Interiors of Small Bodies: Foundations and Perspectives Richard P

Home , 1998 KY26, Castalia (spacecraft), Rubble pile

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Planetary and Space Science 51 (2003) 443–454 www.elsevier.com/locate/pss

Interiors of small bodies: foundations and perspectives Richard P. Binzela;∗, Michael A’Hearnb, Erik Asphaugc, M. Antonella Baruccid, Michael Beltone, Willy Benzf , Alberto Cellinog, Michel C. Festouh, Marcello Fulchignonid, Alan W. Harrisi, Alessandro Rossij, Maria T. Zubera

aDepartment of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 01239, USA bAstronomy Program, University of Maryland, College Park, MD 20742, USA cEarth Sciences Department, University of California, Santa Cruz, CA 95064, USA dLESIA, Observatoire de Paris a Meudon, 92195 Meudon, Cedex, France eBelton Space Exploration Initiatives, Tucson, AZ 85716, USA f Physikalisches Institut, University of Bern, Bern, Switzerland gINAF-Osservatorio Astronomico di Torino, Pino Torinese 10025, Italy hObservatoire Midi-PyrenÃ ees,Ã Toulouse, France iSpace Science Institute, 4603 Orange Knoll, La Canada, CA 91011, USA jISTI-CNR, CNR - Area della Ricerca di Pisa, Via Moruzzi 1, Pisa 56124, Italy Received 29 November 2002; accepted 18 March 2003

Abstract

With the surface properties and shapes of solar system small bodies (comets and asteroids) now being routinely revealed by spacecraft and Earth-based radar, understanding their interior structure represents the next frontier in our exploration of these worlds. Principal unknowns include the complex interactions between material strength and gravity in environments that are dominated by collisions and thermal processes. Our purpose for this review is to use our current knowledge of small body interiors as a foundation to deÿne the science questions which motivate their continued study: In which bodies do “planetary” processes occur? Which bodies are “accretion survivors”, i.e., bodies whose current form and internal structure are not substantially altered from the time of formation? At what characteristic sizes are we most likely to ÿnd “rubble-piles”, i.e., substantially fractured (but not reorganized) interiors, and intact monolith-like bodies? From afar, precise determinations of newly discovered satellite orbits provide the best prospect for yielding masses from which densities may be inferred for a diverse range of near-Earth, main-belt, Trojan, and Kuiper belt objects. Through digital modeling of collision outcomes, bodies that are the most thoroughly fractured (and weak in the sense of having almost zero tensile strength) may be the strongest in the sense of being able to survive against disruptive collisions. Thoroughly fractured bodies may be found at almost any size, and because of their apparent resistance to disruptive collisions, may be the most commonly found interior state for small bodies in the solar system today. Advances in the precise tracking of spacecraft are giving promise to high-order measurements of the gravity ÿelds determined by rendezvous missions. Solving these gravity ÿelds for uniquely revealing internal structure requires active experiments, a major new direction for technological advancement in the coming decade. We note the motivation for understanding the interior properties of small bodies is both scientiÿc and pragmatic, as such knowledge is also essential for considering impact mitigation. ? 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Asteroids; Comets; Interiors

1. Introduction Spacecraft have provided resolved images of 1P/Halley, 951 Gaspra, 243 Ida, 253 Mathilde, 433 Eros, and 19P/Borrelly Asteroids and comet nuclei (which we collectively re- while radar observations are allowing shape modeling for fer to as small bodies) are emerging from the domain of dozens of small near-Earth objects (NEOs) and main-belt astronomers into the realm of geology and geophysics. asteroids. Thus, these bodies are no longer just “star-like” or dust/gas obscured points of light viewed through a tele- scope. They are becoming individual worlds that beg for de- ∗ Corresponding author. Tel.: +1-617-253-6486; fax: +1-617-253- 2886. tailed exploration, explanation, and geological/geophysical E-mail address: [email protected] (R.P. Binzel). understanding.

As our knowledge base and understanding of shapes and are even more dominated by an abundance of joints and surface characteristics advance, we are naturally drawn to cracks. The relative tensile strength (deÿned as the eIec- “deeper” questions about the nature of the interior. Explor- tive tensile strength divided by the tensile of its constituent ing and understanding the interior properties of small bod- material) may approach zero. As in the case of fractured ies is challenging from a classical geophysics perspective bodies, the original structure may remain mostly in place. because so many diIerent formation and evolutionary pro- Bodies that are shattered with rotated components have cesses are at work. For example, objects that we consider been thoroughly fractured (by collisions or tidal stresses) to be “asteroid-like” formed in the more dense and hotter such that original units within the interior have been some- regions of the solar nebula inside the orbits of the newly what displaced and reoriented. Rubble piles are bodies that forming giant planets. More “comet-like” small bodies have been completely shattered and reassembled, where the acquired increasing volatile content as they formed in the new structure may be completely disorganized relative to region of the giant planets themselves and even far beyond. the original. If the units somehow become attached or ce- Superimposed on this diversity of formation environments mented to one another, the resulting structure is a coherent and compositions is the relentless role of collisions. Here rubble-pile. (More generally, Richardson et al. denote any again a diversity of outcomes is likely the rule. In the aster- aggregate structure that undergoes any re-attachment or ce- oid belt collisional outcomes may be the single most dom- menting as a coherent aggregate.) A “thermally modiÿed inant factor in controlling present day interior structures. primitive aggregate” might also increase or decrease in bulk At further heliocentric distances, collisions can dictate not porosity and in eIective strength. An increase in the strength only the interior structure but can also modify the original of contact between units could be described as a “lithiÿed pristine state of the volatile content. primitive aggregate.” The challenges and the complexities of understanding small body interiors relative to the vacuum of our knowledge is what draws us to their scientiÿc study. We also recognize 2. Science questions that the long-term survival of human civilization may also depend on our having suJcient knowledge of how to miti- For small bodies, the answers to fundamental science gate a potential impact. In these early stages, learning about questions can be greatly dependent on factors such as: size, these objects is the same thing as doing something about composition, presumed formation location, homogeneity them. Herein, we focus on the scientiÿc motivation for their of the original body (in terms of both composition and study and organize our review by posing a set of fundamen- structure), thermal history, collisional history, and dynami- tal science questions to serve as a basis for assessing the cal (orbital) history. Here, we frame