How Many Moons Does Mars Have? A Look at Phobos and Deimos

How Many Moons Does Mars Have? A Look at Phobos and Deimos
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Two Moons Are Better Than One

Although similar in many ways, there are vast differences between the world we are familiar with – Earth – and the closest planet near us – Mars. Among the most radical of differences is the existence of two moons surrounding the red planet.

After millions of years of human existence, the fact that our planet is orbited by a single satellite has had both a physical and cultural impact on our development. How would our world be different if we had two moons? By looking at both satellites of Mars, Phobos and Deimos, one can not only speculate but take into account the unique factors which make these bodies interesting celestial objects.

Discovering the Moons

486px-Professor Asaph Hall

Navy Telescope

For years before they were finally identified, scientists and writers speculated on the existence of Martian moons. Ironically, many early conjectures centered around the orbit of two bodies. Most notable was the work of Johannes Kepler, Voltaire and Jonathan Swift.

It took a young man from Connecticut to finally disprove the erratic stories and formulate a legitimate location. Asaph Hall, an orbital expert at the Harvard College Observatory, secured his place in history in 1877. Taking a post with the United States Naval Observatory, Hall traveled to Washington, DC to take charge of the 26-inch (66-cm) refracting telescope, the largest in the world at the time.

Increasingly fascinated by the suppositions regarding their existence, Hall became obsessed with locating and identifying the red planet’s moons. Through intense research and long observations, on the morning of August 10th Hall believed he had finally found one of the moons. However, the weather was poor, prompting the astronomer to duplicate his process days later.

Roughly 48 hours later, on August 12, Asaph Hall’s diary entries confirm that he did indeed identify the Martian moon of Deimos. This was soon followed by Phobos, viewed by Hall on the 18th. This discovery through open the doors to astronomy unlike others in decades. The search was on for more moons from even more planets. The solar system was beginning to take shape. Despite this sudden success, or possibly because of it, many scientists continued to search endlessly for “lost” Martian moons.

Technical Data on Phobos

Phobos colour 2008

  • Date of Discovery: August 18, 1877
  • Mythology: Son of Ares, Greek God of War; Personification of Fear & Panic
  • Dimensions: 27 x 22 x 18 km
  • Radius: 11.1 km
  • Density: 1.876 g/cm3
  • Speed of Orbit: 2.138 km/s
  • Length of Day: 7.653 hours
  • Semi-Major Axis: 9,377.2 km
  • Surface Gravity: 0.0084 to 0.0019 m/s2
  • Escape Velocity: 11.3 m/s
  • Inclination to Equator: 1.093 degrees
  • Orbital Eccentricity: 0.0151
  • Albedo: 0.071
  • Surface Temperature: -70°F to 25°F (-112°C to -4°C)

Technical Data on Deimos


  • Date of Discovery: August 12, 1877
  • Mythology: Son of Ares, Greek God of War; Personification of Fighting
  • Dimensions: 15 x 22 x 10.4 km
  • Radius: 6.2 km
  • Density: 1.471 g/cm3
  • Speed of Orbit: 1.35 km/s
  • Length of Day: 30.2976 hours
  • Semi-Major Axis: 23,460 km
  • Surface Gravity: 0.0039 m/s2
  • Escape Velocity: 5.6 m/s
  • Inclination to Equator: 0.93
  • Orbital Eccentricity: 0.0002 to 0.0005
  • Albedo: 0.068
  • Surface Temperature: -40°F to 62°F (-40°C to 17°C)

Egg-Shaped and Pot-Marked: Moon #1

Beyond simple astronomical factors and sheer numbers, there are many unique factors which make Phobos a scientific marvel:

  • The large crater on the tip of Phobos is named Stickney. The Mars Global Surveyor discovered it is filled with a fine, dust-like powder most likely from broken down meteoroids impacting its surface over millions of years.

  • The larger of the two moons, Phobos orbits Mars so closely that it cannot be seen from certain areas of the planet.

  • Phobos is on a collision course with Mars, advancing toward each other at a rate of 1.8 meters every 100 years. This means they will collide in approximately 50 million years.

  • It is believed that the high level of heat loss on Phobos is a direct result of its surface being primarily made from a fine dust. This dust is referred to as C-type rock, found most commonly on asteroids made from carbonaceous chondrite.

