Contributor: Joshua Emery (NASA-Ames), F. Marchis (UC-Berkeley) 3.2.3.1 Scientific Background
Asteroids constitute the debris left over from the formation of the Solar System. Because of their small to moderate sizes (as compared to the planets), they have generally not undergone any late- stage endogenic alteration. Their surfaces therefore still sport the scars of early and late-stage collisional evolution and early-stage geologic processes, along with other ongoing exogenic surface processes (i.e. space weathering). Adaptive optics observations of asteroids can play a key role in revealing what this debris has to show us about the formation and evolution of the Solar System.
This section first discusses three specific areas of asteroid research that can be addressed by disk- resolved observations. This short list is not meant to be exhaustive; many additional applications of improved AO to asteroid science could be included and will undoubtedly be pursued as more scientists consider the possibilities. The section ends with an overview of the improvement offered by NGAO in terms of increased number of asteroids that will be resolved.
3.2.3.1.1 Collisional Evolution of the Asteroid Belt
Imaging of asteroids with improved spatial resolution can significantly impact the understanding of the accretional and collisional evolution of the Solar System. The presently observed properties of the Main Belt depend on many factors, including the initial conditions (e.g., total initial mass in Main Belt, compositional distribution of this mass, timing of Jupiter’s formation) and evolution processes (e.g., collisional and fragmentation laws, migration of giant planets, degree of mixing).
These are complex processes that are being modeled with ever increasing sophistication, but require observational constraints. Fortunately, the asteroids themselves, when properly observed, provide the many clues that are necessary to unravel the different factors. As stated by Bottke et al. (2005b), “Like archaeologists working to translate stone carvings left behind by ancient civilizations, the collisional and dynamical clues left behind in or derived from the Main Belt, once properly interpreted, can be used to read the history of the inner Solar System.”
One strong constraint would be the asteroid cratering record, particularly the occurrence of large craters on large asteroids. For example, imaging by HST with a spatial resolution of ~36 km/pixel has revealed a large impact basin (~460 km diameter) at the south pole of the basaltic (differentiated) asteroid 4 Vesta, which itself has a diameter of ~560 km (Thomas et al. 1997).
The existence of this single large impact basing on Vesta has already been used as a primary constraint in multiple collisional evolution models (e.g., Bottke et al. 2005a, O’Brien and Greenberg 2005). The argument used is that large collisions should be frequent enough that the impact on Vesta is not too unlikely, but not so frequent that many large impacts should have occurred. While this is insightful use of recent observational data, one must always be wary of statistics drawn from a sample size of one. Vesta’s surface could potentially be a statistical outlier, in which case extending its properties to the entire asteroid belt would be an astronomical red herring.
Spatially resolved imaging of other large asteroids is critical in order to place the results for Vesta into context and to derive truly reliable statistical constraints on large collisions throughout the Main Belt. Observations of the 15 or 20 largest asteroids would provide the statistics necessary to put much stronger constraints on the frequency of these large collisions. We estimate that 20 Main Belt asteroids will be resolved with sufficient resolution with NGAO in R-band (33 in V-band) for mapping comparable to that done previously for 4 Vesta. This compares with only one (Ceres) that is available from the current Keck AO (K-band). The criterion for these results is that the fractional resolution (spatial resolution divided by diameter) be equal to or smaller than for the HST observations of Vesta (36km/560km = 0.065). The NGAO resolution in R-band on Vesta is
~11 km, an improvement of more than a factor of three over the HST data. The largest part of the improvement is the extension of high Strehl diffraction limited performance to shorter wavelengths. Comparing imaging results for large asteroids of different taxonomic types (and therefore presumably different compositions) will also reveal information about how surface structure and strength varies among asteroids (e.g. O’Brien et al. 2006).
