Meteorite NWA 16788 sold at auction in New York, despite protests from Government of Niger.

The planet Mars has been of great interest to planetary scientists for as long as the discipline has existed. The planet has been extensively studied by telescope for centuries, and in recent decades by a series of robotic probes. These probes have taught us a great deal more about the planet than could be determined by remote sensing alone, but are only able to carry a limited amount of instrumentation, which cannot be changed, updated, or repaired once the probe has left Earth. To this end the planned Mars Sample Return program aims to bring samples from the red planet back to Earth, where they can be studied with a full range of laboratory techniques. However, this program is still in its very early stages, with no fixed target on Mars or spaceship design settled upon, and it is by no means settled that the mission will go ahead at all.

Until such time as samples are returned from Mars to Earth, the only way in which terrestrial scientists can gain direct access to material from Mars is by the examination of Martian meteorites, pieces of rock from the surface of the planet Mars which have been ejected into space, usually as a consequence of other large bodies impacting the planet. These can be confirmed as having come from Mars by a distinct mineralogy and the presence of isotope ratios detected on the planet by remote probes and not found elsewhere in the Solar System (this is not peculiar to Mars, each large body in the Solar System has its own unique isotope signature). To date, the Meteoritical Society has confirmed 402 meteorites as being of Martian origin.

The majority of Martian Meteorites have been discovered in desert environments, with 343 (85.3%) coming from the Saharan region (61 from Algeria, 1 from 'Algeria or Mali', 1 from 'Algeria or Mauritania', 4 from 'Algeria or Morocco', 1 from 'Algeria or Western Sahara', 1 from Egypt, 15 from Libya, 13 from Mali, 1 from 'Mali or Mauritania', 1 from 'Mali or Niger', 19 from Mauritania, 184 from Morocco, 1 from Niger, 1 from Nigeria, 1 from Tunisia, 15 from Western Sahara, and 23 from unknown 'Northwest African' countries).

Of the 343 known Martian Meteorites from the Sahara, 184 (53.4%) come from Morocco, with a further 15 (4.4%) from the Western Sahara, a disputed territory occupied by Morocco since 1976. This is not because Morocco is more prone to Martian Meteorite falls than other countries, but rather to a difference in the law. While most countries in the region ban the trade in, and export of, meteorites, Morocco allows a trade by licensed dealers, as long as all meteorites are registered with the Moroccan Geological Survey, and a sample of the material is deposited with them. 

This has led to the development of a successful meteorite market in Morocco (which also has a similar trade in fossils). However, there is also a suspicion that many of the meteorites traded through Morocco might originate in other countries (although, given the willingness of the international meteorite community to trade in meteorites either with no known point of origin, or known to have been smuggled out of countries where their trade is forbidden, this scarcely seems worth the effort).

The unregulated way in which meteorites (including Martian Meteorites) are traded also means that many are held in private collections, rather than by public bodies where they can be accessed by scientists (some private collectors do allow scientists to examine their material, but this is of limited value unless it can be guaranteed that all scientists in the field have, and will continue to have, access).

The largest Martian Meteorite discovered to date is NWA 16788 (North West Africa 16788), with a mass of 24.665 kg. This is not just important because bigger meteorites are more impressive; the body from which this was derived is likely to have been significantly larger, which means that the event which caused it to be ejected from Mars must also have been particularly large, giving scientists a reasonable hope of connecting this meteorite to a specific geographic location on Mars.

NWA 16788, the largest individual Martian meteorite recovered thus far. Franza et al. (2024).

NWA 16778 was (allegedly) discovered on 16 July 2023 near Kefkaf in Niger. It was confirmed as being a Martian Meteorite on the basis of samples sent to the Shanghai Astronomy Museum, with the Meteoritical Society being informed that the meteorite was being held in the collection of the Purple Mountain Observatory in China. However, in 2024 the meteorite appeared in a private collection in Arezzo, Italy. Two small samples of the meteorite were donated to the University of Firenze, and it was loaned to the Italian Space Agency during the 2024 European Researchers’ Night, on 27 September, an event intended to boost public engagement with science.

On 8 July 2025 NWA 16788 was placed on display at the auction house Sotherby's in New York, ahead of a planned auction on 16 July, at which it was predicted to fetch US$2-4 million; it was eventually sold to an anonymous buyer for US$4.3, which is likely to amount to a final cost to the buyer of about US$5.3 million once fees and taxes are taken into account.

