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increasing the accuracy of the measurement by increasing the intensity. As individual photons are refl ected from a movable mirror, it will recoil, and the shot noise of individual photon arrivals will lead to a fl uctuating force on the mirror; it will begin to shake ever more violently as the intensity is increased, limiting the gain in accuracy due to increasing intensity. This is the radiation pressure shot noise, and it is the physical mechanism enforcing the Heisenberg uncertainty principle in an optomechanical position transducer. Balancing the effects of shot noise and radiation pressure noise leads to the standard quantum limit for an optomechanical position transducer. Large interferometers designed to detect gravitation waves are not yet at this limit, which would require more laser power than the optical components could stand, yet it will need to be taken into account in the next generation of detectors. Purdy et al. report the fi rst direct demonstration of the radiation pressure shot noise. Their study is a landmark in the emerging fi eld of quantum optomechanics ( 4), where the objective is to coherently control the PLANETARY SCIENCE A Unique Piece of Mars Munir Humayun Following the pioneering Mars Exploration Rovers, NASA’s Curiosity rover is actively exploring the crustal rocks of Mars. Despite the exciting results returned by the rovers, there is no substitute for a hand sample of crustal rock. Because such samples will not be returned to Earth anytime soon, geochemists who want a piece of Mars in their labs must satisfy themselves with martian meteorites ( 1). These comprise a group of igneous rocks with telltale signs of martian alteration products ( 2) and have provided ground truth for the information returned by the rovers. Oddly, however, the hundred or so known martian meteorites are chemically unrepresentative of the martian crust determined by missions ( 3). On page 780 of this issue, Agee et al. ( 4) put an end to this conundrum with the fi nding of a new martian meteorite, Northwest Africa (NWA) 7034, a basaltic breccia unique among Department of Earth, Ocean and Atmospheric Science and National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA. E-mail: humayun@ magnet.fsu.edu quantum motion of a collective vibrational degree of freedom of a mechanical element. In their experiment, the mechanical element is a 7-ng square dielectric membrane placed inside a Fabry-Perot optical interferometer. The bulk mechanical resonance frequency is above 1 MHz with a linewidth less than 1 Hz (see the fi gure). As the membrane moves, it modulates the frequency of the optical resonance and consequently modulates the amplitude and phase of the transmitted light. This motion can be detected with high effi ciency by means of optical detection. Fluctuations of the membrane due to radiation pressure noise appear in the noise power spectrum of the optical signal. In the experiment, two optical modes were used; one with high power, the signal, is the source of the radiation pressure shot noise, while the other, weaker probe beam monitors the displacement of the membrane to detect the radiation pressure noise. The experiment can also be thought of as a quantum nondemolition measurement of the photon number in the strong signal beam using a coupling to the signal beam mediated by the mechanical element. known martian meteorites with respect to age, oxygen isotopes, and petrology. One might wonder whether such a rock should even be construed to be of martian origin. Oxygen isotopes are the meteoritic equivalent of DNA fingerprinting, each unique signature implying a different planetary reservoir ( 5). One of the big surprises in NWA 7034 is that its oxygen isotopes are shifted to greater 16 O depletion, and to heavier isotopic compositions, than those of the SNC (shergottite, nakhlite, and chassignite) martian meteorites. Had it not been for the alertness of Agee et al., NWA 7034 would likely have been classifi ed as a unique achondrite (an asteroidal sample) on the basis of its distinct oxygen isotope composition. However, they showed that the mineral chemistry of pyroxenes from NWA 7034 plot on the distinct FeO versus MnO trend defi ned by other martian meteorites. Another distinguishing characteristic of martian meteorites over achondrites is their young radiometric ages [
