Документ взят из кэша поисковой машины. Адрес оригинального документа : http://selena.sai.msu.ru/Symposium/sym-pap-a.doc Дата изменения: Mon Oct 6 18:09:54 2008 Дата индексирования: Thu Feb 27 20:25:16 2014 Кодировка: Поисковые слова: п п п п п п п п п

REMOTE DETERMINATION OF LUNAR SOIL MATURITY

P.C.Pinet1, V.V.Shevchenko2, S.Chevrel1, Y.Daydou1, T.P.Skobeleva2,
O.I.Kvaratskhelia3, C.Rosemberg1
1UMR 5562 "Dynamique Terrestre et Planetaire"/CNRS/UPS, Observatoire Midi-
Pyrenees, 14 Av.E.Belin, Toulouse, 31400 France; 2Sternberg Astronomical
Institute, Moscow University, Moscow, 119899, Russia, 3Abastumany
Astrophysical Observatory, Georgian Academy of Sciences, Georgia.

Space weathering processes on the Moon (such as micrometeorite
bombardment and solar-wind ion bombardment) affect the optical properties
of an exposed lunar soil. The main spectral/optical effects of space
weathering are a reduction of reflectance, attenuation of the 1-?m ferrous
absorption band, and a red-sloped continuum creation (Fisher and Pieters,
1994). Fisher and Pieters (1994, 1996) developed one approach to determine
the maturity of lunar soil from Clementine spectral data. This method is
based on a relationship between the values of 750nm/950nm of reflectance of
lunar soils and their measured Is/FeO values.
Lucey et al. (1995, 1998a, 1998b) proposed to estimate the maturity
of lunar soils from Clementine UVVIS data using a method which decorrelates
the effects of variations in Fe2+ concentration from the effects of soil
maturity. The method calculates the Euclidean distance from 750nm
reflectance and values of 950nm/750nm ratio to the hypothetical
"hypermature very dark and very red endmember", ("optimized origin") and
takes this value as an estimate of optical maturity defined as parameter ?
(high values correspond to immature materials, low values correspond to
mature materials) ( Jolliff, 1999).
The amount of fused glassy particles and others agglutinates in the
lunar upper layer is the direct index of the soil reworking caused be the
micrometeorite bombardment. Besides, this micrometeorite bombardment is
also responsible for the mechanical process through which the large
particles are broken down into smaller ones. McKay et al. (1991) showed
that increasingly mature soils become progressively finer-grained, better-
sorted, and composed of a greater proportion of agglutinates.
Altogether, the increasing rate of the fused glassy fragments, of
agglutinates, and of fine size fraction in the regolith affects the
polarization of the light reflected by an exposed lunar soil. Therefore,
polarimetric properties of the lunar regolith may be modified by the soil
reworking process in the course of time. In 1966 Dollfus already showed
that the maximum of polarization for powders, laboratory taken as lunar
soil analogs irradiated by protons flux (simulation of the solar wind
radiation on the Moon), is reduced in the red part of the spectrum. So, the
determination of the maturity level of a lunar soil could be based on the
spectropolarimetric properties of the regolith upper layer.
Shevchenko et al. (1993), Shevchenko (1994), and Pinet et al. (1997)
developed the method to determine the maturity of lunar soil by using
spectropolarimetric ratio Pmax(B)/Pmax(R) for blue (B) and red (R) spectral
regions. On the basis of known laboratory results and telescopic data, it
was found that spectropolarization ratio Pmax(419nm)/Pmax(641nm) could be
used as a remote sensing parameter of lunar soil maturity. This parameter
does not correlate with the soil chemical composition (for example, with
FeO content) but a good anticorrelation (r = -0.951) was found with Is/FeO
values for Apollo and Luna landing sites (Shevchenko, 1994; Shevchenko et
al., 1999). The data show a strong dependence between this parameter and
the exposure age of the lunar surface layer. So, it is possible to consider
spectropolarization ratio Pmax(419nm)/Pmax(641nm) as an independent remote
sensing index of the lunar soil maturity level.
A detailed remote sensing survey of ten lunar regions of mare and
highland types has been carried out by means of Clementine spectro-imaging
data with the purpose of establishing the regional distribution of the
maturity state and weight percent of iron content in the lunar soils. The
spectral dataset has been instrumentally calibrated and a radiometric
calibration using previous telescopic spectra has been made, resulting in
the production of absolute reflectance spectra organized in regional image
cubes (Pinet et al., 1993; Pinet et al., 1995; Pinet et al., 1996; Pinet et
al., 1997).
The data are used to obtain a scale of conformity between spectral
index of maturity ? and spectropolarization index established by Shevchenko
et al. (1999). A special optimization technique has been developed on the
basin of maximal likelihood, to locate very precisely the
spectropolarimetric telescopic observations available (Kvaratskhelia, 1988)
in the Clementine regional mosaics. In Fig. 1a, is shown the Clementine
image of crater Proclus which is a very young lunar crater, with an
extensive ray system. The boxes inside the crater exhibit for the process
for the search of the real site position operates from the reference
catalog (Shevchenko et al.,1999). Size of the each box is 5.6x5.6 km that
is resolution of the telescopic observations for these sites. In Fig 1b, is
shown another regional example related to the Reiner-? formation - crater
Reiner region. The box sizes are the same. The red boxes indi