This sale has prompted a protest by the Government of Niger, which pointed out that since 1997 Nigerien law has prohibited the unlicensed export of a range of heritage items including 'mineralogical specimens', and that the meteorite appeared to have been illegally trafficked out of the country. Sotherby's has denied any wrong-doing, noting that the legislation in question does not specifically mention meteorites.

Since the meteorite was placed on sale, a number of prominent international scientists have come forward to support the Nigerian Government's position, including palaeontologist's Steve Brusatte of the University of Edinburgh, who has raised concerns about the loss of valuable scientific specimens into the private vaults of oligarchs, and Paul Sereno of the University of Chicago, who believes that the term 'mineralogical specimens' clearly covers meteorites, and that the sale represents a clear breach of international law. Sereno, who has led expeditions to fossil sites in Niger for many years, and who founded the organisation Niger Heritage with the intention of building a museum in the country's capital, Niamey, further went on to point out that the removal of heritage items, cultural or natural, from a country without that country's consent is reflective of a colonial attitude which the world should have moved on from.

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Mars approaches perigee.

The planet Mars will reach perigee, the closest point on its orbit to Earth, slightly after 1.30 pm on Sunday 12 January 2025, when it will be 0.67 AU (i.e. 0.67 times the average distance between the Earth and the Sun) from the Earth. Mars orbits the Sun at an average distance of 1.52 AU, but at a slower speed (the speed at which a body orbits another body is determined by the distance between them, with a greater distance resulting in a slower speed), so that a Martian year is 687 days long. Most outer planets (i.e. planets which are in our Solar System, but further from the Sun than the Earth) tend to be at their closest to the Earth close to when they are at opposition (i.e. when they are directly on the opposite side of us to the Sun), but because the orbits of all planets are elliptical (Mars veries between 1.38 and 1.67 AU from the Sun, while the Earth varies between 1.47 and 1.52 AU from the Sun) these seldom match up, with the effect that this year Mars will be at opposition four days after its perigee, on Thursday 16 January.

The relative positions and orbits of Mars,  Earth, Venus, and Mercury at 1.00 pm GMT on Sunday 12 January 2025. JPL Small Body Database.

While the relative positions of the planets have no direct influence on life on Earth, the perigee and opposition of a planet do present the best oportunites for observations of by Earth-based observers. Between 12 and 16 January 2025, Mars will appear as a bright object in the constellation of Gemini, although a Full Moon on Monday 13 January, in the constellation of Cancer, will hamper viewing somewhat.

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Eclipse observed on Mars.

NASA's Perseverance Rover has observed an eclipse from the surface of Mars, caused by the moon Phobos passing in front of the Sun. Phobos is significantly smaller than our Moon, only 17 km in diameter compared to 3475 km, but orbits much closer to Mars than the Moon does to Earth, an average of 9376 km compared to 384 400 km, which combined with the smaller apparent size of the Sun seen from Mars, which is roughly 1½ times as far from the Sun as the Earth is, means that eclipses are still an observable phenomenon. 

Solar eclipse on Mars, caused by the moon Phobos passing in front of the Sun, recorded by the Perseverance Rover. Images are at 10 second intervals, with the whole event lasting much less time than an eclipse on Earth, due to the closer orbit of Phobos, and the correspondingly higher speed of the Martian moon, which takes only 7.6 hours to orbit the planet. NASA.

The eclipse, which occurred on 30 September 2024, was the second such eclipse observed by Perseverance this year, with the Curiosity and Opportunity rovers also having Martian eclipses. These eclipses are common because, unlike that of our Moon, the orbit of Phobos has very little tilt, effectively remaining above the planet's equator at all times.

An eclipse caused by the passage of Phobos in front of the Sun recorded by the Perseverance Rover on 8 February 2024. NASA.

Mars's second moon, Deimos, does not cause such obvious eclipses, as it is only 6.2 km in diameter, and orbits at an altitude of 23 460 km, making it almost invisible from the planet's surface.

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Curiosity rover detects elemental sulphur on Mars.

