Linda Rowan

Touch and Go

### Contents #### Research Articles and Reports The Spirit Rovers Athena Science Investigation at Gusev Crater, Mars S. W. Squyres et al. [Pancam Multispectral Imaging Results from the Spirit Rover at Gusev Crater][1] J. F. Bell II et al. [Surficial Deposits at Gusev Crater Along Spirit Rover Traverses][2] J. A. Grant et al. [Wind-Related Processes Detected by the Spirit Rover at Gusev Crater, Mars][3] R. Greeley et al. Spirit at Gusev Crater: Plates Localization and Physical Properties Experiments Conducted by Spirit at Gusev Crater R. E. Arvidson et al. Textures of the Soils and Rocks at Gusev Crater from Spirits Microscopic Imager K. E. Herkenhoff et al. [Magnetic Properties Experiments on the Mars Exploration Rover Spirit at Gusev Crater][4] P. Bertelsen et al. Chemistry of Rocks and Soils in Gusev Crater from the Alpha Particle X-ray Spectrometer R. Gellert et al. Minerology at Gusev Crater from the Mossbauer Spectrometer on the Spirit Rover R. V. Morris et al. Initial Results from the Mini-TES Experiment in Gusev Crater from the Spirit Rover P. R. Christensen et al. [Basaltic Rocks Analyzed by the Spirit Rover in Gusev Crater][5] H. Y. McSween et al. See related [News story][6]. T he escapades of the Mars Exploration Rover Spirit have given new meaning to the expression “touch and go.” Spirit landed on the floor of Gusev crater on 4 January 2004 universal time. This special issue presents the research results of the first 90 sols (martian solar days). Although the entry, descent, and landing of Spirit were perilous, science operations began soon after, when the mast of the rover popped up and the two instruments on the mast, the panoramic camera (Pancam) and the miniature Thermal Emission Spectrometer (mini-TES) scoped out the morphology and gross mineralogy of the landing site. Pancam and mini-TES provided scouting reports and detailed investigations throughout the 90 sols. On sol 12, the fully unfolded and fully loaded rover left its landing shell to begin unprecedented mobile exploration. Besides the mast, with two instruments and a suite of magnets to analyze magnetic particles, Spirit has a robotic arm with a microscopic imager to view textural details, an alpha-particle x-ray spectrometer to measure elemental abundances, a Mossbauer spectrometer to distinguish iron-bearing minerals, and a Rock Abrasion Tool (RAT). Spirit drove about 600 meters from the Columbia Memorial Station to the rim of Bonneville crater in 90 sols. Along the way, the rover performed quick analyses, called touch-and-go operations, in which the instruments on the arm touched and analyzed a feature, but no brushing or grinding was done. At more interesting features such as the rocks Adirondack, Humphrey, and Mazatzal, Spirit stayed longer, using the RAT and the other instruments to acquire more data. Exploration has changed since the era of nautical exploration by large sailing vessels, when “touch and go” is thought to have originated to describe a ships keel touching the seafloor briefly but not getting stuck. Certainly Spirits exploration has been successful, as the following 11 papers describe, taking some of the peril and fear out of touch-and-go operations, although robotic space exploration remains difficult and human space exploration remains distant. Spirit is still going strong, extending its mission to over 200 sols and setting the distance record driven by a rover on another planet to over 3000 meters as of 28 July. There is more to come. You go girl! [1]: /lookup/doi/10.1126/science.1100175 [2]: /lookup/doi/10.1126/science.1099849 [3]: /lookup/doi/10.1126/science.1100108 [4]: /lookup/doi/10.1126/science.1100112 [5]: /lookup/doi/10.1126/science.3050842 [6]: /lookup/doi/10.1126/science.305.5685.770

GEOCHEMISTRY: Exotic Extraterrestrial Carbon

GEOCHEMISTRY Graphite is a relatively rare mineral phase in meteorites. Some of the graphite found in some of the most primitive meteorites, the chondrites, originated from other stars. Semenenko et al. describe seven unusual graphite-bearing xenoliths found within the Krymka chondrite. The

A Three-Body Solution

ASTROPHYSICS In the center of the Milky Way lies a supermassive black hole called Sagittarius A* (Sgr A*). This black hole (inside the central white patch) has been detected by means of emissions just beyond its event horizon and through the motion of very close and very young massive stars. About 10 stars orbit the black hole within 0.04 parsecs and about 40 orbit within 0.1 parsecs, yet no model has been able to explain how these stars can be so close, so young, so massive, and in such relatively stable orbits. Alexander and Livio propose a three-body dynamical interaction model to knock these stars into their orbits, like billiard balls. The young stars form far from Sgr A* and are scattered into eccentric orbits by the massive black hole. Over time, these stars cross paths with stellar mass black holes (SBHs) that are clustered near Sgr A*. Every so often, a star, a SBH, and Sgr A* undergo a three-body exchange, in which the SBH is ejected from the system and the star gets captured in a tight orbit around Sgr A*. Although the mass and the size of Sgr A* are being defined with increasing precision (see Bower et al. , this issue, p. [704][1]), the number and distribution of SBHs are less certain, so how broadly this ingenious model applies will depend on improved specification of the SBHs. — LR Astrophys. J. 606 , L21 (2004). [1]: /lookup/doi/10.1126/science.1094023

ASTROPHYSICS: A Planetary System Is Born?

