This project aims to develop new approaches for testing the hypothesis that the solar system underwent a massive re-organization several hundred million years after its beginning, including the migration of Jupiter and Saturn and scattering of a large number of small bodies. This event is imagined to have re-shaped the orbits of many comets and asteroids and caused the ‘late heavy bombardment’ if intense meteoric impact that effected the Earth and other inner solar system bodies.
Our strategy is to develop a focused, mechanistic and testable hypothesis for the mineralogical, chemical and spectral changes that occurred to the small, icy outer solar system bodies that experienced this orbital re-organization, based on telescopic observations of these bodies and laboratory experiments conducted on analogues. This work will lead to a method for identifying specific bodies that underwent dramatic migration, and to predictions regarding the chemical and isotopic composition of near-surface materials that would be found on such a body if it were examined with a suite of in-situ analytical instruments. Our broader purpose is to define the specific goals and instrumentation needs of a future mission to such a body. Our hope is that this work will lead to a successful proposal for such a mission, and thus catalyze a large advance in our understanding of solar system evolution and the nature and history of the many small icy bodies that populate the outer solar system.
Our current understanding of the evolution of the solar system based on recent dynamical models (e.g. the Nice Model) suggests radical rearrangement in the first hundreds of millions of years of its history, changing the orbital distances of Jupiter, Saturn, and a large number of small bodies. Our goal is to build a methodology that can be used to concretely tie individual solar system bodies to dynamical models using observables, providing evidence for their origins and evolutionary pathways. Ultimately, one could imagine identifying a set of chemical or mineralogical signatures that could quantitatively and predictably measure the radial distance at which individual bodies first accreted.
We have chosen to focus our efforts on a specific population of bodies: the Jupiter Trojan asteroids. The Trojans were predicted by the dynamical models (e.g. the Nice Model) to have initially formed in the outer solar system (Kuiper Belt) and later been scattered inward to co-orbit with Jupiter during the large-scale rearrangement events. We seek to identify measurable signatures that can tie the Trojan populations to these origins and evolutionary pathways, providing experimental support of dynamical models, and providing testable hypotheses that can feed into the design of experiments that might be performed on potential future missions to these and other primitive bodies.
Our strategy is a fourfold approach to the problem:
We begin with the current state of knowledge of the Trojan asteroids. The Trojans exhibit two distinct populations, characterized by their “red” and “less red” spectral slopes1. The only spectral features present in infrared spectra from telescopic observations resemble comet-like amorphous or fine-grained silicates2. We consider a formation scenario, following the line of reasoning developed in the Nice model, where The two Trojan asteroid populations formed in the Kuiper Belt (or trans-Neptunian disk, the parent region of today’s Kuiper belt) and migrated inward. Furthermore, it is possible to consider the two populations observed in the Kuiper belt, designated here as “Red” and “Very Red” as the parent bodies of the two Trojan populations (See Figure 1). In such a scenario, we hypothesize that the original volatile composition of these bodies, and their subsequent processing by irradiation and heating, determines the nature of the surfaces of these bodies today. We seek to find the combination of volatiles that would be expected on the surface for different formation scenarios, perform laboratory experiments by creating mixtures of these ices in the laboratory and processing them accordingly. These laboratory measurements include spectra that can be compared to telescopic observations providing a means for determining whether our hypothesis is feasible.
Figure 1. Cartoon representing the relative redness of Jupiter Trojans and KBOs.
At the heart of our hypothesis is the idea that the bimodal distribution of colors in the Jupiter Trojans as well as KBOs stems from surface differences resulting from chemical differences in the nonvolatile crusts that formed on their surfaces. The presence of a sharp dividing line between red and less red objects can be attributed to differences in their formation location, namely their position relative to the stability line of a key ice component that can cause reddening with environmental processing (irradiation, heating).
Figure 2. Sublimation lines in the early solar system as a function of formation distance. Our hypothesis is that the red Trojans would have originated from a distance somewhere on the right side of the H2S stability line and the less-red Trojans would have originated from a distance somewhere on the left side of the H2S stability line.
We begin with theoretical predictions of the relevant ice composition for objects as a function of radial distance from the Sun. The question is, for a Trojan sized object, what is the composition of ices that would have been stable long enough to form an irradiated crust (that still remains today in some form) as a function of distance? The method is as follows: All bodies begin with a volatile fraction similar to a notional comet. We assume that the first ~ 100 meters of material are in contact with the surface and can be lost at the surface temperature loss rate. Jeans escape and direct volatile escape are calculated, and the faster of the two is taken as the time required to deplete the surface of volatiles. If the irradiation crust can form before a given volatile is depleted, it can remain on the surface of the bodies in question today, affecting their spectral properties.
