Scientists propose new concept of terrestrial planet formation

A team of scientists from NASA, Hampton University and the University of Hong Kong propose a new way of understanding the cooling and transfer of heat from terrestrial planetary interiors and how that affects the generation of the volcanic terrains that dominate the rocky planets. Based on the present dynamics of Jupiter's tidally heated moon, Io, the scientists hypothesize that the geological histories of the solar system's terrestrial bodies, specifically Mercury, Venus, Moon and Mars, are consistent with a mode of early planetary evolution involving heat-pipes. They further propose that heat-pipe cooling is a universal process that may explain the common features seen on the surfaces of terrestrial planets.

Io with a volcanic plume at the top [Credit: The University of Hong Kong]

"We believe that the concept of a heat-pipe mode of planet formation is important and will help explain the evolution of all rocky planets," said Dr. Justin Simon, NASA Planetary Scientist, Center for Isotope Cosmochemistry and Geochronology in the Astromaterials Research and Exploration Science Division at NASA's Johnson Space Center in Houston, Texas and one of the coauthors of the paper. "If shown to be correct, it will be discussed along with the theories of plate tectonics, planetary 'magma oceans' and the 'giant impact theory for the origin of the moon.'"

The scientists hypothesize heat-pipe cooling was involved in the evolution of all terrestrial planets including early Earth and represents the transition from the magma ocean to the rigid-lid or plate tectonic modes of planetary evolution. Heat-pipes transport heat from the interior to the surface via mantle melting and magma ascent. The resulting eruptions lead to global volcanic resurfacing by which older volcanic layers are progressively buried and pushed downward to form thick, cold and strong mechanical lithospheres.

The authors review the observations relevant to the formation of the surfaces of each of the terrestrial planets and current models that have been proposed to explain them. They then discuss the major outstanding problems and show how the heat-pipe hypothesis can resolve these in a consistent way across all planets.

"The terrestrial bodies in our solar system look different enough that the classical view is that they all formed differently, at least in terms of making their outer shells. If our analysis holds merit, it points in the direction of a universal model for the early development of terrestrial planets, across our solar system and beyond," said Dr. Alexander Webb, Associate Professor, The University of Hong Kong.

Dr Alex Webb in the field of Isua, Greenland, studying 3.8 billion year old rocks that may have been produced 
via heat-pipe processes on Earth [Credit: The University of Hong Kong]

The authors note that Mercury was globally resurfaced early in its evolution by volcanic eruptions emplacing smooth plains with few identifiable eruption centers. The authors conclude that the geological observations of the planet point to an episode of heat-pipes operating for somewhat less than the first billion years of Mercury's evolution. The surface of Venus is also dominated by lavas with broad plains made up of numerous flows spanning hundreds of kilometers at low slope with few identifiable source structures. Venus does not display sufficient volcanic flux to currently experience active heat-pipe cooling, but the authors conclude that the thick, stagnant lithospheric lid is a relict of heat-pipe operation that ceased rapidly several hundred million years ago.

Among the most important surface features on Mars are its large volcanos, ancient cratered terrains and the crustal dichotomy between the elevated southern hemisphere and the depressed northern hemisphere. It remains unclear which processes were responsible for the formation of the dichotomy, but the authors conclude that a strong ancient lithosphere created by heat-pipe volcanism would have aided in the preservation of this ancient feature. Similarly, the Moon stands out as having a shape that is dramatically out of hydrostatic equilibrium, but preserving a disequilibrium shape requires a strong, early-formed lithosphere. The authors argue that a strong lithosphere is precisely the expected behavior of a body experiencing heat-pipe cooling.

The team brought together geological, geochemical and geochronological evidence from the terrestrial bodies in our solar system to show that heat-pipes may have provided the primary mechanism of crustal formation and resurfacing. The heat-pipe hypothesis provides a uniform explanation for common features of the known terrestrial planets that have not undergone plate tectonics and should be considered an important aspect of their evolution.

"The development of this theory is a great example of how exploration of our planetary neighbors, in this case [Jupiter's moon] Io, has led to a deeper understanding of Earth as well as rocky planets across the galaxy," said Dr. William Moore, professor of atmospheric and planetary sciences, Hampton University, USA.

