Looking for the Chinguetti Meteorite.

In 1916 a young French army officer called Captain Gaston Ripert reported being taken to see a giant meteorite in the Mauritanian desert, south of Chinguetti. The story is a strange one, with Ripert claiming he was taken blindfolded, at night, on a ten hour Camel ride into the desert, where he observed a huge iron structure 100 m long and 40 m wide, recovering a smaller, 4.5 kg meteorite from its surface. Shortly after returning to Chinguetti, where he was commander of the local Camel corps, Ripert reported that his guide, a local chief, was poisoned, leaving him unable to relocate the site.

The eccentric nature of this story led many people to dismiss it out of hand. It was not unusual for western travellers of the time to make up tales of wild adventure; some even paid ghostwriters to create particularly entertaining tales. However, officers in colonial armies were supposed to refrain from such nonsense, and some aspects of Ripert's story were hard to rectify with the story being complete fiction. 

During the past century a number of expeditions have sought to locate Ripert's meteorite, with the first in 1924, although by this time Ripert was stationed in Cameroon, and could only be communicated with by letters. This meant that the early searches concentrated on the area to the southwest of Chinguetti, although Ripert later clarified that the area he was taken to was probably to the southeast. The French naturalist and explorer Théodore Monod mounted a number of expeditions to find the meteorite, starting in 1934, but was unable to locate it. In the 1950s an expedition by the French army used a declinometer  (instrument for measuring magnetic declination) in a search for the meteorite, without success, and in the 1990s a team from the British TV station Channel 4 used a magnetometer during a search for the meteorite, but took only a few measurements.

Despite all this, there are a number of elements of the story suggest that it was not complete fiction, not the least of this being Ripert's willingness to talk to experts about his journey for the rest of his life. The smaller rock which Ripert recovered did prove to be a meteorite, albeit one which, when subjected to radionuclide analysis in 2001 was shown not to have been part of a larger body (radionuclides form near the surface of asteroids due to a constant bombardment by cosmic rays, but these can only penetrate a little way, so the radionuclides they form are absent from the interior of large bodies). Finally, Ripert reported observing metallic needles protruding from the large meteorite, which he tried unsuccessfully to break off, finding that they were too ductile (able to be deformed without losing toughness) for the tools he had at hand). In 2003, the American geologist and meteorite specialist William Cassidy reported similar ductile metal needles protruding from nickel-rich zones of iron meteorites, but this was clearly unknown to science in 1916.

A fragment of the smaller meteorite brought back by Gaston Ripert in the collection of the Smithsonian National Museum of Natural History. Wikimedia Commons.

In a paper published on the arXiv database at Cornell University on 21 February 2024, Robert Warren  of Salisbury in England, Stephen Warren of the Astrophysics Group at Imperial College London, and Ekaterini Protopapa of the Department of Physics at the University of Oxford, describe the results of a more recent search for the Chinguetti Meteorite, and the prospects for either discovering its existence or proving its non-existence in the future.

Warren et al. began by collating remote-sensing data covering the region from multiple sources; they are reasonably confident that other researchers will have searched Google Earth for signs of the meteorite,  but they also accessed data from other sources, including the Shuttle Radar Topography Mission, the Advanced Land Observing Satellite (ALOS), and Landsat.

Using the reasoning that the only way a 40 m high meteorite could have disappeared in the deserts of Mauritania is for it to have been covered by a sand dune, Warren et al. began by searching for a region of high dunes which could be reached from Chinguetti by Camel in under 10 hours. There are two bands of dunes close to Chinguetti; the Les Boucles field, most of which is within 20 km of the city, and the Batraz field, which is between 40 and 60 km to the southeast. Much of the intervening area is also covered by sand dunes, but these are not large enough to describe an object as that described by Ripert.

Map showing the high sand dunes, greater than 30 m height, to the south of Chinguetti. Warren et al. (2024).

Warren et al. made two trips into the desert from Chinguetta, in the company of experienced local chameliers, one lasting eleven days and one lasting six. They found that Camels typically travel at speeds of between 2.0 and 3.6 km per hour, assuming good terrain, with the maximum speed achieved by unburdened Camels being about 5.0 km per hour. 

However, even assuming that Ripert and his guide were riding Camels unburdened by anything other than themselves, it is unlikely that this maximum speed would have been achieved for 10 hours, because the primary concern of the chameliers is for the welfare of their Camels, which are not only the most important assets they own, but also their only way of getting back to safety should a problem arise. This meant that if Warren et al.'s chameliers expected a journey to take four hours, they would travel for two hours, then give the Camels a three hour break to rest and feed, before completing the journey, something they were quite inflexible about. Neither would they travel in a straight line on anything other than the flattest terrain, but instead would zig-zag to avoid taking the Camels over steps and ledges, and would never take their Camels over the tops of dunes.

