Outgassing history of Venus and the absence of water on Venus

Similarities in the size and mean density of Earth and Venus encourage the use of Earth-analogue models for the evolution of Venus. However, the amount of water in the present Venus atmosphere is miniscule compared to Earth's oceans. The 'missing' water is thus one of the most significant problems related to the origin and evolution of Venus. Other researchers proposed that Venus accreted with less water, but this was challenged. The high D/H ratio in Venus' atmosphere is consistent with an earlier water mass more than 100 times higher than at present conditions and is often cited to support a 'wet' Venus, but this amounts to only 0.01 to 0.1 percent of the water in terrestrial oceans and the high D/H ratio on Venus could easily reflect cometary injection. Nevertheless, many authors begin with the premise that Venus once had an oceanlike water mass on its surface, and investigate the many possible mechanisms that might account for its loss. In this paper we propose that Venus degassed to lower degree than the Earth and never had an oceanlike surface water mass

Acceptance speech for the 1993 Clarke Award

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31698/1/0000634.pd

The influence of high worry on static and dynamic insular functional connectivity

Worry is a form of repetitive negative thought. High worry-proneness is one risk factor leading to anxiety disorder. Several types of research indicated that anxiety disorder was highly associated with disrupted interoception. The insula is consistently considered to play a key role in interoception. However, the relationship between worry and the interoception network is poorly investigated in worry-prone individuals. Thus, it is essential to identify the neural characteristic of high worry-proneness subjects. A total of 32 high worry-proneness (HWP) subjects and 25 low worry-proneness (LWP) subjects were recruited and underwent magnetic resonance imaging scanning. Six subregions of insula were chosen as regions of interest. Then, seed-based static and dynamic functional connectivity were calculated. Increased static functional connectivity was observed between the ventral anterior insula and inferior parietal lobule in HWP compared to LWP. Decreased static functional connectivity was found between the left ventral anterior insula and the pregenual anterior cingulate cortex. Decreased dynamic functional connectivity was also shown between the right posterior insula and the inferior parietal lobule in HWP. Moreover, a post-hoc test exploring the effect of changed function within the insular region confirmed that a significant positive relationship between static functional connectivity (ventral anterior insula–inferior parietal lobule) and dynamic functional connectivity (posterior insula–inferior parietal lobule) in LWP but not in HWP. Our results might suggest that deficient insular function may be an essential factor related to high worry in healthy subjects

Reaction kinetics, geospeedometry, and relaxation theory

This paper explores the application of homogeneous reaction kinetics to geospeedometry and to structural relaxation theory. Numerical simulations of reaction kinetics during cooling for some first- and second-order elementary reactions have been carried out to systematically examine the effects of kinetic parameters and cooling histories on the final speciation. An analytical solution for a special case of first-order reaction A B has also been obtained. On the basis of both the analytical solution and the numerical simulation, the cooling rate (-dT/dt) at the apparent equilibrium temperature (Tae, obtained by measuring `quenched' speciation) and the relaxation time scale ([tau]r, the time for the departure from equilibrium to decrease to 1/e of the initial departure) at Tae can be approximately related as follows: where R is the gas constant, and max(Ef, Eb) is the greater of Ef and Eb, which are the activation energies of the forward and backward reactions. This approximation works well when the initial temperature (T0) is high such that [tau]r(T0) is small compared to the cooling time scale. Using the above relation, the cooling rate at Tae can be estimated easily and the cooling history of a natural assemblage can be inferred if enough homogeneous reactions are characterized.Relaxation of homogeneous reactions during the quench of a silicate melt into a glass can be studied quantitatively using the above method if the kinetic parameters are known. Melt-glass transitions probably involve the quenching of many homogeneous reactions that may have different Tae for a given quench rate, and are thus complicated. Authors have previously discussed relaxation of a reaction during cooling using the idea of a melt-glass transition. Because glass relaxation is complicated and the theory is still in its infancy, whereas reaction kinetics can be understood and quantified, it may be more productive to study the relaxation of homogeneous reactions directly and to study glass relaxation using the concept of reaction kinetics instead.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31680/1/0000616.pd

Water in the Oldest Lunar Rocks: Moon is "Wetter" than Previously Thought

No abstract availabl

Hydrous species geospeedometer in rhyolite: improved calibration and application,

Abstract-The hydrous species geospeedometer is based on the homogeneous interconversion reaction between molecular H 2 O species and OH species in silicate melts and glasses. Here we report new data for the calibration of the geospeedometer in rhyolitic glass, extending the coverage of quench rate to 94 K/s and of H 2 O t to 7.7 wt.% by using a piston-cylinder apparatus at low pressures (200 -500 MPa) to prevent bubble growth and to generate high and monitored quench rates. The new experimental data at pressure are highly reproducible and consistent with earlier data at 0.1 MPa, indicating negligible pressure effect on the relation between speciation and quench rate at P Յ 500 MPa. In order to avoid calibration uncertainties, the original infrared data are used to represent species concentrations and the equilibrium constant. With new data at higher total H 2 O, the linear relation does not hold anymore. Furthermore, the new data show that the linear relation between ln QЈ and ln q does not hold at high q. Hence, the geospeedometry model of Zhang et al. can be used for interpolation, but extrapolation may lead to large errors. A new geospeedometry model using the combined data set is presented in this work and applied to natural rhyolitic glasses. The new geospeedometer can be used to quantify cooling rates in a quench medium or an experimental apparatus. Furthermore, it can be used to determine the cooling rates of individual pyroclasts, different parts of a lava flow, and melt inclusions in phenocrysts, thus allowing inference of rich details of volcanic processes

