Exploring the Bimodal Solar System via Sample Return from the Main Asteroid Belt: The Case for Revisiting Ceres

Abstract: Sample return from a main-belt asteroid has not yet been attempted, but appears technologically feasible. While the cost implications are significant, the scientific case for such a mission appears overwhelming. As suggested by the “Grand Tack” model, the structure of the main belt was likely forged during the earliest stages of Solar System evolution in response to migration of the giant planets. Returning samples from the main belt has the potential to test such planet migration models and the related geochemical and isotopic concept of a bimodal Solar System. Isotopic studies demonstrate distinct compositional differences between samples believed to be derived from the outer Solar System (CC or carbonaceous chondrite group) and those that are thought to be derived from the inner Solar System (NC or non-carbonaceous group). These two groups are separated on relevant isotopic variation diagrams by a clear compositional gap. The interface between these two regions appears to be broadly coincident with the present location of the asteroid belt, which contains material derived from both groups. The Hayabusa mission to near-Earth asteroid (NEA) (25143) Itokawa has shown what can be learned from a sample-return mission to an asteroid, even with a very small amount of sample. One scenario for main-belt sample return involves a spacecraft launching a projectile that strikes an object and flying through the debris cloud, which would potentially allow multiple bodies to be sampled if a number of projectiles are used on different asteroids. Another scenario is the more traditional method of landing on an asteroid to obtain the sample. A significant range of main-belt asteroids are available as targets for a sample-return mission and such a mission would represent a first step in mineralogically and isotopically mapping the asteroid belt. We argue that a sample-return mission to the asteroid belt does not necessarily have to return material from both the NC and CC groups to viably test the bimodal Solar System paradigm, as material from the NC group is already abundantly available for study. Instead, there is overwhelming evidence that we have a very incomplete suite of CC-related samples. Based on our analysis, we advocate a dedicated sample-return mission to the dwarf planet (1) Ceres as the best means of further exploring inherent Solar System variation. Ceres is an ice-rich world that may be a displaced trans-Neptunian object. We almost certainly do not have any meteorites that closely resemble material that would be brought back from Ceres. The rich heritage of data acquired by the Dawn mission makes a sample-return mission from Ceres logistically feasible at a realistic cost. No other potential main-belt target is capable of providing as much insight into the early Solar System as Ceres. Such a mission should be given the highest priority by the international scientific community

Ion-selective electrodes for sodium and potassium: a new problem of what is measured and what should be reported

For clinicalpurposesthe activitiesof Na and K obtained with ion-selective electrodes in undiluted whole blood or serum should be multiplied by an appropriate factor to obtain the same values as the substance concentrations obtained by flame photometry. The factor is primarily dependent on the massconcentration of water in normal plasma divided by the molal activity coefficient of Na (or K) of normal plasma. We discuss the value of the molal activity coefficient of Na obtainedby theoreticalcalculationsand by direct measure-ment. The discrepanciesbetween theory and measurement (+ of 0.747 and 0.73, respectively)may be due to some binding of Na (protein binding or ion pair formation), a small and variable residual liquid-junction potential, or uncertainty about the appropriate value for the ionic strength of normal plasma (0.16 mol/kgor somewhathigher)

A Strategy for Automated Meaning Negotiation in Distributed Information Retrieval

The paper reports on the formal framework to design strategies for multi-issue non-symmetric meaning negotiations among software agents in a distributed information retrieval system. The advancements of the framework are the following. A resulting strategy compares the contexts of two background domain theories not concept by concept, but the whole context (conceptual graph) to the other context by accounting the relationships among concepts, the properties and the constraints over properties. It contains the mechanisms for measuring contextual similarity through assessing propositional substitutions and to provide argumentation through generating extra contexts. It uses presuppositions for choosing the best similarity hypotheses and to make the mutual concession to common sense monotonic. It provides the means to evaluate the possible eagerness to concede through semantic commitments and related notions of knowledgeability and degree of reputatio

The effectiveness of the Arctic Council

Created in 1996, the Arctic Council has now been in operation long enough to justify a systematic effort to assess its effectiveness. To explore this topic, we created a questionnaire and circulated it to a large number of individuals who have participated in the work of the council in one capacity or another or who have followed the work of the council closely. This article analyses the quantitative and the qualitative input of those who responded to the questionnaire. The main conclusions are that: (1) the council has achieved considerable success in identifying emerging issues, framing them for consideration in policy venues and raising their visibility on the policy agenda and (2) changes now occurring in the Arctic will require significant adjustments to maintain the effectiveness of the council during the foreseeable future