The moon of Earth stands out as a prominent feature in our night sky. Scientists largely agree that during the early stages of Earth’s formation, a smaller, planet-like object collided with Earth, ejecting a substantial amount of material into space. This debris was subsequently pulled into orbit around Earth due to gravity and maintained a slow enough speed to become trapped in Earth’s gravitational field. However, the
giant impact hypothesis
has provided clarity on the origin of our moon. In contrast, the origins of other moons in our solar system, like the Martian moons Phobos and Deimos, remain a topic of debate.

An alternate theory suggests that two small celestial bodies approached Mars early in its existence and collided with the gas and dust clouds left from its formation. This surrounding dust could have decelerated them sufficiently for Mars’ gravity to capture them. This theory is referred to as the
gas drag capture hypothesis
and may account for the existence of Phobos and Deimos. Furthermore, they are composed of
different materials
than those found on Mars
, which raises additional questions.

One challenge to this theory is that the dust density around Mars would have to be several times greater than current models of solar system formation indicate, to slow down approaching objects effectively. Additionally, there’s a question of probability. Although Phobos and Deimos both have orbits that lie within 2° of the Martian equator, the odds of both objects aligning with Mars at an angle that matches the equator is around only 0.00001%.

To investigate the viability of this scenario, two scientists from Japan developed a model aimed at calculating the trajectory of a Phobos-sized object approaching Mars. The aim was to show, through various challenges, that the gas drag trap hypothesis might not be as implausible as previously believed.

Initially, the researchers defined the pertinent equations of motion to include in their model. This included variables such as the angular velocity of an object approaching Mars, its distance from the planet, its potential energy, and the drag force that reduces its speed. Additionally, they factored in Mars’ mass and the state of the surrounding matter at the time, which they referred to as the primitive atmosphere of Mars. They estimated this atmosphere’s temperature at 200 Kelvin (approximately -73°C or -100°F) and its density at 4.7 × 10.^-7 kilograms per cubic meter, increasing near the Martian surface and decreasing exponentially with height.

Next, the team needed to establish the initial orbit of the incoming satellite, testing eight different speeds ranging from 20 meters/second to 160 meters/second (about 45 miles/hour to 360 miles/hour) in 20 meters/second increments. There were 4,096 angles of incidence to be tested relative to Mars’ equator and poles, leading to a total of 32,768 initial trajectory combinations for objects approaching Mars.

Their findings indicated three potential outcomes for objects entering Mars’ primordial atmosphere: they could escape Mars’ gravitational grasp, become temporarily trapped, or be permanently ensnared. Remarkably, nearly all objects approached at the slowest speeds were captured in some capacity, while only around 10% of those at the highest speeds were captured. The researchers posited that about 1 in 50 incoming objects would be permanently secured by Mars, particularly if they lost enough energy, limiting their orbits to within 10 degrees of Mars’ equator.

The research team proposed a potential history for Phobos and Deimos, suggesting that due to their composition, they likely formed in the outer solar system, possibly within or beyond the asteroid belt. Over time, they may have been scattered by Jupiter’s gravitational influence, gradually approaching Mars at the right angles and speeds to be captured by its gas, resulting in their current eccentric orbits. Eventually, their orbits became slower, more circular, and moved closer to Mars.

This proposed scenario aligns well with current observations of Phobos and Deimos. The research team anticipates that future
Mars satellite exploration
missions will further investigate these moons. The planned mission will orbit Mars and then Phobos, conducting detailed observations and remote sensing while collecting surface samples to return to Earth, enhancing our understanding of these moons’ origins. The mission is set to launch in 2026, with Phobos samples expected to arrive back on Earth in 2031.

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