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Physics Newsletter September #2

  • Bhavya Goel
  • Sep 21, 2024
  • 5 min read

Physics Pulse: Physics Newsletter

By: Bhavya Goel - Researcher




Gigantic asteroid impact shifted the axis of Solar System's biggest moon 



Around 4 billion years ago, a massive asteroid hit Ganymede, the largest moon of Jupiter and the Solar System. Recently, a scientist from Kobe University discovered that this impact was so powerful it shifted Ganymede’s axis, the imaginary line it spins around. The asteroid was about 20 times bigger than the one that caused the dinosaurs to go extinct, making it one of the largest impacts known in our Solar System.


Ganymede, which is even bigger than Mercury, is an icy moon with oceans of liquid water beneath its surface. Like Earth’s moon, Ganymede is tidally locked, meaning it always shows the same face to Jupiter as it orbits. On its surface, scientists noticed large furrows, or narrow trenches, that form circular patterns around one specific spot. These furrows were thought to be the result of a massive asteroid strike.


Dr. Hirata, the scientist behind this recent discovery, noticed something new. He realised that the spot where the asteroid hit Ganymede is almost exactly on the opposite side of the moon from Jupiter. This was important because a similar event happened on Pluto, where an asteroid impact shifted Pluto’s axis. Using this clue, Dr. Hirata concluded that Ganymede’s axis must have shifted because of the asteroid.


Dr. Hirata specialises in running computer simulations that show what happens when asteroids hit moons or planets. These simulations allowed him to figure out how big the asteroid was. He estimated that the asteroid was about 300 kilometres wide, which is about 20 times larger than the asteroid that hit Earth 65 million years ago and wiped out the dinosaurs. The impact formed a transient crater (a temporary hole before the ground settles) that was between 1,400 and 1,600 kilometres wide. The collision likely changed the way Ganymede’s mass was distributed, causing its axis to shift.


Dr. Hirata is curious about how this impact affected Ganymede’s internal structure and its layers beneath the surface, including its subsurface oceans. He hopes to study the thermal and structural effects (how heat and the physical make-up changed) from the asteroid impact.


In the near future, more information may become available to help answer these questions. The European Space Agency’s JUICE space probe is scheduled to reach Ganymede in 2034. JUICE will orbit Ganymede for six months, gathering data and sending back detailed information to Earth that could help scientists like Dr. Hirata learns even more about the history and structure of Ganymede.



Doughnut-shaped region found inside Earth's core deepens understanding of planet's magnetic field



Scientists from the Australian National University (ANU) have discovered a doughnut-shaped region deep inside Earth’s outer core, which sheds new light on how our planet’s magnetic field is generated. This region, found thousands of kilometres below the surface, sits near the equator and slows down seismic waves, the vibrations caused by earthquakes, as they pass through the Earth's core. The structure was previously undetected because it can only be observed by studying seismic waves long after an earthquake occurs, which is different from the usual method of observation.


Earth’s core consists of two main layers: the solid inner core and the liquid outer core. The outer core is mostly made up of molten iron and nickel, and it’s this moving liquid that creates the planet’s magnetic field—a crucial shield that protects us from harmful solar winds and radiation. The newly discovered doughnut-shaped region contains lighter chemical elements, which mix with the molten iron and nickel. This combination of elements, along with temperature variations, helps stir the liquid in the outer core, affecting how the magnetic field is produced.


The research team, led by Professor Hrvoje Tkalčić, discovered that this doughnut-shaped region plays a role in slowing down seismic waves. Because of this, they inferred that the region must have a unique composition compared to the rest of the outer core. Understanding this region could help scientists unlock more mysteries about Earth’s magnetic field, such as how it could change over time or what could cause it to weaken or stop.


The magnetic field is essential for life on Earth because it forms a protective barrier against dangerous solar radiation. This discovery helps scientists gain a clearer picture of the processes deep inside Earth that keep this magnetic shield intact. It also highlights how seismic waves can reveal hidden structures in the planet's interior, deepening our understanding of Earth's inner workings.



Physicists discover “hidden turbulence” throughout van Gogh’s Starry Night 



Vincent van Gogh’s The Starry Night (1889) continues to captivate both art lovers and scientists alike, with its swirling vortices in the night sky offering a unique blend of artistic mastery and scientific insight. While many interpret the painting’s swirling patterns as a reflection of van Gogh's inner turmoil, physicists see a remarkable representation of atmospheric turbulence. A recent study published in Physics of Fluids reveals that the illusion of movement in the painting is due not only to van Gogh’s depiction of turbulence but also to the scale of his paint strokes, uncovering a second type of “hidden turbulence” at the microscale.


According to co-author Yongxiang Huang, van Gogh’s intuitive understanding of natural phenomena, such as the movement of clouds and air, might have informed his precise depiction of turbulence. The painting's flowing brushstrokes convey this effect, and earlier research has even suggested similarities between the turbulence in The Starry Night and patterns seen in space, such as molecular clouds where stars are formed.


In physics, turbulence refers to chaotic fluid movements, often involving eddies and vortices. Mathematicians and physicists, including Andrei Kolmogorov in the 1940s, have attempted to describe turbulence mathematically. Kolmogorov’s scaling law describes how energy cascades from large turbulent eddies to smaller ones in a fluid system. Van Gogh’s painting has been shown to align with this law, highlighting how his artistic technique captured the essence of turbulent flows.


A 2019 analysis even found that the luminance of van Gogh’s brushstrokes mirrors the turbulent features of clouds, sharing similarities with Kolmogorov's scaling in nature. This recent study by Huang’s team goes further, revealing that the microscale brushstrokes in The Starry Night also align with a different phenomenon called Batchelor scaling, which describes fluctuations at small scales before diffusion takes over. The discovery of both types of scaling in van Gogh's painting is rare, underscoring the artist's unique ability to capture the complexities of nature in his work.


These findings suggest that van Gogh’s intuitive grasp of fluid dynamics could offer fresh insights into the study of turbulence, opening new pathways for understanding similar phenomena across various scales in nature.



Physics race pits Usain Bolt against Jurassic Park dinosaur



Scott Lee, a physics professor at the University of Toledo, has developed a unique approach to inspire enthusiasm in introductory physics students. His latest activity, published in The Physics Teacher by AIP Publishing, poses a fun question: "Is Usain Bolt faster than a 900-pound dinosaur?" This exercise challenges students to apply 1D kinematics concepts—displacement, speed, velocity, and acceleration—using spreadsheets to determine whether Bolt could beat Dilophosaurus wetherilli in a 100-metre race.


Lee, who has long been fascinated by dinosaurs, realised the potential of using them to engage students in physics. The Dilophosaurus was chosen because its speed was comparable to Bolt's, making for an interesting race. More famous dinosaurs like Tyrannosaurus rex were slower and wouldn't have made for a compelling competition.


The activity also introduces students to Newton's second law, focusing on how mass and force impact acceleration. Bolt's smaller size gives him an early advantage, ultimately allowing him to beat the dinosaur by 2 seconds. The exercise concludes with real-world examples, like how lionesses use their acceleration to catch faster prey.


Lee hopes this creative approach will encourage other educators to think outside the box when designing physics problems that connect with students' interests.



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