When rain hits the dirt in your backyard, complicated stuff happens inside each water droplet. The energy of the impact causes things like “shock compression,” “fast granular flows,” and “capillary interactions.” And don’t even get me started on the Reynolds hydrodynamics!
If you don’t know what any of that means, that’s okay, because scientists have just found a much simpler way to model raindrop strikes. According to a group of researchers from the University of Minnesota, raindrop impacts are remarkably similar to asteroid impacts. To find that out, the researchers used high-speed photography and lasers to measure what happens when water drips into a pile of tiny glass beads.
In the video above, the water droplet hits the particles slowly, spreading out horizontally and creating a small crater. The water then retracts, pulling with it a layer of granular particles, and it bounces back up. At higher impact velocities, the water spreads out more on the surface and picks up more sand, which increases the weight of the droplet and reduces the height of the jump.
Increasing the energy of the impact causes the water to take on finger-like projections. These pick up so many particles that the retraction gets interrupted; it no longer forms a perfect sphere.
At very high impact speeds, the finger-like projections spread even further and pick up enough grit that they stop bouncing back.
Understanding how water droplets interact with small particles is important, because it helps scientists understand and control things like soil erosion and the effectiveness of drip irrigation. Raindrop craters have also been used to infer the Earth’s air density 2.7 billion years ago.
Even though asteroids are about a bajillion times bigger than raindrops (and also made of rock), the way their energy gets distributed during impact — and the shape of the resulting craters — are very similar. For example, the researchers found that the ratio of the depth of the crater to its diameter is about 0:20, which is spot-on for what’s observed in the simple craters on the moon, Mars, and Mercury.
While the asteroid data is helping the Minnesota researchers model the dynamics of a water droplet, the authors suggest that the water droplet data may also be helpful in modeling asteroid impacts. That’s because when an asteroid crashes into a big object, such as a planet, the extreme heat and pressure liquefies or vaporizes the asteroid.
The authors caution against drawing too close of a link between the two phenomenon, since the impact energy of an asteroid is 18 orders of magnitude larger than that of a raindrop, so different physical processes are likely at play. “Nevertheless,” they write, “the remarkable similarity between the two processes indicates that they may share common mechanisms that are worth further investigation.”