Robustness Analysis Weisberg, Michael
Philosophy of science,
12/2006, Letnik:
73, Številka:
5
Journal Article, Conference Proceeding
Recenzirano
Modelers often rely on robustness analysis, the search for predictions common to several independent models. Robustness analysis has been characterized and championed by Richard Levins and William ...Wimsatt, who see it as central to modern theoretical practice. The practice has also been severely criticized by Steven Orzack and Elliott Sober, who claim that it is a nonempirical form of confirmation, effective only under unusual circumstances. This paper addresses Orzack and Sober’s criticisms by giving a new account of robustness analysis and showing how the practice can identify robust theorems. Once the structure of robust theorems is clearly articulated, it can be shown that such theorems have a degree of confirmation, despite the lack of direct empirical evidence for their truth.
Abstract Comet 81P/Wild 2 is a ∼4.5 km-sized primordial object that almost has not been modified by internal heating by 26 Al decay. Its nucleus could have been formed by hierarchical agglomeration ...or gravitational collapse of pebble swarms concentrated by streaming instability. To shed light on the cometesimal formation mechanism from laboratory sample analysis, we reexamined the 26 Al– 26 Mg isotope systematics of the plagioclase-bearing fragment, Pyxie (from Wild 2 track 81), with significantly improved analytical precision. The revised upper limit of the initial ( 26 Al/ 27 Al) 0 of Pyxie is ≤1.5 × 10 −6 , 2 times smaller than those estimated from other Wild 2 fragments. Assuming homogenous distribution of 26 Al in the early solar system, the minimum crystallization age of Pyxie is estimated to be >3.6 Ma after calcium–aluminum-rich inclusions. Additional petrologic examination demonstrated that it is a chondrule fragment formed in disk environments enriched in moderately volatile elements comparable to the Si-rich rim of CR chondrules before accreting by comet Wild 2. The late accretion of the Wild 2 nucleus with most silicates likely from a common source are not favored by the hierarchical agglomeration model that considers early and continuous accretion. Instead, the results are more in line with comet formation by gentle gravitational collapse of pebbles when the 26 Al abundance is extremely low ( 26 Al/ 27 Al ≤ 1.5 × 10 −6 ) before gas dispersal.
Because contemporary scientific research is conducted by groups of scientists, understanding scientific progress requires understanding this division of cognitive labor. We present a novel ...agent‐based model of scientific research in which scientists divide their labor to explore an unknown epistemic landscape. Scientists aim to find the most epistemically significant research approaches. We consider three different search strategies that scientists can adopt for exploring the landscape. In the first, scientists work alone and do not let the discoveries of the community influence their actions. This is compared with two social research strategies: Followers are biased toward what others have already discovered, and we find that pure populations of these scientists do less well than scientists acting independently. However, pure populations of mavericks, who try to avoid research approaches that have already been taken, vastly outperform the other strategies. Finally, we show that, in mixed populations, mavericks stimulate followers to greater levels of epistemic production, making polymorphic populations of mavericks and followers ideal in many research domains.
Al-Mg isotope systematics of twelve FeO-poor (type I) chondrules from CR chondrites Queen Alexandra Range 99177 and Meteorite Hills 00426 were investigated by secondary ion mass spectrometry (SIMS). ...Five chondrules with Mg#’s of 99.0–99.2 and Δ17O of −4.2‰ to −5.3‰ have resolvable excess 26Mg. Their inferred (26Al/27Al)0 values range from (3.5 ± 1.3) × 10−6 to (6.0 ± 3.9) × 10−6. This corresponds to formation times of 2.2 (−0.5/+1.1) Myr to 2.8 (−0.3/+0.5) Myr after CAIs, using a canonical (26Al/27Al)0 of 5.23 × 10−5, and assuming homogeneously distributed 26Al that yielded a uniform initial 26Al/27Al in the Solar System. Seven chondrules lack resolvable excess 26Mg. They have lower Mg#’s (94.2–98.7) and generally higher Δ17O (−0.9‰ to −4.9‰) than chondrules with resolvable excess 26Mg. Their inferred (26Al/27Al)0 upper limits range from 1.3 × 10−6 to 3.2 × 10−6, corresponding to formation >2.9 to >3.7 Myr after CAIs. Al-Mg isochrons depend critically on chondrule plagioclase, and several characteristics indicate the chondrule plagioclase is unaltered: (1) SIMS 27Al/24Mg depth profile patterns match those from anorthite standards, and SEM/EDS of chondrule SIMS pits show no foreign inclusions; (2) transmission electron microscopy (TEM) reveals no nanometer-scale micro-inclusions and no alteration due to thermal metamorphism; (3) oxygen isotopes of chondrule plagioclase match those of coexisting olivine and pyroxene, indicating a low extent of thermal metamorphism; and (4) electron microprobe data show chondrule plagioclase is anorthite-rich, with excess structural silica and high MgO, consistent with such plagioclase from other petrologic type 3.00–3.05 chondrites. We conclude that the resolvable (26Al/27Al)0 variabilities among chondrules studied are robust, corresponding to a formation interval of at least 1.1 Myr.
