41st Lunar and Planetary Science Conference (2010) 1740.pdf

THERMAL MODELING OF MAFIC AND ULTRAMAFIC IGNEOUS ROCKS A. H. Jowell 1, S. J. Davis 2, A. J. Stein 3, Z. S. Erb 1, B. L. Oliver 3, A. M. Eckert-Erdheim 3, 1Durham Academy Upper School, Durham NC 27705, 2Chapel Hill-Carrboro City Schools, Chapel Hill, NC 27516 3Durham Public Schools, Durham, NC 27702 Introduction: Research over the past few decades er albedo; and the water-altered ultramafic peridotite has indicated that the majority of surface rocks on would have the highest thermal inertia due to water’s Mars are mafic or ultramafic basalts [1]. Just as on presence. Earth, Mars has both vesicular and massive basaltic Analytical Approach : We obtained a massive and rocks. Vesicular basalts form when gas bubbles are vesicular mafic basalt from the Columbia River Basin trapped in cooling magma. When the gas escapes the in Washington and Oregon, and we acquired an ul- magma a vesicle is created [2]. It is important to study tramafic peridotite containing hydroxylated silicates the thermal properties of vesicular basalts because sol- from the Falls Lake Melange in Wake Country, North id bedrock of vesicular basalt could be mistaken for Carolina. After collecting these rocks, we made sure fine sediments if they both exhibit similar thermal that they each had a weathered and fresh surface. We properties. measured the heating and cooling curves of these rocks Scientists predict that there are ultramafic rocks to determine if their thermal properties could be util- containing hydroxylated silicates on Mars. It is critical ized to assess geological events on Mars. to study the thermal characteristics of a rock with hy- We used a 75-watt heat lamp in order to provide droxylated silicates because rocks containing hydroxy- constant and consistent light on the rocks that we lated silicates indicate the past presence of aqueous tested. The apparatus consisted of a rock placed with environments [3, 4]. its flat surface facing upwards (to minimize slope ef- Although many of the surface rocks on Mars are fects), a heat lamp positioned 40 cm above the center not highly weathered [5], some areas such as the of the rock, and an infrared thermometer clamped to a younger lowlands contain basalts which may have tripod with its tip pointed 30 cm from the center of the been weathered under submarine conditions [6]. As a rock’s surface. When determining the heating and result, it is important to determine the effects of weath- cooling curve of a rock, we measured the initial tem- ering on the thermal signature of rock samples since perature of the rock before turning on the heat lamp. weathering results in changes to the thermal properties The temperature of a given rock was recorded every 30 of rocks [7]. seconds until the rock’s temperature stabilized. Once The thermal inertia of a rock is the measure of a the rock reached a stable temperature, we turned off surface’s ability to absorb heat during the day and re- the heat lamp. We defined a stable temperature as the lease it at night, and it has been used to determine the temperature of the rock hovering within half a degree mineralogy of rocks present on the Martian landscape Celsius for 20 readings. We continued to record the [8, 9]. Albedo (the measure of reflectivity of an object) temperature until the rock reached a stable cooling temperature. In addition, the fresh surfaces of all three affects the thermal inertia of a rock for it causes the rocks were heated for about 20 hours to determine the rock either to absorb or to reflect light depending on maximum temperature that could be attained in our the rock’s color. During the MONS program, we con- experimental settings. A complete cool down curve ducted an experiment to determine the thermal proper- was also recorded. ties (including thermal inertia and albedo) of different Results: An analysis of the thermal properties of rocks on Mars. We tested and compared the thermal the fresh surfaces of the vesicular and massive basalt properties of vesicular versus mafic basalts, ultramafic showed that the vesicular basalt heated and stabilized versus mafic compositions, a rock containing hydroxy- more rapidly and to a higher temperature (Figure 1). lated silicates, and how weathered and fresh surfaces This is most likely due to air present in the vesicles, influence the thermal properties of rocks. since air heats much faster than solid rock. However, We hypothesized the following: the vesicular basalt after 20 hours, the massive basalt reached the same would heat up more quickly and would not retain heat temperature as the vesicular basalt (64 ° C for massive as effectively as the massive basalt due to its lower basalt and 63.8 ° C for vesicular basalt); thus, the ther- thermal inertia; the weathered surface of the ultramafic mal property of the two rocks differed only in the rate peridotite would reach a higher temperature and have a of heating and not in the maximal attainable tempera- higher thermal inertia than the non-weathered surface ture. Furthermore, when the rocks were cooled after 20 because of its darker color and lower albedo; the mafic hours of heating, the vesicular basalt cooled faster than basalt's weathered surface would have a lower thermal the massive basalt. This was due to the lower thermal inertia than the non-weathered surface because the inertia of the air bubbles located in the vesicular basalt. lighter colored silt covering the basalt results in a high- 41st Lunar and Planetary Science Conference (2010) 1740.pdf

switched off completely and the room became progres- sively warmer and warmer; this altered the starting temperature of the rocks.

