Non-Cognitive Predictors of Student Success:A Predictive Validity Comparison Between Domestic and International Students

Non-Cognitive Predictors of Student Success:A Predictive Validity Comparison Between Domestic and International Students

INTRODUCTIONDiscoveries of silicate-metal worlds around other stars haveinspired diverse geophysical models of their plausiblestructures and tectonic regimes [1]. These models areseverely hampered, however, by inexact assumptions aboutlong-lived radiogenic heat production (235,238U, 232Th, 40K) andinventories of rock-forming elements (e.g. Si, Al, Fe, Ca, Na,Mg).

Host stars of planets ought to broadly reflect the overallcompositions of planetary systems [2,3; Figure 1]: to date, theonly confirmed connection between stellar host abundancesand planets is the presence of Jupiter-mass worlds aroundstars with enriched [Fe/H] content [e.g. 4]. Otherwise, stellarmetallicity is a poor predictor of the likeliness of hostingterrestrial-type planets [5; Figure 2]. Planets form in acompositional gradient dictated by thermodynamicalequilibrium in the disc and the chemical composition andtemperature of the newly formed central star, and mixing-inof intrinsic and extrinsic material in the stellar birth cluster.

Key elements within the planet govern mantle processes, andthus the nature of its crust, composition of atmosphere andretention of a hydrosphere [e.g. 6,7]. For example, withoutplate recycling dictated by mantle rheology (internal heat,mineralogy, water content) a carbon cycle is unlikely.

To better understand the geodynamical nature of exoplanetsto make predictions about observations of such phenomenaand M-r and atmospheric compositions, we must understandhow differences in stellar host compositions would bereflected in the evolution of exoplanets [e.g. 8].

Here, I show how Galactic Chemical Evolution (GCE) models ofstar (and planet) age and composition yield different effectson geodynamical regimes. The geodynamical consequences ofthese differences are discussed in the following sections.

EARTH-LIKE EXOPLANETARY MANTLESRecent [8-10] GCE codes: (1) improve models for theevolution of radiogenic heating in rocky exoplanets and (2)assess the geophysical effects of different rock-formingelement inventories (e.g. Mg/Si; Figure 3) emphasizing factorsthat affect geodynamic regimes (e.g. mantle properties, heatproduction, crust type).

Cosmochemically Earth-like exoplanets

Stephen J. Mojzsis1,2

mojzsis@colorado.edu http://isotope.colorado.edu

1. CRiO, Geological Sciences, University of Colorado2. Hungarian Academy of Sciences

SEE THE PRESENTATION: EPSC-DPS2019-1460 | Orals | EXO2Constraining the range of bulk terrestrial exoplanet compositions, and their effects on coupled interior-atmosphere evolution

by Robert Spaargaren, Maxim Ballmer, Stephen Mojzsis, Daniel Bower, Caroline Dorn, and Paul TackleyTue, 17 Sep, 11:15–11:30 Venus (Room 6)

Figure 1. The similarity of meteoritic and photosphericabundances is evident in this figure for our solar valuesvs. terrestrial compositions. Abundances have beennormalized to 106 atoms of Si. Points along the diagonalhave near-identical abundances in both meteoritic andphotospheric data sets. Elemental abundances above thediagonal are depleted in CI chondrites (N, C, O, H andnoble gases). Elemental abundances below the diagonalare depleted in the Sun (Li). [from ref. 3]

Figure 2. Mass-radius relation for known low-massexoplanets. The spread indicates a variety in density. Here,the very large uncertainties for radius and mass are omitted.Simple interior structure estimates are also shown by coloredlines. The intensity in color of the dots indicates themetallicity (Fe/H ratio) of the host star as shown in the legendbar at right. (Data source: www.exoplanet.eu).[see ref. 5]

Figure 3. observed variations in stellar compositions of FGKM starsfrom spectrographic measurements (translucent red dots).Compositions are taken from the Hypatia catalogue [11]. Solar-system composition is marked by the yellow diamond [12].Subsequent work reported here neglects the M-stars in the analysis.

EPSC-DPS2019-1952 | EXO2Tue, 17 Sep, 17:15–18:45 | L2.67

THEORY AND PRACTICEKey processes related to the geodynamics of “Earth-like”rocky planets begin with mantle compositions which resultin different geodynamical states [6-8]. The most importantphysical property for planetary interiors is viscosity, whichcan differ by orders of magnitude between differentminerals. The six most abundant rock-forming elements inthe Sun are Si, Mg, Fe, S, Al and Ca. The first mostimportant ratio for mineralogy is the magnesium to silicon(MgO:SiO2) ratio.

