The Many Moons Of Mars

Mysterious and enticing–a true Wonderland world–Mars has sung its scientific siren’s song for years to those who seek to understand its many long-held secrets. This small, rocky world with an intriguing red hue, gets its rusty color from the large amount of iron oxide that coats its surface. Much of this small world’s charm comes from its reputation of being the happy abode of “little green men”–Earth’s neighboring planet that plays host to life as we know it. However, Mars has many captivating features and bewitching mysteries, in addition to the somewhat dated idea that it is the most likely world in our Solar System–other than our Earth–to host living creatures. The duo of small potato-shaped Martian moons, Phobos and Deimos, are frequently considered to be captured asteroids that the Red Planet’s gravity snared when they were making an ancient and unfortunate journey through interplanetary space from their place of birth in the Main Asteroid Belt, located between the orbits of Mars and Jupiter. The duo of rocky objects now circle their adopted parent-planet–at least, for the time being. However, in July 2016, a team of astronomers proposed an alternative viewpoint, suggesting that the two little moons were born from an ancient impact on the Martian surface by a crashing primordial object–along with many other now long-lost little moons.

Ever since their discovery in 1877, Phobos and Deimos have both bewildered and bewitched astronomers trying to answer the question of how Mars ended up with its duo of misshapen little moons. However, this perplexing riddle might have been solved by a multidisciplinary study conducted by French, Belgian, and Japanese scientists.

Astronomers have for years contemplated two competing hypotheses explaining the origin of the Martian moons. The first proposes that Phobos and Deimos are, indeed, escapees from the Main Asteroid Belt. Alas, this viewpoint begs the question of why they should have been so cruelly captured by their adopted parent-planet in the first place. An alternative theory points to the possibility that the moons were born from the debris left by a violent collision between Mars and a primordial protoplanet–a baby planet still under construction. However, this theory also suffers from uncertainty because it does not explain precisely how this particular tragic mechanism gave rise to Phobos and Deimos.

“A major difficulty has been to explain why a giant impact on Mars would have left two moons so different from our own Moon, a huge single mass, that also formed from Earth undergoing such an impact,” explained Dr. Sebastien Charnoz in a July 4, 2016 Centre National de la Recherche Scientifique (CNRS) Press Release. Dr. Charnoz is a planetary scientist at the Institut de Physique Du Globe De Paris (IPG) who contributed to the new research.

The Giant-Impact Theory, alternatively termed the Theia Impact, or Big Splash Theory, proposes that Earth’s Moon was born from the debris remaining from a catastrophic collision, that occurred about 4.3 billion years ago, between the primordial Earth and an unfortunate protoplanet, that was about the size of Mars. The Earth’s Moon-forming collision would have occurred when our Solar System was still forming during the Hadean eon. The Hadean eon occurred about 20 to 100 million years after our Solar System emerged from its frigid, dark natal cloud of gas and dust. The doomed impacting protoplanet, often called Theia, received its name in honor of a Titan in Greek mythology who was the mother of Selene, the Moon goddess. An analysis of lunar rocks, published in 2016, indicates that this catastrophic crash was a direct hit–causing a thorough mixing of both Earth-stuff and Theia-stuff. The Giant-Impact Theory is the favored scientific explanation for the birth of Earth’s Moon.

When a moon is in an orbit around its parent-planet, all is well–just as long as the gravity that is holding the moon together in one piece exceeds the powerful, relentless pull of its planet. Alas, if a moon wanders too close, and the tidal forces of the parent-planet exceed the gravitational bind of the unlucky moon, the moon will fall apart. This is termed the Roche limit. Earth’s relatively large Moon is a very fortunate natural satellite, and the limit here is a bit under 10,000 kilometers–while our Moon is a safe 385,000 kilometers away from our planet.

