Wednesday 30 November 2022

Eruption on Mauna Loa Volcano, Hawai'i.

Mauna Loa Volcano, located on Big Island Hawai'i, and considered to be the world's largest active volcano, began erupting at about 11.30 pm local time on Sunday 27 November 2022 (about 9.30 am on Monday 28 November GMT), producing effusive lava eruptions from the Moku‘āweoweo Caldera. By the following morning, the eruptive activity had migrated to the upper Northeast Rift Zone, producing lava streams descending the volcano's northern flank. An initial ash advisory was issued but quickly withdrawn, and no properties are currently threatened by the lava flows, which are all contained within the Hawai‘i Volcanoes National Park.

Lava flows moving northeast downslope of Mauna Loa volcano from the Northeast Rift Zone eruption, observed from Saddle Road at 6.00 am Hawai'i time on 29 November 2022. United States Geological Survey.

Mauna Loa is a shield volcano (broad, dome-shaped volcano) made up largely of successive layers of basalt lava, which have been flowing from fissures on roughly the same spot for about 700 000 years. The volcano has risen from a seafloor 5000 m beneath the surface to its current altitude of 4170 m above sealevel, making it the world's tallest mountain of any sort (Everest reaches higher, at 8848 m above sealevel, but rises from the Tibetan Plateau, more than 5000 m above sealevel, so it's height is much lower). The volcano covers an area of about 5271 km², and makes up about half the area of Big Island.

Aerial photograph of the dominant fissure 3 erupting on the Northeast Rift Zone of Mauna Loa, taken at approximately 8.00 am Hawai'ian Standard Time on Tuesday 29 November 2022. Fountains were up to 25 m high, and the vent was feeding the main lava flow to the northeast. United States Geological Survey.

Despite being a highly active volcano (with 33 eruptions since 1843), Mauna Loa is not considered to be a particularly dangerous volcano, and injuries associated with its eruptions are rare. This is because most of the activity on the volcano is in the form of slow moving lava flows, which most people are capable of simply walking away from. The last eruption on Mauna Loa occurred in 1984, when lava flows reached within 8 km of the town of Hilo, the largest settlement on Big Island, causing no damage or injuries. However, since the 1980s the population of Big Island has more than doubled, increasing the risk of lava flows reaching populated areas.

Aerial photo captured during an overflight of the Northeast Rift Zone eruption of Mauna Loa between 5.00 and 6.30 pm Hawai'ian Standard Time on 28 November 2022. This photo shows fissure vents erupting above 3 km above sealevel on the Northeast Rift Zone of Mauna Loa. Civil Defence Patrol/United States Geological Survey.

The islands of Hawai'i have formed as a result of hotspot volcanism, with a mantle plume hotspot currently located under Big Island, Hawai'i, and each of the other islands being the result of previous activity from the same hotspot, with the oldest Islands in the northwest and newest in the southeast. A volcanic hotspot is an area where magma from deep inside the Earth is welling up through the overlying plate (in this case the Pacific) to create volcanism at the surface. Volcanoes move as they erupt, swelling as magma enters their chambers from bellow, then shrinking as that magma is vented as lava.

The position of the Hawai'i Hotspot relative to the islands of Hawai'i. Joel Robinson/USGS/Wikimedia Commons.

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Tuesday 29 November 2022

Investigating burial customs in Bronze Age Finland.

Bronze age Scandinavian burial practices are well documented from southern Norway and Sweden, as well as Denmark, where a wide range of burial styles, including large elite burial mounds, gallery graves, cairns and flat ground cemeteries. Away from this area the range of practices is much less diverse. In Finland, where the Bronze Age is thought to have lasted from about 1800 to about 500 BC, the only form of burial known from this period is the cairn. There are, however, a very large number of these cairns, some estimates suggesting as many as 10 000, of which only a tiny fraction have ever been excavated. Furthermore, many of the cairns that have been investigated have yielded no archaeological material. Where material has been found beneath cairns, the most common thing recovered is burned Human bone, apparently produced when the dead were cremated. Since many of the cairns which have been investigated were excavated at a time when methods in archaeology were rather less vigorous, it is possible that many of these also contained burned bone, but that this was overlooked. However, there are other possible explanations for the boneless cairns; they may have formerly contained unburned burials in wooden coffins (something known from southern Scandinavia), which would have decomposed far more quickly than burned bone in the acidic soils present across most of Finland. Alternatively, the interpretation of these sites as burial cairns may be completely wrong, they may simply represent piles of rocks formed as agricultural land was cleared, as landmarks used by sailors, indicators of land ownership, or sacred sites of some other, non-burial related nature. These possible explanations have been debated for over a century, without any evidence being found to favour one theory over another. 

Bronze Age cairns in Finland, particularly those found close to the coast, tend to be different to those constructed in the Stone Age (9000-1800 BC), being more monumental in nature. The largest of these Bronze Age cairns are the largest prehistoric monuments in Finland. The practice of cremating the dead appears to have been introduced to Finland at the beginning of the Bronze Age, and to have persisted till the end of the Iron Age, which in Finland is placed at 1050 AD. However, the way in which the cremated remains were buried changed considerably over that time, starting with large stone cairns in the Early Bronze Age, which shrank over time. Later a soil infill between the stones appeared, eventually progressing to the remains being buried beneath the ground in flat cemeteries. 

