2nd International Planetary Caves Conference (2015) 9017.pdf

WOOD VALLEY PIT CRATER CAVE MICROCLIMATE: A POSSIBLE ANALOG FOR MARS. T. N. Titus1, G. E. Cushing1, C. Okubo1 and R. G. Vaughan1, 1U.S. Geological Survey, Astrogeology Science Center, Flagstaff, AZ 86001 ([email protected])

Introduction: Caves are subterranean voids that recorded using LiDAR [10]. Thermal infrared images provide access to near-surface and sub-surface geology of the cave entrance over an ~8-hour period were col- [e.g. 1], as well unique microclimates that are often lected to increase our understand of how the cave mi- exceptionally stable and benign. . It has been proposed croclimate may affect surface temperature and the re- that martian caves may preserve evidence of past life mote detectability of the cave entrance. While this lim- or even harbor extant life [e.g. 2,3]. Caves may also ited amount of thermal imagery is not sufficient to provide shelter for future human Mars explorers, accomplish this goal, it does provide a baseline. shielding them from extreme temperatures and radia- Temperature: Rock and air temperatures were con- tion [e.g. 4-6]. Caves may provide access to resources currently recorded using HOBO data logger U23-003s, necessary for human exploration, e.g. water ice [5]. with 2 external probes. These sensors have an absoulte The suitability of caves for preservation of past life, accuracy of 0.2°C and a relative accurancy of 0.02°C. evidence of extant life, or even the usefulness of a Humidity: Humidity was measured throughout the cave for human exploration (either as a shelter or as a cave complex using HOBO data logger U23-001s, resource) will greatly depend on the cave’s microcli- where air temperature and humidity are measured with mate. Many factors can influence cave microclimates internal sensors. These sensors are not accurate at rela- such as thermal conduction of geothermal heat, cave tive humidities above 95%. breathing (the exchange of inside and outside air), per- Pressure: Barometric Pressure was measured near colation of water (important for terrestrial caves), cold- the cave entrance and at the back of the cave using trapping and evaporation/condensation of volatiles and HOBO S-BPA-CM10 pressure sensors. liquids within the cave. While some of these processes may not apply to planets other than Earth, a valid model of these processes is important to fully under- stand the effects and differences that will occur if a computer model of a cave is “moved” to another planetary surface. Before exploration of planetary caves can begin, whether with robots or humans, a better understanding of cave microclimates will be needed. Terrestrial caves and caverns provide a wide variety of analog sites for such studies. Figure 1: Temperatures from the upper level of the cave. Analog Site: We analyzed data collected from This is ~60 m underground which is ~10 m below the floor of the pit. The red line is rock temperature while the green Wood Valley Pit Crater (WVPC) cave. WVPC cave and blue lines are air temperatures. The difference between formed above a dike located along Kīlauea’s south- the green and blue lines is that the green line is an external west rift zone on the island of Hawai’i [7]. Access to temperature probe that is located next to the rock surface the dike is through a ~50 m deep collapse pit followed while the blue line is an internal probe contained within the by an additional 50 m of scrambling through a con- humidity sensor and is less affected by the rock temperature. necting cave network. Dikes, unlike lava tubes which form near the surface, are usually inaccessible for study. This cave is unique in that its entrance is located at the bottom of a collapse pit (or Atypical Pit Crater) similar to some features observed on Mars [e.g., 7-10]. Data: A unique set of cave mapping and microcli- mate data have been acquired for this inactive and par- tially drained dike located ~100 m underground. The data collected over a period of one year consisted of rock and air temperatures, relative humidity and baro- metric pressure from within the drained section of the Figure 2: Temperatures from the lower level ~90 m under- dike (i.e., the cave). Data was generally collected eve- ground. The red line is rock temperature while the green and ry 15 minutes. Additionally, the topology of the cave blue lines are air temperatures. and the locations of the climatology sensors have been 2nd International Planetary Caves Conference (2015) 9017.pdf

Results: Preliminary results suggest a cave system Humidity. Throughout the lower portion of the that is actively breathing where climatic conditions are cavern, the climate remains saturated year round. The dominated by percolation of rain water and cold- transient appearance of “fog” deep in the cave indi- trapping. cates that the air becomes supersaturated at times. The Temperature: The diurnal variation in temperature amount of supersaturation can vary on timescales of a decreases as one goes deeper into the cave (Figs. 1-2). few hours. However, we did not have proper instru- The mean temperature also decreases suggesting that mentation to either quantify or characterize this effect. cold-trapping is an important process controlling the Thermal Detectability. Thermal imaging was ac- cave’s microclimate. quired during morning and afternoon hours. The imag- Pressure. The pressure data acquired at the back of es were taken from several locations during the 2012 the cave and at the base of the cave entrance indicates and 2014 field expeditions. Fig. 5 is an example of strong solar tide (S1, S2) component (Fig. 3). This thermal imaging taken at close range to one of the cave effect has been observed in a cave in Brazil but the entrances. The cave entrance does show about a one- pressure variation was only ~1 Pascal [11], whereas degree Celsius decrease when compared to other shad- the pressure variations at WVPC cave can approach 4 owed images (not shown). Pascal. Preliminary analysis suggests that this “pit” may act as a hydraulic system (albeit an inefficient system) that increases the pressure variation due to the solar tide activity in the atmosphere. Over a period of 7-10 days, the pressure gradient within the cave structure, as determined from the two pressure instruments, cycles from inhalation to exhala- tion and back to inhalation. This cycle of cave breath-

ing is mostly likely due to normal surface pressure Figure 5: Visible and thermal view of one of the cave en- changes from island weather systems. trances. The white box in the visible image marks the area observed by the FLIR shown on the right. Temperature is in degrees Celsius. Conclusions: The geologic setting of the WVPC cave provides a unique analog site where microclimate studies can be conducted. Access to an evacuated dike 100 meters underground is rare. The cave entrance, located at the bottom of a 50-meter-deep pit crater, is a suitable analog for features seen on Mars and the Moon, and provides an element of security for instru- mentation. Figure 3: Ten-day sample of pressure from just below the Further research and characterization is needed to cave entrance and the back of the cave. The 12-hour period fully quantify the effects and implications of supersat- is due to the solar tides (S1, S2). The pressure treads show uration. Air movement (i.e. wind) can be felt within evidence of breathing within the ~7-8 day cycle. the long, but constricted, tunnel that connects the “fur- nace room” to the back of the basaltic dike (see Fig. 1 in Okubo et al. this workshop, [10]). This airflow needs to be measured and compared to pressure data to further characterize the “cave breathing” process. References: [1] Rodriguez J. A. P. et al. (2005) Icarus, 175, 36-57. [2] Boston P. J. et al. (2006) GeCAS, 70, A60. [3] Boston, P. J. et al. (2001) aSbIO [4] Levelle R. J. and Datta S. (2010) PSS, 58, 592–598. [5] Williams K. E. et al. (2010) Icarus, 358–368. [6] Wynne J. J. et al. (2008) EPSL, 272, 240-250. [7] Favre G. (2014) Stalactite, 64, 14–25. [8] Figure 4: A portion of the humidity data. The white line Cushing et al. (2015) [9] Cushing G. E. (2012) J. Cave Karst show data acquired just below the cave entrance while the Stud., 74, 33–47. [10] Okubo et al. (2015) 2IPCC, Abstract red line show data acquired deeper in the cave. The humidity quickly saturates. Phase shifts in humidity indicate that the #9005. [11] Sondag F. et al. (2003) JHyd., 273, 103-118. cave does breath.