I have had this thought in my head for a while and after sitting in the lab writing out sample plans, reading over literature, and having skype conversations with the field team I wanted to reach out and get an outside scientific opinion from you guys. I have been planning field work for about a month and wanted to get outside opinions on the subject. I decided to write this blog to reach out to everyone who reads my content. Below is a description of the field work, what questions I am are wanting to answer, and my thoughts on how to tackle the question.
For those who do not know, I have been tasked to plan the logistics for the Canadian Space Agency FAST Volcanic Analogues for the Exploration of Mars field deployment at the Holuhraun lava field in Iceland (see figure 1 below) from the 24th of July to the 8th of August. A team of five, including myself, are to spend 14 days in Iceland, 10 of which will be in the field, studying the surface roughness of lava flows using UAVSAR, LiDAR, and field observations and measurements. With this deployment being part of my PhD thesis I wanted to ensure I had a strong field plan before leaving in July. Using the science objectives written out in the FAST proposal I began planning.
Figure 1. Google Earth image of the Holuhraun lava field in Iceland. Vent is at the SW corner of the lava field and a majority of the lava flows have a general SW-NE flow direction.
One of the science objectives stated, "Determine the rheological rationale for the observed lava textures on Earth and Mars.", which discusses what can we interpret about the physical and chemical properties of lava flows using their surface roughness. This brought up some controversy and concerns with the team since it is very difficult to know exactly how a lava flow behaved based off field and remote sensing data. Generalized inferences can be made, for example clinkered a`a lava flows had to overcome a yield strength in order to cause the lava flow the continue moving on the surface. This would have ultimately caused the surface to fragment and viscously-tear. Rubbly pahoehoe lava flows would have formed when a solidified pahoehoe crust experienced mechanical fracturing from either inflation plateaus, increase in effusion rates, or disrupted by underlying topographic roughness. So we can get a general idea, but can never really know all of the details unless we observed and studied the lava flow during the eruption.
I began to start thinking, if we have difficulty correlating surface morphology and roughness to the rheology of lava with confidence then we should focus on using remote sensing and field data to infer the surface morphology and roughness of lava flows on the Moon and Mars (which is one of the other science objectives in the proposal). Radar remote sensing techniques have been used to study the surfaces of the Moon and Mars and have discovered lava flows with similar morphologies to terrestrial examples. When radar backscatter data was analysed for the first time, planetary scientists noticed that the results show some resemblances to terrestrial volcanic radar data. Campbell et al. (1996, 2007, 2008, 2010, and 2014) and Morgan et al. (2016) used Arecibo P-Band and S-Band backscatter data (70 cm) to describe the morphology of lunar mare lava flows, and Harmon et al. (2012) studied volcanic features on Mars using Arecibo S-Band backscatter data (12.6 cm). Figure 2 shows the backscatter radar data highlighting the flows margins of the Imbrium Mare buried beneath a layer of lunar regolith. The radar brightness contrast appears more distinct in the P-Band data because the longer wavelength is able to penetrate further into the regolith and interact with deeply buried subsurface features. The radar bright lava flows indicate a rough surface. If compare to terrestrial lava flows the surface may look similar to rubbly pahoehoe or a`a. The radar dark lava flows indicate smooth surfaces, similar to terrestrial smooth pahoehoe lava flows. When we look at Mars, we observe similar radar backscattering (Harmon et al. 2012) as observed on Earth and the Moon. The Tharsis region is radar bright (see figure 3 below), with the rough volcanic features on the planets surface situated within the northern hemisphere. Some of the radar backscatter data is indicative of rough slabby pahoehoe flows and very rough blocky flows.
With access to radar datasets of other planetary surfaces such as the Moon and Mars, we should continue to study volcanic analogue sites on Earth where radar data with similar wavelength scales are available.
Figure 2. Two radar maps from Morgan et al. (2016). Left radar map is P-Band backscatter coverage of Imbrium Mare and the right radar map is S-Band backscatter coverage of Imbrium Mare.
Figure 3. Arecibo S-Band data of the Tharsis region on Mars. Radar bright features are the volcanic regions, which cover a majority of the northern hemisphere. Source: Harmon et al. (2012).
Now, I want to open the floor to you guys and start a discussion. Directly comparing terrestrial lava flows to Martian and lunar lava flows is a delicate subject since we are under the assumption that magmatic and volcanic processes on other planetary bodies are very similar to ones studied on Earth. However, we cannot deny that radar remote sensing data has shown similarities in surface morphology and roughness between terrestrial and Martian and lunar lava flows. AIRSAR and UAVSAR L-Band data (24 cm) of lava flows from Craters of the Moon and Holuhraun show some surface morphologies and roughness to exhibit similar CPR values (circular polarization ratios, values used to quantify radar roughness), making them indistinguishable without additional ground-truth information. High resolution imagery of these lava flows could distinguish them but such imagery in not currently available for other planetary bodies. HiRISE and LRO NAC images have max resolutions of 0.25 m/pixel and 0.5 m/pixel, which is useful for metre-scale roughness features but not for decimetre to centimetre-scale features.
Would you say it is applicable to study the morphology and roughness of lava flows at all surface size-scales in detail to improve our understanding on how they backscatter radar signals? Could this help with current and future volcanic analogue research?
Hope to start discussing this with you guys soon :)
Campbell, B. a, Shepard, M.K., 1996. Lava flow surface roughness and depolarized radar scattering. J. Geophys. Res. 101, 18,941-18,951.
Campbell, B.A., Campbell, D.B., Margot, J.-L., Ghent, R.R., Nolan, M., Chandler, J., Carter, L.M., Stacy, N.J., 2007. Focused 70-cm wavelength radar mapping of the Moon. IEEE Trans. Geosci. Remote Sens. 45, 4032–4042.
Campbell, B.A., Carter, L.M., Hawke, B.R., Campbell, D.B., Ghent, R.R., 2008. Volcanic and impact deposits of the Moon’s Aristarchus Plateau: A new view from Earth-based radar images. Geology 36, 135–138. https://doi.org/10.1130/G24310A.1
Campbell, B.A., Carter, L.M., Campbell, D.B., Nolan, M., Chandler, J., Ghent, R.R., Ray Hawke, B., Anderson, R.F., Wells, K., 2010. Earth-based 12.6-cm wavelength radar mapping of the Moon: New views of impact melt distribution and mare physical properties. Icarus 208, 565–573. https://doi.org/10.1016/j.icarus.2010.03.011
Campbell, B., Hawke, B., Morgan, G., Carter, L., Campbell, D., Nolan, M., 2014. Improved discrimination of volcanic complexes, tectonic features, and regolith properties in Mare Serenitatis from Earth‐based radar mapping. J. Geophys. Res. Planets 119, 313–330. https://doi.org/10.1016/S0733-8619(03)00096-3
Harmon, J.K., Nolan, M.C., Husmann, D.I., Campbell, B. a., 2012. Arecibo radar imagery of Mars: The major volcanic provinces. Icarus 220, 990–1030. https://doi.org/10.1016/j.icarus.2012.06.030
Morgan, G.A., Campbell, B.A., Campbell, D.B., Hawke, B.R., 2016. Investigating the stratigraphy of Mare Imbrium flow emplacement with Earth-based radar. J. Geophys. Res. 121, 1498–1513. https://doi.org/10.1002/2016JE005041.Received