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- Goal I Supplement A: Asteroids Major Science Questions
- Planetary Mission Priorities
- Research and Analysis Contributions
- Key Facilities and Programs
- Goal I Supplement B: Comets Major Science Questions
- Goal I Supplement C: Meteorites Major Science Questions
- Goal I supplement D: Phobos and Deimos Major Scientific Questions
- Goal I Supplement E: Outer Solar System Planetesimals Major Science Questions
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06/26/2015 ----------------- DRAFT ----------------- First draft for SBAG community comment Goal I: Science Send comments to: Dr. Tim Swindle SBAG Goal I Lead tswindle@lpl.arizona.edu Comments due by: 09/19/2015 Also feel free to contact Dr. Nancy Chabot, SBAG chair: Nancy.Chabot@jhuapl.edu Goal I Subcommittee: T. Swindle (lead), K. Carroll, J. Castillo-Rogez, W. Grundy, E. Kramer, J. Nuth, C. Raymond, H. Smith ----------------- DRAFT -----------------
The small bodies now present in the Solar System represent remnants of the building blocks of the planets. As such, they are our best windows into the processes that occurred during the earliest history of the Solar System. But they also, as a result of their large numbers, represent test particles that have survived 4.5 billion years of evolution of the Solar System, and bear the results of many processes that have occurred during that evolution. From their orbital characteristics to their chemical compositions and their interior structures, they contain myriad clues to the history of the Solar System, often retaining information that the larger planets have lost. They also contain clues to the history of the habitability of the planets, not only because they have a common pre-solar and early nebular history, but also because the bombardment of the planets by small bodies has been a significant part of the planets’ histories. There are several different categories of “small bodies” in the Solar System that we consider, but these groups are interrelated, and the boundaries are not always clear. Small bodies we consider include: 1) Asteroids, remnants of terrestrial planet accretion that are found both in the Main Belt and as Near-Earth Objects; 2) Comets, bodies that outgas volatiles as they pass through the inner Solar System but that usually originate in the icy outer Solar System; 3) Outer Solar System Planetesimals, primarily Kuiper Belt objects but also including the Centaurs and icy small moons of outer planets that presumably originated in the Kuiper Belt; 4) Meteorites, remnants of Near-Earth Objects that have collided with Earth, providing samples that can be analyzed with laboratory instruments; 5) Phobos and Deimos, the enigmatic moons of Mars whose origin is unclear, but which may be more closely related to asteroids than to the planet they orbit; and 6) Dust, whose high surface-to-volume ratio makes it atypically bright through telescopes and also makes it possible for a micrometer-sized object to be captured by Earth’s atmosphere without being destroyed. Scientific goals, most of which apply to more than one of these types of objects, are:
A critical part of understanding the history of the small bodies in the Solar System, and hence the history of the Solar System itself, is understanding exactly what is present. Size-frequency distributions, distributions of chemical and spectral properties of astronomical objects, and distributions of properties of all sorts in laboratory samples have to be known before they can be explained, but knowledge of the existence of these bodies is a zeroth-order requirement.
Because of their small sizes, small bodies are inherently difficult to even identify with telescopes, much less study. Although the bright tails of comets mean that they have been observed since antiquity, every other type of small body orbiting in the solar system has been discovered via telescopes. Most of the discoveries have been the result of systematic search programs, whether for near-Earth objects, Kuiper Belt objects, or small moons of the outer Solar System planets. Furthermore, since objects in different regions of the Solar System orbit at vastly different rates, the optimal search parameters for one type of object (e.g., Kuiper Belt objects) may be completely inapplicable for some other type (e.g., near-Earth objects).
Laboratory analysis provides a level of detail that is inaccessible to studies using telescopes or even spacecraft. However, the level of knowledge of the Solar System that we can gain from laboratory analysis is limited by the samples available. Hence, to fully understand the small bodies of the Solar System, we need samples from as many different objects as we can acquire, including meteorites of as many different types as possible, micrometeorites, interplanetary dust, and samples from comets (of both silicate and icy materials), asteroids, the martian moons, and as many other small bodies as become accessible to spacecraft technology.