our science questions in current foundation and limits of our knowledge as well as a sequence that principally addresses objects from larger to to identify prospects for advancement. smaller sizes, but in reviewing current answers and providing perspective we attempt to take into account all of these factors. Fundamentally we wish to understand in what cases 1.1. Terminology “primitive aggregates” may still be found where the transi- tions may reside between bodies that have been and have Alas, in any emerging ÿeld there is often a proliferation not been aIected by “planetary processes.” We also wish to of terms that are used to describe similar phenomena or know how to distinguish between those objects, unaIected structures. In an eIort to promote convergence of terminol- by planetary processes, which retain primitive properties in ogy, we adopt (and extend, with new terms in quotations) their interior, i.e., “accretion survivors,” and those that are the lexicon proposed by Richardson et al. (2002) to de- hopelessly modiÿed by 4.5 billion years of collisional and scribe the full range of possible interior states. Monoliths thermal evolution. In order to achieve this, we take it as a are essentially wholly intact units having a strength that is given that the modiÿcation processes and their eIects must eIectively equal to the tensile strength (maximum force per be well understood and therefore intensively studied. unit area that a body can withstand before fracture or rup- ture occurs) of its constituent material. Bodies that are not 2.1. At what size scales do “planetary processes” shape monoliths are referred to as aggregates whose categoriza- internal structure? tion is described in terms of decreasing coherence (increasing number of boundaries or interfaces between units) and Loosely, we deÿne “planetary processes” as those that al- increasing bulk porosity. A “primitive aggregate” describes low some degree of internal diIerentiation which may not a body for which the lack of connection between diIerent necessarily culminate in core formation. Within the outer units is the result of its primordial formation process, rather solar system, it remains only speculation on the degree than the result of fractures propagated by collisions. Frac- to which the largest trans-Neptunian objects (TNOs) may tured bodies have a suJcient number of cracks or faults that have undergone some diIerentiation. Pluto–Charon have the their tensile strength is reduced, yet their original structure most precisely determined diameters and densities (2350 ± remains intact. Shattered bodies have interior structures that 50 km, 1250 ± 50 km, 1:99 ± 0:07, and 1:66 ± 0:15, respec- R.P. Binzel et al. / Planetary and Space Science 51 (2003) 443–454 445 tively; Tholen and Buie, 1997) and models for their interiors LINEAR (C/1999 S4) into a power-law distribution of frag- (McKinnon et al., 1997) suggest some diIerentiation into ments (cumulative slope 1.74; MNakinen et al., 2001) prior a rock/ice core. The possible diIerentiation of Pluto (and to perihelion. Charon) may also be the result of the formation and tidal Thermal stresses may also aIect asteroids, but in a dif- evolution of this binary system. We note with excitement ferent way. Besides the reradiation forces (Yarkovsky and that Pluto–Charon are not unique as binary objects in the YORP) which provide gentle non-gravitational thrusts and outer solar system (e.g., 1998 WW31; Veillet et al., 2002). spins to asteroids, as discussed (above/below) thermal ex- The upper size limit for which volatile-rich objects have pansion was postulated by Dombard and Freed (2002) as likely undergone any substantial diIerentiation remains un- a mechanism for the production of pervasive fractures on constrained. (Kissel et al., 1986) did not ÿnd any evidence Eros, as it moved from its place of origin in the main belt of anomalous 26Mg in comet 1P/Halley, which may sim- into near-Earth space. ply indicate that comets formed after all the initial 26Al had For bodies formed in the transition region between disappeared. Measurements of the ortho- to para-hydrogen Mars and Jupiter (classically the asteroid belt), our in- ratio in water lines and other hydrogenated species suggest sights into diIerentiation processes are substantially more that comets have been preserved at low temperatures, con- enhanced through the availability of meteorite sam- sistent with the analysis by Lewis (1971) that shows that ples. Cooling rates from iron meteorites generally sug- objects smaller than 300 km should have internal tempera- gest bodies in the diameter range of about 100 km and tures of less than 25 K. The presence of the S2 molecule is larger (though some rapid cooling rates suggest sizes also considered as a clue that comet nuclei have been ther- down to ∼ 20 km) were capable of undergoing sub- mally preserved at very low temperatures. stantial diIerentiation in the early solar system (Haack Apart from primordial heating that may be experienced et al., 1990; Mittlefehldt et al., 1998). Vesta, with a 500 km by volatile-rich bodies, substantial thermal evolution (and diameter and basaltic (igneous) crust (McCord et al., 1970), consequently internal evolution) can occur as a result of appears to be the only “intact” surviving diIerentiated par- orbital evolution. The outcomes can be as fundamentally ent body within the asteroid region. Spectroscopic colors, important as the classical case of planetary diIerentiation. albedos, and radar reQectivities (GaIey et al., 1989; Ostro For example, a body dynamically evolving from the Kuiper et al., 2000) are all consistent with objects such as 16 Psy- belt into an orbit typical for a Jupiter-family comet experi- che and 216 Kleopatra being 100–200 km remnant iron ences both a seasonal thermal wave due to each perihelion cores of subsequently demolished parent bodies. passage, which ultimately couples to a secular increase in Not all large asteroids appear to have undergone sub- the interior temperatures. The details of this process are very stantial diIerentiation, with the nearly 1000 km diameter sensitive to unknown parameters of cometary nuclei, but Ceres displaying spectral and albedo characteristics consis- representative calculations suggest that the seasonal thermal tent with a relatively primitive (unheated) class of carbona- wave propagates tens of meters below the surface (e.g., com- ceous chondrite meteorites (GaIey et al., 1989). Ceres and pare Podolak and Prialnik, 1996; Benkhof and Boice, 1996 other similarly common “C-class” objects in the outer re- for the case of P/Wirtanen’s orbit). These are the depths to gions of the asteroid belt may have accreted slowly enough which gaseous transport is expected to take place assuming for radiogenic heat (principally from 26Al) to have been ad- appropriate porosity. Below this depth, temperature gradu- equately dissipated without diIerentiation. Particularly im- ally increases to come into equilibrium with the tempera- portant for Ceres may have been the increasing abundance ture expected for a given semi-major axis but thermal con- of volatiles with increasing heliocentric distance. These ad- ductivity in these layers is slower and the time scale might ditional volatiles may have been essential for quenching well be longer than the time scale for dynamical evolution. (through the heat of fusion for water ice) the diIerenti- This thermal evolution can lead to diIerentiation of ices, ation process within Ceres (Grimm and