The argument: (i) Earth’s Primitive Mantle [Mg/Si] is ~1.03(CI=0.93); (ii) The dominant upper mantle (UM) phase ofthat composition is olivine (Mg,Fe)2(SiO4) for which nolattice site can accommodate Fe3+; (iii) If Earth had inheriteda slightly lower [Mg/Si] (e.g. <0.9), pyroxene ((XY(Si,Al)2O6,where X represents divalent Ca, Mg and Fe, and Yrepresents trivalent Cr, Al, Fe) would dominate. Pyroxenetakes up Fe3+ into its structure and with substitutionsmaintains low activity of Fe3+ and a very low oxygenfugacity; (iv) Owing to (ii), the Fe3+ present in the Earth’sUM goes into spinel ((Mg,Fe)Al2O4) such that there is amodal phase imposing a high oxygen fugacity (~FMQ) ongases in equilibrium with rock; (v) Consequently, for theentirety of the geologic record it seems that Earth’s UMalways degassed a relatively oxidized form of carbon (CO-CO2) rather than an alternative mantle which would degasCH4-CO [e.g. 13].

The consequences: The Earth’s lower mantle containsdifferent minerals and is dominated by bridgemaniteperovskite (MgSiO3) which has a several orders-of-magnitude higher viscosity than the second most importantmineral, magnesiowüstite (MgO). In this case, an increase inMgO fraction would lead to magnesiowüstite dominatingthe rheology and the overall strength of the LM being muchweaker. These arguments consider only MgO and SiO2;other important oxides such as FeO, CaO, Al2O3 also affectthe mineralogy and physical properties and warrant study.An increase in viscosity results in a hotter planet with moremelting, whereas a decrease in viscosity results in a coolerplanet. In turn, along with age, this affects the likelihood ofthe planet developing and sustaining plate recylcing.

CONCLUSIONSIn metal+silicate planets, subtle changes in [Mg/Si] vs.[Fe/H] for different stellar sources of different ages shouldmake the difference between a CH4 vs. CO2 atmosphere anda fluid, convecting (pure olivine) interior vs. a stiff, non-convecting (pure pyroxene), mantle [Figures 4-7].

REFERENCES[1] Laughlin G. and Lissauer J.L. (2015) Treatise on Geophysics, 2nd edition.[2] Goldschmidt V. M. (1937) Norske vidensk. – akad. Oslo, Mat.-Nat. Klass,4, 1-148.[3] Wang H.-Y. et al. (2019) MNRAS 482, 2, 2222–2233.[4] Fischer D.A. and Valenti J. (2005) Astrophys. J. 622, 1102-1117.[5] Berg et al. (2009) Astrophys. J. 702, L172.[6] Ringwood A.E. (1989) EPSL, 95, 1-7.[7] Palme H. and O'Neill H. St. C. (2014) Treatise on Geochemistry, 2ndedition. DOI/10.1016/B978-0-08-095975-7.00201-1.[8] Frank E.A. et al. (2014) Icarus 243, 274-286.[9] Côté B. et al. (2016) Astrophys. J. 824, 82. doi:10.3847/0004-637X/824/2/82[10] Lugaro M. et al. (2018) Prog. Part. Nucl. Phys. 102, 1-47.[11] Hinkel N.R. et al. (2014) AJ. 148 (3) L54.[12] Asplund M. et al. (2005)[13] Trail D. et al. (2011) Nature, 480, 79-82.[14] Adebekyan V. Zh. et al. (2012) A&A 545, A32.

Figure 4. Mass fractions of some major mantle-formingelements. In the analytical Clayton model described in ref.8, these are primary species that experience a constantstate of enrichment in the gas since the gas is initiallymetal-free and have concentrations that are fit to CI valuesat Solar System formation.[from ref. 8]

Figure 5. Mass fraction evolution of Fe in gas. Initially, it isonly produced in massive stars, but 1 Gyr after formation,Type Ia supernovae begin contributing Fe to the gas,boosting its production rate.[8]

Figure 6 Plot of the Mg/Si ratio in solar systems that form aroundSun-like stars using a new multi-zone GCE calculationsincorporating stellar lifetimes. As with Fe in Figure 5, initially, Si ismore effectively produced in massive stars, and as Type Iasupernovae begin contributing more Si to the ISM gas, the ratio ofMg/Si declines with the boosted concentration of Si.[unpublishedmanuscript]

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Figure 7 (A). Si/Fe mass fraction ratio over galactic history. Early in the Galaxy’s history (t < 1 Gyr), the ratio decreaseswhile Si and Fe are both being produced in massive stars, but when Type Ia supernovae begin exploding, the productionof Fe increases, leading to declining Si/Fe. (B). Model output of Mg/Si vs. Fe/H [from ref. 8]. (C). [Mg] and [Si] as well as(D) [Al] and [Ca] plotted against [Fe/H] for 1111 FGK stars from ref. 16. (D). Data compilation of ratios, compare to (B).

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