Saturday, 16 December 2017

Determining the origin of scoriaceous micrometeorites.

Micrometeorites are particles of extra-terrestrial material less than 2 mm across. These have been collected from a number of environments, including Antarctic blue ice and snow, deep sea sediments and even rooftops, and are the most abundant form of meteorite material available to scientists. Scoriaceous micrometeorites are micrometeorites dominated by micron-sized equant iron-rich olivine crystals within a glassy mesostasis, which is thought to be indicative of having undergone caused by superheating from friction with the Earth's atmosphere due to the orbital momentum of the asteroid, which is greater than that caused by simply falling.

In a paper published in the journal Geology on 17 August 2017, Matthew Genge and Martin Suttle of the Impact and Astromaterials Research Centre at Imperial College London, and the Earth Sciences Department at The Natural History Museum, and Matthias Van Ginneken of Earth System Science at the Vrije Universiteit Brussel, present the results of a study in which they examined scoriaceous micrometeorites in order to attempt to determine their origin and the stresses that they been subjected to.

Scoriaceous micrometeorites contain clusters of clusters of forsterite and enstatite crystals, which Genge et al. believe to have been formed by the fragmentation of larger crystals due to superheating, as other micrometeorites lack these, suggesting that the the original crystals cannot be fractured in this way by shockwaves passing through the minerals as the parent bodies fragment in the atmosphere, impact the ground, or undergo some process in space unrelated to their eventual decent to Earth. They suggest that the most likely cause of such fragmentation is thermal stress, as different minerals within the micrometeorite expand at different rates when heated.

Backscattered electron images of micrometeorites. (A) Highly vesicular scoriaceous micrometeorite containing a cluster of forsterite (FO) relicts and eskoliate (CR). (B) A scoriaceous micrometeorite with a well developed external magnetite rim and clusters of forsterite (FO). Small triangular shards of forsterite are present close to larger crystals. This particle is similar to a micro-porphyritic olivine cosmic spherule. An expanded inset shows a cluster of small enstatite crystals. (C) A scoriaceous micrometeorite containing abundant vesicles and a magnetite rim. Two areas of relicts occur, one enstatite (FE) exhibits abundant fractures partially infilled with mesostasis (MV). (D) A scoriaceous micrometeorite containing two clusters of forsterites (FO) consisting of numerous individual crystals. (E) An unmelted finegrained micrometeorite with an external igneous rim (IR) surrounding an unmelted core (UC). A forsterite relict (FO) is present that truncates the igneous rim ((f) shows expanded view) and contains numerous fractures partially infilled with melt and is surrounded by a magnetite rim (MR). Fracturing within the crystal is most abundant in the part closest to the surface of the particle. Scale bars are 50 μm, except in (f) where it is 5 μm. Genge et al. (2017).

Genge et al. were able to construct a model of the rate at which forsterite expands due to heating, and from this determine the temperature to which these minerals had been raised, and the difference in temperature across the mineral grain; suggesting that in some cases this temperature difference may be as much as 4000 K per μm, resulting in a high degree of shear stress, caused by different parts of the crystal expanding at different rates, and causing the crystal to shatter.

Such a high temperature difference within a micrometeorite requires some explanation, as most minerals conduct heat fairly well. Genge et al. theorise that this may have been caused by the parent bodies having been comprises of at least 5% phyllosilicates (sheet minerals such as micas, chlorite, serpentine, talc, and the clays), which conduct heat poorly, and which would have been destroyed by the very high temperatures implied. 

This in turn suggests that the original material from which these micrometeorites were derived was similar in composition to that of a CI1 or CM2 carbonaceous chondrite, meteorites with a high composition of phyllosilicates. CM2 chondrites have previously been shown to lose phyllosilicates due to dehydration (the loss of hydrogen and oxygen from the mineral as water) at high presures, suggesting that scoriaceous micrometeorites may be formed specifically from the fragmentation of such chondrites, rather than being the products of any meteorite raised to the correct temperature.

About half of all micrometeorite-sized particles entering the Earth's atmosphere are thought to be derived from members of the Veritas Asteroid Family (a group of asteroids in the Outer Main Belt thought to have formed about 8.5 million years ago by the break-up of a large parent-body), with much of the remaining material derived from the Koronis Family Asteroids (a group of asteroids in the Central Main Belt thought to have formed by the collision of two large bodies about two billion years ago), and a small contribution from the Themis Family (a group of asteroids in the Outer Main Belt thought to be the main source of carbonaceous chondrites).

The Koronis Family Asteroids is thought to produce only ordinary chondritic material, while both the Veritas and Themis families are thought to produce carbonaceous chondritic material. Genge et al. suggest that the likely carbonaceous origin of scoriaceous micrometeorites implies that these are most likely to have originated from the Veritas and Themis asteroid groups.

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Three confirmed dead as Tropical Storm Kai-Tek sweeps across the Philippines.