One of the most fundamental properties of an object is its chemical composition. Its chemical composition not only speaks to the processes involved in its formation (for example, does it represent material that would have condensed at high or low temperatures?) but also to the possible paths its evolution may take (a body that forms with frozen volatiles may undergo processes that will not happen on an object made of more refractory material). Different modes of study lend themselves to different types of analysis. Chemical compositions can be measured on a grain-by-grain basis for returned samples or laboratory samples of meteorites or interplanetary dust, while infrared spectroscopy is one of the most effective tools for telescopic observation, even though what it really determines is mineralogy. Spacecraft, meanwhile, can make direct elemental determinations with techniques like gamma-ray spectroscopy, but without the spatially resolution of laboratory samples, or can use techniques like infrared spectroscopy to make measurements with higher spatial resolution than that of ground-based telescopes, but often at the price of poorer spectral resolution.
Knowing what happened when is critical to understanding processes. At one level, knowing the onset and durations of events, whether it is chondrule formation, aqueous alteration, or impacts, constrains what processes might be possible candidates. For example, what is the relation of chondrule formation to the formation of calcium-aluminum-rich inclusions, in either time or space? How does the distribution of impact ages for meteorites from Main Belt asteroids compare to the distribution for samples from the Moon, and what does that say about the dynamical processes at work?
Is the Solar System typical or anomalous? One of the best ways to test is to figure out what happened when in the early Solar System and then compare that to what is seen in planetary systems that are currently forming around other stars. Ironically, although it can be very difficult to observe planets around other stars, small bodies, or, more accurately, the dust that they generate, can be much easier to detect. If we know things like when most of the gas cleared from the Solar System and how frequent collisions were, we can look at other systems to see if the same behavior is exhibited.
Because of their sizes, the orbits of small bodies, particularly those that are in heliocentric orbit, are affected by processes that may affect planetary-sized bodies little, if at all. For example, the volatile jetting that can drive changes in cometary orbits and the Yarkovsky Effect that can move small objects around the inner Solar System are both processes that would be of no importance to the orbital evolution of Earth or Mars, but are major factors in the current architecture of the Solar System. On the other hand, the distribution of orbits within the Main Asteroid Belt, the Kuiper Belt and the Oort Cloud probably all reflect planetary migration, to some degree. Theoretical studies provide the underpinning for understanding these processes, but these theoretical models need to be tested, both by experiments (either at the laboratory level or by spacecraft on actual small bodies) and by very high-precision measurements of the short-term evolution of orbits of small bodies, particularly near-Earth objects.
While there are many processes that could alter the orbits of small bodies, the current architecture of the Solar System reflects one specific history. That history, in turn, affects the planets, including Earth, both through impacts of small bodies whose orbits were greatly perturbed, and through interactions between the planets. Small bodies provide the best chance to fully understand the history of planets like Earth and Mars. The Main Asteroid Belt, for example, can be thought of as an ensemble of test particles, and the present architecture of the Main Belt must be a reflection of the same history that the terrestrial planets experienced.
Some of the effects that can alter the orbits of small bodies, most notably the Yarkovsky Effect, can be large enough on short enough timescales that they can be tested by simply following the orbits of specific objects with enough precision. But other effects, including planetary resonances, close encounters with planets, and even aspects of the Yarkovsky and YORP Effects occur on longer timescales, or infrequently enough that we cannot directly observe them. However, the recent orbital history of an object is recorded on the surface of small bodies, as a result of the bombardment by meteoroids and micrometeoroids and of solar and galactic charged particles, and even tidal effects (during planetary encounters). Determining the extent to which all of these secondary effects have occurred can provide constraints on the strength and nature of the orbital processes.
As our knowledge of other planetary systems explodes, so, too, do models of the evolution of such systems. It is crucial to ask the question of what those models would imply for the best-studied system we have, the Solar System. Just as studies of the Solar System can lead to predictions that can be tested at other planetary systems, so too can predictions based on observations from other systems be tested on the Solar System.
Our direct analysis of small bodies, whether via spacecraft, telescope or laboratory analysis of samples, is generally limited to material that has been at or near the surface of some body, at least in the most recent past. Therefore, it is essential to understand the processes that affect the surfaces, particularly those known as “space weathering,” and how they affect various properties. However, these processes are also worthy of study in their own right.
