Shergottites
The most common Martian meteorites: young basaltic and olivine-rich rocks.
- Classification
- Stony meteorites › Achondrites › Planetary achondrites › Martian meteorites › Shergottites
- Share of witnessed falls
- about 0.3%
- In our catalogue
- 99 meteorites, 3 seen to fall
- Heaviest
- Zagami, 18 kg
- Earliest recorded fall
- Shergotty, 1865
- Most recent fall
- Tissint, 2011
- Type specimen
- Shergotty, fell in India in 1865
Shergottites, named after Shergotty in India (fell 1865), make up about three-quarters of Martian meteorites. They are basaltic, olivine-phyric or 'lherzolitic' (poikilitic) igneous rocks that crystallised surprisingly recently, mostly between about 150 and 600 million years ago. Their minerals are often converted to glass (maskelynite) by the shock of being blasted off Mars.
Mineralogy and chemistry
Basaltic shergottites, such as Shergotty, Zagami and Los Angeles, are made mostly of pyroxene (pigeonite and augite) and plagioclase turned to maskelynite glass by shock. Olivine-phyric shergottites, such as Tissint, Dhofar 019 and Elephant Moraine A79001, contain large olivine crystals in a fine basaltic groundmass. Poikilitic shergottites, such as Allan Hills A77005 and Yamato 793605, have olivine enclosed in large pyroxene crystals and formed as cumulates; they were formerly called lherzolitic.
Shergottites are also divided by trace elements and isotopes into depleted, intermediate and enriched kinds, depending on how much of the incompatible elements their source in the Martian mantle contained. This variety shows that Mars's mantle has had distinct reservoirs since it formed.
Origin and formation
Shergottites are lavas and shallow intrusions from Mars's volcanic regions, perhaps from the great volcanic provinces of Tharsis or Elysium. Most crystallised between about 150 and 600 million years ago, though a few are about 2.4 billion years old. Many show strong shock and pockets of impact melt; it was in such glass in Elephant Moraine A79001 that Martian atmospheric gas was found in 1983. They were launched from Mars in several separate impacts, with cosmic-ray exposure ages ranging from about 1 to 20 million years.
Shergottites are the main source of what we know about the Martian mantle and its volcanoes. Their minerals record water and chlorine in Martian magmas, and their isotopes show that Mars separated into core, mantle and crust within a few tens of millions of years of forming.
Abundance, type specimen and famous examples
Shergottites make up about three-quarters of Martian meteorites. Shergotty, the type specimen, fell near Sherghati in Bihar, India, on 25 August 1865; about 5 kg was recovered. Zagami fell in Nigeria in 1962 as a single stone of about 18 kg, the largest Martian meteorite seen to fall. Tissint fell in Morocco on 18 July 2011, and its fresh stones, collected within months, preserve Martian gases and minerals uncontaminated by long exposure on Earth. Los Angeles was found among a collector's rocks in California and recognised in 1999.
How to recognise one
Shergottites are greenish-grey to black basalts, often with clear or white maskelynite and dark shock veins. Olivine-phyric ones show yellow-green olivine crystals. Fresh Tissint stones have a shiny black crust. They are weakly to moderately magnetic and need laboratory confirmation.
Famous examples
- Shergotty Martian (shergottite), India, 1865 · 5 kg
- Zagami Martian (shergottite), Nigeria, 1962 · 18 kg
- Tissint Martian (shergottite), Morocco, 2011 · 7 kg
- Elephant Moraine A79001 Martian (shergottite), Antarctica, 1979 · 7.94 kg
- Allan Hills A77005 Martian (shergottite), Antarctica, 1977 · 483 g
- Los Angeles Martian (shergottite), 1999 · 698 g
- Dhofar 019 Martian (shergottite), Oman, 2000 · 1.06 kg
Related groups
- Nakhlites Augite-rich volcanic rocks from Mars, about 1.3 billion years old.
- Chassignites Olivine-rich Martian rocks; the first known fell in France in 1815.
- Martian meteorites Rocks from Mars, recognised by gas from the Martian atmosphere trapped inside them.
- ALH 84001 and other Martian rocks The ancient Martian rock once claimed to hold fossils, and 'Black Beauty'.
Photos shared under Creative Commons licences (Wikimedia Commons).