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Listing of Talk Abstracts
| The Sun in its Youth |
| Dr.
Alicia Aarnio (University of Michigan) |
| Pre-main sequence, solar-mass stars and their circumstellar disks provide unique astrophysical laboratories for studying the solar system and its properties preceding and during the epoch of planet formation. At the same time, the present day Sun yields a wealth of data giving insight to the microphysics needed to understand the processes involved in stellar evolution. X-ray surveys of young clusters have greatly advanced our understanding of activity on young stars, while optical studies have allowed us to measure rotation periods and infer the properties of the environs in which these systems are developing. Spectral energy distributions, radiative transfer modeling, and interferometry have constrained circumstellar disk properties, leading to ever improving understanding of protoplanetary disk structure and evolution. Spectropolarimetry has provided detailed maps of the magnetic field structures of young Suns, and progress has been made to characterize the far-reaching effects of the field in a star+disk system. In this talk, I review our present picture of the Sun in its youth as we observe in T Tauri stars via the methods mentioned here, as well as how our knowledge of the Sun has shaped our interpretations of these observations. |
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| Constraints on the Birth Environment of the Solar System
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| Fred Adams (University of Michigan) |
| Most stars -- and hence most solar systems -- form within groups and
clusters. The first objective of this talk is to explore how these
star forming environments affect solar systems forming within them.
The discussion starts with the dynamical evolution of young clusters
with N = 100 - 3000 members. We use N-body simulations to study how
evolution depends on system size and initial conditions. Multiple
realizations of equivalent cases are used to build up a robust
statistical description of these systems, e.g., distributions of
closest approaches and radial locations. These results provide a
framework from which to assess the effects of clusters on solar system
formation. Distributions of radial positions are used in conjunction
with UV luminosity distributions to estimate the radiation exposure of
circumstellar disks. Photoevaporation models determine the efficacy of
radiation in removing disk gas and compromising planet formation. The
distributions of closest approaches are used in conjunction with
scattering cross sections to determine probabilities for solar system
disruption. The result of this work is a quantitative determination of
the effects of clusters on forming solar systems. The second objective
of this talk is to use these results to place constraints on the
possible birth environments for our solar system. |
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| Helioseismology and the Early Solar Luminosity
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| Sarbani Basu (Yale University) |
| Helioseismology has been extremely successful in revealing what the structure of the Sun is like. Helioseismic analyses have shown that current solar models reproduce the structure of the Sun to withing fractio of a percent. However, helioseismic analysis is restricted to studying the current Sun. In this talk I shall review attempts that have been made to model the Sun in a manner so that it had high luminosity early in its life but conforms to the current helioseismically determined solar structure.
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| The Independency of Stellar Mass-Loss Rates on Stellar X-ray Luminosity and Activity Level Based on Solar X-ray Flux and Solar Wind Observations |
| Ofer Cohen (Harvard-Smithsonian CfA) |
| Stellar mass-loss rates are an important input ingredient for stellar evolution models since they determine stellar evolution parameters such as stellar spin-down and increase in stellar luminosity through the lifetime of a star. Due to the lack of direct observations of stellar winds from Sun-like stars stellar X-ray luminosity and stellar level of activity are commonly used as a proxy for estimating stellar mass-loss rates. However, such an intuitive activity - mass-loss rate relation is not well defined. In this paper, I study the mass-loss rate of the Sun as a function of its activity level. I compare in situ solar wind measurements with the solar activity level represented by the solar X-ray flux. I find no clear dependency of the solar mass flux on solar X-ray flux. Instead, the solar mass-loss rate is scattered around an average value of 2 × 10-14 Msun yr-1. This independency of the mass-loss rate on level of activity can be explained by the fact that the activity level is governed by the large modulations in the solar close magnetic flux, while the mass-loss rate is governed by the rather constant open magnetic flux. I derive a simple expression for stellar mass-loss rates as a function of the stellar ambient weak magnetic field, the stellar radius, the stellar escape velocity and the average height of the Alfvén surface. This expression predicts stellar mass-loss rates of 10-15 to 10-12 Msun yr-1 for Sun-like stars. |
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| Winds of Change and Rays of Truth: Does the Sun Really Have a Mid-Life Crisis?
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| Jeremy Drake (Smithsonian Astrophysical Observatory) |
| The dependence of stellar luminosity on mass for stars like the Sun is quite steep, varying approximately as L~M^5.
