|
ScienceWeek
PLANETARY SCIENCE: ON THE COMPLEXITIES OF ASTEROIDS
The following points are made by Erik Asphaug (Science 2004 306:1489):
1) Twelve years ago, astronomers obtained the first close look at asteroids, when the Galileo mission en route to Jupiter acquired high-resolution images of Gaspra and Ida [1,2]. Since then, much has changed but little has solidified. Even following a year-long rendezvous by NASA's NEAR Shoemaker orbiter at asteroid Eros [3], asteroid science remains at a crossroads. The surface remote sensing and imaging techniques applied to date have yet to resolve a single fundamental question of asteroid geophysics or chemistry.
2) A detailed new model for asteroid seismology, reported by Richardson et al [4), demonstrates how acoustic reverberations from ]impacts can cause asteroid topography to flatten, diffusing small-scale features and erasing small craters. Like other recent models [5], this work also illustrates how seismological experiments -- akin to those conducted by Apollo astronauts on the Moon -- may soon reveal information about the structure and evolution of comets and asteroids.
3) Asteroids are famously menacing, and the movie script requires them to be tamed or destroyed. The hazard posed by asteroids has focused attention on them, but their essence is the more interesting question. Asteroids are not mere rocks; their own self-gravitation, however minuscule, is central to their evolution. Nor are they planets: Most asteroids are undifferentiated (never melted) precursors to planets, or fragments of these. Others are fragments of differentiated planet precursors that were catastrophically disrupted long ago. Most are very porous, spin rapidly, and are irregular in shape, suggesting a tumultuous history. Contradictory attempts have been made to correlate their visible and infrared colors to the confusing taxonomy of meteorites.
4) As for asteroid geophysics, the most basic terminology is undecided. Conflicting definitions exist for terms such as regolith, rubble pile, contact binary, comet, asteroid, and even planet. (The largest known asteroid, Ceres, a target of NASA's Dawn mission, has a diameter of 930 km and is arguably a planet if Pluto is one).
5) Help with understanding asteroids, when it comes, will be in the form of orbiters that image the inner structure of an asteroid, and landed experiments including sample returns. Seismomechanical probes of the deep interior can also reveal how regolith (planetary soil) behaves under microgravitational conditions. These experiments will not only solve one of the most interesting and important puzzles in planetary science; they will also tell us how to tame any dangerous rogue asteroid.
References (abridged):
1. M. J. S. Belton et al., Science 257, 1647 (1992)
2. M. J. S. Belton et al., Science 265, 1543 (1994)
3. J. Veverka et al., Science 289, 2088 (2000)
4. J. E. Richardson, H. J. Melosh, R. Greenberg, Science 306, 1526 (2004)
5. J. D. Walker, W. F. Huebner, Adv. Space Res. 33, 1564 (2004)
Science http://www.sciencemag.org
--------------------------------
Related Material:
PLANETARY SCIENCE: ON ASTEROID SPACE-WEATHERING
The following points are made by Robert Jedicke et al (Nature 2004 429:275):
1) Asteroid collisions in the main belt eject fragments that may eventually land on Earth as meteorites(1-3). It has therefore been a long-standing puzzle in planetary science that laboratory spectra of the most populous class of meteorite (ordinary chondrites, OC) do not match the remotely observed surface spectra of their presumed (S-complex) asteroidal parent bodies. One of the proposed solutions to this perplexing observation is that "space weathering" modifies the exposed planetary surfaces over time through a variety of processes (such as solar and cosmic ray bombardment, micro-meteorite bombardment, and so on). Space weathering has been observed on lunar samples(4), in Earth-based laboratory experiments(5), and there is good evidence from spacecraft data that the process is active on asteroid surfaces.
2) Neither laboratory nor spacecraft measurements have measured the rate of space weathering on asteroids. In the laboratory it is difficult to calibrate the techniques used in simulating the process (for example, rapid laser heating of crushed OC meteorite samples(5)) with the presumed actual mechanism (micro-meteoroid-induced heating of regolith grains). Estimates of characteristic time scales for asteroidal space weathering based on laboratory results vary from 5 x 10^(4) yr to 10^(8) yr. From the spacecraft's vantage point it is possible to distinguish young from old terrains on the basis of crater counting or other geological morphology (for example, rock slides) but it is difficult to date the surface's absolute ages, because cratering rates on asteroid surfaces vary with time, orbital elements, size and so on. Space weathering on Eros seems to reach maturity on timescales of several tens of millions of years (C. R. Chapman, unpublished work).
3) The authors report they have developed a technique for measuring the rate of space weathering on main-belt asteroids. Their method uses asteroid families that are genetically related groups of objects that were created in the collisional disruption of a larger asteroid. The surfaces of asteroids in a family must all have the same age as they are all "re-set" by the catastrophic disruption that generated the group. When the collision takes place, many large fragments are ejected and they re-accumulate a layer of material on their surfaces that was formerly in the interior of the parent asteroid.