Ripert himself mentioned taking several detours during his journey, which makes a journey 50 km in a straight line from Chinguetta even less plausible. However, for the sake of convenience, Warren et al. take the area within 50 km of the city as a search area. This includes the more distant Batraz Dune Field, which Warren et al. consider unlikely, although they do concede that there is a route along a dry river bey which could bring a determined Camel rider this far in 10 hours if breaks were neglected. They also rule out the area of the Les Boucles Dune Field which lies within 10 km of the city, reasoning that Ripert, who was in charge of the local Camel Corps, would have recognised a location in this area. 

A sand dune in the Les Boucles Dune Field to the south of Chinguetta. Bruno Locatelli/Google Maps.

Having defined their search area, Warren et al. then searched their dataset for dunes large enough to have covered the meteorite described by Ripert. According to Ripert's description, the northeastern side of the meteorite was already covered by a dune at the time when he visited. The area is noted for its strong, prevailing winds, which blow northeast to southwest more-or-less constantly all year round, causing sand dunes to migrate in the same direction, and Ripert stated in 1932 that he thought it possible that the meteorite would already have been covered by the dune. Taking Ripert's estimate that the meteorite was 40 m high, it would require a dune more than 40 m high to cover it.

Sand dunes in a desert do not typically stack up against one-another; instead, they are usually discrete structures, with flat spaces between them. Warren et al. identified dunes higher than 30 m high in their remote sensing dataset, in order to give an error of margin, creating a map showing dunes which meet this criterion within the two dune fields. Since dunes are unlikely to have moved more than 100  m since 1916, the meteorite, if buried, must be within 100 m of the western edge of the dune covering it. 

Since a height of 30-40 m is reached within 300-400 m of the western flank of the dunes, it would in theory be possibly for a walk along the western flank of the dunes with a magnetometer (a passive instrument that measures changes in the Earth's magnetic field), and be confident of passing within 500 m of the meteorite, a distance at which it ought to be highly detectable.

Warren et al. also not that a magnetic survey of the area has been carried out by aircraft on behalf of the Mauritanian Ministry of Petroleum Energy and Mines by the Fugro geological surveying company, using funds provided by the World Bank, and this data has subsequently been made available to teams of scientists working on other projects. With this in mind, Warren et al. wrote to the Ministry requesting access to the data, but have yet to receive an answer.

Between 13 and 17 December 2022 Warren et al. carried out a magnetometer survey of the eastern part of the Les Boucles Dune Field on foot, covering the western edges of six large dunes, based upon which they are confident that the presence of a large iron meteorite beneath these dunes can be ruled out. Based upon the time this took, they estimate that a survey of all the potential dunes would require an expedition lasting three weeks. 

See also...

Evidence of recent liquid water at low latitudes on Mars.

The surface of Mars today is a hyperarid desert, yet in many places has features apparently formed by liquid water. It is now generally accepted that liquid water was present on early Mars, when the planet had a very different atmosphere. However, that early atmosphere has subsequently disappeared, and it is now assumed that the atmospheric pressure on Mars is now to low for liquid water to form.

This being the case, it came as a great surprise to scientists when, in March 2009, droplets of liquid were observed on one of the robotic arms of NASA's Phoenix Rover. Studies of the data recovered by Phoenix eventually concluded that the conditions for hypersaline water could exist on Mars at high latitudes during the summer, when the temperature rose above the melting point of such solutions (which is significantly lower than 0°C), resulting in a freeze-thaw cycle that might help to explain some of the features seen in these regions. 

However, the wider presence of apparent water features, such as slope lineae and gullies, is harder to explain. This had led scientists do develop alternative explanations, under which such features could have developed under dry conditions, since it is difficult to understand how sufficient liquid water could be present on the surface of Mars to explain features such as slope flows hundreds of metres in length. This disparity between a theoretical presence of small amounts of water on Mars, and a necessity to invoke dry formation of features we would typically associate with the presence of large amounts of water on Earth, makes it necessary to study smaller (less than one metre) features on Mars's surface in order to understand the planet's hydrological conditions properly. This is particularly true of features at lower (i.e non-polar) latitudes, where milder conditions make a freeze-thaw cycle unlikely, and there is a higher potential for the presence of some form of microbial life.

The China National Space Administration's Zhurong Rover landed on the southern Utopia Planitia on 15 May 2021, and spent nine months exploring the Late Hesperian northern lowlands, including studying the microstructure and chemical composition of the dune features observed there.

