Martian Moons – The Two Moons of Mars Phobos and Deimos
In the August of 1877 an American astronomer was looking at Mars in hopes of discovering a moon. He instead discovered two Martian Moons; Deimos and Phobos
Phobos and Deimos—the mysterious moons of Mars—have intrigued scientists for generations. These tiny satellites orbiting the planet Mars offer clues to the planet’s past and its potential future in human space exploration. In this article, we dive deep into the Martian satellites Phobos and Deimos, unravel fascinating facts about Phobos, and learn how NASA science continues to probe their origins and significance.
Whether you’re a space enthusiast, a student of astronomy, or just curious about what orbits the surface of Mars, this article will offer a thorough overview of the Mars’ moons, their orbits, potential missions, and why they’re crucial for future Mars exploration.
Article Outline
What Are the Moons of Mars and Why Are They Important?
How Were Phobos and Deimos Discovered?
What Are the Main Differences Between Phobos and Deimos?
How Do the Martian Moons Orbit Mars? Could Phobos Crash Into Mars?
What Has NASA Learned from Mars Missions?
What Is the Origin of the Martian Moons?
What Are the Plans for Future Moon Exploration?
How Do Phobos and Deimos Compare to Earth’s Moon?
Why Is Phobos Key to a Mars Sample Return Mission?
What Are the Moons of Mars and Why Are They Important?
Mars has two small moons, Phobos and Deimos, which are thought to be either captured asteroids or remnants from a collision. These tiny moons are some of the smallest moons in the solar system, but they carry outsized importance in the quest for understanding the origin of the martian moons and potential for human presence on the martian surface.
Understanding the moons of Mars helps scientists learn more about planetary formation, orbital dynamics, and the history of impacts in the solar system. Plus, their proximity to Mars makes them ideal targets for future moon exploration and stepping stones for crewed Mars missions.
How Were Phobos and Deimos Discovered?
The two moons of Mars, Phobos and Deimos, were discovered in 1877 by American astronomer Asaph Hall. He first discovered Deimos, followed shortly by Phobos, while observing from the U.S. Naval Observatory. His discoveries added new chapters to the study of moons around planets.
These martian satellites Phobos and Deimos are unique not just because of their size but because of their odd shapes and mysterious origins. Since then, numerous missions—like NASA’s Mars Global Surveyor and Mars Reconnaissance Orbiter—have provided observations of Phobos and helped refine what we know.
What Are the Main Differences Between Phobos and Deimos?
Phobos is the larger of the two moons, measuring about 22 km across, whereas Deimos is only about 12 km wide. Phobos is much closer to Mars and orbits mars every 7.7 hours, three times a day, while Deimos takes 30 hours for a single orbit around Mars.
Despite both being small and close to Mars, their surfaces and compositions are different. The surface of Phobos is covered in dust and regolith, marked by the large Stickney crater, while Deimos was most probably hit in the past but shows smoother terrain due to dust movement.
How Do the Martian Moons Orbit Mars?
Both Phobos and Deimos have nearly circular orbits, unusual for captured asteroids. This has led to debates about the origin of the moons. Phobos lies just 6,000 km from the surface of Mars—closer than any other moon in the solar system—and phobos orbits closer to its planet than any other known moon.
While Deimos has a more stable path, phobos orbits mars so fast that it rises in the west and sets in the east—much faster than Mars rotates. These orbital dynamics are a central point in understanding how martian moons behave around Mars.
Could Phobos Crash Into Mars?
Yes, Phobos could eventually either crash into Mars or break apart, forming a ring around Mars. Its low orbit causes it to spiral slowly inward due to tidal forces. In about 50 million years, this Mars moon Phobos, may disintegrate entirely or collide with the planet Mars.
This catastrophic future makes Phobos an interesting object of study—not just as a moon, but as a case study in orbital decay. Scientists are watching its orbit around the planet closely using data from the mars reconnaissance orbiter and other missions.
What Has NASA Learned from Mars Missions?
NASA’s Mars missions, including Mars Express, Mars Global Surveyor, and Mars Reconnaissance Orbiter, have contributed massively to our understanding of Phobos and Deimos. The surface of the moon Phobos has been mapped in high resolution, revealing fractures and grooves that hint at internal structure and origin.
NASA science has shown that Phobos may be a good site for a sample return from Phobos—a goal many scientists believe could unlock secrets of the solar system. Meanwhile, Deimos remains less explored, but its distant orbit and smoother surface make it intriguing.
What Is the Origin of the Martian Moons?
The origin of Phobos and Deimos is still debated. Some scientists argue they are captured asteroids, while others suggest they formed from debris following a massive impact. A newer theory proposes a common progenitor of Phobos and Deimos that broke apart.
Regardless of which theory holds true, the nature of Phobos and its orbit suggest a violent past. The question of how the moons formed relates to broader ideas about how moons are made in the solar system. Understanding the progenitor of Phobos and Deimos could reshape how we see planetary evolution.