NASA's Curiosity Rover has detected crystals of elemental sulphur on the surface of Mars, the first time sulphur has been detected as a pure element on the planet. The crystals were observed at a location within Gale Crater called Convict Lake on 7 Jun 2024, and form a patch about 12 cm across. The crystals are thought to have been exposed by the rover itself driving over a rock and crushing it several days previously.

A patch of minerals including crystals of pure elemental sulphur on the surface of Mars. The image has been colour enhanced to for the benefit of Human eyes; the rover used an X-ray spectrophotometer to detect the element. NASA/JPL/CalTech/Malin Space Science Systems. 

The presence of sulphur on Mars is hardly surprising not surprising. The element is one of the most common in the universe and has been detected on all planets in the Solar System, as well as meteorites, asteroids, and comets. But most sulphur previously found on Mars has been in the form of sulphate salt evaporites, which formed as lakes and other bodies of water dried out on the surface of the planet long ago.

On Earth, sulphur deposits typically take the form of sulphates (the oxidised form of the mineral) or sulphites (the reduced form) with elemental sulphur forming in sedimentary rocks through the actions of sulphur-reducing micro-organisms in anaerobic (i.e. oxygen free) environments, and in volcanic rocks by the reaction of gaseous hydrogen sulphide and sulphur dioxide. The geology of Gale Crater is dominated by sedimentary deposits, including evaporites, but is generally low in sulphates. 

It is possible that the Convict Lake rock is of volcanic origin, and reached the Gale Crater locality as ejecta. However, the images of the rock resemble the surrounding sedimentary rocks, making it more likely that it is local in origin. This makes it likely that the sulphur has been derived from an original sulphate source in some way, although this does not necessarily imply the presence of sulphur reducing micro-organisms, as in the oxygen-free atmosphere of Mars, abiotic reducing reactions impossible on Earth become far more likely.

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Dust devil observed from the surface of Mars.

NASA's  Perseverance Rover has observed and recorded an dust devil from the surface of Mars. These storms are well recorded from space-based observations made by satellites orbiting Mars, but ground-based observations are much rarer, in part because rovers on Mars are usually closed down when dust storms approach them, to prevent them from being damaged.

NASA’s Perseverance rover captured this Martian dust devil moving east to west at a speed of about 19 kmph along Thorofare Ridge on  30 August 2023. The video has been sped up 20 times, is composed of 21 frames taken four seconds apart, and has been digitally enhanced in order to show maximal detail. NASA/JPL/Caltech.

Dust devils are a phenomenom known on Earth as well as Mars, although the lower gravity and thinner atmosphere of Mars means that the Martian devils tend to be both taller and slower moving than their Earthly equivalents. This storm was about 4 km away from the rover when it was observed, and was moving along a portion of the western rim of Jezero Crater known as Thorofare Ridge at a speed of about 19 kmph. The storm of the dust devil is not visible, but based upon its shadow, it is estimated to have been about 2 km high, with the basal portion being about 60 m wide.

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Evidence of prolonged wet-dry cycling on early Mars.

Mars has an extensive sedimentary record, dating back at least 4.3 billion years, which shows that early Mars had a very difficult climate from today, with permanently wet environments, and possibly even the conditions for life. Whether-or-not this early climate also included areas with episodic or periodic wet intervals has to date, however, been unclear. Such fluctuations in hydrodynamic conditions leave distinctive traces in the sedimentary record, such as cracks, however they are also easily eroded away, and models of the early Martian climate have been ambivalent about the existence of such conditions.

In a paper published in the jornal Nature on 9 August 2023, William Rapin of the Institut de Recherche en Astrophysique et Planétologie at the Université de Toulouse 3, Gilles Dromart of the Laboratoire de Géologie de Lyon Terre, Planètes, Environnement at the École normale supérieure de Lyon, Ben Clark of the Space Science Institute, Juergen Schieber of Indiana University, Edwin Kite of the University of Chicago, Linda Kah of the University of Tennessee, Knoxville, Lucy Thompson of the University of New Brunswick, Olivier Gasnault, Jeremie Lasue, and Pierre-Yves Meslin, also of the Institut de Recherche en Astrophysique et Planétologie at the Université de Toulouse 3, and Patrick Gasda and Nina Lanza of the Los Alamos National Laboratory, report the presence of a well-preserved polygonal mud-crack pattern on strata dated to about 3.6 billion years ago (dating to the Hesperian Eon of Mars), which they believe to be evidence of a wet-dry cycling system, and therefore to provide useful new insight into the early climate of Mars.