McSween, 1989, phase changes in the ices (most notably a phase change 1993). Certainly if accretion time scales and/or quenching from amorphous to crystalline ice at temperatures around proved suJcient to arrest diIerentiation within the outer 130–150 K), and formation of mantles due to the release of regions of the asteroid zone, “single” (non-binary) Tro- volatiles (Weissman 1990). jan asteroids and TNOs also would be expected not to be We also point out that thermal stresses can lead to the diIerentiated. loss of the body. Weissman (1980) found that 10% of dynamically new comets from the Oort cloud split, while 4% of returning comets and 1% of short period comets split 2.2. Which bodies are “accretion survivors”, i.e., bodies during any given encounter. These breakups tend to oc- whose current form and internal structure are not cur long before perihelion, so there is no known mecha- substantially altered from the time of formation? nism for explaining these random splitting events. Samaras- inha (1999) has proposed expanding volatiles, propagating Here, we seek to understand which individual bodies or from the sun-warmed exterior to the interior of a coarsely classes of bodies are observable today that provide actual porous comet, in order to explain the disruption of comet windows back to the shapes and internal structures created 446 R.P. Binzel et al. / Planetary and Space Science 51 (2003) 443–454 in the early solar system. In other words, are there currently (C/1999 S4) is suggested by the breakup of this comet far observable remnants of the ÿnal outcomes of accretion? from any massive body. The presence of fragments of sizes As we discuss in the following section, it seems likely of order 50–100 m in diameter indicates that this body en- that virtually all asteroids have been thoroughly shattered by tered the inner solar system as a surviving accretion aggre- collisions over the age of the solar system. Intriguingly and gate. perhaps counter intuitively, collisions that severely fracture Thermal evolution of comets, primarily as a result of the interior (without causing total disruption) may be essen- perihelion passages, complicate the inferences we may tial for a body’s long-term survival. While Ceres, by virtue make on their formation process and interior structure from of its size and Vesta by virtue of its thermal evolution may remote-sensing observations (e.g., Keller et al., 1986; Bell be best termed as surviving “protoplanets”, asteroids in the et al., 2000) or direct impact experiments (A’Hearn, 1999). intermediate size range (a few hundred km) are most likely The thermal skin depth is related to the rotation period of shattered survivors from the time of their accretion. Evi- the nucleus, its thermal conductivity and its density. For dence abounds that such survival is stochastic, as evidenced pure water ice, the skin depth is only 20 cm and is probably by the widespread number of asteroid families that are al- lower in reality given the presence of an insulating layer of most certainly the remnants of catastrophically disruptive porous dusty material and not ice. The presence of a mantle collisions of parent bodies having originals sizes of less than probably makes the comet stronger against collisional dis- 100 km to greater than 200 km (Hirayama, 1918; Zappala ruption perhaps by providing an additional layer structure et al., 2002; Tanga et al., 1999). to render even more ineJcient the propagation of impact Trojan asteroids at the L4 and L5 Lagrange points of shock waves. Jupiter have long been suspected of being primordial remnants retaining structures from the time of their formation 2.3. At what characteristic sizes are we most likely because of their dynamical isolation and their inferred (from to ÿnd: rubble-piles, substantially fractured (but not extreme lightcurve variations) elongated or possible con- reorganized) interiors, and intact monolith-like bodies? tact binary shapes (Cook, 1971; Hartmann and Cruikshank, 1978; Cellino et al., 1985). Intuitively, the “binary argu- Understanding the complex interplay between strength ment” is that a large pair (as opposed to “just” a primary and gravity is the basis for answering this question, where with a small satellite) is easiest and most likely to form in the answers are not necessarily intuitive for inhabitants of a an early accretion period when relative encounter velocities gravity-dominated planet. In some ways, a small body may are low. Any substantial amount of subsequent collisional be thought of as a planet having a lithosphere extending into evolution is likely to disrupt or dislodge the binary pair, its deep interior. At present, we are generally left to infer making binaries a telltale signature of survivors from the the nature of this lithosphere through measurements of the accretion epoch. If this logic holds, then the discovery by bulk density, through observations of collisional outcomes direct imaging showing the L4 object 617 Patroclus to be (both in the laboratory and asteroid families), through obser- a ∼ 100 km binary pair (Merline et al., 2001) supports the vations of the consequences of tidal, rotational, or thermal presumption of the largest Trojans being accretion survivors. stresses (cometary breakups), and through analytical and This survivor size range inferred from binaries is consistent numerical simulations. The current wisdom on the overall with Trojan collisional evolution and population size distri- collisional evolution of the asteroid belt (e.g., Davis et al., bution models by Marzari et al. (1997), who estimate that 1989, 1994, 1999, 2002) is that all sizable bodies are thor- this transition occurs around 50–100 km. oughly shattered (Melosh and Ryan, 1997). What eludes us Perhaps of greatest interest to planetesimal studies is iden- is an understanding of the arrangement or coherency of these tifying the smallest objects that may be accretion survivors. interiors. Whether TNOs are shattered or not is less clear. While we are observationally challenged to resolve or even Although collisions are as frequent in the inner Kuiper belt detect km-sized bodies in the Trojan region, Kuiper Belt, as in the asteroid belt, they generally occur at signiÿcantly and Oort cloud, comets in the inner solar system are in lower velocities and thus are more likely to have their eIects fact small-sized outer solar system representatives that are conÿned to the near-surface layers. Since the internal struc- close enough for study. Their inferred strengths, or lack ture is also unknown, the threshold energy for “shattering” thereof, within their internal structure provides the clearest throughout is also unknown. Davis and Farinella (2001) ar- evidence for most comets being well described as accre- gue that “many” of the TNOs are shattered. tion survivors having a “rubble-pile” structure (Weissman, Bulk density measurements require a determination for 1986). From its tidal encounter with Jupiter, prior to its both the mass and the volume, parameters that have been his- plunging demise, the separation of the components within torically diJcult to determine. Classically, asteroid masses comet D/Shoemaker-Levy-9 has given the basis for esti- have been determined by their interactions with one another mating not only its internal strength (as the maximal tidal (e.g., Schubart and Matson, 1979; Viateau, 2000) while stress at periapse was no greater than 1000 bars), but also their volumes are inferred from direct measurements