Mercury’s Weather-Beaten Surface: Understanding Mercury in the Context of Lunar and Asteroidal Space Weathering Studies

Mercury’s regolith, derived from the crustal bedrock, has been altered by a set of space weathering processes. Before we can interpret crustal composition, it is necessary to understand the nature of these surface alterations. The processes that space weather the surface are the same as those that form Mercury’s exosphere (micrometeoroid flux and solar wind interactions) and are moderated by the local space environment and the presence of a global magnetic field. To comprehend how space weathering acts on Mercury’s regolith, an understanding is needed of how contributing processes act as an interactive system. As no direct information (e.g., from returned samples) is available about how the system of space weathering affects Mercury’s regolith, we use as a basis for comparison the current understanding of these same processes on lunar and asteroidal regoliths as well as laboratory simulations. These comparisons suggest that Mercury’s regolith is overturned more frequently (though the characteristic surface time for a grain is unknown even relative to the lunar case), more than an order of magnitude more melt and vapor per unit time and unit area is produced by impact processes than on the Moon (creating a higher glass content via grain coatings and agglutinates), the degree of surface irradiation is comparable to or greater than that on the Moon, and photon irradiation is up to an order of magnitude greater (creating amorphous grain rims, chemically reducing the upper layers of grains to produce nanometer-scale particles of metallic iron, and depleting surface grains in volatile elements and alkali metals). The processes that chemically reduce the surface and produce nanometer-scale particles on Mercury are suggested to be more effective than similar processes on the Moon. Estimated abundances of nanometer-scale particles can account for Mercury’s dark surface relative to that of the Moon without requiring macroscopic grains of opaque minerals. The presence of nanometer-scale particles may also account for Mercury’s relatively featureless visible–near-infrared reflectance spectra. Characteristics of material returned from asteroid 25143 Itokawa demonstrate that this nanometer-scale material need not be pure iron, raising the possibility that the nanometer-scale material on Mercury may have a composition different from iron metal [such as (Fe,Mg)S]. The expected depletion of volatiles and particularly alkali metals from solar-wind interaction processes are inconsistent with the detection of sodium, potassium, and sulfur within the regolith. One plausible explanation invokes a larger fine fraction (grain size <45 μm) and more radiation-damaged grains than in the lunar surface material to create a regolith that is a more efficient reservoir for these volatiles. By this view the volatile elements detected are present not only within the grain structures, but also as adsorbates within the regolith and deposits on the surfaces of the regolith grains. The comparisons with findings from the Moon and asteroids provide a basis for predicting how compositional modifications induced by space weathering have affected Mercury’s surface composition.     

Structural health management technologies for inflatable/deployable structures: Integrating sensing and self-healing

Inflatable/deployable structures are under consideration as habitats for future Lunar surface science operations. The use of non-traditional structural materials combined with the need to maintain a safe working environment for extended periods in a harsh environment has led to the consideration of an integrated structural health management system for future habitats, to ensure their integrity. This article describes recent efforts to develop prototype sensing technologies and new self-healing materials that address the unique requirements of habitats comprised mainly of soft goods. A new approach to detecting impact damage is discussed, using addressable flexible capacitive sensing elements and thin film electronics in a matrixed array. Also, the use of passive wireless sensor tags for distributed sensing is discussed, wherein the need for on-board power through batteries or hardwired interconnects is eliminated. Finally, the development of a novel, microencapuslated self-healing elastomer with applications for inflatable/deployable habitats is reviewed.     

Synthetic Aperture Radar

The general theory of side-looking synthetic aperture radar systems is developed. A simple circuit-theory model is developed; the geometry of the system determines the nature of the prefilter and the receiver (or processor) is the postfilter. The complex distributed reflectivity density appears as the input, and receiver noise is first considered as the interference which limits performance. Analysis and optimization are carried out for three performance criteria (resolution, signal-to-noise ratio, and least squares estimation of the target field). The optimum synthetic aperture length is derived in terms of the noise level and average transmitted power. Range-Doppler ambiguity limitations and optical processing are discussed briefly. The synthetic aperture concept for rotating target fields is described. It is observed that, for a physical aperture, a side-looking radar, and a rotating target field, the azimuth resolution is ?/? where ? is the change in aspect angle over which the target field is viewed, The effects of phase errors on azimuth resolution are derived in terms of the power density spectrum of the derivative of the phase errors and the performance in the absence of phase errors.     

Output of a Balanced Frequency Discriminator Driven by a Sine Wave and Gaussian Noise

The output of a realizable balanced frequency discriminator is calculated for an input consisting of a sine wave plus Gaussian noise. Explicit autocorrelation and power spectra are found for one practical embodiment of the discriminator for various input carrier-to-noise ratios, with the carrier tuned to center frequency and also off-center. The formulas also permit the calculation of the output for the case of a spectrum of noise slowly swept through the discriminator. Although qualitatively similar to results previously obtained with an ideal discriminator, substantial differences are also found. Measurements are made that closely verified the theoretical results. No limiting is assumed.     

