Kramer G.Y., Kring D.A., Nahm A.L., Pieters C.M. (2013), Spectral and Photogeologic Mapping
Connor Lynch
Kramer G.Y., Kring D.A., Nahm A.L., Pieters C.M. (2013), Spectral and photogeologic mapping of Schrodinger Basin and implications for post-South Pole-Aitken impact deep subsurface stratigraphy, Icarus 223, 131-148
This paper by Kramer et al. takes an in-depth look into the Schrodinger impact basin in order to gain a better understanding of the stratigraphy of the lunar in and around SPA. Using data from M^3, LRO, and crater-scaling relationships previous models using Clementine etc. were improved upon thus revealing new insights about the crust on the lunar farside, SPA effects regionally, and peak-ring formation.
The main spectral analysis indicated that there are six main lithologies in Schrodinger five of these being dominated by a single mineral phase (only one can be unambiguously identified). The authors go on to elaborate on each of these lithologies and their appearance and locations in the M^3 data. Even though pyroxene spectrally dominates the surface of the moon, plagioclase is more volumetrically abundant. More data may be needed but it cannot be assumed that any area of the Moon is devoid of plagioclase.
Three main units of Schrodinger were identified as the spectrally distinct unit, the mare basalt, and the pyroclastic deposit. The fourteen or so specific geological units are explained in detail and many questions remain concerning surrounding heavily cratered terrain among other things.
All of the data in the paper can give us more insight about the object that formed Schrodinger and the stratigraphy of the lunar crust near SPA. Using Clementine data Bethell and Zuber (2009) defined the outer basin rim numerically as well as the outer rings and ejecta boundaries. The model presented here suggests strongly that the dominance of orthopyroxene in the spectra of the crater floor, walls, and proximaejecta is indicative of noritic crust that reaches depths of 20+ km below the surface.
The surface of SPA ejecta has also been identified as noritic. One question that is brought up is that between the time of the SPA and Schrodinger impact events there were other numerous events. These events’ mixing of the upper 8 km of surface material would imply that SPA melt preserved within Schrodinger should occur as excavated samples in the ejecta or exposed in the cross section of the megaregolith in the basin walls.
Schrodinger is the most well preserved basin of its size and therefor could be an ideal place to collect evidence to support theories of the basin-forming process. SPA impact melt should be found in the walls of Schrodinger and thus sample return could be necessary to make further discoveries in the lunar geology.
1. The plateau of heavily cratered terrain that extends outward past the SE rim of Schrodinger has an unclear origin. We are uncertain if the source is Schrodinger itself to the NW or somewhere further SE. If Schrodinger is the source of this secondary crater field (open to debate) then why are these secondary craters found in this specific form only in this location? Does this say anything about the projectile that created Schrodinger or lunar surface processes over time?
2. Kramer et al. argue for a pre-Nectarian designation for the peak ring due to the spectral signature of pure anorthosite, the fact that material that makes up the peak ring was brought to the surface from a depth greater than the cumulative ejecta thicknesses from basins around Schrodinger and ejecta from craters on Earth.
Do we agree with their argument here or are there other factors that they are not taking into consideration when dating Schrodinger's peak ring?
3. Wieczorek and Phillips (1999) calculated excavation cavities for 11 complex craters and basins on the Moon and assumed that basin-related gravity anomalies are directly related to amount of material excavated and subsequent Mohoisostatic rebound and fitting a parabola to the pre-impact surface depression. Thus SPA's transient crater diameter is calculated to be 2099 km as proposed by Wieczorek and Phillips (1999). Since this diameter would happen to bisect Schrodinger, why don't we see evidence for some sort of structural/compositional contrast? Wouldn't something be apparent in the peak ring or ejecta?
4. Since Schrodinger's impact site was originally target crust covered with around 6 km of SPA ejecta and 8+ km of ejects from surrounding impact basins, much of the excavated material at the time of Schrodinger had previously been excavated. What effects do these properties have on the ensuing structure of Schrodinger and its ring formation and depth?
5. Sample return and/or higher resolution mapping may be needed, but as of now how can we interpret areas of the peak ring that exhibit strong unanimous mineral compositions? Some areas are primarily anorthite-rich and some strongly olivine-rich and few areas are mixed. One theory proposed here is the presence of layered sequences of magmatically differentiated material.
6. Using Head (2010), Collins et al. (2002), and Cintala and Grieve (1998) what model of basin formation concerning the origin of the peak ring material is more likely? One suggests a mid-crustal origin (greater than 21 km excav. depth) and the other from the lower crust/upper mantle.
7. Which of the three reasons or combination thereof given at the very end of the paper seem most plausible in explaining the lack of evidence for an upper anorthositic crust in the ejecta of any of the craters surrounding Schrodinger?
8. Besides the useful age differences between SPA and Schrodinger as well as the questions involving the layering of the lunar crust and upper mantle and peak ring formation, why is Schrodinger a possible candidate for a sample return mission?
