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2.5 Studies of asteroids, impacts, and their effects on the development of biospheres and their planetary environs (Kyte, Newman, Lowe, Byerly)

	Figure 2.5.1. Cr isotope data for terrestrial rocks, carbonaceous chondrite meteorites, with 53 < 0, and K-T boundary, and Archean spherule beds (S2-S4). The spherule beds have isotopic compositions similar to the carbonaceous chondrites and are clearly distinct from terrestrial materials.

Members of the UCLA Center for Astrobiology are directly involved in exploring several areas of this important subject. This work involves direct studies of terrestrial impact events and critical events in Earth history, analytical studies of the evolution of planets and asteroids through time to explore the possibility of planetary forcing of clustered impacts, and modeling large impacts to understand their influence on the evolution of planetary volatile inventories (i.e., oceans and atmospheres).

2.5.1 Extraterrestrial impact history on Earth

This area of research is led by Frank Kyte, an established expert on the study of sediment deposits formed by large-body impacts and co-chair of the NAI Impact Focus Group. Kyte plans to engage in collaborative research with a several research groups to expand our understanding of the impact history on Earth and potential links to perturbations in the Earth's evolutional history. This research involves Kyte's expertise in characterizing an impact signature using chemical and mineralogical analyses, in collaboration with experts in diverse fields ranging from sedimentology to isotope geochemistry. Several of the collaborations are described briefly below.

Early Archean (3.5 to 3.2 Ga) impact deposits that were first reported by Lowe and Byerly (1986) in sedimentary rocks from the Barberton Greenstone Belt are now clearly established as derived from mega-impacts, based on their anomalous Cr-isotopic compositions (Figure 2.5.1, Kyte et al., 2003b). These rocks provide the earliest confirmed record of impacts on Earth, and provide a link to the Late Heavy Bombardment impacts that were recorded on the Moon. This research includes field and laboratory studies with UCLA lead-team members Lowe (Stanford) and G.R. Byerly (LSU), as detailed below.

The Triassic-Jurassic boundary is one of the "Big Five" mass extinctions in the Phanerozoic. Detailed studies of new stratigraphic sections have demonstrated a significant stable isotope shift (Ward et al., 2001) and a small Ir anomaly (Olsen et al., 2002), suggesting a possible impact link to these extinctions. We are engaging in analyses of new sections with P.D. Ward (University of Washington) and other workers, to further examine this record.

Figure 2.5.2. Clinopyroxene-bearing spherules from a late Eocene impact deposits in the western Indian Ocean (Ocean Drilling Program site 709). They are commonly believed to have come from the impact that formed the 100 km Popigai impact structure in Siberia. These spherules may have an isotopic signature that can be traced to a cometary source.

Multiple Late Eocene impacts might be caused by a comet shower, according to Farley et al. (1998) who reported an increased flux of ³He to sediments at that time. We are using chemical, mineralogical, and isotopic studies of these ejecta deposits to sort out the provenance (e.g., cometary, impact plume, interplanetary dust?) of various physical components at several sites around the world. This work should help to constrain the comet shower hypoothesis.
The late Pliocene impact of the Eltanin asteroid is the only known deep-ocean (5 km) impact (e.g., Gersonde et al., 1997) and is characterized by high-energy deposits with high concentrations of meteoritic materials (Kyte et al., 2002a-c).

A recent oceanographic expedition to the impact site recovered 17 new sediment cores with ejecta deposits. This unique deposit might be used to constrain models of impacts on early Earth that are proposed as a potential source of organic matter for the origin of life (Pierazzo and Chyba, 1999; Kyte et al., 2003a). Kyte and colleagues are also attempting to obtain precise ages on this impact to link it to the climate record at 2.4 Ma, a time of rapidly deteriorating climate.

2.5.2 Exploring the early Archean impact record and the consequences for early life

Over the next few years, the UCLA lead-team plans to investigate three main aspects of the Barberton Greenstone Belt (BGB) impacts:

Archean impact rates: The presence of at least four major impact layers, each probably marking the impact of bolides 20-50 km in diameter, suggests that there must have been many more impacts of lesser size. The presence of condensed sections of cherty sediments in the BGB, representing deposits that accumulated slowly over long intervals of time, offers the possibility of searching for physical and geochemical evidence of these smaller impacts and of more directly estimating the impact flux at 3.5-3.2 Ga. This in turn will help in assessing the possible role of impacts on biological evolution, environmental stability, and crustal development of the early Earth. To evaluate this flux we propose to provide additional geochronological constraints on critical sections within the BGB, especially within the upper Onverwacht, which is composed of over 100 million years of sedimentary and volcanic rock with no internal age dates. The Stanford-USGS SHRIMP will be used on several samples previously collected from the upper Onverwacht. Within the upper Onverwacht we will also collect closely spaced samples, and in places obtain continuous cores, from the condensed sedimentary sections. Preliminary analyses for chromium anomalies will be used to screen for possible impact layers. Subsequent analyses for iridium anomalies and possibly chromium isotopic anomalies will be used to distinguish chondritic impact materials from the very similar komatiitic volcanic materials.

