SOLAR WIND VOLATILE PRESERVATION Samantha R. Jacob Department of Geology and Geophysics University of Hawai i at Mānoa Honolulu, HI 96822 ABSTRACT Because the Moon has a negligible atmosphere and magnetosphere, its surface is constantly bombarded with extra-lunar particles. Internal geologic activity has long been quiet on the Moon, so solar wind particles, galactic cosmic ray particles, and original material ejected from the inner Solar System planets have likely been preserved on the lunar surface for billions of years (Rumpf et al., 2008). After emplacement, the regolith may become covered by lava protecting the particles from further bombardment beneath the layer of lava. Examination of these materials could give us understanding of the history of the inner Solar System. In an effort to identify deposits that may contain paleoregoliths, I combined several different data sets, such as Lunar Reconnaissance Orbiter (LRO) visible imagery and Clementine spectral data, into a Geographic Information System (GIS) map to study areas of the Moon that contain multiple lava layers. I identified individual lava units within a region then calculated the thickness of each lava flow. This work will be used in conjunction with numerical modeling completed by Dr. Sarah Fagents and Elise Rumpf to determine the probability of discovering intact paleoregolith deposits in the study area. This is especially important as it is one of the goals in NASA s Strategic Plan, to better understand the Sun s family of planets (NASA, 2006). BACKGROUND Due to bombardment by meteoroids, solar wind particles, and cosmic rays the bedrock on the Moon becomes modified and fragmented. The result is lunar regolith, which is a mixture of mineral fragments, impact glasses, and breccias. On newly exposed rock, regolith accumulation rates are high. As the upper layer of regolith gets thicker, accumulation slows down, because the upper regolith layer shields the underlying bedrock from continued impact. The current rate of regolith formation is ~1 mm/ma, however it is believed to have been higher in the Moon s early history due to a higher rate of impacts. Because of the constant implantation and mixing that creates regolith, the Moon is believed to hold critical information regarding the early history of the inner Solar System. The regolith has been implanted with particles that penetrate the layer at different depths. Particles such as high-energy electrons, protons and neutrons penetrate to depths of centimeters to meters. These particles cause nuclear reactions to take place in the regolith, which leave behind tracks of crystal damage (Fagents et al., 2010). Products of these reactions such as CO, CO 2, H 2 O, and N 2 can be used to measure the particle influx, surface exposure age and burial depth of the regolith (Fagents et al., 2010). However the majority of particles implanted in the regolith are ionized atoms from the Sun s plasma. These are mostly hydrogen and helium atoms. These particles only penetrate the regolith to depths of microns to millimeters. Theories suggest that the intensity and composition of the solar wind has varied throughout the history of the Solar
System. Further studies of the particulate regolith, such as the one proposed here, could provide further evidence of such variability. Such studies could also provide evidence for or against the hypothesis of a more massive early Sun. In order for solar wind particles to be preserved they must be covered and protected from the solar environment. It is important that they are protected from continued bombardment and mixing with newer particles to maintain a clear record. Burial by lava flows is one mechanism by which the implanted particles can be preserved as it provides a cover that protects the particles (Fig. 1). The most ideal scenario is for the paleoregolith to be sandwiched between two lava flows. In this scenario both lava flows can be isotopically dated to give a precise date of the paleoregolith exposure. Fig. 1: Visualization of how volatiles would be emplaced and preserved in the regolith There is, however, a problem with volatile preservation due to burial by lava flows, specifically the heat associated with the flows. The particles underneath the lava are exposed to and therefore must survive temperatures that exceed 1100 degrees Celsius (Fagents et al., 2010). Volatiles such as CO, CO 2, N 2, and Xe are lost at temperatures greater than 700 degrees Celsius. Other volatiles such as Ne, CH 4, and Ar are lost at temperatures greater than 500 degrees Celsius, while H 2 and He are lost above 300 degrees Celsius (Table 1). The eruption temperature of lunar basalts is between 1170 and 1440 degrees Celsius (Fagents et al., 2010). However there is a limiting factor to how deep the regolith can be heated. According to Fagents et al. (2010) particulate regolith has a low thermal conductivity and specific heat capacity due to the presence of interstitial voids and vacuum conditions. The low conductivity of the regolith restricts the depth to which the substrate can be heated. The thermophysical properties of the regolith change with varying temperatures; however it is not known exactly how the properties are correlated to temperature variation. Table 1: Temperatures at which certain volatiles will be lost.
