Transcript Regoliths and Space Weathering

Schedule
• Remember: Papers are due November
4
Regolith
• Regolith: Greek
rhegos (blanket) +
lithos (stone) the
mantle of fragmental
and unconsolidated
rock material that
nearly everywhere
overlies bedrock.
• This includes the soil
of the Earth.
• On rocky objects,
particularly asteroids
and moons, what you
see on the surface is
regolith.
Almost all Solid Objects Have
Regolith
• Asteroids are essentially
all regolith or megaregolith
– Most are likely rubble piles
– Some may include accreted
fragments from impactors
(example Almahata Sitta)
– Only very rapidly spinning
asteroids may be regolithfree
•
•
•
•
Mars
Moon
Mercury
Earth
• During the early phases of
the Apollo Moon landing
program, Tommy Gold of
Cornell raised a concern
that the dust layer at the top
of the regolith would be
very thick (10’s of meters),
low density, and fluffy.
• The worry was that the
lunar landing module (with
the astronauts) would sink
beneath the surface.
• This caused NASA to fly the
a lunar robotic lander
program (Surveyor), at a
cost ~ $3 billion, before
Apollo.
The Lunar Regolith
• We know the most about the Moon, so lets
start there.
• Impacts at all scales form and alter the
regolith.
• Micrometeorites form a fine (<1mm) soil)
• Basin-forming impacts excavate a colossal
amount of material….this is what forms the
surface units of the Moon and other planets.
– Take the Imbrian basin
• 1200 km in diameter
• Ejecta was up to 400 meters thick 600 km from ring edge.
• Fractured the crust down to ~25 km
– With smaller bodies, a single major impact can
resurface the entire object (i.e. Vesta)
•
Regolith “Soil”: The upper layer of
regolith.
– Fine particles, very loose, very fluffy,
created by micrometeorite bombardment.
– About 20 cm deep
– Density about 0.9-1.1 g/cm3. Increases
with depth to about 1.9 g/cm3. Porosity
about 45%.
– The regolith becomes progressively
more compacted with depth.
•
Depth of regolith varies with the age of
the surface
– On the moon Mare has about 4-5 meters,
Highlands about 10 meters
– Overturn is very slow, 7 cm of overturn
can take 109 years.
•
Megaregolith: Deep shattered layer
– The rubble from basin and heavy
bombardment ejecta. About 2 km deep
in highlands
– Structurally disturbed and displaced
crust. Between 2~10 km deep.
– Bedrock fracturing from the impacts.
About 25 km deep.
Terms
Lunar Soils
• Accumulate at a rate of
~1.5 mm/million years
• Dominated by <1 mm
particles
• Mean particle size
between 40 to 130 μm
• Average particle size of
~65 to 70 μm
• Grain density of 3.1
g/cm3
• Bulk density ranges
from 1.45 to 1.79 g/cm3,
depending on depth
Regolith Processes: Comminution
• Comminution: breaking
of rocks and minerals
into smaller particles
– Impacts at all scales
grind down particle size.
– Major impacts produce
ejecta blocks
– Micrometeorites grind
down gravel and blocks
to dust (remember they
impact with an order of
magnitude more force
than a bullet)
Regolith Processes: Agglutination
• Agglutination: welding of mineral
and rock fragments together by
micrometeorite-impact-produced
glass.
• High-velocity impacts produce
enough heating in Lunar soils to
melt material and weld fragments.
• This process is limited to the Moon
(and probably Mercury) since
impact speeds need to be ~10 km/s
• Agglutinates are NOT found in
meteorites….. Average impact
velocities in the asteroid belt are ~
5 km/s. Too low to produce
melting and agglutinates.
Regolith Processes: Agglutination
• Agglutination works against
comminution since it joins
small particles to form bigger
particles.
• This is why Tommy Gold was
proved to be wrong…..
Impact Gardening
• Ejecta is excavated by
the impacts and
spread over the
surface, adding to the
regolith.
• This process mixes
the upper layers of the
regolith, depositing
fresh material on the
surface.
• With impact basins,
the gardening can be
huge…..
Regolith Processes: Comminution
• For asteroids
comminution has an
additional twist
6 meters
Eros
– Low gravity
– Low escape velocity
• As asteroids get smaller
– Low gravity allows
progressively larger
ejecta debris to escape
– Smaller asteroids should
have courser regoith soil
Itokawa
Regolith Processes:
Solar Wind Effects
• Spallation: formation of elements as a result of cosmic ray impacts
that cause protons and neutrons to spall off.