3.2.3.1.2 Size Distribution
The size distribution of the Main Belt as a whole and of various sub-populations is a major property that must be properly explained by any model. The initial size distribution of the Main Belt was set by the accretion process – the number of objects of a given size that grew during that stage. Collisional and dynamical erosion since then have left their marks as well, altering the initial distribution. The size distributions of other populations likewise depend on their formation and evolutionary environment. The distributions within asteroid families are initially set by fragmentation laws, which are themselves uncertain and vary for different compositions. The size distribution of near-Earth objects is set by the delivery mechanism from the Main Belt, which is very likely size dependent.
Without accurate knowledge of the sizes of asteroids, it is impossible to decode the information contained in the size distributions. Visible, disk-integrated photometry is not able to determine the size of an object, only the brightness – the size and albedo cannot be unraveled without additional information. Direct imaging is the most straightforward means of size determination. Other methods, such as radiometry – in which the thermal emission is measured at the same time as visible reflected flux – depend on a large number of parameters that are generally poorly known.
The radiometric method in particular was used to derive the sizes of a large number of Main Belt asteroids, but it first had to be calibrated because of uncertainties in several effects, including thermal inertia, thermal-IR phase functions, and “beaming” (due to surface roughness) (Lebofsky et al. 1989).
The calibration used for large, Main Belt asteroids has been shown to be inappropriate for smaller objects, and especially for near-Earth objects, which are often observed at high phase angles (Walker 2003, Delbo et al. 2003, Wolters et al. 2005). The most straightforward approach would be a large, direct imaging campaign of thousands of asteroids. This is probably not feasible on the Keck telescopes because of the time involved, but NGAO will provide the capability to directly measure sizes for a significant sub-sample that spans the range of sizes, compositions, shapes, orbital classes, dynamical families, and viewing geometries. These observations can then anchor the distributions of each subgroup, recalibrating the results of other methods to make them more reliable. With NGAO in R-band, there would be 1193 observable objects to choose from (Table 2).
We estimate that ~300 directly imaged asteroids, if well chosen, would be adequate to provide such an anchor. Marchis et al. (2006) initiated such survey with the Keck NGS AO and observed 30 asteroids over a few half-nights. Considering an overhead of ~20 min per object and an integrations time of 5-15 min per object, such ambitious program could be completed in 12 nights.
Well-calibrated size distributions of asteroid families will in turn allow the investigation of the physics of disruption and fragmentation, which is a key uncertainty in evolutionary models. The same is true for a properly anchored size distribution of near-Earth objects. In fact, there are currently very few NEOs with known sizes. This also presents a problem for hazard mitigation
(i.e., detecting and stopping potentially devastating impactors) since the number of objects in near- Earth space that could cause regional catastrophes is currently unknown.
3.2.3.1.3 Geologic Properties and Surface Heterogeneity
The largest asteroids have been, and possible still could be, geologically active bodies in their own right. It appears that some large asteroids differentiated – Vesta has a basaltic crust and the M- type asteroids are thought to be remnant cores of disrupted, differentiated asteroids – but many others did not. These differences are still unexplained. Some hypotheses pose that volatile content was an important inhibitor of differentiation, others point to the change in silicate mineralogy with heliocentric distance, and still others suggest that the heat source (e.g., radioisotopes or induction heating) was somehow not uniformly distributed among asteroids. Direct observation of large asteroids, both differentiated and not, is the best approach to understand this current conundrum.
Imaging can directly discover surface heterogeneities in the form of albedo variations across the surface. These can be strong clues to different geologic units (e.g., lava flows on Vesta, carbonate/organic/water/clay deposits on Ceres). Detailed shape analysis can also provide information the internal composition and structure. As an example, the very nearly spherical shape of Ceres as determined by HST imaging has been used to infer that it is actually a differentiated icy object, with an H2O mantle surrounding a rocky core (Thomas et al. 2005). The non- homogeneous shape of Vesta, on the other hand, reflects the different rheologies (Thomas et al.
1997, 2005). Accretion and later collisional evolution were not uniform across the inner Solar System, as generally modeled, but were affected by the different materials present at different distances from the Sun. NGAO imaging will allow an investigation of the results of these differences through shape as well as albedo mapping.
