Wednesday, 8 December 2021

The December Monocerotid Meteor Shower.

The December Monocerotid meteor shower is visible each year between 5 and 20 December, and is expected to peak at about 4.00 am GMT on Thursday 9 December this year, when about two meteors per hour can be expected, although these should be quite bright. The shower takes its name from the constellation of Monoceros, which is faint but lies on the celestial equator, making the shower visible from both hemispheres, although they are likely to be easier to see in the northern hemisphere than the southern. The peak of activity will fall before the first quarter Moon on 11 December 2021, which should prevent observations from being greatly hampered by moonlight.

The radiant point of the December Monocerotid Meteors. In The Sky.

Meteor showers are thought to be largely composed of material from the tails of comets. Comets are composed largely of ice (mostly water and carbon dioxide), and when they fall into the inner Solar System the outer layers of this boil away, forming a visible tail (which always points away from the Sun, not in the direction the comet is coming from, as our Earth-bound experience would lead us to expect). Particles of rock and dust from within the comet are freed by this melting (strictly sublimation, transforming directly from a solid to a gas due to the low pressure on it's surface) of the comet into the tail and continue to orbit in the same path as the comet, falling behind over time.

The Earth passing through a stream of comet dust, resulting in a meteor shower. Not to scale. Astro Bob.

The December Monocerotid Meteor Shower has been shown to be caused by the Earth passing through the trail of the comet C/1917 F1 Mellish, where it encounters thousands of tiny dust particles shed from the comet as its icy surface is melted (strictly sublimated) by the heat of the Sun. 

How the passage of the Earth through a meteor shower creates a radiant point from which they can be observed. In The Sky.

C/1917 F1 Mellish was discovered on 20 March 1917 by American amateur astronomer John Mellish. The name C/1917 F1 Mellish implies that it is a comet (C/), and that it was the first such body (1) discovered in the second half of March 1917 (1917 F - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).

C/1917 F1 Mellish completes one orbit every 143 years on an eccentric, orbit tilted at 32.7° to the plane of the Solar System, that takes it from 0.19 AU from the Sun (19% of the average distance at which the Earth orbits the Sun, and inside the orbit of the planet Mercury, which also makes it one of the closest Sun-approaching comets known) to 54.6 AU from the Sun (54.6 times as far from the Sun as the Earth, and considerably outside the orbit of the planet Neptune). As a comet with a period of more than 20 years but less than 200 years, C/1917 F1 Mellish is considered to be both a Periodic Comet, and a Halley-type Comet.

The calculated orbit and current position C/1917 F1 Mellish.  JPL Small Body Database.

This close approach of C/1917 F1 Mellish to the Sun is particularly conducive for the creation of meteors (lots of material is ablated of the surface of the comet as it passes close to the Sun), but tends to make the orbit of the comet a bit unstable (large amounts of material ablating off the surface of a comet can push it onto a new orbital trajectory). The upshot of this is that C/1917 F1 Mellish appears to be associated with several different minor meteor showers, with the December Monocerotids and November Orionids (or ξ-Orionids) confidently assigned to the comet, and the December Canis Minorids (sometimes the 11 Canis Minorids) and April ρ-Cygnids also thought likely to be caused by material derived from C/1917 F1 Mellish. The Geminid Meteor Shower has also previously been linked to C/1917 F1 Mellish, although this is now usually associated with Asteroid 3200 Phaethon.

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Monday, 6 December 2021

Eoazara xerrii: A new species of Elasmotheriine Rhinoceros from the Late Miocene of Morocco.

Today only five species of Rhinoceros survive, all of which are highly endangered, but the fossil record contains about fifty genera of these Animals, which, even allowing for some exaggeration due to descriptions based upon fragmentary specimens, makes them among the most successful large Mammals of the Neogene. Rhinoceroses can be divided into two distince evolutionary lineages, the Rhinocerotinae and the Elasmotheriinae, although Early and Middle Miocene forms can be hard to assign to either group with confidence, by the Late Miocene the two Rhinoceros types had diverged enough to make differentiation fairly simple.