• Implantation: See next slide
• Vaporization: See next slide
• Sputtering: atoms are ejected from a solid target material due to
bombardment of the target by energetic particles. The sputtered
atoms mostly recondense on grain surfaces.
• Charging: Solar ultraviolet and X-ray radiation are energetic
enough to knock electrons out of the lunar soil. Positive charges
build up until the tiniest particles of lunar dust are repelled and
lofted anywhere from m’s to km’s high. Eventually they fall back
toward the surface where the process is repeated. On the night
side, the dust is negatively charged by electrons in the solar wind.
Regolith Processes:
Solar Wind Implantation and Vaporization
•
The elements making up the solar wind are
implanted onto the surfaces and shallow
interiors of the regolith.
– The wind is mostly H and He, so these
dominate
•
•
The buildup of solar wind H can change the
chemistry of the regolith, creating reducing
conditions.
When the regolith is briefly heated by
impacts, the implanted H drives reduction
reactions.
– Iron-rich silicates (olivine and pyroxene) are
converted to reduced iron and iron-poor
ensitite.
– This produces particles of submicron Fe which
when suspended in agglutinate glass is a
powerful reddening agent.
•
Vaporization: Low-temperature phases can
be vaporized during impact and will
recondense on surfaces
Space Weathering
• This term covers the alterations suffered by solid
materials when exposed to the space
environment.
– Crystal damage and spallation from cosmic rays
– Irradiation, implantation, and sputtering from solar wind
particles
– Bombardment and vaporization by different sizes of
meteorites and micrometeorites.
– Or almost any regolith process…..
• The effects of space weathering depend on the
chemistry of the target material. For lunar
materials and ordinary chondrites, one effect is to
darken the material and reddening the spectra.
Space Weathering Agent: Solar Wind
•
•
Bombardment of helium ions
on olivine under vacuum
conditions in the lab
simulates space weathering
of asteroids and other airless
bodies.
The observed effects are:
–
–
–
–
reddening of the spectral slope
slight darkening of the olivine
attenuation of the 1 μm
absorption band
formation of metallic iron in the
outer layer of the mineral
surface in powder and flat slab
PSRDpresents
http://www.psrd.hawaii.edu/Aug09/solarwind.helium.html
Space Weathering Agent: Solar Wind
The more we understand the processes and
timescales of space weathering, the better we can:
• Interpret reflectance spectra
of the surfaces of airless
planetary bodies
• Compare spectra of
meteorites to spectra of
asteroids, to determine
meteorite parent bodies
PSRDpresents
http://www.psrd.hawaii.edu/Aug09/solarwind.helium.html
Our “Type Section” for Space
Weathering has been the Moon
• Lunar weathering
generates nano-phase Fe
(amongst other things) on
the grain surfaces and in
glassy rims.
• This is EXTREMELY
optically active and
produces the characteristic
lunar “red slope” in the
visible and near-IR spectra.
• Another effect is the
darkening of the
reflectance and attenuation
of the absorption bands.
Try a little forward modeling
• But it is not just the Moon that is
exposed to this environment.
• Space weathering can be viewed
as the response to energetic
inputs that drive the surface
composition away from
equilibrium.
– The result are chemical reactions
and evolution that can be
understood from underlying
thermodynamic driving forces.
Look at the Chemistry….
• By using techniques and insights developed
in materials science and physics we can:
– Assess the environment of the common asteroidal
and planetary materials
– Forward model the weathering reactions
– Make testable predictions about processes and
products.
• Start with the idea that key weathering
reactions are driven by:
– The chemical environment of space (hard vacuum,
low fO2, solar wind H, sputtering)
– Thermal energy supplied by micrometeorite
impacts.
– Follow the chemical products……
Take Olivine
• Turns out that the
thermodynamics of olivine
make it very susceptible to
weathering.
• In a reducing environment when
you add energy (heat) olivines
lose oxygen and metallic
cations (producing nano-phase
Fe)
– The mineral becomes more
disordered and less optically
active.
– At the same time, the weathering
product (npFe0) creates a powerful
optical component (i.e. lunar red
slope).
We have done a
few experiments
• Olivine weathering has been
simulated a number of ways.
– Most commonly with laser
zapping.
• We did this by changing the
chemistry and the kinetics
(reducing environment and
warming things up)
• The result is lunar-like
spectra
From T. Kohout et al.,
Icarus 2014
But this is just a
start….
