December 13, 2012
image by Mark Wieczorek and collegaues (2012) (NASA / JPL / GSFC / MIT / IPGP)
Emily Lakdawalla, one of the best space science writers, has published an excellent tutorial on the geophysics of gravity anomalies and the recent GRAIL results. While reading her account I was reminded of my initial reaction when learning of the results by Mark Wieczorek's team which modelled the data to extract this map of bulk densities of the lunar highlands crust. Maria are excluded because they are places where the crust is thin and has relatively recent (3.5-2.5 b.y. old) dense lava flows. Based on this analysis Mark found that the highland crust's average density - the green on the map - is only 2550 kg/m^3 (2.55 gm/cc). This is surprising because the highlands rocks brought to Earth by Apollo astronauts are 2800 to 2900 kg/m^3, considerably denser. Mark's group calculated that this difference could be explained if the top few kilometers of the crust had 12% void space. As an analogy think of a bag of marbles. Each marble has a certain density but the bag of marbles has a lower density because of all the empty space between the touching spherical marbles. Moon rocks aren't spherical so something other than packing caused the void spaces, and Mark proposed that it was intense fracturing from the early heavy bombardment of the lunar crust. This is regarded as a prime discovery of GRAIL but its seems to be a confirmation of what was first deduced 40 years ago. Using data from the System of Lunar Craters catalog, Bill Hartmann calculated that the highlands must have been intensely fractured by all the impacts that produced those and earlier craters. He and later researchers determined that the mega-regolith, the pulverised crust, was about two km thick. The nice thing about the GRAIL results is that they arrive at the same understanding using a totally different approach with completely different data.
Hartmann W.K. 1973, Ancient lunar megaregolith and subsurface structure. Icarus 18, 634-636.
Yesterday's LPOD: Happy 12/12/12!
Tomorrow's LPOD: Liftoff To the End