In a paper published in the International Journal of Osteoarchaeology on 9 June 2022, Kati SaloJarkko Saipio, Maddie Hentunen,  Kristiina Mannermaa, and Markku Oinonen of the University of Helsinki, present a review of cremated bone remains from cairn burials in Bronze Age Finland held in the National Museum of Finland, and provincial museums in Finland. They examine the number of burials beneath each cairn, the location of the cairns in relationship to resources such as agricultural land, and the health of those buried, as reflected by pathologies detectable on the cremated remains. They also look at the distribution of the cairns across Finland, and their timescale, introducing a number of new carbon-dates, and compare the burials to Bronze Age burials elsewhere in Scandinavia and around the Baltic Sea. Salo et al. aim to understanding the relationship between the changing use of food resources, health of the population, and burial customs over the course of the Bronze Age in Finland.

Bone material from 218 cairns was examined, including material from 76 cairns in the Satakunta Region of central Finland, where the densest concentrations of cairns are found, 50 cairns from the northern coast of Ostrobothnia, most of which have been shown to be Iron Age in origin, 36 cairns from the Southwest Finland Region, 33 cairns from the southern coast of the Uusimaa and Kymenlaakso regions, 16 from inland regions, and 7 from the Åland Islands.

Location of the cairn burial sites in this study. Dashed lines represent land uplift isobases. As the land uplift is greatest in the Ostrobothnian area (northern coast), these cairns have also mostly been located near the coastline in the Bronze Age. Wesa Perttola in Salo et al. (2022).

The study concentrates on the Bronze Age, and includes material from 118 cairns dating to this period, but also includes material from 57 Iron Age cairns, 37 cairns that may be either Bronze Age or Iron Age, and 6 cairns that have been shown to contain material from both the Bronze and Iron ages.

The cairns range from 1.6 to 21 m in width, 1 to 20 m in width, and 0.2 to 3 m in height, with an 'average' cairn being roughly 8 m long, 7 m wide, and 1 m high. Ninety seven of the cairns are built on top of identifiable stone rings, made of larger stones than the rest of the cairn, 31 are built around large individual boulders, and 24 are built around stone cists (small stone-built coffin-like boxes or ossuaries used to hold the bodies of the dead).

Bronze Age cairn in Satakunta (Kaupinvuori in Rauma). Leena Koivisto in Salo et al. (2022).

The most common item recovered from the cairns other than bone was ceramic, with 58 cairns producing ceramic fragments. This was followed by iron artefacts, recovered from 42 cairns, bronze objects, recovered from 41 cairns, quartz, 41 cairns, flint, 18 cairns, other lithic items, 37 cairns, and burnt clay, 14 cairns. However, some of the ceramic, stone, and burnt clay items may have come from Stone Age settlements overlain by the cairns, while some of the metal items may post-date cairn construction.

Salo et al. obtained 15 new carbon dates from bone fragments recovered from 14 cairns during the course of the study. These were added to previously obtained carbon dates, contributing to a growing chronological database for Bronze Age Finland. Altogether, 67 dates have now been obtained from 43 cairns. This dating of cairns in Finland has enabled the connection of cairn-building activities to be connected to other events in ancient Finland, such as shifting shorelines.

Nine of the new dates obtained by Salo et al. were from Satakunta, and six were from the Åland Islands. Thirteen were obtained from cremated Human bone, one from cremated Dog bone (which was found with cremated Human bones), and one from a Sheep or Goat bone. The two bones dated from the same cairn were the Dog bone and a Human bone, both from a cairn in the Åland Islands, with both dating from the Late Bronze Age. The Sheep or Goat bone was of Iron Age origin.

The dates obtained showed that many late Bronze Age cairns in Satakunta were built on top of Late Neolithic or Early Bronze age settlements, something which had previously been suspected, but which archaeologists had been unable to confirm by now. Late Bronze Age cairns in Satakunta were also often built around large solitary stones, generally glacial erratics (large stones deposited away from their source after being carried by glaciers). Furthermore, they demonstrate a connection between the distribution of Late Bronze Age cairns and agricultural land; in the Early Bronze Age cairns tended to be built on high up on prominent features, while Late Bronze Age cairns are typically located close to good agricultural land, and often known Bronze Age settlements. In Gotland (Sweden) the Late Bronze Age saw the appearance of permanent settlements, and across southern Scandinavia population levels are known to have risen from about 1000 BC onwards, leading people to expand into new areas. These changes are likely to have been linked to changing economic circumstances. An increase in the number of bronze artefacts originating from Scandinavia has also been recorded in Satakunta in the Late Bronze Age, something not observed in other parts of Finland. 

Salo et al. were able to identify the remains of a minimum of 212 individual Humans from 164 cairns. The majority of the cairns (132) nappeared to hold only a single burial, with 32 cairns holding the remains of between two and five individuals. The single cairn that contained the remains of at least five individuals also yielded Iron Age material, but with a single Bronze Age metal item. Two cairns in the Luistari cemetery in Eura were shown to contain at least four individuals. Both appear to have been repeatedly used over a long period of time, but with their oldest remains dating to the Late Bronze Age. 

Five left zygomatic bones (cheekbones) from the Salo Palomäki Cairn, indicating a minimum of 5 individuals were buried beneath the cairn. Kati Salo in Salo et al. (2022).