In a paper published in the journal Acta Palaeontologica Polonica on 19 October 2021, Denis Geraads of the Centre de Recherche en Paléontologie du Paris at the Sorbonne Université and the Muséum National d’Histoire Naturelle, and Samir Zouhri of the Department of Geology at the Hassan II University of Casablanca, describe a new species of Elasmotheriine Rhinoceros from the Late Miocene deposits northeast of Skoura in the Ouarzazate Basin of Morocco.

Miocene Mammals are collected by commercial fossil hunters to the north of a section of road between Skoura and Tizi N’Tadderht, roughly 50 km to the northeast of Ouarzazate. Two genera of Rhinocerotine Rhinoceroses have been tentatively identified from these deposits, based upon teeth, a partial mandible, and some post cranial remains, as well as remains ascribed to Elephants, Goats, Ostriches, Turtles, Crocodiles, Horses, Pigs, and Giraffes. Late Miocene Mammal faunas are rare in Africa, which makes the dating of deposits by Mammal remains difficult, thus the exact age of these fossils is unclear, and it is likely that they are not all completely contemporaneous, although they probably do not encompass a very great time-span. However, Geraads and Zouhri favour an early Late Miocene age for the fossil-bearing strata.

The new species is described on the basis of material purchased from a fossil hunter by Serge Xerri in Rabat, and donated by him to the Aïn Chock Faculty of Sciences at Hassan II University of Casablanca. It is named Eoazara xerrii, where 'Eoazara' derives from 'Eo' the Greek for 'dawn' and 'azara' the Amazigh (Berber) for 'Rhinoceros', and 'xerrii' honours Serge Xerri. The material upon which the description is based comprises a skull with the associated mandible, a partial humerus, a lunar, and a trapezoid.

Elasmotheriine Rhinoceros, Eoazara xerrii, from the upper Miocene of Skoura, Morocco. Skull (FSC-Sk-250) in left latero-dorsal (A₁) and right lateral (A₂) views. Antero-ventral view of the front part, showing the right lower incisor, premaxillae, and nasals (A₃). Stereo view of the right auditory area (A₄). (A₃) and (A₄) in oblique view, not to scale. Geraads & Zouhri (2021).

Eoazara xerrii was a large Animal, similar in size to the largest living Rhinos, albeit somewhat more slender. The skull is relatively well preserved, lacking most of the occiput, but retaining the right occipital condyle and basicranium. Most of the right mandible is preserved, as is part of the left; these are attached to the cranium and cannot be separated. The skull is slightly crushed dorsoventrally, but not so much as to greatly alter its proportions. The right mandible is in life position at the articulation with the skull, but shifted by about 30 mm at the anterior, with the lower second premolar being in front of the upper. The teeth of the lower right mandible are preserved, but hard to access, the upper jaw retaining only the second and third molars, which are heavily worn.

The skull is longer than it is broad, with long fused nasal bones which are inflated and terminate in a tip that points towards the premaxillae. A large rugose patch on the dorsal surface of these nasals ndicated the former presence of a single horn. It is possible that there was also a smaller frontal horn, but there is no evidence for this. The premaxillae are long and slender, and lack teeth. 

The postcranial material was derived from the same block as the skull, and the absence of any duplicate parts combined with a similar size is taken to indicate that these came from the same Animal. 

Elasmotheriine Rhinoceros, Eoazara xerrii, from the upper Miocene of Skoura, Morocco. A. Second left metacarpal (FSC-Sk-45) in proximal (A₁), anterior (A₂), and lateral (A₃) views. (B) Imperfectly preserved right second metatarsal (FSC-Sk-53) in anterior (|B₁) and lateral (B₂) views. (C) Left first upper molar (FSC-Sk-33) in occlusal view. Scale bars: (A) 50 mm; (B), (C) 100 mm. Geraads & Zouhri (2021).