Fig. 1a


Fig. 1b

display the most probable positions of the sites.
The Reiner-? formation (swirl) is one of the most young feature on the
lunar surface. Center of bright rays system - crater Proclus is probably
very young lunar object too. Given the likely recent origin of these
features, one may consider the age of their formation as nearly equal as to
the exposure age of their soils.
On the other hand, soils of old formations such as highland craters
have been exposed for an extended period of time. It means that most of the
petrographic and chemical parameters of maturity should have reached steady-
state values with exposure time. In that case, any local variation seen in
the soil maturity should be explained as the result of space weathering
process. Examples of such lunar regions are given for highland craters
Alphonsus and Gassendi examined here. Relative mature soil can be found in
these formations.
As mentioned above, ten lunar surface zones are chosen for the purpose
of this study. The list includes the highland crater Alpetragius, mare
units in Mare Humorum and in Oceanus Procellarum near by Aristarchus, post-
mare craters Aristarchus, Herodotus, and Reiner, which have different ages
of emplacement.
On the bases of these data, a scale of conformity between the two
types of maturity index is obtained. The diagram, depicted in Fig. 2, plots
the spectropolarization index Pmax(419nm)/Pmax(641nm) versus the spectral
index of maturity ? (Lucey's parameter). The


Fig. 2

interval of maturity index covers the geological formation time span from
recent impact to old highland craters.
These quantities display a good correlation (exponential regression)
with a correlation coefficient r = 0.980 for the interval of spectral
index of maturity, ranging from ? = 0.22 to 0.38. Shevchenko et al. (1999)
shows that the spectropolarization index represented, as ratio
Pmax(419nm)/Pmax(641nm), correlates directly with maturity index Is/FeO
established by Morris (1978). The correlation coefficient between the
average values of Morris' parameter for Apollo and Luna landing sites and
the Pmax(419nm)/Pmax(641nm) values for the same places derived from
telescopic observations is r = -0.951. Making use of the dependence
mentioned above it is then possible to build a graph of maturity index
Is/FeO versus the spectral index of maturity ?.
Fig. 3 shows the type of relationship between Is/FeO and ?. The Apollo
and Luna landing sites data, combined with the ten selected lunar features
data, are used to establish the graph in the interval of ? from about 0.23
to 0.38. The data concerning the most mature surface soils arise from the
individual sample stations at the Apollo-17 landing site and are used to
establish the graph in the range ? = 0.12 to 0.23. The estimates of
parameter ? are derived from Clementine UV-VIS reflectance values for
Apollo 17 landing-site sample stations published recently by Jolliff (1999,
Tab. 2). Average values of Is/FeO for the sample stations are compiled by
Jolliff (1999) from Morris' data (1978). Part of the plot from ? = 0 to
0.12 is extrapolated from the linear trend observed data of Apollo 17
stations. It is needed to point out that value ? = 0 (Lucey's "optimized
origin") asymptotic behavior only.
As a matter of fact, a few lunar fine fraction samples (soils with
grain size < 250 ?m) from Apollo-15, Apollo-16, and Apollo-17 set of lunar
soils display a maturity corresponding to the measured values of Is/FeO ~
100; for other samples, values of Is/FeO decrease to a few units (Morris,
1978).
So, the modelled relationship between Is/FeO and ? proposed for remote
sensing data in Fig. 3 relies on laboratory estimates produced for Is/FeO.