Figure 1 : Fresh surface thermal behavior of vesicular mafic basalt (colored blue), massive mafic basalt (colored green), and fresh ultramafic peridotite altered with hydroxylated silicates (colored purple). Figure 2 : Weathered surface thermal behavior of We also found that an ultramafic peridotite contain- vesicular mafic basalt (colored blue), massive mafic ing hydroxylated silicates heated to a lower tempera- basalt (colored green), and fresh ultramafic peridotite ture than the mafic basalts (Figure 1). Even after 20 altered with hydroxylated silicates (colored purple). hours of heating, the ultramafic rock reached a maxi- mum temperature of only 54.5 ° C compared to 64 ° C Future Work: We would like to continue studying for the mafic basalts. A possible reason for this obser- the thermal characteristics of rocks with varying com- vation could be that the ultramafic hydroxylated sili- positions, surfaces, and vesicular and massive proper- cate peridotite was altered with water which resulted in ties in the MONS program. We hope to develop a more a higher thermal inertia. Therefore, the ultramafic hy- accurate model and understanding of the heating and droxylated silicate did not reach as high a temperature cooling trends of the qualities listed above, and we as the mafic basalts and retained heat more effectively. hope to work towards identifying different types of Another possible reason for this observation could be rocks on Mars using our models as a comparison. the higher albedo of the ultramafic rock. As opposed to Acknowledgements: This experiment was per- the darkly colored mafic basalts, the fresh surface of formed by the MONS (Mars Outreach for North Caro- the ultramafic rock was almost white in color, resulting lina Students) Program. We would like to thank the in a higher rate of reflection. directors of this program, Howard Lineberger and Sam Finally, we compared the thermal properties of the Fuerst, who have provided us with this wonderful fresh and weathered surfaces of the three rocks (Figure learning experience. We would also like to thank Dr. 2). Our results showed that the thermal signatures of Jeff Moersch of the University of Tennessee at Knox- all three weathered surfaces were very similar. The ville, who inspired and helped us design this experi- vesicles present on the weathered surface of the ve- ment. Additionally, we would like to thank the Bur- sicular basalt were filled with silt and other sediments, roughs Wellcome Fund of Research which generously effectively removing the air pockets which had previ- sponsors the MONS program. Without the support and ously retained much of the heat. The weathered sur- dedication from these individuals and institutions, we faces of all three rocks were also very similar in color would not have the opportunity to participate in the (reddish-brown) and the albedo effects of the light col- MONS program. ored fresh surface of the ultramafic rock were negated. References: [1] McSween H.Y. (2004) Science This in turn resulted in a stronger resemblance of the 305, 842-845. [2] Gusev Rocks Solidified From Lava. weathered curves. (2006). Retrieved from: http://marsrovers.nasa.gov/- In conclusion, since many of the rocks on Mars are not weathered, determination of thermal signatures gallery/press/spirit/20060127a.html. [3] Milliken, R. E. could be used to differentiate between the types of (2008) Geology 36, 847-850. [4] Squyres, S. W. (2008) rocks. However, it would not be a differentiating factor Science 320, 1063–1067. [5] McSween H.Y. (2009) for weathered surfaces on Mars. Science 324, 736-739. [6] Wyatt, M.B. (2002) Nature One needs to take into consideration when observ- 417, 263-266. [7] Jeffrey, G.T. (2002) The Tricky ing the data that some noise occurred during the data Business of Identifying Rocks on Mars , Hawaii Insti- collection process: the air conditioning system periodi- tute of Geophysics and Planetology. [8] Mellon M.T. cally switched on and off, and doors were opened and et al. (2000) Icarus , 148 , 437-455. [9] Putzig N.E. and closed during data collection. Additionally, by the end Mellon M.T. (2007) Icarus 191, 52-67. of the first week, the air conditioning system was