Tuesday 15 October 2024

Magnitude 6.2 Eaarthquake off the coast of Guanacaste Province, Costa Rica.

The United States Geological Survey recorded a Magnitude 6.2 Earthquake at a depth of 16.0 km, off the west ciast of Costa Rica, roughly 41 km to the west of the town of Tamarindo in Guanacaste Province, Costa Rica, slightly before 11.45 am local time (slightly before 5.45 pm GMT) on Saturday 12 October 2024. There are no reports of any damage or casualties associated with this event, but it was felt across much of northern Costa Rica and southern Nicaragua.

The approximate location of the 12 October 2024 Costa Rica Earthquake. USGS.

Costa Rica lies on the southern margin of the Caribbean Plate; to the south of the country the Cocos Plate, which underlies part of the eastern Pacific Ocean) is being subducted under the Middle American Trench, passing under Central America as it sinks into the Earth's interior. This is not a smooth process, and the plates often stick together until the pressure builds up enough to force them to shift suddenly, causing Earthquakes. As the Cocos Plate sinks deeper if is partially melted by the friction and the heat of the Earth's interior. Some of the melted material then rises up through the overlying Caribbean Plate, fuelling the volcanoes of Central America.

Diagram showing the passage of the Cocos Plate beneath Costa Rica (not to scale). Carleton College.

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Monday 14 October 2024

Dwarf Planet 136199 Eris approaches opposition.

The dwarf planet 136199 Eris will reach opposition (i.e. be directly opposite the Sun seen from Earth) on Thursday 17 October 2024 at 7.50 pm GMT. This means that it will both be at its closest to the Earth this year, about 94.65 AU (94.65 times the average distance between the Earth and the Sun, or about 14, 159 000 000 km), and completely illuminated by the Sun. While it is not visible to the naked eye observer, the planets have phases just like those of the Moon; being further from the Sun than the Earth, 136199 Eris is 'full' when directly opposite the Sun. As this coincides with the Full Moon, the prospects for viewing for those equipped with suitable telescopes is not as good as it might be. The planet will be in the constellation of Cetus and at its highest point in the sky at about midnight local time from anywhere on Earth (this is because the rising and setting of objects in the sky is caused by the Earth's rotation, not the movement of the object). (Even at it's very brightest 136199 Eris will only have a Magnitude of 18.6, making it almost impossible to see with any but the largest of Earth-based telescopes, and where resolvable it will only be possible to see it as a point of light indistinguishable from a faint star.

The orbit and position of 136199 Eris at 8.00 pm GMT on Thursday 17 October 2024. JPL Small Body Database Browser.

136199 Eris orbits the Sun on an eccentric orbit tilted at an angle of 44.1° to the plane of the Solar System, which takes it from 35.9 AU from the Sun (35.9 times the average distance at which the Earth orbits the Sun) to 97.5 AU from the Sun (97.5 times the average distance at which the Earth orbits the Sun. With an average distance of 67.74 AU, 136199 Eris completes one orbit around the Sun every 558 years. This means that the planet is almost stationary compared to the faster moving Earth, so that it reaches Opposition only four days later each year than the year before, and reaches Solar Conjunction (when it is directly on the opposite side of the Sun to the Earth), roughly six months later.

An artist's impression of the Dwarf Planet 136199 Eris. NASA.

136199 Eris was discovered on 5 January 2005 by a team led by Mike Brown of the Palomar Observatory in California. With a diameter of 2326 km it is considered to be the second largest dwarf planet in the Solar System (after 134340 Pluto) as well as the sixteenth largest body in the Solar System, excluding the Sun (though several moons, including our own, are larger). It is also the largest body in the Solar System never to have been visited by a spacecraft (again, with the exception of the Sun). 136199 Eris has a single moon, Dysnomia, which has a diameter of about 700 km and obits at a distance of about 37 350 km.

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Sunday 13 October 2024

Genetic analysis of individuals from the Oakhurst Rockshelter suggest 9000 years of genetic isolation in South Africa.

Modern Southern African populations contain genetic diversity which records the deepest branching events known in the genetic history of extant Humans. The region also has a long archaeological record, with archaic Homo sapiens first appearing here around 260 000 years ago and Anatomically Modern Humans around 120 000 years ago. Because of this, the ancient Human populations of Southern Africa have been the subject of numerous archaeological and palaeogenomic studies, by scientists hoping to gain insights into population structures during the later stages of Human evolution. However, this interest in the most ancient Human genomes recoverable has left somewhat of a gap in the study of more recent, Holocene populations.

