, indicating that the lava flows are no more than 10 to 100 million years old. The Elysium region of Mars is a smaller region of basaltic volcanism that also appears to have been active for much of martian history, including the geologically recent past.
There are three energy sources that drive convective flow in the mantle: heat flowing from the core into the base of the mantle, heat from the decay of radioactive elements in the mantle, and specific heat released during the cooling of the planet. The latter two sources are known collectively as internal heating. Internal heating dominates the energetics of the flow and produces broad-scale upwellings. Heating at the base of the mantle produces narrow upwelling mantle plumes, such as Hawaii and Iceland on Earth. On Mars, the broad Tharsis and Elysium plateaus are related to the internal heating component and individual volcanos probably are (or once were) fed by mantle plumes. Volcanism in Tharsis and Elysium is caused by pressure-release melting within the upwelling plumes. The long history of volcanism in these areas indicates that convective upwelling has occurred in these regions for most of the history of Mars.
I have calculated the magma production that is associated with the thermal fields in my mantle convection simulations. The convection simulations are performed using finite element methods in spherical axisymmetric geometry, which is a good approximation to the large-scale structure of Tharsis. The amount of melting is calculated by comparing the model temperature field with the results of experimental melting studies on Mars-analog compositions. The simulations have been performed assuming a range of abundances for the radioactive elements that heat the interior of Mars.
Major results from these studies include:
- In order for present-day volcanism to occur on Mars at the rates inferred from geologic mapping, mantle convection must still be moderately vigorous, with a thermal Rayleigh number of 106-107, based on the volumetrically averaged mantle viscosity. Roughly half of the total radioactivity is in the mantle and the other half in the crust.
- Melting occurs by adiabatic decompression and is restricted to the heads of hot, upwelling mantle plumes (the white region in the image above). This is consistent with the spatially localized nature of recent volcanism on Mars. Magma production is also intermittent in time.
- Typically only small degrees of partial melting occur in these models. This is consistent with observations of rare earth elements in the shergottite meteorites, which require small amounts of partial melting. The shergottites are a class of igneous meteorites that came from Mars. Many of the shergottites formed around 180 million years ago.
- The temperature dependence of the mantle's viscosity is constrained in these models by laboratory measurements of olivine. This results in a cold, stiff, high viscosity lithosphere in the outer 100-300 kilometers. The most successful models predict a heat flux of 14-18 mW/m2 into the base of the crust in the vicinity of the plumes. This is consistent with estimates of the recent heat flux at large volcanos on Mars made by the Mars Global Surveyor science team.
- The thickness of the lithosphere varies significantly across the surface of Mars. The lithosphere is thinnest near the upwelling plumes and thickest far from the plumes. On Mars, mantle plumes appear to concentrate near the equator (Tharsis and Elysium). The thick lithosphere predicted away from the plume upwellings is consistent with measurements by Mars Reconnaissance Orbiter of thick lithosphere at the poles of Mars.
- The very low present-day heat flux out of the core in these models is consistent with the lack of a present-day magnetic dynamo on Mars.
Mantle Plume Magmatism on Present-day Mars (Geophys. Res. Lett., 2007).
Mantle Convection and Lithospheric Thickness on Mars (Geophys. Res. Lett., 2009).
Melt Propagation on Mars (J. Geophys. Res., 2007).
Martian Thermal Evolution, Mantle Degassing, and the Magnetic Dynamo
The time history of the heat flux out of the core of Mars for several different initial abundances of water in the martian mantle. When the core heat flux drops below a critical value near 10 mW m-2, the core is no longer able to generate a magnetic field. Image based on Figure 3a of Sandu and Kiefer (Geophys. Res. Lett., 2012).
Convection is the primary process for transporting heat through the interior of Mars. The vigor of convection depends strongly on both the temperature and the water content of the mantle - as Mars cools off and loses water to the surface, the viscosity goes up, which slows the subsequent rate of heat loss. The mantle solidus (the temperature at which the mantle begins to melt) is also dependent on the water content of the mantle. Because the mantle water content affects both the viscosity and the melting temperature, there are a set of strong feedback loops between magma production, volcanic degassing of the mantle, and the thermal evolution of Mars.
We have explored these feedbacks using a parameterized convection thermal evolution model that incorporates laboratory constraints of the effects of water content on viscosity and on melting rate. One interesting result of this study is that water loss from the mantle of Mars may have controlled the termination of the magnetic dynamo on Mars. Observations by Mars Global Surveyor showed that portions of the old, highly cratered crust of Mars have a magnetic signature but that younger regions of the crust do not. These observations indicate that there was a magnetic dynamo in the martian core shortly after Mars formed and that dynamo activity ceased roughly 600-800 million years after Mars formed.
Generation of a magnetic dynamo in the core of Mars requires that the core is convecting and that the heat flux out of the core exceeds a critical value of 5-10 mW m-2. In our thermal evolution models, the core heat flux is controlled by the vigor of convection in the overlying mantle. As the mantle degasses water, the viscosity rises and the convective vigor in both the mantle and core decreases. For plausible choices of initial mantle temperature and initial mantle water content, these models can explain termination of the magnetic dynamo during the observed time frame.