Upper limit of the total magnetic flux in an active region according to the photometric-magnetic dynamical model

The photometric-magnetic dynamical model handles the evolution of an individual sunspot as an autonomous nonlinear, though integrable, dynamical system. One of its consequences is the prediction of an upper limit of the sunspot areas. This upper limit is analytically expressed by the model parameters, while its calculated value is verified by the observational data. In addition, an upper limit for the magnetic strength inside the sunspot is also predicted, and then, we obtain the following significant result: The upper limit of the total magnetic flux in an active region is found to be of about 7.23 × 10²³ Mx, namely, phenomenologically equal to the magnetic flux concentrated in the totality of the granules of the quiet Sun, having a typical maximum magnetic strength of about 12G. Therefore, the magnetic flux concentrated in an active region cannot exceed the magnetic flux concentrated in the photosphere as a whole.     

The effects of composting on the nutritional composition of fibrous bio-regenerative life support systems (BLSS) plant waste residues and its impact on the growth of Nile tilapia (Oreochromis niloticus)

Utilization of bio-regenerative life support systems (BLSS) plant waste residues as a nutritional source by Nile tilapia (Oreochromis niloticus) has proven problematic as a result of high concentrations of fibrous compounds in the plant waste residues. Nutritional improvement of plant waste residues by composting with the oyster mushroom (Pleurotus ostreatus), and the effects on growth and nutrient utilization of Nile tilapia fed such residues were evaluated. Five Nile tilapia (mean weight = 70.9 ± 3.1 g) were stocked in triplicate aquaria and fed one of two experimental diets, cowpea (CP) and composted cowpea (CCP), twice daily for a period of 8 weeks. Composting of cowpea residue resulted in reduced concentrations of nitrogen-free extract, hemi-cellulose and trypsin inhibitor activity, though trypsin inhibitor activity remained high. Composting did not reduce crude fiber, lignin, or cellulose concentrations in the diet. No significant differences (P < 0.05) were observed in weight gain, specific growth rate, survival rate, daily consumption, and food conversion ratio between tilapia fed CP and CCP. These results suggest that P. ostreatus is not a suitable candidate for culture in conjunction with the culture of Nile tilapia. Additional work is needed to determine what, if any, benefit can be obtained from incorporating composted residue as feed for Nile tilapia.     

The Cassini Visual And Infrared Mapping Spectrometer (Vims) Investigation

The Cassini visual and infrared mapping spectrometer (VIMS) investigation is a multidisciplinary study of the Saturnian system. Visual and near-infrared imaging spectroscopy and high-speed spectrophotometry are the observational techniques. The scope of the investigation includes the rings, the surfaces of the icy satellites and Titan, and the atmospheres of Saturn and Titan. In this paper, we will elucidate the major scientific and measurement goals of the investigation, the major characteristics of the Cassini VIMS instrument, the instrument calibration, and operation, and the results of the recent Cassini flybys of Venus and the Earth–Moon system.This revised version was published online in July 2005 with a corrected cover date.     

Optimization of moisture content for wheat seedling germination in a cellulose acetate medium for a space flight experiment.

The Porous Tube Plant Nutrient Delivery System (PTPNDS), a hydrophilic, microporous ceramic tube hydroponic system designed for microgravity, will be tested in a middeck locker of the Space Shuttle. The flight experiment will focus on hardware operation and assess its ability to support seed germination and early seedling growth in microgravity. The water controlling system of the PTPNDS hardware has been successfully tested during the parabolic flight of the KC-135. One challenge to the development of the space flight experiment was to devise a method of holding seeds to the cylindrical porous tube. The seed-holder must provide water and air to the seed, absorb water from the porous tube, withstand sterilization, provide a clear path for shoots and roots to emerge, and be composed of flight qualified materials. In preparation for the flight experiment, a wheat seed-holder has been designed that utilizes a cellulose acetate plug to facilitate imbibition and to hold the wheat seeds in contact with the porous tube in the correct orientation during the vibration of launch and the microgravity environment of orbit. Germination and growth studies with wheat at a range of temperatures showed that optimal moisture was 78% (by weight) in the cellulose acetate seed holders. These and other design considerations are discussed.     

Organizing and managing satellite solar power

Building an organization and management structure to create, launch, utilize and protect a satellite solar power energy system will require a global policy for the beneficial use of SSP. The fundamental organizational tasks are: (1) R&D, achieved through a project organization characterized by the integrated management of applied science, development research and construction engineering; (2) investment, generated by a series of groups creating financial vehicles for public and private investment; (3) transmission and distribution, characterized by attention to an engineering and maintenance process emphasizing high reliability; and (4) crisis response, demanding readiness for instant response to potential internal or external scenarios. A differentiated global organization spanning the long timeframe of SSP will need to have a central management core representative of all parts of the organization, with the capacity for self-renewal and re-adaptation. To be successful over its long timeframe, the SSP organization will need to build continuity and public confidence through intergenerational communication, public education, and community outreach. Integrating structures must be created at all levels of the organization, and should encompass joint work tasks and information-sharing among both industrial and government members. Developmental and alliance partners who support the formation and financing of a differentiated satellite solar power organization will share commensurately in the technologies and competencies that are created.