and its density and probable structure (Asphaug and Benz, 1994, from modeling of their albedos and diameters (e.g., Tedesco 1996). Similarly, a low interior strength for comet LINEAR et al., 1992). Apart from using spacecraft in Qyby mode to R.P. Binzel et al. / Planetary and Space Science 51 (2003) 443–454 447 simultaneously measure mass and volume (e.g., Yeomans Britt et al. (2002) ÿnd their solutions distinguishable into et al., 1997), perturbations of Mars as measured by the three rough groups. The ÿrst group, having eIectively zero Viking mission have yielded mass estimates for the largest macroporosity is comprised by the three largest asteroids: asteroids (Standish and Hellings, 1989). In the future, the Ceres, Vesta, and Pallas. With diameters near 1000, 500, planned GAIA astrometric mission is expected to provide and 500 km, respectively, these bodies appear to have suf- masses for ∼100 main-belt asteroids based on astrometric ÿcient self-gravity to eliminate any signiÿcant macroporos- measurements to detect mutual perturbations with unprece- ity. Their inferred nature as protoplanets surviving over the dented precision. An alternative approach for density estima- age of the solar system certainly implies they have experi- tion is based on knowledge of shape and rotational periods enced substantial collisions such that their interiors are ei- for bodies which can be assumed to have equilibrium shapes ther fractured or shattered (in the lexicon of Richardson et (Farinella et al., 1981, 1982). The most obvious candidates al.). However, the lack of macroporosity and the preserva- are the large bodies exhibiting short rotation periods and tion of Vesta’s basaltic crust imply that these largest bodies large lightcurve amplitudes, for which an overall rubble-pile are not rubble-piles. structure (coherent aggregate) has been long proposed. Also A second group of objects has macroporosities ranging in this case, the observational challenge is to measure with from 15% to 25% over a diameter range from about 30 to high accuracy the overall shapes of these objects, and this 300 km. Eros, the most thoroughly studied small body to can be best done currently by taking advantage of the ad- date, falls in the middle of this range (macroporosity ∼ 20%; vancement in high-resolution techniques such as adaptive Wilkison et al., 2002) and thereby provides the best oppor- optics and speckle interferometry. tunity for insight. All indications are that Eros is a heavily Apart from the above possibilities, the existence and de- fractured or shattered body, but not one that was previously tectability of small body satellites (e.g. Belton et al., 1995; disrupted and reaccumulated as a rubble pile. Eros appears Merline et al., 1999, 2001; Margot et al., 2002; Veillet et al., to have a highly homogeneous structure because the oI- 2002; see Weidenschilling et al., 1989 for a summary of pos- set between the center of mass from the gravity and shape sible earlier detections) represents the breakthrough with the models only amounts to 30–50 m. (Miller et al., 2002; Zuber greatest new potential for determining masses and estimating et al., 2000; Thomas et al., 2002). Even with its irregular bulk densities for a wide diversity of objects. Mass, volume, shape (diameter dimensions ranging from 9 to 32 km) and and density determinations for comets are extremely prob- relatively fast 5:27 h spin period, Eros’ interior is not in ten- lematic with density estimates for 1P/Halley falling within sion due to rotation or precession wobble as the spin axis the range 0.2–1:5gcm−3 (Sagdeev et al., 1988). Comet and principal body axis are aligned within 0:02◦ (Miller density estimates based on observed non-gravitational accel- et al., 2002). Even though the interior is likely heavily frac- eration (deduced from successive apparitions) and estimates tured or shattered, surface expressions of structural features of the reaction force from outgassing have been calculated (ridges, grooves, etc.) demonstrate that still Eros retains by several authors but are sensitive to model assumptions. some signiÿcant degree of tensile strength (Prockter et al. Perhaps, the best constraints on those assumptions are for 2002). Other evidence for structural coherence of Eros in- 19P/Borrelly (Soderblom et al., 2002) for which Farnham cludes its high mean density, twisted platform, clustered re- and Cochran (2002) (Icarus, in press) deduce a density be- gions of surface slopes above the angle of repose, subdued tween 0.3 and 0:8gcm−3. or missing crater rims, and the continuity of long grooves Inferring something about the interior structure of small and fractures (Zuber et al., 2000). bodies requires an additional step of being able to esti- A third group of objects, ranging in diameter between mate the bulk porosity. Essential input to estimating the about 50–250 km (setting aside Phobos and Deimos, whose bulk porosity is knowledge of the “grain density” of the residence within the Mars gravity well may make them constituent material, i.e., the average density of the solids special cases), have macroporosities in excess of 30%. (“grains”) that make up the body. Laboratory measurements These appear to be the best candidates for “rubble-pile” ob- of primitive and diIerentiated meteorites (Consolmagno and jects that have been substantially disrupted and reaccreted Britt, 1998; Britt and Consolmagno, 2000) yield both the such that their original internal arrangement has undergone grain density and the “microporosity” arising from frac- moderate or substantial displacement. Most bizarre is the tures, voids, and pores on the scale of tens of microns. Britt inferred ¿ 70% macroporosity for 16 Psyche (Viateau, et al. (2002) examined the bulk porosities for nearly 20 small 2000). Psyche, a ∼ 250 km diameter object, has an M-type bodies, including Phobos and Deimos. Utilizing the grain reQectance spectrum that is interpreted as being analogous density and microporosity for the meteorite material most to very strong and very dense (7:4gmcm−3 grain density) consistent with the composition inferred by telescopic spec- iron meteorites. A very high radar reQectivity for Psyche tra, these researchers derive “macroporosity” (large-scale (Magri et al., 1999) provides the highest weight evidence for porosity) estimates that range from nearly zero to about an iron meteorite analog. A composition of such extremely 70%. For the purpose of inferring large scale internal struc- strong material may be essential for maintaining such a high ture, macroporosity constitutes one of the most fundamental porosity, where for a value ¿ 70% the volume is dominated parameters that we seek to determine. by voids greatly exceeding the abundance of holes in Swiss 448 R.P. Binzel et al. / Planetary and Space Science 51 (2003) 443–454 cheese. Belskaya and Lagerkvist (1996) note that there continue by examining ways in which we are likely to fur- are possibly other compositional interpretations for M-type ther improve our answers over the time scale of a decade or asteroids. If Pysche’s actual grain density is substantially more. Most interesting, of course, are the ÿndings we cannot lower than 7:4gmcm−3, the inferred value for the macro- predict—ÿndings which completely revamp our current un- porosity would be correspondingly lower. High material derstanding and dramatically change our science questions strength may not be a prerequisite for achieving 40–50% and perspectives. We are well advised not to be presumptu- macroporosity values, as three of the other four objects in ous in our current knowledge. For example, an accounting this Britt et al. grouping have C-type spectral properties. of the diJculties and uncertainties arising in measuring These objects are interpreted as likely analogs to weaker a mass, deriving a volume, and interpreting a basic com- carbonaceous chondrite meteorites. 