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Determining how and when Mars formed has been a long-standing challenge for planetary scientists. The size and orbit of Mars are difficult to reproduce in classical simulations of planetary accretion, and this has inspired models of inner solar system evolution that are tuned to produce Mars-like planets. However, such models are not always coupled to geochemical constraints. Analyses of Martian meteorites using the extinct hafnium–tungsten (Hf–W) radioisotopic system, which is sensitive to the timing of core formation, have indicated that the Martian core formed within a few million years of the start of the solar system itself. This has been interpreted to suggest that, unlike Earth’s protracted accretion, Mars grew to its modern size very rapidly. These arguments, however, generally rely on simplified growth histories for Mars. Here, we combine likely accretionary histories from a large number of N-body simulations with calculations of metal–silicate partitioning and Hf–W isotopic evolution during core formation to constrain the range of conditions that could have produced Mars. We find that there is no strong correlation between the final masses or orbits of simulated Martian analogs and their 182W anomalies, and that it is readily possible to produce Mars-like Hf–W isotopic compositions for a variety of accretionary conditions. The Hf–W signature of Mars is very sensitive to the oxygen fugacity (fO2) of accreted material because the metal–silicate partitioning behavior of W is strongly dependent on redox conditions. The average fO2 of Martian building blocks must fall in the range of 1.3–1.6 log units below the iron–wüstite buffer to produce a Martian mantle with the observed Hf/W ratio. Other geochemical properties (such as sulfur content) also influence Martian 182W signatures, but the timing of accretion is a more important control. We find that while Mars must have accreted most of its mass within ~5 million years of solar system formation to reproduce the Hf–W isotopic constraints, it may have continued growing afterwards for over 50 million years. There is a high probability of simultaneously matching the orbit, mass, and Hf–W signature of Mars even in cases of prolonged accretion if giant impactor cores were poorly equilibrated and merged directly with the proto-Martian core.

Decompressive craniectomy (DC) is a common neurosurgical intervention for severe traumatic brain injury (TBI), as well as malignant stroke, malignancy and infection. DC necessitates subsequent cranioplasty. There are significant demographic differences between TBI and non-TBI patients undergoing cranioplasty, which may influence their relative risk profiles for infection, aseptic bone flap resorption (aBFR) and re-operation. Perform a meta-analysis to determine the relative infection, aBFR and re-operation risk profiles of TBI patients as compared to other indications for DC. A systematic review and meta-analysis was performed in accordance with the PRISMA guidelines. PubMed, MEDLINE, EMBASE and Google Scholar were searched until 26/11/2020. Studies detailing rates of infection, re-operation and/or aBFR in specific materials and the post-TBI population were included, while studies in paediatrics or craniosynostosis repair were excluded. Twenty-six studies were included. There was no difference in relative risk of infection between TBI and non-TBI cohorts (RR 0.81, 95% CI 0.57–1.17), with insignificant heterogeneity (I2 = 33%). TBI was a risk factor for aBFR (RR 1.54, 95% CI 1.25–1.89), with no significant heterogeneity (I2 = 13%). TBI was a risk factor for re-operation in the autologous sub-group (RR 1.49, 95% CI 1.05–2.11) but not in the alloplastic sub-group (RR = 0.86, 95% CI 0.34–2.18). Heterogeneity was insignificant (I2 = 11%). TBI is a risk factor for aBFR and re-operation following cranioplasty. Use of an alloplastic graft for primary cranioplasty in these patients may partially mitigate this increased risk.

Determining how and when Mars formed has been a long-standing challenge for planetary scientists. The size and orbit of Mars are difficult to reproduce in classical simulations of planetary accretion, and this has inspired models of inner solar system evolution that are tuned to produce Mars-like planets. However, such models are not always coupled to geochemical constraints. Analyses of Martian meteorites using the extinct hafnium–tungsten (Hf–W) radioisotopic system, which is sensitive to the timing of core formation, have indicated that the Martian core formed within a few million years of the start of the solar system itself. This has been interpreted to suggest that, unlike Earth’s protracted accretion, Mars grew to its modern size very rapidly. These arguments, however, generally rely on simplified growth histories for Mars. Here, we combine likely accretionary histories from a large number of N-body simulations with calculations of metal–silicate partitioning and Hf–W isotopic evolution during core formation to constrain the range of conditions that could have produced Mars. We find that there is no strong correlation between the final masses or orbits of simulated Martian analogs and their (182)W anomalies, and that it is readily possible to produce Mars-like Hf–W isotopic compositions for a variety of accretionary conditions. The Hf–W signature of Mars is very sensitive to the oxygen fugacity (fO(2)) of accreted material because the metal–silicate partitioning behavior of W is strongly dependent on redox conditions. The average fO(2) of Martian building blocks must fall in the range of 1.3–1.6 log units below the iron–wüstite buffer to produce a Martian mantle with the observed Hf/W ratio. Other geochemical properties (such as sulfur content) also influence Martian (182)W signatures, but the timing of accretion is a more important control. We find that while Mars must have accreted most of its mass within ~5 million years of solar system formation to reproduce the Hf–W isotopic constraints, it may have continued growing afterwards for over 50 million years. There is a high probability of simultaneously matching the orbit, mass, and Hf–W signature of Mars even in cases of prolonged accretion if giant impactor cores were poorly equilibrated and merged directly with the proto-Martian core.

3617 Background: Transforming growth factor beta (TGFβ) is an immunosuppressive cytokine upregulated in colorectal cancer. Preclinical data demonstrated improved response to chemoradiation with TGFβ blockade in colorectal adenocarcinoma. Here we report the results of our single arm Phase II study combining the TGFβ type I receptor kinase inhibitor, galunisertib, with neoadjuvant chemoradiation in patients with locally advanced rectal adenocarcinoma. Methods: Eligible patients had T3+ or N+ rectal adenocarcinoma planned for surgical resection. Enrolled patients completed a 14-day course of galunisertib, followed by chemoradiation with continuous infusion 5-fluorouracil or capecitabine with radiation to 50.4-54Gy in 28-30 fractions. On day 30, patients underwent another 14-day course of galunisertib concurrent with ongoing chemoradiation. Five to nine weeks after completing neoadjuvant therapy, patients underwent response assessment. Those with complete response by physical exam, proctoscopy, and MRI, could opt for non-operative management and proceed to mFOLFOX6. Those with less than a complete response underwent surgical resection. The primary endpoint was complete response rate (CR), which was a composite of pathologic complete responses in those patients who proceeded to surgery, and clinical complete responses maintained at 1 year after completion of therapy for those who chose non-operative management. Using a Simon Two-Stage approach with 90% power and α = 0.05, to detect a 20% improvement in complete response rate compared to historical control (15% vs 35%); we reject the null hypothesis if ≥10/38 patients have a CR. 38 patients were enrolled with 35 patients evaluable. Results: Median age was 51y, 68% of patients were male, 87% of patients were Stage III, 97% of patients were node-positive, and 97% were proficient in mismatch repair. 95% of patients completed study therapy. Toxicity attributed to galunisertib was Grade 1-2. 28 patients went to surgery, with a mean neoadjuvant rectal (NAR) score (NAR = 5pN-3(cT-pT)+12]2/9.61) of 11.29, and pCR in 7 patients. 7 patients underwent non-operative management, with 5 achieving cCR at 1 year. Therefore, complete responses were observed in 12 patients. Peripheral immune monitoring revealed percent change in CD3+CD4-CXCR3+ T cells and activated CD8EM T cells correlated with response to therapy. 2y PFS and OS were 81.5% and 97%. Conclusions: The addition of TGFβ inhibition to neoadjuvant chemoradiation in locally advanced rectal cancer markedly improved response to therapy, was well tolerated, and warrants further investigation. Clinical trial information: NCT02688712.

