[meteorite-list] Lunar Crater Rays Point to a New Lunar Time Scale

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Sep 28 18:42:41 2004
Message-ID: <200409282230.PAA15800_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Sept04/LunarRays.html

Lunar Crater Rays Point to a New Lunar Time Scale
Planetary Science Research Discoveries
September 28, 2004

--- Optical maturity maps of rays, derived from Clementine multispectral
data and calibrated with lunar sample analyses, provide a new way to
define the two youngest time stratigraphic units on the Moon.

Written by Linda M. V. Martel
Hawai'i Institute of Geophysics and Planetology

The Lunar Time Scale should be reevaluated -- suggest remote sensing
studies of lunar crater rays by B. Ray Hawke (University of Hawai'i) and
colleagues at the University of Hawai'i, NovaSol, Cornell University,
National Air and Space Museum, and Northwestern University. These
scientists have found that the mere presence of crater rays is not a
reliable indicator that the crater is young, as once thought, and that
the working definition of the Copernican/Eratosthenian (C/E) boundary
should be reconsidered. The team used Earth-based spectral and radar
data with FeO, TiO2, and optical maturity maps derived from Clementine
UVVIS images to determine the origin and composition of selected lunar
ray segments. They conclude that the optical maturity parameter, which
uses chemical analyses of lunar samples as its foundation, should be
used to redefine the C/E boundary. Under this classification, the
Copernican System would be defined as the time required for an immature
surface to reach full optical maturity.

Reference:

Hawke, B.R., Blewett, D.T., Lucey, P.G., Smith, G.A., Bell III, J.F.,
Campbell, B.A., and Robinson, M.S. (2004) The origin of lunar crater
rays. Icarus, v. 170, p. 1-16.

------------------------------------------------------------------------

Investigating Lunar Crater Rays

Lunar crater rays are those obvious bright streaks of material that we
can see extending radially away from many impact craters. Historically,
they were once regarded as salt deposits from evaporated water (early
1900s) and volcanic ash or dust streaks (late 1940s). Beginning in the
1960s, with the pioneering work of Eugene Shoemaker, rays were
recognized as fragmental material ejected from primary and secondary
craters during impact events. Their formation was an important mechanism
for moving rocks around the lunar surface and rays were considered when
planning the Apollo landing sites. A ray from Copernicus crater crosses
the Apollo 12 site in Oceanus Procellarum. Rays of North Ray and South
Ray craters cross near the Apollo 16 site in the Descartes Highlands and
a ray from Tycho crater can be traced across the Apollo 17 site in the
Taurus-Littrow Valley on the eastern edge of Mare Serenitatis. There is
still much debate over how much ejecta comes from the primary impact
site or by secondary craters that mix local bedrock into ray material.

To sort out the nature of rays, Hawke and his colleagues focused their
study on rays associated with four craters: Tycho, the Messier crater
complex, Lichtenberg, and Olbers A (see photograph below). They combined
Earth-based observations with Clementine-derived maps to investigate ray
compositions, maturities, and modes of origin, and to assess the
consequence of their findings on the lunar time scale.

crater ray locations The white boxes on this photograph show the
locations of the crater rays in the investigation, clockwise from the
left: Olbers A ray system, Lichtenberg crater rays, Messier Crater
Complex, Tycho ray in Mare Nectaris, and Tycho ray in the southern
highlands. Prominent maria and Tycho crater are also labeled.

------------------------------------------------------------------------

The Role of Cosmochemistry

Chemical analyses of lunar samples provide "ground truth" to understand
the geologic processes on the Moon. Specifically, the discoveries by
cosmochemists of nanophase-iron grain coatings, Fe-Si phases, and other
space weathering products in lunar rocks and meteorites have enabled us
to better understand the physical, chemical, and optical changes that
occur over time as the lunar surface is exposed to the space environment
and matures. Older surfaces in which these changes have reached a steady
state are said to be fully mature. Younger surfaces are called immature.