253 Mathilde (50 km position highlights the challenge for determining a bulk diameter) is a prototype example with an estimated macroporosity as a starting point for inferring interior structure. porosity near 40%. Weaker strengths for carbonaceous-like To our advantage, however, we have a number of vantage material may contribute to their likelihood of being sub- points for trying to improve our understanding of small stantially disrupted into rubble-pile structures. body interiors. There appears to be no characteristic size range (within the present sample) for a transition between fractured/shattered objects and bodies whose rearranged interiors classify them 3.1. Outside looking in—the view from afar as rubble piles. It appears that stochastic collision history and basic composition play a greater role than size. Measuring and deÿning shapes and spin states, detecting However, a characteristic size does seem to be revealed binaries and satellites and measuring their orbital dimen- between shattered/fractured bodies and monoliths hav- sions and periods, and inferring parent body interiors from ing tensile strengths. Asteroid lightcurve studies pushing the collisional remnants within asteroid families all provide to progressively smaller sizes through measurements of opportunities for insights from a distant perspective. As dis- near-Earth objects (e.g., Pravec et al., 2000) have revealed a cussed in the previous section, the ∼ 100 m limit against set of remarkably fast rotating objects with periods as short the spin up of small bodies may be the most fundamental as a few minutes (as opposed to ∼ 2 h for the previous short insight gained by rotation studies. The rate at which an ob- period record). An analysis by Pravec and Harris (2000) ject in a non-principal axis or “excited” state loses its ro- ÿnds a transition near 100 m where fast rotations (periods tational energy—i.e., the rate at which it relaxes toward its shorter than ∼ 2 h) are rare or absent for objects larger than ground state (pure spin) is a strong function of the size of ∼ 100 m, although the mean tension ∼ R2 (!2 − G) the object (Burns and Safronov, 1973) depending on the in- across their plane of maximum stress is generally miniscule. verse square of an eIective radius. Thus new insights may This transition may represent a barrier where ¿ 100 m perhaps be gained through unveiling a boundary size be- bodies have such a high degree of fracturing (and a cor- low which excited spin predominates. The clearest signa- responding lack of tensile strength) that self-gravity is the ture of excited spin in lightcurve observations is the simul- dominant force for their coherency. This transition size is taneous presence of two independent periodicities. Unfor- also found in numerical simulations which show that large tunately, nature is not always kind and the independence remnants from a catastrophic disruption of a larger parent of the periodicities is not always clear. For example, in the body are no longer monolithic fragments once the latter case of 1P/Halley the periodicities seen in the lightcurve exceeds 100–300 m in radius (Benz and Asphaug, 1999). are, to the limits of accuracy, all harmonically related to Below ∼ 100 m almost all objects observed to date rotate the 7.4 day periodicity. Our certainty about the excitation faster than this limit, requiring their internal structure to be of Halley’s spin state comes from spacecraft observations governed by their tensile strength. But we note that this ten- of the orientations of the nucleus at the Vega and Giotto sile strength still may be incredibly weak: for the ∼ 11 min encounters, not from the lightcurve. If we depended only period and ∼ 30 m diameter of 1998 KY26 (Ostro et al., on the lightcurve information it is almost certain that we 1998) the required tensile strength is ∼ 300 dyn cm−2 would have convinced ourselves by now that Halley spin (presuming ∼ 1:3gcm−3 for this C-type), orders of was fully relaxed with a 7.4 day period. Numerical calcu- magnitude weaker than snow. Nevertheless, it appears that lations show why this can be the case. As the spin state ∼ 100 m may represent a characteristic size transition evolves for an elongated object torqued by jet activity it is between “intact” strength dominated monoliths and even found that the evolution tends to get hung up whenever the weaker (eIectively strengthless?) fractured/shattered or precession and rotation periods in the spin become commen- rubble-pile bodies predominantly held together by gravity. surable, i.e., harmonically related. In order to make full use of lightcurve information to analyze spin states of cometary 3. Perspectives nuclei, simultaneous information on nucleus shape and the distribution (i.e., coma morphology—orientations and cur- Having set out a fundamental set of science questions vatures of molecular and dust jets) and strength of coma and having brieQy outlined our current understanding, we activity is also required. For asteroidal bodies, the boundary R.P. Binzel et al. / Planetary and Space Science 51 (2003) 443–454 449 size and rotation rate below which such “tumbling” occurs these bodies with implications for interior structure. These provides a measure of the material properties of interiors, insights include: primarily rigidity and speciÿc dissipation factor (Q). Still to be unraveled is the mystery of a separate population of • The greater relative abundance of NEO binaries may be very slow rotators (Harris, 2002). Functionally, this popu- the result of tidal disruptions (Âa la comet Shoemaker-Levy lation ÿts a distribution like N(¡f) ˙ f, rather than the 9). If correct, this argues that many NEOs may have very 3D Maxwellian form that ÿts the main population of spin weak rubble-pile structures that have been formed in the rates within the asteroid belt (Pravec and Harris, 2000). recent solar system. Insights into the interior structure and tidal dissipation ca- • Main-belt binaries may be formed by collisional pro- pabilities might be gleaned if this subpopulation is a rem- cesses. A satellite such as Dactyl might plausibly have nant of massive unstable binaries that gravitationally dis- started out as a clump of unescaped ejecta from a major integrated, in the process using up most of the primary’s impact, with the orbit circularized and expanded by tidal spin energy. dissipation before it could chance to reimpact the primary. After decades, even centuries, of speculation and dubi- Such an accumulated ejecta process for forming satellites ous claims, asteroid satellites (binaries) have at last gained would explain why satellites of main-belt asteroids tend a ÿrm footing. As referenced above, binaries have been to be small compared to their primary. found among practically every class of small body, includ- • TNO binaries may rely on the solar tide to provide the ing NEOs, main-belt asteroids, a Trojan, and several TNOs. “second burn” to put collisionally disrupted components The holy grail within binary systems, of course, is to have into orbit about each other. In any case, the huge dimen- suJcient