A Meteorite From Mars

The Nakhla Martian meteorite is a jewel in the National Space Centre’s collection. There is something almost magical about its green crystals and varnish-like fusion crust, which immediately suggests it is a piece of another world.

But how did a chunk of the Red Planet come to be on Earth? How can we be sure it really did come from Mars? And what secrets might it reveal?

Fire in the sky

On the morning of 28 June 1911, villagers in the area of El-Nakhla el-Bahariya, Egypt were going about their usual business. Around 9:00am a huge fireball appeared in the north-western sky. The arrival of the glowing space rock was accompanied by several loud booms that echoed around the region. Startled residents observed a trail of white smoke tracing the meteorite’s path through the sky.

The rock, travelling around 14 kilometres a second, generated a huge amount of heat as it plunged into the atmosphere, compressing the air in front of it. It lost an estimated 65% of its mass during this violent encounter, breaking apart under the pressure, and falling as a shower of stones. The individual pieces ranged in weight from 20 grams to 1.8 kilograms.

Dr William Fraser Hume, the minister of the Geological Survey of Egypt, soon visited the site. He collected a dozen specimens along with numerous eyewitness accounts (luckily no one was hurt). Hume determined that the stones fell over an area 4.5 kilometres in diameter. Around 40 stones were eventually recovered and distributed to research facilities around the world. The total mass of the stones came to 10 kilograms.

A long journey

The fall witnessed by the people of El-Nakhla el-Bahariya was the culmination of a journey that began long ago, when a piece of Mars was blasted off the planet by an enormous collision. The energy of this event was such that some of the debris reached speeds of more than 5 kilometres a second, enough to escape the gravity of Mars. Once in space, these Martian rocks were continually bombarded with high-energy particles called cosmic rays. Eventually, one of these rocks happened to land on Earth.

Cosmic rays can interact with certain atoms within rocks, changing the number of protons and neutrons in their cores. By counting the number of these changed atoms, scientists determined that Nakhla must have been exposed to cosmic rays for 11 million years as it travelled the Solar System. Working backwards from 1911, Nakhla was ejected from Mars when the Andes Mountain range in South America was starting to form.

The National Space Centre’s 7 gram piece of Nakhla came to us via Professor Monica Grady, who at the time was the curator of the UK’s national collection of meteorites at the Natural History Museum in London. In 1998, she arranged for a sample of Nakhla (catalogued as BM1913,25) to be split and distributed from the Johnson Space Center in Houston, Texas. The original 637 gram piece, which was completely covered in fusion crust, was broken into 65 samples. It is one of these samples that is now on display in our Planets gallery.

How do we know Nakhla is from Mars?

In 1911, when Nakhla fell to Earth, there was no reason for anyone to suspect it was from Mars. As Cari Corrigan, a geologist with the Smithsonian Institution explains, “We didn’t know that any of these were from Mars. All we knew was that they were different from the rest of the meteorites that we had.”

The majority of meteorites are classified as ‘chondrites’. They consist of dust and small grains (called chondrules) from the early Solar System. This primitive material has never been changed by melting. Another common type of meteorite consists of iron-nickel alloys. These ‘iron meteorites’ show obvious signs of melting, presumably in the cores of the asteroids from which they came.

Nakhla doesn’t fit into either of these categories. It does not contain iron-nickel alloys, and its chemistry points to several events of partial melting, followed by the growth of new crystals. This complex process, known as fractionation, can only occur on large planetary bodies where volcanic activity occurs and magma is buried deep below the surface.

The age of Nakhla is also very different to those of other meteorites. Tests show that Nakhla last crystallised into solid rock 1.38 billion years ago, making it much younger than meteorites that come from asteroids, which have ages around 4.5 billion years.

Another clue comes from the types of elements that Nakhla contains. Chondrites formed in the same cloud of gas and dust in which the Solar System originated. They have experienced little change since then, so their abundance of rare-Earth elements closely matches that of the Sun’s atmosphere. Nakhla’s more complex abundance of rare-Earth elements points to its origin on a planet, where geological processes modify the abundance of these elements.

These differences all point to Nakhla’s origin on a planet, but which one? Venus is highly unlikely because its thick atmosphere acts as a protective shield against incoming objects, and would also slow any material ejected by an impact. Its proximity to the Sun’s gravitational pull also means that any material from Venus is far more likely to head towards the Sun rather than out to Earth. The same applies to the closest planet to the Sun, Mercury.

By simple process of elimination, Mars is the favoured planet for the origin of Nakhla, along with similarly unusual meteorites, known collectively as the SNC group. But the conclusive evidence comes from missions to Mars that have sampled the planet’s atmosphere. This was first achieved by the Viking landers in the 1970s, and more recently, and to a higher degree of precision, by the Curiosity Rover.

When samples of Nakhla and other suspected Martian meteorites are heated, the gases they give off can be analysed in great detail. Gases such as xenon and argon are found in more than one form. The ratios of different forms of gases act like a unique fingerprint. Nakhla’s fingerprint is a match for the ratios detected in the Martian atmosphere, confirming that it comes from the Red Planet.

Using this technique, Nakhla was officially recognised as a piece of Mars in 1983. Thanks to new data from the Curiosity rover, this finding was confirmed again in 2013.

Nakhla's secrets

Nakhla continues to be studied by scientists around the world. One paper in 2001 explored Nakhla’s magnetic properties and found that Mars had a magnetic field as recently as 1.3 billion years ago. Other studies have focused on the key question of water on Mars. Veins within Nakhla have been found to contain clay minerals, as well as salts left by evaporation. Both findings indicate the presence of water on Mars when the rock formed.

By far the most controversial research highlights potential traces of biological activity within the Nakhla meteorite. In 2006, scientists from the Natural History Museum and the Open University shared findings of carbon-rich structures in a freshly prepared sample of Nakhla. The structures resembled those left by the activity of microbes in volcanic glass from Earth’s ocean floor. In 2014, a team of scientists concluded that the carbon-rich structures found in Nakhla were most likely caused by non-biological processes, but their presence demonstrated that the Martian subsurface contains niche environments where life could develop.

The debates around various findings in Nakhla, and other Martian meteorites, are likely to continue. Questions of contamination from terrestrial sources will inevitably hang over any results claimed. Perhaps we will only ever be confident that we have answered the question of life on Mars when we visit the planet ourselves, collect truly pristine samples and analyse them under laboratory conditions.

Until then, meteorites such as Nakhla are a precious gift, providing clues that lead to new questions, and stimulating debate over one of our oldest questions – ­are we alone in the universe?