Water, fluorine, and sulfur concentrations in the lunar mantle

The concentrations of volatile elements in the moon have important implications for the formation of the earth–moon system. There is currently a debate regarding the water content of the lunar mantle: Authors studying H_2O in lunar pyroclastic glass beads and in olivine-hosted melt inclusions in such pyroclastic samples and in plagioclase crystals in lunar highland anorthosites infer hundreds of ppm H_2O in the lunar mantle. In contrast, authors studying Zn/Fe ratios infer that the H_2O concentration in the lunar mantle is ≤1 ppm, and they argue that the glassy lunar basalts are a local anomaly. We contribute to a resolution of the debate by a broader examination of the concentrations of H_2O and other volatile components in olivine-hosted melt inclusions in a wider range of lunar mare basalts, including crystalline melt inclusions that are homogenized by melting in the laboratory. We find that F, Cl, and S concentrations in various lunar melt inclusions (including those in glassy lunar basalts) are similar to one another, and previously studied glassy lunar basalts are not a local anomaly in terms of these volatile concentrations. Furthermore, we estimate the pre-degassing H_2O/Ce, F/Nd, and S/Dy ratios of mare basaltic magmas to be at least 64, 4.0 and 100 respectively. These ratios are lower than those of primitive earth mantle by a factor of 3, 5, and 4 respectively. The depletion factors of these volatile elements relative to the earth's primitive mantle do not correlate strongly with volatility or bonding energy, and indeed they are roughly constant and similar to those of other volatile elements such as Li, Cs, Rb and K. This approximate constancy of volatile depletion in the moon relative to the earth can be explained by assuming that both the earth and the moon acquired volatiles from a similar source or by a similar mechanism but the earth was more efficient in acquiring the volatiles. We estimate the H_2O, F and S concentrations in the primitive lunar mantle source to be at least 110, 5.3, and 70 ppm, respectively – similar to or slightly lower than those in terrestrial MORB mantle

Evidence for a "Wet" Early Moon

The Moon was thought to have lost its volatiles during impact(s) of a Mars-size planetesimal with the proto Earth [1] and during degassing of an early planet-wide magma ocean [2]. This view of an anhydrous Moon, however, has been challenged by recent discoveries of water on its surface [3-5] and in lunar volcanics [6-10] and regoliths [11]. Indigenous water is suggested to be heterogeneously distributed in the lunar interior and some parts of lunar mantle may contain as much water as Earth's upper mantle [6,10]. This water is thought to have been brought in part through solar wind implantation [3-5,8,11] and meteorite/cometary impacts [3,4,8,12] after the formation of the primary crust. Here we measured water in primary products of the Lunar Magma Ocean (LMO) thereby by-passing the processes of later addition of water to the Moon through impact events or during mantle overturn as suggested by previous studies (e.g., [8,12]). So far, ferroan anorthosite (FAN) is the only available lithology that is believed to be a primary product of the LMO [2]. It is generally accepted that plagioclase, after crystallization, floated in the LMO and formed FAN as the original crust [2]. Therefore, any indigenous water preserved in FAN was partitioned from the LMO. These data can be used to estimate the water content of the magma ocean at the time of plagioclase crystallization, as well as that of the mare magma source regions

Dynamics of gas-driven eruptions: Experimental simulations using CO_2-H_2O-polymer system

We report exploratory experiments simulating gas-driven eruptions using the CO_2 - H_2O system at room temperature as an analog of natural eruptive systems. The experimental apparatus consists of a test cell and a large tank. Initially, up to 1.0 wt % of CO_2 is dissolved in liquid water under a pressure of up to 735 kPa in the test cell. The experiment is initiated by suddenly reducing the pressure of the test cell to a typical tank pressure of 10 kPa. The following are the main results: (1) The style of the process depends on the decompression ratio. There is a threshold decompression ratio above which rapid eruption occurs. (2) During rapid eruption, there is always fragmentation at the liquid-vapor interface. Fragmentation may also occur in the flow interior. (3) Initially, the top of the erupting column ascends at a constant acceleration (instead of constant velocity). (4) Average bubble radius grows as t^(2/3). (5) When viscosity is 20 times that of pure water or greater, a static foam may be stable after expansion to 97% vesicularity. The experiments provide several insights into natural gas-driven eruptions, including (1) the interplay between bubble growth and ascent of the erupting column must be considered for realistic modeling of bubble growth during gas-driven eruptions, (2) buoyant rise of the bubbly magma is not necessary during an explosive volcanic eruption, and (3) CO_2-driven limnic eruptions can be explosive. The violence increases with the initial CO_2 content dissolved in water