Mantle Degassing and the End of the Martian Dynamo (Geophys. Res. Lett., 2012).
Melting the Martian Mantle
Basaltic rocks from the Homeplate outcrop in Gusev Crater. NASA Planteary Photojournal image PIA06102.
The mantle convection studies discussed above use magma production rates and crustal thickness as some of the primary constraints for determining successful choices of model parameters. In order to make the magma production calculations as realistic as possible, it is important to use melting phase relationships for mantle compositions that are likely to be present on Mars.
Between 2004 and 2010, the Mars Exploration Rover Spirit explored the floor of Gusev crater. One area of intense interest was Homeplate, an outcrop of pyroclastic rocks that formed about 3.6 billion years ago. These rocks are thought to be unmodified from when they first melted in the martian mantle. Spirit's measurements of Homeplate provide important constraints on the chemistry of volcanism on early Mars. Experimental melting studies of material with this composition show that the mantle began melting at a temperature of 1480-1530 oC at a depth between 325 and 380 km. The magma reached a maximum melt fraction between 13-23% and was extracted to the surface from a depth of about 105 km. These results help to constrain our long-term thermal evolution models for Mars.
The shergottites are a group of meteorites whose chemical characteristics indicate that they formed in volcanos on Mars. The samples currently known on Earth formed between 180 and 500 million years ago, so they represent relatively young volcanism, forming during the last 4-10% of martian history. A particular sub-type, the olivine phyric shergottites, appear to most closely represent the composition of the martian mantle. We have studied the melting behavior of one such olivine phyric shergottite, Yamato 980459, which was discovered in Antarctica in 1998. Our experimental melting results indicate that the mantle of Mars melted at a temperature of about 1540 oC at a depth of about 100 km to form the magma that produced this meteorite. Because Mars is smaller than Earth, it cooled faster than Earth and its lithosphere (the cold, outermost layer) is probably now 200-300 km thick in most places. The hot melting temperature and relatively thin lithosphere required to produce the Yamato 980459 magma during the geologically recent past are best explained by a hot mantle plume rising from deep in Mars.
Although Mars and Earth are likely to be similar in composition, observations of the martian meteorites suggest some subtle but important differences in the mantle compositions of the two planets. In particular, Mars appears to have somewhat more sodium and iron in its mantle. Experiments show that these differences allow the mantle of Mars to melt at a temperature of 30-50 oC lower than would occur at the same pressure on Earth. Inclusion of these results into our mantle plume models shows that these differences increase the rate of magma production in upwelling mantle plumes by a factor of 3-10. The small differences in mantle composition between Earth and Mars are clearly important and may be the difference between a Mars that is still slightly active volcanically and a planet that might otherwise be magmatically dead at present.
Melting the Olivine Phyric Shergottite Yamato 980459 (Meteoritics Planet. Sci., 2006).
Melting the Gusev Crater Basalt Fastball (Geophys. Res. Lett., 2010).
Thermochemical Evolution of Mars (Mars Mantle Workshop, 2012).
Gravity Models of Martian Highland Volcanos
This image shows the topography of the summit region of Syrtis Major, an ancient basaltic shield volcano on Mars. The caldera is 150 by 250 km across and is up to 2 km deep relative to the rim. Red is high and blue and purple are low. The image is 590 km across. Image is based on Figure 1 of Kiefer (Earth Planet. Sci. Lett., 2004).
The strength of a planet's gravity varies slightly from place to place. These small variations can be measured by tracking the motions of spacecraft in orbit around the planet. NASA spacecraft such as Mars Global Surveyor and Mars Reconnaissance Orbiter spacecraft have made detailed measurements of both the gravity and the topography of Mars. These observations can be used to look inside of Mars and "see" regions of more dense and less dense rocks.
I have used these gravity observations to study several large highland volcanos on Mars, Syrtis Major, Apollinaris Patera, Tyrrhena Patera, and Hadriaca Patera. For each of these volcanos, the observed topography is insufficient to explain the gravity anomaly. In addition, a high-density subsurface load must also be present beneath the volcano. A possible explanation is that the dense material is cumulate minerals that collected in now-solidified magma chambers beneath each volcano. Pyroxene is probably the dominant cumulate mineral, but olivine may also be present. In each case, the subsurface load is several hundred kilometers across and at least several kilometers thick. The Bushveld Complex, a layered mafic intrusive complex in southern Africa, is similar in size and thickness and is the best terrestrial analog to these structures. Most of these volcanos show evidence for the presence of surface water during the the time the volcanos were active. Interaction of water with the volcanically-heated crust may cause hydrothermal alteration of the crust, resulting in other dense minerals such as hematite or pyrrhotite. These minerals could contribute both to the gravity anomaly as well as to magnetic anomalies seen at some of these volcanos. In essence, the gravity observations are giving us our first look at the magmatic plumbing system on Mars.
Results for Syrtis Major (Earth and Planetary Science Letters, 2004).
Results for Tyrrhena Patera and Hadriaca Patera (Lunar Planet. Sci. Conf., 2011).
For a more general overview of Mars:
The Red Planet: A Survey of Mars
Explore: Mars Inside and Out
Dr. Kiefer's Home Page LPI Home Page
Walter S. Kiefer, firstname.lastname@example.org