Although most of our observations of small bodies deal with the surfaces, most of the material composing those bodies is below the surface. To truly understand those bodies, we need to understand the interiors, and whether the surfaces are representative of the entire bodies, or whether there are significant heterogeneities. While we cannot yet directly access the interiors, they dominate properties such as density, porosity, and gravity, some of which can be estimated from ground-based measurements or flybys, others of which could be measured using geophysical techniques such as gravimetry and radar sounding. Many other properties, such as spin rate and, to the extent that it can be determined, thermal history, are highly dependent on properties of the interior.
It is not enough to know what the interiors of small bodies are like at present – we also need to know how their interiors have evolved to their current states. Our analysis of the current interiors is necessarily limited, but we have bodies that once were in the interiors of larger bodies, most notably differentiated asteroids and meteorites. In addition, theoretical models of the interiors all kinds of small bodies predict evolutionary paths that can be compared to the current observed states.
Life as we know it is based on volatile elements (such as C, N, H, O, and S) and compounds (including water and organic molecules). In addition, the presence of volatiles can alter the history of an object, altering minerals, causing outgassing that can affect orbits, and even contributing to resurfacing. Also, volatiles in small bodies represent potentially valuable extractable resources.
A first-order goal is to understand the present distribution of volatiles in the Solar System. Volatiles can be measured easily in laboratory samples using a variety of high-precision techniques, although contamination is often a problem; techniques such as neutron and gamma-ray spectroscopy can be used by spacecraft to search for volatiles, and volatile compounds often have distinctive spectral signatures, particularly in the IR, that can be used to detect them from the ground. The early stages of contemplated asteroid-mining activities may result in bulk extraction of volatiles, for example through excavation and heating, which if analyzed during that stage of processing could provide scientifically valuable composition estimates of uncontaminated samples.
We would like to know the present distribution of volatiles in the Solar System, but to understand the origin and evolution of the Solar System, we also need to understand what volatiles were present in small bodies at the beginning. Isotopic measurements are crucial, since many processes that can cause volatile loss will also cause isotopic fractionation, particularly for volatiles that end up in planetary atmospheres, such as the noble gases, carbon dioxide, nitrogen and water, among others. Isotopes can be measured most precisely in the laboratory, but some volatile compounds can be readily measured by ground-based telescopes if they are outgassed from a small body. Understanding the relationship between the amount and isotopic compositions of various volatile species in various types of small bodies should, in principle, make it possible to piece together the initial Solar System inventory and composition of volatiles.
Even among objects that are basically similar, volatile contents can vary greatly. Some meteorites are rich in hydrated materials, while others are almost completely unequilibrated despite having volatiles present. Similarly, some asteroid types have both hydrated and OH-free members. Gas-to-dust ratios among comets vary widely. These are all providing clues to the processes that occurred, but we need to find ways to decipher the clues, which requires, in part, finding as many of the clues – obtaining as much data from distinct objects and samples – as possible.
Volatiles are crucial to the histories of planets and moons in the Solar System, but their origin on these bodies is not necessarily well understood. As just one example, the source of water on Earth remains controversial, but almost certainly involves small bodies, whether in the form of late impactors or in the small objects that accreted to become the Earth. In the supplements that follow, we discuss these scientific goals as they apply to particular objects or classes of objects, highlighting some of the major scientific questions at present, and pointing out major missions, research programs, and facilities that are key to addressing the scientific goals. We note that when we address missions, we explicitly limit ourselves to discussion of New Frontiers-level missions. Discovery missions have been extremely successful in addressing the questions surrounding small bodies, and we strongly endorse the continuation of such missions, but do not wish to intervene in the historically-successful competitive selection process by highlighting specific missions at that scale. Goal I Supplement A: Asteroids Major Science Questions
Planetary Mission Priorities The Decadal Survey identified Trojan Tour as a candidate for the next New Frontiers mission. Missions to investigate multiple main belt asteroids, and in particular, main belt comets, should be a priority for future exploration. In-situ exploration and sample return, particularly from objects not well-represented in the meteorite population, would expedite progress towards the science objectives espoused above. Research and Analysis Contributions Recent progress in modeling the dynamic evolution of the early solar system has yielded predictions of the impact fluxes and compositional gradients that can be tested by missions to visit and map asteroids, and to some extent telescopic data. Research on the physics and chemistry of ices has yielded important inferences on the evolution of icy asteroids and their surfaces. Comprehensive models of the evolution of small bodies, including