Background Cranioplasty is a ubiquitous neurosurgical procedure consisting of reconstruction of a pre-existing calvarial defect. Many materials are available, including polymethylmethacrylate in hand-moulded (hPMMA) and prefabricated (pPMMA) form, hydroxyapatite (HA), polyetheretherketone (PEEK) and titanium (Ti). Objective To perform a network meta-analysis (NMA) to assess the relationship between materials and complications of cranioplasty. Methods PubMed/MEDLINE, Google Scholar, EMBASE, Scopus, and The Cochrane Library were searched from January 1, 1990 to February 14, 2021. Studies detailing rates of any of infections, implant exposure, or revision surgery were included. A frequentist NMA was performed for each complication. Risk ratios (RRs) with 95% CIs were calculated for each material pair. Results A total of 3620 abstracts were screened and 31 full papers were included. Surgical revision was reported in 18 studies and occurred in 316/2032 cases (14%; 95% CI 11-17). PEEK had the lowest risk of re-operation with a rate of 8/157 (5%; 95% CI 0-11) in 5 studies, superior to autografts (RR 0.20; 95% CI 0.07-0.57), hPMMA (RR 0.20; 95% CI 0.07-0.60), Ti (RR 0.39; 95% CI 0.17-0.92), and pPMMA (RR 0.14; 95% CI 0.04-0.51). Revision rate was 131/684 (19%; 95% CI 13-25; 10 studies) in autografts, 61/317 (18%; 95%CI 9-28; 7 studies) in hPMMA, 84/599 (13%; 95% CI 7-19; 11 studies) in Ti, 7/59 (9%; 95% CI 1-23; 3 studies) in pPMMA, and 25/216 (12%; 95% CI 4-24; 4 studies) in HA. Infection occurred in 463/4667 (8%; 95% CI 6-11) and implant exposure in 120/1651 (6%; 95% CI 4-9). Conclusion PEEK appears to have the lowest risk of cranioplasty revision, but further research is required to determine the optimal material.

Transforming growth factor beta (TGFβ) is a multipotent immunosuppressive cytokine. TGFβ excludes immune cells from tumors, and TGFβ inhibition improves the efficacy of cytotoxic and immune therapies. Using preclinical colorectal cancer models in cell type-conditional TGFβ receptor I (ALK5) knockout mice, we interrogate this mechanism. Tumor growth delay and radiation response are unchanged in animals with Treg or macrophage-specific ALK5 deletion. However, CD8αCre-ALK5flox/flox (ALK5ΔCD8) mice reject tumors in high proportions, dependent on CD8+ T cells. ALK5ΔCD8 mice have more tumor-infiltrating effector CD8+ T cells, with more cytotoxic capacity. ALK5-deficient CD8+ T cells exhibit increased CXCR3 expression and enhanced migration towards CXCL10. TGFβ reduces CXCR3 expression, and increases binding of Smad2 to the CXCR3 promoter. In vivo CXCR3 blockade partially abrogates the survival advantage of an ALK5ΔCD8 host. These data demonstrate a mechanism of TGFβ immunosuppression through inhibition of CXCR3 in CD8+ T cells, thereby limiting their trafficking into tumors., TGFβ has a role in cancer immunosuppression but the exact mechanisms haven’t been fully elucidated. Here, using mouse models deficient in TGFβ-signaling, the authors show that loss of ALK5 in CD8 + T cells enhances their tumour trafficking and cytotoxicity suggesting that ALK5 inhibitors may have clinical utility.

Ceres is the largest asteroid belt object, and the Dawn spacecraft observed Ceres since 2015. Dawn observed two morphologically distinct linear features on Ceres’s surface: secondary crater chains and pit chains. Pit chains provide unique insights into Ceres’s interior evolution. We interpret pit chains called the Samhain Catenae as the surface expression of subsurface fractures. Using the pit chains’ spacings, we estimate that the localized thickness of Ceres’s fractured, outer layer is approximately ≥58 km, at least ~14 km greater than the global average. We hypothesize that extensional stresses, induced by a region of upwelling material arising from convection/diapirism, formed the Samhain Catenae. We derive characteristics for this upwelling material, which can be used as constraints in future interior modeling studies. For example, its predicted location coincides with Hanami Planum, a high-elevation region with a negative residual gravity anomaly, which may be surficial evidence for this proposed region of upwelling material.

High-resolution mapping of Ceres, Vesta and the icy satellites of Saturn, Uranus and Pluto reveals a rich variety of well-preserved impact crater morphologies on these low gravity bodies. These objects provide a natural laboratory to study effects of composition on crater formation processes under similar surface gravity conditions (though mean impact velocities vary by several factors). Simple craters occur on all these bodies but subtle differences in morphology on Ceres and Vesta are recognized. Immature complex craters (with large floor mounds but not terraces or conical central peaks) occur on Vesta and while smaller than predicted are consistent with its silicate composition. Asymmetric simple craters (with incomplete scarp development) on all bodies are likely related to differential overburden stresses in the rim, and their occurrence is consistent with lower crustal strength on icy bodies including Ceres. Immature and mature complex craters exhibit increasing degrees of complexity, including spiral floor deformation patterns (related to failure in converging floor material), central peaks, and impact melt. Cerean crater morphologic types and simple-complex transition diameters are smaller than on Vesta but similar to those on icy satellites, indicating a much weaker rheology for Ceres' outer layers under impact conditions. These are consistent with geophysical indications of a low-density water ice and probably clathrate rich outer shell. Fluidized floor deposits (impact melt or melt-solid mixtures) are significant in craters >25 km across on Ceres but absent on Saturn satellites. Central pit craters are common on Ceres (at diameters of ~75 to 150 km consistent with gravity scaling from the larger Galilean satellites) but are absent on Saturnian satellites and Charon. The contrasting impact melt and central pit behaviors on Ceres and Saturn's moons is contrary to expectation given the higher impact velocities at Saturn but might be related to lower internal temperatures, or the higher fraction of non-ice material on Ceres. The correlation or scaling of transition diameters to surface gravity is near −0.65 rather than −1, perhaps due to increased porosity on lower gravity bodies. The fundamental similarity of crater morphologies on Ceres and icy satellites, however, indicates that the weaker rheology of water ice results in similar craters even if the non-(ice+clathrate) components are as high as ~30 vol%.

