Monday, 18 December 2017

Furthest Lunar Apogee of 2017.

On Tuesday 19 December 2017, at 1.28 am GMT, the Moon will be at its furthest point from the Earth in 2017, a distance of 406 604 km. The Moon orbits the Earth every 27.5 days, and like most orbiting bodies, its orbit is not completely circular, but slightly elliptical, so that the distance between the two bodies varies by about 3% over the course of a month. This elliptical orbit is also not completely regular, it periodically elongates then returns to normal, making some perigees closer than others. Because this is an elongating and contracting elliptical orbit, rather than a change in the average distance between the Earth and the Moon, the most extreme Lunar Perigee and Apogee of each year typically happen in the same Lunar Month; though this year the closest Lunar Perigee occurred 26 May at 1.24 am GMT, with the second closest occurring to weeks ago at 8.43 am on Monday 4 December.

Diagram showing the relationship of the Lunar orbit and Lunar month. Southern Astronomical Delights.

Although this is the furthest point from the Earth that the Moon will reach in 2017, it is not exceptional. The Moon reached 406 659 km from the Earth on 31 October 2016, and will reach 406 634 on 16 August 2023.
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Sunday, 17 December 2017

Landslide kills at least five in Santiago Province, Chile.

Five people have been confirmed dead and another fifteen are missing following a landslide that hit the town of Santa Lucia in central Santiago Province, Chile, on Saturday 16 December 2017. The deceased have not been named, but have been identified as a 68-year-old-woman, a 35-year-old man, a foreign tourist and two other people. The incident happened at about 9.20 am local time, after over 11 cm of rain fell within 24 hours, liquefying soil on the hillside above the town, and resulting in a massive mudflow that swept through the town, destroying about a third of the buildings, and triggering a fire that destroyed several more.

The town of Santa Lucia in Santiago Province, after being hit by a landslide on 16 December 2017. HO/AFP/Getty Images.

Landslides 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. December is usually a very dry month in Santiago Province, which usually receives less than 2.5 mm of rain in the entire month, making this weeks event truly exceptional.

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Terpios hoshinota: Tracking the progress of the Black Death Sponge on Okinoerabu-jima Island.

Sponges, Porifera, form an important part of many benthic marine communities, both for their contribution to the structure of reefs and their forming of symbiotic relationships with a variety of other organisms, including Prokaryotes, Shrimp, Worms, Hydroids, Zoantharians, and Fish. However not all Sponges are beneficial to the communities that host them. The Heteroscleromorph Demosponge Terpios hoshinota forms symbiotic relationships with a variety of Cyanobacteria, spreading rapidly by photosynthetic growth. This enables it to grow at a rate of several milometers per day, forming a thin black or grey crust that rapidly overgrows and kills Hard Corals such as Lobophylia, Montipora, Acropora, Merulina, and Goniastrea

The first known outbreak of Terpios hoshinota was recorded on Guam in 1973, since when it has spread to the Northern Mariana Islands, Western Caroline Islands, the Philippines, American Samoa, southern Taiwan, the Great Barrier Reef, Sulawesi, Java, the Maldives, Mauritius and the Ryukyu Islands. The first outbreak of the Sponge in the Ryukyu Islands hit the island of Tokunoshima in 1985-86, where it rapidly spread across the reefs of the Yonama Coast, eventually covering 87.9% of the reefs, and gaining the name 'Black Death Sponge'. 

 A colonoy of Terpios hoshinota overgrowing a Coral on Guam. Florida Museum.

The Sponge was detected on the reefs of Okinoerabu-jima Island, about 50 km to the southwest of Tokunoshima in 2010, raising concerns about the fate of the reefs there, however a survey carried out at this time found that Terpios hoshinota had completely disappeared from Tokunoshima, where reefs were now dominated by Hard Corals of the genus Acropora, suggesting that the appearance of the Sponge does not necessarily represent the end of a Coral Reef community.

