|
SLIDES & TRANSCRIPT: EXPLORATION GEOPHYSICS ON MARS:
A TALE OF MINERALS AND WATER |
EXPLORATION GEOPHYSICS ON MARS: A TALE OF MINERALS AND WATERMichael PuruckerRaytheon ITSS at Geodynamics Branch, GSFC/NASA, USA. E-mail: purucker@geomag.gsfc.nasa.gov 
|
Abstract |
To date, the exploration of Mars has identified widely distributed concentrations of water (as ice) in the upper few meters. Concentrations of specular hematite, likely within a 600 m thick layered sequence, have been identified in the Terra Meridiani region near the present Martian equator. We suggest here the presence of deep concentrations of iron in the Terra Cimmeria and Sirenum regions, and the possible presence of near surface Cu-Fe sulfides in the Apollinaris Patera region.
Note to the reader. As an aid I have collected all the large pictures into one zip-file, so that the plots can be downloaded and printed in one step, which is much easier than having to click-and-save all the illustrations from the web page. Download large plots (2.8 Mb). |
Slide 1: Title and Acknowledgments |
Good afternoon, Ladies and Gentleman. I'd like to thank Pat Millegan for inviting me today.
For stimulating discussions concerning this topic, I'd like to acknowledge Tiku Ravat and Dave Clark.
Fugro Airborne Surveys, Gedco, LaCoste and Romberg, and Bird Geophysical
helped to defray the costs of my travel here. This research,
and that of my students, was made possible with support from NASA, and the French and
Danish national space agencies.
|
Slide 2: Outline and Goals |
The goals of this talk are two-fold: to review what our sensors have told us about
possible mineral resources on Mars, and to prompt you to think about promising
mineral exploration techniques that might be applied to Mars.
|
Slide 3: Mars Exploration Status |
The US currently has two orbiter missions (Mars Global Surveyor, sometimes
called MGS, and Mars Odyssey remotely sensing the Mars
environment.
MGS arrived in 1997, and most of its instruments are still functioning flawlessly five years later. It carries five experiments: a magnetometer/electron reflectometer, a camera, a thermal emission spectrometer, a laser altimeter, and gravity field experiments. Mars Odyssey arrived last October. It carries a gamma ray (and neutron) spectrometer, a thermal emission imaging experiment, and a radiation environment experiment.
|
Slide 4: Mars Odyssey Experiments |
The gamma-ray spectrometer measures gamma rays coming from the surface of Mars.
Individual emission lines are characterstic of certain elements, and the
strength of those emission lines is a function of the abundance of those elements.
Associated with the gamma-ray instrument are two instruments which directly
detect neutrons. Those neutrons are sensitive to hydrogen in the upper few meters of
the surface, and so are a proxy for the amount of ice water present.
The thermal emission imaging system maps the visible and infrared spectral bands with a resolution of 20-100 meters at the surface. Nine spectral bands are scanned in the infra-red. This imaging system will lead to better knowledge of the mineralogy of the surface layers. The Martian radiation environment system measures charged particles in the 15 MeV to 500 MeV range, which encompasses the range which is most critical to life. |
Slide 5: Evidence for water at Mars |
This map by Boynton, W.V. et al. (2002), made using the neutron spectrometer component of the gamma-ray instrument,
shows the presence of intermediate-energy neutrons. Hydrogen-enriched soil is shown
in blue, corresponding to a low abundance of epithermal, or intermediate-energy neutrons.
An uncalibrated rainbow scale, not shown here, shows progressively lesser amounts of
hydrogen as you
approach red. The deep blue areas near the south pole may contain up to 50 percent water
ice in the upper one meter. In the equatorial regions, light blue regions probably
indicate the presence of chemically or physically bound H in minerals other than ice.
H in the far north was not visible when this map was made, using data from the first
three months of the mission, because of a cover of carbon dioxide ice.This map
first appeared in the July 5th, 2002 issue of Science.
|
Slide 6: Thermal Emission Imaging system |
The lower images, taken with the thermal emission imaging system, cover parts of the
north wall of Valles Marineris, the deepest known canyon in the solar system. The
image at the lower left is a false color image made using multiple infra-red bands.
