Selected Articles from Issue Number 6/3 June 1999

Messengers from Mars

A talk by Monica Grady given on 15th April 1999

About 30 members assembled to hear Monica Grady give an absolutely superb talk on Martian Meteorites. She has kindly given us permission to reproduce the text of an article she wrote for the Natural History Museum's web page, essentially covering her talk, so that those of you who didn't go can have a flavour of what you missed. This is a slightly edited version of the article. The full text (with illustrations) is available on http://www.nhm.ac.uk/mineralogy/grady/mars.htm

There are twelve meteorites in our collections that in all probability come from Mars. Of course we cannot be absolutely certain of this proposition until a rock is returned directly from the planet to Earth via a space mission. Of the twelve Martian meteorites, six have been found in Antarctica, not because there is a higher incidence of their falling there, but because there is a greater chance of them being collected there.

All the Martian meteorites are igneous rocks, representing crystallisation at different depths.  Some of the rocks have been altered by fluids, others appear to be dry. Many of the twelve are shocked, but all have one thing in common: they come from the same parent planet.

How do we know they are from Mars? There are several features that point to a Martian origin. The Solar System formed ~4550 million years ago from a turbulent cloud of gas, ices and dust. The dust aggregated into increasingly larger bodies, forming the inner, rocky planets, whilst gas and ices accumulated in the giant planets Jupiter, Saturn, Uranus and Neptune. The bodies of the asteroid belt were prevented by Jupiter's gravity from aggre-gating. The date of their crystallisation is thus ~4550 million years ago, the age at which the Solar System formed. This is the age of all "regular" meteorites from the Asteroid Belt (and, indeed, the age of the Sun, and planets like Earth and Mars). Not all the bodies in the Solar System have completely solidified - the Earth is a prime example, and we have almost daily displays of active volcanoes. The solidified rock from a contemporary eruption has a formation age of zero. Igneous rocks from different periods in Earth's history have different ages, but all are younger than 3800 million years - the original rocks that formed the Earth have all been recycled through geological processes. Eleven of the twelve Martian meteorites have young crystallisation ages, between 180 Myr and 1300 Myr. In other words, they have come from a body that supported molten rocks as recently as 180 Myr ago - i.e. not the Asteroid Belt. There are few rocky bodies in the Solar System to which this applies: Venus, Earth, Moon, Mars, and some of the satellites of the giant planets. It is dynamically difficult (although not impossible) to project rocks from Venus to Earth. It is unlikely that ejecta from the satellites of Jupiter or Saturn would escape the gravitational attraction of the parent planets. So, by a process of elimination, Earth, Moon and Mars are left as prime candidates for the origin of these meteorites. Earth and Moon can be ruled out on the grounds of the chemistry of the rocks: the Martian meteorites all have similar oxygen isotopic compositions to each other, but different from the Earth, or the Moon. Oxygen isotopic composition (the relative amounts of the three stable isotopes of oxygen present in a rock) is a characteristic signature of a parent body. Since the oxygen isotopic composition of the Martian meteorites is different from that of the Earth, they cannot have come from our home planet, leaving Mars as the most viable proposition for the meteorites' origin. This may all sound rather circumstantial - however, there is other evidence. The Viking landers of 1976 returned data on the chemical and isotopic composition of Mars' atmosphere. Analysis of one of the Martian meteorites (EET A79001, a basalt from Antarctica collected in 1979) showed the sample to contain, within pockets of shock-produced glass, small quantities of Martian atmospheric gases.

In 1984, the ANSMET team of US scientists paid its annual visit to Antarctica, to search for meteorites on the Allan Hills ice-field. Amongst the rocks found that year was ALH 84001, but it was ten years before the rock was sent for oxygen isotope analysis, and found to be Martian. However, ALH 84001 differs from other Martian meteorites: it is old, having crystallised not long after Mars accreted. Although it is an igneous rock, it has suffered extensive alter-ation, leading to the formation of large patches of bright orange carbonates. It also has more carbon in the form of organic compounds than any other Martian meteorite. The organics might be Martian or terrestrial, but the carbonates have a distinctive carbon isotopic composition, indicating that they came from fluids in contact with Mars' atmosphere.