Space weathering products in the lunar material, which can only be
discovered by sample analysis, affect the spectral signatures of the
Moon's surface. Detailed studies of the variation of spectra with extent
of space weathering have recently been made by Larry Taylor (University
of Tennessee, Knoxville), Carl? Pieters (Brown University) and their
colleagues. Using scanning electron microscopy, they've measured, for
example, the abundance of agglutinate glass and major minerals in
grain-size separates of lunar regolith (see photographs below).

lunar soil sample from Apollo 17 drill core

This is a photograph in reflected light of a polished thin section of
regolith from the Apollo 17 drill core. The big particle with round
bubbles in it, in the center of the photograph, is an agglutinate
(impact glass bonded with rock, mineral, and glass fragments). Many
small agglutinates are also visible. The abundance of these glassy
particles increases with the amount of space weathering, or maturity.
And average grain size decreases as the regolith matures.

 
SEM at Univ. of Hawaii

Larry Taylor and his colleagues used a scanning electron microscope like
the one pictured above to analyze lunar soils in exquisite detail.

------------------------------------------------------------------------

Putting It Together With Remote Sensing Data

Three types of data were compiled and analyzed by the research team to
study the lunar rays.

    * Maps of FeO, TiO2, and optical maturity derived from Clementine
      UVVIS images (415-1000 nanometers) and sample analyses, using the
      methods developed by Paul Lucey (University of Hawai'i) and
      co-workers. Spatial resolution was ~100 meters/pixel.

    * Near-IR reflectance spectra between 0.6-2.5 ?m (600-2500
      nanometers) collected at the University of Hawai'i 2.24-meter
      telescope at Mauna Kea Observatory. Spacial resolutions (called
      spot sizes) were ~1.5 km and 4.5 km in diameter. Spectral curves
      showed absorption bands near 1 ?m characteristic of iron-bearing
      silicate minerals. They used the shape and position of this band
      to determine the composition and relative abundance of pyroxene
      and olivine minerals, which helped them distinguish rock types.

    * Radar data at three wavelengths: 3.0 cm, 3.8 cm, and 70 cm
      transmitted and received by radar antennae at Haystack Observatory
      (Massachusetts) and Arecibo Observatory (Puerto Rico). Surface and
      subsurface scattering properties of the Moon were analyzed using
      these radar backscatter images. The 3.0 cm and 3.8 cm radar data
      are sensitive to roughness on the scales of 1 to 50 cm within the
      upper meter of the regolith. The 70 cm data show roughness on
      scales from 50 cm to 10 m within 5 to 10 meters depth. Rough
      (blocky) ray deposits are bright in radar images. As these young
      rays are exposed to space weathering, they mature and become
smoother so they appear darker in the radar images.

------------------------------------------------------------------------

Making Sense of It

Take a look below at the Clementine albedo images and derived maps of
FeO, TiO2, and optical maturity parameter (OMAT) for each ray segment
studied by Hawke and colleagues. Toggle between maps by moving your
cursor over the three rectangular buttons [FeO, TiO2, and OMAT] below
the images.

General point to consider: Bright areas in the albedo images are either
due to the presence of low FeO material or an immature surface. The dark
lunar maria are lava plains composed of an Fe-rich rock called basalt.
The brighter highlands are made of rocks much lower in FeO.

Following the images, we will discuss the results of the integration of
Clementine data with the Earth-based near-IR and radar data for the
crater rays.

------------------------------------------------------------------------
Clementine albedo image of Lichtenberg rays


      LUNAR CRATER RAYS

The Clementine 750 nm images of each ray segment studied by Hawke and
colleagues are shown here. Move your cursor over the three buttons
located below the images to view the crater ray data. FeO maps, TiO2
maps, or Optical Maturity parameter images will appear at the same time
for each ray.

