My Science School

There are two reasons why people started believing that Martian moons may be artificial satellites. One was a scientific theory and the other, an April Fool’s day joke!

Astronomers and scientists observing Phobos in the late 1950s saw that the moon was slowly spiralling closer and closer to Mars. Back then, they did not have enough information to understand why Phobos was behaving this way. A famous Russian astrophysicist named losif Shklovsky came up with a possible explanation - if Phobos was capable of getting slowed down by Mars’ thin atmosphere, and then it should be having a very low density, meaning it might be hollow, and may even be an artificial satellite! The U.S. President’s scientific advisor at the time, Fred Singer, too seemed to support this argument, while also raising doubts about the accuracy of the measurements the theory was based on. But the “if’s” and “might be’s” in their statements were ignored by a few who were fascinated by the possibility of Phobos being the evidence of extra-terrestrial life on Mars!

At around the same time, an American space enthusiast and columnist, Walter Houston, published an article in the April Fool’s edition of a magazine. He wrote that the moons of Mars were actually space stations made by a group of super-intelligent Martians! Even though this was meant to be a joke, it soon became “viral!”

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If you look at the photographs of Deimos and Phobos you will notice one point in common - their colour. Both the Martian moons look dark in colour, darker than Mars. In fact, they are said to be two of the darkest bodies in our solar system. On closer inspection, scientists discovered that they are made up of a peculiar type of carbon-rich material and ice - a composition they share with the space rocks drifting in the main asteroid belt (that lies between the orbits of Mars and Jupiter) of our solar system. This gave rise to the idea that Deimos and Phobos may be two runaway asteroids that wandered too close to Mars! After all, asteroids do break out of the main asteroid belt occasionally due to collisions or Jupiter’s gravitational effects. And during their journey afterwards, if they happen to pass by a planet like Mars, they could easily get pulled into its orbit!

But in such a case, the orbits of these asteroids would be elliptical at first. It would take them many, many years to settle into the kind of near-circular orbits that the Martian moons now have. Also, the thin atmosphere of Mars would perhaps not be enough to slow down the asteroids before they could fall into orbit. All these points make the asteroid-capture theory unlikely, but not impossible!

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The question of the origin of the Martian moons is something that has been worrying scientists for decades. The asteroid-capture theory offers a likely explanation. But is it good enough to be true? Only under certain very unique circumstances, scientists say.

Another theory is that Deimos and Phobos formed after a large space rock - something big enough to create the major craters that we now see on Mars - crashed into Mars millions of years ago. The impact of the collision sent material flying out of the crater, and into an orbit around Mars. Over time this ejecta collected together and formed the Martian moons. The evidence for this theory is a mineral (a type of silicate) seen on Phobos’ surface; it is found on Mars too! This would also easily explain Deimos’ and Phobos’ orbits - their alignment with Mars’ equator, and their circular shape - something that asteroid-capture theory cannot fully justify. But this theory too has its drawbacks; scientists are not sure whether both Martian moons are old enough to have formed during the asteroid impact period.

This has led scientists to yet another possibility. What if Mars already had a moon long back which was broken up into Deimos and Phobos, maybe by an asteroid impact around 1- to 2.7 billion years ago? It is the latest in a long list of theories.

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The paths of Deimos and Phobos are nearly circular, and they both lie on the plane of Mars’ equator. They also show the same face to the Red Planet as they revolve around it. But there ends the similarity!

The average orbital distance of Deimos, the smaller of the two moons, is around 23,500 kilometres! So from Mars on a full-moon night, Deimos looks like a bright star, just about one-twelfth the size of our Moon (as seen from the Earth)! It takes around 30.3 hours to complete an orbit (lesser than the rotational period of Mars, 24.7 hours) and so it rises in the east and sets in the west.

In comparison, Phobos, with an orbital distance of nearly 9,400 kilometres, travels quite close to Mars. This is why, despite its (relatively) bigger size, Phobos is not visible from the polar regions of Mars! It makes Phobos quite fast too - its orbital period is only about 7.7 hours - and so, if you are standing on Mars, you will see Phobos rise from the west and set in the east twice a day! But an unfortunate side-effect of this proximity to Mars is that Phobos is slowly getting pulled closer to the planet, and will be destroyed within a span of 50 million years or so. Deimos, on the other hand, is moving further away from Mars, and will one day be lost in space!

Not very! The dimensions of Phobos are said to be 27 by 22 by 18 kilometres -quite an odd shape for a moon, isn’t it? This is why we simply say that it has an average diameter of around 22 kilometres! Its mass is around 11 quadrillion kilograms, nothing compared to the Earth’s 6 septillion kilograms.

Further, the gravitational pull on Phobos is only about 1/1700th that on the Earth. This means that if you weigh 50 kilograms on the Earth, on Phobos you will hardly weigh 30 grams - the “Earth weight” of six grapes!

With such a low gravity, Phobos does not have an atmosphere either. But why is this Martian moon so light? For one, Phobos has a low density, a little less than twice that of water. (For comparison, the Earth’s density is 5.5 times that of water!)

