Exploring Mars' Moons: Phobos and Deimos - Josh Habka

This article covers fundamental information about Phobos, and Deimos, the moons of Mars, including history, discovery, composition, and orbits.
Exploring Mars' Moons: Phobos and Deimos - Josh Habka

This comprehensive article delves into the intriguing moons of Mars, Phobos and Deimos. This article explores their origins, physical characteristics, orbital dynamics, and potential implications for Martian exploration. By synthesizing data from planetary missions and theoretical models, this study aims to provide a thorough understanding of these enigmatic satellites.


Mars: The Red Planet

Mars, aptly named the "Red Planet" for its distinctive rusty hue, has long been a focal point of scientific inquiry and human fascination. Situated as the fourth planet from the Sun, Mars occupies a pivotal position in the solar system, offering valuable insights into planetary evolution and the potential for extraterrestrial life. Its barren landscapes, ancient river valleys, and towering volcanoes evoke a sense of wonder and curiosity, beckoning explorers and scientists to unravel its mysteries.

Phobos and Deimos: Martian Moons

Amidst the vast expanse of space surrounding Mars orbit two diminutive moons: Phobos and Deimos. Discovered over a century ago by the pioneering astronomer Asaph Hall, these enigmatic satellites have captured the imagination of scientists and space enthusiasts alike. Despite their modest size and unassuming appearance, Phobos and Deimos hold significant scientific interest due to their unique characteristics and potential implications for Martian exploration.


Phobos is the larger of Mars' two moons, discovered in 1877 by American astronomer Asaph Hall. It orbits Mars closely, completing a revolution in just 7 hours and 39 minutes. With a heavily cratered surface and a mean radius of approximately 11.1 kilometers, Phobos is primarily composed of regolith. Tidal forces are gradually causing Phobos to spiral inward, leading to predictions of its eventual destruction within the next 30 to 50 million years.


Deimos, the smaller of Mars' moons, was also discovered by Asaph Hall in 1877. It orbits Mars at a greater distance than Phobos, completing a revolution in approximately 30 hours and 18 minutes. With a relatively smooth surface and a mean radius of approximately 6.2 kilometers, Deimos is composed of carbonaceous chondrite material. Its low surface gravity makes it a challenging target for exploration, but future missions may seek to study its composition and origins within the Martian system.

Discovery and Naming

The Discovery

In the annals of astronomical history, the year 1877 marked a significant milestone with the discovery of Phobos and Deimos by the eminent American astronomer Asaph Hall. Armed with the formidable capabilities of the United States Naval Observatory's 26-inch refracting telescope, Hall embarked on a quest to explore the heavens above with unparalleled precision and diligence.

Asaph Hall, American astronomer who discovered Deimos and Phobos in 1877 and calculated their orbits(Brittanica).

The Observatory's Role

Situated atop a hill overlooking the bustling city of Washington D.C., the United States Naval Observatory served as a beacon of scientific inquiry, its towering telescope peering into the depths of space with unwavering resolve. Guided by meticulous observations and unwavering determination, Hall meticulously scanned the night sky, driven by a fervent desire to uncover the secrets hidden among the celestial bodies.

The Naming Conundrum

In homage to their newfound celestial subjects, Hall turned to the rich tapestry of Greek mythology for inspiration, seeking names that would aptly encapsulate the enigmatic nature of these Martian moons. Drawing from the pantheon of gods and goddesses, he bestowed upon them the names of Phobos and Deimos, symbolic of the sons of Ares, the fearsome deity of war.

Symbolism in Mythology

Phobos: The Personification of Fear

In Greek mythology, Phobos personifies fear itself, a relentless force that grips the hearts of mortals and gods alike with an ironclad grasp. As the embodiment of terror and trepidation, Phobos casts a shadow of apprehension wherever he treads, instilling dread in the hearts of all who dare to gaze upon his ominous visage.

Deimos: The Harbinger of Terror

Deimos, the lesser-known sibling of Phobos, embodies the essence of terror and dread, his presence heralding impending doom and calamity. With a name that strikes fear into the hearts of mortals, Deimos stands as a silent sentinel of the night, his ominous silhouette casting a pall of foreboding over the Martian landscape.

Physical Characteristics

Mean Diameter22.2 kilometers12.4 kilometers
Orbital Period7 hours and 39 minutes30 hours and 18 minutes
Surface CompositionPrimarily regolithCarbonaceous chondrite
Surface FeaturesNumerous impact cratersSmooth surface with craters
Surface Gravity0.0057 m/s²0.003 m/s²

Phobos, the larger and closer of the two moons, exhibits a heavily cratered surface, with Stickney crater being the most prominent feature. In contrast, Deimos displays a smoother surface with fewer visible impact craters. Both moons are irregularly shaped and exhibit low surface gravity, making them unique among the moons in the solar system.

Origins and Formation

The origin of Phobos and Deimos remains a subject of debate among scientists. One prevailing hypothesis suggests that they are captured asteroids, originating from the asteroid belt between Mars and Jupiter. Alternatively, some theories propose that Phobos and Deimos may have formed from debris ejected during a giant impact event on Mars or from accretion within the Martian system.

Captured Asteroids Hypothesis

One prevailing hypothesis suggests that Phobos and Deimos are captured asteroids, originating from the asteroid belt located between Mars and Jupiter. According to this theory, these moons were gravitationally captured by Mars during the early stages of the solar system's formation, subsequently adopting their current orbits around the Red Planet.

Giant Impact Event

Alternatively, some theories propose that Phobos and Deimos may have formed from debris ejected during a catastrophic giant impact event on Mars. This scenario posits that a powerful collision between Mars and a celestial body resulted in the expulsion of material into orbit, ultimately coalescing to form the moons we observe today.

Accretion within the Martian System

Another hypothesis suggests that Phobos and Deimos may have formed through the process of accretion within the Martian system itself. According to this theory, the moons may have gradually accreted from dust and debris present in the vicinity of Mars, eventually coalescing into their current form over geological timescales.

Orbital Dynamics

Phobos and Deimos orbit relatively close to Mars, with Phobos being the closer of the two, at an average distance of 9,378 kilometers, while Deimos orbits at an average distance of 23,460 kilometers. Their orbital dynamics are influenced by Mars' gravitational pull, resulting in tidal forces that are gradually causing Phobos to spiral inward towards Mars.

GIF showing the basic orbits of Phobos and Deimos compared to Mars Express and Martian Reconnaissance Orbiter(MRO). ESA

Potential Implications for Martian Exploration

The moons of Mars hold significant implications for future exploration missions to the Red Planet. Phobos, in particular, has been proposed as a potential staging ground for human missions to Mars, offering a convenient platform for scientific research and resource utilization. Additionally, studying the composition and geology of Phobos and Deimos could provide valuable insights into the history and evolution of the Martian system.

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