Scientific Justification

Transcription

1 Scientific Justification Introduction The eight planets overwhelmingly dominate the solar system by mass, but their small number, coupled with their stochastic pasts, makes it impossible to construct a unique formation history from the dynamical or compositional characteristics of them alone. In contrast, the huge numbers of small bodies scattered throughout and even beyond the planets, while insignificant by mass, provide an almost unlimited number of probes of the statistical conditions, history, and interactions in the solar system. Studies of these small bodies have been exploited for many years in the inner part of the solar system, where combined dynamical and compositional observations of asteroids have been used to trace chemical gradients, study early radioactivity, and detect and analyze collisional histories in the region of the terrestrial planets. While a combined dynamical-compositional study of the Kuiper belt would offer similar promise for understanding the formation of the region of the giant planets, the typical objects in the Kuiper belt are 10 4 times fainter than those in the asteroid belt, so this promise has been hampered by the difficulty of obtaining concrete observations of the surface compositions of these objects. Instead, formation and evolution studies of the Kuiper belt have been largely dynamical simulations where a hypothesized starting condition is evolved under the gravitaitonal influence of the early giant planets and an attempt is made to reproduce the current dynamical populations. With little compositional information known for the real Kuiper belt, the test particles in the simulation are free to have any formation location and history that places them at the correct ending point. Forcing compositional constraints into these studies would add an entire new dimension to our understanding of the formation and evolution of the outer solar system. New visible-infrared capabilities of WFC3 allow such compositional information of a large number of Kuiper belt objects to be obtained for the first time. Here we propose to exploit these capabilities to perform the first ever large-scale dynamical-compositional study of Kuiper belt objects (KBOs) and their progeny to study the the chemical, dynamical, and collisional history of the region of the giant planets. Kuiper belt compositions: a new probe of the outer solar system For many years, few infrared spectra of KBOs were available, and the only quantitative information on the surfaces of KBOs was their optical colors. While colors are a poor proxy for composition, they have proved a fascinating early tracer of the dynamical homogeneity or lack thereof of the Kuiper belt. The earliest photometric observations () suggested that KBOs came in a wide variety of colors and that there was no relationship between the color and any orbital or physical parameter of the object. To date this great heterogeneity remains unexplained, though it clearly points to a wide diversity of formation or evolutionary histories throughout the Kuiper belt. Subsequent observations of larger numbers of KBO colors eventually showed that one dynamical subset of the Kuiper belt, the cold classical KBOs on dynamically cold low inclination and eccentricity orbits, consists exclusively of objects that are red. While red 1

2 is impossible to interpret compositionally without more spectral information, the existence of this red grouping has been used to argue that the cold classicals are a unique population whose dynamical coherence has been maintained through the dramatic evolution of the outer solar system. The need to retain this group of objects is one of the key constraints on and sometimes the death knell of models of the evolution of the outer solar system. While these red cold classical KBOs stand out as a unique dynamical class, the actual reason for these red colors remains unknown. Connecting and understanding dynamical and compositional groupings requires more than optical colors. Infrared spectroscopy allows a direct probe of the surface ices common in the outer solar system. For many years few infrared spectra were available, as few KBOs were bright enough for even low resolution spectroscopy. This difficulty was partially alleviated by our wide field search for the largest KBOs, which provided a moderate number of bright observable objects, and by long term programs at VLT and Keck to slowly obtain spectra of the very brightest of these. The most systematic survey to date is our Keck survey, which obtained 1.5 to 2.5 µm spectra of more than 30 objects in the outer solar system. Four major results from this limited sample provide examples and details of what could be expected from a much larger survey. A giant primordial collision. Even without detailed spectral analysis of the objects, one small set of objects stood as as appearing to have surfaces which looked almost precisely like laboratory spectra of water ice. Finding any such objects in the outer solar system was unexpected, as all laboratory simulations to date have suggested that any pristine water ice on the surface of a KBOs will be spectrally degraded on 100 Myr time scales. More surprising, however, was that all of the objects with laboratory water ice specra had nearly identical orbits. Moreover, the largest of these water ice objects, 2003 EL61, had previously been speculated to have suffered a giant impact at some point in its past which gave it its rapid spin and system of at least two moons. The combination of these properties made it clear that the additional water ice objects were the fragments of the giant impact that had shaped 2003 EL61. The existence of this family of objects all formed in a single collision provides a wealth of probes into the outer solar system. First, while the identification of the family was clear, the discovery itself was unexpected. In the current Kuiper belt, the probability that such a giant impact would occur is of the order of Even in an earlier, more populated Kuiper belt, the probability is only And yet the collision clearly occured. This simple observations suggests that an entirely different process was operating at some point in time that we have currently not considered. After the discovery, at least one hypothesis has been proposed, that the collision occured during the clearing phase of the solar system between objects that were, in fact, undergoing clearing at the time, a process which had not previously been considered. If this process indeed led to the collision that created the 2003 EL61 family, the implications of this previously unconsidered process are profound. A significant fraction of the high inclination, moderate eccentricity population could have been formed in collisions between objects that were being cleared, leading to an entire reinterpreation of the formation mechanism of significant fraction of the objects in the outer solar system. 2