  • Light and dark are extreme factors on Phobos. NASA compares the daylight climate as similar to that of Chicago on a winter’s day, while the dark side (only a couple of miles away) feels like Antarctica.

  • Phobos possesses absolutely no atmosphere.

  • Flybys conducted by NASA and other space agencies have shown that Phobos is most likely a porous object circling the red planet. According a 2010 Mars Express test on the gravitational field, scientists believe that 25 to 35 percent of the moon must be a void under the surface.

Small, But Fascinating: Moon #2

Smaller than it’s brother, Deimos nonetheless also possesses some intriguing features:

  • While Phobos is a patchwork of craters, Deimos only has two noticeable imprints on its surface. These are known as Swift and Voltaire and both measure under a mile across.

  • When a celestial body’s surface is hit by a meteorite, it throws dust and other material into the atmosphere and it lands back around the crash site. However, this dust is essentially non-existent on Deimos as the material escapes the low gravity into the vacuum of space.

  • Deimos has a very unique pattern in its orbit around Mars. Instead of a relatively normalized rising and setting convention which matches the planet, the moon keeps a time of rising every 2.7 days.

  • The second moon is located so far away from its parent planet that it merely appears from the surface as simply another star in the sky.

  • Due to its distance from the red planet, solar eclipses are a regular occurrence on Mars. Deimos passes between the planet and the Sun very often. However, this appears as nothing more than a black dot due to the diminutive stature of the moon.

That’s No Moon, That’s a Space Station!

Walter Scott Houston Stellafane

Due to the overall fascination of Mars as a planet outside of Earth likely to support life, the Great Plains Observer printed an article in 1959 that stated both Martian moons were found to be artificial satellites. In the report, a fabricated University of the Sierras’ Dr. Arthur Hayall claimed he had evidence that pointed to Phobos and Deimos being space stations of incredible advancement and size.

It turned out that this publication was merely a bogus April Fool’s hoax perpetrated by Walter Scott Houston, an English teacher and amateur astronomer from Wisconsin. The article garnered so much national and international attention from the scientific community that it was reported famed Soviet scientist Iosif Shklovsky took questions about the topic during an interview for a Russian tabloid.

So Where Do These Moons Come From Anyway?

After discovering a new object in the Solar System and studying it to find out all the details possible, the next logical thing for astronomers to do is to learn its origins. The same is obviously true regarding where Phobos and Deimos came from. While the debate regarding the formation of our own moon has taken up countless volumes in the annals of astronomy, the moons of Mars are still a reflection of what we do not know.

The real challenge with these objects is the fact that while they are both orbiting the fourth planet in the Solar System, they have little in common with their parent. In addition, while they have similarities to each other, they are as different as two siblings can be. This makes an already complicated investigation even more so.

As mentioned earlier, both of the Martian moons are composed of materials similar to asteroids. This C-type rock possesses the same traits as many of those found within the asteroid belt, notably the albedo and density. Many scientists point to this fact to support the theory that these are moons originally captured from the belt.

However, this would require both Phobos and Deimos to have orbited Mars due to a specific cause millions of years ago. They would also need to have been pulled into orbit of the red planet. In order to do this, the energy of the asteroids would need to dissipate. While it all depends on the trajectory of the moons, this is questionable due to the fact that the present-day atmosphere of Mars may be too thin to provide the required force to slow down the moons.

Other scientists believe that at one time Mars was surrounded by countless objects of similar size and shape as Phobos and Deimos. Over millions of years, each object simply broke away or was destroyed for various reasons.

This is supported by the fact that much of the composition of the moon Phobos also contains phyllosilicates, a substance that is most readily found on the surface of the planet rather than any asteroids in the system. Proponents of this theory point to the natural production of a moon rather than the wandering satellite concept.

Regardless of where Phobos and Deimos originated, the fact that they are there and are among some of the more fascinating bodies in the Solar System is the important fact. While some will simply say the two satellites are simply more moons to be explored, many other scientists and amateur explorers will dream about the fact that these two celestial bodies represent the unknown. The Martian moons are two strange objects orbiting the infamous red planet.

They may be a pair of asteroids somehow breaking from the belt and claiming their rightful place in the Solar System. They are two dark brothers, birthed from the God of War and positioned in battle with the forces of the Universe. So how many moons does Mars have? There may technically only be two, but they are some of the most fascinating and diverse bodies in the Solar System.