The images at lower right are single band IR images. When calibrated, this instrument
will provide insight into the minerals present in these outcrops of the canyon wall
here. This instrument onboard Mars Odyssey is very similar to the
Aster
instrument
onboard Terra, a component of the Earth Observation System.
|
Slide 7: Radiation environment at Mars |
Shown here are dose rates, in millirads/day, for July of this year. Notice that
typical dose rates are a factor of 20 larger than that experienced at the
surface of the earth. Beginning on July 16, and continuing for almost a week,
a large solar particle event bombarded the spacecraft. Because of the low
atmospheric pressure, and the irregular but generally weak magnetic field,
these values will be little attenuated at the surface. The lesson: any humans
will need to be protected against this radiation. While a thin layer of syrofoam
may suffice for many of the solar particle events, high-energy cosmic rays
are a problem that at present has no easy solution.
|
Slide 8: Earth and Mars compared: Topography |
I next want to turn to some comparative planetology, so you'll have an idea of how
some of your prospecting tools might be able to be used at Mars.
Both Earth and Mars have what are termed 'planetary dichotomies', with most of the planetary elevations concentrated at two heights, an upper, older one and a lower, younger one. In the case of Mars, the upper, older elevation is largely confined to the southern hemisphere. In the case of the Earth, the upper, older elevation is confined to the continents. There are caveats. In the case of Mars, there has been a resurfacing event, largely in the northern hemisphere, which has only partially obliterated the older record, recorded in full in the southern hemisphere. The MOLA topographic record recorded by MGS shows clear evidence for an era of earlier cratering in the northern hemisphere, nearly as old as the cratering in the south (Frey et al. 2001). This is analagous to the ability of satellite radar altimeters to sense sea surface heights on earth, yet at the same time revealing unmistakable evidence for the topography of the sea floor some 4 km lower. On Mars, the resurfacing event created a mantle that is probably less than 1 km thick. |
Slide 9: Earth and Mars compared: Magnetics |
Here we're comparing the magnetic fields of lithospheric origin at Mars and the Earth.
On Earth this includes degrees 15-40+, with the main field of core origin dominating
between degrees 1-13. On Mars, there is very little power in those longest wavelengths,
and all of the signal seems to originate in the lithosphere. Notice that the Mars field
of lithospheric origin observed at satellite altitude (400 km) is an order of magnitude
larger than the Earth's field of lithospheric origin. The contrast is probably larger
than is apparent here because the Earth's lithospheric field on the continents is
probably dominated by a magnetization induced by the earth's core field, whereas the
Martian lithospheric field is dominated by a remanent magnetization, because there is
no apparent core field.
|
Slide 10: Comparative magnetic properties |
The Martian magnetic map is interpreted in terms of a primordial magnetic field which shut
off within the first billion years of Martian pre-history (Acuna et al., 1999).
That primordial field magnetized the Martian lithosphere as it cooled through the
Curie temperature(s)of its constituent minerals.
The strength of the magnetic field measured at satellite altitude is a function of the magnetization of the lithosphere times its thickness, and will scale linearly for small changes in those parameters. The magnetization of the lithosphere is a function of the strength of the primordial field, the magnetic mineralogy and petrology, and the bulk iron content of the crust. The magnetization scale will be a function of the orogenic scale. The strength of the primordial field is proportional to the square root of the core fluid density and its rotation rate, and inversely proportional to the square root of the electrical conductivity (Stevenson, 2001). Estimates for some of these parameters are shown in the table here. It is difficult to explain the strength of the Martian field in terms of what we think we know about Mars. It is likely that one or more of these parameters is significantly different from our expectations, or that our model of an exclusively lithospheric magnetization is incorrect. |
Slide 11: Earth and Mars compared: Gravity-1 |
The Martian gravity field, known through Degree 60 globally, and higher locally,
is significantly rougher than that of the Earth's. This reflects the lack of
compensation of Mars' large volcanic features. Estimates of crustal thickness
based on the gravity data, and additional constraints and models, lead some
researchers (Zuber, 2001) to suggest that the crust may be only 30 km
thick beneath the southern highlands where the largest magnetic anomalies are
located. Others (Spohn et al., 2001) suggest a much thicker crust with smaller thickness
variations.
|
Slide 12: Earth and Mars compared: Gravity-2 |
The more subdued terrestrial gravity field, of which the EGM96 model shown here
is representative, has extrema associated with subduction zones.
|
Slide 13: The role of impact cratering |
Looking in more detail at the spatial character of the Martian magnetic field,
you can see the weak magnetic fields associated with the large impact events
at Isidis and Hellas. This has been interpreted to represent a demagnetization
caused by the impact event in the absence of a large planet-wide magnetic field
(Acuna et al., 1999).