Iron Age cairns were more likely to contain multiple burials than Bronze Age cairns, although it is also possible that all the cairns contained more individuals than have been recorded, since the numbers are based upon the minimum number of individuals that could have produced the recovered remains. One Iron Age site, Cairn 89 from Rieskaronmäki, in Nakkila, in the Satakunta Region, is thought to contain five-to-six individuals, buried at different locations within the cairn over a period of about 270 years, based upon radiocarbon dates.

Single-burial cairns are also common in Bronze Age Sweden, although here double-burials are more common. Salo et al. also note that several other burial types are present in Bronze Age Sweden, and that in some of these multiple burials are more common. 

Ninety five cairns were shown to contain identifiable Animal bones, 53 of which also contained Human remains. A further 15 cairns contained unidentifiable Animal remains; although in all of these cases the sample of material was very small, i.e. less than 3 g. The majority of these Animal bones were uncremated; 27 of the cairns were found to contain cremated Animal bones alongside cremated Human remains, but these were all dated to the Iron Age. Additionally, some of the cremated Animal bones found in Bronze Age cairns may actually come from older, Stone Age, settlements covered by the monuments. This absence of cremated Animal remains from Bronze Age cairns in Finland appears to be significant, implying that the burning of Animals with the deceased and/or the deposition of burned Animal remains alongside the deceased, was not a common practice. These practices were common in Middle and Late Iron Age burials in Finland, and in Bronze Age burials elsewhere in Scandinavia. This difference in timing may be linked to the later adoption of field-cultivation in Finland than elsewhere in Scandinavia.

Iron Age cairns in Finland were also more likely to contain artefacts than Bronze Age cairns, and were more likely to be built on top of older settlements, which may also reflect improving agricultural knowledge.

Between fifteen and eighteen individuals from fifteen Bronze Age cairns and seven individuals from Iron Age cairns could be diagnosed with porotic hyperostosis; a pathological condition in which patches of spongy bone form on the cranium as a result of anemia, which in turn may be a result of malnutrition or a genetic condition. A further nineteen cairns, ten of which could be dated to the Bronze Age, produced remains with signs of osteoarthritis.

Porotic hyperostosis in Rauma Huhdanniska (KM2800:17A), Eura Junnila (KM8307:2), Parainen Trollberg (KM20434:2), Harjavalta Kaasanmäki (KM5104:12), Laihia Murhaasto (KM10858:1), and Vöyri Viskusbacken (KM9385:14). Cribra orbitalia from Nakkila Kuusisto site (KM6126:38). Kati Salo in Salo et al. (2022).

Two Iron Age Cairns and two Bronze Age cairns produced remains with signs of having lost teeth before death. Two Bronze Age cairns produced remains with signs of periapical lesions (tissue produced by a bone or tooth in response to an infection), and one Bronze Age and one Iron Age cairn produced remains with signs of  periosteal bone formation, which is generally provoked by an injury. Osteochondritis dissecans, caused by repetitive trauma to a joint, was observed in remains from a Bronze Age cairn, and may also be present in remains from an Iron age Cairn. One Bronze Age cairn produced a vertebra with a possible Schmorl's node, a form of spinal disk herniation, which would probably have been caused by repetitive injury.

Joint conditions. Possible Schmorls node from Eura Kivimäki site (vertebral body, KM 7412:4). Signs of degerative joint disease from Uusikaarlepyy Råbacken (vertebral body, KM24015:20), Nakkila Rieskaronmäki (articular facet of a rib, SatM16455:4), Laihia Riitasaari (body of a cervical vertebra, KM10435:1), Pedersöre EsseLillmossbacken (scapula, glenoid KM10105:9), and Isokyrö Kaaminmäki (atlas, articular facet for dens axis KM10678:60). Possible osteochondritis dissecans (first hand phalanx, proximal) from Eura Uotinmäki (KM5629:232) and Laihia Riitasaari (KM 10435:1) sites. Kati Salo in Salo et al. (2022).

The commonest form of pathology seen in archaeological material is dental. However, teeth seldom survive cremation, and the material used in Salo et al.'s study was no exception to this, with only small fragments of tooth found. Some of the alveolar fragments found showed signs of dental problems - tooth loss and periapical lesions - showing that these conditions were present in the population, but providing little other information. 

Cribra orbitalia and porotic hyperostosis, spongy bone formation around the orbit and cranium, respectively, both of which are caused by chronic iron deficiencies, were observed in several sets of remains. These can be caused by a direct shortage of iron in the diet, by other dietary problems, such as a lack of vitamin B12, or genetic conditions, something which today is most common among people living around the Mediterranean Basin. Similar conditions can also be caused by Malaria, something common in Finland until the early twentieth century.

The mostly densely populated areas in Bronze Age Finland were around the coast, making it highly likely that Fish were an important dietary resource. Remains from archaeological sites close to the coast around Europe over a wide range of times have been shown to be more prone to porotic hyperostosis and cribra orbitalia than inland populations, and it has been suggested that these conditions might have been caused by parasites contracted from Fish. 

The adoption of agriculture has been widely linked to declining health in many Human populations, and the Bronze Age is thought to have been the period during which agriculture became widespread in Finland.

Cribra orbitalia and porotic hyperostosis have been shown to be rare in populations from the Neolithic-Bronze Age transition in southern Sweden, and the Late Bronze Age of Estonia. These conditions have been shown to be very common in Early Bronze Age Poland, where they are found in more than 20% of the population, however, this is in a sample of remains with much better overall preservation, so direct comparison is difficult.