Consequently, if the Sun were only a few percent more massive when it was 1 Gyr old, its luminosity would have been be sufficient to obviate the need for other solutions to the faint young Sun problem. Just a small amount of mass loss over 3 billion years is needed - but this small amount is huge compared with the current mass loss rate of the Sun. We know the young Sun was much more magnetically active when it was younger and perhaps losing mass at a much higher rate. Evidence from the spin-down rates of younger stars has tended to fall considerably short of the mass loss required. But traditional mass loss and spin-dpwn treatments are simplistic. Here we look at the observational evidence from stars and more sophisticated wind model treatments and show that significantly more mass loss than previously thought could have occurred and the paradox could be.... We also examine claims that cosmic ray fluxes were much lower in the vicinity of Earth at 3 Gya and discuss the implications if cosmic rays can affect climate through cloud seeding as some researchers have suggested.
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| Environments on Early Mars: Constraints from the Geologic Record |
| Prof.
Bethany Ehlmann (Caltech, JPL) |
| Approximately 50% of the exposed rock units on Mars date from its earliest Noachian and Hesperian periods. Consequently, the Martian geologic record provides an important data point on changing environmental conditions during the first billion years of solar system history. In addition to long-standing geomorphic evidence for liquid water on the surface of Mars, at least episodically, during its early history, new data on unit mineralogy from orbital infrared spectroscopy also reveal diverse aqueous geochemical environments that changed through time. Here, I will review the timeline of evolution of early Martian environments based on this combined geomorphic and mineralogic record. The drivers of change are not fully understood, but I will discuss hypotheses for exogenous drivers of change (e.g. solar luminosity, impact bombardment) versus endogenous drivers of change (e.g. secular cooling, rates and styles of volcanism). |
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| Climate Modeling of the Early Earth: What Happened When and How Well Do We Know It? |
| Prof.
James Kasting (Penn State University) |
| Archean climate, 2.5-3.8 Ga, has always been something of a mystery. Oxygen isotopic data from cherts seem to indicate that the mean surface temperature was very high, ~70oC, whereas glacial deposits at ~2.4 Ga and 2.9 Ga seem to indicate that it was relatively low, probably <20oC. (By comparison, today’s value is 15oC.) From a theoretical standpoint it is unlikely that the mean surface temperature went up and down by 50 degrees multiple times; thus, one or the other of these geologic datasets has been misinterpreted. Theory also tells us that the Sun was 20-25% less bright during this time period; hence, the climate would have been very cold had not the greenhouse effect been stronger than today, or the planetary albedo lower. Albedo changes have been suggested multiple times in the literature, most recently by Rosing et al. (Nature, 2010). This change alone, however, is unlikely to have been able to keep the early Earth warm. A stronger greenhouse effect is therefore indicated. CO2 and H2O alone could have provided sufficient warming; however, paleosol evidence suggests that CO2 concentrations were below the required value of ~0.1 bar, or 300 PAL (times the Present Atmospheric Level). The required CO2 amount can be reduced to ~0.02 bar by the addition of 1000 ppmv of CH4, which ought to have been there anyway if methanogens were an important part of the Archean ecosystem, as seems likely. One can reduce this CO2 partial pressure by an additional factor of 2 if the N2 partial pressure was significantly higher, as suggested by Goldblatt et al. (Nature Geoscience, 2009). This puts pCO2 at ~30 PAL, which is within the limits of 10-50 PAL estimated from paleosols at 2.7 Ga (Driese et al., Precambrian Res., 2011). The mean surface temperature assumed for these estimates is 15oC, like today. Thus, Archean climate can be explained self-consistently if the mean surface temperature was indeed close to the modern value. |
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| Isotopic Evidence for High Surface Temperatures on the Archean Earth |
| L Paul Knauth (Arizona State University) |
| Oxygen isotope data show an increase of about 1% in the 18O/16O ratio for cherts in sedimentary sequences over the past 3.8 billion years. There are 4 possible explanations for this trend which has now also been observed in sedimentary carbonate rocks: 1) Cherts somehow lost 18O during diagenesis (burial transformations) and/or metamorphism. 2) Unlike younger examples, cherts on the early Earth formed by hydrothermal emanations around sea floor vents. 3) The 18O content of the hydrosphere has increased over geologic time by this amount. 4) Surface temperatures have declined with time.