Since we infer that both of these impact events occurred within the first few hundred
million years after Mars was formed, the primordial magnetic
field must have been very short-lived. The magnetic map shown here, and in the two
subsequent slides, is a composite of maps by Cain, Arkani-Hamed, and myself.
|
Slide 14: Old terranes lacking large-scale magnetic fields |
There are two large terranes lacking large-scale magnetic fields. Those two terranes
coincide with the 'young' northern lowlands, and with a swath of the southern
highlands shown here. Some of the boundaries can be seen to be abrupt, as for
example near the South pole, and may be tectonically controlled. Although the
absence of large-scale magnetic fields over the large craters Hellas and Argyre
seems reasonable, the absence of those magnetic fields in the intervening area
is puzzling.
|
Slide 15: Are there unrecognized impact or thermal events here? |
One suggestion is that there are unrecognized thermal or impact events in the region
between Hellas and Argyre. Such events might be recognized in the MOLA
topographic data, shown on the left, which so clearly showed evidence of old craters
in the resurfaced northern plains.
|
Slide 16: Power spectra compared |
The figure at left compares the magnetic spectra of Earth and Mars. It illustrates
the dominance of the internal static field of lithospheric origin at Mars, in
contrast to the dominant role played by the core field on the Earth. The solid
lines represent a fit to a shell of random dipolar sources, which at Mars is
calculated to be 46 km below the mean planetary radius. The figure at right also
demonstrates that the 'internal' field is not purely static but depends on the
local time of the data, indicating the existence of ionospheric current systems
below the satellites.
|
Slide 17: Mars' most intense magnetic fields |
The region of most intense magnetic fields at Mars is found in the Cimmeria region of
the southern highlands. The largest crater in this image, Copernicus, is some 300 km
in diameter and dates from Mars' earliest epoch, the Noachian. The magnetic profiles
on the right show that this crater has effectively demagnetized the Martian crust.
Note that the boundary between the positive and negative radial fields (shown in
red and blue, respectively) is a great circle arc, as indicated on the gnomonic
projection in the lower right, consistent with it being a structural boundary.
|
Slide 18: Structural control of Valles Marineris |
The magnetic field measured over Valles Marineris, and to the north over Ganges
Chasma, seems to be truncated by these features, suggestive of structural control
(or the removal of large amounts of magnetic material from the Valley). Valles
Marineris is the deepest canyon in the solar system, with over 6 km of relief
from the rim to the floor of the canyon. For comparison, the Grand Canyon has about
1.5 km of relief.
|
Slide 19: Earth and Mars compared: Bulk chemical composition |
Bulk lithospheric chemical compositions are suggestive of several significant
differences between the planets. The Martian measurements are based on the
Viking and Pathfinder lander measurements, both of which sampled only the
weathered surface layer. Significant enrichments of Iron, Phosphorous, and Sulfur
at Mars are suggested by these meausrements, even after accounting for surficial
weathering.
|
Slide 20: Bulk rock compositions |
Results from the thermal emission spectrometer experiment onboard MGS have been
interpreted in terms of a basaltic composition for the dark regions of the southern
hemisphere. North of the dichotomy boundary, those same results have been interpreted
in terms of a basaltic andesite (Plagioclase feldspar + K-rich volcanic glass). An
alternative interpretation is that altered (in the presence of water) basalt is
dominant north of the dichotomy boundary, consistent with the presence of an ocean
there. However, no unequivocal evidence for shorelines has been detected at the crustal
dichotomy, the expected location.
|
Slide 21: Exploiting local resources at Mars |
Now that we've set the stage with this comparative planetology of Mars and the Earth,
let's go on to the prospects for identifying concentrations of useful resources on Mars.
With payload costs of 1 million US dollars/kilogram, it would seem wise to identify
potential local resources for use in future human or robot exploration and development.
To date, the thermal emission spectrometer on MGS has identified surficial hematite concentrations in Terra Meridiani (Christensen et al., 2000). The area in which the hematite enrichment was found has abundant thin layered deposits, possibly of volcanic ash fall or flow origin (Hynek et al.,2002). The lack of surface water, and the early switch from crustal recycling to a stagnant lid, single plate planet, restricts the number of ore-forming environments that may have been present. Those that exist may be considerably larger because Mars has been through fewer orogenic episodes. Ore-forming enviroments may continue to exist, in the liquid water layer below the cryosphere, and in local hot spots associated with volcanism. |
Slide 22: Fe-rich, highly magnetic ore associations |
Although hematite deposits have been identified in the Terra Meridiani area utilizing
the Thermal Emission Spectrometer onboard MGS, there is no evidence for
strong magnetic fields associated with these deposits. The strong magnetic fields
in the southern hemisphere, particularly in Terra Sirenum and Cimmeria, suggests
the presence of significant amounts of Fe.
Iron is commonly associated with a number of other economically important metals and deposit types, as shown in the figure here. However, the inferred strong concentrations of iron and associated metals in Terra Sirenum and Cimmeria are apparently deeply buried, and so will not be amenable to extraction. |
Slide 23: Resource models for Mars |
I'll briefly discuss two resource models for Mars, for copper and for phosphates.
