Dr. Patrick J. McGovern
MARS RELATED PROJECTS
The thermal history (interior evolution) of Mars
The internal structure of Mars is difficult to study, because as of now we can only observe the planet from the outside. On Earth, we can probe our planet's interior with seismic waves from earthquakes and by analyzing rocks that are brought up from great depths by volcanic or tectonic activity. However, there is one type of data collected by spacecraft orbiting Mars that can tell us about what's inside Mars: precise measurements of variations in the pull of gravity!
The pull of gravity is caused by the mass of a planet. Variations in mass, such as from a large volcano (excess) or chasm (deficit), will
accelerate or decelerate an orbiting spacecraft, and the resulting changes in velocity can be detected from Earth by tracking the shift in frequency of a transmitted radio wave: the famous Doppler shift. If we keep track of where the spacecraft was at every measurement, we can assemble a map of the gravity anomaly of Mars.
But we're not finished there: one thing we've learned about planets is that mass excesses at the surface tend to be adjusted for, or "compensated", by mass deficits somewhere beneath the surface. The gravity by itself doesn't tell us about this compensation, so we need some extra information. This comes from the topography: measurements of the elevation of the Martian surface. For Mars, the best map of the topography was provided by the laser altimeter called "MOLA" aboard the Mars Global Surveyor (MGS) spacecraft.
Once we know the topography, we can estimate its gravitational effect and subtract it out, such that whatever remains must reflect mass anomalies at depth within the planet. In practice, we calculate models of the planet's response to forces that the surface mass anomalies exert on the "lithosphere", or strong outer layer of a planet. A weak lithosphere allows downward sagging of the crust, producing a compensating mass deficit at depth and lowering the gravity signal at the surface. A strong lithosphere will prevent sagging, leading to little compensation and a high gravity signal. To see these effects, we plot the ratio of gravity to topography, called the "admittance", as a function of the scale of the variations (to separate large features from small ones).
This plots shows that for these three areas in the ancient southern highlands of Mars, the gravity/topography admittance observed by our spacecraft (dark line with points marked by error bars) gives the best match to models with low lithosphere thicknesses (the lowermost dashed curves), which means that these terrains are highly compensated.
OK, great, but what does that tell us about the interior of Mars? Well,it turns out that lithospheric thickness is related to the flow of heat
from the interior of the planet. If the lithosphere is thick, the heat flow is low, and if the lithosphere is thin, the heat flow is high. Applying our study to various regions of Mars, we found that in general, older regions had low admittances, and therefore low lithospheric thickness and high heat flux. Conversely, relatively young regions had high admittances, and therefore high lithospheric thickness and low heat flux.
Our findings were consistent with the idea that a planet like Mars starts out rather hot (due to the combined effects of heat of accretion and decay of radiogenic isotopes), but cools down rapidly with time, such that most relatively young features are uncompensated (supported by the thick lithosphere). There are exceptions, such as some of the Tharsis Montes volcanoes, that may reflect an enhanced heat flux required to drive volcanism.
McGovern et al., 2002, J. Geophys. Res.
McGovern et al., 2004, J. Geophys. Res.
to Scientific Staff Page
December 14, 2007