4) In summary: The authors present a measurement of the rate of space weathering on S-complex main-belt asteroids using a relationship between the ages of asteroid families and their colors. Extrapolating this age-color relationship to very young ages yields a good match to the color of freshly cut OC meteorite samples, lending strong support to a genetic relationship between them and the S-complex asteroids.
References (abridged):
1. Vokrouhlick, D. & Farinella, P. Efficient delivery of meteorites to the Earth from a wide range of asteroid parent bodies. Nature 407, 606-608 (2000)
2. Gladman, B. J. et al. Dynamical lifetimes of objects injected into asteroid belt resonances. Science 277, 197-201 (1997)
3. Wisdom, J. Meteorites may follow a chaotic route to earth. Nature 315, 731-733 (1985)
4. Adams, J. B. & McCord, T. B. Alteration of lunar optical properties: Age and composition effects. Science 171, 567-571 (1971)
5. Moroz, L. V., Fisenko, A. V., Semjonova, L. F., Pieters, C. M. & Korotaeva, N. N. Optical effects of regolith processes on S-asteroids as simulated by laser shots on ordinary chondrite and other mafic materials. Icarus 122, 366-382 (1996)
Nature http://www.nature.com/nature
--------------------------------
Related Material:
ASTROPHYSICS: ON THE YARKOVSKY EFFECT
The following points are made by S.R. Chesley et al (Science 2003 302:1739):
1) The Yarkovsky effect is a weak non-gravitational acceleration believed to act on asteroids and meteoroids. According to theory (1-5), absorbed solar radiation is re-emitted in the infrared with some delay, which is related to the thermal inertia of the surface. This delay, in concert with the object's orbital and rotational motion, offsets the direction of the thermal emission and its associated recoil force from the Sun's direction, resulting in a slow but steady drift in the semimajor axis of the object's orbit. Over millions of years, this drift can move main-belt asteroids and meteoroids until they reach a resonance, at which point gravitational perturbations take over and reroute them into the inner solar system (3).
2) The Yarkovsky effect also explains meteorite cosmic-ray exposure ages that are too long for the classical delivery scenarios (3) and the large dispersion of asteroid family members that would otherwise have required unrealistically large collisional ejection velocities. It can also limit the long-term predictability of possibly hazardous close-Earth approaches.
3) The Yarkovsky effect has been detected in the motion of artificial Earth satellites but not for any natural bodies. Vokrouhlick et al (2000) explored the possibility of direct detection by means of the precise determination of near-Earth asteroid (NEA) orbits and concluded that such a detection would be feasible for NEAs up to a few kilometers in size, given precise radar astrometry spanning a decade or more. In particular, they predicted that radar ranging in May 2003 to the asteroid 6489 Golevka (which has a 530-m diameter) would reveal direct evidence for Yarkovsky accelerations.
4) The authors report the outcome of that radar experiment, which confirms Yarkovsky-induced modification of asteroid orbits. Measurements of the distribution of radar echo power in time delay (range) and Doppler frequency (radial velocity) constitute two-dimensional images that can spatially resolve asteroids. The fine fractional precision of radar time-delay measurements and their orthogonality to optical plane-of-sky angular astrometry make them powerful for refining orbits. Radar observations of Golevka were conducted during its close-Earth approaches in 1991, 1995, and 1999. Delay-Doppler measurements were made at Arecibo, PR, and Goldstone, CA, in 1991 and extensively at Goldstone in 1995 (18).
5) In summary: Radar ranging from Arecibo, Puerto Rico, to the 0.5-kilometer near-Earth asteroid 6489 Golevka unambiguously reveals a small nongravitational acceleration caused by the anisotropic thermal emission of absorbed sunlight. The magnitude of this perturbation, known as the Yarkovsky effect, is a function of the asteroid's mass and surface thermal characteristics. Direct detection of the Yarkovsky effect on asteroids will help constrain their physical properties, such as bulk density, and refine their orbital paths. Based on the strength of the detected perturbation, the authors estimate the bulk density of Golevka to be 2.7 grams per cubic centimeter.
References (abridged):
1. D. P. Rubincam, J. Geophys. Res. 100, 1585 (1995)
2. D. P. Rubincam, J. Geophys. Res. 103, 1725 (1998)
3. P. Farinella, D. Vokrouhlick, W. K. Hartmann, Icarus 132, 378 (1998)
4. D. Vokrouhlick, Astron. Astrophys. 335, 1093 (1998)
5. D. Vokrouhlick, Astron. Astrophys. 344, 362 (1999)
Science http://www.sciencemag.org
ScienceWeek http://scienceweek.com
|