The Holocene has seen significant changes in technology and culture within Southern Africa. During the last 2000 year new populations have migrated into the area, bringing with them pastoralism and crop-farming. This began with the arrival of herders from East Africa, and was followed by farming populations from West Africa, who also brought the Bantu language group to the region. As well as  setting up  new communities with new ways of living, both of these groups contributed to the genetic structure of the original populations, so that all  extant San and Khoe populations draw at least 9% of their genetic material from outside Southern Africa. 

In a paper published in the journal Nature Ecology & Evolution on 19 September 2024, Joscha Gretzinger of the Department of Archaeogenetics at the Max Planck Institute for Evolutionary AnthropologyVictoria Gibbon of the Division of Clinical Anatomy and Biological Anthropology at the University of Cape Town, Sandra Penske also of the Department of Archaeogenetics at the Max Planck Institute for Evolutionary Anthropology, Judith Sealy of the Department of Archaeology at the University of Cape Town, Adam Rohrlach, also of the Department of Archaeogenetics at the Max Planck Institute for Evolutionary Anthropology, and School of Computer and Mathematical Sciences at the University of AdelaideDomingo Salazar-García of the Department of Geological Sciences at the University of Cape Town, and the Departament de Prehistòria, Arqueologia i Història Antiga at the Universitat de València, and Johannes Krause and Stephan Schiffels, again of the Department of Archaeogenetics at the Max Planck Institute for Evolutionary Anthropology, present the results of a study in which they obtained genetic samples from a series of individuals from the Oakhurst rockshelter in South Africa, and compared these to other genetic samples from historic and living populations in South Africa.

The Oakhurst Rock Shelter is located 7 km from the southern coast of South Africa, close to the town of George in Western Cape Province. It was first excavated in the 1930s, and has yielded a remarkable sequence of archaeological remains, now known to represent about 12 000 years of accumulation. The Early Holocene layers here have yielded an assemblage of macrolithic tools which has been named the 'Oakhurst Complex' in reference to the site, which has been discovered at many sites across South Africa. Around 8000 years ago, this Oakhurst Assemblage was replaced by a set of microlithic tools, which have been named the Wilton Assemblage, which persisted throughout the remainder of the Middle and Late Holocene, with minor variations. Around 2000 years ago, ceramics also begin to appear at the site. 

As well as the numerous cultural artefacts, the Oakhurst Rock Shelter has also yielded 46 sets of Human remains, adult and juvenile, deposited throughout the archaeological sequence, including the oldest dated set of Human remains to have yielded DNA in South Africa, which are 10 000 years old. Gretzinger et al. obtained genetic material from 13 individuals from the Oakhurst Rock Shelter, all of which have been radiocarbon dated from their bone collagen, yielding ages of between 10 000 and 1300 years; nine of these dates are from previous studies, while four are new dates obtained by Gretzinger et al.. The generic sex was determined for all thirteen individuals, with the mitochondrial haplogroup obtained for nine individuals and the Y chromosome haplogroup for five.

Because mitochondrial DNA is found in the mitochondria, organelles outside the cell nucleus, it is passed directly from mother to child without being sexually recombined each generation, enabling precise estimations of when individuals shared common ancestors, at least through the female line. It is also possible to trace direct ancestry through the male line, using DNA from the Y chromosome, which is passed directly from father to son without sexual recombination.

Gretzinger et al. next created a haplotype population tree including ancient DNA from the nine Oakhurst individuals with mitochondrial DNA haplotypes, as well as samples from other archaeological sites in Africa, and modern populations. Most of the samples used were from previous studies, and are publicly available, however, some of the sequences were obtained from San skeletal material held by the University of Cape Town, and used only with permission of the San communities from which they were obtained. Access to this data is only available to other researchers with the permission of the University of Cape Town Skeletal Repository Committee and the relevant San communities. 

This recovered the Oakhurst individuals as being on the deepest branching limb of the living Human tree, which also includes living San populations, but closest to other ancient individuals from South Africa than to any living population. They also note that they recovered an ancient divide between San populations living north and south of the Kalahari, and that all the ancient South African populations, including the Oakhurst individuals, are on the same branch as the San populations from south of the Kalahari.