In a paper published in the journal Science Advances on 28 April 2023, Xiaoguang Qin of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology andGeophysics of the Chinese Academy of Sciences, Xin Ren of the Key Laboratory of Lunarand Deep Space Exploration at the Chinese National Astronomical Observatories, Xu Wang, also of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Jianjun Liu, also of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, Haibin Wu, again of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and of the  College of Earth and Planetary Sciences at the University of Chinese Academy of Science, Yong Sun of the Institute of AtmosphericPhysics of the Chinese Academy of Sciences, Zhaopeng Chen, again of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, Shihao Zhang, also of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Yizhong Zhang, Wangli Chen, Bin Liu, Dawei Liu, and Lin Guo, again of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, Kangkang Li, again of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Xiangzhao Zeng, Hai Huang, Qing Zhang, Songzheng Yu, and Chunlai Li, again of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, and Zhengtang Guo, once again of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences,  present the results of a study of the surficial microstructure, morphology, and chemical compositions of dunes studied by the Zhurong Rover, and the implications of these results for the possibility of liquid water having existed on the surface of Mars at low latitudes in the recent past.

A series of detached barchan (crescent-shaped) dunes with sinuous profiles are present in the Zhurong Rover landing area, each completely detached from its neighbour. The Zhurong Rover encountered four of these as it made a north-to-south transect of the area in the first four months after it landed. 

The rover discovered that the dunes were covered by two very different types of sand, one light and one dark, with the dark sand overlying the lighter dunes, implying a second generation of deposition. The barchan dunes are composed of the lighter sand, and are 15-30 m long and 3-10 m wide. The darker sand matches the surrounding soils, and forms longitudinal dunes and ridges running over the barchans, most commonly at northwest orientated longitudinal dunes crossing the western flank of the barchan. These longitudinal dunes appear more recent, and are likely to have formed under the current modern conditions.

Exploration route of Zhurong Rover and cracks on bright sand dunes. (A) Map of the exploration route of Zhurong from May to September 2021. The HiRIC photo (0.7-m resolution) was taken by the Tianwen-1 orbiter. Dunes 1 to 4, marked by white rectangles, were measured in situ on Sols 45, 64, 92, and 99, respectively. (B) Panorama mosaics acquired by Zhurong Rover's Navigation and Terrain Camera of longitudinal dunes on barchan Dune 2, with white rectangles indicating positions of the cracks. (C) and (D) Cracks developed on the southwestern slope of longitudinal dune on the western wing of Dune 2, with a white arrow pointing to one of the cracks. (E) Panorama mosaics acquired by Navigation and Terrain Camera of barchan Dune 3, with white rectangles indicating positions of the cracks. (F) Cracks on the northern slope of Dune 3. Qin et al. (2023).

Close examination of the surface of these dunes shows more that one form of cementation holding the particles together, with a continuous crust having formed on the light dues and the particles of the darker ridges held together in agglomerated clusters. Examination of the agglomerated particles suggests the presence of hydrated sulphates, hydrated silica (particularly opal), iron oxide minerals, and possibly chlorides. The hydrated sulphates and hydrated silica are most likely to be forming the cements holding these particles together.

Water traces on bright sand dunes. (A) Topographic contour map of the environs where the trace is located. The coordinate system is east-north-up local Cartesian coordinate, and the origin is that of the rover coordinate system. The background digital orthophoto map photo was taken by the Navigation and Terrain Camera. (B) Multispectral Camera bird’s-eye-view photo showing a strip-like trace and a likely water-soaked fragmented soil block. (C) Enlarged photo showing polygonal cracks and bright polygonal ridges. (D) Enlarged photo showing circular region with the strip-like trace as a part. (E) Navigation and Terrain Camera three-dimensional image of an interdune depression between two dark longitudinal dunes. (F) A cross section of the dune along the profile of the white dash line in (E). Qin et al. (2023).

Compositionally, both the light and dark sands have high iron and magnesium contents, although silica remains the most abundant material, comprising between about 52% and about 90% of all samples. Oxides make up about 15% by weight of the light sand and about 10% by weight of the dark sand. The instrumentation used is known to be incapable of detecting volatile elements such as sulphur, chlorine, and phosphorus, as well as hydrogen and hydroxide ions.