Life in ALH 84001?

Dave McKay and his team studied ALH 84001 using a variety of techniques. They concentrated on the patches of carbonate that have an indisputable Martian origin. Using a microprobe two-step laser mass spectrometer to release and analyse organic molecules from within the meteorite, it was quickly appreciated that, firstly, most of the organics were closely associated with the carbonates, and secondly, that they were polycyclic aromatic hydrocarbons, or PAHs. If the carbonates are Martian, argued the authors, then so too must be the organics. The carbonates were studied by high resolution transmission and scanning electron microscopy (TEM and SEM), the former technique looking at slices through the carbonate, the latter examining the outer surfaces. Results from TEM showed that concentrated in zones within the carbonates were tiny grains of magnetite (iron oxide) and iron sulphides. The individual magnetite grains had shapes not usually observed in meteoritic magnetites: cuboid and tear-drop. The association of magnetite and iron sulphides with dissolution hollows in the carbonate grains also suggested to McKay et al. that they were observing a disequilibrium mineral assemblage such as is often characteristic of biogenic processes.

Examination of the outer surface of the carbonate areas by SEM showed that the surface had an irregular texture, not consistent with growth or cleavage surfaces. Also on the surface were oval-shaped and elongated structures. These, by analogy with terrestrial systems were interpreted as the remains of nanobacteria. Other ovoid features occur in the iron-rich rims of the carbonates, and might therefore be associated with the magnetite and sulphide grains, and thus represent the products of microbial activity. In McKay et al.'s opinion, the most likely explanation that satisfies all the observations is that the carbonate patches, PAHS, the magnetite and the microstructures "could be the fossil remains of a past Martian biota".

This opinion is not universally accepted - at the press conference held to announce the findings, Professor William Schopf, a microfossil expert, pointed out several flaws in the arguments. To start with, the magnetite occurs as discrete grains, and not as a sequence of several connected grains, such as might be found in a terrestrial organism. The grains are also ~100 times smaller than those characteristic of bio-genic processes on Earth. They do not appear to contain voids or cavities, such as is observed in terrestrial microfossils, neither have structures that could be interpreted as cell walls been identified.  In addition to flaws with the microstructure argument, the identification of PAHs is also not a good biomarker. Although PAHs are abundant in coal and petroleum deposits, they are not a primary biological product, but are a result of secondary diagenesis.

So, given the evidence presented by McKay et al., and the observations of other commentators, have the remnants of life on Mars been found? On balance, we think not - the microfossil evidence is inconclusive, and might be explicable by inorganic processes. But this does not mean that we rule out the possibility that life does not, or did not exist on Mars. The building blocks of life were undoubtedly present on the planet. It has had a thicker atmosphere in the past, allowing water to flow on its surface: we can still see the dried up remnants of water channels on satellite images. However, when Mars lost its atmosphere, the surface waters also disappeared. Mars is now a dry and sterile planet, its surface bathed by the sun's ultra violet radiation. However, we do not know what lurks below the surface.

Whatever McKay et al. observed in ALH 84001, primitive microfossils or non-biogenic molecules, they have pushed open further the door to exploration of our neighbouring planet and raised public awareness of how close we are to finding signs of life (extinct or dormant) on other bodies within the solar system. Recent reports from the Galileo probe that the Jovian satellite Europa might be rich in ice, and parts of Io covered in slush reinforce the claims that life might exist in other corners of our solar system, although the images of microstructures in ALH 84001 remind us that any possible life forms are likely to be very primitive. We might not be alone ..... but we don't yet have anyone to talk to!

Monica Grady

Vale of Fernhurst Geowalk

25th April 1999, led by Brian Harvey

In spite of the gloomy weather forecasts, a group of members met in an isolated car park on Woolbeding Common, south west of Fernhurst. The walk was complementary to the geowalk at Hindhead Common last September. The viewpoint across from the car park was an escarpment of the Hythe Beds.