measurements to accurately solve for the mass of sion of the range of stable orbits is such that very little the system (the most diJcult step toward a density determi- subsequent (tidal) evolution should have occurred. nation). In many cases, long-term measurements with large • Time scales of tidal evolution may provide constraints aperture telescopes (often utilizing adaptive optics system) on internal properties of rigidity and energy dissipation, are essential for yielding the prize of well-determined or- and/or time since formation. bital parameters. Precise determination of the total volume • The shape, spin and density of some asteroids (best de- of the bodies (allowing for the presence of concavities) is termined for Eros, but almost certainly also the case for also a challenging step toward achieving density determina- Ida) imply Roche lobes so closely hugging the present tions. Given the rate at which new satellites and binaries are surfaces of those asteroids that almost anything dislodged currently being discovered (e.g., Veillet et al., 2002 estimate from the surface would likely end up in orbit, at least for that 1% of TNOs may be binary), over the next decade the a while. greatest growth in the number of available density measure- • For larger main-belt binaries, one can plausibly point back ments will come from this area. Because the uncertainties to the early solar system to explain their formation. For that make precise density determinations diJcult will per- example, 90 Antiope might have formed as a blob of sist, analyses for inferring interior properties (such as the nearly dispersed ejecta, from some super-large collision, rigidity parameter Q=k2) will be best accomplished on a sta- satisfying the Jeans criterion for coalescence into a single tistical basis using a large sample. gravitationally bound system but with too much angular Still to be unraveled, or even conÿrmed, is the appear- momentum to exist as a single body. Such a blob would ance that the “average” properties of binaries in diIerent have little choice but to settle into a nearly contact binary orbital regions diIer remarkably. NEO binaries tend to conÿguration of two similar mass components. have similar-sized components, with moderate orbital sep- arations. TNO binaries are likewise close to equal sized, Having been produced by the catastrophic disruption of but with huge orbital spacings. Main-belt asteroid satellites large parent bodies, asteroid families continue to present us tend to be smaller relative to their primaries (with one re- with observable outcomes of natural collision experiments. markable exception, 90 Antiope, that is a nearly equal mass, By studying families we are probing the pieces of small nearly contact binary). One might ascribe these diIerences body interiors. Still not yet understood in terms of early to observational selection eIects (e.g., only nearly equal solar system accretion and diIerentiation processes is why sized, widely spaced binaries could possibly be detected in asteroid families observed to date (Bus, 1999; Cellino et al., the TNO region). However, if main-belt binary statistics 2002) are remarkably uniform in their spectral properties, were like either NEO or TNO statistics, binary asteroids indicating little evidence for substantial diIerentiation. This would have been discovered a century or so ago from reg- contrasts with the remnant metal core interpretation for large ular telescopic observations. Thus, some diIerences must M-type asteroids like 16 Psyche. Burbine et al. (1996) sug- be real, and therefore call for explanation. We are only gest an explanation that for families formed from diIerenti- just beginning to have a suJcient number of examples to ated parent bodies, the crust and mantle material may have attempt generalizations as to mechanisms of formation and been “battered to bits” over the age of the solar system—but evolution. At the moment we can oIer only speculations to the extent of this “battering” is constrained by their re- stimulate new insights into the formation and evolution of maining a reasonable chance that Vesta’s basaltic crust has 450 R.P. Binzel et al. / Planetary and Space Science 51 (2003) 443–454 survived. Probing the spectral properties to smaller and in need of new constraints that can be best supplied by the smaller sizes may reveal the missing pieces of disrupted types of experiments described in Section 3.4. diIerentiated parent bodies. Indeed the discovery of a small Given these diJculties, and while we await measure- (15 km) basaltic asteroid located far from (and very un- ments providing new constraints, our present task is to unlikely related to) Vesta by Lazzaro et al. (2000) may be derstand the properties of classes of models rather than just the tip of the iceberg for understanding the diverse focus on speciÿc details. As an example we can exam- diIerentiation history of inner solar system bodies. ine how pre-existing fractures and/or porosity on micro- or More directly related to inferring interior strengths and macroscopic scales aIect the overall strength of a body or structures are the size and velocity distributions within fam- its ability to transmit shock waves (e.g., Asphaug et al., ilies (Cellino et al., 1999; Zappala et al., 2002). In princi- 1998). One of the most important perspectives these mod- ple these are related to the energetics and geometry of the els are giving us is that the eIective strength of a body impact and the strain-rate release of the fragments during strongly depends upon these characteristics. For example, a the break-up and dispersion process. Tantalizingly, we now body large enough to be in the gravitational regime (greater view only the ÿnal state of this complex process. An addi- than roughly 1 km), that is pre-fractured in a small num- tional challenge now being faced is the extent to which the ber of pieces without much void between them is weaker ejection velocities of the fragments (inferred from their or- (in the sense that it is easier to disrupt) than a similar-sized bital displacement from the center of mass) is aIected by monolith. On the other hand, the same body thoroughly shat- other process which may spread their orbits. Understand- tered with large amounts of voids is more diJcult to dis- ing processes like the Yarkovsky eIect (Bottke et al., 2000) rupt than the corresponding monolith despite the fact that have a direct bearing on our being able to utilize the poten- the shattered body is “weaker” in the sense that it has al- tial information contained within asteroid families. Indeed, most no tensile strength! Thus, “survival of the weakest” the observed size-proper elements relations seen in many may be the rule of the collisional evolution jungle as the families tend to suggest that properties of the original ejec- principal eIect for a shattered and porous interior is to at- tion velocity ÿelds of the fragments are still recognizable tenuate the stress wave generated in an impact. As a strong in present families. In particular, the trend exhibited by the shock propagates through a porous target, the energy is in- size (or absolute magnitude) versus proper inclination plots eJciently transferred across the fracture interfaces and pore is something that is not expected to have been substantially spaces, eIectively localizing the energy dissipation. Com- inQuenced by the Yarkovsky eIect. paction may also be very eIective in dissipating the energy in a porous target, as proposed for