3 As expected from its possible formation from objects which would have otherwise been cleared from the solar system, the 2003 EL61 family is itslef in an unstable region of space. In Ragozzine and Brown (2007), we exploited these instabilites to develop a chronometer to determine the time of the 2003 EL61 imapct. To date, with the small number of family members known, we can only place a lower limit of 1 Gyr on the age. But with more objects discovered we should be able to more precisely date this impact, and thus date the time of solar system clearing. While almost all models to date assume that major clearing occured 4.5 Gyr ago, the new and relatively successful Nice model () possits that solar system clearing was delayed by?? Gyr and did not largely occur until the time of the Late Heavy Bombardment. The fact that the family is at least 1 Gyr old and perhaps primordial has led to another insight about the surface compositions of KBOs belt objects. While we still believe that surface with water ice and even small amounts of hydrocarbons will darked and redden in relatively short timescales, it appears that the 2003 EL61 family must be essentially pure water ice with essentially no contamination by hydrocarbons. Given the prevelence of hydrocarbons in the solar nebula, this exclusion is not expected. The only way we can think to exclude hydrocarbons from the 2003 EL61 family is if the original parent body of the family were so big that it was thoroughly differentiated, with a rocky core, and icy mantle, and the light hydrocarbons pushed to the surface. The family member that we are seeing, in this case, are the fragments of the icy hydrocarbon-free mantle. This hypothesis is supported by the unusually high density of 2003 EL61 itself, which suggests that it is a mostly rocky body with a thin veneer of water ice on the surface. This diversity of results from the first clearly understood correlation of surface composition and dynamics in the outer solar system is an excellent example of the types of results expected from such studies. The evolution of the centaurs. One other dynamical class of outer solar system objects the Centaurs also sticks out, not by virtue of having a distinct spectral type, but by viture of having a bifurcation of spectral types. Centaurs are objects which are on unstable giant planet crossing orbits which are thought to be former KBOs belt objects which had close encounters with Neptune that eventually brought the object into the middle part of the solar system. Eventually many of these objects will become Jupiter family comets in the inner solar system. The original hope in studying the spectra of the Centaurs was that they were small objects recently derived from the Kuiper belt and thus would be a good proxy for Kuiper belt compositions. Clearly, however, the Centaurs or at least some of the Centaurs have been modified as they have moved closer to the sun. If all Centaurs showed a particular spectral characteristic, we could hope to understand this processing and use it to help understand the surface and its evolution as an object moves from being a KBO belt object to a comet. But the bifurcation of spectral types is much more difficult to understand. One interesting recent dynamical development is the discovery by Malhotra there are a small number of dynamically longer-lived centaurs embedded within the otherwise short-lived population. With the small numbers of objects known it is difficult to tell if there is any correlation between this 3

4 dynamical age and the compositional bifurcation, but with compositional measurements of a larger number of Centaurs we would be able to determine if the bifurcation is due to the temporal evolution of the surface characteristics of this population which has lived longer in the middle solar system. Such a finding could provide the first concrete clues of the compositional consequences of the evolution of objects from the outer solar system and would allow objects which had undergone such processing to be traced throughout the solar system. The methane giants. A third clear result of the systematic study of outer solar system spectra is the realization that the most likely spectral differences are as much a function of heliocentric distance as they are of object size. Schaller and Brown () suggested that a small number of the largest and most distant objects have enough surface gravity to maintain their volatiles against loss to space over the age of the solar system. In their model, the final loss to space is controled by the slow leackage of Jean s escape from a vapor-pressure controlled atmosphere. The results of the model predictions to date have been almost perfect: almost everything that the model suggests should have volatiles onthe surface (predominantly methane) does, and nothing that the model suggests shouldn t have volatiles has been found to have volatiles. This success suggests that we have, at least, a first-order understanding of atmospheric loss on KBOs, and opens the possibility of being able to find outliers with unusual dynamical or compositional histories by finding objects whose predictions don t fit. Indeed, the one object which doesn t fit the model prediciton is 2003 EL61, the parent of the collisional family, where we presume that the impact took away most of the volatiles on the outermost fragments. Even if we had know nothing else about 2003 EL61, its failure to have a predictable surface composition would have drawn attention to it. Overall spectral diversity If the 2003 EL61 family and centaurs are removed from the sample, no apparent compositional-dynamical correlation or pattern is seen in the remaining?? objects. While the compositions of asteroids are strongly stratified as a function of heliocentric distance, the KBOs have no such stratification. Just as objects with different optical colors are jumbled throughout the Kuiper belt, so are objects with different infrared spectra. Unlike the asteroid belt, however, where compositional differences are glaring, in the Kuiper belt the visible and infrared spectra show that, with only a small number of exceptions, the spectra of KBOs fit along a smooth continuum with the only differences being the amount of absorption due to water ice and the optical color. The simplicity of the spectra allow them to be parameterized using only these two parameters (fig). Oddly, however, little correlation appears between the optical colors and the amount of water ice absorption, totally destroying the commonly held conceptual view that KBO surfaces are a simple balance between red colors due to irradiated organics and blue colors due to fresh water ice exposed by collision (Jewitt) or that KBOs can be compositionally classified by optical colors alone (Barruci). While the cause of this diversity is unknown, the broad possibilities are limited: the surfaces can reflect either primordial differences in the objects, or subsequent evolution of the objects, or both. Primordial differences would likely reflect formation location, evolution could reflect both thermal and collisional history. Whatever the cause of the surface composition variability, understanding the reason 4

