My Mars research focuses on two themes. I am developing numerical simulations of the interior of Mars, focusing on the thermal structure and convective flow in the martian mantle. I am also analyzing gravity and topography observations of several large volcanos to learn about the structure of the magma chambers beneath these volcanos.

Right: In this model, 45% of the total radioactivity has been differentiated into the crust and 55% remains in the mantle. As a result, the mantle is hotter and melting can occur in the head of the hot, rising mantle plume. In this image, the region in white is above the solidus temperature for the martian mantle.

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. Some lava flows imaged at very high resolution by Mars Global Surveyor virtually lack impact craters, indicating that the lava flows are no more than 10 to 30 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 plateau is related to the internal heating component and individual volcanos probably are (or once were) fed by mantle plumes. Volcanism in Tharsis is caused by pressure-release melting within the upwelling plumes. The long history of volcanism in this area indicates that convective upwelling has occurred in this region 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. The observation that present-day Mars still produces some volcanic activity sets a lower bound on the internal temperature of the planet and thus also sets a lower bound on the abundance of radioactive elements in the mantle of Mars.

Major results from this study include:

• In order for present-day volcanism to occur on Mars, the radioactive heating rate in the mantle must exceed 1.6x10^-12 W/kg. The preferred model has half of the total radioactivity in the mantle and the other half in the crust. This corresponds to a thermal Rayleigh number for the mantle that exceeds 10⁶, which indicates that mantle convection remains moderately vigorous on Mars.

• Melting is restricted to the heads of hot, upwelling mantle plumes. 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.

• These models include a stiff, high viscosity layer in the outer 200 kilometers. The most successful models predict a heat flux of 18-20 mW/m² into the base of the crust. This is consistent with estimates of the recent heat flux on Mars made by the Mars Global Surveyor science team.

Melting in the Martian Mantle (Meteoritics and Planetary Science, 2003).

Right: A model for the Syrtis Major gravity anomaly. The red line is the observed gravity up to spherical harmonic degree 50. A model using just surface topography and its flexurally supported root is a poor fit (white line). The yellow line is the model using both surface topography and the buried high density load. The inferred subsurface magma chamber is centered below the caldera and is 300 by 600 km across, which is about twice as large as the caldera but only a small fraction of the 1100 km volcano diameter. Both the caldera and the magma chamber are elongated in the north-south direction.

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's Mars Global Surveyor spacecraft has 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 three large highland volcanos on Mars, Syrtis Major, Tyrrhena Patera, and Hadriaca Patera. For each of these volcanos, the observed topography is insufficient to explain the gravity anomaly. In addition, a buried, high-density load must also be present beneath the volcano. The most likely 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 buried 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. 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 (6th International Mars Conference, 2003).