1M.M.Hedman
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.04.014]
1Physics Department, University of Idaho, 875 Perimeter Drive, MS 0903, Moscow, ID 83844-0903, United States of America
Copyright Elsevier
Given current uncertainties in the cratering rates and geological histories of icy objects in the outer solar system, it is worth considering how the ages of icy surfaces could be constrained with measurements from future landed missions. A promising approach would be to determine cosmic-ray exposure ages of surface deposits by measuring the amounts of cosmogenic Lithium, Beryllium and Boron at various depths within a few meters of the surface. Preliminary calculations show that ice that has been exposed to cosmic radiation for one billion years should contain these cosmogenic nuclei at concentrations of a few parts per trillion, so any future experiment that might attempt to perform this sort of measurement will need to meet stringent sensitivity requirements.
The icy worlds of the outer solar system exhibit a complex array of geological structures that reflect diverse geological histories, but there is still a great deal of uncertainty regarding when various surface features formed. Absolute ages of geological formations on icy bodies have traditionally been estimated based on the observed crater densities for particular surface units. Different models of the impactor flux yield different age estimates, but in general these calculations indicate that the most heavily cratered icy surfaces are comparable in age to the Solar System (Di Sisto and Zanardi, 2016; Kirchoff et al., 2018; Kirchoff and Schenk, 2010; Zahnle et al., 2003). However, recent studies of the dynamical history of Saturn’s satellites suggest that many of that planet’s heavily-crateredmid-sized moons may be substantially less than a billion years old (Asphaug and Reufer, 2013; Ćuk et al., 2016). At the moment, it is not clear how to reconcile these dynamical arguments with the moons’ cratering records, which highlights how little direct information we have about the ages of solid surfaces in the outer solar system.
Even if future analyses of the currently-available data are able to settle the debates regarding the ages of heavily cratered worlds, there are several objects in the outer solar system that have complex geological histories extending up to the present day, including Europa, Enceladus, Titan, Triton and Pluto (Brown et al., 2010; Pappalardo et al., 2009; Schenk et al., 2018; Schenk and Zahnle, 2007; Spencer et al., 2009; Stern et al., 2015). For these objects, a key unanswered question is how long they have been active and how long fresh material can be exposed on their surfaces before it is buried or recycled. This question is not only relevant for efforts to understand the geological history of these bodies, it also determines how long ago materials on the surface were in contact with liquid water reservoirs, which has implications for efforts to assess their habitability. It is therefore worth considering what types of future experiments could help constrain the age of icy surfaces.
The most direct way to measure the absolute age of any solid material is with in-situ measurements of unstable isotopes and/or their daughter products. These sorts of radiometric dates have been obtained from laboratory measurements of both meteorites and lunar samples, and have yielded many important insights into the formation and history of solid bodies in the inner Solar System (Davis et al., 2003; Stöffler and Ryder, 2001). Furthermore, the Mars Science Laboratory recently measured the first radiometric age on another planet, demonstrating that such measurements can be performed by space missions (Farley et al., 2014). There has also been a great deal of recent interest in a mission that would land on Europa and conduct extremely sensitive measurements of surface composition in order to ascertain whether life could exist beneath that world’s surface (Hand et al., 2017). Hence it is worth examining whether a lander on an icy world could perform experiments that would yield information about the age of its surface deposits.
The fact that the surfaces of icy bodies are composed primarily of water ice makes many of the commonly-used radiometric dating systems problematic. Even allowing for the possibility that the water ice on various worlds could have substantial amounts of methane, ammonia, and various organic compounds, the elemental composition of their surfaces would still be dominated by the light elements Hydrogen, Carbon, Nitrogen and Oxygen. The longest-lived unstable isotope of these elements is 14C, which has ahalf-life of about 5700 years (http://www.nndc.bnl.gov/nudat2). While this isotope has been extensively used to date events from the Quaternary period here on Earth (http://intchron.org), and has even been proposed as a way to probe carbon transport processes on Titan (Lorenz et al., 2002), its half-life is far too short to probe the geological history of surfaces that are millions or billions of years old. Indeed, most commonly-used techniques for radiometrically dating rocks involve nuclei with half-lives of order 1 billion years, such as 40K, 87Rb and various isotopes of Uranium. While elements with such long-lived unstable isotopes like Potassium could be present near the surfaces of some icy worlds, a dating technique that relies on such contaminants would require more detailed information about the surface composition of these bodies than we currently have.
In the absence of long-lived isotopes, the most promising way to date ancient icy surfaces is with cosmic-ray exposure ages. The basic idea behind these dates is that any exposed surface is constantly being bombarded with high-energy cosmic rays that cause nuclear reactions within the material. The concentration of the resulting cosmogenic nuclei near the surface therefore increases over time and can provide information about the age of the surface deposit. Such cosmic-ray exposure ages have been used to date various surface deposits on both the Earth and the Moon, and to determine when meteorites broke free from their parent asteroids (Dunai, 2010; Eugster, 2003; Herzog and Caffee, 2014).
Of course, cosmic-ray exposure ages do depend on the assumed flux of cosmic rays, which can vary over time. Indeed, records of cosmogenic nuclei like 14C and 10Be in terrestrial ice cores show that the cosmic ray flux here on Earth has varied byroughly a factor of two on timescales of hundreds to thousands of years (Steinhilber et al., 2012). Fortunately, these variations appear to average out over the longer timescales relevant for dating astronomical bodies. For example, analyses of the cosmic-ray exposure ages from stony meteorites (which range between 1 and 100 million years) indicate that the average cosmic ray flux has varied by less than about 10% over the past 10 million years (Wieler et al., 2013). Exposure ages of iron meteorites extend back 1–2 billion years and can potentially constrain the cosmic ray flux on even longer timescales. However, the interpretation of these data is still uncertain, with some recent analyses suggesting the flux has varied by less than 50%, while others argue for factor of 3 flux variations over timescales of 500 million years (Alexeev, 2016; Alexeev, 2017; Ammon et al., 2009; Wieler et al., 2013). Such long-term flux variations would certainly need to be better constrained before cosmic-ray exposureages could provide accurate geological histories for icy moons. However, even if the long-term variations were as large as a factor of 3 over 500 million years, cosmic-ray exposure ages could still be used to determine if the heavily-cratered icy surfaces in the Saturn system are comparable to the age of the solar system or just a few hundred million years old.
Nuclear reactions induced by high-energy cosmic rays usually generate nuclei with atomic numbers comparable to or less than that of the original nuclei. This again means that only elements with low atomic numbers are likely to be available. Among these, isotopes of Hydrogen, Carbon, Nitrogen and Oxygen are probably not viable because these elements should be common native constituents of the ice, and so distinguishing any cosmogenic material will be extremely challenging. On the other hand, any Helium generated by cosmic rays should diffuse through ice on geological timescales, so this material will probably escape the surface. This leaves Lithium, Beryllium and Boron as the most promising elements for cosmic-ray exposure dating of icy surfaces. These elements are all found at very low concentrations in chondrites, Earth’s crust and ocean water (see Table 1), and they are all chemically reactive species that can be retained in-situ for geological timescales (Indeed, cosmogenic Beryllium in terrestrial ice cores have been used to trace variations in the cosmic ray flux into Earth’s atmosphere over the past 10, 000 years, e.g. (Steinhilber et al., 2012)).
