GEOL 2810 – February 9, 2015
New morphometric measurements of craters and basins on Mercury and the Moon
from MESSENGER and LRO altimetry and image data: An observational framework
for evaluating models of peak-ring formation (Baker and Head, 2013)
Objective: Understand the transition from complex crater to impact basins
(“structures exhibiting at least one sizeable interior ring of peaks in addition to the
ring crest”) by quantitatively assessing the morphology of craters and basins
spanning the transition on the Moon and Mercury.
Methods:
 An updated catalog of protobasins and peak-ring basins is created using MDIS
(Mercury) and LRO (Moon).
 Craters and basins in these catalogs are assigned degradation classes from I,
freshest, to V, most degraded; only craters in classes I and II are used for
measurements.
 Rim crest, base of crater wall, and base of interior peaks are mapped for all fresh
craters and basins. Elevations from altimetry data are used to calculate depth,
height of central peak or peak ring, and peak-to-rim-crest distance. Area of crater
interior, floor, and peaks are measured. Finally, crater volume is measured using
the plane fit to rim-crest elevations and morphometry of the interior. Qualitative
trends in these parameters are discussed with respect to changes in rim-crest
diameter.
 Morphometric analysis of crater to basin transition is then used to discuss
different formation mechanisms.
Major Takeaways:
 Crater/Basin Shapes: The authors observe a decrease in crater depth to diameter
ratios from complex craters to peak-ring basins. This transition can be abrupt,
suggesting the processes of formation are different (seen best in Fig 5a and 5b).
Floor area increases with respect to total interior area as a function of rim-crest
diameter, and it is possible to statistically separate peak-ring basins, protobasins
(of a restricted diameter) and complex craters on the basis of floor area/total
interior area. A volume is defined based on a plane fit to the rim-crest for the Moon
and these volumes are found to be ~40% of the size of excavation cavities
estimations (Weicczorek and Phillips, 1999).
 Central Peak and Peak Rings: There is an abrupt transition between central-peak
diameter and peak-ring diameter (Pike, 1988; Baker et al., 2011a; Baker et al.,
2011b). This observation holds for peak diameter, height, area, and volume; there
is an increasing trend for total peak area and volume. There is no systematic
change in peak complexity with rim-crest diameter.
 Topography: Topographic profiles show a central cavity bounded by a peak-ring
with a sharp inner wall and sloped outer wall.
 Moon vs. Mercury: Mercury has a greater number of peak-ring basins than the
Moon, but fewer > 300 km basins. Peak-ring basins form at smaller diameters on
Mercury and there is more overlap in craters and peak-ring basins, the authors
GEOL 2810 – February 9, 2015
suggest this is a result of larger distribution of impact velocities on Mercury. There
are significantly different depth-to-diameter ratios for complex craters on the
Moon and Mercury, but protobasins and peak-ring basins show similar depth-todiameter ratios. Floor area/interior area is greater on Mercury.
 Modeling: Trends in size (height, area, volume) of central peak on both Mercury
and the Moon generally agree with the dynamic collapse model. This model
requires uplift of the central peak a few km over the rim-crest elevation, which is
not consistent with the observed peak heights even in impacts that record the
crater to basin transition. A more abrupt transition is needed if this model is to be
applicable. The nested melt cavity model requires central peak dimensions to
decrease with rim-crest diameter due to increased depth of melt, which is also not
observed. Topographic profiles on the Moon and Mercury, however, support the
development of an interior melt cavity an important step in the nested melt-cavity
model.
Discussion Questions
1. Can degradation class of the craters and their distribution tell us about the
impact or the target? By removing degraded craters are we biasing our
population (on the other hand by including degraded craters how would the
parameters be biased; are there advantages of having a statistically
significant population over one that includes the effects of degradation?)
2. What added insight do we get from measuring crater volume that we don’t
get from depth?
3. Mercury does not show the same decrease in rim-crest circularity with rimcrest diameter as observed on the moon. One of the hypotheses is that as
basins get larger, they incorporate more pre-existing topography and are
likely to become irregular. Does this work with what we know about
Mercury’s underlying topography compared to the Moon’s (crater density,
volcanism etc.)?
4. How do measured parameters compare with parameters we are interested in
understanding, such as the transient cavity or excavation depth? Are there
alternative parameters that should be measured? In general, how do we use
morphometric properties to learn more about crater formation or are we
confined to understanding only the end stage?
5. How does using data from multiple studies and multiple instruments
influence the findings in this study?
6. Can we use composition (melt vs. excavated material) in addition to
morphology for the complex craters, protobasins, and peak-ring basins to put
additional constraints on models?