Asteroids are the remnants of planet formation, and as such, they represent a record of the physical and chemical conditions in the early solar system and its evolution over the past 4.6 billion years. Asteroids are relatively accessible by spacecraft, and thus may be a source of the raw materials necessary for future human exploration and settlement of space. Those on Earth-crossing orbits pose impact hazards for which mitigation strategies must be developed. For these reasons, several missions to asteroids are in progress or planned with the support of the National Aeronautics and Space Administration (NASA) and other national space agencies. The study of asteroid composition and physical surface properties is vital to both our scientific understanding of the solar system's formation and evolution and to the development of asteroid missions and resource utilization schemes. This dissertation uses infrared spectroscopy to investigate the composition and physical properties of main-belt asteroid surfaces. Our efforts are focused on two populations that are especially relevant to constraining thermal and collisional processes in the asteroid belt: the "M-type" asteroids and primitive asteroid families. To investigate volatiles in the M-type asteroids, we obtained 2-4 micron spectra of six M-type asteroids using NASA's Infrared Telescope Facility. We find spectral signatures of hydrated minerals on all six asteroids, with evidence for rotational variability of hydration in one target. Diversity in the shape of the 3-micron feature in our sampled asteroids suggests there are different modes of hydration in the M-type population. Next, we carried out a thermal and compositional study of M-type asteroid (16) Psyche using 5-14 micron spectra from the Spitzer Space Telescope. Psyche is suspected to be a remnant iron core, and it is the target of an upcoming NASA mission. Using thermophysical modeling, we find that Psyche's surface is smooth and most likely has a thermal inertia of 5-25 J/m^2/K/s^(1/2), and a bolometric emissivity of 0.9, although a scenario with an emissivity of 0.7 and thermal inertia up to 95 J/m^2/K/s^(1/2) is possible if Psyche is somewhat larger than previously determined. From comparisons with laboratory spectra of silicate and meteorite powders, Psyche's emissivity spectrum is consistent with the presence of fine-grained ( < 75 micron) silicates. These silicates may include a magnesian pyroxene component. We conclude that Psyche is likely covered in a fine silicate regolith, which may also contain iron grains, overlying an iron-rich bedrock. Finally, we compared the mid-infrared properties of two primitive asteroids families, ancient Themis (~2.5 Gyr) and young Veritas (~8 Myr). Visible and near-infrared studies show spectral differences between the two families attributed to different degrees of space weathering. To test whether these differences are apparent in the mid-infrared, we analyzed the 5-14 micron Spitzer Space Telescope spectra of 11 Themis-family asteroids and 9 Veritas-family asteroids. We detect a broad 10-micron emission feature, attributed to fine-grained and/or porous silicate regolith, in all 11 Themis-family spectra and six of nine Veritas-family asteroids, with 10-micron spectral contrast ranging from 1% +/- 0.1% to 8.5% +/- 0.9%. Comparison with laboratory spectra of primitive meteorites suggests these asteroids are similar to meteorites with relatively low abundances of phyllosilicates. We used thermal modeling to derive diameters, beaming parameters and albedos for our sample. Asteroids in both families have beaming parameters near unity and geometric albedos in the range 0.031-0.14. Spectral contrast of the 10-micron silicate emission feature is not correlated with asteroid diameter; however, higher 10-micron contrast may be associated with flatter spectral slopes in the near-infrared. There is a slight trend of increasing 10-micron contrast with decreasing albedo in the Veritas asteroids, but not the Themis asteroids. Overall, our results indicate the Themis and Veritas family members show variation in regolith texture and/or structure within both families that is not directly related to family age.

