[meteorite-list] Did Martian Meteorites Come From These Sources?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 29 Jan 2007 12:08:43 -0800 (PST)
Message-ID: <200701292008.MAA29535_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Jan07/MarsRayedCraters.html

Did Martian Meteorites Come From These Sources?
Planetary Science Research Discoveries
January 29, 2007

--- Researchers find large rayed craters on Mars and consider the
reasons why they may be launching sites of Martian meteorites.

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

Large rayed craters on Mars, not immediately obvious in visible light,
have been identified in thermal infrared data obtained from the Thermal
Emission Imaging System (THEMIS) onboard Mars Odyssey. Livio Tornabene
(previously at the University of Tennessee, Knoxville and now at the
JUniversity of Arizona, Tucson) and colleagues have mapped rayed craters
primarily within young (Amazonian vollcanic plains in or near
Elysium Planitia. They found that rays consist of numerous chains of
secondary craters, their overlapping ejecta, and possibly primary ejecta
from the source crater. Their work also suggests rayed craters may have
formed preferentially in volatile-rich targets by oblique impacts. The
physical details of the rayed craters and the target surfaces combined
with current models of Martian meteorite delivery and cosmochemical
analyses of Martian meteorites lead Tornabene and coauthors to conclude
that these large rayed craters are plausible source regions for
Martian meteorites.

Reference:

    * Tornabene, L. L., J. E. Moersch, H. Y. McSween Jr., A. S. McEwen,
      J. L. Piatek, K. A. Milam, and P. R. Christensen (2006)
      Identification of large (2-10 km) rayed craters on Mars in THEMIS
      thermal infrared images: Implications for possible Martian
      meteorite source regions. Journal of Geophysical Res., v. 111,
      doi: 10.1029/2005JE002600).

------------------------------------------------------------------------
Finding What They're Looking For

There are currently 34 Martian meteorites identified out of the 24,000+
that have been cataloged. The numbers are growing as a result of ongoing
searches primarily in the world's desserts (for example see PSRD
article: Searching Antarctic Ice for Meteorites
<http://www.psrd.hawaii.edu/Feb02/meteoriteSearch.html>). Cosmochemists
have determined that these rocks came from basaltic igneous sources with
young (by planetary standards) crystallization ages no more than 1.3
billion years (with the one exception: ALH84001 with an age of 4.5
billion years) and were ejected from Mars by impact cratering events
between 600,000 and 20 million years ago. While these rocks provide
invaluable direct 'ground truth' that scientists are using to help piece
together the chemical and geological history of Mars, the question
remains where exactly did these rocks come from? Knowing their
provenance will add significant details to our understanding of how the
planet formed, differentiated, and evolved geologically.

One approach to answering the question has been to search orbital
multispectral datasets to find volcanic terrains on Mars that match the
mineralogy and spectral properties of Martian meteorites. These locales
must be sufficiently dust-free to allow spectral analysis of the surface
compositions and must also have at least one impact crater of
appropriate size and age that could have ejected rocks at greater than
Mars' escape velocity of ~ 5 kilometers per second. Previous work by
Vicky Hamilton (University of Hawaii) using data from the Mars Global
Surveyor Thermal Emission Spectrometer (TES)
pointed to Eos Chasma, a branch of the Valles Marineris canyon system,
as a possible source for unique Martian meteorite ALH84001. Hamilton's
work with Ralph Harvey (Case Western Reserve University) identified
Syrtis Major as a possible source region of nakhlite/chassignite
meteorites. This is exciting on-going work to
find meteorite source regions.

Alternatively, an answer to Martian meteorite sources may well come from
studies of some uncommon Martian craters that, until just a few years
ago, had gone unnoticed. In 2003, using new Mars Odyssey
Thermal Emission Imaging System (THEMIS)
thermal infrared data, Alfred McEwen (University of Arizona) and
colleagues reported the first discovery of a rayed crater, Zunil, in the
southern Elysium region of Mars. More recently, Livio Tornabene and
colleagues have identified an additional four large rayed craters and
three more they deem probable. Their detailed observations of the
craters combined with the known geochemistry of the meteorites and
models of how meteorites are ejected off the planet add up to a
compelling story that these rayed craters could have supplied Martian
meteorites.