We investigate the effects of varying temperature and chemical potential on the optical absorption spectrum of (001) surface states of topological crystalline insulator SnTe using a four-band effective k ⋅ p Hamiltonian. The spectrum is characterized by a narrow peak at 52 meV and a shoulder feature at 160 meV. Both absorptions have maximal intensity at 0 K or when chemical potential is located at the charge neutrality point. Then, as temperature increases or as chemical potential diverges, they both decrease in intensity. The 52 meV peak originates from transitions between high density of states regions surrounding van Hove singularities and is the spectrum’s most prominent feature. Additionally, a third absorption from 110 meV to 150 meV, initially absent at 0 K or chemical potential at charge neutrality point, gradually builds in intensity as temperature increases or as chemical potential diverges. This absorption arises from transitions between low and high energy bands of opposite helicity. Importantly, we find that all distinct spectral features are diminished if the magnitude of chemical potential diverges to values above the van Hove singularity energies. If a given sample’s chemical potential is well-controlled, conventional infrared spectroscopy may be used to identify the spectral signatures of SnTe (001) surface states at room temperatures and without use of large magnetic fields.

Asteroid 2008 TC3 (approximately 4 m diameter) was tracked and studied in space for approximately 19 h before it impacted Earth’s atmosphere, shattering at 44–36 km altitude. The recovered samples (>680 individual rocks) comprise the meteorite Almahata Sitta (AhS). Approximately 50–70% of these are ureilites (ultramafic achondrites). The rest are chondrites, mainly enstatite, ordinary, and Rumuruti types. The goal of this work is to understand how fragments of so many different types of parent bodies became mixed in the same asteroid. Almahata Sitta has been classified as a polymict ureilite with an anomalously high component of foreign clasts. However, we calculate that the mass of fallen material was ≤0.1% of the pre-atmospheric mass of the asteroid. Based on published data for the reflectance spectrum of the asteroid and laboratory spectra of the samples, we infer that the lost material was mostly ureilitic. Therefore, 2008 TC3 probably contained only a few percent nonureilitic materials, similar to other polymict ureilites except less well consolidated. From available data for the AhS meteorite fragments, we conclude that 2008 TC3 samples essentially the same range of types of ureilitic and nonureilitic materials as other polymict ureilites. We therefore suggest that the immediate parent of 2008 TC3 was the immediate parent of all ureilitic material sampled on Earth. We trace critical stages in the evolution of that material through solar system history. Based on various types of new modeling and re-evaluation of published data, we propose the following scenario. (1) The ureilite parent body (UPB) accreted 0.5–0.6 Ma after formation of calcium-aluminum-rich inclusions (CAI), beyond the ice line (outer asteroid belt). Differentiation began approximately 1 Ma after CAI. (2) The UPB was catastrophically disrupted by a major impact approximately 5 Ma after CAI, with selective subsets of the fragments reassembling into daughter bodies. (3) Either the UPB (before breakup), or one of its daughters (after breakup), migrated to the inner belt due to scattering by massive embryos. (4) One daughter (after forming in or migrating to the inner belt) became the parent of 2008 TC3. It developed a regolith, mostly ≥3.8 Ga ago. Clasts of enstatite, ordinary, and Rumuruti-type chondrites were implanted by low-velocity collisions. (5) Recently, the daughter was disrupted. Fragments were injected or drifted into Earth-crossing orbits. 2008 TC3 comes from outer layers of regolith, other polymict ureilites from deeper regolith, and main group ureilites from the interior of this body. In contrast to other models that have been proposed, this model invokes a stochastic history to explain the unique diversity of foreign materials in 2008 TC3 and other polymict ureilites.

As part of systematic global mapping of Vesta using data returned by the Dawn spacecraft, we have produced a geologic map of the north pole quadrangle, Av-1 Albana. Extensive seasonal shadows were present in the north polar region at the time of the Dawn observations, limiting the ability to map morphological features and employ color or spectral data for determination of composition. The major recognizable units present include ancient cratered highlands and younger crater-related units (undivided ejecta, and mass-wasting material on crater floors). The antipode of Vesta’s large southern impact basins, Rheasilvia and Veneneia, lie within or near the Av-1 quadrangle. Therefore it is of particular interest to search for evidence of features of the kind that are found at basin antipodes on other planetary bodies. Albedo markings known as lunar swirls are correlated with basin antipodes and the presence of crustal magnetic anomalies on the Moon, but lighting conditions preclude recognition of such albedo features in images of the antipode of Vesta’s Rheasilvia basin. ‘‘Hilly and lineated terrain,’’ found at the antipodes of large basins on the Moon and Mercury, is not present at the Rheasilvia or Veneneia antipodes. We have identified small-scale linear depressions that may be related to increased fracturing in the Rheasilvia and Veneneia antipodal areas, consistent with impact-induced stresses (Buczkowski, D. et al. [2012b]. Analysis of the large scale troughs on Vesta and correlation to a model of giant impact into

Liu and Chou (2012) presented a two-step theory to describe reasoning both on the four traditional conditional syllogisms and on four parallel syllogisms that replace the conditional with a disjunction. This article proposes that although Liu and Chou show that the words if and or play an important role in generating the data they report, their theory provides no explanation of how these words play any role. The article also corrects an assertion by López Astorga (2013) that mental-logic theory accounts for the findings of Liu and Chou by interpreting if as the material conditional. Instead I show why mental-logic theory rejects the material conditional and accounts for the data using a procedural-semantics approach to the meaning of logic particles. The article concludes that a theory like that of Liu and Chou requires something like mental-logic theory to account for their data.

