Geology Reference
In-Depth Information
Table 9.5.
Ejection and terrestrial ages (
T
Ej
and
T
Terr
) of selected martian meteorites found in the Antarctic.
a
Meteorite
Classification
T
Ej
(Ma)
T
Terr
(ka)
ALH A77005
Shergottite
3.32 ± 0.55
190 ± 70
ALH 84001
Orthopyroxenite (unique)
14.4 ± 0.7
13 ± 1
EET A79001
Shergottite
0.65 ± 0.20
12 ± 1
LAR 06319
Shergottite
~3.3
—
LEW 88516
Shergottite
4.1 ± 0.6
21.5 ± 1.5
MIL 03346
Nakhlite
9.5 ± 1.0
—
QUE 94201
Shergottite
2.6 ± 0.5
290 ± 50
RBT 04261
Shergottite
3.0 ± 0.6
<60
RBT 04262
Shergottite
2.0 ± 0.5
710±60
b
Y 793604
Shergottite
4.4 ± 1.0
—
Y 980459
Shergottite
1.1 ± 0.2
—
Y 000027
Shergottite
4.9
—
Y 000593, 749, 802
Nakhlite
12.1 ± 0.7
55 ± 20
a
References to the original data for these and other martian meteorites may be found in
Jull
[2006],
Meyer
[2012], and
Herzog and Caffee
[2014].
b
Jull
, unpublished data.
estimates of the average age of the martian surface based
on crater counting. This discrepancy constitutes what is
often called the martian age paradox. Any of several
explanations could help resolve it: (1) The material in
younger martian terrain may be better suited to survive
launch into space by virtue of its mechanical properties.
(2) The crater counting dates may be wrong. (3) The ages
reported for martian meteorites may date a local resetting
event that is, somehow, not reflected the cratering record,
rather than recent crystallization from a magma.
None of the martian meteorites has a CRE age older
than 20 Ma (Figure 9.7). Taken as a group and with only a
few exceptions (lunar, CI, and CM meteorites), the CRE
ages of martian meteorites are younger than those of most
other stony meteorites. The difference can be explained in
terms of the energetics of the collisions that produced the
meteorites and the orbital dynamics of the meteorites'
subsequent transport.
Gladman
[1997] modeled the orbital
evolution of a chaotic swarm of fragments blasted by a
collision from the surface of Mars. In accord with the distri-
bution shown in Figure 9.7, they found that the orbits of the
fragments evolved rapidly, within 1 Ma, into Earth-crossing
orbits and in such a way that most of the fragments fell
into the Sun with 10 Ma. The older CRE ages of many
asteroidal meteoroids reflect the lower energies of aster-
oidal collisions and the longer times they take to reach the
resonances that propel them into Earth-crossing orbits.
0
4
8
12
16
20
Olivine-phyric shergotties
5
4
Basaltic shergotties
4
Lherzolitic shergotties
Nakhlites
4
0
4
8
CRE age (Ma)
12
16
20
Figure 9.7.
Cosmic-ray exposure ages of martian meteorites.
Open and filled bars are for non-Antarctic and Antarctic mete-
orites, respectively.
9.3.2. Lunar Meteorites
et al
., 2001;
Park et al
., 2014]. That the radiometric ages
greatly exceed the CRE ages is no surprise as launch must
postdate the formation of the rocks. More puzzling
is that the radiometric ages are much younger than
The identification of ALH A81005 (Plate 64) as the first
meteorite from the Moon was a major event in meteoritics.
As noted in the
Antarctic Meteorite Newsletter
, vol. 5, No. 4,
November 1982, the meteorite “has been characterized as