Back in the 1950s, there were even rumours that Phobos might be hollow within and may be an artificial satellite made up of sheet metal! But this was later disproved.

Today, scientists say that Phobos is not a solid body but one made up of pieces of rock. It is quite porous and may even have ice hidden beneath its surface!

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The first thing anybody would notice about this Martian moon is the large depression on the middle-western side of its face. An impact crater nearly nine kilometres wide, it covers nearly half the width of Phobos. This is Stickney, named in 1973 in honour of a mathematician, Angeline Stickney, for her contributions that led to the discovery of the Martian moons by her husband, Asaph Hall. The crater Stickney was first photographed by National Aeronautics and Space Administration’s (NASA) Mariner 9 spacecraft. Phobos has many more, smaller craters, and a few of them have been named either after astronomers, or characters in Johnathan Swift’s book Gulliver’s Travels!

The second thing about Phobos’ appearance is the large number of cuts and grooves on its surface. Some of them seem to spread out from Stickney. Scientists say they may have been created by material that was thrown out of the crater (called ejecta), when a space rock hit Phobos. Heavier ejecta may have rolled down the sides of the moon creating these grooves.

The third is the regolith making up Phobos’ surface. It literally flows around as this moon orbits Mars! What we know today about Phobos’ mysterious surface largely comes from the photographs taken during various missions to Mars. But this Martian moon definitely hides many more secrets beneath its surface. Hopefully at least some of them will be revealed to us when the Japanese MMX spacecraft reaches Phobos!

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Deimos, like Phobos, was named after a son of the Greek god of war, Ares (remember, planet Mars was named after the Roman god of war).

Legend has it that he accompanied his father and brother to battle - not a surprise considering that Deimos literally means “dread” while Phobos means “fear”! But thankfully his planetary namesake is not all that “dreadful” in size or form. With an average diameter of about 12 kilometres, and a mass of around 1.5 quadrillion kilograms, Deimos is tiny even when compared to its sibling-moon, Phobos!

Its irregular shaped body (having dimensions of 15 by 12 by 11 kilometres) seems to be made up of a carbon-rich material generally found on certain types of asteroids. Its surface is covered with a dry and loose layer of dust and broken rocks known as regolith (a combination of the Greek words for “blanket” and “rock”) which may be up to 100 metres thick.

Though Deimos has many impact craters, its surface is smoother than Phobos’. As on date, only two main features on it have been identified and named.

Both are impact craters said to be around three-kilometres wide. They are called Swift and Voltaire in honour of the famous Irish and French writers who wrote about the two Martian moons more than a century before their discovery! The remaining craters are much smaller in size.

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The story of the “first discovery” of Martian moons is a funny one! In 1609, the Italian astronomer and scientist, Galileo Galilei, started making telescopic observations of celestial bodies. He was quite good at it, because a year later, he became the very first person to see Mars and four of Jupiter’s largest moons through a telescope! It was during this time that Johannes Kepler, a famous German astronomer, received a letter from Galileo. It contained a coded message (back then, scientists used to regularly communicate with each other this way). To Kepler, it seemed to say “Mars has two moons.” But in reality, Galileo was talking about Saturn’s moons (which were later proven to be its rings)!

The next “clue” came in 1726 in Jonathan Swift’s book Gulliver’s Travels! If you have read it you probably remember the flying island, Laputa, and its astronomy-loving residents who “discover” the two moons of Mars! Maybe Swift was inspired by Kepler, or he simply made a guess based on the number of moons Mercury, Venus, the Earth and Jupiter were then known to have. Twenty-six years later, another writer too used this two-Martian-moons idea in his story, Micromegas, and his name was Voltaire.

These famous works of fiction popularized the concept of Mars having two moons well before they were actually discovered by Asaph Hall in 1877 - more than a century after their publication!

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Though the two moons of Mars were discovered in 1877, it took 92 years until we could get their close-up photographs! This again, thanks to Mariner 7, the U.S. National Aeronautics and Space Administration’s (NASA) spacecraft launched in 1969 to study Mars. Even in the following years, we did not have any Deimos or Phobos explorations - we had to be satisfied with their photographs taken during Mars missions such as those by NASA, the European Space Agency (ESA), and the Indian Space Research Organization (ISRO). But this is not to say that Martian moon exploration programmes were not planned.

Even as early as 1988, humans had dreamt of sending probes to Phobos, the larger of the two moons of Mars. The Soviet Union’s (present-day Russia) Phobos 1 and Phobos 2 were designed to do just that! But while Phobos 1 failed before it could reach its destination, contact with Phobos 2 was lost just before it could reach within 50 metres of the moon to release its landers. In 2011, the Russian Space Agency made plans to collect samples from Phobos during the Fobos-Grunt (meaning ‘Phobos-Ground’ in Russian) mission. Unfortunately that too failed during the launch stage.

But hope still remains in the form of Japanese Aerospace Exploration Agency’s (JAXA) Martian Moons Exploration (MMX for short)! This mission, developed along with ESA, the German Aerospace Centre (DLR), the (French) National Centre for Space Studies (CNES), and NASA, will be launched in 2024. It will hopefully collect many samples from Phobos, and also study Deimos and Mars remotely.