Alas, this is not the case with Phobos and Deimos. Phobos, the larger of the duo of little Martian moons, at 22 kilometers in diameter, is lazily tumbling towards Mars and will approach the Roche limit in about 20 million years. At this tragic point, it will shatter into fragments that will ultimately form a spectacular ring around its planet. Only Deimos will remain–bereft of its companion. Deimos is the smaller moon of the pair, and circles its parent-planet further out. This last remaining little moon will be a lonely object lingering in the Martian sky at this sad point–but it was not always thus. The new 2016 study suggests that Mars once possessed a very complex system of many moons.

In order to shed new light on the mysterious origins of the surviving duo of Martian moons, the researchers conducting this study combined their expertise in astrophysics, planetary science, computer science, and mathematics in order to create complex supercomputer models. The models ran a range of hydrodynamic and numerical simulations able to recreate the sequence of ancient events. Their findings strengthen the hypothesis that a horrific blast in the past formed the moons of Mars–originally a collection of moons and moonlets.

For a very long time, planetary scientists favored the scenario that the duo of potato-shaped Martian moons were probably snared asteroids. However, the pair’s circular orbits at the equator indicated otherwise. The orbits of the little moons suggested that they had really formed from a giant impact billions of years ago. The new research, published in the July 4, 2016 issue of Nature Geoscience, proposes that a massive 2,000 kilometer protoplanet crashed into the primordial Mars. The horrendous impact resurfaced most of the Martian surface and hurled a mass of debris, more than 100 times the mass of both Phobos and Deimos, into orbit around the Red Planet.

Mars may be circled by many moons smaller than 160 to 330 feet in diameter, and a ring of dust has been predicted to circle Mars between Phobos and Deimos.

The “Fire Star”

The first observations of Mars as an object traveling in Earth’s night sky was recorded by the ancient Egyptian astronomers, and by 1534 BCE the ancient astronomers were familiar with the retrograde movement of the planet. By the time of the Neo-Babylonian Empire, the Babylonian astronomers were making regular records of the positions of the planets, as well as systematic detections of their behavior. In the case of Mars, the ancient astronomers realized that it made 42 circuits of the zodiac every 79 years. These scientists of long ago even invented arithmetic methods so that they could make minor corrections pertaining to the predicted positions of the planets inhabiting our Solar System. The ancient astronomers referred to the planets as “wandering stars”.

In the fourth century BCE, Aristotle recorded that Mars vanished behind Earth’s Moon during an occultation. This suggested that the planet was farther away than our Moon. The Greek astronomer, Ptolemy, who lived in Alexandria, attempted to solve the problem of the orbital motion of the Red Planet. Ptolemy’s collective works and model on astronomy was presented in his multi-volume collection, titled the Almagest. The Almagest became the authoritative work on Western astronomy for the next 400 years. Ancient Chinese astronomers were also aware of the existence of Mars by no later than the fourth century BCE. In the fifth century CE, the Indian astronomical work titled Surya Siddhanta proposed a measurement of the estimated diameter of Mars. In East Asian cultures, Mars is usually referred to as the “fire star”–based on the Five Elements: fire, wood, metal, water, and earth.

The astronomer Tycho Brahe, during the 17th century, measured the diurnal parallax of Mars that Johannes Kepler had used in order to make a preliminary calculation of the relative distance to the Red Planet. When the earliest telescopes to be used for astronomical purposes finally became available, the diurnal parallax of Mars was measured again in an attempt to determine the distance between our Sun and Earth. Giovanni Domenico Cassini was the first to make this measurement in 1692–but the early parallax measurements were hindered by the primitive quality of the instruments. The only occultation of Mars by the planet Venus was observed on October 13, 1590, by Michael Maestlin at Heidelberg. In 1610, Mars was viewed by the great astronomer Galileo Galilei, who was the first to make use of a primitive telescope for astronomical purposes. The Dutch astronomer Christiaan Huygens was the first to draw a map of Mars that showed terrain features.

Mars hasn’t always looked the way it does today. The planet suffered a monumental tilt billions of years ago. Before this great tilt occurred, the Martian poles were not where we see them now.