The other common pathology found in the collection is marginal osteophytes, or signs of osteoarthritis. This is found in 10 individuals, who are typically older than the majority of the samples. Of the 10 individuals, all but one were found in Satakunta, and all but one were found in cairns built on top of former settlements. The majority of the remains with osteophytes appear to have been male, which is common in ancient populations. This may be a sign that the individuals had been undertaking hard physical labour, associated with agriculture, which is believed to have been adopted in Satakunta before other regions of Finland. The practice of building cairns on top of former dwelling sites appears to have been linked to the adoption of agriculture, something which had happened by the Late Bronze Age in Satakunta, but which did not happen until the Iron Age in other parts of Finland. Other studies have shown that early agriculturalists were particularly prone to osteoarthritis of the vertebral joints, which seems to be the area most affected in the individuals from Satakunta.

Another practice that appears to have been adopted earlier in Satakunta than other areas is that of placing more than one individual beneath the same cairn. These cairns with more than one internment were also the ones which had the highest rates of osteophytes and porotic hyperostosis, with these individuals also being more likely to be male. Thus these were older male individuals who had been involved with hard manual labour, probably agriculture related, and were suffering from iron-deficiency, something also associated with the adoption of agriculture, which led to lower levels of meat consumption. Analysis of Animal bone from Late Bronze Age cairns in Satakunta suggests that Seal meat was disappearing from the diet at this time. 

Other pathologies, such as trauma or periostitis, were much less common, but this does not mean that they were absent from the population. Other studies of cremated remains have shown that these are generally much less common, suggesting that this is related to the cremation process, rather than the health of the population, possibly because new bone growth tends to split away from older bone when burned. Degenerative joint disease and porotic hyperostosis are more likely to survive cremation, and have been shown to be more common in other populations where cremation was practised.

It is likely that future excavations will uncover more remains from cairns in Finland, and that this will lead to a more detailed understanding of cairn-building people and the lives they lead. Salo et al. suggest that more detailed studies of Iron Age cairns may lead to a better understanding of the transition to an agricultural lifestyle across Finland. 

Strontium isotope analysis could potentially be used to determine the origin of the individuals within the cairns. Studies using this method have been carried out in Estonia, and Gotland (Sweden), where genetic analysis of Bronze Age burials has also been undertaken, although this is not likely to be possible in Finland, where cremation appears to have been a universal practise, as DNA cannot usually be recovered from cremated remains.

The Bronze Age is the earliest period in Finland where sufficient remains exist for a large scale comparison between sites, and Salo et al.'s study provides insights into this little-known area of the European Bronze Age. The common Bronze Age practise appears to have been to bury a single individual beneath a large cairn, although this appears to have changed over time, with multiple burials appearing in the Late Bronze Age and becoming more common in the Iron Age, apparently reflecting a change in burial custom associated with the spread of agriculture. These single burials appear to have been much less likely to have been accompanied by Animals or artefacts than contemporary burials in southern Scandinavia, probably reflecting cultural and economic differences between the two areas. Porotic hyperostosis is more common than in other Bronze Age populations around the Baltic Sea, and osteophytes are seen to appear earlier in Satakunta than other areas of Finland, apparently connected to an earlier adoption of agriculture.

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Thursday 24 November 2022

Acanthaster benziei: A new species of Crown-of-thorns Starfish from the Red Sea.

Crown-of-thorns Starfish, Acanthaster spp., are highly distinctive Starfish found across the tropical Indo-Pacific region from the east coast of Africa to the west coast of Mexico, which get their popular name from the covering of long, venomous spines found in most species. They are typically corallivorous, feeding on Coral Polyps by extruding their stomachs and digesting them externally. Notably, Crown-of-thorns Starfish can undergo sudden rapid population increases, known as outbreaks, which can lead to large areas of Coral Reefs being denuded of their living Polyps, something of great concern to conservationists at a time when Coral Reefs are facing a range of other threats, which has led to them being one of the most extensively studied groups of Marine Invertebrates.

Crown-of-thorns Starfish were first described by the German naturalist Georg Eberhard Rumphius in 1705, and given their own generic name, Acanthaster, by the French palaeontologist François Louis Paul Gervais  in 1841. For a long while, only two species were described within the genus, Acanthaster planci, the typical, long-spined, venomous, corallovorous form, and Acanthaster brevispinus, a shorter-spined, non-venomous form, which does not feed on Corals. However, genetic studies carried out within the past three decades have shown that Acanthaster planci is in fact a species cluster, made up of a number of physically very similar species (cryptospecies), which are nevertheless genetically distinct, which often appear to have diverged from one-another a long time ago. 

Based upon this, it was suggested that the original species should be split into four different species, each inhabiting a different geographical area; the Pacific, the Southern Indian Ocean, the Northern Indian Ocean and the Red Sea, which each of these species probably needing further division into several subspecies. Subsequent studies have indeed confirmed that the Pacific, North Indian Ocean, and South Indian Ocean populations are in fact separate species, although genetic material from the Red Sea population has not, until now, been available.

