Monday 20 May 2024

New images of Jupiter's moon Amalthea.

NASA has released a pair of images of Jupiter's moon Amalthea, taken by the Juno Spacecraft during a close flyby on 7 March 2024. At the time of the flyby the spacecraft was only 265 000 km from the Jovian moon (about 69% of the distance between the Earth and its Moon), though Amalthea still appears as a tiny speck, since it is only about 84 km in diameter.

Images of the Jovian moon Amalthea taken by the Juno Spacecraft on 7 March 2024. NASA/JPL/Caltech/Southwest Research Institute/Main Space Science Systems/Gerald Eichstädt.

Amalthea was discovered on 9 September 1892 by American astronomer Edward Emerson Barnard, then working at the Lick Observatory on Mount Hamilton in California. Other than the four moons discovered by Galileo Galilei in 1610, Amalthea is the only Jovian moon to have been discovered by direct observation. The remaining 90 know moons of Jupiter have been discovered either by photographic surveys or spacecraft flybys.

Amalthea is irregularly shaped, being roughly 250 km long, 146 km wide, and 128 km deep, and tidally locked to Jupiter, so that it always has the same side facing towards the planet and the same end forward, with its long axis aligned with its orbit.

Images of Amalthea captured by the Galileo Spacecraft; (left) from a distance of 446 000 km on 12 August 1999, (right) from a distance of 374 000 km on 26 November 1999. NASA/JPL/Wikimedia Commons.

Amalthea orbits Jupiter at a distance of 181 000 km, making it the third closest known moon to the planet; since the Roche Limit of Jupiter (distance below which a satellite will be torn apart by tidal forces) is about 80 000 km, it is unlikely that there are many undiscovered moons closer to the planet than Amalthea. The current orbit of the moon is extremely circular and has a very low inclination, but it has been calculated that it goes through periodic episodes of resonance with Io, the innermost of the Galilean Moons, during which its orbit becomes more eccentric and tilted.

The surface of Amalthea is extremely red, and brighter than the other inner satellites of Jupiter, with a leading hemisphere 1.3 times as bright as the trailing hemisphere. This has been theorized to be because of an accumulation of sulphur ejected from Io onto the surface of Amalthea, although without visiting the moon it is unlikely that this can ever be established.  

Amalthea as imaged by the Voyager 1 Spacecraft on 5 March 1979. Calvin Hamilton/NASA/Wikimedia Commons.

A number of large craters have been observed on the surface of Amalthea. The largest of these has been named Pan, and is 100 km wide and at least 8 km deep. Another crater, Gaea, is only about 80 km wide, but thought to be at least twice as deep as Pan.

Largely inside the orbit of Amalthea lies the Amalthea Gossamer Ring, a faint dust ring which extends from about 129 000 km above Jupiter to about 182 000 km, which reaches its maximum thickness, about 2300, close to the orbit of Amalthea. The Galileo Spacecraft passed through this ring in 2002-2003, finding that it is made of particles between 0.2 μm and 5.0 μm in diameter, with some larger bodies close to the moon. It is thought that the ring is made up of material ejected from the surface of Amalthea, and that the larger objects may represent debris from a collision.

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Sunday 19 May 2024

Exclusion area around Mount Ibu, Halmahera Island, extended following new eruption.

The Indonesian Center for Volcanology and Geological Hazard Mitigation has extended the exclusion zone around  Mount Ibu, a 1325 m stratovolcano on the northwest coast of Halmahera Island, following a new eruption on 18 May 2024. A two kilometre exclusion zone had already been put in place after the volcano began erupting on 28 April, but this has been increased to seven kilometres following the most recent eruption, which produced an ash column which rose to 4 km above the volcano's summit, resulting in seven villages needing to be evacuated. 

An ash column and lava fountain above Mount Ibu on the Indonesian island of Halmahera on 18 May 2024. Several lightning discharges can be seen within the ash column; ash columns can build up considerable charge variations leading to frequent electrical discharges. Indonesian Center for Volcanology and Geological Hazard Mitigation/Reuters.

The Halmahera Island chain is a volcanic arc formed where the Halmahera Plate, a northeaster extension of the Molucca Sea Plate is being subducted beneath Philippine Plate from the east and the Eurasian Plate from the west, with the underlying plate being melted by the heat of the Earth's interior, and lighter minerals bubbling up through the overlying plate to form volcanoes. The Halmahera volcanoes are located where the Philippine Plate is overriding the Molucca Sea Plate; to the west the Sangihe Islands lie where the Molucca Sea Plate is being overridden by the Eurasian Plate.

Diagrammatic representation of the subduction zones beneath Halmahera (middle), plus the Philippines (top) and Sulawesi (bottom), with the Eurasian Plate to the left, the Molucca Sea Plate in the middle, and the Philippine Plate to the right.  Hall & Wilson (2000).

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Thursday 16 May 2024

Oxynoemacheilus kottelati: A new species of Stone Loach from the Aegean drainage of Anatolia.

Stone Loaches of the genus Oxynoemacheilus are found across the Eastern Mediterranean, the southern Caucasus, Anatolia, Mesopotamia, and Central Iran. To date, 67 species have been assigned to the genus, of which are found in Turkish inland waters, and 36 entirely endemic to the country. Sixteen species are found within the Tigris and Euphrates basins, 14 within rivers and streams draining into the Mediterranean, six in rivers and streams draining into the Black Sea, four in rivers and streams draining into the Caspian, two species within the Konya Basin, two within the Marmara Basin, and one within the Van Basin. 