------------------------------------------------------------------------
Rayed Craters Defined and Located

Tornabene and coauthors define a crater ray as filamentous (thread-like)
elements in radial to subradial lineaments that spread out from a source
crater like spokes from the center of a wheel. A ray contrasts with the
surrounding, underlying surface. We are used to seeing crater rays on
the Moon in visible-light images where this contrast is recognized as
albedo. Lunar rays are brighter than the
underlying surface. On Mars crater rays are not distinctive in visible
light but are apparent in the thermal infrared because of a
thermophysical (temperature-related) contrast with the surroundings.
Martian rays appear brighter or darker than the underlying surface
depending on the relative thermal properties of the materials when (day
or night) the images were taken (see images below).

Moon Mars
lunar nearside rayed craters <
http://photojournal.jpl.nasa.gov/catalog/?IDNumber=PIA00302> Martian
Gratteri crater rays < http://themis.asu.edu/features/gratteri>

LEFT: Clementine images show albedo (reflectivity) variations on the
nearside of Earth's Moon. Extensive bright ray systems surround craters
Copernicus (upper left center) and Tycho (near bottom). Click the image
for more information in a new window. Bright crater rays have also been
observed on Mercury and the icy Galilean moons.

RIGHT: Crater rays on Mars show
up in the thermal infrared and can be dark or bright. This is a
nighttime thermal infrared image of Gratteri crater and its dark rays.
Rocky areas are brighter because they retained daytime warmth into the
night. In contrast, the dark rays and patches show where finer-grain
materials cooled after local sunset. Click the image for more
information in a new window.

------------------------------------------------------------------------
[thermal infrared vs. visible]

Pairs of images illustrate how Martian crater rays appear in different
wavelengths of light. The left hand images are THEMIS nighttime thermal
infrared images. TOP LEFT (a): Gratteri crater rays are revealed as dark
streaks (see figure caption above for more explanation). BOTTOM LEFT
(c): Warmer (brighter) Gratteri ejecta around the crater and colder
(darker) rays are revealed. The right hand images are contrast-stretched
MOC visible images of the same area. RIGHT (b
and d): Crater rays are not easily seen. There is very little albedo
difference between rays and the surrounding plains.

In their global survey for rayed craters, Tornabene and colleagues
studied both THEMIS nighttime and daytime thermal infrared (TIR)
brightness temperature images derived from band 9 (wavelength of 12.57
micrometers). This wavelength is used because it has the highest signal
to noise value and is relatively transparent to atmospheric dust. Image
resolutions ranged from 32 to 256 pixels per degree. They found that
daytime TIR images are not as useful as nighttime TIR images for
identifying rays. The effects of albedo, surface slope, and time of day
all affect daytime surface temperatures more than at night. Moreover,
the survey focused on latitudes specifically between 45 oN and 45 oS
(see map below) because surface radiance and diurnal (a single daily
cycle) thermal contrast generally decrease poleward of the equator. They
note that lower surface radiance translates as lower signal to noise
detected at the spacecraft's instrument, which generates poor quality
images (especially at night when temperatures are much colder) making it
more difficult to recognize rays with certainty at the higher latitudes.
Their survey resulted in an additional four, and another three probable,
rayed craters.

[locations of Martian rayed craters]

The five known rayed craters (black circles with white name lables) and
three probable rayed craters (white circles) are located on this MOLA
shaded relief map of Mars. The rayed craters
cluster in two groups; two craters are located south of Tharsis (the
largest volcanic region on the planet) and the other six, including the
probables, are in or near Elysium Planitia (the second largest volcanic
region).

