On the origin of mascon basins on the Moon (and beyond)

Mascon basins on the Moon are large craters that display significant positive free-air and Bouguer gravity anomalies. An important question is why is not every large crater a mascon, as less than half have been previously determined to be. We detrend the free-air, topographic, and Bouguer gravity anomalies and find that most large basins (28 of 41) display mascon characteristics (e. g., strong positive Bouguer anomalies narrower than the surface rims). Negative free-air gravity annuli surrounding the central highs generally are absent in the Bouguer gravity, implicating surface topography. We propose that beneath a forming large basin, the relatively narrow transient crater drives mantle uplift, while upward and inward collapse forms the surface topography. Furthermore, the nonmascon basins are all ancient and heavily degraded, indicating a postimpact evolutionary process. Our results suggest that mascon formation is the standard for large impacts on the Moon and by extension on other terrestrial planets

Impact basin relaxation on Rhea and Iapetus and relation to past heat flow

Evidence for relaxation of impact crater topography has been observed on many icy satellites, including those of Saturn, and the magnitude of relaxation can be related to past heat flow (e.g. Moore, J.M., Schenk, P.M., Bruesch, L.S., Asphaug, E., McKinnon, W.B. [2004]. Icarus 171, 421-443; Dombard, A.J., McKinnon, W.B. [2006]. J. Geophys. Res. 111, E01001. http://dx.doi.org/10.1029/2005JE002445). We use new global digital elevation models of the surfaces of Rhea and Iapetus generated from Cassini data to obtain crater depth/diameter data for both satellites and topographic profiles of large basins on each. In addition to the factor of three lower amplitude of global topography on Rhea compared to Iapetus, we show that basins on Iapetus >100 km in diameter show little relaxation compared to similar sized basins on Rhea. Because of the similar gravities of Rhea and Iapetus, we show that Iapetus basin morphologies can be used to represent the initial, unrelaxed morphologies of the Rhea basins, and we use topographic profiles taken across selected basins to model heat flow on both satellites. We find that Iapetus has only experienced radiogenic heat flow since formation, whereas Rhea must have experienced heat flow reaching a few tens of mW m(-2), although this heat flow need only be sustained for as little as several million years in order to achieve the observed relaxation magnitudes. Rhea experienced a different thermal history from Iapetus, which we consider to be primarily related to their different formation mechanisms and locations within the saturnian system. A recent model for the formation of Saturn's mid-sized icy satellites interior to and including Rhea (Charnoz, S. et al. [2011]. Icarus 216, 535-550) describes how Rhea's orbit would have expanded outwards after its accretion from a giant primordial ring, which would have instigated early heating through rapid despinning and tidal interaction with Saturn and other satellites. Rhea's basins would therefore be required to have formed within the first few tens of Myr of Rhea's formation in order to relax due of this heating, and if so may provide an important anchor point for Saturn system chronology. None of these heating mechanisms are viable for Iapetus in its isolated position far from Saturn, and as such it has remained dynamically inert since formation, confirming conclusions based on thermal modeling of Iapetus' interior. Rapid and complete relaxation and subsequent erosion by bombardment of a 'first generation' of large basins on Rhea is regarded as an explanation for the lower counts of large basins on Rhea relative to Iapetus, and the overall lower amplitude of topography on Rhea compared to Iapetus

Delayed formation of the equatorial ridge on Iapetus from a subsatellite created in a giant impact

The great equatorial ridge on Saturn’s moon Iapetus is arguably the most perplexing landform in the solar system. The ridge is a mountain range up to 20 km tall and sitting on the equator of Iapetus, and explaining its creation is an unresolved challenge. Models of its formation must satisfy three critical observations: why the ridge (1) sits exactly on the equator, (2) is found only on the equator, and (3) is thus far found only on Iapetus. We argue that all previously proposed models fail to satisfy these observations, and we expand upon our previous proposal that the ridge ultimately formed from an ancient giant impact that produced a subsatellite around Iapetus. The orbit of this subsatellite would then decay, once Iapetus itself had despun due to tides raised by Saturn, until tidal forces from Iapetus tore the subsatellite apart. The resultant debris formed a transient ring around Iapetus, the material of which rained down on the surface to build the ridge. By sequestering the material in a subsatellite with a tidally evolving orbit, formation of the ridge is delayed, which increases the likelihood of preservation against the high-impact flux early in the solar system’s history and allows the ridge to form on thick, stiff lithosphere (heat flow likely <1 mW m 2) required to support this massive load without apparent flexure. This mechanism thus explains the three critical observations

Flanking fractures and the formation of double ridges on Europa

Europa, a satellite of Jupiter, is one of the most intriguing worlds in the solar system. Its dearth of impact craters and plethora of surface morphologies point to a dynamic evolution of its icy shell in geologically recent times. Double ridges are a common landform and appear to have formed over a significant fraction of the satellite’s observed geologic history. Thus, understanding their formation is critical to unraveling Europa’s history, and many models have been proposed to explain their creation. A clue to the formation of ridges may lie in evidence for flexure of the lithosphere in response to a load imposed by the ridge itself (marginal troughs and subparallel flanking fractures). When this flexure has been modeled, a simple elastic lithosphere has typically been assumed; however, the generally thin lithospheres suggested by these models require very high heat flows that are inconsistent with Europa’s expected thermal budget (of order 1 W m-2 vs. of order 10 mW m-2). Each of the proposed formational models, however, predicts a thermal anomaly that may facilitate the flexure of Europa’s lithosphere. Here, we simulate this flexure in the presence of these anomalies, as a means to evaluate the different models of ridge formation. We find that nearly all models of double ridge formation are inconsistent with the observation of flexure (specifically the flanking fractures), except for a cryovolcanic model in which the growing ridge is underlain by a cryomagmatic sill that locally heats and thins the lithosphere