The view west and north was across the Vale of Fernhurst, a spectacular breached anticline, where the Hythe Beds had been eroded to expose Atherfield Clay and Weald Clay. The geological map looks complicated because of individual lenses of small exposures. This could be compared to Hindhead where there had been erosion through the Hythe Beds to form the Devil's Punchbowl.

Brian pointed out with one of the diagrams in his handout that the pericline extended eastwards across the Weald and into France, which oil companies had comprehensively explored. Looking at the geological cross section revealed that below this anticline lies the "true" Weald Basin - a syncline of Jurassic rocks. There had been basin inversion with the topographic high points now the low points, due to successive compression, tension and compression. By the end of the Cretaceous, this area was pushed up to form a topographic high.

The Weald Clay was deposited in shallow lakes while the Atherfield Clays were marine, the sea having broken in from the north-west, with erosion of the London platform to the north. There had also been extensive bioturbation in the shallow water.

Leaving the viewpoint near the car park, we walked down the escarpment to investigate the first feature of erosion in this area. The Hythe Beds outcrop showed a massive rotational slip, which moves when there are long periods of heavy rain (as recently). The house below looked in a very precarious position.

The rocks contained a dark green mineral (glaucon-ite) which is diagnostic of marine sediments. Also found was a small piece of a fossil, although sandstone is not a good medium for preservation.

As we descended eastwards, around the north of Telegraph Hill, we passed through a narrow outcrop of Atherfield Clay, a marine sediment which weathers easily. At this level (roughly 150 metres above sea level), we were walking along the junction between the Hythe Beds and Atherfield Clay. We noted the change of slope and of vegetation at the junction, and the many points where small streams issued.

As an illustration of the great variation in the Weald "Clay", we came across a small working used to extract building sand at the edge of one wooded area.

On crossing the Weald Clay we encountered many small areas of woodland that had obviously been coppiced. Coppicing occurred every 5 to 10 years, and was a renewable energy source for the iron industry, for this was the industrial heartland of the country. At its height, 32,000 men were employed coppicing for industry and domestic fuel. Nowadays, a lot of charcoal-burning occurs for the barbecue industry.

The Weald oak forest was used for shipbuilding which proved a particular problem because of the large number of trees required - 2,000 trees for one man-o-war. Thirty oxen were required to move one tree to south coast.

Walking across the Weald Clay with the scarp slope of Hythe Beds to the south illustrated to us how an anticline weakens rocks, leading to them being eroded (breached). Continuing our walk through grass farmland exemplified how to spot changes of lithology by the vegetation - grassland on Weald Clay, compared to heathland of the Hythe Beds. Looking north-eastwards we could see Blackdown in the distance - a much larger slip than the one we had seen earlier, active from the Quaternary.

On the river floodplain there was an exposure of Weald Clay containing sand probably from the granite of the Cornubian Mountains (fine-grained, no glauconite). After making a detour to avoid a very muddy path, we came across a large house (Lower Hawksfold) with solar panels, producing electricity for their own use.

Now it was time for lunch, and Brian had found us a very pleasant stopping place by a stream where we could have our picnic. After lunch, the walk entered one of the best surviving ironworks in West Sussex - Northpark Furnace at the downstream end of a large hammer pond. Before going around the remains of the ironworks Brian recounted the history of iron making in the Weald:

When the Romans arrived in AD 43, they found a well-developed iron industry, which they expanded. The climate was hot and humid which allowed chem-ical weathering to redistribute ferruginous material. Iron was produced in small clay furnaces with manually operated bellows. The ore was extracted from bell pits with shafts up to 10m deep and about

2m diameter, widening at the base. The iron industry declined after the Romans departed until a revival in the 13^th century with the use of water-wheel driven hammers. The ore was still smelted in clay ovens to produce a "bloom" of pasty iron (hence the term bloomeries for the clay ovens) which could be hammered into shape (wrought iron). In 1254 there were 60,000 nails and 30,000 horse-shoes produced in the Weald. Brian told us of the secret recipe used for making swords with tempered iron: a black goat's or young red-headed boy's urine.

Laurie Baker

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Last modified: November 13, 2001