Small Lava Caves as Possible Exploratory Targets on Mars: Analogies Drawn from UAV Imaging of an Icelandic Lava Field
Abstract
:1. Introduction
- To perform UAV-based high-resolution imaging survey for the part of a lava flow showing all the main morphologies and abundance of small caves;
- To design a sequential methodology for identifying and characterizing the small cave openings on the UAV images with respect to the lava flow morphology;
- To perform a high-resolution comparison of the Icelandic lava flow with some examples from Mars.
2. Study Area
3. Materials and Methods
3.1. UAV Imaging System
3.2. Flight Planning to Mitigate Systematic Error in Absence of Ground Control Points (GCPs)
3.3. Generation of DTM and Orthomosaic
- Photograph alignment (bundle adjustment): Agisoft PhotoScan aligns the photos from a UAV survey using the camera location coordinates and algorithms, automatically detects stable common features among the overlapping images, and determines the location and alignment of each camera position with respect to others [48,49]. This process of bundle adjustment generates a 3D sparse point cloud using the stereo-imaging, projection, and intersection of pixel rays from the different positions [49]. Using a very high computing hardware system (Intel Xeon E5-2650 v4, 12 cores, 24 threads central processing unit, 256 GB random-access memory, and Nvidia Geforce Titan XP 12 GB GDDR5X graphics card), we employed highest processing parameters within Agisoft PhotoScan workflow to derive the best possible results. For photograph alignment, we opted for the “Highest” accuracy and the highest possible numbers of tie points and key points in the processing tool window. The results of the alignment process are shown in Figure 1c.
- Geometry building and dense point cloud generation: A densification technique is applied within the software on the already generated sparse point cloud through the bundle adjustment to derive a 3D dense point cloud using multi-view stereopsis (MVS) or depth mapping techniques [54]. The model geometry is corrected by the intrinsic process of matching features to complete the final phase of geometry building to generate an accurate high-resolution 3D dense point cloud [49]. For this step, we opted for the “Ultra high” processing parameter and “Aggressive” depth filtering to derive the best possible results.
- Texture building and DTM generation: In this step, the generated 3D dense point cloud provides a continuous surface that can be triangulated and rendered with the original imagery to build a textured 3D mesh and create the final DTM [49] and, subsequently, the orthomosaics. For the DTM generation, the dense point cloud was selected as the source data, with enabled interpolation and a pixel resolution of 2 cm/pixel, and WGS 1984 UTM Zone 28N was assigned as the coordinate system for the final outputs. For orthomosaic generation, the DTM were selected as surface data, with enabled hole filling and 2 cm/pixel output resolution.
3.4. Morphometry
3.5. Cave Identification
4. Results and Discussion
4.1. Lava Flow Morphologies and Terrain Parameters
4.2. Cave Opening Distribution and Characterization
4.3. Possible Analogies with Martian lava flows
5. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Parameter | Value |
---|---|
Flight altitude | 70 m |
Flight plan | Double grid |
Battery used/flight | 1 |
Side overlap | 80% |
Front overlap | 85% |
Camera Angle (from vertical) | 0°, 20° |
Total flight time/flight | ~16 min |
Total area captured | ~334,000 m2 |
Total images captured | 990 |
Lava Morphology | Description | Mean Slope (°) | Mean Roughness (mm) | Field Photo | Reference |
---|---|---|---|---|---|
1. Pahoehoe lava | |||||
Shelly pahoehoe | Characterized by fragile gas cavities, small tubes, and buckled fragments of the surface crust. Lobes often form the margin of the sheet flow. | 23.95 | 24.93 | | [34,72] |
