In-Depth Look

Thirty-one unusual meteorites are almost certainly pieces of Mars that were blasted off the red planet by meteoroid impact. They have been called SNC meteorites after the three type samples, Shergotty, Nakhla, and Chassigny, or now simply martian meteorites. All 31 meteorites are igneous rocks crystallized from lava in the crust of a parent body. They are, however, distinct from typical igneous meteorites from asteroids in ways that suggest that the SNC meteorites come from a much larger body, a planet. All but one of these meteorites are very young (1.3 Ga or less) compared to ancient ages for other igneous meteorites (about 4.5 Ga). They also have higher oxygen fugacities and contents of water and other volatiles, contain minerals with ferric iron, and form a distinct trend in oxygen isotopic composition. The conclusive evidence that SNC meteorites are from Mars is the analysis of gases trapped in glass inclusions in EETA79001, which chemically and isotopically match gases measured in the unique martian atmosphere by the Viking lander spacecraft (Figure 1).

The martian meteorites represent five different types of igneous rocks, ranging from simple plagioclase-pyroxene basalts to almost monomineralic cumulates of pyroxene or olivine. The meteorites and their rock types are listed in Table 1 below. Photographs of whole rocks and thin sections of a basalt and a cumulate are illustrated below. All of the meteorites solidified near the martian surface by crystallization from a cooling magma. Some of the shergottite basalts have close to magma compositions, while the other martian meteorites are dominated by accumulation of olivine and/or pyroxene. None of the martian meteorites are surface samples in that they have not been exposed to extensive weathering or irradiation by cosmic rays. The martian soil analyzed by Viking appears to be a weathered basalt which could have been of shergottite composition.

Figure 1. Comparison of Viking-measured Mars atmosphere
to trapped gases in Shergottite meteorite EETA79001

The only natural process capable of launching martian rocks to Earth is meteoroid impact. To be ejected from Mars a rock must reach the escape velocity of 5 km/sec, which is more than five times the muzzle velocity of a hunting rifle. During impact the kinetic energy of the incoming projectile causes shock deformation, heating, melting, and vaporization, as well as crater excavation and ejection of target material. The martian meteorites show low to moderate degrees of shock that appear to require a special mechanism to boost them to the escape velocity and eject them from Mars. The impact and shock provide an explanation for why the martian meteorites are all igneous rocks. Martian sedimentary rocks, and certainly soil, may not be sufficiently consolidated to survive the impact as intact rocks which might later land on Earth as meteorites.

EETA79001, a basaltic shergottite, has light-colored xenoliths and dark glasses containing trapped martian atmosphere.	ALHA77005 lherzolitic shergottite has a splotchy dark-light structure.
ALH84001 orthopyroxenite has afractured zone where carbonate weathering products are more abundant.	QUE94201 basaltic shergottite is tiny and the newest martian meteorite.
Thin section of basaltic Shergottite shows melt texture of pyroxene and plagioclase mineral.	Thin section of dunite Chassigny shows the cumulate texture of olivine in polarized light.

Table 1 - Martian Meteorites

Name	Classification	Mass (kg)	Find/Fall	Year
Shergotty	S-basalt (pyx-plag)	4.00	fall	1865
Zagami	S-basalt	18.00	fall	1962
EETA79001	S-basalt	7.90	find-A	1980
QUE94201	S-basalt	0.012	find-A	1995
ALHA77005	S-lherzolite (ol-pyx)	0.48	find-A	1978
LEW88516	S-lherzolite	0.013	find-A	1991
Y793605	S-lherzolite	0.018	find-A	1995
Nakhla	N-clinopyroxenite	10.00	fall	1911
Lafayette	N-clinopyroxenite	0.80	find	1931
Gov. Valadares	N-clinopyroxenite	0.16	find	1958
Chassigny	C-dunite (olivine)	4.00	fall	1815
ALH84001	orthopyroxenite	1.90	find-A	1993
Dar al Gani 476 Dar al Gani 489 Dar al Gani 670 Dar al Gani 735 Dar al Gani 876 Dar al Gani 975 Dar al Gani 1037	S-basalt	2.02 2.15 0.59 1.62 0.006 0.028 4.01	find	1998 1997 1996-97 1998-99 1998 1999 1999
Y980459	S-basalt	0.082	find-A	1998
Los Angeles 001 Los Angeles 002	S-basalt	0.45 0.25	find	1999 1999
Sayh al Uhaymir 005 Sayh al Uhaymir 008 Sayh al Uhaymir 051 Sayh al Uhaymir 094 Sayh al Uhaymir 060 Sayh al Uhaymir 090 Sayh al Uhaymir 120 Sayh al Uhaymir 150 Sayh al Uhaymir 125 Sayh al Uhaymir 130 Sayh al Uhaymir 131	S-basalt	1.34 8.58 0.44 0.23 0.042 0.094 0.075 0.108 0.031 0.278 0.17	find	1999 1999 2000 2001 2001 ? 2002 2002 2003 2004 2004
Dhofar 019	S-basalt	1.06	find	2000
GRV 99027	S-lherzolite	0.010	find-A	2000
Dhofar 378	S-basalt	0.015	find	2000
NW Africa 480 NW Africa 1460	S-basalt	0.028 0.070	find	2000 2001
Y000593 Y000749 Y000802	N-clinopyroxenite	13.7 1.3 0.02	find-A	2000 2000 ?
NW Africa 817	N-clinopyroxenite	0.104	find	2000
NW Africa 1669	S-basalt	0.036	find	2001
NW Africa 1950	S-lherzolite	0.797	find	2001
NW Africa 856	S-basalt	0.32	find	2001
NW Africa 1068 NW Africa 1110 NW Africa 1775	S-basalt	0.65 0.12 0.025	find	2001 2002 2002
NW Africa 998	N-clinopyroxenite	0.46	find	2001
NW Africa 1195	S-basalt	0.32	find	2002
NW Africa 2046	S-basalt	0.063	find	2003
MIL03346	N-clinopyroxenite	0.715	find-A	2003
YA1075	S-lherzolite	0.055	find-A	?

Classification: S=shergottite, N=nakhlite, C=chassignite, ALH84001 is none of these.
find-A designates Antarctic meteorites (all recent finds).
Year is recovery date for non-Antarctic meteorites and date of martian classification for Antarctic meteorites.