Internal and Tectonic Evolution of Mercury

Mercury's geological and internal evolution presents an interesting enigma: are there conditions that allow for both apparently limited radial contraction over the last 4 billion years and sufficiently rapid core cooling at present to permit a hydromagnetic dynamo? To address this question, we simulate the coupled thermal, magmatic, and tectonic evolution of Mercury for a range of parameters (e.g., mantle rheology, internal heat production, core sulfur content) in order to outline the set of assumptions most consistent with these two conditions. We find that among the models tested, the only ones strictly consistent with ∼1-2 km of radial contraction since 4 Ga and a modern magnetic field generated by a core dynamo are those with a dry-olivine mantle rheology, heat production provided primarily by Th (negligible U or K), and a bulk core sulfur content >6.5 wt%. However, because of the limited coverage and resolution of Mariner 10 imaging and derived topography, the tectonic history of an entire hemisphere is unknown. The potential for other mechanisms (e.g., long-wavelength lithospheric folds) to accommodate contraction remains untested, limiting the ability to restrict models on the basis of accumulated strain. Furthermore, Mercury's magnetic field may be a consequence of a thermoelectric dynamo or even crustal remanence; neither hypothesis places strong constraints on current heat flux from the core. Spacecraft observations of Mercury are needed to elucidate further the internal structure and evolution of the planet

Thank You to Our 2022 Peer Reviewers

On behalf of the journal, AGU, and the scientific community, the editors of Geophysical Research Letters would like to sincerely thank those who reviewed manuscripts for us in 2022. The hours reading and commenting on manuscripts not only improve the manuscripts, but also increase the scientific rigor of future research in the field. With the advent of AGU\u27s data policy, many reviewers have also helped immensely to evaluate the accessibility and availability of data, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU\u27s data policy. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. We received 6,687 submissions in 2022 and 5,247 reviewers contributed to their evaluation by providing 8,720 reviews in total. We deeply appreciate their contributions in these challenging times

Thank You to Our 2019 Peer Reviewers

On behalf of the journal, AGU, and the scientific community, the editors would like to sincerely thank those who reviewed the manuscripts for Geophysical Research Letters in 2019. The hours reading and commenting on manuscripts not only improve the manuscripts but also increase the scientific rigor of future research in the field. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. With the revival of the “major revisions” decisions, we appreciate the reviewers’ efforts on multiple versions of some manuscripts. With the advent of AGU’s data policy, many reviewers have helped immensely to evaluate the accessibility and availability of data associated with the papers they have reviewed, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU’s data policy. Many of those listed below went beyond and reviewed three or more manuscripts for our journal, and those are indicated in italics.Key PointThe editors thank the 2019 peer reviewersPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/162718/2/grl60415.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162718/1/grl60415_am.pd

Investigating Europa’s habitability with the Europa Clipper

The habitability of Europa is a property within a system, which is driven by a multitude of physical and chemical processes and is defined by many interdependent parameters, so that its full characterization requires collaborative investigation. To explore Europa as an integrated system to yield a complete picture of its habitability, the Europa Clipper mission has three primary science objectives: (1) characterize the ice shell and ocean including their heterogeneity, properties, and the nature of surface–ice–ocean exchange; (2) characterize Europa’s composition including any non-ice materials on the surface and in the atmosphere, and any carbon-containing compounds; and (3) characterize Europa’s geology including surface features and localities of high science interest. The mission will also address several cross-cutting science topics including the search for any current or recent activity in the form of thermal anomalies and plumes, performing geodetic and radiation measurements, and assessing high-resolution, co-located observations at select sites to provide reconnaissance for a potential future landed mission. Synthesizing the mission’s science measurements, as well as incorporating remote observations by Earth-based observatories, the James Webb Space Telescope, and other space-based resources, to constrain Europa’s habitability, is a complex task and is guided by the mission’s Habitability Assessment Board (HAB)

Long-wavelength topography on Mercury is not from folding of the lithosphere

Previous work suggested that the lithosphere of Mercury could undergo folding in response to global contraction, and indeed observations from the MESSENGER mission revealed several regions where long-wavelength topography is present. Here, we test, via finite-element simulations that use a more realistic rheological model than that earlier work, lithospheric folding as a formation mechanism for long-wavelength topography on Mercury from interior secular cooling over the last 3.8 Gyr. This radial contraction has been estimated from geological observations to be less than 10 km, which translates into small amounts of horizontal shortening of < 0.3%. Under expected surface temperatures of ∼440 K, the development of even modest fold amplification in such low strain environments is untenable. The scenarios under which there is this positive fold amplification begin with a fully compensated crust, but amplifications are small (factors < 1.1). Under other, non-compensated scenarios (e.g., a constant thickness crust), the collapse to isostasy overwhelms any folding instability. In order to produce lithospheric fold amplitudes that match those observed on Mercury, unrealistically large amounts of horizontal shortening (in excess of 10%, corresponding to hundreds of kilometers of radius change) are required. Therefore, we find that lithospheric folding cannot produce the observed long-wavelength topography on Mercury, and conclude that this topography must be buoyantly supported

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