NASA's Curiosity Rover has documented hundreds of metres of sediments deposited in lakes., rivers, intermittent lakes, and lake-margin settings within Gale Crater on Mars. The vast majority of these have been smectite (silicone and aluminium rich clay) mudstones, but the rover recently encountered a sulphate-bearing unit, apparently marking a major environmental transition foumd in stratified terrains across Mars. At this horizon the rover found an apparent sulphate evaporite deposit.

Context of observations in Gale crater, Mars. Stratigraphic context (left) of the lower portion of Mount Sharp and map (right) showing Curiosity Rover traverse (white) on the High Resolution Imaging Science Experiment (HiRISE) base map overlaid with Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) S-index, which tracks sulfates (shaded yellow). Red rectangle shows the location of close-up map and detailed stratigraphy. Rapin et al. (2023).

The unit at the base of the sulphate-bearing strata has widespread centimetre-scale polygonal patterns, formed of straight lines intersecting at triple junctions. All observations of this were made within an 18 m interval, with some variations apparently caused by subsequent alteration of the same basic pattern. These polygons appear to persist for tens of centimetres, and can be seen on stepped blocks of sediment. Where visible on bedding plains the polygons are made of raised ridges are about 1 cm high, with the polygons having an average diameter of 4 cm. Junctions have an average angle of 120°. The ridges are made up of aligned nodules, of variable size, and juxtaposed in a variety of ways, and apparently made of calcium sulphate and magnesium sulphate minerals. The sediment in which these ridges are emplaced is generally sulphate-poor, though with some patches with raised calcium sulphate levels.

In situ observations of polygonal ridges. (a) General view of bedrock surrounding the rover on sols 3154 to 3156 showing widespread polygonal ridges. (b) Close-up showing ‘stepped’ exposure of polygons within large bedrock blocks. (c) View of bedrock with polygons and locations of ChemCam analysis on ridge (red rectangle) and Alpha Particle X-Ray Spectrometer analysis on smooth host bedrock (dotted circle). (d) Remote micro-image of cemented ridge with spots analysed by ChemCam (reticles 1 to 5), highlighting details of nodular texture. (e), (f) Bedrock with polygonal pattern (e) and interpretative overlay (f) that shows prominent ridges (solid red lines), less certain ridges (dotted red lines) and cross-cutting later-stage calcium-sulphate-filled veins (white areas). Rapin et al. (2023).

On Earth, polygonal ridge patterns can form on evaporite deposits as a result of subsurface salinity convection, but Rapin et al. believe this to be unlikely, as this generally only occurs on pure salt crusts, and produces polygon patterns with much larger diameters (which would presumably be larger still if they formed under Mars's low gravity conditions). They instead suggest that the more likely explanation is that the ridges formed originally as cracks within a drying sediment, which was then infilled by salt-rich water, which evaporated leaving the nodules, which are more resilient to erosion than the surrounding sediment, leading to raised ridges as this is eroded away. Newly formed desiccation cracks generally have T-junctions between blocks, but where they undergo repeated cycles of wetting and drying, these tend to reform into more even 120° Y-junctions, with about 10 cycles of wetting and frying typically needed on Earth for 120° to be reached, giving a pattern of even hexagons.

Formation model for sulphate-enriched polygonal ridges. (a)–(c) Repeated cycles of desiccation (a), recharge (b) and flooding (c) form a vertically propagating hexagonal pattern of sulphate enrichment. (a) Evaporation (grey arrows) desiccates and cracks near-surface sediment, triggering salt crystallization (red) at and near cracks where the subsurface brine (purple) concentrates. (b) Water recharge heals cracks by sediment hydration. (c) Flooding dissolves excess salts at the surface but subsurface brine and intrasediment sulphate salts are preserved and siliciclastic sediment is deposited on top. (d) Sediment is buried with saturated brine in pore spaces and sulphates are mostly preserved. (e) Later diagenesis partially dissolves intrasediment sulphate salts and late diagenetic fractures are filled with calcium-sulphate (white). (f) Sulphate-cemented polygonal ridges become visible during exhumation as the softer host bedrock is preferentially removed during weathering. Rapin et al. (2023).

