1. Are the Grooves on Phobos a Sign That It Is Breaking?
Phobos orbits Mars, and its surface is covered in many long grooves. For a long time, these were thought to be cracks caused by collisions. However, researchers at NASA, including one named Hord, proposed a different idea. They suggested the grooves might be "stretch marks" caused by tidal forces deforming the moon. [3]
This theory depends on what Phobos is made of. It is not a solid rock. Instead, it is thought to be a "rubble pile"—loose rocks held together weakly. The surface is covered in a layer of dusty sand about 100 meters thick, called regolith. If this model is correct, the grooves are the first signs that Phobos is starting to fall apart. [3]
It is important to note that this is just one model. It has not definitively proven why the grooves exist. But it connects to Phobos’ future. As it moves closer to Mars, the planet’s pull gets stronger. Why does getting closer cause a moon to break?
2. Why Does a Planet's Tidal Force Tear Moons Apart?
The reason a moon breaks when it gets too close is called tidal force. A planet pulls harder on the side of the moon that is closer to it. It pulls less on the side that is farther away. This difference in pull stretches the moon toward the planet. [1]
At the same time, the moon’s own gravity tries to keep it in a round shape. If the planet’s stretching force becomes stronger than the moon’s own gravity, the moon can no longer hold its shape. The point where this happens is called the Roche limit. It is named after a French astronomer, Edouard Roche, who calculated it in the mid-19th century. [1]
The Roche limit is not a single fixed distance. It changes based on what the moon is made of. If the moon is soft and fluid-like, the limit is about 2.44 times the planet’s radius, adjusted by the density of the planet and moon. If the moon is solid and hard, the factor is about 1.26. [2]
Think of it like a snowball versus an ice ball. If you pull them with the same force, the snowball breaks much easier than the ice ball. Similarly, a solid moon can stay closer to a planet than a loose one.
3. How Did Jupiter Break Up Comet Shoemaker-Levy 9?
We have seen this happen in real life. On July 8, 1992, Comet Shoemaker-Levy 9 passed very close to Jupiter. It came within about 40,000 kilometers of Jupiter’s clouds. The tidal forces were so strong that the comet broke into more than 20 pieces. [5]
Interestingly, humans did not see the moment it broke. The comet was discovered in March 1993, already split into a line of fragments. These pieces looked like a string of pearls. They crashed into Jupiter in July 1994. [5]
The International Astronomical Union’s educational glossary uses this comet as a key example of the Roche limit. [6] Some chains of craters on moons, called catenae, are also thought to be caused by asteroids breaking up before hitting the surface. However, not all crater chains are formed this way.
4. What Happens to the Pieces of a Broken Moon?
What happens to the pieces after a moon breaks? Inside the Roche limit, small particles struggle to clump together into a new moon because the planet’s gravity keeps pulling them apart. They tend to spread out and form rings. Outside the Roche limit, particles can gather together to form moons. [2]
Most known planetary rings are inside their planet’s Roche limit. For example, some of Saturn’s rings are within 2.3 times Saturn’s radius. Jupiter’s rings are within 1.8 times Jupiter’s radius. Jupiter’s moon Amalthea orbits just outside this limit. [1]
There are exceptions. Some moons inside the limit, like Pan at Saturn or Metis at Jupiter, do not break. They stay together because they are solid, not just held by gravity. The Roche limit applies to objects held together only by their own gravity. [2]
A model from Purdue University suggests Phobos will reach the limit and break into a ring around Mars. This ring might eventually fall into Mars or gather into small moons. This cycle may have happened 3 to 7 times in the past, but this is still a hypothesis. [4]
5. Could Saturn's Rings Be the Remains of a Broken Moon?
Are Saturn’s rings also the remains of a broken moon? Since some rings are inside the Roche limit, this is a likely possibility. But when did they form? [1]
In 2019, data from the Cassini spacecraft suggested the rings are relatively young. Based on their mass, scientists estimated they formed 10 to 100 million years ago. Saturn itself is about 4.5 billion years old. If Saturn were 45 years old, its rings would be like a baby, only 1 month to 1 year old. [7]
There are two main ideas about how they formed. One is that a comet broke up. The other is that an icy moon broke up. A model by Canup in 2010 suggests a moon the size of Titan, with an icy mantle and rocky core, approached Saturn. The Roche limit stripped away the icy mantle to form the rings, while the core fell into Saturn. This explains why the rings are mostly pure water ice. [8]
However, in December 2024, a team including Ryuki Hyodo proposed a different view. They said that tiny meteorites hitting the rings create charged particles that fall into Saturn or its atmosphere. This means the rings might be older than they look, possibly as old as the solar system itself, about 4.5 billion years. The age of Saturn’s rings is still debated. [9]
6. How Can We Calculate Phobos' Movement Toward Mars?
You can use a calculator to see how fast Phobos is moving. It moves 2 meters every 100 years. Over 30 million years, that is 300,000 cycles of 100 years. So, 2 meters times 300,000 equals 600,000 meters, or about 600 kilometers. Over 50 million years, it would be about 1,000 kilometers. [3]
This is a rough estimate. It assumes Phobos moves at a constant speed, which may not be true. Moving 600 kilometers closer does not automatically mean it breaks instantly. It depends on when it reaches the Roche limit. To understand how long 30 million years is, try calculating the distance for shorter times. In 1,000 years, it moves 20 meters. In 10,000 years, it moves 200 meters. [3]
These numbers help us imagine the scale. The same force that creates grooves on Phobos, splits comets, and debates the age of Saturn’s rings is tidal gravity. How far away is Phobos from Mars right now? That is the next question to investigate.