I've chosen these two because they will be critical for any advanced civilization
requiring electronic communication (copper) and for terra-forming the planet (phosphate
is the rare ingredient of the fertilizer trio). I'll not discuss energy resources,
because Robert Zubrin, a former NASA engineer now with the International Mars Society,
has developed a plan to produce rocket fuel utilizing the Mars atmosphere. Although
this technology has not been demonstrated at Mars, a Mars Scout mission
"Mars Exploration with a Self-Refueling Hopper" has been
proposed to test this concept.
Terrestrial resource models have become increasingly sophisticated, and there are now expert systems that apply Artificial Intelligence concepts to the identification of likely resources. A recent US Geological Survey publication ( DDS-64, USGS Mineral Deposit Models, 2000) is a compilation of 29 previously published terrestrial mineral deposit models and related reports by the USGS. PROSPECTOR is an example of an expert system that has been implemented on computer (in Lisp, for example) and might be used as a template for exploration on Mars that would be conducted by our surrogates. |
Slide 24: Copper-1 |
Copper is commonly associated with sulfur on earth. It is also concentrated during
the igneous cycle in basaltic and gabbroic rocks. There are multiple terrestrial
deposit types recognized, but the richest deposits are likely to be associated
with veins and contact metamorphic deposits, and this deposit type will probably
also be recognized at Mars, in contrast to some of the deposit types in which
liquid water plays a major role.
|
Slide 25: Copper-2 |
The prospecting techniques for Copper used on the Earth should apply equally well
to Mars, when account is taken of the additional crustal conductivity and
dispersion provided by the abundant surficial iron oxides.
|
Slide 26: Copper-3 |
Although we have yet to measure copper at Mars, the higher Fe and S levels found
in the Martian upper crust are encouraging signs for the presence of significant
copper. If we use the magnetic and gravity data at Mars to guide us in the search
for prospective locations in basaltic and gabbroic terrane, we note the presence
of significant gravity and magnetic anomalies associated with the shield volcano
Apollinaris Patera (10 South, 175 East). This is one of the few areas on Mars
in which surface geologic features show a strong relation to the gravity and
magnetics observations. The margins of this shield volcano may have promise for
the presence of veins or contact metamorphic copper sulfide deposits.
|
Slide 27: Phosphate-1 |
Phosphate is the rare component of the terra-forming trio of K, N, and P. On
earth, it occurs largely in minerals of the apatite group. Most of the large
terrestrial deposits are associated with liquid water, or biological activity.
Small, but rich, deposits are found in igneous apatites, commonly in alkaline
rocks.
|
Slide 28: Phosphate-2 |
Although terrestrial prospecting techniques commonly rely on the association of P
with radiometric materials, on Mars it may be easier to target the alkaline host rocks
with a orbital mapping spectrometer.
|
Slide 29: Terrestrial EM spectrum |
EM surveys are an excellent way of identifying both water and metallic resources.
In a review article in JGR-Planets, Grimm(2002), points out that iron ore deposits
could easily be confused with subsurface liquid water on the basis of conductivity.
Proper differentiation may rely on defining the 3-D shape of the conductor. While
liquid water should be an easy target for EM on Mars, the detection of ice will be
difficult unless it is massively segregated.
EM surveys can use either active or passive probes. Utilizing an active probe means bringing a substantial power source from Earth, but active probes are capable of higher resolution. A passive probe would utilize the natural EM spectrum present at Mars. The terrestrial EM spectrum used for passive EM probing is shown here. The Spheric band, associated with lightning, will probably be much weaker at Mars, if it exists at all. Possible sources may be the dust storms which periodically envelop the planet. The geomagnetic band is associated with the interaction of the dynamic Earth with the sun's magnetic field. On Mars, this areo-magnetic band, associated with the interaction of the Sun's magnetic field with the magnetized and unmagnetized planet, is another source of signals. |
Slide 30: Natural sources for EM Exploration |
The interaction of the mini-magnetospheres associated with the large magnetic features
in the southern highlands with the interplanetary magnetic field is another source of
natural signals which can be used to probe the subsurface.