Using relationships between chondrule (26Al/27Al)0, Mg#, and Δ17O, we interpret spatial and temporal features of dust, gas, and H2O ice in the FeO-poor chondrule-forming environment. Mg# ≥ 99, Δ17O ∼ −5‰ chondrules with resolvable excess 26Mg initially formed in an environment that was relatively anhydrous, with a dust-to-gas ratio of ∼100×. After these chondrules formed, we interpret a later influx of 16O-poor H2O ice into the environment, and that dust-to-gas ratios expanded (100× to 300×). This led to the later formation of more oxidized Mg# 94–99 chondrules with higher Δ17O (−5‰ to −1‰), with low (26Al/27Al)0, and hence no resolvable excess 26Mg.
We refine the mean CR chondrite chondrule formation age via mass balance, by considering that Mg# ≥ 99 chondrules generally have resolved positive (26Al/27Al)0 and that Mg# < 99 chondrules generally have no resolvable excess 26Mg, implying lower (26Al/27Al)0. We obtain a mean chondrule formation age of 3.8 ± 0.3 Myr after CAIs, which is consistent with Pb-Pb and Hf-W model ages of CR chondrite chondrule aggregates. Overall, this suggests most CR chondrite chondrules formed immediately before parent body accretion.
NWA 8785 is a remarkable, recently identified, unequilibrated enstatite chondrite. It was classified as an EL3 but contains highly unusual characteristics not observed in any other EL3, including a ...high abundance of FeO‐rich matrix and metal‐rich nodules that are texturally and mineralogically different from those in other EL3s. We characterized the mineral assemblages and compositions of metal‐rich nodules in a thin section of NWA 8785 and compared them to nodules in other EL3s to evaluate models for formation of metal‐rich nodules in EL3s. Of a total of 40 metal‐rich nodules, 10 were selected for detailed study. These metal‐rich nodules vary in their physical structure, texture, and mineral assemblages. Some contain the rare Al‐poor, alkali‐rich silicate mineral roedderite, a first discovery in an EL3, as well as the Cl‐bearing sulfide djerfisherite. The diversity of metal‐rich nodules in NWA 8785 suggests each nodule formed independently and supports their origin by primary processes prior to accretion. The high abundance of FeO‐rich matrix and the unique qualities of its metal‐rich nodules call into question classification of NWA 8785 as an EL3, but the Si content in its kamacite and Cr and Ti content in its troilite, and the presence of alabandite, support its classification as an EL3; thus, it is an EL3‐anomalous. Although alternative hypotheses exist, the presence of roedderite, as well as a magnetite‐rich matrix and sodalite, may provide the first evidence of extensive metasomatic alteration on the EL3 parent body.