Five decades of observations of Ceres suggest that the dwarf planet has a composition similar to carbonaceous meteorites and may have an ice-rich outer shell protected by a silicate layer. NASA’s Dawn spacecraft has detected ubiquitous clays, carbonates and other products of aqueous alteration across the surface of Ceres, but surprisingly it has directly observed water ice in only a few areas. Here we use Dawn Framing Camera observations to analyse lobate morphologies on Ceres’ surface and we infer the presence of ice in the upper few kilometres of Ceres. We identify three distinct lobate morphologies that we interpret as surface flows: thick tongue-shaped, furrowed flows on steep slopes; thin, spatulate flows on shallow slopes; and cuspate sheeted flows that appear fluidized. The shapes and aspect ratios of these flows are different from those of dry landslides—including those on ice-poor Vesta—but are morphologically similar to ice-rich flows on other bodies, indicating the involvement of ice. Based on the geomorphology and poleward increase in prevalence of these flows, we suggest that the shallow subsurface of Ceres is comprised of mixtures of silicates and ice, and that ice is most abundant near the poles. Despite evidence for an ice-rich outer shell, little water ice has been observed on the surface of Ceres. Lobate morphologies observed on Ceres that are increasingly prevalent towards the dwarf planet’s poles are consistent with ice-rich flows.

International audience; A new model for terrestrial planet formation (Hansen [2009]. Astrophys. J., 703, 1131-1140; Walsh, K.J., et al. [2011]. Nature, 2011, 206-209) has explored accretion in a truncated protoplanetary disk, and found that such a configuration is able to reproduce the distribution of mass among the planets in the Solar System, especially the Earth/Mars mass ratio, which earlier simulations have generally not been able to match. Walsh et al. (Walsh, K.J., et al. [2011]. Nature, 2011, 206-209) tested a possible mechanism to truncate the disk-a two-stage, inward-then-outward migration of Jupiter and Saturn, as found in numerous hydrodynamical simulations of giant planet formation. In addition to truncating the disk and producing a more realistic Earth/Mars mass ratio, the migration of the giant planets also populates the asteroid belt with two distinct populations of bodies-the inner belt is filled by bodies originating inside of 3 AU, and the outer belt is filled with bodies originating from between and beyond the giant planets (which are hereafter referred to as 'primitive' bodies). One implication of the truncation mechanism proposed in Walsh et al. (Walsh, K.J., et al. [2011]. Nature, 2011, 206-209) is the scattering of primitive planetesimals onto planet-crossing orbits during the formation of the planets. We find here that the planets will accrete on order 1-2% of their total mass from these bodies. For an assumed value of 10% for the water mass fraction of the primitive planetesimals, this model delivers a total amount of water comparable to that estimated to be on the Earth today. The radial distribution of the planetary masses and the dynamical excitation of their orbits are a good match to the observed system. However, we find that a truncated disk leads to formation timescales more rapid than suggested by radiometric chronometers. In particular, the last giant impact is typically earlier than 20 Myr, and a substantial amount of mass is accreted after that event. This is at odds with the dating of the Moon-forming impact and the estimated amount of mass accreted by Earth following that event. However, 5 of the 27 planets larger than half an Earth mass formed in all simulations do experience large late impacts and subsequent accretion consistent with those constraints.

According to the generally accepted scenario, the last giant impact on the Earth formed the Moon and initiated the final phase of core formation by melting the Earth's mantle. A key goal of geochemistry is to date this event, but different ages have been proposed. Some argue for an early Moon-forming event, approximately 30 million years (Myr) after the condensation of the first solids in the Solar System, whereas others claim a date later than 50 Myr (and possibly as late as around 100 My) after condensation. Here we show that a Moon-forming event at 40 Myr after condensation, or earlier, is ruled out at a 99.9 per cent confidence level. We use a large number of N-body simulations to demonstrate a relationship between the time of the last giant impact on an Earth-like planet and the amount of mass subsequently added during the era known as Late Accretion. As the last giant impact is delayed, the late-accreted mass decreases in a predictable fashion. This relationship exists within both the classical scenario and the Grand Tack scenario of terrestrial planet formation, and it holds across a wide range of disk conditions. The concentration of highly siderophile elements (HSEs) in Earth's mantle constrains the mass of chondritic material added to Earth during Late Accretion. Using HSE abundance measurements, we determine a Moon-formation age of 95 +/- 32 Myr since condensation. The possibility exists that some late projectiles were differentiated and left an incomplete HSE record in Earth's mantle. Even in this case, various isotopic constraints strongly suggest that the late-accreted mass did not exceed 1 per cent of Earth's mass, and so the HSE clock still robustly limits the timing of the Moon-forming event to significantly later than 40 My after condensation., Includes Supplemental Material

On October 6, 2008, the small (~4 m) asteroid 2008 TC 3 was discovered and predicted to hit Earth within ~19 hours. Photometric data and a reflectance spectrum were obtained. The asteroid fragmented at ~37 km altitude above Sudan. Approximately 700 centimeter-sized fragments were recovered and constitute the meteorite Almahata Sitta. It is a unique meteorite breccia, consisting of ~50–70% ureilitic materials, plus samples of nearly every major chondrite group. The reflectance spectrum of 2008 TC 3 is closest to that of F-class asteroids, not previously associated with any meteorite type. 2008 TC 3 /Almahata Sitta records a complex history of fragmentation, migration, and reaccretion of materials in the Solar System.

[1] The Framing Camera (FC) on the Dawn spacecraft provided the first view of 4 Vesta at sufficiently high spatial resolution to enable a detailed correlation of the asteroid's spectral properties with geologic features and with the vestoid (V-type) asteroids and the Howardite-Eucrite-Diogenite (HED) class of meteorites, both of which are believed to originate on Vesta. We combine a spectral analysis of the basin with visible and near-IR spectroscopy of vestoids and with archived data over the same spectral range for HED meteorites. The vestoids are only slightly more akin to the Rheasilvia basin than to Vesta as a whole, suggesting that the crustal material ejected is a well-mixed collection of eucritic and diogenitic materials. The basin itself is more diogenitic, implying Vesta is differentiated and the impact that created Rheasilvia uncovered a mineralogically distinct layer. The Rheasilvia basin exhibits a larger range in pyroxene band strengths than Vesta as a whole, further implying that the basin offers a view into a complex, differentiated protoplanet. The discrepancy between the spectral properties of the HED meteorites and Vesta, in particular the meteorites' deeper pyroxene absorption band and the redder color of the vestoids, can be explained by the abundance of smaller particles on Vesta and by the addition of low-albedo exogenous particles to its surface, which in turn are due to its larger gravity and longer exposure time to impact processing. Solar phase effects are slight and do not explain the spectral discrepancies between the HEDs, Vesta, and the vestoids.