Three people have been confirmed dead after Tropical Storm Kai-Tek swept across the Philippines on Saturday 16 December 2017. The storm made landfall on Samar Island, where 77 000 people have been evacuated from low lying areas, and seven people are known to have been injured amid widespread flooding. All three confirmed deaths occurred on the neighbouring island of Leyte, and include a woman killed by a landslide, a three-year-old boy who drowned and another person who was sucked down a manhole. Two further deaths have been reported on the islands of Biliran and Dinagat, though authorities have not yet been able to confirm these.

Flooding in Eastern Samar Province on Samar Island, the Philippines, in the wake of Tropical Storm Kai-Tek. Rhoda Baris/Rappler.

Tropical storms are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere.These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.

 The passage of Tropical Storm Kai-Tek till 12.00 noon GMT on Saturday 16 December 2017  (thick line) with its predicted future path (thin line, circles represent the margin of error on the predictions). Colours indicate the strength of the storm. Tropical Storm Risk.

The low pressure above tropical storms causes water to rise there by ~1 cm for every millibar drop in pressure, leading to a storm surge that can overwhelm low-lying coastal areas, while at the same time the heat leads to high levels of evaporation from the sea - and subsequently high levels of rainfall. This can cause additional flooding on land, as well as landslides, which are are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall.

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Possible metoerite impact near Thunder Bay, Ontario.

Police officers called to a suspect explosion in the Thunder Bay area of Ontatio, Canada, late on the evening of Thursday 14 December 2017, have reported a possible meteorite impact. The officers reported finding a hole in the snow about a meter across, close to Highway 61, with a small amount of  'rock-like' material in the centre. Since there were no Human footprints close to the site they suspect a meteorite impact was the most likely cause of the event. The site was inspected on Wednesday 15 December by Stephen Kissen of the Geology Department at Lakehead University, who could not find any traces of meteorite material, though he does not rule out a meteorite as the cause of the incident. Small meteorites are often completely vapourised in explosions caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid, and such a superheated small object would have a high chance of being destroyed if it hit a snowfield.

Stephen Kissen of the Geology Department at Lakeview University at the site of the possible Thunder Bay meteorite impact. Lakehead University.

Local media has speculated that the impact might be associated with the Geminid Meteor Shower, however this is unlikely as this is comprised of dust-sized particles from the surface of an object called 3200 Phaethon, which is classed as an Apollo Asteroid (an asteroid with an orbit that crosses that of the Earth), which burn up high in the atmosphere, while objects capable of reaching the ground need to be much larger.

The Thunder Bay 'meteor crater'. Thunder Bay Police Services.

Objects of this size probably enter the Earth's atmosphere several times a year, though unless they do so over populated areas they are unlikely to be noticed. They are officially described as fireballs if they produce a light brighter than the planet Venus. The brightness of a meteor is caused by friction with the Earth's atmosphere, which is typically far greater than that caused by simple falling, due to the initial trajectory of the object. Such objects typically eventually explode in an airburst called by the friction, causing them to vanish as an luminous object. However this is not the end of the story as such explosions result in the production of a number of smaller objects, which fall to the ground under the influence of gravity (which does not cause the luminescence associated with friction-induced heating).
The approximate location of the 14 December 2017 Thinder Bay 'meteorite impact'. Google Maps.
These 'dark objects' do not continue along the path of the original bolide, but neither do they fall directly to the ground, but rather follow a course determined by the atmospheric currents (winds) through which the objects pass. Scientists are able to calculate potential trajectories for hypothetical dark objects derived from meteors using data from weather monitoring services.
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Friday, 15 December 2017

Magnitude 6.5 Earthquake beneath West Java, Indonesia.

The United States Geological Survey recorded a Magnitude 6.5 Earthquake at a depth of 91.9 km under the district of Cipatujah in West Java, Indonesia, slightly after 11.45 pm local time (slightly after 4.45 pm GMT) on Friday 15 December March 2017. The event was felt across most of Java, as well as on Bali and Christmas Island. The event is reported to have caused a number of building collapses and several deaths, though the extent of the damage is as yet unclear.

The approximate location of the 15 December 2017 West Java Earthquake. USGS.
The Indo-Australian Plate, which underlies the Indian Ocean to the south of Java, Bali and Lombok, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate which underlies the islands and neighbouring Sumatra, along the Sunda Trench, passing under the islands, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fuelling the volcanoes of Java and neighbouring islands.
 Subduction along the Sunda Trench beneath Java, Bali and Lombok. Earth Observatory of Singapore.
Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.
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Shark remains from the Early Cretaceous of Argentina.