thermal evolution, hydrothermal and low-temperature chemistry, and surface processes are key to understanding how to interpret the properties we can measure and to understand the possible habitats for life in this transitional region between the terrestrial planets and ice outer solar system bodies. Key Facilities and Programs Ground-based facilities provide a wealth of data on the asteroid population and its characteristics, including the Arecibo and Goldstone solar system radars, the Keck and IRTF telescopes on Mauna Kea (Hawaii), Pan-STARRS, Catalina Sky Survey (CSS) and the impending Large Synoptic Survey Telescope (LSST), and well as an international network of smaller telescopes. The Minor Planet Center and the JPL NEO office record, track and catalog the asteroid population and support planetary defense assessments. The Hubble Space Telescope and Spitzer Observatory also provide unique and valuable data on small bodies in the main belt. The Antarctic Search for Meteorites is the largest source of diverse extraterrestrial materials to support solar system studies. Goal I Supplement B: Comets Major Science Questions
Planetary Mission Priorities The Decadal Survey identified the Comet Surface Sample Return mission as a top candidate among future New Frontiers missions. While the material collected in the Stardust mission has provided a wealth of insights, the volume of material collected was small, and the high velocity collection technique altered the particles from their original form. A mission that returns a much larger sample of material from the surface of a comet would likely revolutionize our understanding of primitive bodies. ESA’s Rosetta mission is already providing a wealth of new information, and will continue to do so as 67P/Churymov-Gerasimenko approaches perihelion.
The use of publicly available archival data is extremely important to long-term characterization of short-period comets, as well as population-wide studies of long-period comets. Ongoing analysis of these data, as funded by R&A programs, helps to leverage additional insights from data already collected by both ground-based and space-based facilities. Additionally, the continued collection of high-quality data on new or returning comets is critical, due to the ever-evolving nature of comets. Key Facilities and Programs There are several key facilities and programs that contribute to cometary science. The IRFT is a critically important facility for cometary study, used to identify surface properties and characteristics, finding the existence of CO and PAHs in comet spectra. Radar observations with Arecibo allow the physical size and dimensions of comets to be measured, which may lead to insights in cometary activity and evolution. Publicly available archival data sets, especially those from surveys (e.g., NEAT, NEOWISE, Spitzer, CFHT), help to characterize long-term cometary behavior. Making more data sets publicly available and easily searchable would contribute substantially to the understanding of cometary behavior. In the coming years, ALMA will provide exceptional data at wavelengths that have not been explored, allowing the study of difficult to observe emission lines such as those from CO and PAHs.
Planetary Mission Priorities: The community of scientists who study meteorites and interplanetary dust has a near-complete overlap with the community that studies samples returned by missions from Apollo onward. These missions, and upcoming sample-return missions, such as OSIRIS-REx and the proposed New Frontiers Comet Surface Sample Return mission, provide the context of a known parent body, a key piece of information, as well as providing samples that have not suffered through atmospheric entry. Research and Analysis Contributions: The meteorite community is funded almost entirely through R&A programs, so these programs, including programs to establish and maintain expensive state-of-the-art analytical facilities, are crucial to progress in this area. Key Facilities and Programs: The Antarctic Search for Meteorites (ANSMET) is crucial to meteorite studies. Although collectors find comparable numbers of meteorites elsewhere, the ANSMET collection represents an unbiased collection of an area, with well-documented collection circumstances, minimal contamination, and maximum accessibility to researchers. Collection programs like ANSMET are particularly crucial for identifying new groups of relatively rare meteorites. Goal I supplement D: Phobos and Deimos Major Scientific Questions: (1) What are the martian moons composed of? What is the elemental and chemical composition of Phobos and Deimos? What are the spectral features on Phobos? Are volatiles (i.e.water) present? Does the composition of the martian moons differ from one another and from Mars? (2) What are the physical and surface properties of Phobos and Deimos? What is the internal structure of each of the martian moons? What physical (geology) processes occur (or have occurred) on the martian moons (weathering, impacts, tidal evolution, groove formation, etc.)? Is the redder unit of Phobos weathered ultramafic material from Mars or transferred material from Deimos? (3) What is the origin of Phobos and Deimos? Are they related to primitive/ ultra-primitive D-type asteroids? Are Phobos and Deimos formed from re-accreted Mars ejecta? Are Phobos and Deimos captured bodies formed elsewhere in the Solar System? If a captured body, where did they originate (the asteroid belt, outer main belt, Kuiper Belt, etc.)? Do the two martian moons have the same origin? (4) How do Phobos and Deimos relate to processes currently or formerly occurring on other bodies in the Solar System? Are Phobos and Deimos similar to the sources of water and other volatiles delivered to terrestrial planets in the early Solar System? Are surface processes on Phobos and Deimos similar to those on S-type and C-type bodies? How does the origin and formation of Phobos and Deimos relate to Mars?