1) Explanations suggesting that cherts lose 18O with time assume that quartz precipitates in the ocean and is then altered during diagenesis. In fact, cherts are phases that form during diagenesis when initially precipitated opal is mobilized and transformed into interlocking crystals of microcrystalline quartz. This happens during early burial for nodular cherts where low 18O meteoric waters intrude into the coastal burial systems. Deep sea oozes transform in pulses into different kinds of quartz chert during early to late burial. Cherts form with a wide range of initial 18O contents. Subsequent deformation and metamorphism can recrystallize cherts and decrease their 18O content if the water/rock ratio is anomalously high. Metacherts are coarsely crystalline, easily identifiable, and not used to deduce history of depositional conditions. Extensive testing of the later alteration scenarios have now been published and demonstrate clearly that the lower 18O values of chert were established in the Archean and before tectonic deformation that concluded the depositional cycle. Also, the highest 18O Archean cherts are lower than younger cherts that form or alter during deepest burial. The alteration scenario is untenable.
2) Quartz that forms in hydrothermal systems is easily recognized and is isotopically distinct from chert. Arguments that Archean cherts are all hydrothermal quartz aprons around sea floor vents are not supported by the now extensively-described geology, sedimentology, and petrography of the Archean sedimentary sequences. Geology simply must be considered.
3) Early claims that 18O of the oceans has undergone large changes have been challenged since the advent of Plate Tectonics when it was shown that isotopic exchange between sea water and ridge material buffers the 18O content of sea water to near its present value. Extensive analyses of ancient igneous rocks that have undergone such exchange as well as those in other settings show no change over geologic time indicating that oceans were not strongly depleted in 18O in the past.
4) When an understanding of how and when chert forms and how it is distinguished from metachert and hydrothermal quartz is considered, climatic temperature change emerges as the simplest explanation for the chert isotopic data. Carbonate and silicon isotope changes over time are consistent with temperature decline as are indications from physical sedimentology that the Archean oceans had lower viscosity. Objections to this interpretation have centered around suggested difficulties for early life, isotopic analyses of trace constituents (hydrogen, phosphate), and the faint young Sun paradox. However, the nature of the fossil record, discovery that earliest common ancestors are optimized at the proposed higher temperatures, and the molecular biology of early organic molecules now actually support the climate change interpretation. Isotopic analyses of trace constituents involve bizarre pre-treatments of samples, non-reproducible data, and incorrect mass spectrometer data corrections in the case of hydrogen, and unsupportable assumptions about how cherts form and the nature of isotopic exchange during metamorphism in the case of phosphate.
No sensible discussion of changes in the 18O composition with time and how it relates to depositional temperatures of the initial silica is possible without serious consideration of how cherts form, how they are distinguished, and how the data correspond to the mode of formation or later metamorphism. These issues have been extensively described and exhaustively discussed in the papers arguing that the trend with time is most readily explained in terms of climatic temperatures. Models for the atmospheric greenhouse evolution remain in flux, and the faint early Sun issue is in the hands of astronomers.
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| The Measurement of Winds in Sun-Like Stars |
| Prof.
Jeffrey Linsky (JILA/University of Colorado) |
| Abstract: The properties of Sun-like star winds, in particular the mass loss rate, are important for the evolution of stellar angular momentum,dynamo generation of magnetic fields, and the chemical composition of planetary atmospheres. The later field has become especially important recently with the discovery of super-Earths in the habitable zones of stars. Until recently, the present day Sun provided the only measured data point concerning mass loss rates of late-type dwarf stars. Brian Wood and collaborators have shown that charge exchange reactions between the ionized stellar wind and the partially-ionized interstellar medium produces a "hydrogen wall" around a star that can be observed as absorption in the stellar Lyman-alpha line to the blue of the interstellar absorption feature. I will describe this technique and its application to late-type stars. Using scaling laws between the mass loss rate and X-ray
flux and between the X-ray flux and age, I will present our presictions of the solar mass loss rate with time beginning at an age of 600 Myr. I will compare these predicted mass loss rates with new models of stellar winds.