Exposure to ultraviolet (UV) radiation is the single most important risk factor for skin cancers. The incidence and severity of skin cancers are on the rise in most parts of the world including Canada. Melanoma is the most aggressive form of skin cancer with a poor prognosis. It is possible to calculate the approximate time required to develop sunburn based on the skin type of an individual and the UV index of the region of residence. A tool was constructed for this purpose using various web technologies such as PHP and JavaScript. The tool was named SUNBUC as an acronym for Sun Burn Calculator. There were two phases of the study: 1. Usability testing and 2. A controlled trial, which was designed to test the impact of the tool on the sun protection behaviour of the respondents over a period of 3 months. The null hypothesis was that tailored information and response independent information has a similar impact on sun safety behaviour as measured by the frequency of usage of sun protection methods such as sunscreen. Ethics board approval was obtained for the study. The usability of the online survey and SUNBUC was tested on five respondents using the think-aloud method and evaluated using the System Usability Scale. The evaluation showed average usability and system modifications were made according to the findings of the think-aloud study. The controlled trial design consisted of the control group with 48 respondents and intervention group with 53 respondents. Post intervention survey responses were obtained from 46(96\%) and 48(91\%) respondents belonging to the control and intervention groups respectively. Having implemented SUNBUC, findings showed no significant difference between the respondents who used the tool and the control group in short-term sun protection behaviour. However, many respondents felt that SUNBUC gave them a sense of control over their behaviour, a proximal determinant of the behaviour itself as per the Theory of Planned Behaviour. / Thesis / Master of Science (MSc)

Student Number : 8325808 - M Family Medicine thesis - Faculty of Health Sciences / Childhood protection from the sun affords lifetime protection from skin cancer. This cross sectional descriptive study was conducted on 184 mothers in 4 medical waiting rooms in Gauteng. The purpose of the study was to determine mothers’ knowledge, attitude and behaviour towards sun protection of their children. Data was collected by means of an anonymous, confidential, self-administered questionnaire. There was a significant correlation between personal behaviour of the mother and behaviour of the use of sun protection on their children. The majority of mothers purchase a sunscreen of SPF factor > 15. Knowledge often does not transfer into behaviour, with many parents still believing that a suntan is healthy. Mothers’ attitudes towards sun protection was positive. Knowing someone with skin cancer does not increase the usage of sun protection. Mothers attain their knowledge of sun protection from magazines and television. Mothers felt they would benefit from more information. Sun avoidance methods need to be taught and the perception of a suntan being healthy needs to be changed in the public media.

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Several techniques exist for retrieving and studying information about the properties of the Solar atmosphere from the polarization state of spectral lines. These are commonly called spectral diagnostics. Among the current problems to which these are applied, one is to understand the interconnection between the solar magnetic field and chromospheric heating. Non-LTE inversion has so far been the most reliable method for inferring chromospheric magnetic fields from high-resolution spectropolarimetric observations. However, if the magnetic field is sufficiently low that the line is in the weak field regime, the weak field approximation is often used as a complement. The latter allows for rapid analysis of large datasets and can be used to infer the vector components of the magnetic field. The reliability of the approximation in highly dynamic and stratified atmospheres has however not been well studied. The purpose of this project is to study and assess the reliability, the validity conditions, and the origin of possible breakdowns of the weak field approximation. This is done by computing the magnetic field of a model chromosphere, performed with realistic three-dimensional magnetohydrodynamics, from synthetic Ca II 8542 polarization profiles. Real magnetic fields are further on inferred from sunspot and plage observations of the same line with the intention to test the method under observational constraints. Stokes I profiles with peculiar shapes are seen in both sunspots and plage, apart from the common quiet profiles. The effect of two such types on the inferred field is studied more closely in this project; raised core profiles, which exhibit a flat core and are common in plage and in the vicinity of bright points, and umbral flashes, which exhibit core emission and are seen in sunspots. The shape of the former is directly connected to presence of steep vertical temperature gradients arising from chromospheric heating, while that of the latter is due to oscillatory motions of the plasma. The weak field approximation works well for observations with high S/N ratio and where quiet profiles are abundant, such as in sunspots. It is vulnerable in plage regions where there is an abundance of raised core (RC) or umbral flash (UF) profiles which lead to failed estimations. Profiles with low S/N tend to yield failed estimations as well. This is common where there is plage, whereby clusters of failed inversions tend to arise in such regions. The vertical component and inclination are well determined, while the horizontal component and azimuth are less well determined. The approximation works well in general, and allows for a rapid and efficient inference of the magnetic field vector as long as the wavelength range is chosen wisely.

Thesis (D. Litt(Semit. Tale)--Universiteit van Pretoria, 2002. / Includes bibliographical references (p. 242-257).

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