Ionospheric currents, such as those suggested by the work of Nils Olsen (2002) in the figure here, may also be a source with which to probe the subsurface. |
Slide 31: Summary |
SEG's ROLE
SEG is planning a special issue for next summer which will highlight space-related issues such as these. If you'd like to contribute an article you can contact Patrick Millegan or D. Ravat. WHAT CAN YOU DO? I brought along about 50 copies of the best available topographic map of Mars, based on the MOLA (Mars Orbiting Laser Altimeter) experiment onboard Mars Global Surveyor. MOLA continues to operate, in a passive radiometer mode, some six years after arrival at Mars. For those of you who would like to make your own topographic maps of Mars, the digital data is available at resolutions of up to 1/128th of a degree from the Goddard Space Flight Center web site . The Mars Society is an organization devoted to the exploration and colonization of Mars. Finally, remember that it's our children and students who will have to be motivated to continue this long-term endeavor. I brought along with me one of the tools I use to teach about Mars in the schools. It consists of a small Mars globe, several small rare-earth magnets that I've dropped in through the polar holes after applying glue to them, and a 3-D magnaprobe that allows me to find those magnets after they've stuck to the bottom of the globe's crust. The globe can also be used to teach concepts of geography, such as latitude and longitude. The longitude system on Mars extends from 0 to 360 West. Can you guess why? Enjoy. |
Slide 32: References and suggested reading |
Acuna, M.H. and 12 other authors, 1999, Global distribution of crustal magnetization discovered by the Mars Global Surveyor MAG/ER Experiment, Science, 284, 790-793. Arkani-Hamed, J., 2001, A 50-degree spherical harmonic model of the magnetic field of Mars, Journal of Geophysical Research, 106, 23197-23208. Arkani-Hamed, J., 2002, Magnetization of the Martian crust, Journal of Geophysical Research, 107, 10.1029/2001JE001496. Bandfield, J.L., V.E. Hamilton, and P.R. Christensen, 2000, A global view of Maritian surface compositions from MGS-TES, Science,287, 1626-1630. Boynton, W.V., and 24 co-authors, 2002, Distribution of Hydrogen in the near-surface of Mars: Evidence for subsurface Ice deposits, Science, 297, 81-85. Cain, J.C., B.B. Ferguson, and D. Mozzoni, in press, An n=90 internal potential function of the Martian crustal magnetic field , Journal of Geophysical Research and associated electronic supplement Christensen, P.R. et al., 2000, Global mapping of Martian hematite mineral deposits: Remnants of water-driven processes on early Mars, Journal of Geophysical Research. Frey, H.R.,J.H. Roark, K.M. Shockey, E.L. Frey, and S.E.H. Sakimoto, 2001, Ancient Lowlands on Mars, Geophysical Research Letters. Grimm, R.E., 2002, Low-frequency electromagnetic exploration for groundwater on Mars, Journal of Geophysical Research,107, 10.1029/2001JE001504. Hynek, B.M., R.E. Arvidson, and R.J. Phillips, 2002, Geologic setting and origin of Terra Meridiani hematite deposit on Mars, Journal of Geophysical Research, 10.1029/2002JE001891. Lemoine, F.G., D.E. Smith, D.D. Rowlands, M.T. Zuber, G.A. Neumann, D.S. Chinn, and D.E. Pavlis, 2001, An improved solution of the gravity field of Mars (GMM-2B) from Mars Global Surveyor, Journal of Geophysical Research, 2001, 106, 23359-23376. Purucker, M., D. Ravat, T.J. Sabaka, C. Voorhies, and M.H. Acuna,2000, An altitude- normalized magnetic map of Mars and its interpretation, Geophysical Research Letters, 27, 2449-2452. Purucker, M. and D. Clark, 2000, Exploration Geophysics on Mars: Lessons from magnetics, The Leading Edge, May 2000, 484-487. Purucker, M., B. Langlais, and M. Mandea, 2001, Interpretation of a magnetic map of the Valles Marineris region, Mars, Extended abstract from the 32nd Lunar and Planetary Conference, March 12-16, 2001, Houston, Texas Spohn,T. and 9 coauthors, 2001, Geophysical constraints on the evolution of Mars, Space Science Reviews, 96, 231-262. Stevenson, D., 2001, Mars' core and magnetism, Nature, 412, 214-219. Voorhies, C.V., T.J. Sabaka, and M. Purucker, 2002, On magnetic spectra of Earth and Mars, Journal of Geophysical Research, 107, 10.1029/2001JE001534. Wanke, H., J. Bruckner, G. Dreibus, R. Rieder, and I. Ryabchikov, 2001, Chemical composition of rocks and soils at the Pathfinder site, Space Science Reviews, 96, 317-330. Wyatt, M.B. and H.Y. McSween, 2002, Spectral evidence for weathered basalt as an alternative to andesite in the northern lowlands of Mars, Nature, 417, 263-266. Zuber, M., 2001, The crust and mantle of Mars, Nature, 412, 220-227. |
NASA Privacy Statement