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| Was the Earth Always at 1 AU (and Was the Sun Always 1 Solar Mass)? |
| David Minton (Purdue University) |
| The migration of planets, planetary embryos, and planet cores is now a commonly-studied process in the early evolution of the solar system and exoplanet systems. Here I review the mechanisms by which planets can migrate and whether or not they are plausible solutions to the Faint Young Sun Paradox (FYSP). I also review calculations of the time-evolution of the solar mass that would be required to solve the FYSP hypothesis. |
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| Assessing the Standard Solar Model |
| Marc Pinsonneault (Dept. of Astronomy, Ohio State University) |
| The standard solar model has been rigorously tested with powerful diagnostics, such as solar neutrinos and helioseismology. We demonstrate that solar models make strong predictions about the luminosity evolution of the Sun, and that mass loss is the most significant potential factor not typically considered in solar models. A physically based prescription scaled to the solar rotation history does not significantly impact the luminosity evolution for the large majority of the lifetime of the Sun. This points to solutions of the faint young Sun paradox which do not alter the predicted luminosity evolution. |
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| Banded Iron Formation: Probes of Archaean Surface Environments |
| Minik Rosing (University of Copenhagen) |
| The energy balance for Earth’s surface environments is strongly influenced by the radiative properties of the atmosphere. At present, we have no proxies to address the total atmospheric pressure on past atmospheres, but we can address the partial pressures of reactive gasses such as O2 and CO2 through the composition of geologic deposits. The widespread preservation of magnetite in Archaean sedimentary rocks place upper limits on the partial pressure of CO2 in the atmosphere due to stability limits of magnetite in the presence of CO2 and because siderite saturation limits the solubility of Fe In seawater at high PCO2. Within the limits defined by magnetite stability and siderite solubility, the greenhouse effect of CO2 and CH4 cannot compensate for a fainter young Sun. We suggest that reduced albedo due to lower cloud albedo and lower surface albedo of virtually continent-free Earth.
The magnetite stability argument has been challenged by Reinhardt and Planavsky (2011) and Dauphas and Kasting (2011) who argue that the mineral compositions of banded iron formations are microbially controlled and do not reflect ocean or atmosphere chemistry. Our view is, that marine chemical sediments form when combinations of solute species in seawater exceed the solubility products of solid phases such as carbonate, sulfate or oxide minerals. Mineral precipitation can be influenced by living organisms, either by direct mediation, or because metabolic activities perturb the aqueous environment and cause abiotic precipitation. The formation of chemical sediments depends on two requirements. The mineral components must be efficiently transported in solution, and an effective vector for precipitation must be present in the environment. The most common chemical sediments on the present Earth are Ca and Mg carbonates. The widespread formation of Ca-Mg carbonate sediments is possible because Ca and Mg are major solutes in modern seawater, and are efficiently transported to sites of carbonate precipitation. It is not possible to assign one unique mode of carbonate precipitation to all carbonate sediment. Some form by changes in temperature, some are biologically mediated while others form due to degassing of CO2 or to evaporation of water from saturated solutions. By analogy, the presence of banded iron formation during the Precambrian can be understood in the light of efficient transport of dissolved Fe in the reducing water under an oxygen poor atmosphere. Just like carbonate precipitation on the present Earth, iron-minerals would have precipitated from ancient oceans due to a multitude of reasons. In some banded iron formations, iron oxidizing phototrophic organisms seem to have provided the chemical vector for precipitation, because they excrete ferric iron, which alters the near environment of the organisms and leads to precipitation of ferric iron minerals such as ferri-oxy-hydroxide, magnetite or green rust, but we can not assume that this is the case for all BIF deposition through the geologic record. Iron formations do not form today because Fe2+ is no longer a major solute in the ocean, and not because the mechanisms of precipitation cannot operate on the modern Earth. A requirement for the formation of BIF is that iron is mobile as a solute in seawater, and can be transported on the scale of a sedimentary basin. This requires a high concentration of Fe2+ or soluble complexes. Absence of atmospheric oxygen is not the only requirement for this. At high pCO2, siderite saturation limits the mobility of iron because the concentration of dissolved iron is reduced to a level similar to that of the present ocean even at anoxic conditions at a pH buffered by basalt. We therefore conclude that the formation of BIF indicate high solubility of Fe and the widespread preservation of magnetite in Archaean BIF suggests that magnetite was thermodynamically stable in contact with seawater: Both are inconsistent with atmospheric partial pressure of CO2 much higher than the 1000 ppm range.
References
1. Reinhard, C. T. & Planavsky, N. J. Mineralogical constraints on Precambrian pCO2. Nature 472, doi:10.1038/nature09959 (this issue).
2. Dauphas, N. & Kasting, J. F. Low pCO2 in the pore water, not in the Archean air. Nature 472, doi:10.1038/nature09960 (this issue).
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| The Case for a Hot Early Earth |
| David Schwartzman (Howard University) |
| I will make the case for a much warmer climate on the early Earth than now. The oxygen isotope record in sedimentary chert and the compelling case for a near constant isotopic oxygen composition of seawater over geologic time support thermophilic surface temperatures until about 1.5- 2 billion years ago, aside from glacial episodes at 2.1-2.4 Ga and possibly one at 2.9 Ga.