The hexagons are located in a sulphate-poor sediment overlaying a sulphate-rich nodular bedrock, making the precipitation of sulphate minerals plausible, although it is unlikely that they originally formed in their current configuration. Most likely, the salts were precipitated in a slurry with sediment particles in original cracks with a T-junction formation, but that then redissolved with each cycle of wetting and re-precipitated with each cycle of evaporation, eventually forming the resilient hexagon-shaped patterns seen today.

Larger colour image of bedrock with polygonal ridges for context. MastCam image and close-ups (a), (b) and (c) with rectangle locations of close-up view. Close-ups (b), (c) show bedrock 10 to 20 meters away where regularly spaced ridges and nodules can be observed supporting lateral extension of the same polygonal pattern although camera resolution prevents detailed geometrical analysis at this distance. Rapin et al. (2023).

The water that caused these patterns was probably brought in by periodic flooding and groundwater recharge, which would have added sediment to the deposits (leading to the depth of strata we see today), while dissolving any surface salts, so that each drying event is likely to have precipitated more salt than its predecessor. This dessication-water recharge cycle is likely to have affected only the upper few centimetres of sediment at any one time, with the hexagons therefore able to move up through the sediment column with repeated cycles. The consistency of the size and shape of the polygons implies a repeated cycle of regular intensity, while the variable size and shape of the nodules within the ridges implies multiple generations of growth.

The repetitive nature of the floods and the limited amount of depth penetration makes it likely that these cycles were seasonal, although a shorter-term cycle cannot be ruled out. The time period over which this cycle occurred is unclear, although the ridges have been identified at multiple points within an 18 m succession, which on a typical Earth floodplain with a sedimentation rate of about 0.01 mm per year, would imply a period of thousands to millions of years, although this might not have been a continuous interval of seasonal flooding; possibly the occurrence of seasonal interludes was itself part of a larger cycle. Individual blocks of polygon propagation are often more than 2 m thick, with no visible boundaries to other polygon-bearing or polygon-free strata, which suggests that if there were longer dry intervals without sedimentation, then little erosion took place in these intervals either. 

The mature hexagon shapes of the polygons indicate they were formed by repeated wet-dry cycles, and the thickness of the strata in which they are found implies that this cycle occurred at least episodically for a very long period of time. Mud crack polygons have also been seen in the underlying Murray Formation, but these typically have a T-junction structure, suggestive of a single drying event, whereas the polygons reported by Rapin et al. appear to be indicative of a repeated wet-dry cycle, which occurred for a sustained period of time on the early surface of Mars. This agrees with models of the early climate of the planet which have suggested that a single event, such as a meteor impact or supervolcano, forming all the water-related features on Mars (one a popular hypothesis) is unlikely. Instead, Mars appears to have had a longer period of Earth-like climate with seasonal flooding events, and evaporite deposits forming in seasonal lakes. 

A climate with wet and dry cycles is considered to be conducive to, and perhaps essential for, the type of prebiotic chemical evolution needed to form the precursors for life. Dessication lowers the amount of water available, thereby increasing the proportion of soluble ingredients within the remaining liquid, raising the rate at which reactions can occur. In particular, nucleotides form more readily from nucleobases in a concentration, and also polymerise to form larger molecules such as DNA or RNA more readily, and amino acids more readily form proteins under such conditions. Dioctahedral smectites, which appear to be ubiquitous clay minerals on the surface of Mars, are capable of tightly adsorbing nucleotides through cation exchange, and have been suggested as having been vital for the formation of the first pre-biotic organic polymers. Thus, seasonal pools on the surface of Mars could have reasonably formed the 'warm little ponds' proposed by Darwin as a location for the appearance of life.

Sediments in Gale Crater have been shown to contain about 500 g of organic material per cubic metre, as well as a variety of other soluble elements. The discovery that this site also once underwent seasonally wet and dry cycles supports the idea that this area was once suitable for prebiotic chemistry, but it is highly unlikely to have been the only place on Mars where such conditions were found, although the discovery does re-enforce the importance of Gale Crater as a site of global importance for understanding the early history of Mars.

The discovery of evidence of wet-dry cycling from a time when organics and volatiles are known to have been accumulating on Mars for over a billion years supports the idea that conditions on Mars during the Noachian–Hesperian transition period may have been favourable for the emergence of life on Mars, possibly more so than the earlier, and apparently wetter, Noachian Eon.

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