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

Characteristics of Martian Rayed Craters

Tornabene and colleagues found that the eight identified rayed craters
all lie within lava plains or adjacent to the two major volcanic regions
(Tharsis and Elysium Planitia, see map above). The table below
summarizes what is known about Martian rayed craters. Each is shown in
THEMIS nighttime infrared images, with North to the top. Crater
locations are listed in latitude, longitude. Crater diameters and the
longest ray length measured for each crater are listed in kilometers.

Martian rayed crater characteristics
Zunil
nTIR of Zunil
Crater Location: 7.7 oN, 166 oE
southeast of Cerberus Planum within Elysium Planitia
in Amazonian-aged lava plains
Crater diameter: 10.1 km

Ray Length: 927 km
Rays are not symmetrically arranged around the crater. White arrows
point to two distinct dark rays to the northwest of Zunil crater.

Tomini
nTIR of Tomini
Crater Location: 16.27 oN, 125.9 oE
southwest of Elysium Mons
in Hesperian-aged ridged volcanic plains
Crater diameter: 7.4 km

Ray Length: 668 km
Curved dark rays are prominent to the west and southeast. A long,
straight ray trends north-northeast opposite the forbidden zone (a
wedge-shaped zone where crater ejecta was never deposited).

Gratteri
nTIR of Gratteri
Crater Location 17.7 oS, 199.9 oE
near Memnonia Fossae southwest of Tharsis
in Noachian-aged volcanic plateau
Crater diameter: 6.9 km

Ray Length: 595 km
More than 30 rays, more than double the number found with any other
rayed crater. Longest rays occur to the northwest and southeast. Region
north of the crater is very dusty, so if rays exist in this region they
lack a theromphysical contrast to the dust.

Zumba
nTIR of Zumba
Crater Location 28.65 oS, 226.9 oE
Daedalia Planum
in Hesperian-aged lava flows
Crater diameter: 3.3 km

Ray Length: 240 km
One of its longest rays trends to the east while another of the longest
rays trends west. Zumba rays are unique because they are also distinct
in daytime TIR images.

Dilly
nTIR of Dilly
Crater Location 13.27 oN, 157.23 oE
Cerberus Planum within Elysium Planitia
in Amazonian-aged volcanic terrain
Crater diameter: 2.0 km with an elliptical shape

Ray Length: 50 km
Dark rays appear to the northwest and southeast in distinctive
"butterfly wing" pattern.

Tomini B
nTIR of TominiB
Crater Location 14.9 oN, 123.25 oE
near Tomini crater
in Hesperian-aged volcanic plains
Crater diameter: 4.2 km

Ray Length: 220 km
Three discernable, but faint rays.

Crater A
nTIR of Crater A
Crater Location 18.1 oN, 155.5 oE
near Zunil and Dilly craters
in Amazonian-aged volcanic terrain
Crater diameter: 5.7 km

Ray Length: 86 km
Faint rays.

Crater B
nTIR of Crater B
Crater Location 15.5 oN, 159.2 oE
near Zunil and Dilly craters
in Amazonian-aged volcanic terrain
Crater diameter:1.5 km

Ray Length: 42 km
Faint rays.

    Table Notes: The four new rayed craters have International
    Astronomical Union approved names of Gratteri, Tomini, Zumba, and
    Dilly named after small towns or villages in Italy, Indonesia,
    Ecuador, and Mali, respectively. The three crater names in italics
    are probable rayed craters. Probable rays were identified by
    Tornabene and coauthors after they used digital image processing to
    stretch the contrast on the nighttime thermal infrared images. The
    technique of contrast stretching reveals the subtle tonal
    differences in the images that represent faint temperature
    signatures of the rays. The white boxes in some of the images refer
    to close-up images in Tornabene and coauthor's published research
    article.