• Turns out that surfaces with
exposed npFe0 are an ideal
environment for catalyzing
further reactions.
• Mineral decompostion (and
the production of catalitic
materials) can be thought of
as the first stage of space
weathering.
• The problem on the Moon is
that there is nothing much
to catalyze.
From T. Kohout et al.,
Icarus 2014
But what if you do have
something else to react with?
• The second stage of weathering
depends upon the presence of
“feedstock” components that can
participate in catalyzed chemical
reactions on exposed surfaces.
• On Volatile-rich Asteroids…..
– Reactive surfaces use the volatiles
coming out of frost-line small bodies
as the feed-stock (CO, H20, NH3) for
catalytic reactions (for example:
Fischer-Tropsch Type (FTT)).
Fischer-Tropsch Reactions?
• Developed in Germany in the 1920s,
this is a catalytic technology to
convert hydrogen and CO into liquid
hydrocarbons.
– The process has widespread industrial
applications, particular in the generation of
liquid fuels from coal or gas.
• (2n + 1) H2 + n CO  CnH(2n+2) + n H2O
• Can use iron, cobalt, and ruthenium
as catalysts.
• Varying pressure and temperature
varies the reaction outputs, typically
long-chained alkanes.
• BUT, if NH3 is in the feedstock FTT
can produce amino acids.
Weathering on Volatile-rich
Carbonaceous Asteroids
• Fischer-Tropsch Type (FTT)
reactions using the heat supplied
by micrometeorite bombardment
would “weather” volatile-rich
surfaces, producing…..
– Long-chain hydrocarbons,
polyaromatic hydrocarbons, amino
acids, various complex organics.
– “Maturity” in this case probably
relates to the abundance and variety of
organics.
• Space weathering on volatile-rich
asteroids produces organics…
– Which is what we see in CI and CM
carbonaceous chondrites.
How does the “weathered”
surface survive?
• This is a surface effect…..how can we end up with
this stuff as part of the interior of a meteorite and
survive atmospheric entry?
• Remember that asteroids are mostly (almost all)
rubble piles.
– They go thru cycles of disruption and reaccretion.
– Impact ejecta buries surface material.
– The stratigraphy gets inverted all the time.
• For example, about 12% of OC falls have solar
wind implanted gases. They are samples of the
regolith.
A few examples
Mifflin (L5)
Fayetteville (H4~6)
Tysnes Island (H4)
Conclusions:
A Theory of Space Weathering
• The basic scheme can be applied to ANY airless body
– Thermodynamics and mineral kinetics are the drivers.
– Outputs depend on the chemistry of the available inputs.
– Decomposition of common rock-forming minerals can create
strongly catalytic regoliths.
• The range of weathering products are not random, but
the predictable (and testable) outcome of the source’s
mineral kinetics and chemical feedstock.
• Weathering products do not have to be optically active.
– Probably most weathering products are spectrally neutral or
even suppress an object’s reflectance spectrum because
decomposition makes the minerals more disordered.
Conclusions:
Volatile-Rich Asteroids
• For volatile-rich asteroids, major weathering
products include a range of carbon-rich compounds
– Weathering in the presence of catalysts transforms
“feedstock” materials into a range of products including
long-chain organics and amino acids.
• The generation of pre-biotic compounds are
probably a routine and predictable by-product of
common space weathering processes.
– The precursors of life are probably abundant in any spaceweathered asteroid belt, in any solar system, and only wait
being accreted to a hospitable environment.
Asteroid, Meteor, Meteorite
In the early morning of October 6, 2008 an
asteroid close to Earth was detected by a
Catalina Sky Survey (CSS) telescope at Mount
Lemmon, Arizona. It entered and broke up in
Earth's atmosphere 19 hours later leaving
behind a luminous train of clouds in the sky
and meteorites on the desert floor.
CSS telescope
discovery images
of asteroid TC3
Train station sign in the Nubian Desert
http://www.psrd.hawaii.edu/April10/AlmahataSitta.html
Asteroid, Meteor, Meteorite
280 meteorite
fragments
(weighing 3.95
kilograms) have
been found.
Almahata Sitta is an anomalous
polymict ureilite: A spectacular
mixture of lithologies giving new
clues to the mineralogy, density,
thermal history, magnetism, and
geologic evolution of its parent
body.
http://www.psrd.hawaii.edu/April10/AlmahataSitta.html
Asteroid, Meteor, Meteorite
http://www.psrd.hawaii.edu/April10/AlmahataSitta.html