Maximum likelihood tree showing genetic affinities between ancient and present-day southern Africans, generated using TreeMix of genome sequences from present-day and ancient populations, excluding populations with evidence of asymmetrical allele sharing with non-Africans indicative of recent gene flow. Branches of ancient individuals/groups are truncated for better readability. Gretzinger et al. (2024).

Looking at the wider genomes, Gretzinger et al. found that San and Khoekhoe populations split into three principle groups, with the Kx`a-speaking Ju|’Hoan and !Xuun forming a northern cluster, Khoe-Kwadi-speaking Nama, and Tuu-speaking ‡Khomani and Karretjiemense forming a southern cluster (Karretjiemense is an Afrikaans word meaning 'people of the cart', but is how these people self-identify), while the Tuu-speaking Taa, Kx`a-speaking ǂHoan, and Khoe-Kwadi-speaking Gǀui and Gǁana form a central group. Eight of the Oakhurst individuals lie within the southern cluster, as do four other Later Stone Age skeletons with published genomes from South Africa, although the oldest individual in the dataset show a slightly greater affinity for the northern cluster. Notably, within the southern cluster, the Oakhurst individuals showed the greatest affinity to populations still living close to the area today, with the youngest individual, OAK007, dated to 1344 years before the present, showing the greatest affinity for living populations, sharing more and longer identical by descent segments with the Karretjiemense and ‡Khomani than with any other tested population.

Comparison of the genomes of the Oakhurst individuals to other, previously published, ancient African genomes, Gretzinger et al. found that all South African Later Stone Age genomes were closer to one-another that to those of any other ancient African. The youngest individual, OAK007, was most closely related to two other Later Stone Age individuals, from St. Helena and Faraoskop, both of which have been dated to about 2000 years before the present. Together, these three individuals form a sister group to two further individuals from Ballito Bay on the eastern coast of KwaZulu-Natal, thought to be of similar age. Older genomes from Oakhurst become steadily less closely related to these individuals as they get older, as well as less closely related to the genomes of historical San samples from Sutherland, Western Cape Province. However, the genome of a 1200-year-old pastoralist from South Africa clustered with Later Stone Age genomes from Malawi, while those of four Iron Age farmers from South Africa clustered most closely with Later Stone Age genomes from Cameroon. 

Next Gretzinger et al. looked for potential ingression of non-San genetic sequences into the Oakhurst individuals, finding no trace of affinity to populations in either East of West Africa, and a consistent grouping with southern rather than northern San groups, the genetic gulf between which groups appears to have been widening steadily since their split around 20 000 years ago, before the drying of the Lake Makgadikgadi palaeo-wetland, which once covered most of central Botswana.

All of the Oakhurst individuals dating to between 10 000 and 1344 years before the present, form part of a single clade (group with shared common ancestry), which also includes individuals from St. Helena, Faraoskop, and Ballito Bay dating to between 2200 and 1300 years before the present. However, the genomes of individuals from South Africa from between 1300 and 1200 years before the present show a significant discontinuity with earlier individuals, with a second discontinuity observed between 1200 and 400 years before the present. Gretzinger et al. attribute these discontinuities to the influxes of first pastoralists from East Africa and then farmers from West Africa into the region. However, they find no trace of West African ancestry in three individuals from Sutherland dating to the late nineteenth century, while about 11% of their genome appears to be of East African ancestry, a proportion similar to that seen in living ‡Khomani individuals from the Northern Cape Province, who typically have genomes comprising about 9% East African genetic material.

Based upon this, Gretzinger et al. observe that no evidence of any genetic influx from outside of modern South Africa recorded at Oakhurst Rock Shelter between 10 000 and 13 000 years before the present, a remarkable period of genetic continuity lasting almost 9000 years. Despite this, the Oakhurst individuals show no signs of being genetically isolated, The level of conditional nucleotide diversity (the  extent to which each member of a pair of chromosomes differs from its partner, used as a measure of inbreeding within a population( maintained within the Oakhurst samples is lower than that found in  Later Stone Age population from Malawi, Kenya and Cameroon, but comparable to other Later Stone Age populations from Western Cape and KwaZulu-Natal, and greater than is seen in ancient hunter gatherer populations from Serbia, Japan, and Brazil, as well as modern San and Khoe populations. This is non consistent with a model of long-term isolation, instead indicating to the presence of a much larger population of Later Stone Age hunters in South Africa before about 1300 years before the present, when other groups are generally accepted to have begun to arrive in the region, and a subsequent dramatic reduction in the size of that population.

