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Cracking a Cosmic Mystery
Seismology offers clues to the interiors of asteroids
By James D. Walker, Ph.D., and Walter F. Huebner, Ph.D.
Suppose your grandparents had a beautiful, shiny, black, rock sphere sitting on their mantle when you were a child. Every time you saw it, you wondered if it were hollow on the inside, but they never let you touch it. Years later, you are going through your parents' attic and find the orb. You hold it up to the light, tap and shake it, but still can't tell what's inside. Can you tell without cutting it open?
While you ask yourself this question, you might wonder how the scientific community would address this problem. There are two basic approaches, similar to your initial attempts: electromagnetic waves that reach inside the orb ("holding it up to the light") or mechanical sound waves that pass through the orb ("tapping and shaking"). Both techniques would work to help you get some idea of what is inside.
Your predicament is similar to the one astronomers and planetary scientists face when they try to determine the internal structure of celestial bodies. Unfortunately, many bodies of interest are pretty big, so it is unlikely that electromagnetic waves would work because the waves are absorbed by large amounts of matter. Thus, sound waves are the technique of choice. Seismology - the study of sound waves in the Earth -- has yielded all the information we have about the Earth's internal structure.
The Threat of Impact
SwRI scientists are very interested in learning more about comets and asteroids and are evaluating the use of seismology as an investigative tool. Initial work to quantify a seismology mission to an asteroid was performed through SwRI's internal research program and is now being funded by NASA.
The internal structure of nearby asteroids and comets could have direct relevance to life on Earth. How? Many believe that an asteroid or comet could hit Earth, just as the pieces of comet Shoemaker-Levy 9 hit Jupiter in 1994. The largest piece of the comet was estimated to be one to two kilometers across. It impacted at 60 km/s (135,000 mph). Soot impact marks in the clouds of Jupiter were the diameter of Earth.
One kilometer is not large as far as near-Earth objects (NEOs) go. Currently some 700 objects have been identified in near-Earth space that are one kilometer across or larger. Families of asteroids, such as the Atens and Apollos, cross Earth's orbit and have the potential, at some time, to pass very close to us. Because of this, there is considerable concern that such a body could hit Earth. Based on Jupiter's experience with impacts, there is reason to be concerned. The U.S. Congress has mandated that 90 percent of NEOs with diameters of one kilometer or greater be found and catalogued by 2008.
Suppose an object is on a collision course for Earth. What can be done about it? The two general ideas are either to move it (change its orbit) or to break it into pieces and spread the pieces apart. To do either requires knowledge of the composition and structure of the object. It is straightforward to push a solid rock - just attach some rocket engines to it. But how do you push a massive piece of cotton candy? If an asteroid is a huge pile of rocks, the best bet may be to blow the rocks apart so that the pieces drift away from each other, some missing Earth and others spreading out enough to avoid causing great damage when they strike. But to accomplish this, the right tools and the right plan are needed.
Scientists at Southwest Research Institute (SwRI) are focusing on characterizing the interior structure and composition of NEOs through seismology, addressing three key topics: how to create the mechanical seismic waves, how to measure the waves after they have traversed the body and how to interpret what is measured.
Internal Structure of Asteroids and Comets
Very little is known about the internal structure of comets and asteroids. Asteroids are thought to be mostly rock. Most meteorites are thought to come from asteroids, and the meteorites recovered on Earth are all rocky: carbonaceous, stony or nickel-iron in composition. An asteroid's density can range quite a lot, from that of water to that of iron.
What is an asteroid made of and what is its density? It is possible to estimate the mass of an asteroid by flying past it, and to get a very accurate value for the mass by going into orbit around it (some asteroids come in pairs, so we can estimate their mass by the time it takes for them to orbit each other). So far, a spacecraft has orbited only one asteroid: the near-Earth asteroid 433 Eros. Eros is potato shaped, roughly 33 by 13 by 13 km (20.5 by 8 by 8 miles). Based on the orbital velocity, the mass of the asteroid is 6.7 x 1015 kg and its density is estimated to be 2.7 g/cm3; therefore, Eros is thought to be a stony asteroid.
However, there is still much that isn't known about Eros or other asteroids. Is it a solid piece of rock? Is it a collection of a small number of large pieces? Is it a loose collection of boulders that hold together through a weak gravitational pull but are coated by a layer of dust so that it appears to be one body? These scenarios are all possible. Current thinking leans towards a collection of large pieces.