[1] The Rheasilvia impact on asteroid 4 Vesta may have been sufficiently large to create disrupted terrains at the impact antipode. This paper investigates the amount of deformation expected at the Rheasilvia antipode using numerical models of sufficient resolution to directly observe terrain modification and material displacements following the arrival of impact stresses. We find that the magnitude and mode of deformation expected at the impact antipode is strongly dependent on both the sound speed and porosity of Vesta's mantle, as well as the strength of the Vestan core. In the case of low mantle porosities and high core strengths, we predict the existence of a topographic high (a peak) caused by the collection of spalled and uplifted material at the antipode. Observations by NASA's Dawn spacecraft cannot provide definite evidence that large amounts of deformation occurred at the Rheasilvia antipode, largely due to the presence of younger large impact craters in the region. However, a deficiency of small craters near the antipodal point suggests that some degree of deformation did occur.

The Dawn mission was designed to test our hypothesis about the origin and evolution of the early solar system by visiting the largest differentiated basaltic asteroid, 4 Vesta, believed to be a survivor from the earliest times of rocky body formation. Observations from orbit show that Vesta is the parent body of the Howardite, Eucrite, Diogenite meteorites. Vesta has an iron core and a eucritic–diogenitic crust. Its surface is characterized by abundant impact craters but with no evident volcanic features. It has two ancient impact basins in the southern hemisphere that are associated with circum-planetary troughs. The northern hemisphere is the more heavily cratered and contains the oldest terrains. The surface of Vesta is diverse, with north-south and east-west dichotomies in the eucrite-to-diogenite ratio. Its surface contains both very bright and very dark material, and its color varies strongly from region to region. Both the mineralogical and the elemental compositions agree with that expected for the HED parent body. Significant OH or H may be present in the upper crust and the presence of pits in “fresh” craters is consistent with the devolatilization of the surface after a collision either brought to or tapped a source of water on Vesta. The presence of dark material on the surface of Vesta suggests efficient transport pathways for organic material, and the mixing of the dark material with the more pristine pyroxene explains the varying albedo across the surface. Vesta has proven to be a reliable witness to the formation of the solar system.

Lunar samples suggest that the inner Solar System was bombarded by asteroids about 4 Gyr ago. Radiometric ages of meteorites suggest an unusual number of high-velocity asteroids at this time, consistent with a dynamical origin of the bombardment in which the asteroids were pushed by outer planet migration onto highly eccentric orbits.

[1] The estimated excavation depth of the huge Rheasilvia impact basin is nearly twice the likely thickness of the Vestan basaltic crust, so the mantle should be exposed. Spectral mapping by the Dawn spacecraft reveals orthopyroxene-rich materials, similar to diogenite meteorites, in the deepest parts of the basin and within its walls. Significant amounts of olivine are predicted for the mantles of bulk-chondritic bodies like Vesta, and its occurrence is demonstrated by some diogenites that are harzburgite and dunite. However, olivine has so far escaped detection by Dawn's instruments. Spectral detection of olivine in the presence of orthopyroxene is difficult in samples with

INTRODUCTION Thermochemical models have predicted that the dwarf planet Ceres has, to some extent, formed a mantle. Moreover, due to viscous relaxation, these models indicate that Ceres should have an icy crust with few or no impact craters. However, the Dawn spacecraft has shown that Ceres has elevation excursions of ~15 km, cliffs, graben, steep-sided mountains, and a heavily cratered surface. ### RATIONALE We used Dawn’s Framing Camera to study the morphology, size frequency, and spatial distribution of the craters on Ceres. These data allow us to infer the structure and evolution of Ceres’ outer shell. ### RESULTS A large variety of crater morphologies are present on Ceres, including bowl-shaped craters, polygonal craters, floor-fractured craters, terraces, central peaks, smooth floors, flowlike features, bright spots, secondary craters, and crater chains. The morphology of some impact craters is consistent with water ice in the subsurface. Although this might have favored relaxation, there are also large unrelaxed craters. The transition from bowl-shaped simple craters to modified complex craters occurs at diameters of about 7.5 to 12 km. Craters larger than 300 km are absent, but low-pass filtering of the digital elevation model suggests the existence of two quasi-circular depressions with diameters of ~570 km (125.56°E and 19.60°N) and ~830 km (24.76°W and 0.5°N). Craters are heterogeneously distributed across Ceres’ surface, with more craters in the northern versus the southern hemisphere. The lowest crater densities are associated with large, well-preserved southern hemisphere impact craters such as Urvara and Yalode. Because the low crater density (LCD) terrain extends across a large latitude range in some cases (e.g., Urvara and Yalode: ~18°N and 75°S; Kerwan: ~30°N and 46°S), its spatial distribution is inconsistent with simple relaxation driven by warmer equatorial temperatures. We instead propose that impact-driven resurfacing is the more likely LCD formation process, although we cannot completely rule out an internal (endogenic) origin. We applied two different methodologies to derive absolute model ages from observed crater size-frequency distributions. The lunar-derived model adapts the lunar production and chronology functions to impact conditions on Ceres, taking into account impact velocities, projectile densities, current collision probabilities, and surface gravity. The asteroid-derived model derives a production function by scaling the directly observed object size-frequency distribution from the main asteroid belt (extended to sizes 300 km in diameter are missing, and there are several areas with LCDs associated with large impact craters (e.g., Yalode, Urvara, Kerwan, Ezinu, Vinotonus, Dantu, and two unnamed craters northeast and southeast of Oxo). Areas A and B are topographic rises with central depressions that also show LCDs. Thermochemical models have predicted that Ceres, is to some extent, differentiated and should have an icy crust with few or no impact craters. We present observations by the Dawn spacecraft that reveal a heavily cratered surface, a heterogeneous crater distribution, and an apparent absence of large craters. The morphology of some impact craters is consistent with ice in the subsurface, which might have favored relaxation, yet large unrelaxed craters are also present. Numerous craters exhibit polygonal shapes, terraces, flowlike features, slumping, smooth deposits, and bright spots. Crater morphology and simple-to-complex crater transition diameters indicate that the crust of Ceres is neither purely icy nor rocky. By dating a smooth region associated with the Kerwan crater, we determined absolute model ages (AMAs) of 550 million and 720 million years, depending on the applied chronology model. [1]: pending:yes

