b'Space rocksFeatureNWA 8657 diabasic shergottite 200 and 150 Ma, they were ejected by a number of impacts between 3.7 and 0.73 Ma. NWA 8657 and its pairs have not As with lunaites, impacts on Mars can ejecta material withthemselves been dated (Irving 2018).sufficient velocity to escape the planet and put it into Earth crossing orbits, where some may eventually fall as meteorites. Much larger impacts are needed to eject material from MarsOther meteorites on displaythan from the Moon, due to the greater Martian gravity (0.38G). A small group of meteorites have long been knownMeteorites are fragments of naturally occurring space material to form a distinct family known as SNC meteorites, after thethat have fallen to any planetary surface through natural original specimens from Shergotty, Nakhala, and Chassigny.processes. Collisions between asteroids and between asteroids At the time of writing (22/7/19), 135 distinct specimens areand larger bodies such as the Moon and planets, can send known, not including paired stones. Demonstrating that thefragments into Earth-crossing orbits, some of these can fall SNC meteorites come from Mars is one of the most remarkableas meteorites (McSween et al. 1987). They can be linked pieces of scientific reasoning in the history of meteoritics. Thespectroscopically. In addition to Earth, meteoritic material two Viking spacecraft that landed on Mars in 1976 measuredhas been found on the Moon (Anders et al. 1973) and Mars the noble gas isotopic ratios of the Martian atmosphere, these(Schrder et al. 2008). A probable terrestrial rock has been found ratios are unique to each planet. In 1983 the SNC meteoriteson the Moon in one of the samples returned by the Apollo 14 were found to have the same ratios. The most common Marsmission (Bellucci et al. 2019). Most meteorites are, however meteorites are Shergottites (118), the remainder are Nakhalitesfrom the asteroid belt (McSween et al. 1987). Figure 4 shows (11) Chassignite (4), single examples of an orthopyroxenite (thea number of asteroids visited or about to be visited by space famous ALH84001 with putative evidence for Martian life), and amissions that represent the most common asteroid classes.regolith breccia (Irving 2019).The Geoscience Australia display (Figure 5) has examples of the Geoscience Australias display specimen is of Northwest Africafollowing meteorites classes:8657 (Figure 3). The meteorite, originally massing 234 g (GAs sample is 8.421 g), was purchased from a Moroccan dealer inType L (low iron) ordinary chondrites: Nine Mile, Kybunga, and 2015. NWA 8657 is described as being predominantly complexlyAllan Hills A76009. Nine Mile (also known as Menindee 003) is zoned, prismatically-twinned clinopyroxene and maskelynitefrom NSW (Meteorite Bulletin database 2019c), Kybunga from (shocked plagioclase glass) with accessory ilmenite, ulvspinel,north of Adelaide in South Australia, respectively (Fitzgerald pyrrhotite, merrillite, chlorapatite and vesicular glass. Minor1979) and Alan Hills A76009 from Antarctica (Meteorite Bulletin components are anorthoclase and altered glass. The sample isdatabase 2019d). Ordinary chondrites are typical of many of classed as a diabasic (doleritic) Shergottite (Meteorite Bulletinthe smaller asteroids, known as group S bodies. An example is database 2019b). It is one of 15 individual specimens linkedasteroid Itokawa (Yoshikawa et al. 2006), visited by the Japanese to the same event (Irving 2019). The meteorite is enrichedHayabusa 1 probe in 2005, and with samples returned to Earth in incompatible elements (Howarth et al. 2017), indicating itat Woomera in 2010. Their name refers to the presence of formed from a fractionated basaltic magma. The Martian sourcechondrules, typically millimetre-scale spherical bodies formed of dated enriched diabasic Shergottites were formed betweenas melt droplets in the solar nebula before accreting to form Figure 4.Example asteroids with compositions representative of different meteorite classes. A) Asteroid Itokawa (535 m long), a typical small asteroid of L ordinary chondritic composition (JAXA image). B) Asteroid Ryugu (1 m diameter), with a carbonaceous chondritic composition (JAXA image). C) Asteroid Vesta (diameter 573 km) visited by the Dawn mission), probably the source of the HED family of meteorites (NASA image). D) Artists impression of asteroid Psyche (diameter 279 km), to be visited by the forthcoming Psyche orbiter mission to be launched in 2022 for a 2026 arrival (Peter Rubin/NASA image).OCTOBER 2019 PREVIEW 46'