In a paper published in the journal Zootaxa on 17 November 2022, Gert Wörheide of the Department of Earth and Environmental Sciences Palaeontology and Geobiology, and the GeoBio-Center at Ludwig-Maximilians-Universität München, and the Bavarian State Collection of Palaeontology and Geology, Emilie Kaltenbacher and Zara-Louise Cowan, also of the Department of Earth and Environmental Sciences Palaeontology and Geobiology at Ludwig-Maximilians-Universität München, and Gerhard Haszprunar, also of the GeoBio-Center at Ludwig-Maximilians-Universität München, and of the Bavarian Zoological State Collections, describe the Red Sea population of Crown-of-thorns Starfish as a new population.

The new species is named Acanthaster benziei in honour of marine biologist John Benzie, for his extensive work on Crown-of-thorns Starfish. The description is based upon four specimens collected from species within the territorial waters of Saudi Arabia by  Sara Campana and OliverVoigt in 2017.

Typical colouration of Acanthaster benziei. (A) GW4081 (Paratype, hiding during the day under a crevice), Al-Lith, Saudi Arabia, (B)–(D) Thuwal Reefs, Saudi Arabia. Approximate diameter of specimens is 25–30 cm. Oliver Voigt & Gert Wörheide in Wörheide (2022).

Acanthaster benziei is a large Starfish with a convex disk and 11-14 arms (the range for the genus being 10-25), of uneven lengths, and tapering to a point. Each arm has two rows of ambulacral tube feet, which have flattened tips and lack suckers. The central disk of the species is 28-65 mm across, with an aboral (upper surface) covered in papulae (pimples) arranged in an apparently random manner. Both surfaces are covered in calcareous ossicles (plates) and spines. These Starfish are grey-green to grey-purple in colour, although the aboral spines are orange or red. The papulae on the aboral surface of the central disk can form darker patterns, giving this surface a 'bulls-eye' appearance.

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Tuesday 22 November 2022

Magnitude 5.6 Earthquake in West Java kills at least 162 people

The Baden Meteorologi, Klimatologi, dan Geofisika recorded a Magnitude 5.6 Earthquake at a depth of 10 km in the Cianjur Districs of West Java Province, Indonesia, slightly after 1.20 pm local time (slightly after 6.20 am GMT) on Monday 21 November 2022. The event triggered a series of landslides, as well as causing a large number of building collapses, and is now known to have killed at least 162 people. 

The approximate location of the 21 November 2022 West Java Earthquake. United States Geological Survey.

Many of those who have died are reported to have been children, with at least one school having been severely damaged by the Earthquake, which struck with very little warning. Many homes have also been destroyed and damaged, a hospital in Cianjur District damaged. Much of the area has been left without power, and many roads have been damaged, making communication with more remote areas very difficult. Nurses from the Indonesian Red Cross are reported to be trying to reach several remote villages on motorbikes.

Damage to a school in the Cianjur District of West Java, Indonesia, following an Earthquake on 21 November 2022. ABC News.

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.

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Saturday 19 November 2022

Understanding the ancient greenhouse climate of Mars.

The surface of Mars today is cold, arid, and heavily oxidised, but covered in surface features that tell of a very different past. The planet is home to hundreds of dry lakes, channels, and other features which must have been laid down in warm, wet conditions. Exactly how this came to be is not completely clear; Mars has always been further from the Sun than the Earth, and the ancient Sun is believed to have been cooler and dimmer than it is today. The most likely explanation is that Mars once had a dense atmosphere, rich in greenhouse gasses such as hydrogen, methane, and carbon dioxide. However, beyond this simple assumption, we know little about the ancient climate of Mars. We do not know whether the features seen on the surface of the planet represent a single warm phase, or whether Mars went through several cycles of warming and cooling, as Earth has done throughout it's history. 

The weathering of minerals on the surface of Mars potentially offer some insight into the ancient climate of the planet. The way in which minerals weather is related to the presence of oxidising chemicals and/or acids at the planet's surface, and on Earth this has been used to interpret ancient climatic conditions. Loosely speaking, the elements calcium, magnesium, sodium, potassium, and manganese are mobile, and tend to be removed from minerals by weathering processes, while the elements titanium, aluminium, and zirconium, are immobile. Over time mobile elements are removed from mineral formations by weathering, while the immobile ones are concentrated by the same processes, forming clays rich in these elements. Many sites on Mars have been shown to have a surface layer of aluminium-rich clays, overlaying a sub-surface layer of iron and magnesium smectites, something which has been interpreted as an weathering-profile, with the iron having been removed from the upper layers. Importantly, iron is usually only a mobile element under reducing conditions, when it becomes soluble in water, potentially providing insights into the ancient climate of Mars. The leaching of iron implies wet, reducing conditions.

The pattern of aluminium-rich clays overlaying iron-rich smectites can be seen at many locations in the Southern Highlands of Mars, although these have not been studied in any detail. Potentially, studies of these deposits could enlighten us about the history of Mars in a number of different ways. These would include determining if the conditions which produced them were global in extent; whether they record a single, or multiple events; and are they all of the same age? 

In a paper published in the journal Communications Earth and Environment on 4 November 2022, Binlong Ye and Joseph Michalski of the Department of Earth Sciences at the University of Hong Kong, Hong Kong, examine 203 exposures, distributed across the Martian surface, at which aluminium- and silicon-rich minerals could be observed overlaying iron- and magnesium-rich minerals, in order to build up a more comprehensive picture of the ancient climate of Mars.