Reconstructing the demographic history of South Africa over the past 2000 years is complicated, with at least two significant prehistoric population influxes, and substantial genetic exchange with both other parts of Africa and other continents following the establishment of the first European settlements in about 1650. To try to address this, Gretzinger et al. created a model using genomes from Later Stone Age hunter-gatherers in South Africa, the Luxmanda archaeological site in Tanzania, which has been dated to about 3000 years before the present, and modern Mende populations from West Africa.

They then developed a best-fit model which enabled them to group populations into primarily West Africa or Primarily East African (excluding populations with a substantial amount of genetic material from both sources), in order to estimate dates for the admixtures of the West and East African components. They found that San and Khoe populations began to absorb genes from East Africa substantially before those from West Africa, with an estimated date of 1068 years before the present. This is consistent with the East African ancestry recovered the 1200-year-old pastoralist remains from Kasteelberg, on the southwest coast of South Africa near St. Helena Bay, and the estimated date of admixture of 1228 years before the present recovered from the nineteenth century Sutherland material. 

The arrival of West African genes in South Africa appears to have been considerably more recent, with living Bantu-speaking groups such as the Herero, Tswana, and Kgalagadi, producing an estimated admixture date around 808 years before the present, while 400-year-old remains attributed to Iron Age farmers from KwaZulu-Natal yielded an estimated admixture date around 832 years before the present. Living San and Khoe groups yielded a more recent estimated admixture date, of about 578 years before present. Gretzinger et al. suggest that this may reflect either several waves of West African arrivals, or a continuous flow, with an initial admixture of San and Khoe genetic material into the ancestors of modern Bantu-speaking groups and a subsequent flow of West African genes into the ancestors of modern San and Khoe populations.

All groups show considerably more Later Stone Age ancestry on their X chromosomes than on their autosomal (non-sex determining) chromosomes, with this signal being stronger in San and Khoe populations than the Bantu-speaking groups. This implies that in most cases, the contribution from Later Stone Age hunter-gatherers was from the female side. The extent to which this is true appeared to vary between living populations, with the living Damara (a Khoekhoe-speaking people from northwestern Namibia) having had about 1.4 female Later Stone Age hunter-gatherers in their ancestry for each male, the ǂHoan (a Kxʼa language-speaking group from Botswana) having about 2.28 Later Stone Age hunter gatherer females per male in their ancestry, the Shua (a Khoe-speaking group from central Botswana) having about 4 Later Stone Age hunter gatherer females per male in their ancestry, the Haiǁom (a Khoekhoe speaking group from Namibia) having about 5.2 Later Stone Age hunter gatherer females per male in their ancestry. This also applies to South Africa Bantu-speaking groups (for whom the overall contribution of Later Stone Age hunter gatherer genetic material is lower), with about 2.1 females per male having contributed genetic material to the extant population. 

This female bias can also be seen in the historical Sutherland genomes and the 1200-year-old pastoralist remains from Kasteelberg, although, surprisingly, not to the four Iron Age KwaZulu-Natal individuals, who have a higher proportion of Later Stone Age hunter gatherer genetic material on their autosomal chromosomes than on their X chromosomes, indicating a higher proportion of male Later Stone Age hunter gatherer ancestors than female ones. This is different to the situation seen in all other groups in South Africa and Botswana for which a trend could be determined, and may reflect a change in the way different groups were integrating in the past 400 years compared to the nature of such interactions during the arrival of the first farmers into the region.