Images showing features of agglomerates and crust on the bright sand and dark sand surfaces. (a), (d) Panorama mosaics of Dunes 1 and 3 acquired by the Navigation and Terrain Camera, where white crosses denote the target positions of the laser-induced breakdown spectrometer . (b, c) The Multispectral Camera images (band centered at 699.2 nm, with a Full Width Half Maximum of 14.8 nm) of dark sand and bright sand regions denoted by the white boxes in the photo in (a). (e) The Multispectral Camera image located at the white box in the photo in (d). The upper side of the image is the dark sand region, and the lower side is the bright sand region. For (b), (c), (e), the imaging distance are 2.67m-2.81m, 2.21m-2.28m and 3.15m-3.39m, respectively and the maximum resolutions are 0.42mm, 0.34mm and 0.51mm, respectively. Qin et al. (2023).

The second and third dunes encountered are covered with polygonal cracks, although these are seen only on the underlying bright sand dunes, not the darker sand ridges running across them. The polygons formed by the cracks have an average area of 55.2 cm², and an average side length of 4.8 cm, far smaller than cracks previously observed on Mars by remote sensing. Assuming that the cracks have a depth to width ratio of between 1/3 and 1.4, this would equate to a depth of 1.25-1.7 cm. The majority of the polygons are pentagons, though they range from triangular to heptagonal in shape. The average internal angle of the polygons is 120°, and intersections between cracks are typically Y-shaped.

The MI images of bright sand and dark sand. (a), (d) Panorama mosaics of Dunes 2 and 3 acquired by Navigation and Terrain Camera. (b), (c) The MI images before and after ablation by e laser-induced breakdown spectrometer at the marked target (cross) on the dark sand surface of a longitudinal ridge on the western flank of Dune 2. The image size is 1024 pixel × 1024 pixel. (e), (f) The MI images before and after ablation by e laser-induced breakdown spectrometer at the marked target (cross) on the bright sand surface of Dune 3. The yellow dashed ellipse encircles the e laser-induced breakdown spectrometer crater. (g) The quartzite used in the laboratory experiment. (h) The MI image of the rock surface lasered by the laser-induced breakdown spectrometer. (i) The MI image of onboard Nontronite calibration target after probing with the laser-induced breakdown spectrometer obtained on Sol 58. The red arrow points to the crater created by laser-induced breakdown spectrometer laser ablation. Qin et al. (2023).

A light-toned, strip-like trace, over 40 cm long and about 1.5 cm wide was observed within the interdune depression of the second barchan dune. This ran along the lowest part of the trough depression, and separates light and dark bands of sand, with a dark sand slope to the north and a light sand slope to the south, with abundant polygonal cracks. The shape of this trace appears to be exactly what would be expected by pooled water, should this be able to exist here, and the underlying crust be impermeable to water.

Based upon the superposition of the features, Qin et al. conclude that the light-coloured barchan dunes were formed first, then became encrusted with sulphates, and possibly chlorides, during a more humid climatic phase. 

In order to determine the age of these dunes, Qin et al. looked at the density of craters on the land-surface they form part of (the rate at which asteroids randomly impact Mars is considered to be approximately constant, so that parts of the Martian surface can be dated by the density of impact craters), concluding that this surface was between 400 000 and 1.4 million years old. 

The polygonal cracks which have formed on the surface of some of these cracks are believed to have been caused by a loss of moisture, either through drying or desiccation, with the dark, longitudinal dunes forming after this, and finally the sand in the longitudinal dunes becoming agglutinated into clumps. The cementing of the sands requires a liquid or gas which was able to fill the pore spaces between them, then transform into a solid state. Such substances would include carbon dioxide gas turning into dry ice, liquid water freezing into ice, or various salts and other hydrated chemicals precipitating out of solution as the water in which they were dissolved evaporates. The conditions around the Zhurong landing site make the formation of dry ice and/or water ice highly improbable, and both of these would be detectable by the laser-induced breakdown spectrometer on the Zhurong Lander, which has found no evidence of their presence. However, hydrous sulfates, opaline silica, ferric oxides, and probably chlorides, have bee detected, and this mixture would provide a suitable cement for the sand grains.

The formation of a cement from a mixture of salts and hydrated minerals requires the presence of liquid water. This could have originated from rain, snow, or frost, or have upwelled from a subterranean source, although the evaporation of groundwater drawn upwards by capillary action seems unlikely, as there are cracked evaporation surfaces on the raised dunes, but not the surrounding flatlands, which makes the precipitation of water, either as rain or frost/snow which then thawed before evaporation, the most likely explanation.

The saturated vapor pressure (point at which the atmosphere can hold no more evaporated water, and it begins to precipitate out) is unrelated to the atmospheric pressure, although the temperature must be above 0°C for liquid rain to fall. At 0°C on Mars the saturated vapor pressure would be 611 pascals, while the atmospheric pressure observed on Mars by the Zhurong Lander  is between 786 and 834 pascals, meaning that the atmosphere would need to be about 72% water for rain to fall. Since the modern Martian atmosphere is about 95% carbon dioxide, liquid precipitation on Mars is currently impossible.