INTRODUCTION Classic volcanism prevalent on terrestrial planets and volatile-poor protoplanets, such as asteroid Vesta, is based on silicate chemistry and is often expressed by volcanic edifices (unless erased by impact bombardment). In ice-rich bodies with sufficiently warm interiors, cryovolcanism involving liquid brines can occur. Smooth plains on some icy satellites of the outer solar system have been suggested as possibly cryovolcanic in origin. However, evidence for cryovolcanic edifices has proven elusive. Ceres is a volatile-rich dwarf planet with an average equatorial surface temperature of ~160 K. Whether this small (~940 km diameter) body without tidal dissipation could sustain cryovolcanism has been an open question because the surface landforms and relation to internal activity were unknown. ### RATIONALE The Framing Camera onboard the Dawn spacecraft has observed >99% of Ceres’ surface at a resolution of 35 m/pixel at visible wavelengths. This wide coverage and resolution were exploited for geologic mapping and age determination. Observations with a resolution of 135 m/pixel were obtained under several different viewing geometries. The stereo-photogrammetric method applied to this data set allowed the calculation of a digital terrain model, from which morphometry was investigated. The observations revealed a 4-km-high topographic relief, named Ahuna Mons, that is consistent with a cryovolcanic dome emplacement. ### RESULTS The ~17-km-wide and 4-km-high Ahuna Mons has a distinct size, shape, and morphology. Its summit topography is concave downward, and its flanks are at the angle of repose. The morphology is characterized by (i) troughs, ridges, and hummocky areas at the summit, indicating multiple phases of activity, such as extensional fracturing, and (ii) downslope lineations on the flanks, indicating rockfalls and accumulation of slope debris. These morphometric and morphologic observations are explained by the formation of a cryovolcanic dome, which is analogous to a high-viscosity silicic dome on terrestrial planets. Models indicate that extrusions of a highly viscous melt-bearing material can lead to the buildup of a brittle carapace at the summit, enclosing a ductile core. Partial fracturing and disintegration of the carapace generates slope debris, and relaxation of the dome’s ductile core due to gravity shapes the topographic profile of the summit. Modeling of this final phase of dome relaxation and reproduction of the topographic profile requires an extruded material of high viscosity, which is consistent with the mountain’s morphology. We constrained the age of the most recent activity on Ahuna Mons to be within the past 210 ± 30 million years. ### CONCLUSION Cryovolcanic activity during the geologically recent past of Ceres constrains its thermal and chemical history. We propose that hydrated salts with low eutectic temperatures and low thermal conductivities enabled the presence of cryomagmatic liquids within Ceres. These salts are the product of global aqueous alteration, a key process for Ceres’ evolution as recorded by the aqueously altered, secondary minerals observed on the surface. ![Figure][1] Perspective view of Ahuna Mons on Ceres from Dawn Framing Camera data (no vertical exaggeration). The mountain is 4 km high and 17 km wide in this south-looking view. Fracturing is observed on the mountain’s top, whereas streaks from rockfalls dominate the flanks. Volcanic edifices are abundant on rocky bodies of the inner solar system. In the cold outer solar system, volcanism can occur on solid bodies with a water-ice shell, but derived cryovolcanic constructs have proved elusive. We report the discovery, using Dawn Framing Camera images, of a landform on dwarf planet Ceres that we argue represents a viscous cryovolcanic dome. Parent material of the cryomagma is a mixture of secondary minerals, including salts and water ice. Absolute model ages from impact craters reveal that extrusion of the dome has occurred recently. Ceres’ evolution must have been able to sustain recent interior activity and associated surface expressions. We propose salts with low eutectic temperatures and thermal conductivities as key drivers for Ceres’ long-term internal evolution. [1]: pending:yes

The asteroid belt is found today in a dramatically different state than that immediately following its formation. It is estimated that it has been depleted in total mass by a factor of at least 1000 since its formation, and that the asteroids' orbits evolved from having near-zero eccentricity and inclination to the complex distributions we find today. The asteroid belt also hosts a wide range of compositions, with the inner regions dominated by S-type and other water-poor asteroids and the outer regions dominated by C-type and other primitive asteroids. We discuss a model of early inner solar system evolution whereby the gas-driven migration of Jupiter and Saturn brings them inwards to 1.5 AU, truncating the disk of planetesimals in the terrestrial planet region, before migrating outwards toward their current locations. This model, informally titled "The Grand Tack," examines the planetary dynamics of the solar system bodies during the final million years of the gaseous solar nebula lifetime--a few million years (Myr) after the formation of the first solids, but 20-80 Myr before the final accretion of Earth, and approximately 400-600 Myr before the Late Heavy Bombardment of the inner solar system. The Grand Tack attempts to solve some outstanding problems for terrestrial planet formation, by reproducing the size of Mars, but also has important implications for the asteroid population. The migration of Jupiter causes a very early depletion of the asteroid belt region, and this region is then repopulated from two distinct source regions, one inside the formation region of Jupiter and one between and beyond the giant planets. The scattered material reforms the asteroid belt, producing a population the appropriate mass, orbits, and with overlapping distributions of material from each parent source region.

A diverse range of terrestrial planet compositions is believed to exist within known extrasolar planetary systems, ranging from those that are relatively Earth-like to those that are highly unusual, dominated by species such as refractory elements (Al and Ca) or C (as pure C, TiC and SiC)(Bond et al. 2010b). However, all prior simulations have ignored the impact that giant planet migration during planetary accretion may have on the final terrestrial planetary composition. Here, we combined chemical equilibrium models of the disk around five known planetary host stars (Solar, HD4203, HD19994, HD213240 and Gl777) with dynamical models of terrestrial planet formation incorporating various degrees of giant planet migration. Giant planet migration is found to drastically impact terrestrial planet composition by 1) increasing the amount of Mg-silicate species present in the final body; and 2) dramatically increasing the efficiency and amount of water delivered to the terrestrial bodies during their formation process.

A persistent difficulty in terrestrial planet formation models is creating Mars analogs with the appropriate mass: Mars is typically an order of magnitude too large in simulations. Some recent work found that a small Mars can be created if the planetesimal disk from which the planets form has an outermost edge at 1.0 AU. However, that work and no previous work could produce a truncation of the planetesimal disk while also explaining the mass and structure of the asteroid belt. We show that gas-driven migration of Jupiter inward to 1.5 AU, before its subsequent outward migration, can truncate the disk and repopulate the asteroid belt. This dramatic migration history of Jupiter suggests that the dynamical behavior of our giant planets was more similar to that inferred for extra-solar planets than previously thought, as both have been characterised by substantial radial migration.

A New Dawn Since 17 July 2011, NASA's spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684 ) use Dawn's observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687 ) report on the asteroid's overall geometry and topography, based on global surface mapping. Vesta's surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690 ) report on Vesta's complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694 ) describe two giant impact basins located at the asteroid's south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697 ) present the mineralogical characterization of Vesta, based on data obtained by Dawn's visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700 ) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet.