5 would allow significant new insights into the evolution of the outer solar system. If the variations are primarily primordial we could reconstruct the initial locations of the nowjumbled Kuiper belt. If the variations are evolutionary we could use them to reconstruct collision or thermal histories of different regions of the Kuiper belt. In either case, with the current small number of objects known it is impossible to determine the cause of the variability, but the promise for discovery is strong. A large scale compositional-dynamical survey. All of the findings from the Keck KBO composition survey are limited by the small number of objects in the sample and by the even more sparse number of examples of each of the subclasses of objects studied. While this survey has greatly expanded our understanding of outer solar system compositions, a significantly expanded set of observations is required to fullfill the promise of using the composition of objects in the outer solar system to guide and constrain our understanding of how this region formed. Ground-based spectroscopy has hit its practical limit, and our survey for bright KBOs is essentially complete, so no significant new compositional information will come from the ground. New capabilities with the WFC3, however, allow us to greatly expand the number of objects for which we can assess surface composition. While the ground based spectra are limited to the?? objects brighter an R 20.5 (each of which takes about half of a night at Keck), in a single orbit with WFC3 we will be able to measure the surface compositon for KBOs down to R 23.4 (see details below), allowing us to potentially reach almost 500 targets, giving us the capabilitiy of optimally selecting a target list for a large survey rather than simply taking the only objects that can be measured. We have indentified a key sample of?? objects with which we will use to obtain a general survey of the composition of the outer solar system. While it is likely that much of the power of this survey will come from previously unanticipated results, we highlight here some of the goals that a based on results from the earlier limited studies: Identify large numbers of 2003 EL61 fragments. This collision, the largest for which we have multiple fragments anywhere in the solar system, will continue to be the subject of intense scrutiny for years to come. The identification of a large number of fragments will allow us to more tightly date the age of the impact and thus likely of the clearing of the solar system itself. The distribution of fragment sizes and velocities will allow a significantly more complete understanding of giant impact physics. The discovery of fragments that have become unstable will help to trace new dynamical pathways in the outer solar system. To date, the 2003 EL61 family spectral signature appears unique and easily identifiable, so finding new family members will work extremely well. Examine thermal evolution. The finding by Malhotra of much longer-lived orbits embedded within the otherwise unstable regions between the giant planets allows a direct determination of the effects of thermal processing on the surfaces of these former KBOs by comparing the compositional statistics of the short- and long-lived populations. If the compositional bifurcation is indeed caused by thermal processing we will have a new tracer of the thermal history of KBOs which can add yet another dimension to 5

6 our understanding of their histories. Find compositional outliers EL61 stands out as a compositional outlier which should have a methane-covered surface but does not. Other compositional outliers would reveal objects with other unique histories. One particular outlier for which we would be on the lookout would be an object with visible volatiles that should have lost all of them from its surface by now. Such a object would be a likely candiate for an object with a freshly excavated crater exposing pristine interior ices. Explore the composition of the cold classical KBOs. The earlier ground based surveys have made no progress understanding the surface composition of the red dynamically coherent cold classical KBOs for the simple reason that almost none of them is bright enough to obtain a spectrum of. This large-scale survey will be the first that allows the exploration of this unique subset of objects. Understand the spectral diversity. The spectral diversity of the KBOs is perhaps the biggest mystery of the surface composition, and solving this mystery could lead to the biggest prize: a tracer of the dynamical and perhaps thermal history of the individual objects in the Kuiper belt. With a greatly expanded sample this mystery can be unraveled. blue icy collisions. red like cold classical from same place? else? similarities to centaurs from when closer? This diversity of specific investigations resulting from this proposed survey is only a sampling based on our current limited knowledge of the studies that it will be possible to undertake with these data. As with any project where the field is potentially rich, our current knowledge level is low, and a new instrument can suddenly exploit vastly larger segments of the population, the potential for discovery both anticipated and not is high. Preparing for future observations. While this survey will provide the first largescale information on compositions of small bodies in the outer solar system, even more will be learned with future facilities on the horizon. The observations proposed here will provide the essential sample of well characterized objects that will allow us both to interpret and select targets for these future facilities. ALMA, for example, will have the capability to finally make radiometric measurements to get the sizes and thus albedos of a very large sample of KBOs. This albedo is the final parameter, along with color and water ice absorption, that would fully characterize the visible-ir spectrum of the surface. A collection of albedos with no spectral information, however, will be interesting but not rich. In contrast, facilities like JWST and the TMT will likely not have the capability to do spectral surveys of large numbers of objects, but will instead do limited targeted observations. Without this initial WFC3 survey we will have a limited range of potential targets and will likely miss ones that would be otherwise interesting. These observations, while providing an instant richness of scientific opportunities by themselves, are critical to collecting a sample that will likely be used for all future studies of Kuiper belt compositions. 6