This work has generated predictions of stability of volatiles vs. radial distance of formation as illustrated in Figure 2. The end result is an ordering of volatiles from the longest to shortest retention time: H2O, CH3OH, HCN, NH3, CO2, H2S, C2H2, C2H6. While the assumptions made about irradiation time scales and surface layers may not be very precise, the ordering of the volatiles is the important parameter for this work.
Based on this work, we have identified H2S as the most likely candidate responsible for the differences between the red and less red Trojans and the very red and red KBOs. The only other candidate in the primordial disk would be the ethane line, which is not considered a viable hypothesis because ethane would not ne expected to cause appreciable changes in reddening. On the other hand, sulfur is a known reddening agent.
Figure 3. Representation of a hypothesized scenario where the presence and absence of H2S is responsible for the very red and less red KBOs after the formation of an irradiation crust. The two KBO populations are formed inside and outside of the H2S evaporation line with exposure and sublimation dictated by the formation distance, leading to the formation of red and very red nonvolatile crusts. When the large scale solar system disruption occurs, these two populations are scattered in to their current location, with one outcome being the two Trojan populations which become red and less red through processing during and after emplacement. At the same time, the current KBO source populations remain and retain their original colors.
We have devised a series of experiments to explore the hypothesis that the presence or absence of H2S is the cause of the difference between the very red and red KBOs. This is a reasonable hypothesis because sulfur is a known reddening agent, and the expected radial distance where H2S becomes unstable is in a reasonable location in the primordial disk for the formation of the two KBO populations on both sides of the line. A representation of the hypothesis is shown in Figure 3. The presence or absence of certain volatiles and their subsequent processing may represent the key to the red and less red populations observed today in both the Kuiper Belt and Trojan populations.
The ice experiments in this work are designed to provide experimental evidence to test this hypothesis. We begin our experiments with the ices that have the longest retention time, starting with water. Since water does not redden with irradiation, it is a control experiment. The next step is to add methanol and so on. As we irradiate, the hypothesis is that at some point the addition of one more ice to the mixture will cause a distinctive reddening, explaining the existence of the red and very red KBO populations.
Figure 4. Infrared spectra comparing three and four ice mixtures at 50 K before and after irradiation with 10 KeV electrons and an electron fluence of 2 x 1021 eV/cm2. The primary difference is the formation of OCS in the four ice mixture.
To date we have performed experiments with H2O, CH3OH, NH3, and H2S. To test the hypothesis that H2S is the reddening agent, we compare experimental results of three ice and 4 ice mixtures (i.e. with and without H2S). Chemical changes are evident between the three ice and four ice mixtures (see Figure 4 and 5). When H2S is included in the starting mixture, OCS forms after irradiation. This is the primary difference in chemistry observed between the 3 ice and 4 ice mixtures. Notably, a marked reddening occurs upon irradiation of the 4 ice mixture when compared with the 3 ice mixture, providing support of the H2S hypothesis (see Figure 6). Experiments are currently underway to characterize the organic residues that remain after irradiation and heating, in order to link sulfur related chemistry with spectral properties. From these non-volatile components we hope to gain further insight into the irradiation products on the surface of the Trojan asteroids today. Initial results show a rich sulfur dominated chemistry present in the residues made from sulfur containing ices that significantly alters spectral properties when compared to non-sulfur containing ices.
Figure 5. The evolution of sulfur containing species in ices during heating to Trojan temperatures showing the retention of OCS and an increase in CS and SO2. None of these features are present in the ice mixtures without sulfur, providing confirmation that they are sulfur-associated species.
Figure 6. Reflectance measurements of 3 ice (top) and 4 ice (bottom) mixtures at various stages in the irradiation experiment. The key feature to note is the marked reddening which occurs in the 4 ice mixture (containing H2S) after irradiation at 50K. When warming to 120K (simulating the current Trojan location) that reddening does not disappear, indicating that non-volatile products have formed that could be responsible for the reddening of the Trojan asteroids as observed today.
1. J.P. Emery, D.M. Burr, D.P. Cruikshank (2011) “Near-Infrared spectroscopy of Trojan asteroids: Evidence for two compositional groups”, Astron J 141, article id 25.
2. J.P. Emery, D.P. Cruikshank, J. Van Cleve (2006), “Thermal emission spectroscopy (5.2 – 38 μm) of three Trojan asteroids with the Spitzer Space Telescope: Detection of fine-grained silicates”, Icarus 182, 496-512.