This week sees the 50th anniversary of the fall of the largest single Martian meteorite. On October 3, 1962, in Zagami, Nigeria, this large meteorite – nearly 18 kg in mass, landed close to a farmer who was out chasing crows from his field. A meteorite found in this way is known as a “fall,” most meteorites are merely “finds.”
Of the tens of thousands of meteorites that have been discovered on Earth, there are 103 named meteorites that have been identified as originating from Mars. They are divided into three groups, named for the first meteorite of each type found, with just the one oddball – Allan Hills 84001. This is the meteorite that hit the news in 1996 with the claim that it contains microfossils which are evidence of extra-terrestrial life!
This Allan Hills meteorite was found in 1984 by a team of meteorite hunters in the Antarctic. The Antarctic is a great place to find meteorites, even if last year they only found 302 meteorites. In the easiest case, the rocks tend to be visible on the ice sheet and with the nearest terrestrial rock being 3km straight down, any rocks on the ice almost certainly fell there from outer space! Allan Hills itself is the source for 1836 meteorites, two of which have come from Mars, although only one is being studied to see if Martian life might be in there! The Antarctic also acts to concentrate meteorites – with ice movement over mountain ranges leading to rich fields of rocks. Unfortunately, some of these rocks are not meteorites – so searchers do have to be quite discriminating. The analysis of the 2011-2012 finds is not yet in, but from 2009 – 2010; 1000 meteorites have been classified. They include three lunar meteorites and three more pieces of a previously known Martian meteorite. This year, there will be two American teams. One eight member team will look for meteorites in ice fields that have previously shown they have a lot! The four member recon team will hunt for them in less well known ice fields, including sites in the Robison Glacier region and then moving to the Amundsen Glacier region. With both teams moving frequently, results will be heavily dependent on the weather.
Meteorites do have some common likely characteristics. The first is a fusion crust – a blackened exterior of the rock due to its passage through the earth’s atmosphere. There are often thumbprint indentations called regmaglyphs on the surface. Meteorites are also heavy for their size, don’t have crystals or holes inside them and don’t have a layered structure. There might be tiny spheres called chondrules, or metal grains of nickel or iron. Terrestrial rocks will tend to have had all their iron rusted away by our moist atmosphere. Do not bring a magnet next to a potential meteorite! Some meteorites are iron and will attract a magnet, but you run the risk of inducing magnetism in the rock.
Martian meteorites are a great way to study Mars, since we haven’t yet got a sample return mission underway. Sure, there are rovers rolling around Mars sending back lovely pics, but there are actual samples of the planet here now! Since 1975, over half of the Martian meteorites discovered have been found in northern Africa. In 2011, in Morocco, a fireball was seen, glowing first yellow, then green as it split into two. A few months later over 12 kg of perfect fusion crust meteorites were found in a remote part of Morocco – they are known as the Tissint meteorites. At the 75th Annual Meteoritical Society Meeting in August, entire sessions were devoted to just these rocks.