7 Description of the Observations Our wide ranging Keck spectral survey has shown that, with small exceptions, the complete visible-infrared spectra of all small bodies in the outer solar system can quantified by two simple parameters: the visible color and the depth of the infrared water ice absorption features. The relatively simplicity of these spectra allows the possibility of targetted photometry to be used as a proxy for full spectroscopy. Broad-band J-H colors from NICMOS can be used to capture the deepest water ice absorptions of the 2003 EL61 families, as the H band stradles the 1.5 µm water absorption features. Unfortunately, for the weaker water ice absorption prevelent in much of the remainder of the outer solar system, the J-H colors are affected by the behavior of the visible absorber in the 1 µm region, so J-H cannot be used except in extreme cases. Fortunately, WFC3 has a pair of medium-band filters specifically designed to isolate water absorptions features. The?? and?? sample the highest point of the continuum and the deepest part of the absorption, without any contamination by regions of the spectrum affected by non-water ice features. Thus a relatively simple two band photometric measurement can provide the same depth of information as a detailed infrared spectrum. WFC3 is also the first dual visible/infrared instrument on HST and provides the capability quickly switch to visibile photometry during the course of an observation. In a single orbit we can thus measure a proxy for the full infrared spectrum and also measure the visible colors. These two measurements provide a full characterization of the visible-infrared spectrum of the KBO belt object. Filling an orbit with?? minutes of??,?? minutes of??,?? minutes of??, and?? minutes of?? allows us to reach a S/N of?? on objects with an R magnitude of Our Keck spectroscopy is limited to objects brighter than R 21, thus we have the capability to sample KBOs two full two magnitudes dimmer than the small sample to date. This capability opens up the possibility of a significant new exploration of the small bodies of the outer solar system. Some of this photometry could be done with ground-based telescopes. While the filters required for the infrared photometry do not exist on any current telescopes they could eventually be acquired. Indeed, we have attempted to identify fainter members of the 2003 EL61 family with ground-based J-H photometry. But precision measurements of the visible- IR colors of objects in the Kuiper belt are hampered both by the movement of the objects across the sky and by the rotation of the objects causing potentially significant photometric variations on time scales as short as several hours. The movement of the objects across the sky makes photometric measurements imprecise unless observations can be taken in short enough individiual images that the object does not smear significantly compared to the background stars. To date, the faintest object to have had even a broad-band infared color measured to better than??% is??, with R???. Measurement of colors in narrower bands would be comparably less accurate. The limit of ground-based infared photometry has been essentially reached. Space-based photometry, on the other hand, is comparably easily accomplished. Objects 7

8 as faint as R 24 have had J and H photometry measured to an accuracy of?? within a single orbit with NICMOS. The visible photometry, in contrast to the infrared photometry, is relatively easily accomplished from the ground for even the faintest objects in our sample. Here, however, we are hampered by potential rotational variation of the KBOs. Smaller objects, in particular, appear to have irregular shapes and thus have photometric modulations as they rotate. Typical rotation rates are 10 hours, so measurements over the course of a single night are rarely hampered, but attempting to do color measurements from observations obtained on different nights leads to considerable uncertainty. This uncertainty is particularly difficult when measuring visible and infrared colors, as these can rarely be measured simulatneously. The VLT has attempted to solve this difficulty by doing the measurements simultaneously with two different telescopes, but attempting to measure visible colors of this entire sample simultaneous with the IR measurements from WFC3 would be impossible. We those instead propose to measure visible and IR colors with WFC3 in quick succession. The visible colors take only a modest amount of additional time, and, again, can easily be fit into a single orbit. Special Requirements None. Coordinated Observations None. Justify Duplications None. 8