the large craters comparable to the radius of Mathilde (Veverka et al., 1999; 3.2. Outside looking in—the digital view Housen et al., 1999). Thus, our current perspective is evolving toward a view that the majority of small bodies are Numerically simulating the properties of interiors, in par- shattered survivors that become increasing diJcult to dis- ticular their response to impacts, presents a powerful tool rupt as sub-catastrophic collisions eIectively add strength by for applying and evaluating a variety of physical models. In adding fractures and by possibly increasing porosity. Stated principle, the simulation of a body’s response to a collision another way, thoroughly shattered and porous interiors, even by means of numerical calculations is a straightforward to the point of rubble-pile structures, are the longest-lived procedure. Once the relevant elasto-dynamical conserva- state for small bodies and may be considered their natural tion equations have been appropriately transformed into end state—thus predicting that when explored, almost all numerical code, initial conditions can be speciÿed and the small body interiors will be found to be thoroughly shattered outcome of collisions computed. The entire procedure can and porous. be checked in detail by comparing numerical results to labo- Particularly relevant toward unveiling the current interior ratory impact experiments (e.g., Fujiwara et al., 1989; Benz structure of small bodies is how fragments from catastrophic and Asphaug, 1995; Ryan and Melosh, 1998; Ryan et al., collisions reaccumulate themselves after the blow, when the 1999). In practice, however, the procedure is considerably impact indeed proves energetic enough to catastrophically more complicated for several reasons: (1) the physics in- disrupt the target. Modeling results by Michel et al. (2001) volved may not necessarily be described by simple laws suggest that the largest members of asteroid families are (fractures, phase changes, rheology, etc.). (2) The equa- likely rubble-piles, composed of fragments dispersed from a tions may be diJcult to make discrete. (3) The computing catastrophically disrupted parent that are able to reaccumu- power available may not be suJcient to allow for the late through their mutual gravitational attraction. If correct, desired or needed resolution. Applying numerical models this work points toward families as the best place to study to real solar system bodies presents serious challenges, in the rubble-pile end state for small body interiors. Nature particular when trying to infer the initial conditions of a could be giving us a helping hand in resolving this ques- body’s interior that have lead to the morphological and/or tion if there actually is a propensity for forming satellites dynamical properties observed today. We are dealing with within asteroid families (Davis et al., 1996; Durda, 1996; a classical under-determined inverse problem that is greatly Doressoundiram et al., 1997; Michel et al., 2001). R.P. Binzel et al. / Planetary and Space Science 51 (2003) 443–454 451

3.3. Outside looking in—the NEAR and near view niques and yield an improved knowledge of the gravity ÿeld (in terms of spherical, or ellipsoidal harmonics). The result Although a model of the geologic structure can uniquely would be a better detection of lateral and internal inhomo- deÿne the gravitational ÿeld of a body, a model of the grav- geneities in the body and set the appropriate reference sur- itational ÿeld cannot uniquely deÿne the geologic structure face (analogous to the Earth’s geoid) to be compared with that produced it. Even though we cannot achieve unique other representation methods (as stated above). solutions, well-constrained solutions in terms of overall homogeneity are achievable with orbiting missions, as demon- 3.4. Up close and personal—active experiments strated by NEAR at Eros and discussed in Section 2.3 above. Promising new approaches based on alternative representa- However clever we are with models and data interpre- tions of the gravity potential (e.g., the direct calculation of tation of gravity ÿelds, the interiors of small bodies will the potential obtained by modeling the body as a polyhedron, remain a mystery until we begin to probe inside directly Werner and Scheeres, 1997) could allow us to model in- through active experiments. Deep Impact (A’Hearn, 1999) teriors even for highly irregular shapes. Mixed approaches, will make the ÿrst such experiment in the most simple way: using both spherical harmonics and polyhedral representa- impacting a projectile of a known mass and velocity and tions (e.g., Scheeres et al., 2000) could help to discriminate observing the consequences. The selection of 9P/Tempel-1 the presence of density layers with relative accuracies of the as the Deep Impact target will directly address our perva- order of 10−9. sive lack of direct and detailed knowledge of the strength, New missions on the horizon promise the exploration of structure, and composition of cometary nuclei. interiors for a wide variety of small bodies. On the pro- Taking further steps toward directly studying small body toplanet scale, the Dawn mission to Vesta and to Ceres interiors will require applying the tools of modern geophys- (Russell et al., 2002) will reveal the presence or absence ical prospecting: radar reQection and transmission tomogra- of a core. CONTOUR’s intended close Qybys of comets phy, seismo-acoustic waveform inversion, magneto-telluric 2P/Encke and 73P/Schwassmann-Wachmann 3 (Bell et al., imaging, and good old-fashioned drilling and blasting. 2000) are examples for a low-cost mission to derive mul- Some of these are at present more feasible than others, but tiple comet densities. Rosetta’s Qight path may include one the overall mission requirements involve spacecraft at least or more asteroid Qybys enroute to its cometary destination. capable of rendezvous and in most cases landing. Radar Rosetta will send a lander to the comet’s surface to perform reQection tomography may be the most cost-eIective tool in situ measurements of its physical and chemical proper- for learning about small body interiors, as it can be done ties. The Rosetta spacecraft is planned to operate for 2 years from an orbiter without a lander. But, it is not likely to within a short distance from the comet in a parallel orbit, provide unambiguous characterization in the absence of providing data on the evolution of a comet nucleus during ground truth. Furthermore, a small body with metallic its approach to perihelion. Qecks (a chondrite) may be opaque to radar, as may one Detailed examination of small body interiors from orbital which is clay rich. Radar plus grenades might be a better missions currently appears achievable through very accurate combination, especially for probing near-surface geology orbit determination by means of multiple frequency track- (fault expressions, crater roots, cohesion of “ponded” ma- ing in conjunction with an accelerometer to reduce the ef- terials, etc.). Radar plus grenades plus a seismic network fects of non-gravitational perturbations on the orbit solu- might reveal whole-body structural characteristics, and tion (particularly critical in the low gravity ÿeld of a small the required landers would allow for radio transmission body). These techniques will be demonstrated on the Bepi- tomography (much like the Rosetta CONCERT sounding Colombo mission to Mercury (Milani et al., 2001). The mul- experiment) unless the material