Gretzinger et al. finally note a recent admixture of male northwest European DNA into San/Khoe and mixed groups from Colesberg and Wellington. The estimated date for these ingressions is 199 years before the present, despite the known arrival of Dutch and British migrants into the region from the mid-1600s onwards, something which led to a collapse in San and Khoe genetic, linguistic and cultural diversity. In addition to severely disrupting existing population structures, the European arrivals introduced a range of new populations into the region, all of which have contributed to modern population structures to some extent. As an example, Gretzinger et al. note that mixed-ancestry South Africans from Colesberg drew an average of 24.4% of their ancestry from South Asia, 2.8% from East Asia, 8.2% from Northern Europe, and about 35.5% from Later Stone Age hunter gatherers. Some San and Khoe groups also have a significant proportion of European ancestry, with the Karretjiemense drawing an average of 5.61% of their ancestry from Europe, the ‡Khomani on average 9.45%, and the Nama on average 6.83%. This suggests that southern San populations were particularly affected by intermixture with Europeans, with these groups having a higher proportion of European ancestry that other San or Khoe populations, comparable to that of the mixed-ancestry South Africans sampled. Thus, the modern populations most closely related to the Oakhurst individuals appear to be particularly affected by genetic ingression from other populations.

Demographic changes in the San and Khoe populations of southern Africa: Summary of the inferred population history of the San and Khoe in southern Africa. Sex symbols indicate male- and female-biased reproduction. Note that pastoralism and farming both appeared in present-day South Africa at about the same time, 2,000 years ago. Gretzinger et al. (2024).

The question of population continuity within Later Stone Age communities in Southern Africa has engaged archaeologists for over a century. In the past two decades, the application of genetic methodology to archaeological problems has helped to unravel the demographic histories of Stone Age populations in Europe, Asia, and North Africa, revealing episodes of large-scale migration in these regions, during which indigenous populations were either replaced by or absorbed into the new population. These biological replacements of populations also appear to have been vectors for the spread of new technologies. In South Africa, in contrast, there appears to have been a surprisingly long period of genetic continuity, with on detected influx of genetic material from elsewhere for at least 9000 years, from the beninning of the Holocene till around 1200 years ago, during which time the Southern San remained isolated from Northern and Central San populations as much as from other populations elsewhere in Africa.

This implies that the cultural changes seen at the Oakhurst Rockshelter, such as the transition from the Oakhurst to the Wilton technocomplex, were a result of entirely local inovation. It has previously been observed that there have been slight fluctuations in craniofacial size in Later Stone Age populations in coastal South Africa, something which has been interpreted as a sign of genetic discontinuity, something which Gretzinger et al.'s results contradict. Since the population was not a small isolated one which might be subject to strong effects from genetic drift, it seems likely that these variations were driven by changes in the local environment.

The 9000 years of genetic and cultural isolation experienced by Later Stone Age hunter gatherers in South Africa seems to have ended quite abruptly, with the spread of herding communities from East Africa shortly followed by the arrival of farming communities from West Africa. Most parts of South Africa do not record any genetic trace of these arrivals before about 1300 years ago. However, there is evidence for changes in settlement patterns and other cultural behaviours in this coastal South Africa from about 2000 years ago, which have been interpreted as a response to the arrival of herding in the area. In Europe, a similar cultural shift is seen at the Neolithic-Mesolithic boundary, with a genetic admixture between the incoming farming population and the extant hunter gatherer population not being recorded for about 2000 years after the cultural shift. This implies that in Europe at least, farming and hunter gatherer populations were able to live alongside one-another for a long period of time before beginning to intermix, something which may also have been true in South Africa. Alternatively, pastoralism may have been culturally transmitted from East Africa to Southern Africa long before the spread of East African populations into the region.

However, from about 1200 years ago onwards, there has been substantial migration into Southern Africa from other regions, and substantial ingression of new genetic material into all populations, with the effect that all living San and Khoe populations are admixed with one or both of East African Pastoralist and West African Farmer ancestry. This process was accelerated by the arrival of European settlers in the mid-seventeenth century, which led to widespread population collapse among hunter gatherer populations in Southern Africa. Combined with a loss of oral traditions, these events have greatly obscured the prehistoric population structure of southern Africa. 

Genetic methods such as those used by Gretzinger et al. provide a way to study these ancient population structures, showing that the San and Khoe inhabitants of South Africa are the direct decendants of the Early Holocene inhabitants of the region, despite considerable disruption to their lifestyle and population structure by later migrants to the region.

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Monday 7 October 2024

Understanding the plasma environment around Mercury.

As the closest planet to the Sun, has the most direct exposure to the solar winds of any planet in the Solar System. The planet has a weak magnetic field, with a magnetosphere strongly linked to the surface and exosphere (a zone around its solid surface within which individual atoms and molecules can be found moving freely, but seldom, if ever, interacting) by a range of processes. These are largely driven by the exchange of energy with the solar winds, and loss of material from the planetary surface. The magnetosphere contains a mixture of ions derived from the solar winds, primarily hydrogen⁺ and helium²⁺ ions, and others derived from the planet's surface via ionization of material in the exosphere. 