We studied a thin section of Lewis Cliff (LEW) 87223, an unusual EL3‐related, enstatite chondrite (EC) that has primary and secondary features not observed in other ECs. We studied its metal‐rich ...nodules, possible shock features, and chondrules, eight of which are Al‐rich chondrules (ARCs). LEW 87223 has petrologic and compositional features similar to EL3s. Enstatite is the dominant mineral; chondrule boundaries are well defined; Si content of metal (0.5–0.6 wt%) is consistent with typical EL3; it has Cr‐bearing troilite, oldhamite, and alabandite; and its O‐isotopic composition is similar to other ECs. However, metal abundance in LEW 87223 (~13 vol%) is slightly higher than in other EL3s and its metal nodules are texturally and mineralogically different from other ECs. Both high and low Ni metals are present, and its alabandite has higher Fe (27.8 wt% Fe) than in other EL3s. Silicates appear darkened in plane polarized light, largely due to reduction of Fe from silicate. A remarkable feature of LEW 87223 is the high abundance of ARCs, which contain Ca‐rich plagioclase and varying amounts of Na‐rich plagioclase along chondrule edges and as veins. This suggests Na metasomatism and the possibility of hydrothermal fluids, potentially related to an impact event. LEW 87223 expands the range of known EC material. It shows that ECs are more diverse and record a wider range of parent body processes than previously known. LEW 87223 is an anomalous EL3, potentially the first member of a new EC group should similar samples be discovered.
We present detailed electron microprobe analyses and oxygen three-isotope measurements by high precision secondary ion mass spectrometry on 45 type I (FeO-poor) chondrules/fragments and 3 type II ...(FeO-rich) chondrule fragments from Meteorite Hills 00426 and Queen Alexandra Range 99177, two of the most primitive CR3 chondrites. Type I chondrules/fragments have Mg#’s (defined as the Mg# of constituent olivine and/or low-Ca pyroxene) ranging from 94.2 to 99.2; type II chondrule fragments have Mg#’s of 53–63. Oxygen three-isotope measurements plot on the slope ∼1 primitive chondrule mineral (PCM) line. Within chondrules, Δ17O (=δ17O–0.52×δ18O) values of coexisting olivine, pyroxene, and plagioclase are homogeneous, with propagated uncertainties of 0.3‰. This indicates each phase crystallized from the final chondrule melt, and that efficient oxygen isotope exchange occurred between ambient gas and chondrule melt. Among type I chondrules there is a well-defined increase in Δ17O, from –5.9‰ to ∼−1‰, as Mg#’s decrease from 99.2 to ∼96; type II chondrule fragments are comparatively 16O-poor (Δ17O: ∼0.2–0.6‰). The relationship between Mg# and Δ17O among type I chondrules confirms that addition of a 16O-poor oxidizing agent to the highest Mg# chondrule precursors resulted in forming lower Mg# CR chondrules. Using aspects of existing equilibrium condensation models and a mass balance we estimate that type I CR chondrules formed at dust enrichments of 100–200×, from dusts with 0–0.8 times the atomic abundance of ice, relative to CI dust. The type II chondrule fragments are predicted to have formed at CI dust enrichments near 2500×.
Oxygen three-isotope ratios of nine crystalline silicate particles from comet Wild 2 were measured to investigate oxygen isotope systematics of cometary materials. We are able to analyze particles as ...small as 4μm using an ion microprobe with a∼1×2μm beam by locating the analysis spots with an accuracy of ±0.4μm. Three particles of Mn-rich forsterite, known as low-iron, manganese-enriched (LIME) olivine, showed extremely 16O-rich signatures (δ18O, δ17O∼−50‰), similar to refractory inclusions in chondrites. The three Mn-rich forsterite particles may have formed by condensation from an 16O-rich solar nebula gas. Other particles consist of olivine and/or pyroxene with a wide range of Mg# =molar MgO/(FeO+MgO) % from 60 to 96. Their oxygen isotope ratios plot nearly along the carbonaceous chondrite anhydrous mineral (CCAM) and Young and Russell lines with Δ17O(=δ17O−0.52×δ18O) values of –3.0‰ to +2.5‰. These data are similar to the range observed from previous analyses of Wild 2 crystalline silicates and those of chondrules in carbonaceous chondrites. Six particles extracted from Stardust track 77 show diverse chemical compositions and isotope ratios; two Mn-rich forsterites, FeO-poor pigeonite, and three FeO-rich olivines with a wide range of Δ17O values from −24‰ to +1.6‰. These results confirmed that the original projectile that formed track 77 was an aggregate (>6μm) of silicate particles that formed in various environments.