In a paper published in the journal Zoosystematics and Evolution on 9 May 2024, Davut Turan of the Faculty of Fisheries at Recep Tayyip Erdoğan University, Sadi̇ Aksu of the Vocational School of Health Services at Eskişehir Osmangazi UniversitySali̇m Serkan Güçlü of the Faculty of Eğirdir Fisheries at Isparta University of Applied Sciences, and Gökhan Kalaycı, also of the Faculty of Fisheries at Recep Tayyip Erdoğan University, describe a new species of Oxynoemacheilus from streams in the Aegean drainage of Anatolia.

The new species is named Oxynoemacheilus kottelati, in honour of the Swiss ichthyologist Maurice Kottelat, for his contributions to our understanding of the world's Fish fauna. The species is described from a series of Fish collected from the Havran and Karınca streams in Balıkesir Province, Turkey, in October 2023.

Oxynoemacheilus kottelati FFR 15655, (a), (b) Holotype, male, 47 mm; FFR 15656; (c) Paratype, female, 49 mm; Türkiye, Balıkesir Province, Havran Stream. Turan et al. (2024).

Specimens of Oxynoemacheilus kottelati range from 35 to 54 mm in length, with a deep body, laterally compressed at the base of the tail. They have a marbled brownish pattern on their flanks and dorsal surface, but are white on the underside. 

Oxynoemacheilus kottelati, FFR 15657, paratypes: (a) male, 47 mm; (b)female, 46 mm; (c) male, 45 mm; Havran Stream; FFR 15656; (d) female, 48 mm; Karınca Stream; Türkiye, Balıkesir Province. Turan et al. (2024).

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Wednesday 15 May 2024

Iron Age Illyrian helmet discovered by archaeologists in Croatia.

Archaeologists from the Korčula Town Museum, the University of Zagreb, and Dubrovnik Museums, have uncovered an Iron Age Illyrian helmet at an ancient graveyard on the Peljesac Peninsula in southern Croatia. The helmet was found upright on its own in a separate chamber to any Human burial, leading the archaeologists involved to conclude that it was placed there as a votive offering (an object of value placed in a sacred place as an offering) rather than as grave goods with a specific individual. 

An Illyrian helmet excavated at an Iron Age graveyard on the Peljesac Peninsula in southern Croatia. Marta Kalebota/Korčula Town Museum.

Surprisingly, this is the second such helmet burial found at the cemetery, implying that this was an important ritual activity for the Illyrians of the Peljesac Peninsula. The previous helmet, uncovered in 2020, was dated to about 400 BC, but the style of the newly discovered helmet suggests it to be slightly older, possibly dating to between 500 and 600 BC.

An Illyrian helmet excavated at an Iron Age graveyard on the Peljesac Peninsula in southern Croatia in 2020. Dubrovnik Museums.

The term 'Illyrian' derives from 'Illyrioi' ('Ἰλλυριοί') was used by the Ancient Greeks to describe the peoples of the northwest Balkan Peninsula, and probably refers to a group of peoples with similar cultures and languages rather than a single group  that would have identified as 'Illyrian'. They were regarded as barbarians by both the Greeks and Romans, but appear to have been organised into several distinct kingdoms with well-understood boundaries and trade networks, ruled from walled cities. They were able to hold their territory against potentially aggressive neighbours such as the Greeks, Macedonians, and Thracians, and for a time were considered a maritime power, presenting a threat to shipping on the Adriatic and raiding coastal communities. They proved less able to defend themselves against later Roman incursions, however, and all the Illyrian kingdoms were conquered by Rome in a series of wars between 229 and 168 BC.

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Asteroid 2024 JN16 passes the Earth.

Asteroid 2024 JN16 passed by the Earth at a distance of about 25 400 km (0.07 times the average distance between the Earth and the Moon, or 0.017% of the distance between the Earth and the Sun, but more than 62 times as far from the Earth's surface as the International Space Station), with a relative velocity of about 9.26 km per second, slightly before 9.50 am GMT on Tuesday 14 May 2024. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2024 JN16 has an estimated equivalent diameter of 2-7 m (i.e. it is estimated that a spherical object with the same volume would be 2-7 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) more than 36 km above the ground, with only fragmentary material reaching the Earth's surface.

120 second image of 2024 JN16 taken with the Celestron 14"-F8/8.4 (356/3000 mm) Schmidt-Cassegrain Telescope at Ceccano in Italy on 5 April 2020. The asteroid is the small point at the centre of the image, indicated by the white arrow, the longer lines are stars, their elongation being caused by the telescope tracking the asteroid over the length of the exposure. Gianluca Masi/Virtual Telescope Project.

2024 JN16 was discovered on 12 May 2024 (two days before its closest approach to the Earth) by Gennady Borisov at the MARGO observatory in Crimea. The designation 2024 JN16 implies that it was the 638th asteroid (object N16 - in numbering asteroids the letters A-Y, 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 N16 = 13 + (25 x 16) = 638) discovered in the first half of May 2024 (period 2024 J - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).

The relative positions of 2024 JN16 and the Earth on at 10.00 am GMT on 14 May 2024. JPL Small Body Database.

2024 JN16 is calculated to have a 456 day (1.25 year) orbital period, with an elliptical orbit tilted at an angle of 5.79° to the plain of the Solar System which takes in to 0.89 AU from the Sun (89% of the distance at which the Earth orbits the Sun) and out to 1.43 AU (1.43 times the distance at which the Earth orbits the Sun, somewhat less than the distance at which the planet Mars orbits). 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). 

The relative positions of 2024 JN16, the Earth, and the planets of the Inner Solar System on at 10.00 am GMT on 14 May 2024. JPL Small Body Database.

This means that 2024 JN16 has regular close encounters with the Earth, with the last calculated to have happened in November 2019, and the next predicted for January 2032. 2024 JN16 also has occasional close encounters with the planet Venus, with the next predicted for June 2025.

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