Heat-pipes should also occur on rocky exoplanets orbiting other stars. A planet twice the mass of Earth should take more than twice as long to cool, because the surface area does not grow as fast as the mass. For large exoplanets, the lifetime of the heat-pipe mode may exceed the lifetime of Sun-like parent stars and thus any subsequent plate-tectonic phase may never be observed. This study forces us to rethink our expectations of what types of surfaces and atmospheres to expect as we expand our exploration of other solar systems.

The team's findings are discussed in a paper recently published in Earth and Planetary Science Letters.

Source: The University of Hong Kong [September 19, 2017]

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Scientists produce best estimate of Earth's composition

Scientists at ANU have produced the best estimate of Earth's elemental composition which will help them understand how the Earth formed 4.6 billion years ago.

Credit: NASA/JPL

The Solar System began as a dense blob in a molecular cloud of hydrogen gas and dust that collapsed under its own gravity, forming the early Sun, Earth and other planets.

Co-researcher Associate Professor Charley Lineweaver said the Earth's chemical composition was set at that early stage of formation.

"The four most abundant elements - iron, oxygen, silicon and magnesium - make up more than 90 per cent of the Earth's mass, but working out exactly what the Earth is made of is tricky," said Dr Lineweaver from the Research School of Earth Sciences and the Research School of Astronomy and Astrophysics at ANU.

"Seismological studies of earthquakes inform us about the Earth's core, mantle and crust, but it's hard to convert this information into an elemental composition.

"Our deepest drilling has only scratched the surface down to 10 kilometres of our 6,400 kilometre radius planet. Rocks at the surface only come from as deep as the upper mantle."

The research is published in the international journal Icarus and is available here.

Lead author ANU PhD scholar Haiyang Wang said the team made the most comprehensive estimates of the Earth's composition based on a meta-analysis of previous estimates of the mantle and core, and a new estimate of the core's mass.

"Our work focused on getting realistic uncertainties so that our reference model can be used in future comparisons of the Earth with the Sun, or with Mars or with any other body in the Solar System," said Mr Wang from the ANU Research School of Astronomy and Astrophysics.

Co-researcher Professor Trevor Ireland from the ANU Research School of Earth Sciences said planetary scientists would find many uses for this new composition record.

"This will have far-reaching importance, not only for planetary bodies in our Solar System but also other star systems in the universe," he said.

Haiyang Wang has received the Prime Minister's Australia Asia Award to support his PhD research at ANU.

Source: The Australian National University [September 18, 2017]

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Changes in Earth's crust caused oxygen to fill the atmosphere

Scientists have long wondered how Earth's atmosphere filled with oxygen. UBC geologist Matthijs Smit and research partner Klaus Mezger may have found the answer in continental rocks that are billions of years old.

Matthijs Smit of the University of British Columbia examines ancient rocks from the 
deep crust in Norway during the summer of 2017 [Credit: Matthijs Smit]

"Oxygenation was waiting to happen," said Smit. "All it may have needed was for the continents to mature."

Earth's early atmosphere and oceans were devoid of free oxygen, even though tiny cyanobacteria were producing the gas as a byproduct of photosynthesis. Free oxygen is oxygen that isn't combined with other elements such as carbon or nitrogen, and aerobic organisms need it to live. A change occurred about three billion years ago, when small regions containing free oxygen began to appear in the oceans. Then, about 2.4 billion years ago, oxygen in the atmosphere suddenly increased by about 10,000 times in just 200 million years. This period, known as the Great Oxidation Event, changed chemical reactions on the surface of the Earth completely.

Smit, a professor in UBC's department of earth, ocean & atmospheric sciences, and colleague, professor Klaus Mezger of the University of Bern, were aware that the composition of continents also changed during this period. They set out to find a link, looking closely at records detailing the geochemistry of shales and igneous rock types from around the world -- more than 48,000 rocks dating back billions of years.

"It turned out that a staggering change occurred in the composition of continents at the same time free oxygen was starting to accumulate in the oceans," Smit said.