The effects of the large impacts on surface environments. Beds S2 and S3 are traceable over long distances within a number of structural belts in the BGB, and we have already documented their deposition under a range of conditions from subaerial fan delta to shallow tidally influenced shelves to deep, quiet-water conditions (Lowe & Byerly, 1986; Lowe et al., 2003). In shallow-water and shelf environments, impact spherules have generally been extensively reworked by impact-produced tsunamis and, in some areas, subsequent environmental currents and waves. In deeper-water settings, the spherules accumulated as direct fall deposits By tracing the beds along individual outcrop belts and among the various structural belts, it will be possible to document the effects of the tsunamis and fall-deposition of enormous volumes of impact-produced debris on local environments and to determine how new environmental conditions that were established following the impacts differed from those prevailing before the impacts. The latter should provide key evidence regarding the longterm effects of such large impacts. We will also examine the sedimentary record immediately above these well documented impact layers for indications of major compositional anomalies that might be associated with impact-induced modifications of the atmosphere and shallow hydrosphere. S2 and S3, for instance, show evidence of rapid chemical sedimentation immediately following deposition of the impact layers.

The effects of impacts on putative biological materials and communities. In a number of localities, black organic cherts occur below and immediately above S2 and S3. The types, abundances, distribution, and isotopic composition of organic grains and mat-like layers (e.g., Walsh and Lowe, 1999) should provide clues about the changes in organic materials and perhaps the organisms that produced them across the impact layers. If the impacts depositing S2 and S3 were sufficiently large, they may have boiled away the surface layer of the oceans (Sleep and Zahnle, 1998). Such a profound environmental effect may be reflected in the character of organic matter across the impact horizons.

Schedule and Workplan: Lowe and Byerly will spend approximately 3 weeks each summer conducting field studies in the Barberton and other Kaapvaal greenstone belts, South Africa, and/or the greenstone belts of the Pilbara, Western Australia. One week each summer will be spent with analytical studies. Byerly will visit Stanford for one week each summer to do geochronology in the SHRIMP Lab. The initial two years will focus on Barberton, and subsequent years will be spent in the Pilbara looking for correlative impact layers.

2.5.3 Solar-System chaos and the frequency of asteroid impacts

Figure 2.5.3. Plot of asteroids (dots) as a function of semi-major axis distance from the Sun and inclination of orbits. The Hungarias is the group closest to Mars and most likely to be affected by changes in the motion of Mars. The n3, n4, and n6 are secular resonances in which an asteroids orbit processes at the same rate as the orbit of a major planet, resulting in very large perturbations of the asteroid’s orbit. As the rates of Earth’s and Mars’orbits change due to chaos, so too do the locations of the unstable orbits of the asteroids depicted by n3, n4, and n6.

Chaos in the motions of the inner planets of the Solar System causes episodic transitions in the way their orbits interact, most notably in the rates with which their point of closest approach to the Sun (perihelion) precesses in space. While chaos makes it difficult to reconstruct or predict these transitions far from the present, our state-of-the-art simulations indicate that a major transition took place about 65 Ma ago, coinciding approximately with the Chicxulub asteroidal impact (Kyte, 1998) that was responsible for the Cretaceous-Tertiary (K-T) mass extinction event. So the question is: Was chaos in the inner Solar System the ultimate cause of the extinction of the dinosaurs? Or, more generally, can chaotic disturbances of inner Solar System dynamics lead to an increased probability of a sizeable asteroid hitting the Earth?

These questions have to be considered in the general framework of asteroid orbital dynamics. The largest changes in the orbits of the major planets and asteroids are regular rather than chaotic. From time to time, the orbits of some asteroids (e.g., 1750 Eckert) are strongly perturbed by the major planets due to certain orbital alignments (see Williams and Hierath, 1987). Furthermore, some regions of the asteroid belt are chaotic independently of chaos in the motions of the major planets (Lecar et al. 2001). The result of large may be collision with another body, ejection from the Solar System, or scattering within the asteroid belt. Most of the current asteroids are the survivors of this constant perturbative erosion. However, chaotic transitions in the motions of the inner planets may change the locations of regions of strong erosion in orbital parameter space and, as a result, long-term survivors may become vulnerable.

In order to determine the magnitude of any chaos-induced increase in asteroid impact probability, Varadi and colleagues propose to carry out a variety of long-term numberical simulations. First, they will determine the types and ranges of possible chaotic transitions by simulating only the motions of the major planets. Next, they will add thousands of randomly placed hypothetical asteroids into the model in order to locate regions in the parameter space which enable them to survive. Finally, Varadi et al. will drive the survivors through regime transitions. One way to approach this problem in the shorter term (which we shall also use) is to take a set of the most vulnerable existing asteroids - those which are already or nearly planet crossing, such as 433 Eros and 1750 Eckert - and simulate their behavior both backwards in time through the observed major transition near the K-T boundary and through an equally sizeable transition that is due to occur about 30 Ma from now.