Vesta and Ceres are the largest members of the asteroid belt, surviving from the earliest phases of Solar System history. They formed at a time when the asteroid belt was much more massive than it is today and were witness to its dramatic evolution, where planetary embryos were formed and lost, where the collisional environment shifted from accretional to destructive, and where the current size distribution of asteroids was sculpted by mutual collisions and most of the asteroids originally present were lost by dynamical processes. Since these early times, the environment of the asteroid belt has become relatively quiescent, though over the long history of the Solar System the surfaces of Vesta and Ceres continue to record and be influenced by impacts, most notably the south polar cratering event on Vesta. As a consequence of such impacts, Vesta has contributed a significant family of asteroids to the main belt, which is the likely source of the HED meteorites on Earth. No similar contribution to the main belt (or meteorites) is evident for Ceres. Through studies of craters, the surfaces of these asteroids will offer an opportunity for Dawn to probe the modern population of small asteroids in a size regime not directly observable from Earth.

We couple the results of N-body simulations of late-stage accretion (O'Brien et al., 2006) to a hafnium–tungsten (Hf–W) isotopic evolution code to investigate the evolution of planetary bodies in the inner solar system. Simulations can simultaneously produce planets having Earth- and Mars-like masses and Hf–W systematics by assuming that the tungsten partition coefficient decreases with increasing semi-major axis (e.g. due to increasing oxidation). Simulations assuming that Jupiter and Saturn occupy circular orbits are more successful at reproducing the Hf–W systematics than those assuming present-day Jupiter and Saturn orbits. To generate Earth-like tungsten anomalies, 30–80% of each impactor core is required to re-equilibrate with the target mantle. Some model outcomes yield a target and final impactor having similar (Earth- and Moon-like) tungsten anomalies. However, in no case can the inferred lunar Hf/W ratio be simultaneously matched. This result suggests that the Moon isotopically equilibrated with the Earth's mantle in the aftermath of the giant impact (cf. Pahlevan and Stevenson, 2007). Alternatively, either the dynamical models which show the Moon being derived primarily from the impactor mantle, or the accretion timescales obtained by the N-body simulations, are incorrect.

No terrestrial planet formation simulation completed to date has considered the detailed chemical composition of the planets produced. While many have considered possible water contents and late veneer compositions, none have examined the bulk elemental abundances of the planets produced as an important check of formation models. Here we report on the first study of this type. Bulk elemental abundances based on disk equilibrium studies have been determined for the simulated terrestrial planets of O'Brien et al. (2006). These abundances are in excellent agreement with observed planetary values, indicating that the models of O'Brien et al. (2006) are successfully producing planets comparable to those of the Solar System in terms of both their dynamical and chemical properties. Significant amounts of water are accreted in the present simulations, implying that the terrestrial planets form "wet" and do not need significant water delivery from other sources. Under the assumption of equilibrium controlled chemistry, the biogenic species N and C still need to be delivered to the Earth as they are not accreted in significant proportions during the formation process. Negligible solar photospheric pollution is produced by the planetary formation process. Assuming similar levels of pollution in other planetary systems, this in turn implies that the high metallicity trend observed in extrasolar planetary systems is in fact primordial., 61 pages (including online material), 12 figures (7 in paper, 5 online). Accepted to Icarus

To date, no accretion model has succeeded in reproducing all observed constraints in the inner Solar System. These constraints include 1) the orbits, in particular the small eccentricities, and 2) the masses of the terrestrial planets -- Mars' relatively small mass in particular has not been adequately reproduced in previous simulations; 3) the formation timescales of Earth and Mars, as interpreted from Hf/W isotopes; 4) the bulk structure of the asteroid belt, in particular the lack of an imprint of planetary embryo-sized objects; and 5) Earth's relatively large water content, assuming that it was delivered in the form of water-rich primitive asteroidal material. Here we present results of 40 high-resolution (N=1000-2000) dynamical simulations of late-stage planetary accretion with the goal of reproducing these constraints, although neglecting the planet Mercury. We assume that Jupiter and Saturn are fully-formed at the start of each simulation, and test orbital configurations that are both consistent with and contrary to the "Nice model." We find that a configuration with Jupiter and Saturn on circular orbits forms low-eccentricity terrestrial planets and a water-rich Earth on the correct timescale, but Mars' mass is too large by a factor of 5-10 and embryos are often stranded in the asteroid belt. A configuration with Jupiter and Saturn in their current locations but with slightly higher initial eccentricities (e = 0.07-0.1) produces a small Mars, an embryo-free asteroid belt, and a reasonable Earth analog but rarely allows water delivery to Earth. None of the configurations we tested reproduced all the observed constraints. (abridged), Accepted to Icarus. 21 pages, 12 figures, 5 tables in emulateapj format. Figures 3 and 4 degraded. For full-resolution see http://casa.colorado.edu/~raymonsn/ms_emulateapj.pdf

The near-Earth Asteroids Eros and Itokawa show a pronounced lack of small (≲100 m) craters, the vast majority of which were formed during their time in the main belt, and this has been cited as possible evidence that small (≲10 m) impactors are efficiently removed from the main belt by the Yarkovsky effect. Using well-tested models for the evolution of the main-belt size distribution and the evolution of crater populations on asteroid surfaces, I show that a pronounced lack of small impactors would require size-dependent removal far stronger than can result from the Yarkovsky effect (or any other known process). Furthermore, such strong removal would lead to wavelike perturbations in the main-belt and near-Earth asteroid size distributions that are inconsistent with their observed size distributions, as well as the cratering records on asteroid surfaces. A more likely explanation is that processes on asteroid surfaces, such as seismic shaking, are responsible for erasing small craters after they form.

Extrasolar planetary host stars are enriched in key planet-building elements. These enrichments have the potential to drastically alter the building blocks available for terrestrial planet formation. Here we report on the combination of dynamical models of late-stage terrestrial planet formation within known extrasolar planetary systems with chemical equilibrium models of the composition of solid material within the disk. This allows us to constrain the bulk elemental composition of extrasolar terrestrial planets. A wide variety of resulting planetary compositions exist, ranging from those that are essentially "Earth-like", containing metallic Fe and Mg-silicates, to those that are dominated by graphite and SiC. This implies that a diverse range of terrestrial planets are likely to exist within extrasolar planetary systems., Comment: 4 pages, 1 figure. Submitted to the proceedings of IAU symposium 265 Chemical Abundances in the Universe: Connecting First Stars to Planets

Total proctocolectomy and ileal pouch–anal anastomosis is the operation of choice for patients with familial adenomatous polyposis. With this operation comes the risk of developing ileal pouch polyps. Although rare, ileal pouch carcinomas may also occur within the pouch. Periodic endoscopic surveillance of the retained rectum and ileal pouch is recommended. Endoscopic polypectomy of medium and large polyps should be performed. Sulindac is effective in the reduction and often in the elimination of numerous smaller pouch polyps. Future studies are necessary to determine the role of sulindac and other chemotherapeutic agents in preventing the development of these polyps.