There is a strong argument that, except for very small asteroids (less than 200 meters across), all asteroids are made of pieces. Some small asteroids rotate fast, but no large asteroids do. The rotational velocities of objects greater than 200 meters across appear to be small enough that gravity, minute though it is for these bodies, is sufficient to hold the object together. If there were large asteroids that were one large solid piece of rock, one would think that at least a few of them would be rotating fast, but none has been observed.
Comet nuclei are composed mostly of ice and dust. Their densities could be quite low (less than that of water) and their structure could range from that of a porous, fluffy snowball to a solid piece of ice.
Seismology in the Cosmos
Seismology has been successfully used in the past on two bodies beyond Earth: the moon and the Sun. (In 1976, the Viking landers carried seismometers to Mars, but there were equipment and interpretation difficulties.)
Seismology studies from the Apollo missions were very successful. Passive seismometers were placed on the moon on four Apollo missions - 12, 14, 15 and 16 - and active seismographic experiments were conducted on Apollo 14, 16 and 17.
There are two sources of human-made seismic signals: explosives and impactors. The Apollo missions used both. The impactors were the Saturn IVB upper-stage rocket body and the Lunar Module ascent stage. By the end of the Apollo program, nine spacecraft impacts had been recorded seismically. In addition to the impacts, three Apollo missions included direct active seismological experiments using explosives as a source. Apollo 14 and 16 carried an extremely successful handheld thumper the astronauts placed on the lunar surface to produce small disturbances and obtain sound speeds through the lunar surface's regolith (soil). Both missions carried mortar packages - each containing four grenades - that were placed on the moon, though they were not used on Apollo 14. Also extremely successful was Apollo 17's active seismology experiment involving the placement of explosives by the astronauts.
Helioseismology is performed through optical observation of the surface of the Sun rather than the mechanical seismometers that were used on the moon and would be used on a comet or asteroid. The Sun is constantly churning, producing enough noise that the globe continuously rings. Many frequencies at which it naturally oscillates ("normal modes") have been determined.
Seismic Sources: Experiments and Computations
Nature produces seismic waves for studies of the Earth and moon through earthquakes and moonquakes. Human-made seismic sources are explosives and impactors. To determine what to use, it is important to quantify impactors and explosives as a seismic source - that is, how much of a signal is produced by a given charge and how much is produced by an impact?
SwRI researchers developed a technique to directly compare the loads produced by impactors and explosives in different configurations. The technique uses what seismologists refer to as the seismic moment tensor to quantify the outward radial load generated in the plane of the surface, and the downward momentum into the body to quantify the downward load produced at the surface.
In SwRI studies, direct comparisons were performed between impactors and explosives. In particular, the role of explosive placement was studied, such as how much is gained by placing the explosive source into the surface rather than just sitting on top. Impactor speed also was studied. In addition to this, researchers experimented with explosives to confirm the computational results, with the explosive placed at various depths into the surface.
Embedding the explosive increased the downward momentum by 40 percent, but more strikingly, it increased the radial moment term by a factor of 5. Thus, there are large gains made relative to the seismic load by placing the charge deeper within the target. Unfortunately, remotely placing a charge at depth requires additional equipment. Thus, for a space mission there are tradeoffs to be studied.
Also, it was shown that often it is possible to design an impactor that produces a seismic pulse indistinguishable from a given explosive configuration. Thus, impactors are a quantifiable seismic source. Again, it comes down to mission tradeoffs as to which will be used.
The cost for either explosives or impactors as seismic sources is the cost of the space mission to move material to the NEO site. The explosives must be soft-landed. Because of the soft landing, the explosive location would be precisely known. Knowing the exact location of the source aids in solving the inverse problem of determining wave speeds and their domains within the asteroid or comet body. Impacting projectiles could be at any obtainable velocity. However, at higher velocities there is more uncertainty as to the impact point, so guidance and navigation of the impactor become of prime importance.
From a mission point of view, it takes time to deploy the seismometers, and it is desirable to have a large amount of time between impacts or explosive events so that the asteroid or comet body can be allowed to stop vibrating, reducing background noise and making it much easier to identify the seismic motion caused by the source.
Seismic Waves, Ringing Spheres and Eros
Researchers will understand the interior structure of the asteroid through analyzing the arrival of seismic waves at the seismometers after they have passed through the asteroid body and perhaps reflected off the far side, an internal fracture surface or an internal surface where there is a change in material from one side to the other. Seismograms on Earth are studied and analyzed in just this fashion to determine properties and internal structure.
