Fig. 1A
(top): Intensity (arbitrary units) vs. m/z during laser
ablation sampling of K-feldspar and analysis on the MMS.
1B
(bottom): Ion current on a quadrupole MS vs. time during
melting of K-feldspar in Li-tetraborate flux at 1000
o
C.
IN-SITU K-Ar GEOCHRONOLOGY: AGE DAT
ING FOR SOLAR SYSTEM SAMPLE RETURN
SELECTION.
J. Hurowitz
1
, O. Aharonson
2
, M. Channon
2
, S. Chemtob
2
, M. Coleman
1
, J. Eiler
2
, K. Farley
2
, J.
Grotzinger
2
, M. Hecht
1
, J. Kirschvink
2
, D. McLeese
1
, E. Neidholdt
1
, G. Rossman, M. Sinha
1
, W. Sturhahn
1
, K. Wal-
tenberg
2,3
, P. Vasconcelos
3
, W. Zimmerman
1
, B. Beard
4
, C. Johnson
4
.
1
Jet Propulsion Laboratory, California Insti-
tute of Technology,
2
Div. of Geological and Pl
anetary Sciences, California Institute of Technology,
3
School of
Earth Sciences, The University of Queensland,
4
Dept. of Geoscience, University of Wisconsin-Madison.
Introduction:
The development of an in-situ
geochronology capability for Ma
rs and other planetary
surfaces has the potential to fundamentally change our
understanding of the evolution of terrestrial bodies in
the Solar System. For Mars
specifically, many of our
most basic scientific questions about the geologic his-
tory of the planet require
accurate knowledge of the
absolute time at which an
event or process took place.
For instance, what was the age and rate of early Mar-
tian climate change faithfully recorded in the mineral-
ogy and morphology of surf
ace lithologies (e.g., [1])?
Currently, our only means of assessing the absolute
age of a surface on a planetary body is through the use
of crater counting statistics. This technique is fraught
with uncertainty for planets with active geologic sur-
faces, on the order of billions of years in some cases
(e.g., [2]). Accordingly, there is much room for im-
provement in our understanding of the absolute chro-
nology of the surfaces of rocky planetary bodies.
Age Characterization Prior to Sample Return:
While returned samples will
receive in-depth analyti-
cal treatment in terrestrial geochronology laboratories,
the ability to characterize the ages of samples in-situ
would provide an invaluable dataset, ensuring that the
samples selected for Earth
return would capture those
periods in the geological evolution of a planet that are
of greatest interest to the scientific community. In Oc-
tober 2009, the Keck Institute for Space Studies and
JPL made a major award to a group of Caltech scien-
tists, and JPL scientists and engineers, respectively, to
investigate a broad range of concepts for in-situ age
dating, with an emphasis on Mars. Below, we briefly
describe one of the more promising in-situ techniques
we are developing using miniaturized flight hardware.
Methodology & Instrument Development
: In the
methodology we are currently developing, a powdered
or fragmental rock sample would be positioned in a
crucible that has been loaded (prior to flight) with a Li-
based fluxing agent and a solid double-spike contain-
ing
41
K and
39
Ar. Under vacuum, the sample-flux-
spike mixture would be fused at low-T (
≤
1000
o
C) via
resistance heating and the
40
Ar
Sample
/
39
Ar
Spike
ratio
measured using a focal plane miniature mass spectro-
meter (MMS), detailed in [3]. The sample would then
be cooled to a glass, and sampled with a 1064 nm
pulsed Nd-YAG laser. The ablated K-neutrals are io-
nized by electron impact and the
39
K
Sample
/
41
K
Spike
ratio
analyzed on the MMS. Whole
rock ages can then be
calculated from measured sample/spike ratios. To date,
we have built testbed instrument systems that have
made measurements demonstrating: (1) low-T Ar-
release, (2) sample-spike equilibration, (3) quench
glass formation, and (4) K-isotope measurement by
laser ablation at ~1 wt% levels. Example results are
shown on
Figs. 1A, B
.
References:
[1] Bibring, J.P., et al. (2006)
Science
312,
400-404. [2] Hartmann, W.K. and Neukum, G.
(2001)
Space Sci. Rev.
96,
165-194. [3] Sinha, M.P.
and Wadsworth, M. (2005)
Rev. Sci. Instrum.
76,
8.
5052
.
pdf
Solar
System
Sample
Return
Mission
(
2011
)