Before oxygenation, continents were composed of rocks rich in magnesium and low in silica -- similar to what can be found today in places like Iceland and the Faroe Islands. But more importantly, those rocks contained a mineral called olivine. When olivine comes into contact with water, it initiates chemical reactions that consume oxygen and lock it up. That is likely what happened to the oxygen produced by cyanobacteria early in Earth's history.

However, as the continental crust evolved to a composition more like today's, olivine virtually disappeared. Without that mineral to react with water and consume oxygen, the gas was finally allowed to accumulate. Oceans eventually became saturated, and oxygen crossed into the atmosphere.

"It really appears to have been the starting point for life diversification as we know it," Smit said. "After that change, the Earth became much more habitable and suitable for the evolution of complex life, but that needed some trigger mechanism, and that's what we may have found."

As for what caused the composition of continents to change, that is the subject of ongoing study. Smit notes that modern plate tectonics began at around the same time, and many scientists theorize that there is a connection.

Smit and Mezger published their findings today in the journal Nature Geoscience. The research was funded by the Natural Sciences and Engineering Research Council.

Source: University of British Columbia [September 18, 2017]

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Tectonic plates 'weaker than previously thought,' say scientists

Experiments carried out at Oxford University have revealed that tectonic plates are weaker than previously thought.

Researchers experimented on olivine crystals to help determine the strength of tectonic plates 
[Credit: Lars Hansen]

The finding explains an ambiguity in lab work that led scientists to believe these rocks were much stronger than they appeared to be in the natural world. This new knowledge will help us understand how tectonic plates can break to form new boundaries.

Study co-author Lars Hansen, Associate Professor of Rock and Mineral Physics in Oxford University's Department of Earth Sciences, said: 'The strength of tectonic plates has been a major target of research for the past four decades. For plate tectonics to work, plates must be able to break to form new plate boundaries. Significant effort has gone into measuring the strength of the key olivine-rich rocks that make up plates using laboratory experiments.

'Unfortunately, those estimates of rock strength have been significantly greater than the apparent strength of plates as observed on Earth. Thus, there is a fundamental lack of understanding of how plates can actually break to form new boundaries. Furthermore, the estimates of rock strength from laboratory experiments exhibit considerable variability, reducing confidence in using experiments to estimate rock properties.'

The new research, published in the journal Science Advances, uses a technique known as 'nanoindentation' to resolve this discrepancy and explain how the rocks that make up tectonic plates can be weak enough to break and form new plate boundaries.

Dr Hansen said: 'We have demonstrated that this variability among previous estimates of strength is a result of a special length-scale within the rocks – that is, the strength depends on the volume of material being tested. To determine this we used nanoindentation experiments in which a microscopic diamond stylus is pressed into the surface of an olivine crystal. These experiments reveal that the strength of the crystal depends on the size of the indentation.

'This concept translates to large rock samples, for which the measured strength increases as the size of the constituent crystals decreases. Because most previous experiments have used synthetic rocks with crystal sizes much smaller than typically found in nature, they have drastically overestimated the strength of tectonic plates. Our results therefore both explain the wide range of previous estimates of rock strength and provide confirmation that the strength of the rocks that make up tectonic plates is low enough to form new plate boundaries.'

The study was an international collaboration involving scientists from Stanford University, the University of Pennsylvania, Oxford University and the University of Delaware.

Dr Hansen added: 'This result has implications beyond forming tectonic plate boundaries. Better predictions of the strength of rocks under these conditions will help inform us on many dynamic processes in plates. For instance, we now know that the evolution of stresses on earthquake-generating faults likely depends on the size of the individual crystals that make up the rocks involved. In addition, flexing of plates under the weight of volcanoes or large ice sheets, a process intimately linked to sea level on Earth, will also ultimately depend on crystal size.'

Source: University of Oxford [September 14, 2017]

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Scientists find fossilised cosmic dust in white cliffs of Dover

Researchers from Imperial College London report that the white cliffs of Dover contain the fossilised remains of cosmic dust.

The white cliffs of Dover [Credit: Imperial College London]

The significance of this discovery, published in the journal Earth and Planetary Science Letters, is that the fossilised cosmic dust could provide a new source of information about the early solar system.