METHODS At the beginning of this project, I looked for locations on the Moon with mare flow boundaries and/or distinct layers of lava. Much of the fall semester was spent looking at images of Lichtenberg Crater from the Narrow Angle Camera (NAC) onboard the Lunar Reconnaissance Orbiter. This site was interesting because the southern half of its ejecta material has been covered by what is presumed to be a lava flow; we have also found a NAC image that shows distinct layers of mare in the crater wall. Later in the semester, I combined the NAC images with the Clementine topographic and multi-spectral data sets into a Geographic Information System (GIS) map. I also downloaded shape files from the Planetary Interactive G.I.S.-on-the-Web Analyzable Database (PIGWAD) and the Planetary Data System (PDS) into my map. Using the combination of all the data sets and the shape files, we were able to determine possible mare flow boundaries. By the end of the semester I was able to draw in some boundaries and make a simple geologic map of Lichtenberg crater (Fig. 2). N Fig. 2: Simple geologic map of Lichtenberg Crater. Lichtenberg Crater is 20km in diameter. The dashed lines indicate mare flow boundaries and the circle is the outline of the crater. The background of the image is the Clementine UVVIS 750nm swath and the lighter swaths are images from the Narrow Angle Camera. During the spring semester I focused on three craters Lichtenberg, Kepler, and Bessel. These were selected because each had exposed layering in the crater walls, which likely represent boundaries between lava flows. At this point, I also started using data from the Lunar Orbiter Laser Altimeter (LOLA) onboard the Lunar Reconnaissance Orbiter (LRO). Using this data set and a few equations I was able to determine average thickness of individual layers in a unit. First I had to determine the height (Ht) of the entire package of stacked flows (Fig. 3) using the difference in elevation of two LOLA points. Then I used ArcMap to measure the ground distance (Dis) covered by the lava package. Plugging these two measurements into equation 1, I was able to determine the slope of the crater wall at each package site. Then, I used equations 2
and 3 to approximate the thickness of individual flows. The variable n indicates number of lava flows in each package. Slope (degrees) = arctan(ht/dis) (1) Thickness (m) = Ht/n (2) Thickness (m) = Dis/n * cos(slope) (3) N N Fig. 3: The first row of images shows a lava package on the SSE walls of Bessel Crater. The package height was measured to be about 171 m. The second row of images shows a lava package on the S wall of Kepler Crater. The height of the package was measured to be 81 m. The images are from the Narrow Angle Camera onboard the Lunar Reconnaissance Orbiter. The images on the left show the outlines of individual flows. The solid lines outline obvious flows, while the dashed lines outline uncertain flows. The right images are the same package just without the flows outlined. The height of each package was calculated in order to determine the thickness of individual flows. The two thickness equations calculate the minimum (2) and maximum (3) flow thickness. They also represent the miniumum and maximum dip of the flow (Fig. 4). Equation 2 assumes the flow is horizontal, whereas equation 3 assumes that the flow is perpendicular to the slope of the crater wall. We originally used these two equations because we could not further constrain the dip of the flow. However this meant that the ranges of thicknesses we measured were quite large. Furthermore, there were a few different uncertainties and errors in our calculations, including uncertainties in the number of flows counted. Another issue was that we didn t always
have LOLA data points directly over the area of exposed layering, this may have produced error in our height calculations of the lava packages. Fig. 4: Visualization of maximum and minimum dips of individual lava flows. In order to minimize the error and ranges of our lava flow thicknesses we needed to better calculate the maximum dip of individual flows. This was done by assuming the flow was parallel to the outer rim of the crater and doing some more calculations. The calculations involved two LOLA data points that were anywhere from 250-520 m apart. Then the change in topography between the two points was calculated. Using the following equation, the slope of the outer rim or dip of the flow was calculated. The number 30298.4 is the number of meters in one degree latitude on the Moon. Slope = arctan(change in topogaraphy/30298.4 * change in latitude) (4) RESULTS The range of individual lava flow thicknesses are shown in Table 2 after calculating a maximum dip using Equation 4. The thickness of the flow is important to know because the regolith beneath the lava is heated differently depending on the thickness of the overlying lava flow. Thus the regolith layer must be a certain thickness for volatiles to survive. Unfortunately our data resolution was not good enough for us to determine how thick the regolith layers were. So we do not know if any volatiles could be found in this area. However, we were able to make a graph, based on our calculated thicknesses, that shows how thick the regolith must be for volatiles to survive (Fig. 5). The 300 C isotherm corresponds to the minimum thickness the regolith must be in order to find a complete sample of volatiles.
