TOPOGRAPHY, GRAVITY AND MAGNETICS: GEOPHYSICAL CONSTRAINTS ON THE EARLY IMPACT AND DYNAMO HISTORY OF MARS

Two of the many major discoveries by Mars Global Surveyor (MGS) were the presence of remanent magnetic anomalies on Mars and a very large population of previously unknown buried impact basins in both the highlands and lowlands of Mars. The first indicated that Mars once had a dynamo and global magnetic field, now gone; the second indicated that most parts of Mars were much older than we had previously believed. Evidence for the buried impact basins came from MOLA topography in the form of Quasi-Circular Depressions (QCDs), most of which had no visible expression. While crater retention ages derived from visible and buried impacts are likely a closer approximation to the actual age of buried surfaces, those ages would still be minimum ages because there could be basins buried so deeply they would not have an expression in topography alone. Some of those more deeply buried basins may be expressed as Circular Thin Areas (CTAs) in crustal thickness models, derived from MGS topography and gravity data. Many CTAs are coincident with previously mapped QCDs, and the ratio of non-QCD CTAs to QCDs is highest in areas of deepest burial, both of which support the idea that many CTAs may be deeply buried impact basins. Crustal thickness data also reveals the presence of several additional very large impact basins, bringing the total > 1000 km in diameter to 20. The N(300) crater retention ages for these basins, from superimposed CTAs and QCDs, indicate that most of the basins formed in a very brief interval of time, perhaps in only 100 million years. This may be a martian equivalent of the “terminal lunar cataclysm”. More interesting, the brief interval when most of the large basins formed also appears to bracket the time when the martian dynamo died: the 5 least magnetized basins all have N(300) < 3.0, compared with the 14 most magnetized basins which have N(300) > 3.0. The change in crustal magnetic field intensity over this time is a factor 10. The switch from dynamo to “no-dynamo” conditions on Mars may have occurred in less than a few 10s of MY, and, given its association with the “spike” in large basin production on Mars, it is worth considering whether the death of the dynamo may have been prompted by the largest of these impacts, 4 of 5 of which may have occurred in the 60 MY interval just prior to the loss of the magnetic field.