ASTROPHYSICS Herbig Ae/Be stars are intermediate-mass, pre-main sequence stars, and some have circumstellar disks in which extrasolar planets and planetesimal precursors may lurk. The very young Herbig Be star LkHα234 is embedded in a nebula within the NGC 7129 star formation region. Five spectra taken by Chakraborty et al. over 33 days show large and abrupt variations in the sodium absorption lines of LkHα234. The authors infer that an asteroid-sized solid object (about 100 km in diameter) was in the process of falling from the disk onto the stellar surface and disintegrated about 0.1 to 2 AU above the star. This infall may be just one among many in a very young planetary system where planetesimals either accrete to form larger planets, fall into the star, get ejected from the system, or get pushed to the edge of the system, like comets in the solar system. — LR Astrophys. J. 606 , L69 (2004).

PLANETARY SCIENCE: Cosmic Crystal Kinetics

PLANETARY SCIENCE Fragments of extraterrestrial space debris experience different degrees of heating in the atmosphere. Spinel crystals with different compositions and shapes are ubiquitous products found in most space debris. Spinel grains can rapidly crystallize at high temperatures from space debris that melts on its way through the atmosphere. Toppani and Libourel produced about 300 different spinels in pulse heating experiments with samples of the Orgueil meteorite and compared these synthetic products with over 130 debris particles. Using the Al2O3 content and FeO/Fe2O3 ratio of the spinels and the composition of the atmosphere, they estimated the entry velocity, angle of entry, duration of the fall, and the altitude at which the spinel formed. From this thermal history they can potentially eliminate the atmospheric effects to estimate the pristine extraterrestrial condition of space debris. — LR Geochim. Cosmochim. Acta 67 , 4621 (2003).

ASTROPHYSICS: To an Instability and Beyond

ASTROPHYSICS Just like Buzz Lightyear speeding through the atmosphere in his own bubble, the Sun creates a Buzz-like bubble of supersonic solar wind that rams into the interstellar medium. This tapering bubble is called the heliosphere and essentially defines the edge of the solar system. The gas

GEOPHYSICS: Subduction and Heat Flow

GEOPHYSICS Quantitating the heat flux at the core-mantle boundary is important for understanding the cooling of the core, the convection of the mantle, and the movement of tectonic plates. This heat flux has been estimated indirectly as the heat flowing out at volcanic hotspots, about 2 terawatts. Labrosse has developed a refined numerical model that includes convection between isothermal layers and internal heating in the mantle. The simulations show that most of the hot plumes upwelling from the boundary do not make it all the way to the surface, so that looking only at hotspots (generated by the hot plumes) will underestimate the total heat flow from the core into the mantle. It appears that the downward flow of cold plumes from the surface toward the core, such as occurs at subduction zones, dominates the permeation of rising plumes and the heat flux across the core-mantle boundary, which is estimated from these simulations to total 6 terawatts. — LR Earth Planet. Sci. Lett. 199 , 147 (2002).

Three Strikes and You're Out

GEOPHYSICS A large bolide hit the Yucatan Peninsula of Mexico about 65 million years ago, as revealed by the Chicxulub impact crater and the layers of debris at the Cretaceous-Tertiary (K-T) stratigraphic boundary in North America, Europe, Africa, New Zealand, and the Pacific and Atlantic Oceans.

ASTROPHYSICS: Floating Through a Cluster

ASTROPHYSICS Candidate extrasolar planets have been found in some unexpected places, principally as isolated objects (free-floaters) in globular clusters. Free-floaters are expected to be rare because the standard planet formation model requires that planets form in disks around stars. Furthermore,

Hydrocarbons in the Solar Nebula

GEOCHEMISTRY Recent studies of interplanetary dust particles and primitive meteorites have identified hydrocarbons in the samples; sometimes these phases were associated with iron and nickel metal. These organic materials must have formed in the solar nebula, but how they did has been unclear. The solar nebula (the flattened disk of debris left over from the formation of the sun) contained abundant gases—carbon monoxide and hydrogen—and dust grains—iron metal and refractory oxide phases—which swirled around the sun and either aggregated into planetesimals or were removed from the disk. Fischer-Tropsch catalysis is the conversion of CO and H2 into simple hydrocarbons by an iron or nickel metal catalyst, and Kress and Tielens have modeled this known surface chemical reaction to try to determine the origin of simple hydrocarbons in the solar nebula. They found that Fischer-Tropsch catalysis could have served to convert CO into CH4 at the time of planetesimal formation. At earlier times, the temperatures would have been too high, and at later times the temperatures too low, for catalysis to be efficient. Thus, their kinetic model offers a plausible route for hydrocarbon formation in the solar nebula and may help provide clues to organic material origins in other astrophysical environments. — LR Meteorit. Planet. Sci. 36 , 75 (2001).