Jupiter's Interior as Revealed by Juno Stevenson, David J
Annual review of earth and planetary sciences,
05/2020, Letnik:
48, Številka:
1
Journal Article
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Jupiter is in the class of planets that we call gas giants, not because they consist of gas but because they were primarily made from hydrogen-helium gas, which upon gravitational compression becomes ...a metallic fluid. Juno, in orbit about Jupiter since 2016, has changed our view: The gravity data are much improved, and the simplest interpretation of the higher order even harmonics implies that the planet may have a diluted central concentration of heavy elements. Jupiter has strong winds extending to perhaps ∼3,000-km depth that are evident in the odd zonal harmonics of the gravity field. Jupiter's distinctive magnetic field displays some limited local structure, most notably the Great Blue Spot (a region of downward flux near the equator), and some evidence for secular variation, possibly arising from the winds. However, Juno is ongoing; it has not answered all questions and has posed new ones.
Juno's mission reveals Jupiter's interior.
A core exists but is diluted by hydrogen.
The mission revealed wind depth and magnetic field.
The Earth's Moon is thought to have formed by an impact between the Earth and an impactor around 4.5 billion years ago. This impact could have been so energetic that it could have mixed and ...homogenized the Earth's mantle. However, this view appears to be inconsistent with geochemical studies that suggest that the Earth's mantle was not mixed by the impact. Another outcome of the impact is that this energetic impact melted the whole mantle, but the extent of mantle melting is not well understood even though it must have had a significant effect on the subsequent evolution of the Earth's interior and atmosphere. To understand the initial state of the Earth's mantle, we perform giant impact simulations using smoothed particle hydrodynamics (SPH) for three different models: (a) standard: a Mars-sized impactor hits the proto-Earth, (b) fast-spinning Earth: a small impactor hits a rapidly rotating proto-Earth, and (c) sub-Earths: two half Earth-sized planets collide. We use two types of equations of state (MgSiO3 liquid and forsterite) to describe the Earth's mantle. We find that the mantle remains unmixed in (a), but it may be mixed in (b) and (c). The extent of mixing is most extensive in (c). Therefore, (a) is most consistent and (c) may be least consistent with the preservation of the mantle heterogeneity, while (b) may fall between. We determine that the Earth's mantle becomes mostly molten by the impact in all of the models. The choice of the equation of state does not affect these outcomes. Additionally, our results indicate that entropy gains of the mantle materials by a giant impact cannot be predicted well by the Rankine–Hugoniot equations. Moreover, we show that the mantle can remain unmixed on a Moon-forming timescale if it does not become mixed by the impact.
•We investigate the initial state of the Earth's mantle after the Moon-forming impact.•An MgSiO3 liquid EOS is implemented for hydrodynamic calculations.•We find that most of the Earth's mantle experiences melting by the impact.•The mantle is not likely to be mixed in the standard giant impact model.•The mantle may be mixed in the recent giant impact models.
The Fuzziness of Giant Planets' Cores Helled, Ravit; Stevenson, David
Astrophysical journal. Letters,
05/2017, Letnik:
840, Številka:
1
Journal Article
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Giant planets are thought to have cores in their deep interiors, and the division into a heavy-element core and hydrogen-helium envelope is applied in both formation and structure models. We show ...that the primordial internal structure depends on the planetary growth rate, in particular, the ratio of heavy elements accretion to gas accretion. For a wide range of likely conditions, this ratio is in one-to-one correspondence with the resulting post-accretion profile of heavy elements within the planet. This flux ratio depends sensitively on the assumed solid-surface density in the surrounding nebula. We suggest that giant planets' cores might not be distinct from the envelope and includes some hydrogen and helium, and the deep interior can have a gradual heavy-element structure. Accordingly, Jupiter's core may not be well defined. Accurate measurements of Jupiter's gravitational field by Juno could put constraints on Jupiter's core mass. However, as we suggest here, the definition of Jupiter's core is complex, and the core's physical properties (mass, density) depend on the actual definition of the core and on the planet's growth history.
Low-mass low-density planets discovered by Kepler in the super-Earth mass regime typically have large radii for their inferred masses, implying the presence of H2-He atmospheres. These planets are ...vulnerable to atmospheric mass loss due to heating by the parent star's XUV flux. Models coupling atmospheric mass loss with thermal evolution predicted a bimodal distribution of planetary radii, which has gained observational support. However, a key component that has been ignored in previous studies is the dissolution of these gases into the molten core of rock and iron that constitute most of their mass. Such planets have high temperatures (>2000 K) and pressures (∼kbars) at the core-envelope boundary, ensuring a molten surface and a subsurface reservoir of hydrogen that can be 5-10 times larger than the atmosphere. This study bridges this gap by coupling the thermal evolution of the planet and the mass loss of the atmosphere with the thermodynamic equilibrium between the dissolved H2 and the atmospheric H2 (Henry's law). Dissolution in the interior allows a planet to build a larger hydrogen repository during the planet formation stage. We show that the dissolved hydrogen outgasses to buffer atmospheric mass loss. The slow cooling of the planet also leads to outgassing because solubility decreases with decreasing temperature. Dissolution of hydrogen in the interior therefore increases the atmosphere retention ability of super-Earths. The study highlights the importance of including the temperature- and pressure-dependent solubility of gases in magma oceans and coupling outgassing to planetary evolution models.