In a paper published in the journal Zoological Studies on 19 April 2017, Masashi Yomogida, Masaru Mizuyama, and Toshiki Kubomura of the Molecular Invertebrate Systematics and Ecology Laboratory at the University of the Ryukyus, and James Davis Reimer, also of the Molecular Invertebrate Systematics and Ecology Laboratory, and of the Tropical Biosphere Research Center at the University of the Ryukyus, describe the results of a long-term study of the Terpios hoshinota outbreak on Okinoerabu-jima Island, based upon a series of surveys carried out between March 2010 and September 2014.

Yomogida et al. carried out a series of transect studies on the Yakomo Coast of Okinoerabu-jima Island, with each survey examing the surface covering of an area of reef measuring 10 m by 1 m. Each survey divided the covering of the reef into nine categories: (1) Terpios hoshinota, (2) Macroalgae (Seaweed) except the Sponge Weed Ceratodictyon spongiosum, (3) the Sponge Weed Ceratodictyon spongiosum, (4) Cyanobacterial mats, (5) living reef-building Corals, (6) dead Coral, (7) other benthic organisms, including Soft Corals, Giant Clams, Sea Cucumbers, and Sea Urchins, (8) sand and gravel, and (9) anything that could not be identified.

(A) Location of Okinaerabu-jima Island, Kagoshima, Japan in the northwestern Pacific and, (B) map of Yakomo coast on Okinoerabu-jima Island. Red dotted box shows the Terpios hoshinota survey area, white dotted lines show the approximate area of Terpios hoshinota along the coast, and red solid lines approximate locations of permanent transects. Google Earth/Yomogida et al. (2017).

Terpios hoshinota covered over 24% of the reef at the outset of the study (March 2010), and remained this high until October of that year, but fell to 17.6% coverage in December 2010. In June 2011 the species underwent a catastrophic die-back, falling to a covering of only 0.02% of the reef. Sponge levels remained low for the next year, having reached only 0.3% coverage by May 2012, but did eventually begin to recover, reaching 11.4% coverage by September 2014.

Coverage of the reef by Macroalgae remained below 10% in all surveys except one, in May 2012, when it reached 13.6%. Cyanobacteria were completely absent from the reef in all surveys except one, in October 2011, when it covered 39.9% of the reef. Sand and gravel remained the dominant coverings of the reef throughout the survey, with coverage varying between 50.2% and 89.4%; none of the other categories ever climbed above a 5% coverage on the reef.

Clearly some event significantly reduced the coverage of Terpios hoshinota in 2011, and came close to removing the Sponge from the reef altogether. Yomogida et al. suggest that the most likely culprit was Typhoon Songda, which passed close to the island on 28 May 2011, and which is recorded as having generated windspeeds of up to 139 kilometres per hour, and wave heights of up to 10.22 m. This event could have removed the Sponge encrustation either by directly tearing it from the reef or covering it in sand or other soft sediments.

This suggests that typhoons could play a major role in inhibiting the ability of Terpios hoshinota to dominate ecosystems, and are likely to have been the cause of the disappearance of the Sponge from Tokunoshima Island. However Yomogida et al. also note that tropical storms may also play a role in the dispersal of Terpios hoshinota, as the larvae of Cyanobacteria-hosting Sponges tend to have rather limited dispersal capacities, suggesting that something else has aided the apparent rapid dispersal of this species. They also note that Terpios hoshinota is now found in both tropical and subtropical seas, and that tropical storms are a feature of only subtropical seas, with areas such as Indonesia and the Maldives, where the Sponge has become established, not effected by these storms.

Yomogida et al. also note that an outbreak of Terpios hoshinota on Pagan Island in the Mariana group was strongly linked to a volcanic eruption on that island, with a large patch of the Sponge appearing with the onset of volcanic activity in 2010, and disappearing when volcanic activity stopped in 2012. They suggest that this might be connected to the deposition of volcanic ash into the waters around Pagan Island, which would have increased the levels of nutrients, particularly iron, to the Sponge and its symbiotic Cyanobacteria. This raised the possibility that Human activities may be facilitating the spread of Terpios hoshinota, if these activities result in extra nutrients being released into the water.

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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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