The researchers found that Martian rays, like their lunar counterparts,
are comprised of densely concentrated and clustered chains of small
secondary impact craters (tens to hundreds of meters in diameter),
ejecta, and surge deposits from the primary and secondary craters. Their
detailed work shows that the rays have two characteristic thermal
signatures: colder areas and warmer small spots. The different
signatures are attributed to different particle sizes and induration (or
hardness) of the materials. The colder areas (darker streaks in the
THEMIS nighttime thermal infrared images shown in the table) are
interpreted to be fine-grained, loosely-packed debris. This material
cools quickly after sunset. On the other hand, the warmer (brighter)
spots are interpreted to be fresh secondary craters (formed from ejecta
blocks originating from the source crater), which excavated courser,
rocky materials upon re-impacting the surface. Rocks or indurated
sediments cool more slowly and consequently appear warmer (brighter)
than their surroundings at night. A variety of other Mars data sets
(such as THEMIS visual, Mars Orbiter Camera (MOC) narrow-angle images,
TES-dervied thermal inertia, albedo, and dust cover maps) were compared
to the THEMIS thermal infrared data to corroborate the interpretations
made by Tornabene and his colleagues.

The research team identified rayed craters in volcanic plains that are
specifically characterized by intermediate values of albedo, thermal
inertia, and dust cover index (previously defined as "thermophysical
Unit C" by Michael Mellon, University of Colorado, Boulder, and
colleagues). Only about 20% of the Martian surface appears to have this
optimal combination of thermophysical properties needed to recognize
crater rays. This suggests that other rayed craters may be present on
Mars, but cannot be readily detected by means of THEMIS thermal infrared
images. For example, if ray debris lies on top of surfaces that have
dark nighttime TIR signatures, such as dust-mantled surfaces, then the
rays are not discernable because there is no thermal contrast. Hence, a
surface covered with a thick dust mantle obstructs our view of crater
rays. Conversely, regions with little to no dust cover would not be able
to produce the cold ray material that we so readily observe in THEMIS
nighttime thermal infrared images. As a consequence, other (more
rigorous and difficult) means may be necessary to detect additional
rayed crater systems on Mars such as linking far-field secondary crater
populations to a single source crater. High-resolution visible images
from cameras like HiRISE (High Resolution Imaging Science Experiment) or
CTX (Context Camera) on NASA's Mars Reconnaissance Orbiter
may be very useful in future ray surveys.

------------------------------------------------------------------------
Ray Patterns and Oblique Impacts

Rays are evidence of high-velocity ejecta. The fact that Martian rayed
craters are among the freshest craters of their size and are found in
young volcanic plains makes some people wonder if some of the ejecta
from these impacts could have escaped from Mars to become Martian
meteorites on Earth. The Mars rock would have to reach the escape
velocity of 5 km/sec. During an impact event the kinetic energy of the
incoming projectile causes shock deformation, heating, melting, and
vaporization, as well as excavation of the crater and ejecta material.
However, Martian meteorites show low to only moderate degrees of shock.
To address this question Tornabene and coauthors examined the ray
patterns to better understand the formation process.

Elliptical crater shapes, forbidden zones (wedge-shaped zones lacking
crater ejecta), and "butterfly wing" ray patterns in four definitive
rayed craters (the exception is Zunil) and all three probables are cited
as evidence that the Martian rayed craters formed by moderately oblique
impact events. During oblique impacts it is possible that some of the
ejecta is released at high velocities but low shock pressures. This is a
process known as spallation and it is likely responsible for creating
some of the ray-forming secondaries. Spallation is also currently the
favored mechanism for ejecting relatively low-shocked rocks off Mars.
Models also show increases in spallation volumes when oblique impacts
strike volatile-rich subsurfaces. This is significant because Tornabene
and colleagues observed fluidized ejecta blankets around the primary
craters or around nearby larger craters--commonly recognized as evidence
for subsurface volatiles (water ice). They concluded the ray formation
process is consistent with spallation models of Martian meteorite delivery.

