Why gophers are cooler than people but not planets 2.0

The average American gopher, weighing roughly 1-2 lbs, lives a solitary life in a burrow consisting of multiple tunnels. Whether used for foraging, storage, or rest, these tunnels can reach up to hundreds of square yards in size, with depths ranging up to six feet below the surface. With an assumption that I can consider myself an average America, this feat is more than I expect to accomplish in my lifetime (Minecraft excluded). 

Figure 1. Gopher, ft. burrow (National Park Service)

Debatably more accomplished than a gopher, the Earth has created tunnels through a plethora of processes; lava tubes and their formation being the most applicable process to my research. Generally seen as a “roofed conduit” (or tunnel) of actively flowing, drained, or plugged magma, lava tubes on Earth are yards to miles in length, 0.5-30 yards in width or height, and anywhere from centimeters to meters below the surface. Now this may feel like it puts the gopher’s efforts to shame, but to put it in perspective Earth weighs around 13,170,000,000,000,000,000,000,000 gophers. Even more shameful is the fact that Lunar and Martian lava tubes have been found to be 1-3 times larger than those on Earth. As of the writing of this blog I do not know of any gophers having conducted interstellar travel.

Volcanoes’ process of digging is a degree more heated than gophers’, with the three main mechanisms being overcrusting, shallow inflation, and deep inflated-entrenched tubes. Overcrusting of magma occurs when the surface chills and solidifies, insulating the molten lava below and allowing it to maintain high temperature and lower thickness for longer. Imagine placing a napkin over freshly microwaved soup where the napkin insulates the liquid, keeping it hot longer than it would stay without a cover. The second form, shallow inflation, results in a bulge morphology. To deviate from the food and animal metaphors, consider that you have a family member who loves packaging peanuts, so you decide to mail them some. As you shove the peanuts into your inadequate envelope it will inflate and form a bulge either in the middle or towards the far end as you keep pushing. 

The magma used as an analogue for the Moon and Mars is termed basaltic due to its composition, and has two broad variants: pahoehoe and a’a, with the first being more liquid and less rough than the latter. For pahoehoe lava flows, the inflation process results in a central bulge of the lava sheet (packaging peanuts relatively evenly shoved into the envelope) and a sequence of elliptical lava tubes form below the surface. In contrast, the shallow inflation bulge for a’a flows is found nearer to the end of the lava flow (packaging peanuts shoved to the opposite side) due to higher pressure, these a’a flows create larger lava tubes than pahoehoe flows. The final variant, deep inflated-entrenched lava tubes, are the largest found on Earth and involve downward erosion. Compared to the previous three lava tube formation mechanisms, deep inflated-entrenched tubes lack identifiable surface features. 

Figure 2. Models of lava tube geometries and associated surface features. Of note are top left corresponding to overcrusting, top right  to pahoehoe shallow inflation, bottom left  to a’a shallow inflation, and bottom right to thermal erosion and breakdown (Sauro et al. 2020).

 

Definitions of technical jargon aside, lava tubes (inferred from the presence of skylights and collapse along winding features) on the Moon and Mars were created through similar processes; overcrusting and shallow inflation for Mars, and deep inflation-entrenchment for the Moon. Unfortunately for Mars, lunar lava tubes are the current candidate for “most likely to be stable and not collapse - as much”. Fortunately for Mars, my research into modelling lava tubes looks past this shortcoming. At its current stage, my research focuses on using mesh (interconnected shapes, a latticework shown below) software to create lava tube analogous geometries - a circle or cylinder representing a tube inside of a rectangle representing the surrounding subsurface. My next step is to import this mesh geometry into a solver software that will allow me to simulate the movement of seismic waves generated by a hammer hitting the surface through the subsurface. As the surrounding rock and vacuum space of the lava tube are different compositions, these simulations will present estimates of the lava tube’s location and dimensions. These estimates will hopefully allow for steps to be taken by astronauts to reach and utilize the tubes. A step not taken by your average American gopher.

Figure 3. Example of a simple lava tube mesh geometry (created by me using GMSH software).

References

A. Bradford. Facts About Gophers, Live Science (2017), 

https://www.livescience.com/57623-gopher-facts.html 

Cool Cosmos. How much does Earth weigh and how is this measured? IPAC. 

https://coolcosmos.ipac.caltech.edu/ask/61-How-much-does-Earth-weigh-and-how-is-this

-measured

F. Sauro, R. Pozzobon, M. Massironi, et al., Lava tubes on Earth, Moon and Mars: A review on 

their size and morphology revealed by comparative planetology,

Earth-Science Reviews (2020), https://doi.org/10.1016/ j.earscirev.2020.103288

SPECFEM2D User Manual (2020). https://specfem2d.readthedocs.io/en/latest/