Several different landers have now taken atmospheric readings on Mars, giving a range of surface temperatures between -105°C and -5°C, a range of atmospheric pressures between 683 and 849 pascals, and a vapor pressure of 0.27 pascals. Under these conditions, the frost point (point at which the temperature drops so low that water absorbed into the atmosphere precipitates out as frost) would be about -74°C, which means frost would be possible at the Zhurong landing area. More widely, it is assumed that frost and snow are relatively common on Mars.

Mixing water ice, from frost or snow, with salts could potentially lead to its melting point being lowered sufficiently for highly saline liquid water to form. Any subsequent raise in temperature could subsequently lead to water evaporation, with seasonal or even daily cycles of frost formation, melting, and evaporation enabling the formation of cements.

The temperature on Mars is thought to rise rapidly between 5.00 and 6.00 am, local true solar time, providing an interval in which frost can sublimate, and potentially also in which it could melt and then evaporate in a hyper-saline environment. This happens seasonally, with steeper rises and higher temperatures achieved in local summer.

Map showing the number of days (noted on contour lines in terms of sols) during a Martian year and locations where the ground temperature is exceeds 0˚C. Contour intervals are 40 sols. The Zhurong landing site marked with a red star. Qin et al. (2023).

The orbital obliquity of Mars (i.e. the angle at which it is turned to the Sun, which determines the severity of the seasons) is thought to have been equal to or greater than it is now throughout the past 1.4 million years, which would mean that the climate of Mars has been comparable to or more humid than Today throughout this interval. This would imply that the formation of liquid water at low latitudes on Mars has remained at least as possible as it is today over this period.

Such a process of repeatedly forming hypersaline solutions would facilitate the dissolution of silica from sand grains to form opal, as well as attacking other minerals, enabling hydrated sulphates and iron oxides to form.

The polygonal cracks on the surface Mars are also almost certainly the result of either freeze-thaw thermal contraction or desiccation, in response to seasonal or daily changers in temperature, with their general shape suggesting the later is more likely. Meteorite impact effects and carbon dioxide freeze/sublimate cycles have been suggested as an origin for similar cracks elsewhere on Mats, but there are no signs of any meteor impacts large enough to have caused these cracks near the Zhurong landing site, and carbon dioxide is unable to freeze out of the Martian atmosphere this far from the Martian poles.

Qin et al.'s study is the first small-scale study of such cracks at low latitudes on Mars. They believe that these features are almost certainly the result of desiccation, but cannot rule out an alternative hypothesis, in which the cracks are formed by the freezing of hypersaline water, causing cracks to form in the crust under tensile stress. 

All of the features seen in the Zhurong landing area point towards the presence of saline water, providing evidence that liquid water can form at low latitudes on Mars. Qin et al. propose that water accumulated on top of the dunes as frost or snow after the atmospheric temperature dropped below the frost point, then melted due to a combination of rising temperatures and contact with salt within the sands. which would in turn facilitate the formation of hydrated silica (opal) and iron oxides. This water would then evaporate away, at fairly low temperatures due to the low atmospheric pressure on Mars, leaving the salts to precipitate out and form a cement between the sand grains, forming cracks on the dune surface as they dried out. This cycle would likely repeat numerous times.

If this hypothesis is correct, then it suggests that the amount of liquid water available on the surface of Mars in the recent past is considerably higher that previously suspected. It has previously been suggested that transient films of water might have formed on the surface of rocks in the recent past due to acid weathering, and that small amounts of water might have formed duricrusts and rock surface coatings over geological timescales. The situation at the Zhurong landing site appears quite different, with apparently mobile sands unlikely to have become cemented together by any process operating on a geological timescale. Rather this appears to be the result of an evaporative process operating over a relatively short period. less than 1.4 million years ago, and possibly less than 400 000 years ago.

This recent presence of water at 'tropical' latitudes on Mars becomes less unreasonable when it is remembered that Mars is thought to have undergone a significant change in the obliquity of its orbit about 5 million years ago, and only to have reached its current, low-obliquity orbital configuration about 3 million years ago. This may suggest that the thick ice caps present at the current Martian poles are a relatively modern feature, a result of a fairly recent transfer of water from lower latitudes, something which may well have still being occurring 1.4 million years ago. This provides further support for the theory that high-obliquity excursions in the Martian orbit might well have provided enough water for gully formation. Thus the presence of sufficient saline water at low latitudes on Mars for evaporite formation in the recent past is in fact in accord with our current understanding of the planet's recent geological past.

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