The Δ17O values of ferromagnesian Wild 2 particles (including data from previous studies) increase from ∼−23‰ to+2.5‰ with decreasing Mg#: Δ17O values of Mn-rich forsterite particles (Mg#=98–99.8) cluster at −23‰, those of FeO-poor particles (Mg#=95–97) cluster at −2‰, and those of FeO-rich particles (Mg#≤90) scatter mainly from −1.5‰ to+2.5‰. Compared to chondrules in primitive chondrites, the systematic trend between Mg# and Δ17O among the Wild 2 particles is most similar to that reported for CR chondrite chondrules. We argue that CR chondrites and some cometary materials share multiple common chemical and isotope characteristics. We suggest that many of the crystalline silicate particles formed in the outer regions of the asteroid belt, or regions that share the common properties, and were transported to comet-forming regions and accreted into comet Wild 2.
► We analyzed oxygen isotope ratios of nine crystalline silicates from comet Wild 2. ► Oxygen isotope ratios are highly variable: 16O-rich to 16O-poor. ► 16O-rich Mn-rich forsterites may be condensates from an 16O-rich solar nebula gas. ► Δ17O-Mg# trend of 16O-poor silicates resembles that of CR chondrite chondrules. ► Many of Wild 2 silicates may have been transported from the outer asteroid belt.
Irritable bowel syndrome (IBS) affects 10%–15% of adults in the United States and 12% of the worldwide population. Gastroenterologists as well as primary care practitioners are likely to be the first ...resource for patients with gastrointestinal (GI) symptoms. IBS is difficult to diagnose, as it is a functional GI disorder, determined after ruling out a myriad of other diagnoses. The 2016 Rome IV criteria define IBS as “a functional bowel disorder in which recurrent abdominal pain is associated with defecation or a change in bowel habits. Disordered bowel habits are typically present (ie, constipation, diarrhea, or a mix of constipation and diarrhea), as are symptoms of abdominal bloating/distension occurring over at least 6 months and not less than 3 months.” Treatment of IBS historically has been through medical management; however, nutrition management of IBS using the FODMAP (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) elimination diet is now a recommended, first‐line therapy. FODMAPs are short‐chain, poorly absorbed carbohydrates that are associated with symptoms in people with IBS. This diet intervention can be quite challenging, and therefore, patients should meet with a qualified dietitian who can provide the complex diet information in a practicable form. Physician‐dietitian collaboration is invaluable for IBS patients to achieve a successful outcome.
Opaque assemblages (OAs) are small (submillimeter) objects composed primarily of metals, sulfides, and oxides that exist in nearly all chondritic meteorite groups as discrete objects in the matrix or ...associated with chondrules. The size, morphology, and petrology of OAs vary greatly between different chondrite groups, with petrologic grade within a single group, and by their apparent textural setting. Two hypotheses may explain the formation of matrix OAs: (1) they were separated from chondrules via surface tension during heating events, or (2) they formed as free‐floating objects in the solar nebula; however, this is the first comprehensive study of the petrology of OAs in ordinary chondrites (OCs) as a group, which seeks to determine if one hypothesis is sufficient to explain all such objects. Here, we use a newly developed machine learning algorithm to show that all OAs from the least equilibrated OC, Semarkona (LL 3.01), are composed of kamacite, taenite, troilite, pentlandite, magnetite, and other minor phases. These OAs form two distinct groups based on their modal mineralogy: one group in and associated with chondrules, and the other group free‐floating in the matrix. Chondrule OAs exhibit a bimodal distribution between sulfide‐ and metal‐rich endmembers in agreement with previous findings. Matrix OAs cluster at roughly equal abundances of sulfides and metals and universally exhibit magnetite rims. The two populations of chondrule OAs cannot be combined to form the modal mineralogies observed in matrix OAs and some matrix OAs exhibit mineralogical layering consistent with fractional condensation. Both observations support the hypothesis that matrix OAs were not formed by expulsion from chondrules and instead formed as free‐floating objects in the solar nebula; however, chondrule OAs must have formed with their host chondrules during heating events.
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