Mr Martin Suttle, lead author and a research postgraduate from Imperial's Department of Earth Science and Engineering, said: "The iconic white cliffs of Dover are an important source of fossilised creatures that help us to determine the changes and upheavals the planet has undergone many millions of years ago. It is so exciting because we've now discovered that fossilised space dust is entombed alongside these creatures, which can also provide us with information about what was happening in our solar system at the time."

The team also report in a separate study published in the journal Geology that they've discovered a way for determining if cosmic dust was clay rich. Clays can only form if water is present, so a method for determining clay content could act like a cosmic divining rod for determining the presence of water rich asteroids in our solar system.

Martin Suttle at the white cliffs taking chalk rock samples 
[Credit: Imperial College London]

Dr Matt Genge, lead author from the College's Department of Earth Science and Engineering, said: "In the distant future, asteroids could provide human space explorers with valuable stop offs during long voyages. Being able to source water is vital because it can be used to drink, to make oxygen and even fuel to power spacecraft. The relevance of our study is that cosmic dust particles that land on Earth could ultimately be used to trace where these water-rich asteroids may be, providing a valuable tool for mapping this resource."

White cliffs of Dover study

Cosmic dust has been previously found in rocks up to 2.7 billion years old. However, until now only cosmic dust that was very well preserved could be studied by researchers. The significance of the their new study says Mr Suttle is that less well preserved fossilised cosmic dust can now also be located and examined in detail.

The intricate microscopic patterns on a fossilised cosmic dust specimen 
[Credit: Imperial College London]

Previously, scientists had not known that the white cliffs of Dover contained fossilised cosmic dust, although it has been located in other rocks before.

The researchers suggest that the reason it has been overlooked is that the fossilisation process masked the true identity of the dust particles. This is because when the dust fossilised it replaced the original mineral content with different materials. At the same time the original minerals in early fossilised creatures were also being replaced with similar materials, masking the identity of the space particles.

The team determined that fossilised cosmic dust was present in the chalk samples by spotting their distinctive spherical structure and christmas tree-like shape of their crystal content.

A cosmic dust fossil with Christmas tree-like crystal structure 
[Credit: Imperial College London]

In geological terms, pristine cosmic dust particles are a relatively recent record of events in the solar system. Now that they've located a new source of cosmic dust, which is much older, the team says it could help them to understand events beyond Earth such as major collisions between asteroids, which have occurred much earlier, perhaps even around 98 million years ago – a time when cosmic dust records have been difficult to unearth.

Cosmic dust divining rod

In the study in the journal Geology, the Imperial researchers have calculated that olivine crystals in cosmic dust act as a proxy for clay particles and the presence of water.

The holes in this cosmic dust particle shows where the water has bubbled 
to the surface and vaporised [Credit: Imperial College London]

As cosmic dust enters the atmosphere it can reach searing temperatures of more than 600 degrees Celsius and this causes its original mineral content to undergo transformations where they turn into glass and crystals. The heat also vaporises any trace of water molecules, making it difficult to determine if it was present.

Now, Dr Genge and his colleagues have analysed past studies and carried out some calculations to determine that shattered pieces of olivine crystals contained in cosmic dust is a proxy for water. This is because the loss of water from the interior particle has a cooling effect leading to extreme differences in temperature between the surface and the core of the particle. Olivine crystals shatter when one part is hotter than the rest because huge stresses develop owing to differences in expansion.

Dr Genge has calculated that around 75 per cent of the cosmic dust that lands on Earth contain shattered olivine crystals. As cosmic dust particles are the pulverised remains of asteroid and comet collisions in our solar system, it suggests that clay content and thus water content of these space rocks are high.

Now that the team knows that clay rich asteroids may be abundant in supply the next step will see them trying to trace the origins of the cosmic dust to asteroids orbiting the solar system. They plan to do this by comparing how cosmic dust and asteroids reflect infra-red radiation to find parent asteroids that match the dust particle's infrared signatures.

Author: Colin Smith | Source: Imperial College London [September 07, 2017]

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