Mars Geophysics Research
My Mars research focuses on numerical simulations of the interior of Mars, particularly of the thermal structure and convective flow in mantle plumes, which are the likely cause of the youngest volcanic activity on Mars. This work has recently been expanded to consider how InSight mission seismic observations can constrain these plume models. Although most of my modeling is geophysical, I have also studied how subtle differences in the composition of the mantles of Mars and Earth can affect the melting temperature and the amount of volcanic activity that is generated. I have also modeled 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 magnetic field of Mars to turn off about 4 billion years ago.
Mantle Plume Volcanism on Mars
Example mantle plume model for Mars, showing a hot plume originating at the core-mantle boundary and rising toward the surface in the center of the image. The color scale shows mantle temperatures from hot (red) to cold (purple). The region in white is above the solidus temperature for the martian mantle. This image is based on Figure 1 of Kiefer and Li (2016).
The Tharsis and Elysium volcanic rises are the most obvious manifestations of present-day mantle upwelling on Mars. They contain numerous large shield volcanos, including Olympus Mons, the largest volcano in the solar system. Volcanic activity in Tharsis and Elysium has occurred during most of the history of Mars and extends to essentially the present day. Radiometric age dating of some igneous martian meteorites indicate that some martian volcanism is no more than 180 million years 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.
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 mantle plume thermal structure and the associated magma production using finite element methods. Major results 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.
- 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 are a class of igneous meteorites from Mars that require small amounts of partial melting.
- The Elysium mantle is quite dry, with a maximum water concentration of slightly more than 50 ppm, consistent with measurements of water in martian meteorites.
- 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. The model lithosphere thickness in Elysium is consistent with InSight mission seismic velocity model, and 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 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.
Martian Thermal Evolution, Mantle Degassing, and the Magnetic Dynamo
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 ~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.
Melting the Martian Mantle
The mantle convection models discussed above use magma production rates and crustal thickness as important constraints for determining successful choices of model parameters. 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. Although Mars and Earth are expected 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 than Earth has. 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.
Return to profile page