Disk-resolved spectroscopy is another powerful means of mapping geology. The extension of NGAO to shorter wavelengths will allow complete characterization of the important 1-m silicate band, permitting the mapping of detailed silicate mineralogy on individual surfaces. A water of hydration band at ~0.7 m can also be mapped to help understand the effects of water on individual asteroids (i.e., were isolated areas altered, perhaps by impacts, or entire asteroids or groups of asteroids by amore wide-spread event?). Additionally, there is recent spectral evidence for silicates on the surface of some M-type (presumably metallic) asteroids. Are these asteroids not metallic, or are they metallic with a silicate covering, perhaps remnant mantle material? Such a remnant mantle might provide only partial coverage, and could therefore be mapped by NGAO disk-resolved spectroscopy.
3.2.3.1.4 Improvements in Number of Resolvable Asteroids by NGAO
Table 4 summarizes the number of asteroids resolvable from visible to near-IR domain and per population (see Appendix. Number of Observable Asteroids for more details). Thanks to the high angular resolution provided in V and R bands, ~800 main-belt asteroids could be resolved and
have their shape estimate with a precision of less than 7%. With current AO system ~100 asteroids, located only in the main-belt, can be resolved. The determination of the size and shape of Trojan asteroids, even if limited to a few of them, will be useful to estimate their albedo. For NEAs, the large number of resolvable objects is a result of very close approaches to Earth. Many of these are unnumbered, and so refined orbits may bring them not nearly as close.
Table 4 Number of asteroids resolvable with Keck NGAO in various wavelength ranges and per population.
Unnumbered asteroids (most of the NEAs) have poorly known orbits.
Resolvable asteroids in each band (numbered and unnumbered)
Orbital type V R I J H K
Near Earth 526 460 376 269 204 152
Main Belt 855 716 526 319 194 100
Trojan 13 11 5 0 0 0
Centaur 1 1 1 0 0 0
TNO 3 3 3 3 3 3
Other 4 2 1 0 0 0
3.2.3.2 References
Bottke, W.F., D.D. Durda, D. Nesvorny et al. 2005a. The fossilized size distribution of the main asteroid belt. Icarus 175, 111-140.
Bottke, W.F., D.D. Durda, D. Nesvorny et al. 2005b. Linking the collisional history of the main asteroid belt to its dynamical excitation and depletion. Icarus 179, 63-94.
Delbo, M., Harris, A.W., Binzel, R.P., Pravec, P., Davies, J.K. 2003. Keck observations of near- Earth asteroids in the thermal infrared. Icarus 166, 116-130.
Lebofsky, L.A. and Spencer, J.R. 1989. Radiometry and thermal modeling of asteroids. In
Asteroids II (R.P. Binzel, T. Gehrels, and M.S. Matthews, Eds.), pp. 128-147, Univ. Ariz.
Press, Tucson.
Marchis, F. Kaasalainen, M., Hom, E.F.Y., et al. 2006. Size, Shape, and multiplicity of main-belt asteroids. I. Keck Adaptive Optics Survey, submitted to Icarus.
O’Brien, D.P., and R. Greenberg 2005. The collisional and dynamical evolution of the main belt and NEA size distributions. Icarus 178, 434-449.
O’Brien, D.P., R. Greenberg, J.E. Richardson 2006. Craters on asteroids: Reconciling diverse impact records with a common impacting population. Icarus in press (available online).
Thomas, P.C., R. P. Binzel, M.J. Gaffey, et al. 1997. Impact excavation on asteroid 4 Vesta:
Hubble Space Telescope results. Science 277, 1492-1495.
Thomas, P.C., J.Wm. Parker, L.A. McFadden, et al. 2005. Differentiation of the asteroid Ceres as revealed by its shape. Nature 437, 224-226.
Walker, R.G. 2003. IRAS diameters and albedos revisited. DPS 35, abstract #34.19.
Wolters, S.D., Green, S.F., McBride, N., Davies, J.K. 2005. Optical and thermal infrared observations of six near-Earth asteroids in 2002. Icarus 175, 92-110.