What Are the Plans for Future Moon Exploration?
A mission to Phobos is currently in planning stages. Japan’s MMX (Martian Moons eXploration) mission plans to visit both Phobos and Deimos and even conduct a sample return mission from Phobos. This would provide crucial data back to Earth and possibly even a piece of Phobos.
NASA has also expressed interest in a mars sample return program, where astronauts or robots could study the moons phobos and deimos in detail. As these smallest moons orbit close to Mars, they could also serve as bases for future crewed mars exploration.
How Do Phobos and Deimos Compare to Earth’s Moon?
When comparing Phobos and Deimos to Earth’s moon, the differences are striking. The moon that orbits Earth is much larger, spherical, and geologically active in its past. In contrast, the Martian moon Phobos is irregularly shaped, dusty, and geologically dead.
Moreover, Earth’s moon formed from a massive impact, but the origin of the martian moons is still not fully clear. Nonetheless, both systems offer insight into planetary history and the processes that shape moons in the solar system.
Why Is Phobos Key to a Mars Sample Return Mission?
Because of its proximity and the data already available, Phobos is an ideal target for a sample return mission. Its low gravity makes landing and takeoff easier than the Martian surface, and its history may contain clues about the planet Mars itself.
Furthermore, a mission to moon Phobos would help prepare for longer-term mars mission plans, including a mars rover or crewed base. As such, the phobos reconnaissance and international mars exploration projects are vital stepping stones to broader ambitions.
Naming the Moons of Mars
In the August of 1877 an American astronomer was looking at Mars in hopes of discovering a possible Martian moon, when he discovered two; first Deimos and then Phobos six nights later. He named them after two of the sons of Ares, the Greek god of war. Phobos, who was the god of alarm and panic1 (and where we get the name phobia), and Deimos the god of fear1.
Shape and Orbit of the Martian Moons
Neither Phobos nor Deimos are spherical like many other moons which is due to their comparably small size. The gravity created by any object with mass, acts upon that body and others by attracting matter to the centre of that object. When an object reaches a certain size their gravity is strong enough to start shaping the object by drawing the matter towards its centre and the shape that is produced is a sphere. As these two moons have a gravitational pull too weak to affect their shape they are both as they have originally formed and have been formed by natural events such as collisions.
The orbit of Phobos is much faster than Deimos as shown in the illustration above.
Origin of the Martian Moons
Phobos and Deimos are of similar material to the asteroids which lie in a belt between Jupiter and Mars. Some scientist believe that the two moons may actually be asteroids caught in the gravitational pull of Mars while some believe that it is the remnants of a large object that was shattered in a collision.
Phobos
Phobos was discovered on the 11th of August 1877 by Asaph Hall whilst working at the US Naval Observatory. The large crater on the right hand side of the image below was named after his wife ‘Stickney’ who encouraged Hall in his work. The Crater Stickney is approximately 9.6km or 6 miles which is almost as wide as the moon itself. The lines on the surface of Phobos are believed to have been caused by the impact that caused the Stickney crater.
Phobos is the larger of the two Martian moons, with a mean radius of 11.1km or 6.9mi (our moon has a mean radius of 1,737.4 km or 1,079.6mi). It travels in orbit at a distance of 6,000km or 3,700mi from the Martian surface which is closer than any other moon in our solar system. This distance is actually decreasing by 1.8 meters every hundred years which will eventually result in Phobos colliding with Mars or breaking up and forming a Martian ring (but not for millions of years).
Phobos orbits Mars three times each day (Martian days last similar length to Earth’s at 24.6hrs as opposed to earth days at 23.9hrs).
This image was taken of Phobos from low Martian orbit by NASA’s Mars Reconnaissance Orbiter using the Compact Reconnaissance Imaging Spectrometer for Mars. The image gives a good indication of how this relatively small but very close moon looks in the Martian night sky.
Deimos
Deimos, like our own moon, is moving away from the red planet and will one day move out of reach of the Martian gravity where it will drift slowly away into space. The surface of Deimos has suffered fewer collisions than Phobos but it too has many craters, some of which are two miles wide.
This image was taken of Deimos from low Martian orbit by NASA’s Mars Reconnaissance Orbiter using the Compact Reconnaissance Imaging Spectrometer for Mars. The image gives a good indication of how the tiny moon looks in the Martian night sky.
Quick Comparison of Martian Moons
Phobos
Deimos
Dimensions
27×21.6×18.8km (12.4×14.3×17.4mi)
15×12.2x11km (6.2×7.5x 9.9mi)
Volume
5,729km3
998km3
Mass
10,658,529,896,187,200kg
1,476,188,406,600,170kg
Mean Distance From Mars
9,380km (5,826mi)
23,460km (14,573mi)
Orbital Period
7.66 hours
30.35 hours
Sources
1.The Dictionary of Mythology by J. A. Coleman