Of the 203 sites Ye and Michalski examined, 54 were new sites, detected during the study. The majority of the sites were in the Mawrth Vallis, Eridania northern basin, Valles Marineris, Nili Fossae, Simois colles/Gorgonum chaos, Noachis Terra and Hellas Basin, at latitudes ranging from 30° north to 40° south. Sites outside this range may exist and not have been observed, or may have been obliterated by polar processes. Other features on Mars which have been associated with an ancient wet climate, such as lake basins or networks of valleys, are also restricted to the range 30° north to 40° south, which might reflect the distribution of ancient rainfall, and are found at altitudes ranging from 3000 m below to 6000 m above the Martian average surface level (lacking sees, Mars has no sealevel). Almost 88% of these exposures are on terrains interpreted as Noachian in age. Of the 203 sites examined, 65 are in units thought to be Early Noachian in age, 85 in units thought to be Middle Noachian in Age, 28 in units thought to be Late Noachian in age, and 18 are in units attributed to the Late Noachian/Early Hesperian transition. Dating based upon the density of craters suggests these sites are between 3970 and 3180 million years old

The relationship between compositional stratigraphy and valley networks and open basin lakes. The green dot represent the presence of weathering profiles in the study. The blue tone indicates the occurrence of the valley network, and the yellow circle represents the location of the open basin lake. Ye & Michalski (2022).

The sites were observed by remote sensing in a wide range of contexts, including impact crater floors, crater rims, crater walls, inter-crater plains and basins, within valley networks, and the knobby terrain of the Eridania deep basin deposits. This method has inherent biases, and is better able to detect steep exposures in dust free regions; it is therefore highly likely that many other similar exposures exist but have been overlooked.

Ye and Michalski combined data on 154 sites studied using the HiRISE  and CRISM instruments on the Mars Reconnaissance Orbiter, and the OMEGA instrument on the Mars Express spacecraft. This data was combined as the HiRISE instrument observes better in the part of the spectrum at which iron minerals are visible, while the CRISM and OMEGA instruments observe better in the part of the spectrum at which aluminium minerals are best detected.

Characteristics of representative geological contacts in martian weathering profiles. HiRISE IRB data (infrared, red, and blue-green) reveal sub-metre compositional differences of geological contacts of weathering profiles in false colour. (a) Mawrth Vallis; (b) Northern Hellas Basin region; (c) Eridania northern basin; (d) Noachis Terra; (e) Nili Fossae region; (f) Terra Tyrrhen; (g) Valls Marineris; (h) Simois colles. Ye & Michalski (2022).

The HiRISE image show contacts between white and red (units in many locations, with the white units always overlaying the red. The contact between the two layers is typically not sharp, and does not follow bedding planes, with the general texture and fabric of the rock being continuous between the two units. Ye and Michalski take this as indicative of iron (the main contributor of red colouration to rocks) having been leached from the upper layers.

On Earth, the crust can be divided into two distinct types, felsic crust, dominated by silicon and aluminium minerals, which makes up the continents, and mafic crust, dominated by iron and magnesium minerals, which makes up the ocean basin. On Mars this differentiation does not exist, with the entire surface of the planet being covered by mafic crust. However, while the long-term differentiation of material that has created the Earth's continental crusts never occurred on Mars, some felsic terrains have been produced in upland areas. Thus, the vast majority of the exposures studied are in mafic terrains, but such exposures within felsic terrains out also to be detectable. 

Such a weathering profiles within felsic terrains were found by Ye and Michalski in the massifs surrounding the Hellas Basin. These massifs are thought to have been formed by uplift or exhumation of the crust following the event which formed the Hellas Basin (actually a giant impact crater) about four billion years ago. The rocks here are very mixed, with a wide variety of lithologies present, but one distinctive rock-type present appears to be anorthasite (calcium-rich plagioclase feldspar) with a high iron content. Where present, this rock-type has a massive structure without bedding planes, probably indicating a plutonic origin. The majority of these rocks appear to be iron-rich silicates, but areas at the top of the massif have absorbance spectra implying the presence of aluminium hydroxides compounds, most likely kaolinite clay, a mineral which on Earth typically forms in hot, moist climates. Similar associations, with felsic terrains having aluminium-clay deposits in their uplands, were also observed in the Xanthe Terra and Noachis Terra regions. 

Example of possible precipitation-driven chemical alteration of felsic materials. (a) The geology context of compositional stratigraphy on the massif of northern Hellas Planitia (66.32 2°E, 25.21°S). MOLA elevation data are draped over THEMIS daytime infrared data (warm colors are at higher elevations and cool colors are at lower elevations). (b) Close-up view of the massif indicates the location of compositional stratigraphy (white arrow). (c) Ratioed CRISM I/F spectra contain aluminium clay minerals, iron/magnesium smectites, and felsic materials. (d) CRISM mineral map shows the distribution of diverse altered minerals: iron/magnesium smectites in red, felsic materials in green and aluminium clay minerals in blue. The different colour arrows show the locations of spectra acquired. Ye & Michalski (2022).

Ye and Michalski considered three possible origin scenarios for these deposits. Firstly, they could all have formed during a single, geologically brief, warm wet episode on ancient Mars. Secondly, Noachian Mars may have had an overall cold and dry climate, but with repeated intervals of warmer, wetter conditions. Such a scenario should, in theory produce some layered deposits in which aluminium rich and iron rich deposits alternate. These would be rarer and harder to find than on the Earth, where sedimentary rock processes are dominated by daily and seasonal water processes, are still likely to be present on Mars even if these events were separated by tens of thousands of years. A third possibility is essentially the same as the second, but in this version the most recent episode will have overwritten earlier events, with available clay always being washed downwards, so that older, lower deposits will contain iron washed down from above.