Fig. 3


According to Morris's classification (Morris, 1976), mature soils are
defined as soils described by Is/FeO > 60, but immature soils are
characterized by Is/FeO < 30. Submature soils are those with intermediate
values.
As it follows from the plot in Fig. 3, a significant inflection of the
curve is seen for values of Is/FeO ~ 70 - 75. As revealed by Fisher and
Pieters (1994), when a soil reaches maturity, optical properties no longer
change with further exposure. The trend shown in Fig. 3, is consistent with
this effect which is accounted here by the curve flattening of the
spectral index of maturity values at Is/FeO > 70 - 75.
Indeed, the coefficient of correlation between the maturity index and
the spectral index of maturity for groups of the Apollo and Luna landing
sites data and of the selected features data is r = -0.963, but for the
group of the most mature Apollo-17 soils data the coefficient of
correlation is only r = -0.668.
The time scale required for a soil to reach maturity (and an optical
steady-state) is probably from one hundred to several hundred million years
for fresh material that has not been previously exposed (Fisher and
Pieters, 1994).
Based on the calculation of the submicroscopic iron production rate,
Morris (1977) obtained a value of exposure age equal to about 100 Myr for
cumulative exposure time required to reach the level of maturity
characterized by value of Is/FeO = 50.
Shevchenko (1994) and Pinet et al. (1997) from correlation between
maturity index and exposure age of a collection of lunar samples found that
exposure age of 100 Myr corresponds to maturity index value of Is/FeO ~ 75.
It is nearly average maturity of the Apollo-11 landing site and the
maturity level of samples 10010 - 10011, the exposure age of which is about
100 Myr (Poupeau et al., 1980).
The present results suggest, that the slope in flexion detected for
Is/FeO values around 75 is indicative of the beginning of the asymptotic
behavior expected for a lunar regolithic soil when it reaches maturity
steady-state. The implication is that one should be careful when
interpreting ? relative estimates less than 0.22 +/-0.02 in terms of local
variations of maturity in the lunar regolith, at the 100 - 200 m resolution
available with Clementine.




References

Dollfus, A., In The nature of the lunar surface, eds. W.Hess, D.Menzel, and
J.O'Keefe, J.Hopkins Press, Baltimore, Maryland, p.155, 1966. Fescher,
E.M., and C.M.Pieters, Icarus, 111, 475-488, 1994; Fescher, E.M., and
C.M.Pieters, J.Geophys. Res., 101, 2225-2234, 1996; Jolliff, B.L., ,
No.E6, 14,123-14148, 1999; Kvaratskhelia, O.I., In Bull. Abastum. Astroph.
Observ., No.66, 312 p., 1988. Lucey, P.G., G.J.Taylor, and E.Malaret,
Science, 268, 1150-1153, 1995: Lucey, P.G., D.T.Blewett, and B.R.Hawke,
J.Geophys. Res., 103, 3679-3699, 1998a; Lucey, P.G., G.J.Taylor , and
B.R.Hawke, In Lunar Planet. Sci. XXIX, # 1356, 1998b; McKay, D.S.,
G.Heiken, A.Basu, G.Blanford, S.Simon, R.Reedy, B.M.French, and J.Papike,
In Lunar sourcebook, Cambr. Univ. Press, Cambrige, pp.285-356,1991; Morris,
R.V., Proc. LPSC 7th, 315-335, 1976; ; Morris, R.V., Proc. LPSC 8th, 3719-
3747, 1977; Morris, R.V., Proc. LPSC 9th, 2287-2297, 1978; Pinet, P.C.,
S.D.Chevrel, and P.D.Martin, Science, 260, 797-801, 1993; Pinet, P.C., .,
V.V.Shevchenko, S.Chevrel, F.Bellagh, and C.Rosemberg, , In Lunar Planet.
Sci. XXVI, 1125-1126, 1995; Pinet, P.C., F.Costard, S.D.Chevrel,
P.D.Martin, F.Bellagh, and J.Blamont, , In Lunar Planet. Sci. XXVII, 1037-
1038,1996; Pinet, P.C., V.V.Shevchenko, S.Chevrel, In Lunar Planet. Sci.
XXVIII, 1115-1116, 1997; Poupeau, G., et al., In Lunar Soil from Mare
Crisium, Nauka Press, 263-663, 1980 (In Russian); Shevchenko, V.V.,
P.Pinet, S.Chevrel, Solar System Research, 27, No. 4, 16-30, 1993;
Shevchenko, V.V., In Lunar Planet. Sci. XXV, 1267-1268, 1994; Shevchenko,
V.V., T.P.Skobeleva, O.I. Kvaratskhelia, In Lunar Planet. Sci. XXX, # 1318,
1999;

-----------------------
[pic]



[pic]