Slabby pahoehoe | Slabs of broken crust, usually less than a meter to few meters across. Formed when the pahoehoe crust is tilted and stretched during flow. | 15.55 | 15.32 | | [73,74] |
Spiny pahoehoe | Flexible crusts ruck into tight folds before cooling. The smooth glassy surface is the result of its formation under very low strain rates, when the lava is extremely crystalline and viscous. Surface resembles segment of coiled rope. | 15.50 | 13.99 | | [75,76,77] |
2. Aa lava | |||||
Cauliflower aa | Appears as bulbous protrusions on the lava surface which breaks to give fragments up to decimeters across. Grey-black, often glassy surfaces rough at millimeter-scale. This is an initial aa lava type in the transformation from pahoehoe to rubbly aa. | 26.31 | 30.04 | | [34,73] |
Rubbly aa | Formed as the crust breaks to yield rounded rubble varying in dimensions from sand to blocks several meters in diameter. Have accumulated fragments, with a clinkery and blocky surface. | 24.97 | 27.42 | | [34,74] |
Elevation Class | Elevation Range (m) | Area (km2) | Number of Caves | Mean Area of Cave Openings (m2) | Lava Morphology |
---|---|---|---|---|---|
High | 563.92–577.45 | 0.03 | 27 | 1.06 | Shelly pahoehoe, Cauliflower aa |
Medium | 558.26–563.92 | 0.17 | 48 | 1.35 | Shelly pahoehoe, Spiny pahoehoe |
Low | 553.81–558.26 | 0.13 | 6 | 0.45 | Slabby pahoehoe, Shelly pahoehoe, Rubbly aa |
Total = 81 |
Type | Description | Sketch | Reference |
---|---|---|---|
Open vertical conduit | These structures have oval or round shaped vertical passageways and are found in recent volcanic rocks, where lava rose to the surface and then waned. The openings are typically marked by a rootless small spatter cone called hornito. | | [18,66] |
Collapsed lava tunnel (Skylights) | Skylights are openings where the roof of the lava tube has collapsed. In an active flow, these skylights allow convective cooling of the lava. | | [79,80] |
Lava rise cave | Lava rise caves are formed as a result of inflation due to fluid lava accumulating under the solidified surface crust. Once the lava drains leaving a deflated center, if the uplifted surface crust can support itself, a flat cave remains under it. | | [18] |
Hidden or tumulus cave | Tumulus lava caves are formed when during volcanic activity below the arching surface crust, liquid lava is injected causing the surface crust to bulge as it solidifies without any horizontal shortening. Once the lava drains, the unstable section of the crust collapses revealing the tumulus cave. | | [18] |
Surface fractures | The observed small surface fractures are deep open cracks that are formed due to tensile stress in lava during and after solidification. | | [71,81] |
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Sam, L.; Bhardwaj, A.; Singh, S.; Martin-Torres, F.J.; Zorzano, M.-P.; Ramírez Luque, J.A. Small Lava Caves as Possible Exploratory Targets on Mars: Analogies Drawn from UAV Imaging of an Icelandic Lava Field. Remote Sens. 2020, 12, 1970. https://doi.org/10.3390/rs12121970
Sam L, Bhardwaj A, Singh S, Martin-Torres FJ, Zorzano M-P, Ramírez Luque JA. Small Lava Caves as Possible Exploratory Targets on Mars: Analogies Drawn from UAV Imaging of an Icelandic Lava Field. Remote Sensing. 2020; 12(12):1970. https://doi.org/10.3390/rs12121970
Chicago/Turabian StyleSam, Lydia, Anshuman Bhardwaj, Shaktiman Singh, F. Javier Martin-Torres, Maria-Paz Zorzano, and Juan Antonio Ramírez Luque. 2020. "Small Lava Caves as Possible Exploratory Targets on Mars: Analogies Drawn from UAV Imaging of an Icelandic Lava Field" Remote Sensing 12, no. 12: 1970. https://doi.org/10.3390/rs12121970
APA StyleSam, L., Bhardwaj, A., Singh, S., Martin-Torres, F. J., Zorzano, M.-P., & Ramírez Luque, J. A. (2020). Small Lava Caves as Possible Exploratory Targets on Mars: Analogies Drawn from UAV Imaging of an Icelandic Lava Field. Remote Sensing, 12(12), 1970. https://doi.org/10.3390/rs12121970