The Mariner 10 spacecraft made three flybys of Mercury in 1974 and 1975, detecting heavy ions thought to be derived from the planet's exosphere. Subsequent observations of Mercury by Earth-based telescopes were able to identify ions such as sodium⁺, potassium⁺, and calcium⁺. Between 2011 and 2015 the MESSENGER spacecraft orbited Mercury, during which time its Fast Imaging Particle Spectrometer was able to detect a variety of ions in the planet's magnetosphere, including hydrogen⁺ and helium²⁺ derived  from the solar winds, as well as heavier ions such as helium⁺, oxygen⁺, water group ions (hydroxide⁺, water⁺, and hydrogen peroxide⁺), sodium⁺, magnesium⁺, aluminium⁺, and silicon⁺, derived from the planet. The helium⁺ ions showed a relatively even distribution around the planet, but the other planetary-derived ions were concentrated around the planetary cusp (i.e. directly facing the Sun) and in the equatorial band of the nightside of the planet, although it was not possible to further define the distribution of individual types of ion.

The BepiColombo spacecraft is a joint project by the European Space Agency and the Japan Aerospace Exploration Agency which was launched in 2018, on aa trajectory which would lead it to make close flybys of Mercury in October 2021, June 2022, June 2023, September 2024, December 2024, and January 2025, before finally entering the planet's orbit in November 2026. 

In a paper published in the journal Communications Physics on 3 October 2024, a team of scientists led by Lina Hadid of the Observatoire de Paris, Sorbonne Université, Université Paris Saclay, École polytechnique, and Institut Polytechnique de Paris, present the result of a study of ion plasma observations made by BepiColombo's Mercury Plasma Particle Experiment instruments, during the flyby of Mercury made on 19 June 2023.

The 19 June 2023 flyby took BepiColombo to about 235 km above the surface of Mercury, enabling sampling of ions within the magnetosphere plasma at low altitudes along the spacecraft's trajectory. During the flyby BepiColombo approached Mercury from its dusk-nightside, passing through the post-midnight magnetosphere close to the equator of the planet, and moving away towards dawn-dayside. 

BepiColombo’s journey through Mercury’s magnetosphere. European Space Agency.

BepiColombo crossed the bow-shock of the planet inbound at 6.44 pm GMT, and outbound at 7.52 pm. Outside of this bowshock region, both before and after the crossing, ions with energies of about 10 and about 20 electronvolts were constantly observed, with mass-per-charge ratios of 1 and 16. This is consistent with hydrogen⁺ and oxygen⁺ ions, derived from water molecules outgassed from the planet.

Projections of BepiColombo’s third Mercury flyby trajectory in the aberrated Mercury–Sun magnetospheric (aMSM) coordinate system. (a) X'–Z' and (b) X'–Y' planes, all expressed in Mercury radii (The radius of Mercury is 2440 km). Note the displacement in (a) of the magnetopause relative to the planetary centre because of the northward offset of the magnetic dipole by approximately 0.2 of the radius of Mercury. In traditional MSM coordinates, the X-axis and Z-axis point to the sun and north pole, respectively, and the Y-axis completes a right-hand system. In the aberrated coordinates, Mercury’s orbital velocity is considered. The X-axis is anti-parallel to the solar wind direction in the rest of the reference frame of Mercury. The aberration angle varies between about 5.5° and about 8.4° assuming a solar wind speed of 400 km per second. The black arrows indicate the viewing direction of instrument during this flyby. The magenta and cyan crosses represent the observed inbound (and outbound) bow shock and magnetopause crossings, respectively. The red dot highlights the closest approach of BepiColombo to Mercury. The black solid and dashed lines represent the modelled dayside bow shock and magnetopause that are obtained from the statistical distribution of observed crossing points, respectively. Hadid et al. (2024).