Although not specifically targeted at Phobos and Deimos the New Frontiers candidate missions for Comet Surface Sample Return and Trojan Tour and Rendezvous would contribute to a better understanding of primitive bodies and surface processes on Phobos and Deimos. Research and Analysis Contributions The majority of Phobos and Deimos research is funded through the research and analysis program within the Planetary Science Division. These NRA’s include: Solar System Workings, Mars Data Analysis, and Emerging Worlds. Key Facilities and Programs Key facilities for investigating Phobos and Deimos are spacecraft orbiting Mars (MRO, Mars Express, Odyssey, MAVEN, MSL, MER) and The Hubble Space Telescope. The Solar System Exploration Research Virtual Institute (SSERVI) is a vital program for Phobos and Deimos investigations. Goal I Supplement E: Outer Solar System Planetesimals Major Science Questions
Planetary Mission Priorities NASA's New Horizons mission visits the Pluto system in 2015. Pluto is among the largest Kuiper belt objects, with a complex satellite system thought to result from a giant impact. Its atmosphere, seasonal volatile transport cycles, and other size-dependent features will shape our thinking about other large Kuiper belt objects. An extended mission could explore a small, undifferentiated Kuiper belt object. Bookending the size distribution by exploring both a planet-sized body and a small planetesimal is an exciting prospect for Kuiper belt science, and is strongly supported by the community. The latest Planetary Decadal report recommended no new starts for Kuiper belt exploration in the larger mission size classes. But of the New Frontiers recommendations, both the Comet Surface Sample Return and the Trojan Tour are valuable to Kuiper Belt science. The first would return a sample from an object that formed as an outer solar system planetesimal (albeit only the refractory components). The second would survey a sample of those planetesimals trapped into Trojan-type orbits 5 AU from the Sun, where they would be less thermally processed than objects in the main asteroid belt (albeit much more thermally processed than their siblings in the Kuiper belt). It could also be possible to encounter a Centaur on the way to Uranus, shedding light on how Kuiper belt objects evolve as they transition to cometary orbits. Two flagship class missions in NASA's Astrophysics Division are especially important for outer solar system small bodies science. Hubble Space Telescope has made many important discoveries in this field over the past quarter-century, and James Webb Space Telescope is also expected to play a significant role when it becomes operational. Research and Analysis Contributions Dynamical modeling, informed by observed orbital distributions of outer solar system small bodies, has completely reshaped our view of the early evolution of the solar system. Continuation of this work is crucial for refining the details of that history via observationally testable predictions and also for linking present-day small body remnant populations to their places of origin in the protoplanetary nebula, enabling their use to study distinct nebular environments. Dynamical modeling needs to be well supported in the R&A program portfolio, and it needs to be clear what portion of the program (Solar System Workings? Emerging Worlds?) will be supporting it. Modeling the mechanical and thermal evolution of planetesimals and the mobility of volatiles within them is crucial for understanding how the objects available for study today relate to those that formed in various regions across the protoplanetary nebula. Further laboratory studies are needed on the fundamental properties of cryogenic materials, as well as low temperature chemistry occurring over very long timescales and the effects of various types of energetic radiation. Key Facilities and Programs Being small and distant, Kuiper belt objects are especially faint and challenging observational targets. Their study depends on access to the most capable telescopes: Hubble, JWST, Keck, ALMA, Spitzer, etc., and can benefit from future facilities currently under construction or still in planning stages. Support for observational studies is at present mostly through the SSO program. Directory: sbag -> goals goals -> Goals and Objectives for the Exploration and Investigation of the Solar System’s Small Bodies Assessment Group (sbag) Download 46.44 Kb. Share with your friends:
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