Sharks first appeared during the Devonian, and are still dominant predators in the seas of today. They form an important part of the marine fossil record in many areas, and are occasionally used for stratigraphy (dating rocks), though often they are usually only represented by their teeth, which are mineralised and grown and shed throughout their lives. Despite this, as with all groups. there are gaps in their fossil record, with areas where, despite apparently having good Shark habitats, Sharks remains are absent from the fossil record. One such area is southwestern Gondwana (South America and Antarctica), where Shark remains are common from the Late Cretaceous and Cainozoic, but almost entirely absent from the Jurassic and Early Cretaceous.

In a paper published in the journal Cretaceous Research on 13 September 2017, Soledad Gouiric-Cavalli and Alberto Cione of the Divisi on Paleontología Vertebrados at the Museo de La Plata, and the Consejo Nacional de Investigaciones Científicas y Técnicas, and Darío Lazo, Cecilia Cataldo, and María Aguirre-Urreta, also of the Consejo Nacional de Investigaciones Científicas y Técnicas, and of the Instituto de Estudios Andinos “Don Pablo Groeber” at the Universidad de Buenos Aires, describe two partial Shark specimens from the Early Cretaceous Agrio Formation of Neuquén Province in Argentina.

The first specimen described is a partial spine from the dorsal fin of a Hybodont Shark, a group of highly distinctive Sharks that appeared in the Devonian and which are generally thought to have become extinct at the End of the Cretaceous. This is incomplete, and broken into two fragments, but is extremely distinctive, with a pulp cavity, a keel on its foreward surface, and a series of denticle bases on its rear surface, features not seen in anything other than the spines of Hybodonts.

Hybodontiformes fin spine. (A) Silhouette of the fin spine in which preserved parts are shown. (B)-(C) Posterior fin spine margin to show the denticle bases. (D) Cross section of the fin spine in their anterior margin at level illustrated in (A) with a circle. (E) Close-up of (B) to show denticle bases. (F) Cross section of the fin spine at level shown in (A). Scale bars are 20 mm in (A) and (C), 2 mm in (D), and 3 mm in (E) and (F). Gouiric-Cavalli et al. (2017).

The second specimen described is a partial tooth assigned to an unknown member of the genus Protolamna, a group of Mackeral Sharks that went extinct at the end of the Cretaceous. This is a small tooth, only 4 mm in height, though the tip of the cusp is missing.

Protolamna sp. (A) Labial view. (B) Lingual view. Scale bar is 2 mm. Gouiric-Cavalli et al. (2017).

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Sorbus cibagouensis: A new species of Rowan from Tibet.

Trees and shrubs of the genus Sorbus are found across temperate the regions of Europe, Asia, and North America, where they are variously known as Rowans, Whitebeams, Service Trees or Mountain Ashes. They are members of the Rose Family, Rosaceae, and produce brightly coloured berries, which are consumed by Birds, helping to spread their seeds.

In a paper published in the Nordic Journal of Botany on 25 January 2017, Zhijian Yin, Mingxu Zhao, Fanglin Tang, and HongYan Sun of the China Forest Exploration and Design Institute and the Southwest Monitoring Center of Natural Reserve and Wildlife of the State Forestry Administration of the Peoples Republic of China, and Hua Peng of the Kunming Institute of Botany of the Chinese Academy of Sciences, describe a new species of Sorbus from the Cibagou National Nature Reserve in Zayü County in southeast Tibet.

The new species is named Sorbus cibagouensis, meaning 'from Cibagou'. It is a small tree, reaching about 6 m in height, with dark grey to black bark, leaves reaching 19 cm in length, but split into distinct leaflets, and produce white blossom. The species was found growing in a mixed conifer and broadleaved forest at an altitude of between 2600  and 2950 m above sealevel, though its distribution is not really known.

Sorbus cibagouensis, wild plant. Zhao Mingxu in Yin et al. (2017).

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Thursday, 14 December 2017

Elonus gruberi: A new species of Ant-like Leaf Beetle from the United States.

Ant-like Leaf Beetles, Aderidae, are a widespread but little-studied group of Beetles found across the globe. The adults, which in some species resemble Ants are found on the underside of the leaves of trees and shrubs, while the larvae are found in rotting wood, leaf litter, and sometimes the nests of social Insects. The group contains about a thousand species, the majority of which are found in the tropics.

In a paper publshed in the journal Zootaxa on 26 October 2017, Nicolas Gompel of the Fakultät für Biologie at Ludwig-Maximilians Universität München, describes a new species of Ant-like Leaf Beetle from the United States, as part of a review of the North American genus Elonus.

The new species is named Elonus gruberi, in honour of Jeff Gruber, a friend of Nicolas Gompel, for his work collecting Beetles in North America. These Beetles range from 2.6 to 3.4 mm in length and are a dark matte brown in colour, with orange markings on the wing-cases. They are found in Alabama, Arkansas, Delaware, Indiana, Iowa, Massachusetts, Mississippi, New Hampshire, Ohio, Texas, Pennsylvania and Wisconsin states.

Elonus gruberi, from Wisconsin. Gompel (2017).

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