PERSPECTIVES 772 Pyroclastic flows ∆ 17 O = +0.6‰ hν *O=C=O *O + C=O *O + O *O 2 3 *O + H O hν H *O + O 3 2 2 2 2 H 2 *O 2 mass-independent isotope fractionations driven by the penetration of ultraviolet light through the thin atmosphere of Mars ( 7, 8). Further, on Earth such processes would be wiped out by plate tectonics, but the absence of subduction on Mars resulted in the preservation of mass-independent isotope anomalies ( 7, 8). Ever since the Viking mission in 1976, it has been known that Mars’ atmosphere is escaping the planet’s gravitational grasp with attendant mass fractionation, but that carbon and oxygen isotopes did not exhibit the extreme mass-dependent fractionation observed in nitrogen. Therefore, a reservoir on Mars must be buffering the C and O isotopes ( 9). Agee et al. now show that the lithosphere may be part of the buffering (at least for oxygen) because the bulk oxygen isotope composition of NWA 7034 has been shifted toward the atmospheric end relative to other martian meteorites (see the fi gure). Incidentally, this is one of the questions Curiosity is designed to tackle on Mars, but Agee et al. may have beaten the rover to the punch line. Upper atmosphere ∆ 17 O ≥ +0.8‰ MgSiO 3 + *O 3 MgSi*O 3 + O 3 Lava flows ∆ 17 O = +0.3‰ Pyroclastic flows? Spirit of exploration. A cartoon of a volcanic pyroclastic fl ow schematically depicting how NWA 7034 may have formed on Mars ( 4) and how photochemical reactions may imprint isotopic signatures in the newly formed rock. (Inset) An image of Gusev Crater viewed by Spirit (PIA02688, 19 February 2006) showing possible pyroclastic deposits. Another interesting aspect of NWA 7034 is its clastic nature, whereby numerous fragments of rock and mineral are bound together. It is unknown whether these clasts all originate from a single (pyroclastic) volcanic eruption, or whether multiple clasts are introduced by impact or some other external agent. Experience with lunar rocks revealed a wealth of information in 2- to 4-mm rock fragments; is this an opportunity to repeat that exercise on Mars? Agee et al. measured 0.6 weight percent water in NWA 7034 with a distinct oxygen isotope composition from the bulk rock, effectively a sample of the martian hydrosphere or permafrost trapped within the matrix of NWA 7034. What are the host minerals? As a rock that appears to have originated at the martian surface, NWA 7034 may contain the elusive hydrous minerals that host water on Mars and their mineralogy might now be determined in the laboratory, with important inputs into directing Curiosity or designing the next generation of Mars probes. 15 FEBRUARY 2013 VOL 339 SCIENCE www.sciencemag.org Published by AAAS And, fi nally, there is macromolecular organic carbon (MMC) recognized in inclusions within feldspar crystals ( 4). It is tempting to wonder whether the volcanic activity associated with the igneous clasts in NWA 7034 provided a warm haven for martian life. If so, this is the place to start a search. Because NWA 7034 is a desert fi nd and not a fresh fall (even though it appears rather fresh by Saharan meteorite standards), an important question is whether the organic matter in NWA 7034 is actually from Mars. This can be settled by measurement of the D/H (deuterium/hydrogen) ratio for the MMC because the martian hydrogen is characterized by a high D/H ratio relative to terrestrial organics ( 10). If an extraterrestrial origin is indeed confirmed, it may yet prove to be meteoritic organics associated with micrometeorite infall on the martian surface. Anyway, the hunt for life on Mars in another meteorite will then be on. If one were to wish for a single martian meteorite, it would be NWA 7034, the fi rst known archetypal crustal rock from Mars. When other such rocks are found, they may help to clarify many remaining questions about the martian surface. For example, has hydrothermal activity occurred on Mars? Have ore mineralizations occurred? Is there evidence of soil (aeolian dust) in the breccia? Is trapped ancient atmosphere (nitrogen, noble gases) present in the amorphous material? Stay tuned for more exciting discoveries. References 1. H. Y. McSween Jr., Meteorit. Planet. Sci. 37, 7 (2002). 2. A. H. Treiman, J. D. Gleason, D. D. Bogard, Planet. Space Sci. 48, 1213 (2000). 3. H. Y. McSween Jr., G. J. Taylor, M. B. Wyatt, Science 324, 736 (2009). UNIVERSITY 4. C. B. Agee et al., Science 339, 780 (2013); 10.1126/science.1228858. 5. R. N. Clayton, T. K. Mayeda, Geochim. Cosmochim. Acta 60, 1999 (1996). 6. T. J. Lapen et al., Science 328, 347 (2010). 7. H. R. Karlsson, R. N. Clayton, E. K. Gibson Jr., T. K. Mayeda, Science 255, 1409 (1992). 8. J. Farquhar, M. H. Thiemens, T. Jackson, Science 280, 1580 (1998). 9. A. O. Nier, M. B. McElroy, Science 194, 1298 (1976). 10. L. A. Leshin, E. Vicenzi, Elements 2, 157 (2006). NASA/JPL-CALTECH/USGS/CORNELL 10.1126/science.1232490 CREDIT: on February 14, 2013 www.sciencemag.org Downloaded from
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