As BepiColombo entered the dusk magnetosphere of Mercury, it encountered ions with energies of around 20 000 electronvolts, but following this the energy of the ions fell to a few tens of electronvolts. Hadid et al. interpret this area as the low latitude boundary layer, the area along the magnetospheric side of a planet's low-latitude magnetopause where plasmas form the magnetosheath and magnetosphere are mixed. This kind of energy dispersion within the plasma mantle is typically seen at high latitudes; its presence close to the equator of Mercury suggests a relationship between the plasma mantle and the low latitude boundary layer, with convection carrying ions deep into the magnetosphere. 

Hydrogen⁺ ions were detected in the low latitude boundary layer region which Hadid et al. interpret as having derived from the duskside magnetosphere (i.e. the part of the magnetosphere where the Sun is setting). The low latitude boundary layer region is also likely to contain heavy ions derived from the dayside exosphere of the planet and transported over the polar caps, although Hadid et al. are careful to emphasise that determining the origin of the low latitude boundary layer is beyond the scope of the current study.

Model of the hydrogen⁺ ion trajectories. (a) Shows various particle trajectory projections in the equatorial plane traced backward in time. (b) Shows the particle kinetic energy versus time. The ions are launched from different locations (closed circles) along BepiColombo’s orbit, and their trajectories are traced backward in time. The colour code depicts the different magnetospheric regions, viz., the Low- Latitude Boundary Layer in green, the umbra in blue, the Plasma Sheet Horns in yellow, and ring current in red. The test hydrogen⁺ ion trajectories were computed using a modified Luhmann–Friesen model for the magnetic field combined with a two-cell convection pattern for the electric field. The full equation of motion was integrated backward in time using a fourth-order Runge–Kutta technique. Hadid et al. (2024).

As BepiColombia enetered the umbra (shadow) of Mercury and inner part of the low latitude boundary layer at 7.24 pm it encountered 'cold' ions with energies as low as 30-100 electronvolts. Hadid et al. suggest that this might be because negatively charged, causing low-energy ions from the exosphere to become attracted towards it. The ions encountered around the low latitude boundary layer include oxygen⁺ and calcium⁺ and/or potassium⁺ ions thought to have originated from the dayside of the planet and lighter hydrogen⁺ and helium²⁺ ions, probably of solar origin.

At 7.28 pm, shortly after leaving the low latitude boundary layer, BepiColombo began to encounter 'hot' ions with energies in the kiloelectronvolt range, in an area corresponding to the 'plasma sheet horns' detected by the MESSENGER spacecraft a decade previously. These ions are thought to originate from the tail of the magnetosphere, and to be accelerated towards the planet by convection currents.

After passing through this region at 7.32pm, BepiColombo encountered a region with intense ion fluxes, with ion energies in the 5-40 kiloelectronvolt range. Because this layer is present at low altitudes in the equatorial region, Hadid et al. interpret this as a tenuous ring current, in which charged particles could become trapped in orbits of the planet at altitudes of 1.3-1.5 times its radius. At this altitude a hydrogen⁺ could orbit Mercury in about four minutes, bouncing back and forth on either side of the equatorial plane throughout its motion around the planet. The presence of such a ring current had been suggested from the MESSENGER data, but the data was rather limited, with particles with energies of no more than 13 kiloelectronvolts being detected. The greater energy range detected in the BepiColombo data provides much better support for the presence of such a ring current, although again it is not sufficient to state definitively that this is what is being detected. 

The high energy particles within this band appear to be hydrogen⁺ and helium²⁺ ions, but BepiColombo also encountered larger particles. The most common of these have energies of around 2 kiloelectronvolts, and are interpreted as oxygen⁺ ions, while more energetic particles, with energies of around 10 kiloelectronvolts, and are interpreted as being predominantly calcium⁺ and potassium⁺ ions, with some sodium⁺ ions also present. BepiColombo also encountered cold ions, with energies of about 15 electronvolts. These cold ions are presumed to have originated from the surface of Mercury, and peaked at the closest to the planet, an altitude of 332 km.

After passing through the post-midnight magnetosphere of Mercury, BepiColombo re-entered the planet's magnetosheath and then moved back into the solar wind. This solar wind comprises a compressed and heated stream of hydrogen⁺ and helium²⁺ ions, although outgasses water group ions could again be detected in this region.

Mercury’s magnetosphere during BepiColombo’s third flyby. European Space Agency.