Elasmotheriine Rhinos were widespread in Eurasia in the Miocene, but much less common in Africa, with most of those that have been recorded coming from East or Southern Africa. In North Africa only a few teeth from a probable juvenile Elasmotheriine are known.

Distribution map of the Elasmotheriinae. The size of the symbols reflects the number of species in each site (one species for the smallest symbols, up to four for the largest one: Guonigou in China). Eoazara xerrii  is from the area called 'Skoura'. Geraads & Zouhri (2021).

Curiously, a phylogenetic analysis carried out by Geraads and Zouhri did not find that Eoazara xerrii was closely related to other African species of Elasmotheriines, nor even European ones, but rather with Asian species, with its closest relatives being Iranian, Chinese, and Siberian species. This suggests a distribution along the southern shores of the Tethys Ocean in the early Late Miocene, with European and other African forms arriving in their locations via separate routes out of Asia.

Majority rule consensus tree of the 16 most parsimonious trees obtained by TNT on the data matrix of the 59 taxa that have at least 50% of the characters scored. Length = 1644; CI = 17; RI = 55. American taxa in green, Eurasian taxa in blue, African taxa in red. The dashed line extends to the taxa that might belong to the Elasmotheriinae as well. Geraads & Zouhri (2021).

The teeth of Eoazara xerrii are not well preserved, but it appears to have had a high cement cover with complex shapes on its occlusal surfaces (i.e. those surfaces which grind against one-another during chewing), which are generally taken as adaptations to grazing on Grasses. This fits well with the other Mammals found at Skoura, which are likely to have inhabbited an open, savanah-like biome. Such Savanah ecosystems are know to have appeared in the Late Miocene in the area between Iran and the Balkans, as well as in Morocco, whereas it is not thought to have reached Iberia until some time later. The presence of an Elasmotheriine Rhinoceros species in Morocco, adapted to this environment and closely related to Asian forms, strongly implies that by this time Savanahs had spread along much of the coast of North Africa.

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Online courses in Palaeontology. 

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Saturday, 4 December 2021

Asteroid 2021 WC1 passes the Earth.

Asteroid 2021 WC1 passed by the Earth at a distance of about 203 500 km (0.53 times the average distance between the Earth and the Moon, or 0.14% of the distance between the Earth and the Sun), slightly after 3.00 pm GMT on Sunday 28 November 2021. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2021 WC1 has an estimated equivalent diameter of 4-12 m (i.e. it is estimated that a spherical object with the same volume would be 4-12 m in diameter), and an object of this size would be expected to explode in an airburst (an explosion 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) between 42 and 30 km above the ground, with only fragmentary material reaching the Earth's surface.

The relative positions of 2021 WC1 and the Earth on 28 November 2021. JPL Small Body Database.

2021 WC1 was discovered on 27 November 2021 (the day before its closest approach to the Earth) by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2021 WC1 implies that the asteroid was the 28th object (asteroid C1 - in numbering asteroids the letters A-Z, excluding I, are assigned numbers from 1 to 25, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 26, A2 = 51, etc., which means that C1 = 25 + 3 = 28) discovered in the second half of November 2021 (period 2021 W - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).

The orbit and current position of 2021 WC1. The Sky Live 3D Solar System Simulator.

2021 WC1 has a 590 day (1.61 year) orbital period, with an elliptical orbit tilted at an angle of 9.01° to the plain of the Solar System which takes in to 0.81 AU from the Sun (81% of the distance at which the Earth orbits the Sun) and out to 1.94 AU (194% of the distance at which the Earth orbits the Sun, and more than the distance at which the planet Mars orbits the Sun). It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer). This means that 2021 WC1 has occasional close encounters with the Earth, with the last thought to have happened in May 2017 and the next predicted in April 2080. 2021 WC1 also has occasional close encounters with the planet Mars, which it last came close to in September 2012, and which it is expected to approach again in December 2023.

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