“Isocrater” impacts: Conditions and mantle dynamical responses for different impactor types

Highlights

• Impactors of different type/size/velocity can produce craters of the same diameter.

• Conditions for such “isocraters” are derived from scaling laws, modeled numerically.

• Response of interior to isocrater impacts varies strongly between impactor types.

• Responses to similar impactors vary strongly with planetary structure.

• Observed geophysical anomalies may allow to resolve non-uniqueness of impactor.

Abstract

Impactors of different types and sizes can produce a final crater of the same diameter on a planet under certain conditions. We derive the condition for such “isocrater impacts” from scaling laws, as well as relations that describe how the different impactors affect the interior of the target planet; these relations are also valid for impacts that are too small to affect the mantle. The analysis reveals that in a given isocrater impact, asteroidal impactors produce anomalies in the interior of smaller spatial extent than cometary or similar impactors. The differences in the interior could be useful for characterizing the projectile that formed a given crater on the basis of geophysical observations and potentially offer a possibility to help constrain the demographics of the ancient impactor population. A series of numerical models of basin-forming impacts on Mercury, Venus, the Moon, and Mars illustrates the dynamical effects of the different impactor types on different planets. It shows that the signature of large impacts may be preserved to the present in Mars, the Moon, and Mercury, where convection is less vigorous and much of the anomaly merges with the growing lid. On the other hand, their signature will long have been destroyed in Venus, whose vigorous convection and recurring lithospheric instabilities obliterate larger coherent anomalies.

Introduction

The cratered surfaces of planetary bodies in the solar system offer abundant evidence that meteorite impacts have been an important geological factor in their evolution, especially in the first few hundreds of millions of years. However, the dynamics of the impact process itself as well as later degradation make it difficult to reconstruct the physical properties of the impactor unambiguously (e.g., Herrick and Hynek, 2017), because the impactor is usually obliterated during the impact, even though numerical models of central peak formation in complex craters suggest that a certain fraction of the impactor material may be preserved to some extent, especially in slow or oblique impacts (e.g., Yue, Johnson, Minton, Melosh, Di, Hu, Liu, 2013, Svetsov, Shuvalov, 2015). The principal features of the remaining crater, i.e., its geometrical characteristics, depend on a combination of several parameters that characterize the impactor and the target. While it is in principle possible to determine the target properties reasonably well, the combination of properties of the projectile (diameter, velocity, density) can generally only be inferred on the basis of morphological studies of the crater and the ejecta (e.g., O’Keefe, Ahrens, 1982, Schultz, Crawford, 2016) or statistical information on candidate impactor types, especially if the crater or basin is very old and its ejecta are obliterated.

There are several candidate impactor classes, which belong to two general groups, namely asteroids and comets; in principle, larger trans-neptunian objects (TNO) which for some reason acquired very eccentric orbits that brought them into realms of the solar system closer to the Sun are a third group, although no such objects are known to exist presently. The asteroids are divided into various classes, of which especially the rocky S-types and the less dense C-types are important due to the relatively large mass and number fractions of the total asteroid population they constitute (DeMeo and Carry, 2013). These two major classes are expected to travel at similar velocities, but differ markedly in their average densities (Carry, 2012). There is even more uncertainty about the density of comets, but it seems to be clear that they are, on average, less dense by at least a factor of two than even C-type asteroids. In the inner Solar System, their velocities cover a wide range at any given distance from the Sun, as they originate from different reservoirs at very different distances from the center.

Direct statistical information about their relative abundance is essentially limited to results from observations of present-day populations. While total meteorite fluxes can be deduced to some extent from cratering statistics, such statistics provide no direct information about the proportions of the different impactor types and their possible temporal evolution. In an attempt to establish a more stringent link with the past, Ivanov et al. (2002) compared the observed size–frequency distribution of the modern main-belt asteroid population with that of the impactors on the Moon as derived from the lunar cratering record and impact scaling laws and found that both have a similar shape. These authors hence argue that at least for the past  ∼ 4 Gy, most impacts in the inner Solar System were caused by main-belt asteroids, or more generally, collisionally evolved bodies. Similar conclusions were also reached by other authors on the basis of cratering statistics and dynamical simulations (e.g., Strom, Malhotra, Ito, Yoshida, Kring, 2005, Rickman, Wiśniowski, Gabryszewski, Wajer, Wójcikowski, Szutowicz, Valsecchi, Morbidelli, 2017). Still, many assumptions made about these issues rely heavily on knowledge of the current state and on models of long-term Solar System evolution including phenomena such as orbital shifts of planets (e.g., Gomes et al., 2005), all of which are still poorly known. Furthermore, the earliest stage of impact history is not constrained by a similar argument, and the case has been made that different impactor populations have existed at different times during the history of the Solar System and are recorded on Mars and on the farside of the Moon, respectively (Bottke et al., 2017).

A frequently made assumption in models of individual impacts on planets and/or their effects on their interiors is that the impactor is a body of the most frequent class, usually assumed to be an S-type asteroid traveling at the average impact velocity corresponding to the target planet and striking at an angle of 45°. Given the need to limit the scope of such studies, this focus on the most frequent and hence most likely category is perfectly legitimate, but it may lead to impacts of other bodies being forgotten. However, all of those other categories are not so rare that they can safely be considered insignificant: the contribution of comets has been estimated to lie between a few per cent and a few tens of per cents, increasing with proximity of the target planet to the Sun (e.g., Chyba, 1987, Olsson-Steel, 1987). It seems therefore rather unlikely that even all of the dozens of large impact basins expected to have existed in the inner Solar System were formed only by S-type asteroids; for instance, it has been suggested that the South Polar–Aitken basin on the Moon or the crater Eminescu on Mercury were formed by cometary impacts (Shevchenko, Chikmachev, Pugacheva, 2007, Schultz, 2017). Geochemical arguments, especially isotopic studies of water and nitrogen for the Earth, the Moon, and Mars, point to a strong dominance of asteroidal impactors but also confirm the existence of a contribution of cometary impactors on the order of a few percent (e.g., Barnes et al., 2016); these authors also invoke carbonaceous chondrites as a major asteroidal source for the water, which would indicate that many impactors were C-type asteroids (Carry, 2012). Near-surface geological aspects of this ambiguity have been addressed in the literature in some cases (e.g., Pugacheva et al., 2016, for the Shackleton crater on the Moon), but the implications for the deep interior evolution have not received much attention so far.

In this paper, we consider different combinations of impactor characteristics that would all result in a crater of the same size (diameter) according to the scaling laws established by the theory of impact dynamics; we will call such events “isocrater impacts”. Related scaling laws indicate that such isocrater impacts may still differ in their effects on the interior of the planet. By combining two-dimensional dynamical models of mantle convection with a parameterization of the principal effects of impacts based on scaling laws, we address the question whether the differences in interior dynamics effects of large isocrater impacts can be large enough to have significant consequences for the long-term evolution of the planet. The general theoretical considerations in the next section also allow us to exclude certain impactor types as causes of large impact basins for a given target body and thus also as important exogenic influences on interior dynamics. Furthermore, the models can also provide hints whether a population of now-extinct impactors might or must be considered to explain observed consequences of certain impacts.