Table 2: Shows calculated average thicknesses of individual lava flow packages in each area. Crater Location Average Thickness (m) Bessel NNE 13-36 Bessel SSW 3-9 Bessel SSE 2-19 Kepler NW 7-27 Kepler S 5-19 Lichtenberg S 6-16 Lichtenberg NE 2-31 Fig 5: This graph shows the depths at which certain volatiles would be lost for different lava flow thicknesses. RELEVANCE TO THE STRATEGIC PLAN FOR NASA This is an important project because it pertains to several of NASA s current goals outlined in its Strategic Plan. One of the goals is to understand the Sun and its effects on Earth and the Solar System (NASA, 2006). By analyzing the record preserved in the lunar regolith, we would be able to understand the faint young Sun paradox. This could also help us predict future variability in the Sun s strength and composition. NASA s plan also stresses continued learning about the formation of the Sun s family of planets (NASA, 2006). Ancient lunar regolith could contain particles from the inner planet s early atmosphere and/or original material from the surfaces of the early planets. Studying these records could lead to a better understanding of the early composition of the inner Solar System. Lastly, because the Moon may be the next stop for manned missions, it is essential to know as much about it as possible. This project will further the understanding of our Moon and help NASA with its efforts to return man to the Moon.
CONCLUSION Because the Moon has a negligible atmosphere and magnetosphere, lunar regolith is constantly bombarded with solar wind and cosmic particles. These particles could provide us with information about the Sun s past intensity and help us predict its future. However, unless the regolith is covered and protected it will be an oversaturated mixture of differently aged particles. Plus, we cannot directly date regolith. In this project we were interested in areas of regolith that were covered by a lava flow. Since we can date lava flows we can also get a date range for the regolith, if it is sandwiched between two lava flows. Possible areas of regolith layerd between lava flows include flow boundaries and crater walls. In this project three craters Bessel, Kepler, and Lichtenberg provided the best study locations. However, being sandwiched by lava is not a perfect solution. The heat from the overlying lava penetrates the regolith and causes different volatiles to escape at certain temperatures. This project employed different techniques to calculate the average thickness of individual lava flows in the three study sites. The thickness of individual lava flows is important because the depth at which heat penetrates the regoligh varies with thickness of the overlying lava. Calculations can be made from the approximated flow thicknesses that show how thick the regolith layer must be for certain volatiles to survive. When man returns to the Moon, samples could be returned and studied that would further our understanding of the early history of our Solar System. REFERENCES Fagents, S. A., et al. Preservation potential of implanted solar wind volatiles in lunar paleoregolith deposits buried by lava flows. Icarus (2010), doi;10.1016/j.icarus.2009.11.033 Lunar Reconnaissance Orbiter Camera. 2008. Arizona State University. Sept. 2010. http://lroc.sese.asu.edu/index.html NASA, 2006. NASA strategic plan. http://www.spacegrant.hawaii.edu/2006nasastrategicplan.pdf Planetary Data System. William Knopf. 2011. National Aeronautics and Space Administration. Nov. 2010. http://pds.nasa.gov/ Rumpf, M.E., Fagents, S.A., Crawford, I. A., and Joy, K.H., 2008. Predicting Volatile Preservation in the Lunar Regolith through Heat Transfer Modeling. Lunar and Planetary Science XXXIX. U.S.G.S Astrogeology: Map-a-Planet. U.S. Geological Survey. Oct. 2010. www.mapaplanet.org U.S.G.S Planetary GIS Web Server PIGWAD. 2011. U.S. Geological Survey. Nov. 2010. http://webgis.wr.usgs.gov/