What do silly putty and ketchup have in common? This question is weird enough, I know, but even weirder — this was answered in my Comparative Planetology class, taught at Florida Tech by Catherine Neish, Ph.D. this semester.
The answer is both silly putty and ketchup are non-Newtonian fluids, meaning their flow properties are different from a normal (Newtonian) fluid, such as water or oil.
Generally speaking, people do not associate silly putty with the word “flow.” But Professor Neish showed us it’s an accurate description: she rolled the putty into a vertical stack and left it standing on a desk. After thirty minutes of lecture, the putty had deformed, flattening seemingly of its own volition, or “flowing.”
Silly putty is actually considered a viscoelastic material because it has both elastic and viscous properties, hence why its flow takes such a long time (when I say “elastic,” I’m referring to the putty’s tendency to act like a solid when thrown; more specifically, like a solid bouncy ball).
However, under the right circumstances, silly putty can be put under too much stress, as show in this video. The amount of time it took the putty to fall is behind its reaction; you can look up Maxwell time if you want to know more.
To better explain what a non-Newtonian fluid is, we watched the below video about ketchup. It describes the frustrating phenomenon of not being able to pour the right amount of ketchup out of the bottle. Either nothing comes out, or way too much comes out. This is actually due to the viscosity of ketchup, which can be affected by how much force you apply to the ketchup bottle or by how long you have been trying to get something to come out of it. Watch this video for more details! Other non-Newtonian fluids include toothpaste, peanut butter, shampoo, blood and paint.
What does any of this have to do with Comparative Planetology? The subject was brought up because rocks and other materials that make up objects in our Solar System have a viscous flow over geologic time scales (a lot longer than a half hour!). More specifically, formations on planet surfaces, such as craters or mountains, have a relaxation time, just as the silly putty relaxed from a vertical position to a flatter one. Comparing different craters shows us some are more defined than others, causing scientists to theorize viscous interiors for observed objects, just as the Earth’s mantle is a viscous layer.
For example, we have noticed significant differences between craters on the Moon and craters on Ganymede, one of Jupiter’s moons. The Moon’s craters are very clear and well-defined; there’s hardly any visible relaxation. Ganymede, on the other hand, has craters that are flattened. Notice in the image below of Ganymede: several of the more recent craters are deep and well-defined, but the others in the surrounding area appear faded. Something had to make those craters deform like this, and viscous flow is our number one theory. Composition is also suspected to play a big part in it, considering Ganymede is an icy satellite while the Moon is just a lump of rock. Ice relaxes much faster than rock, corroborated by these observations.
So there you have it. Silly putty and ketchup helped us learn about the behavior of materials, which have been used to help us observe our Solar System. By observing other objects, we hope to eventually figure out the best theory for our own planet’s formation, which we actually do not know as much about as people would think! Comparative Planetology is turning out to be quite an interesting class, and hopefully all you aspiring space science majors out there will get the chance to take it.