Clementine albedo image of Messier rays
Clementine albedo image of Olbers A rays Clementine albedo image of
Tycho rays in S. Highlands Clementine albedo image of Tycho rays in
Nectaris

FeO maps <#data> TiO2 maps <#data> Optical maturity maps <#data>
FeO wt.%
dark is lower and
bright is higher TiO2 wt.%
dark is lower and
bright is higher OMAT Parameter
dark is mature and
bright is immature

------------------------------------------------------------------------

Messier Crater Complex. Near-IR spectra as well as the FeO, TiO2, and
maturity maps indicate that the south and west rays of the Messier
complex are composed of debris from immature mare basalts. They appear
bright in radar images. Highlands material is not present in these rays,
hence they are bright because they are immature.

Tycho Ray in Mare Nectaris. The portion of Tycho ray in Mare Nectaris is
composed largely of immature mare basalt with little to no detectable
Tycho ejecta material. The ray is dominated by fresh local material
excavated and emplaced by the secondary craters as well as fresh
material that is constantly exposed on the crater walls due to
landslides. This ray segment is bright because it is immature.

Tycho Ray in Southern Highlands. This ray segment is composed of
relatively immature highland debris. But it is not possible to determine
how much of the material is local and how much is projectile material
blasted in from Tycho Crater. This ray segment is bright because it is
immature.

Lichtenberg Crater Rays. FeO and TiO2 abundances for these rays are
consistent with highland rocks. The optical maturity map demonstrates
that these highlands-rich ejecta deposits and rays are fully mature.
Hence, these rays are bright because of their composition.

Olbers A Ray. This high-albedo ray, which was deposited on a mare
surface, has reduced FeO and TiO2 abundances, consistent with the
presence of a large non-mare component. Much of the ray is not distinct
in the optical maturity map but some areas are bright in the OMAT (see
arrows A, B, and C) suggesting that these areas are not as mature as the
adjacent terrain. This ray has a significant amount of highlands ejecta
debris and is bright because of composition and immaturity, and is a
good example of a "combination" ray.

The work by Hawke and others shows that the brightness of rays is due to
immaturity and/or compositional differences.

cartoon of ray evolution
This cartoon shows the evolution of a lunar combination ray. In panel A
the ray brightness is due to the presence of immature, high-albedo
highlands material and immature mare debris excavated by secondary
craters. Panel B shows a ray in transition. The ray is less bright
because of increased maturity. In panel C the ray has reached complete
optical maturity and its brightness is only due to the fact that
highlands-rich ray material is brighter than the darker material of the
mare surface below.

------------------------------------------------------------------------

Redefining the C/E boundary

The working definition of Copernican age craters (larger than a few
kilometers in diameter) is that they have sharp features and bright
rays. Craters with slightly subdued form and have no rays have been
traditionally given an Eratosthenian age. It is generally accepted that
the boundary between Copernican and Eratosthenian is 1.1 billion years
ago (see chart below) though different people have defined different
durations for the systems. Copernicus crater itself (about 0.8 billion
years old, based on dates obtained from Apollo 12 samples) is considered
a good early Copernican marker, but it does not mark the base of the
system.

lunar time scale defined

There are craters with compositional rays whose ages are significantly
greater than 1.1 billion years. Lichtenberg is a prime example. This
20-km-diameter crater is embayed on the southeast by mare basalt flows.
Harry Hiesinger (Brown University) and his coworkers have estimated the
age of these flows on the basis of the number of craters formed on them.
They report an age of 1.6-2 billion years, distinctly older than the
nominal C/E boundary age of 1.1 billion years. It follows that the mere
presence of rays is not a reliable indicator of crater age. And it is no
longer valid to assign a Copernican age to craters based only on the
presence of rays.

Hawke and others conclude that a new method using the optical maturity
parameter is required to distinguish Copernican from Eratosthenian
craters. They acknowledge a problem of not knowing the time required for
a surface to reach full optical maturity; no such age has been firmly
established.

