(this is from Nick Hoffamn's link below: http://www.agu.org/pubs/pip/2004JE002343.pdf)

<quote>

It is shown in this paper that 5 giant impact basins of Mars, Argyre, Hellas, Isidis,
Thaumasia, and Utopia, are located on a single great circle. This suggests that the
projectiles’ orbital planes coincided with the equator of Mars at the time of impact. The
projectiles were lager fragments of a heliocentric asteroid that broke apart as entered the
Roche limit of Mars. The mass of the asteroid was at least ~9.8x1020 kg, ~1.5 x10-3 times
mass of Mars.

</quote>

The idea that impact basins trace former equatorial regions on Mars has been around a long time, based in part on the number and alignment of elongate craters on the surface of Mars. This is the first time I've run into an extensive study of the possibility that the giant basins may delineate a former equatorial region. (of course I still cling to the radical notion that Hellas isn't an impact basin, and Utopia certainly isn't an impact basin, but I could be wrong!)

Previous studies suggested that the crust of Mars has slid over the interior. Are there other mechanisms that could account for significantly different equatorial regions than we see now, given that plate tectonics apparently doesn't operate on Mars?

Could the "seam" on the equator of Iapetus be related to the phenomenon of giant impacts and/or low angle impacts into former equatorial regions of Mars?