------------------------------------------------------------------------
Launch Sites for meteorites from Mars

Cosmochemical analyses show that Martian meteorites came from basaltic
igneous sources (by crystallization from cooling magma) with young (<1.3
billion year) crystallization ages (with the one exception of 4.5
billion year old ALH84001) and were blasted off Mars by impact cratering
events 600,000 to 20 million years ago. Taking into account the
uncertainties in ages derived by crater counting for Martian terrains
(e.g. see review by William Hartmann, Planetary Science Institute,
Tuscon), Tornabene and coauthors have suggested matches between certain
rayed craters (based on surface ages) and the crystallization age
groupings of the Martian meteorites. Based on their studies, Tornabene
and colleagues suggest rayed craters within Elysium are possible sources
for the shergottites and the two rayed
craters outside of Elysium (Zumba and Gratteri) are possible sources for
nakhlites, chassignite, and ALH 84001.
Specifically, Zumba is in late Hesperian-age volcanic terrain and could
be a source crater for the nakhlites and chassignites, which are about
1.3 billion years old. Gratteri, which is in older volcanic terrain
(Noachian-age), is suggested as a possible source for the oldest Martian
meteorite known, ALH 84001.

As the research by Tornabene and coauthors shows, observations of the
rayed craters and target surfaces combined with current models of
Martian meteorite delivery and cosmochemical analyses of Martian
meteorites suggest these large rayed craters are plausible source
regions for Martian meteorites. Finding meteorite source regions will
continue to pique our interest as researchers look further into the
spectral signatures of the surfaces where rayed craters have been
identified to help define and compare the compositions to the only field
samples we have.

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

ADDITIONAL RESOURCES

    * Christensen, P. R., N. S. Gorelick, G. L. Mehall, and K. C.
      Murray, THEMIS Public Data Releases, Planetary Data System node,
      Arizona State University. http://themis-data.asu.edu.
    * Hamilton, V. E. (2004) Detailed investigation of a globally
      unique, orthopyroxene-rich deposit in Eos Chasma, Mars, Eos Trans.
      AGU, 85, Fall Meet. Suppl., Abstract P11A-0959.
    * Hartmann, W. K. (2005) Martian cratering 8: Isochron refinement
      and the chronology of Mars, Icarus, v. 174, p. 294-320.
    * Harvey, R. P. and V. E. Hamilton (2005) Syrtis Major as the source
      region of the Nakhlite/Chassigny group of Martian meteorites:
      Implications for the geological history of Mars, Lunar Planet.
      Sci., XXXVI, Abstract #1019, Houston (CD-ROM).
    * Mars Meteorites, comprehensive site from Ron Baalke, Jet
      Propulsion Lab.http://www2.jpl.nasa.gov/snc/.
    * Martel, L. M. V. (2002) Searching Antarctic Ice for Meteorites.
      Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/Feb02/meteoriteSearch.html.
    * McEwen, A. S., E. Turtle, D. Burr, M. Milazzo, P. Lanagan, P.
      Christensen, J. Boyce, and the THEMIS Science Team (2003),
      Discovery of a large rayed crater on Mars: Implications for recent
      volcanic and fluvial activity and the origin of Martian
      Meteorites, abstract, 34th Lunar and Planetary Science Conference,
      Lunar Planet. Inst., Houston, TX.
    * McEwen, A. S., B. S. Preblich, E. P. Turtle, N. A. Artemieva, M.
      P. Golombek, M. Hurst, R. L. Kirk, D. M. Burr, and P. R.
      Christensen (2005), The Rayed Crater Zunil and Interpretations of
      Small Impact Craters on Mars, Icarus, v. 176, p. 351??? 381.
    * McSween Jr., H. Y. (1999) Meteorites and Their Parent Planets. 2nd
      Edition. Cambridge University Press, 310 p.
    * Mellon, M. T., B. M. Jakosky, H. H. Kieffer, P. R. Christensen
      (2000) High-reolution thermal inertia mapping from the Mars Global
      Surveyor Thermal Emission Spectrometer, Icarus, v. 148, p. 437-455.
    * Taylor, G. J. (2005) Martian meteorites record surface
      temperatures on Mars. Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/July05/Mars_paleotemp.html.
    * THEMIS Feature Image: Gratteri Crater's Far-Flung Rays
      <http://themis.asu.edu/features/gratteri>.
Received on Mon 29 Jan 2007 03:08:43 PM PST