Another approach to the seismic analysis of a small body is to use a charge or impact to excite the normal modes and then examine the frequencies. On Earth it takes a huge earthquake to excite low frequency normal modes. On an asteroid it will be possible to do so with a human-made source. Though it has been demonstrated mathematically that one cannot "hear the shape of a drum," the observed outer geometry combined with determining the vibrational frequencies of a cohesive body will provide considerable information about its interior structure and properties.
SwRI researchers have directly shown that the seismic analysis of the normal modes of an asteroid can reveal details about its internal structure. To verify the technique, researchers applied this approach to a computer simulation of an 11-km rock sphere and compared the vibrational frequencies to those produced by an analytic solution. For the low-frequency modes, the frequencies were within 1 percent of the expected values.
With the technique verified, using NASA funding, SwRI scientists carried out a conceptual seismic study of Eros, a near-Earth asteroid that was visited by the Near Earth Asteroid Rendezvous (NEAR) spacecraft in February 2001. SwRI researchers took surface data geometry from the NEAR mission and constructed a three-dimensional solid model of the asteroid. Two versions of the model were then produced - one where the material was homogeneous throughout the interior and a second that contained a large crack. Eulerian computations were performed of an explosive seismic source on a surface with hypothetical material properties for Eros. The load (tangential moment and downward momentum) was computed and transferred to the Lagrangian code that contained the full Eros model, the seismic computation was computed for Eros with the Lagrangian code and the data from the seismometers were then analyzed to determine the frequencies of the normal modes. For the two different Eros interiors these frequencies were different, showing that seismology can differentiate between an Eros with and without a cracked interior. Thus, seismology can help us understand the internal structure of this asteroid in particular, and asteroids in general.
Most theories regarding comet origins imply that comets are very low-density bodies composed mainly of ice and dust. It is reasonable to wonder whether a seismology approach might yield information for such bodies. SwRI scientists performed a low-density aluminum foam test. The foam was 6 percent the theoretical density, or 0.16 g/cm3. Computations again agreed very well with tests. The computations allowed scientists to determine the size of the signal had solid aluminum rather than foam been tested. Passage through the foam reduced the amplitude of the signal by a factor of 100. This result implies that there is considerable question as to how much signal could be sensed if the comet body was of very low density.
Two questions arise with regard to seismometers for an asteroid or comet. How sensitive must they be? And, how should they be attached to the surface? Essentially, seismometers exist - and are light enough for space missions -- that can detect the small accelerations and ground motions relevant to the seismology problem on a small body with a small source. Readings on an asteroid or comet seismology mission would be of surface motions less than Earth's background noise. Seismometers have been built for space applications with reasonably small masses (less than 0.5 kg or one pound) and volumes with the required sensitivities. Scientists at SwRI are currently looking into MEMs-related seismometers and accelerometers that have been developed for other applications and applying them to seismometry on asteroids and comets.
Of great concern is how to attach the seismometer to the asteroid or comet surface so that the required mechanical measurements can be made. For all seismometers used to date, the attachment has always been with friction and gravity; however, for a small body, its gravity is likely to be insufficient to maintain good enough frictional contact with the surface material to allow measurement of ground motion. Work is ongoing at the Institute to examine attachment methods, such as spikes embedded into the surface or fluids released onto the surface that harden to produce a footprint that is attached to the ground.
Currently, SwRI is participating in two mission proposals. The Deep Interior mission involves traveling to two nearby asteroids to perform a seismological experiment on each. The first asteroid visited will depend on the final flight schedule, but the second will be 4015 Wilson-Harrington, a NEO that is roughly 4 km across.
The second mission proposal is a return mission to Eros. SwRI's contribution would be a seismological experiment including seismometers, seismic sources (probably explosives) and the subsequent analysis of the seismic signals. In particular, scientists hope to learn Eros' interior structure and composition using this experiment. Understanding one near-Earth asteroid will provide a solid foundation for a better understanding of other asteroids and celestial bodies.
And who knows? If someday we see an asteroid or comet heading our way, as Shoemaker-Levy 9 did for Jupiter, then understanding the internal composition and structure of these bodies may be vital to us here on Earth.
Comments about this article? Contact Walker at (210) 522-2051, or firstname.lastname@example.org.
The authors greatly acknowledge Erick Sagebiel of SwRI who assisted with the computations and produced a number of the figures for this story.
Published in the Spring 2004 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.