The process of tide-driven walking, proposed as a major mechanism for strike–slip displacement on Europa, is modeled using a finite-element numerical simulation of the behavior of viscoelastic material. For material parameters that are plausible for the water ice composing Europa's crust, the simulation confirms earlier analytic results for strike–slip displacement along a crack that penetrates down to the liquid water substrate. The finite element code permits testing other cases as well. Of considerable interest is whether tidal walking can operate if a crack penetrates not to liquid but only as far as warm, relatively viscous ice. In such a case, significant displacement can be driven, but only if the threshold value of the compressive force needed to lock the fault is near the value of the overburden stress at the bottom of the crack. Such special conditions are not needed for displacement if the crack penetrates to the underlying ocean.

The dwarf planet (1) Ceres, the largest object in the main asteroid belt with a mean diameter of about 950 kilometres, is located at a mean distance from the Sun of about 2.8 astronomical units (one astronomical unit is the Earth-Sun distance). Thermal evolution models suggest that it is a differentiated body with potential geological activity. Unlike on the icy satellites of Jupiter and Saturn, where tidal forces are responsible for spewing briny water into space, no tidal forces are acting on Ceres. In the absence of such forces, most objects in the main asteroid belt are expected to be geologically inert. The recent discovery of water vapour absorption near Ceres and previous detection of bound water and OH near and on Ceres (refs 5-7) have raised interest in the possible presence of surface ice. Here we report the presence of localized bright areas on Ceres from an orbiting imager. These unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible. Of particular interest is a bright pit on the floor of crater Occator that exhibits probable sublimation of water ice, producing haze clouds inside the crater that appear and disappear with a diurnal rhythm. Slow-moving condensed-ice or dust particles may explain this haze. We conclude that Ceres must have accreted material from beyond the 'snow line', which is the distance from the Sun at which water molecules condense.

The asteroid belt helps us in reconstructing the origin and the evolution of the solar system, probably better than the planets themselves. This is because the asteroid belt provides several key constraints that can be used to effectively guide the development, calibration, and validation of evolutionary models. Compared to other small-body populations, such as the Kuiper belt or Oort cloud, the constraints provided by the asteroid belt are probably more stringent, due to the fact that the number and the properties of the asteroids are better known, thanks to groundbased observations, space missions, and meteorite analysis. The structure of this review chapter is therefore as follows. We start by reviewing in section 2 what the most important observational constraints on the asteroid belt are and what they suggest. Then, in section 3, we will review the main models proposed, from the oldest to the most recent, and from the earliest to the latest evolutionary phases they address. In section 4, we will discuss several implications for asteroid science from our current preferred view of the dynamical evolution of the asteroid belt. The dynamical evolution of the asteroid belt has already been the object of a review chapter by Petit et al. (2002) in the Asteroids III book. This review therefore has an important overlap with that chapter. Nevertheless, both our

O'Brien and Greenberg [O'Brien, D.P., Greenberg, R., 2005. Icarus 178, 179–212] developed a self-consistent numerical model of the collisional and dynamical evolution of the main-belt and NEA populations that was tested against a diverse range of observational and theoretical constraints. In this paper, we use those results to update the asteroid cratering model of Greenberg et al. [Greenberg, R., Nolan, M.C., Bottke, W.F., Kolvoord, R.A., Veverka, J., 1994. Icarus 107, 84–97; Greenberg, R., Bottke, W.F., Nolan, M., Geissler, P., Petit, J., Durda, D.D., Asphaug, E., Head, J., 1996. Icarus 120, 106–118], and show that the main-belt asteroid population from the O'Brien and Greenberg collisional/dynamical evolution modeling is consistent with the crater records on Gaspra, Ida, Mathilde, and Eros, the four asteroids that have been observed by spacecraft.

New observations of Titan by the Cassini spacecraft suggest the presence of cryovolcanism on the surface. Cryovolcanism has important astrobiological implications, as it provides a means of exposing Titan's organics to liquid water, transforming hydrocarbons and nitriles into more evolved and oxidized prebiotic species. One possible cryovolcano – the 180 km structure Ganesa Macula – resembles the pancake domes seen on Venus by the Magellan spacecraft. To assess the potential of Ganesa Macula for prebiotic chemistry, we estimate its height using radarclinometry and other methods, and calculate the freezing timescale assuming an initially completely liquid dome. Given height constraints of ~200 m to 4 km, we find that liquid water or water–ammonia environments could be sustained in Ganesa Macula for timescales of the order of 102–105 years. These timescales open a window for prebiotic chemistry far wider than can be explored in terrestrial laboratory experiments.

Impact-induced seismic vibrations have long been suspected of being an important surface modification process on small satellites and asteroids. In this study, we use a series of linked seismic and geomorphic models to investigate the process in detail. We begin by developing a basic theory for the propagation of seismic energy in a highly fractured asteroid, and we use this theory to model the global vibrations experienced on the surface of an asteroid following an impact. These synthetic seismograms are then applied to a model of regolith resting on a slope, and the resulting downslope motion is computed for a full range of impactor sizes. Next, this computed downslope regolith flow is used in a morphological model of impact crater degradation and erasure, showing how topographic erosion accumulates as a function of time and the number of impacts. Finally, these results are applied in a stochastic cratering model for the surface of an Eros-like body (same volume and surface area as the asteroid), with craters formed by impacts and then erased by the effects of superposing craters, ejecta coverage, and seismic shakedown. This simulation shows good agreement with the observed 433 Eros cratering record at a Main Belt exposure age of 400 ± 200 Myr , including the observed paucity of small craters. The lowered equilibrium numbers (loss rate = production rate) for craters less than ∼100 m in diameter is a direct result of seismic erasure, which requires less than a meter of mobilized regolith to reproduce the NEAR observations. This study also points to an upper limit on asteroid size for experiencing global, surface-modifying, seismic effects from individual impacts of about 70–100 km (depending upon asteroid seismic properties). Larger asteroids will experience only localized (regional) seismic effects from individual impacts.