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Figure 1: Map showing location of the Mud Creek slide. Before and after photos are present showing the spectacular movement of material. Also present are other similar landslides and instrumentation

Geologic time is slow. Very slow. When geologists talk about mountains rising, such as the Himalayas, they speak on the millimeter per year scale. Every so often, however, drastic movement occurs. On May 17^th2017, the Mud Creek landslide occurred off of Highway 1 in Big Sur (fig.1). The slide was so large, it added 13 new acres to the coastline of California in an instant (fig. 2); amazingly, no one was injured. Since then, scientists have been able to research the slide to figure out how and why the slope failed so dramatically. 

Figure 2: Oblique time lapse view of the slope before failure, after, and the process of rebuilding the road.  

This event was what’s known as a bedrock landslide, which occur at large scales and often move very large, deep masses of earth. Bedrock landslides are highly dependent on groundwater rather than surface water. If the water table rises high enough into the soil that overlies the bedrock, it will effectively cause the overlying rock and soil to float. Water finds its way between individual grains and exerts a pressure on each grain, pushing them away from each other. This force destabilizes the rock, leading to rapid and catastrophic slope failure. 

It takes time for groundwater to move and cause the water table to rise. Groundwater flow isn’t like a raging river of water moving through the subsurface: in reality, groundwater saturates deeper rocks, sort of like a sponge, and only slowly creeps along if there is sufficient water pressure. Because of this, bedrock landslides can occur on seemingly innocuous, sunny days often months after a large rainfall event. In the case of the Mud Creek landslide, the last large rainfall events occurred in February, March, and April (fig. 3), but the slide didn’t occur until mid-May.

Figure 3: Hydrograph showing discharge of the Big Sur River. Discharge can be used as a proxy for rainfall events by looking for spikes in discharge. The landslide occurred in May when there was no rainfall occurring. 

 The Mud Creek landslide is interesting because it was actually moving steadily before the catastrophic slide occurred. Using remote sensing satellites, researchers were able to track the motion of the Mud Creek landslide for the past several years before the big slide occurred. Before total failure, the slope was moving at a stable rate, creeping along at about .24 to .43 m/yr (fig. 4). During this time (2009-2017), California was in a historic drought. Small rainfall events during the winters would cause the velocity of the slide to increase slightly, but it wasn’t until the historically wet winter of 2017 that the slide started to accelerate and deviate from past years’ motion (fig.4).

Researchers attribute the sudden catastrophic failure of the Mud Creek landslide to rapid changes in climate. The fast transition from a drought to very intense rain events caused the Mud Creek landslide to become unstable very quickly. Other similar, slow moving slides in the area remained stable, likely due to their smaller slope angles. 

Figure 4: Top: Downslope velocity of 3 creeping landslides. Notice rapid acceleration and subsequent failure of the Mud Creek slide. Bottom: Solid lines are precipitation values. The dotted line is the calculated pore fluid pressure. Notice how pore fluid pressure rapidly increases before the point of catastrophic failure.  

            California is currently experiencing another historically wet winter after a small drought, and Sierra snowpack currently stands at about 146% of what it was last year. What will this mean for catastrophic slope failure in California? If the Mud Creek landslide is any indication, it will likely mean more landslides during the spring months as snow melts and percolates through the subsurface. Keep a close eye on the coastal ranges of California this coming year and hopefully we’ll see some geology in action! 

Handwerger, A.L., Huang, M., Fielding, E.J., Booth, A.M., and Bürgmann, R., 2019, A shift from drought to extreme rainfall drives a stable landslide to catastrophic failure: Scientific Reports, p. 1–12, doi:10.1038/s41598-018-38300-0.

https://earthobservatory.nasa.gov/images/144552/a-strong-start-to-sierra-snowpack?utm_source=TWITTER&utm_medium=NASA&utm_campaign=NASASocial&linkId=63616203

NASA Cassini Spacecraft Discovered Ingredients That Could Sustain Life on One of Saturn’s Moons

Fig. 1. Enhanced image taken of Saturn’s moon Enceladus showing the erupting geyser spray of fine particles that the Cassini spacecraft flew through. (NASA/JPL)

Evidence is mounting that Saturn’s ice-covered moon Enceladus may be able to support microscopic life. NASA’s Cassini spacecraft flew through an erupting geyser on Enceladus (Fig. 1) and detected large amounts of molecular hydrogen, or H₂. This chemical signature is the critical ingredient needed to support a chemical reaction that feeds certain microbes on Earth (Fig. 2), called methanogenisis. Methanogenisis produces methane from hydrogen and water and creates usable energy for the microorganisms.

Fig. 2. Electron microscope image of methanogens. Methanogens are microbes that get their chemical energy from a reaction that makes methane from hydrogen and water. Recent discoveries from NASA suggest that Enceladus may be able to support such life (Maryland Astrobiology Consortium, NASA, and STSci)

This discovery also reveals information about what the subsurface environments on Enceladus could look like.  According to a recent Science article by NASA scientists (Waite et al., 2017), the detection of H₂ is most plausibly caused by ongoing water-rock hydrothermal reactions at Enceladus’ seafloor (Fig. 3). In such a scenario, hot fluids would flow over and through cracks in rocks releasing H₂ into the overlying ocean. Hydrothermal vents on Earth host massive communities of simple life forms, further strengthening the idea that Enceladus is ripe for life.

Fig. 3. Graphic illustration of the hydrothermal reactions that NASA scientists think are occurring at the bottom of the ocean of Enceladus, producing H₂ (NASA/JPL)

H₂ will only form under specific environmental conditions. Therefore scientists can also infer that the pH ranges of Enceladus’ subsurface ocean are likely fairly basic, ~9-11 (Fig. 4).

Fig. 4. Graphic illustration of what scientists expect the environmental conditions of Enceladus’ oceans to be. The orange region identifies the H₂ chemical signature detected by the Cassini spacecraft. The dark blue diagonal lines show constant ocean pH values.  The highlighted blue region identifies what pH ranges best coincide with what is expected for Enceladus. (Waite et al., 2017)

In contrast, the most common hydrothermal vents found on Earth are acidic not basic. For example, Figure 5 shows a cloudy acidic plume erupting from a hydrothermal vent in near Guam. However, there are unique hydrothermal systems in Hawaii, specifically the Lōʻihi Seamount that have pH’s similar to the estimates for Enceladus. Therefore, before heading all the way back to Enceladus, NASA scientists are planning to first explore closer to home by sending underwater submarines to the Lōʻihi Seamount. One such research project is the NASA SUBSEA or Systematic Underwater Biogeochemical Science and Exploration Analog project. SUBSEA will explore Lōʻihi this August to September to learn more about how the seamount is capable of supporting life.

Fig. 5. Erupting cloudy plume from a hydrothermal vent near the Island of Guam. (NSF)

The discovery of H₂ doesn’t mean that life currently exists on Enceladus, simply that Enceladus may contain chemical food sources capable of supporting microscopic life. The discovery of H₂ is nonetheless very exciting and makes Enceladus a top choice for future space missions. The discovery of potential microscopic life beyond Earth may soon be within reach.