The 19 June 2023 flyby of BepiColombo has provided us with our first reasonably detailed view of the structure of Mercury's magnetosphere, demonstrating that it is not greatly different from that of the Earth. Both low and high energy ions were observed in the planet's magnetosphere, including the deepest parts encountered, suggesting that ion sputtering (the dislodging of low energy ions from the surface of the planet by the impact of high energy ions from the Sun) plays a significant role in the system. The evidence collected by the spacecraft supports the presence of a ring current encircling Mercury, and for the first time demonstrate the presence of a low latitude boundary layer. 

The magnetosphere of Mercury will remain a subject of study for the remainder of the BepiColombo mission, including the planned further flybys and orbital stage, which should serve to greatly enhance our understanding of the planet's magnetic environment.

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Saturday 5 October 2024

Outbreak of Marburg Virus reported in Rwanda.

On 27 September 2024 the  Rwanda Ministry of Health confirmed that an outbreak of Marburg Virus Disease was present in the country, following the detection of the Virus in the blood of two patients by real-time reverse transcription polymerase chain reaction analysis at the National Reference Laboratory of the Rwanda Biomedical Center, according to a press release issued by the World Health Organization on 30 September 2024.

As of 29 September 2024, 26 cases of the disease have been reported in seven of the country's thirty districts (Gasabo, Gatsibo, Kamonyi, Kicukiro, Nyagatare, Nyarugenge and Rubavu), with eight people having died of the disease, a case fatality rate of 31%. The majority of the patients are healthcare workers from two health facilities in Kigali; this is not uncommon with outbreaks of the Marburg and Ebola viruses, with the highly transmittable nature of these diseases often resulting in aa high mortality rate in healthcare workers around the initial locus of the outbreak.

Contract tracing has led to the screening of about 300 contacts of diagnosed patients, one of whom had travelled to Belgium, with all found to be healthy and not a threat to public health. The initial source of the outbreak is still under investigation.

Marburg Virus Disease is a haemorrhagic fever, similar to the closely related Ebola Virus Disease. Both are caused by single-strand negative-sense RNA viruses of the Filoviridae family. Both are easily spread though contact with bodily fluids, and can also spread by contaminated clothing and bedding. 

Negative stained transmission electron micrograph of a number of filamentous Marburg Virions, which had been cultured on Vero cell cultures, and purified on sucrose, rate-zonal gradients. Erskine Palmer/Russell Regnery/Centers for Disease Control and Prevention/Wikimedia Commons.

Marburg Virus has an incubation period of between two and 21 days, manifesting at first as a high fever, combined with a severe headache and a strong sense of malaise. This is typically followed after about three days by severe abdominal pains, with watery diarrhoea and vomiting. In severe cases the disease develops to a haemorrhagic stage after five-to-seven days, manifesting as bleeding from some or all bodily orifices. This typically leads to death on day eight or nine, from severe blood loss and shock. There is currently no treatment or vaccine available for Marburg Virus, although a number of teams are working on trying to develop vaccines. 

Previous outbreaks of Marburg Virus have been reported in Rwanda, as well as the neighbouring Democratic Republic of Congo and Tanzania. The Virus has also been reported in a number of other African countries, including Angola, Equatorial Guinea, Ghana, Guinea, Kenya, and South Africa. The most recent outbreaks occurred in January 2023, with unrelated epidemics in Tanzania and Equatorial Guinea. 

The high rate of infection of healthcare workers seen in Marburg Virus is particularly alarming, as this tends to weaken communities ability to resist the Virus. The Virus can spread quickly in healthcare settings, infecting people whose immune systems are already stressed by other conditions, and creating aa reserve which can feed infections in the wider community. This makes it important to screen all people potentially infected with the disease as quickly as possible, and to arrange for patients to be treated in isolation, as well as quickly tracing all known contacts of any cases, and screening them for infection too.

Marburg Virus is a zoonotic infection (disease transferred from Animals to Humans), with a wild-reserve of the Virus known to be present in Egyptian Fruit Bats, Rousettus aegyptiacus, which are found across much of Africa, the Mediterranean region, the Middle East, and South Asia. These Bats form large colonies in caves or sometimes mines. They are frugivores, and can be major pests of farmed fruits, bringing them into conflict with Humans, and are sometimes hunted for food, all of which create potential avenues for the Marburg Virus to pass from a Bat host to a Human one.

A colony of Egyptian Rousette Bats, Rousettus aegyptiacus. Giovanni Mari/Flikr/iNaturalist.

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