What is the Oxygen Isotope Composition of Venus? The Scientific Case for Sample Return from Earth’s “Sister” Planet

Venus is Earth’s closest planetary neighbour and both bodies are of similar size and mass. As a consequence, Venus is often described as Earth’s sister planet. But the two worlds have followed very different evolutionary paths, with Earth having benign surface conditions, whereas Venus has a surface temperature of 464 °C and a surface pressure of 92 bar. These inhospitable surface conditions may partially explain why there has been such a dearth of space missions to Venus in recent years.The oxygen isotope composition of Venus is currently unknown. However, this single measurement (Δ17O) would have first order implications for our understanding of how large terrestrial planets are built. Recent isotopic studies indicate that the Solar System is bimodal in composition, divided into a carbonaceous chondrite (CC) group and a non-carbonaceous (NC) group. The CC group probably originated in the outer Solar System and the NC group in the inner Solar System. Venus comprises 41% by mass of the inner Solar System compared to 50% for Earth and only 5% for Mars. Models for building large terrestrial planets, such as Earth and Venus, would be significantly improved by a determination of the Δ17O composition of a returned sample from Venus. This measurement would help constrain the extent of early inner Solar System isotopic homogenisation and help to identify whether the feeding zones of the terrestrial planets were narrow or wide.Determining the Δ17O composition of Venus would also have significant implications for our understanding of how the Moon formed. Recent lunar formation models invoke a high energy impact between the proto-Earth and an inner Solar System-derived impactor body, Theia. The close isotopic similarity between the Earth and Moon is explained by these models as being a consequence of high-temperature, post-impact mixing. However, if Earth and Venus proved to be isotopic clones with respect to Δ17O, this would favour the classic, lower energy, giant impact scenario.We review the surface geology of Venus with the aim of identifying potential terrains that could be targeted by a robotic sample return mission. While the potentially ancient tessera terrains would be of great scientific interest, the need to minimise the influence of venusian weathering favours the sampling of young basaltic plains. In terms of a nominal sample mass, 10 g would be sufficient to undertake a full range of geochemical, isotopic and dating studies. However, it is important that additional material is collected as a legacy sample. As a consequence, a returned sample mass of at least 100 g should be recovered.Two scenarios for robotic sample return missions from Venus are presented, based on previous mission proposals. The most cost effective approach involves a “Grab and Go” strategy, either using a lander and separate orbiter, or possibly just a stand-alone lander. Sample return could also be achieved as part of a more ambitious, extended mission to study the venusian atmosphere. In both scenarios it is critical to obtain a surface atmospheric sample to define the extent of atmosphere-lithosphere oxygen isotopic disequilibrium. Surface sampling would be carried out by multiple techniques (drill, scoop, “vacuum-cleaner” device) to ensure success. Surface operations would take no longer than one hour.Analysis of returned samples would provide a firm basis for assessing similarities and differences between the evolution of Venus, Earth, Mars and smaller bodies such as Vesta. The Solar System provides an important case study in how two almost identical bodies, Earth and Venus, could have had such a divergent evolution. Finally, Venus, with its runaway greenhouse atmosphere, may provide data relevant to the understanding of similar less extreme processes on Earth. Venus is Earth’s planetary twin and deserves to be better studied and understood. In a wider context, analysis of returned samples from Venus would provide data relevant to the study of exoplanetary systems

A general method for the localization of enzymes that produce phosphate, pyrophosphate, or CO2 after polyacrylamide gel electrophoresis

Previous workers have stained gels for enzymes that produce inorganic phosphate by using the insolubility of calcium phosphate. This method can also be applied to enzymes that produce pyrophosphate or CO2. The white bands of the precipitated calcium salt are clearly visible when viewed against a dark background and can be photographed or scanned. The method can be used at pH 6 and above; the level of Ca2+ required is reduced at higher pH values. The sensitivity of the method is tested by injecting the various anions into presoaked gels; as little as 10 nmol of phosphate or pyrophosphate and 100 nmol of CO2 produce clearly visible precipitates

Diurnal changes in the properties of phosphoenolpyruvate carboxylase in Bryophyllum leaves: a possible co valent modification

In plants that show Crassulacean acid metabolism, phosphoenolpyruvate carboxylase catalyses the key step of CO2 fixation at night. We show here that the properties of this enzyme from Bryophyllum fedtschenkoi undergo marked changes between night and day; the night form is much less sensitive to feedback inhibition by malate than is the day form. Incubation of leaves with 32Pi followed by extraction and immunoprecipitation of phosphoenolpyruvate carboxylase showed that only the night form contained 32P. This suggests that the activity of the enzyme is controlled by a covalent modification mechanism

Persistent circadian rhythms in the phosphorylation state of phosphoenolpyruvate carboxylase from Bryophyllum fedtschenkoi leaves and in its sensitivity to inhibition by malate

Phosphoenolpyruvate carboxylase (EC 4.1.1.31; PEPCase) from Bryophyllum fedtschenkoi leaves has previously been shown to exist in two forms in vivo. During the night the enzyme is phosphorylated and relatively insensitive to feedback inhibition by malate whereas during the day the enzyme is dephosphorylated and more sensitive to inhibition by malate. These properties of PEPCase have now been investigated in leaves maintained under constant conditions of temperature and lighting. When leaves were maintained in continuous darkness and CO2-free air at 15°C, PEPCase exhibited a persistent circadian rhythm of interconversion between the two forms. There was a good correlation between periods during which the leaves were fixing respiratory CO2 and periods during which PEPCase was in the form normally observed at night. When leaves were maintained in continuous light and normal air at 15°C, starting at the end of a night or the end of a day, a circadian rhythm of net uptake of CO2 was observed. Only when these constant conditions were applied at the end of a day was a circadian rhythm of interconversions between the two forms of PEPCase observed and the rhythms of enzyme interconversion and CO2 uptake did not correlate in phase or period

PEP carboxylase kinase is a novel protein kinase controlled at the level of expression

Phosphoenolpyruvate (PEP) carboxylase plays a number of key roles in the central metabolism of higher plants. The enzyme is regulated by reversible phosphorylation in response to a range of signals in many different plant tissues. The data discussed here illustrate several novel features of this system. The phosphorylation state of PEP carboxylase is controlled largely by the activity of PEP carboxylase kinase. This enzyme comprises a protein kinase catalytic domain with no regulatory regions. In many systems it is controlled at the level of expression. In C4 plants, expression of PEP carboxylase kinase is light‐regulated and involves changes in cytosolic pH, InsP3 and Ca2+ levels. Expression of PEP carboxylase kinase in CAM plants is regulated by a circadian oscillator, perhaps via metabolite control. Some plants contain multiple PEP carboxylase kinase genes, probably with different expression patterns and roles. A newly discovered PEP carboxylase kinase inhibitor protein might facilitate the net dephosphorylation of PEP carboxylase under conditions in which flux through this enzyme is not required

The circadian clock in <i>Arabidopsis</i> roots is a simplified slave version of the clock in shoots

The circadian oscillator in eukaryotes consists of several interlocking feedback loops through which the expression of clock genes is controlled. It is generally assumed that all plant cells contain essentially identical and cell- autonomous multiloop clocks. Here, we show that the circadian clock in the roots of mature &lt;i&gt;Arabidopsis&lt;/i&gt; plants differs markedly from that in the shoots and that the root clock is synchronized by a photosynthesis- related signal from the shoot. Two of the feedback loops of the plant circadian clock are disengaged in roots, because two key clock components, the transcription factors CCA1 and LHY, are able to inhibit gene expression in shoots but not in roots. Thus, the plant clock is organ- specific but not organ- autonomous