A possible solution was proposed by Jennifer Grier (formerly at the
University of Arizona and now at the Harvard-Smithsonian Center for
Astrophysics) and Al McEwen (University of Arizona) and colleagues.
Their work showed that if the ejecta of Copernicus crater were slightly
more mature, it would be impossible to tell apart from the optically
mature bedrock. Since the commonly accepted age of Copernicus is about
0.8 billion years, then perhaps full optical maturity occurs at about
0.8 billion years. More work is necessary and future studies will look
more closely at optical maturity maps of the Copernicus crater region to
better define the C/E boundary in the lunar time scale.

Apollo 17 image of Copernicus Crater
This oblique photograph was taken by the Apollo 17 crew in 1972 with a view
looking south across Mare Imbrium. Copernicus crater, 93 kilometers in
diameter, is seen in the distance. Hummocky ejecta, rays, and several
chains of small secondary craters from Copernicus are visible in the
foreground, as well as the crater Pytheas, 20 kilometers in diameter.

One particularly useful way to pin down the ages of specific craters is
to determine their absolute ages by isotopically dating samples returned
from them. Cosmochemists could date either samples of impact melt from
the floors of the craters or samples of mare basalts that embay ejecta
from the craters, as at Lichtenberg.

painting of lunar lander Artist's concept of a lander on the Moon.
Future automated sample-return missions to the Moon could allow
cosmochemists to date specific impact craters or lava flows that embay
craters. Such targeted sample return missions also allow us to address
many other important problems in lunar science, such as the age of the
youngest lava flows on the Moon.

ADDITIONAL RESOURCES

Gaddis, L., Tanaka, K., Hare, T., Skinner, J., Hawke, B.R., Spudis, P.,
Bussey. B., Pieters, C., and Lawrence D. (2004) A new lunar geologic
mapping program. Lunar and Planetary Science Conference XXXV, abstract 1418.

Grier, J.A. and McEwen, A.S. (2001) The lunar record of recent impact
cratering. In Accretion of Extraterrestrial Matter Throughout Earth
History. B. Peuckner-Ehrenbrink and B. Schmitz (Eds.), Kluwer Academic
Publishers, Norwell, MA, p. 403-422.

Grier, J.A., McEwen, A.S., Lucey, P.G., Milazzo, M., and Strom, R.G.
(2001) Optical maturity of ejects from large rayed lunar craters.
Journal of Geophys. Res., v. 106, p. 32847-32862.

Hawke, B.R., Blewett, D.T., Lucey, P.G., Smith, G.A., Bell III, J.F.,
Campbell, B.A., and Robinson, M.S. (2004) The origin of lunar crater
rays. Icarus, v. 170, p. 1-16.

Lucey, P.G., Taylor, G.J., and Malaret, E. (1995) Abundance and
distribution of iron on the Moon. Science, v. 268, p.1150-1153.

Lucey, P.G., Blewett, D.T., and Jolliff, B.L. (2000) Lunar iron and
titanium abuncance algorithms based on final processing of Clementine
UV-VIS data. Journal of Geophys. Res., v. 105, p. 20297-20305.

Lucey, P.G., Blewett, D.T., Taylor, G.J., and Hawke, B.R. (2000) Imaging
of lunar surface maturity. Journal of Geophys. Res., v. 105, p. 20377-20386.

Martel, L.M.V. (2004) New mineral proves an old idea about space
weathering. Planetary Science Research Discoveries.
http://www.psrd.hawaii.edu/July04/newMineral.html.

Taylor, G.J. (1997) Moonbeams and elements. Planetary Science Research
Discoveries. http://www.psrd.hawaii.edu/Oct97/MoonFeO.html.

Taylor, L.A., Pieters, C., Keller, L.P., Morris, R.V. McKay, D.S.,
Patchen, A., and Wentworth, S. (2001) The effects of space weathering on
Apollo 17 mare soils: Petrographic and chemical characterization.
Meteoritics & Planetary Science, v. 36, p. 285-299.
Received on Tue 28 Sep 2004 06:30:41 PM PDT


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