New Formation Models for the Kepler-36 System Bodenheimer, Peter; Stevenson, David J.; Lissauer, Jack J. ...
The Astrophysical journal,
12/2018, Letnik:
868, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Formation of the planets in the Kepler-36 system is modeled by detailed numerical simulations according to the core-nucleated accretion scenario. The standard model is updated to include the ...dissolution of accreting rocky planetesimals in the gaseous envelope of the planet, leading to substantial enrichment of the envelope mass in heavy elements and a non-uniform composition with depth. For Kepler-36 c, models involving in situ formation and models involving orbital migration are considered. The results are compared with standard formation models. The calculations include the formation (accretion) phase as well as the subsequent cooling phase, up to the age of Kepler-36 (7 Gyr). During the latter phase, mass loss induced by stellar XUV radiation is included. In all cases, the results fit the measured mass, 7.84 M⊕, and radius, 3.68 R⊕, of Kepler-36 c. Two parameters are varied to obtain these fits: the disk solid surface density at the formation location and the "efficiency" factor in the XUV mass-loss rate. The updated models are hotter and therefore less dense in the silicate portion of the planet and in the overlying layers of H/He, as compared with standard models. The lower densities mean that only about half as much H/He is needed to be accreted to fit the present-day mass and radius constraints. For Kepler-36 b, an updated in situ calculation shows that the entire H/He envelope is lost, early in the cooling phase, in agreement with observation.
We present a novel mode of cultural evolution whereby some forms of transmission may be modelled as quasispecies. The model incorporates the effect of high rates of error in certain forms of ...communication; while also building on the structural similarities between biological molecules and written language. Firstly, both written language and key biological molecules, such as RNA and proteins, are modular. Within these molecules, structural domains may be recombined, while retaining their function. Likewise, sentences are structured as combinations of clauses, in which each clause contains a domain of information. The clausal structure permits the recombination of information to adopt different meanings, while allowing each unit to retain its identity. Secondly, by virtue of intrinsically-high error rates, we show that some, but not all, aspects of communicated culture information exists as rapidly evolving clouds within the population. These clouds of cultural information behave as quasispecies, which we model with varying mutation rates and suitable selection coefficients. We then integrate these ideas with the application of Shannon Diversity Index to produce a more holistic view of culture that is centred on the evolution of its information. Re-imagining culture, as evolving clouds of information, unifies the mode in which information is stored culturally and biologically, and opens up new avenues of comparative analysis.
Oxygen levels in the placental microenvironment throughout gestation are not constant, with severe hypoxic conditions present during the first trimester. This hypoxic phase overlaps with the most ...critical stages of placental development, i.e., blastocyst implantation, cytotrophoblast invasion, and spiral artery remodeling initiation. Dysregulation of any of these steps in early gestation can result in pregnancy loss and/or adverse pregnancy outcomes. Hypoxia has been shown to regulate not only the self-renewal, proliferation, and differentiation of trophoblast stem cells and progenitor cells, but also the recruitment, phenotype, and function of maternal immune cells. In this review, we will summarize how oxygen levels in early placental development determine the survival, fate, and function of several important cell types, e.g., trophoblast stem cells, extravillous trophoblasts, syncytiotrophoblasts, uterine natural killer cells, Hofbauer cells, and decidual macrophages. We will also discuss the cellular mechanisms used to cope with low oxygen tensions, such as the induction of hypoxia-inducible factor (HIF) or mammalian target of rapamycin (mTOR) signals, regulation of the metabolic pathway, and adaptation to autophagy. Understanding the beneficial roles of hypoxia in early placental development will provide insights into the root cause(s) of some pregnancy disorders, such as spontaneous abortion, preeclampsia, and intrauterine growth restriction.
We investigated the biotransformation of four common dietary polyphenols, rutin, quercetin, chlorogenic acid and caffeic acid, in an in vitro mixed culture model of human intestinal microbiota, to ...determine effects on human gut bacteria. All four compounds were biotransformed rapidly, disappearing from the medium within 0.5 h and later replaced by known phenolic acid breakdown products, at concentrations up to hundreds of micromolar, much higher than in no-polyphenol control experiments. Quantitative PCR was used to measure effects of the polyphenols on the balance between the major groups of intestinal bacteria that are known to influence gut health, i.e., Bifidobacterium spp., Bacteroidetes, and Firmicutes. Fermentation of polyphenols stimulated proliferation of bifidobacteria and decreased the ratio of Firmicutes to Bacteroidetes, relative to controls. Polyphenols also stimulated short chain fatty acid production by the bacteria. Pure bifidobacterial cultures were treated separately with either fermented media isolated from the incubations, the pure test polyphenols, or the biotransformation products detected in the fermentations. Growth stimulation was observed only with fermented polyphenol media and the pure biotransformation products. It appears that dietary polyphenols may have the ability to modify the gut microbial balance, but this effect is indirect, i.e., it is mediated by biotransformation products, rather than the original plant compounds.
•We incubated four dietary polyphenols with human fecal bacteria.•Fermenta was rich in short-chain fatty acids such as butyrate.•Fermentation increased bifidobacteria and decreased Firmicutes:Bacteroides ratio.•Metabolites in the fermenta also specifically increased bifidobacterial growth.•Metabolites included 3-hydroxyphenylacetic acid and 3-hydroxyphenylpropionic acid.
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