Other evidence includes the following:
1) Melting temperatures of proteins resurrected from sequences inferred from robust molecular phylogenies give paleotemperatures at emergence consistent with a very warm early climate.
2) High atmospheric pCO2 levels in the Archean prior to about 2.8 Ga are consistent with high climatic temperatures at the triple point of primary iron minerals in BIFs, the formation of Mn-bicarbonate clusters leading to oxygenic photosynthesis and generally higher weathering intensities on land. These higher weathering intensities would not have occurred if seafloor weathering dominated the carbon sink, pulling down the temperature, hence this empirical evidence supports a hot climate and high carbon dioxide and possibly methane levels.
3) The inferred viscosity of seawater at 2.7 Ga is consistent with a hot Archean climate (Fralick and Carter, 2011).
4) A temperature constraint held back the emergence of major organismal groups, starting with phototrophs. Oxygen levels alone cannot explain the big delay in the appearance of the “higher” kingdoms, given the likely presence of oxygen in microenvironments in the Archean, as well as the plausibility of early anoxic metazoa.
5) A cold Archean is hard to explain taking into account the higher outgassing rates of carbon dioxide, smaller land areas and weaker biotic enhancement of weathering than present in the context of the long term carbon cycle, taking into account the fainter Archean sun in climate modeling.
References
Fralich, P. and J.E. Carter, 2011, Neoarchean deep marine paleotemperature: Evidence from turbidite successions Precambrian Research 191 (2011) 78– 84
Knauth L.P., 2005, Temperature and salinity history of the Precambrian ocean: implications for the course of microbial evolution. Palaeogeogr. Palaeoclimatol. Palaeoecol. 219:53–69.
Schwartzman, D., 1999, 2002 (updated), Life, Temperature and the Earth: The Self-Organizing Biosphere. Columbia Univ. Press.
Schwartzman, D.W. and L. P. Knauth, 2009, A hot climate on early Earth: implications to biospheric evolution. In: K.J. Meech, J.V. Keane, M.J. Mumma, J.L. Siefert and D.J. Werthimer (eds.) Bioastronomy 2007: Molecules, Microbes, and Extraterrestrial Life, Astronomical Society of the Pacific Conference Series Vol. 420, San Francisco, pp. 221-228.
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| Precambrian CO2 Constraints from Paleosols |
| Dr.
Nathan Sheldon (University of Michigan) |
| While photochemical models have provided valuable theoretical constraints on Precambrian atmospheric conditions, pCO2 levels have also previously been estimated using simple thermodynamic models based upon the mineral assemblages in paleosols and banded ironr formations (BIFs). However, thermodynamic approaches often require unreasonable or poorly constrained assumptions, and their results are highly dependent on the quality of the thermodynamic data available. In particular, re-evaluation of the paleosol thermodynamic model using more recent thermodynamic data demonstrates that that approach is unreliable and does not provide a significant constraint for atmospheric pCO2 reconstruction. Similarly, the timing of mineral formation in BIFs makes it unclear whether the observed mineral assemblage is representative of authigenic or diagenetic conditions.
As an alternative, a new model based upon paleosol mass-balance during weathering has been proposed. The paleosol mass-balance model gives replicable results from multiple and widely distributed contemporaneous paleosols from the Paleoproterozoic (~2.2 Ga ago) and Mesoproterozoic, and gives results that are consistent with independent proxies based on microfossils. The pCO2 curve generated by this method spans from 2.7–0.96 Ga ago and indicates that: 1) late Archean and early Paleoproterozoic pCO2 levels were similar, suggesting no significant change in response to the Great Oxidation Event; 2) from at least 2.7–1.8 Ga ago, pCO2 levels were broadly consistent at 20–40 times pre-industrial levels (PIL); 3) between 1.8 and 1.1 Ga ago, there was a significant drop in pCO2 to less than 10 times PIL, coincident with a change in calcification and stromatolite abundance in the oceans. Climate model results using the paleosol-derived pCO2 values indicate consistently equable conditions from 2.7–0.96 Ga ago and are also permissive of a Paleoproterozoic “snowball” Earth event, which suggests that the paleosol mass-balance model provides a quantitatively valuable constraint on Archean and Proterozoic pCO2 levels. More poorly constrained data from older Archean paleosols/weathering surfaces are also consistent with high, but not ultra-high (>100x PIL) pCO2 levels, which suggests that even under essentially anoxic conditions, the "Faint Young Sun" paradox can not be solved with CO2 alone.
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