is too opaque or the body tiple frequency X and Ka band link of BepiColombo will too large. Seismology on small bodies might prove very provide tracking precision of the order of 10 cm in range challenging if their near-surface layers are highly porous and 10−5 mm s−1 in range rate (RMS values). Simulations and strongly attenuative. Blasts are very poor seismogenic performed for the BepiColombo mission show that, together activators to begin with; they may create holes in the with the accelerometer data, this ultra-accurate tracking will regolith and little more. A deep penetrator with seismic allow the determination of the spacecraft position with an thumper and several deep-anchored receivers may be re- actual error of a few tens of centimeters. This will lead to quired for high-quality seismic imaging of larger objects—a an unprecedented accurate knowledge of the gravity ÿeld very complex mission goal that may not be achievable for of a solar system body (beyond the Earth) and will ÿrmly several decades. Due to the potential for strong attenua- constrain the presently unknown nature of Mercury’s core tion of electromagnetic and seismic energy, the smallest (Milani et al., 2001). As a comparison, the Eros gravity ÿeld objects may be the best ÿrst candidates for seismology and derived by NEAR is deemed reliable up to degree and order radar. NEOs derived from cometary source material up to 6(Miller et al., 2002) whereas a reliable ÿeld of Mercury up a few km across may also be good candidates for radar. to degree 25 should be attainable. A similarly instrumented Where seismic imaging is challenging, radio imaging may mission around a small body could employ the same tech- be optimal, and perhaps vice versa. An ideal candidate may 452 R.P. Binzel et al. / Planetary and Space Science 51 (2003) 443–454 be those objects near the transition size of ∼ 100 m. One that the only object we need to understand in great detail might ÿre large projectiles (Âa la Deep Impact) to reveal is the one that will cross our path. We disagree. From the nearly global consequences. One might try to spin up an standpoint of practicality, the science questions we address object, causing it to rotate so fast that it cracks or otherwise and the methods we employ to answer them are essential Qings itself apart. The list of creative possibilities goes on for learning how to learn about the structure and interiors and on. of small bodies. Without a sound knowledge base gained Only the most simple entrees from the above menu of by the scientiÿc study of these objects, we have no con- possibilities for directly measuring small body interiors are text within which to formulate realistic mitigation strategies. likely to ÿt within the cost envelope of Discovery class mis- While it is unlikely that an actual mitigation will have to sions within the United States. Low-cost (Discovery class) be performed for generations, the advancement of our sci- missions with focused science goals in this area are likely to entiÿc understanding through the exploration of small body be competitive. More extensive and more expensive small interiors is certain. body interior missions will ÿnd it increasingly diJcult to compete for Qight selection in the face of many compelling mission opportunities within all of planetary science. Feasi- 4. Conclusions bility studies for future missions to NEOs are being funded within ESA’s Aurora Program. The goal of these studies is Although the study of small body interiors is in its to give preliminary ÿndings relevant to preparing and devel- infancy, we are able to begin to formulate fundamental oping a mitigation plan. The governments of ESA’s member scientiÿc questions whose answers are as diverse as the countries will have the opportunity to select and to ÿnance solar system small body population as a whole. We have one of these missions. Among the mission concepts are those the opportunity to explore and understand objects ranging that deal with (i) the discovery and follow up of potentially from diIerentiated protoplanets to unaltered remnant plan- dangerous objects, (ii) the knowledge of their bulk compo- etesimals. The same collision processes that are likely to sition through the determination of the taxonomy of a large have thoroughly weakened and shattered interiors, in some sample of the NEOs population, and (iii) the determina- cases creating rubble-piles, may have eIectively made their tion of the bulk physical properties (mass, density, surface targets virtually impossible to destroy completely. These composition, and microscopic and macroscopic roughness). processes also may be responsible for creating satellites, Two other mission studies (named Don Quixote and Ishtar) whose orbital motions provide the ÿrst key for unlocking are addressing understanding the detailed internal structure their interior secrets. Only the smallest bodies may physi- of select NEO’s. Don Quixote proposes to send penetrators cally behave as monoliths, but their interior structures are with seismometers to perform tomography of the asteroid likely complex as well. Our spacecraft exploration of these target by monitoring the response to shooting a known mass worlds is only just beginning, with steps toward increas- at the surface. Ishtar plans an in-orbit radar global survey ingly sophisticated experiments falling within our vision. of few small asteroids to characterize the internal structure The scientiÿc motivations for the understanding of small together with their bulk geophysical properties. Overall, the body interiors, perhaps fueled by a practical need to under- natural evolution of our increasing science knowledge about stand the requirements for impact mitigation, put us on a small bodies and careful and rational (fact, not fear) assess- new and exciting threshold for exploration and discovery. ment of the impact hazard provides the most likely path by which extensive interior investigations will become a reality. Acknowledgements

3.5. Learning how to learn: the science role for mitigation We thank the International Scientiÿc Workshop Center of the Observatoire de Paris at Meudon for hosting an “Interior As we state in the Introduction, our motivations for the Structures of Small Bodies” workshop 10–14 June 2002, study of small body interiors are the science questions and which formed the basis for the synthesis and prospectus the diverse range of answers that can give us broad new presented here. insights across the ÿeld of planetary science. We note that nearly all of the answers gained from the science questions are both fundamental and essential to the “practical” problem of mitigation, should an NEO be discovered on a de- References cidedly hazardous trajectory. Fortunately, the odds greatly favor that there is no sizable threatening object (or objects) A’Hearn, M.F., 1999. Deep Impact. Bull. Am. Astron. Soc. 31, 1114. on course for impact within the next century or more. Yet, Asphaug, E., Benz, W., 1994. On the tidal disruption of periodic comet Shoemaker-Levy 9. Nature 370, 120–124. prudence dictates that we should search so as to be sure and Asphaug, E., Benz, W., 1996. Size, density and structure of comet that we should have an understanding of how to mitigate Shoemaker-Levy 9 inferred by the physics of tidal breakup. Icarus a hazardous object should it be necessary. One can argue 121, 225–248. R.P. Binzel et al. / Planetary and Space Science 51 (2003) 443–454 453

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