The more recently obtained data concerning the Red Planet comes from seven active probes that either roam the Martian surface or orbit around the planet. The seven spacecraft include a quintet of orbiters and a duo of rovers. This collection includes 2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, MAVEN, Mars Orbiter Mission, Opportunity, and Curiosity.

Dozens of crewless spacecraft, including landers, rovers, and orbiters, have been dispatched to Mars by the Soviet Union, the United States, Europe, and India to observe the planet’s climate, surface, and geology. Since the year 2000, cameras circling in orbit around Mars have sent back to Earth a treasure chest overflowing with revealing pictures of the “fire star.” These wonderful images have displayed a Martian surface etched with small valleys and carved into slopes. These features are eerily similar in their shape to gullies carved by gushing water flowing on Earth. The Martian gullies are believed to be less than a few million years old–a tiny wink of the eye on geological time scales. In fact, some of the gullies even appear to be younger than that! These detections are enticing. This is because the observations hint to planetary scientists that great quantities of liquid water may still be lingering on Mars at present–and that this gushing water might be what carved out the gullies.

There are ongoing studies assessing the past habitability potential of the Red Planet, as well as the possibility of life.

The Many Moons Of Mars

The research published in the July 4, 2016 issue of Nature Geoscience highlights the main factor differentiating moon-birth around Mars and Earth: the differing rotation speeds of the two planets prompted “completely different tidal actions,” Dr. Charnoz explained in the July 4, 2016 CNRS Press Release. Dr. Charnoz proposes that at the time of their respective impacts, “Earth took less than four hours to spin on its axis whereas Mars rotated very slowly over a 24-hour period.” The result of this important difference caused Earth to hold on to its single, large Moon, while the Martian collision created a dozen smaller moons alongside a larger moon. As time went by, Martian tidal action–resulting from the planet’s slow rotation rate–caused most of the moons, including the largest one, to crash back down to the surface of their parent-planet. As a result, only the two most distant moons, Phobos and Deimos, survived as testimony to the ancient catastrophe.

The vanished, ill-fated, large moon could have been a few hundred kilometers in diameter. The large moon would also have allowed a handful of other smaller moons to form–including the duo of small, shapeless ones that survive today. However, this large inner moon would have been born close to or within the Roche limit. This is why it is likely that it crashed into Mars as a result of tidal forces within several million years–and the collection of other small moons followed their leader. Only Phobos and Deimos kept their distance.

If this ancient catastrophic impact really did occur, there should be deposits of these tragic moons on the Martian surface. The Japanese Aerospace Exploration Agency (JAXA) is currently planning a sample return mission to Phobos and Deimos, the Martian Moons Explorer, and NASA has plans to eventually return samples to Earth from the surface of Mars– perhaps as soon as the 2020s. At the conclusion of their paper, the authors note, “Our scenario provides further motivation for a sample return mission to the Martian satellites.”

JAXA has announced a space mission scheduled to begin in 2022, with an expected return to Earth in 2026. “Its objective is to carry out close-up remote sensing and in-situ observations of both Phobos and Deimos, and to bring back samples from Phobos,” commented Dr. Ryuki Hyodo in the July 4, 2016 CNRS Press Release. Dr. Hyodo is a planetary scientist, originally from Kobe University in Japan, and he is also currently collaborating with the IPG. “High-resolution impact simulations are still needed to understand more about the disk structure,” he continued to explain to the press.

The current study’s Franco-Belgian-Japanese collaboration looks forward to this mission. JAXA plans to enlist them to conduct tests on the Martian samples when they are returned to Earth. The samples will help the scientists determine whether Phobos is indeed made up of a mixture of Martian mantle and debris left in the wake of the tragic crash of the doomed, vanished protoplanet–as suggested by their supercomputer simulations.

“More generally, our findings clarify how giant impacts give birth to satellites and can create a diverse variety of satellite systems,” Dr. Charnoz told the press on July 4, 2016. He added that the team could apply their method to other regions of our Universe:

“Uranus and Neptune and–why not?–systems of satellites around exoplanets that we may identify in the future.”

https://youtu.be/7KzPDdo4pRs