Hypotheses for the pattern within weathering profiles for aluminium/silicon materials and iron/magnessium clays. A scenario of a single climate transition (left) and another case of multiple repeated climate transitions spread out over geologic time (centre panel). A third hypothesis is that multiple events occur, but the latest event chemically overwrites older weathering profiles as iron migrates downward in the section. The blue tone unit refers to aluminium/silicon (iron-poor) materials, and the warm brown colour indicates the occurrence of iron/magnesium smectites. Ye & Michalski (2022).

Of the 203 profiles studied by Ye and Michalski, 201 showed only a single aluminium-rich layer overlaying iron-rich deposits. However, at the remaining two locations, one on Meridiani Planum and the other in the southern part of Coprates Chasma, iron-rich smectite deposits could be observed overlaying aluminium-rich clay deposits. 

Evidence of multiple pedogenic events in southern Meridiani Planum. (a) The geologic context of weathering profiles on an interfluve in southern Meridiani Planum. MOLA elevation data draped over THEMIS daytime infrared data (warm colours are higher elevation and cool colours are lower elevations). (b) CRISM data extracted from regions of interest are shown as offset ratio spectra compared to laboratory spectra of relevant minerals. (c) A CRISM mineral parameter map shows the distribution of iron/magnesium smectites and aluminium clay minerals (iron/magnesium smectites in red and aluminium clay minerals in blue). (d) 3D view of weathering outcrops is shown in the rectangle of (c) with 5 times vertical exaggeration. The different colour arrows show locations of spectra acquired. (e) Close-up view of HiRISE image shows the morphology and contact of clay-rich outcrops. (f) The subset of HiRISE images exhibits pervasive boxwork veins on the aluminium clay minerals unit. Ye & Michalski (2022).

The example on Meridiani Planum is particularly clear, and close to the area explored by the Opportunity rover. Here, there is an area of intense erosion and impact cratering, where high-standing 'knobs' of material have been left by erosive action. On these knobs two geological units can be observed. The upper of these is a relatively dark unit about 10 m thick, which is spectrally consistent with an iron/magnesium smectite. The lower unit is also massive, but with polygonal fractures (indicative of drying sediment, usually clay) and box-work veins (a feature produced by water percolating through a deposit, dissolving and redepositing minerals), possibly of some sulphate material. This unit is about 80 m thick, and is again comprised principally of iron/magnesium smectite, but with the upper portion apparently being an aluminium-rich clay. This appears to imply at least two phases of deposition, with the lower smectite layer being deposited first, having the soluble metal ions washed out of its upper portion, and then a second smectite layer deposited above it.

The vast majority of the exposures detected show only a single climate transition. However, this does not preclude there having been two or more such transitions, due to the possibility of such events having been over-written by subsequent events. Furthermore, most of the exposures are of a limited size; ideally, to detect repeated climate cycles in the rock record, geologists would look for exposures hundreds or even thousands of metres deep. 

The small size of the majority of the exposures detected also makes it hard to date these exposures using impact crater counting. However, the majority of the profiles are associated with cap units (i.e. the uppermost units in any succession), which gives them wider horizontal exposure, and makes it possible to establish a minimum age by this method.

Using computational stratigraphy to date cap units in the Martian Southern Highlands gives ages of between 3.8 and 3.6 billion years, consistent with previous results. 

The youngest example found by Ye and Michalson is in the floor of the Orson Welles Crater in Xanthe Terra, which is is breached by a series of fissures and graben to the southwest and by the Shalbatana outflow channel to the northeast, and which has been dated to 3.57 billion years before the present, based upon crater density analysis of the rim and ejecta deposits associated with the main crater, or 3.18 billion years, based upon similar analysis of sediments within the crater. The chaotic terrain associated with this crater has several outcrops with aluminium clays overlaying iron/magnesium smectites; notably, the upper layer also appears to contain opaline silica or allophane/imogolite, features that form by the weathering of volcanic ashes in cool environments with a limited water supply. 

The youngest known example of compositional stratigraphy on Mars. (a) The geology context of compositional stratigraphy on the chaotic materials in the Orson Welles crater. MOLA elevation data draped over THEMIS daytime infrared data (warm colours are higher elevations and cool colours are lower elevations). The white arrows indicate the presence of impact ejecta. (b) The CTX shows the overview of layered light-toned compositional stratigraphy. (c) Close-up HiRISE image shows the morphology and texture of compositional stratigraphy. (d) CRISM spectra extracted from regions of interest are shown as offset ratio spectra compared to laboratory spectra of relevant minerals. (e) The CRISM parameter map shows the distribution of alteration minerals, iron/magnessium phyllosilicate in red/yellow and aluminium phyllosilicate in blue. The different colour arrows show locations of spectra acquired. Ye & Michalski (2022).

Ye and Michalski are careful to point out that the age of a deposit and the age of weathering on that deposit are not necessarily the same; thus if a 3.18 billion-year-old deposit shows signs of weathering, then it can be said that the weathering is not older than 3.18 billion-years-old, but no minimum age can be assumed.