Varadi et al. use a specialized code to accurately reconstruct the orbital and rotational history of planets and asteroids for the past 100 million years. The numerical integration scheme is a version of the classical Stormer-Cowell integrator which has been optimized to reduce the long-term effects numerical round-off errors. In our ongoing project, the physical model is successively refined to take into account small corrections in the equations of motions due to General Relativity, the finite size of the lunar orbit etc. The UCLA lead team have obtained an improved analytical representation of the lunar orbit which is a significant step toward increased accuracy. The simulation results are used not only to investigate a possible connection between inner solar system orbital chaos and asteroid impacts, but also to couple the orbital and rotational dynamics of Mars in order to understand long-term changes in Martian climate (§2.6.1). The same code and others derived from it are and will be used to simulate the long-term orbital evolution of asteroids. Emphasis will be placed on dynamical effects that can push asteroids onto collisional orbits with Earth.

Activities in model and code development and also in Educational and Public Outreach will be combined through the creation of distributed, modular cross-platform software in the form of intelligent screen savers which can do background computations in a client-server setup. As opposed to serving only the needs of the UCLA projects, the screen savers will have the capability to carry out a number of tasks initiated and controled by the local user. Our team will provide the means for the general public to explore the difficult questions beyond pictures. Another important feature is that the software will be modular and configurable. It will have basic libraries and will be driven by scripts. This will offer a great deal more flexibility than single-task software. The server will be able to re-configure the client screen savers without the need of downloading new executable code. This will also make porting between platforms much simpler and the clients more secure.

2.5.4 Impacts and the evolution of atmospheres

Large vertical impacts (10 times K/T events and greater) may not erode planetary atmospheres (Newman et al., 1999). Here Newman and colleagues consider the energetics of large bolide impacts on atmosphere-free planets to determine whether such impacts will contribute to the planet's volatile inventory. Newman and colleagues have shown, for a wide range of impact speeds and for all but the most highly oblique impact angles, that volatile material will be added to the planet's inventory, especially if the planet’s lithosphere has been depleted in volatiles as we assume Earth was following the Giant Impact (Mischna and Newman, in prep). For Venus, the situation is much the same as well as for Mars, though for a much reduced range of impactor speeds. Volatile retention depends primarily on the planet's escape velocity and the bolide's impact speed and angle of entry.

Gravity plays a dominant role in these impacts. Following the Giant Impact, it is possible to acquire enough volatiles through impacts without requiring significant degassing of surface volatiles and mantle outgassing, both of which seem unlikely following the Giant Impact. The importance of gravity can be summarized in a very simple scenario: an impactor strikes a planet at 20 km/s and vaporizes approximately three times its own mass in target (i.e., surface) material (O’Keefe and Ahrens) (O'Keefe and Ahrens, 1977), hence the vapor cloud has four times the mass of the impactor. Excluding all other sinks of energy (i.e. the impact has perfect conversion of kinetic energy from bolide to vapor cloud), the mean speed of the cloud will be reduced to 10 km/s, less than Earth’s gravitational escape velocity. If we subsequently add more realistic sinks of energy (i.e., latent and sensible heat, seismic radiation, etc.), the expansion velocity of the cloud will decrease further, and more vapor will be retained. We are therefore calculating a strict lower bound on retained material. More inclusive representations of energy sinks will increase the amount of vapor retained.

We have created a simple, physical model derived from the self-similar model of vapor cloud density of Zel’dovich and Raizer (1966) for expansion of gas in a vacuum, the model used previously by Vickery and Melosh (1990). We also assume the amount of target material excavated by an oblique impact based on the findings of O’Keefe and Ahrens (1977) as well as Pierazzo and Melosh (2000). By considering both the expansion speed of the cloud as well as the bulk drift of the cloud due to lateral motion of the oblique impactor, we can solve for the fraction of the expanding cloud for which the total (vector) velocity is less than the escape velocity of the planet. We observed (Mischna and Newman, in prep) that most impact events on airless worlds provide substantial inventories of volatiles, thereby producing the atmospheres and hydrospheres necessary for life.

We intend to perform first-principles hydrodynamic simulations of oblique impacts for a variety of impact speeds (i.e., solar system positions of origin), target planet, impact angles, crustal volatile content (ranging from ocean-impacts to dry, Venus-like lithospheres). The simulations will be performed in collaboration with Los Alamos collaborator Eugene Symbalisty using LANL and Sandia codes, some of which have now been ported to desktop workstations, to develop more accurate quantitative estimates of the efficiency of the volatile retention process. Finally, we will consider how these results can be applied to extrasolar planetary systems.

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