The oldest examples in Ye and Michalski's study are apparently rain-weathered pyroclastic-sediments on the flanks of two volcanoes in Thaumasia Planum. These volcanoes have been dated to 3.97 and 3.83 billion-years-old, respectively, and show signs of remnant crustal magnatization, which would indicate a pre-Noachian or early Noachian origin. Again, it is impossible to determine if the weathering is as old as the deposits, but these are the oldest sediments in which this sort of weathering can be seen.

The Orson Welles Crater and Thaumasia Planum deposits provide a time bracket for the deposition of sediments altered by chemical weathering driven by precipitation on Mars. These span the whole of the Noachian and Hesperian periods on Mars.

Exactly how long wet conditions persisted on Mars has been a source of speculation among scientists for many years. A number of methods have been used to approach this, including geomorphic analyses, numerical climate modeling, and  chemical alteration models. Ye and Michalski's results add further information to this debate.

The presence of thick clay-bearing pedogenic profiles is indicative of the presence of water and therefore a climate warm enough to allow liquid water. Ye and Michalski have used remote sensing to detect outcrops of such clays tens of metres thick (smaller deposits would be undetectable using available methods). Such deposits would be considered substantial on Earth, where similar profiles are common, but generally less than a metre thick, although it is quite possible that ancient Earth would have been host to similar massive pedogenic profiles, which have been overlaid by subsequent events.

Clays form on Earth with at a typical rate of about 0.01 mm per year, with the fastest known examples exceeding 0.05 mm per year. Assuming similar rates of formation on Mars, a deposit 120 m thick containing about 15% clay would require less than 40 million years, or 10% of the duration of the Noachian, to form. As few of the clay-profiles discovered exceed 100 m in thickness, and most are in the region of 50-60 m, the time needed to form these deposits is potentially much shorter, perhaps 1-10 million years, only 1-2% of the time-frame within which these deposits are thought to have formed.

The thickness of the deposits and estimated proportion of clay present can provide only a very rough estimate of the duration of weathering events. Rain-driven chemical erosion is caused by the dissolution of cations such as iron or magnesium in water draining through the rock. However, water cannot absorb an infinite amount of such cations; the amount that can be absorbed is determined by the diffusion rate, which in turn relates to temperature. Loosely speaking, the colder it is, the less cations can be absorbed. Another problem is physical erosion; chemically weathered sediments are more prone to disaggregating. These add significant complications to the estimated timeline for the production of weathered clay profiles on Mars, though Ye and Michalski estimate that shorter, warmer, climate excursions would produce thicker clay-deposits than longer-lived, but cooler, excursions.

The rate at which rocks are eroded by water is also dependent on their lithology, permeability, and porosity, which are hard to determine on Mars, although it is likely that many of these deposits were a mixture of volcanic ashes and impact glass. Such volcanic deposits are known to be more prone to chemical alteration by water than other rock-types, which may shorten the time in which these Martian deposits could have formed.

Some of the profiles seen are close to other features associated with running water, such as cross bedding, layered sediments, and channels, which imply they formed as part of a complex pattern of water-rock interactions. The presence of water-deposited sediments is likely to also be indicative of water-related erosion of the parent rocks from which these sediments are derived.

Evidence of sedimentary process on or within weathering profiles on Mars. Layer structures of weathering profiles (a)–(d). Inverted craters (e), (f) and inverted channels on weathering outcrops (g), (h). Ye & Michalski (2022).

Ye and Michalski's Martian global survey has produced over 200 examples of ancient chemical weathering on Mars. The youngest of these appear to be Hesperian in age, although the majority are Noachian, i.e. more than 3.7 billion years old. These profiles range widely in altitude, from 11 km above the Martian average surface level, to 5-6 km below, which appears to support the idea that their formation was driven by top-down water transportation, and was global in extent. 

Almost all of the profiles show only a single transition between climate-states, however, examples of more than one transition are present, and the lack of more widespread multiple transitions can be explained by chemical 're-setting' of the rocks during wet phases. These deposits occur in a wide-variety of mineralogical contexts, suggesting a complex history of water-mineral interactions on Mars.

The most probable origin of these deposits are a series of geologically short-lived (1-10 million years) warm climate excursions driven by reducing greenhouse gasses, such as hydrogen or methane. A wet anoxic environment is ideal for the mobilisation of iron ions in water, while leaving aluminium ions in place, creating the the profiles found by Ye and Michalski. 

The time needed to form the observed profiles is much shorter than the duration of the Noachian, making a single climate transition unlikely. This being the case, the chemical-resetting scenario is the most plausible for the origin of these deposits, which means that each example probably records the last wet interlude in the area where it is found.

Many of the weathering profiles are associated with water-generated sedimentary structures such as degraded impact craters, valley networks, closed-basin lakes, and open-basin lakes. In some cases it appears possible that the weathered profiles have themselves been cut through by water features, although the most plausible scenario is that the features are contemporary in nature, and that most of the weathering profiles are associated with a pulse of valley-network formation in the Late Noachian-Early Hesperian. 

The channels and lakes could have been formed in as little as about a thousand to about a million years, while the weathering profiles are thought to have taken 1-10 million years to form. Thus the weathering profiles widen the interval for the potential presence of water on Mars. Furthermore, although the interval for the formation of each weathering feature is relatively short, the total time in which they could have formed is very long. The presence of top-down weathering profiles in a wide range of geological contexts, apparently formed over a long period of time, suggests multiple formation events over an extended period of Martian history, driven by repeated spikes in the reducing greenhouse gas content of the ancient Martian atmosphere.

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