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Have you ever wondered how the night sky would appear from the Red Planet? Some say it would be pretty similar to that on a clear Earth night, but with one key difference - instead of just one moon, you would see two. One might resemble a bright star and the other, a pale space potato! And through a telescope they look nothing like our Moon; they are highly uneven masses covered with lumps, bumps and huge craters. But like our Moon, they always show the same face to their planet. Meet Deimos and Phobos, the only natural satellites of Mars!

Only around 12 kilometres wide, Deimos is the smaller of the two. It takes a little more than 30 hours to complete an orbit around Mars. Phobos measures 22 kilometres across (for comparison, the diameter of our Moon is 3,474 kilometres). However, it orbits very close to Mars, and takes only about eight hours to complete a revolution!

Unfortunately for Phobos, it is coming closer and closer to Mars - its orbital distance is reducing by about 1.8 centimetres every year. At this rate, one day it will either crash into the Red Planet, or get pulled apart by the gravitational effect of Mars and form a ring of debris around it. Either way, this will be a spectacular event. But we will have to wait 50 million years more to see it happen!

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Perseverance pays! Maybe that was what the American astronomer, Asaph Hall, felt after he discovered the Martian moons in 1877!

Asaph Hall was just 33 years old when he joined the prestigious U.S. Naval Observatory (USNO) in Washington, D.C. in 1862 as an astronomer. Though the very next year he was made the Professor of Mathematics in the USNO, Hall retained an active interest in the study of planets, moons, stars and their orbits. The year 1877 brought Mars especially close to the Sun and the Earth in a rare phenomenon called the “perihelic opposition.” During this time, Hall was in charge of his observatory’s 26-inch refracting telescope (telescope that uses lens for magnification), the world’s largest during that time, and he decided to use it in his search for Martian moons.

Initially it must have been quite frustrating for Hall, because after catching a glimpse of what appeared to be a moon on the 10th of August he could not find it again. He was about to give up. But his wife, Angeline Stickney (who was also a mathematician), motivated him to keep trying. Finally on the night of 12 August 1877, Hall discovered Deimos, and on 18 August 1877, Phobos! The biggest feature on Phobos, a nine-kilometre-wide impact crater, was named “Stickney” in the light of Hall’s wife’s contribution to the discovery of the two Martian moons.

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The formation of Valles Marineris was a puzzle that left many scientists scratching their heads. Erosion by an ancient melting glacier? Tectonic action? Volcanic activity? Theories were many. But what everyone agreed upon was that the story of this canyon system was closely tied to the history of the Tharsis region!

Around four billion years ago, when Mars was at its ‘volcanic’ best, enormous quantities of magma collected underneath its north western part. The ground swelled up to form the Tharsis bulge. Over the years, as more and more lava spilled out, volcanoes grew in size to become giants like the Olympus Mons, Ascraeus Mons, Pavonis Mons and Arsia Mons. Finally, the pressure build-up was so great that it cracked the Martian crust! These fractures expanded in time, forming the system of chasmata that we know as the Valles Marineris - this is the commonly accepted theory today.

These cracks also released the water stored below the Martian surface. The escaping fluid washed away the sides of the chasmata making it still wider. The channels found in chaotic terrain on the eastern end of Valles Marineris may be the result of such flooding events. But we still don’t have all the details. Was it a single large flood episode followed by smaller ones later, or a series of floods within a short period of time? Nobody knows!

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Scientists say there may be mainly two factors that help Martian volcanoes grow enormous.

One, Mars has a weak gravitational pull (which is only about one-third that of the Earth’s) and a thin atmosphere. Magma, or the molten material under a planet’s surface before it becomes lava, contains not only melted minerals but also gases trapped within it. As magma approaches the surface, the effects of low atmospheric pressure helps gas bubbles expand faster. This pushes out huge quantities of hot ash and other debris in a large explosion called a Plinian eruption. The low gravity also makes it easier for magma to collect under the Martian surface in magma chambers, larger and deeper than those found on the Earth. So when a volcano erupts on Mars, there is more lava produced and the low gravity keeps it flowing for longer periods.

Two, Mars does not have any active tectonic plates. The Earth’s crust (along with the upper mantle) is in the form of huge jigsaw-puzzle-like pieces known as tectonic plates. As these plates gradually move, their boundaries (which are points of weakness) may get aligned above areas of the underlying mantle that are hotter than the surrounding regions, permitting magma to move up and form volcanoes. When the plates move again the flow is cut off. In Mars, however, there is hardly any tectonic movement. So lava is able to continuously flow and pile up at a single location.

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Did you think that the area occupied by Olympus Mons (300,000 square kilometres) was unimaginably big? Wait till you hear how much of Martian land the shield volcano Alba Mons covers. At least 5.7 million square kilometres! That’s more than half the size of Europe!

With respect to area, Alba Mons may be the largest volcano on Mars. But it stands only 6.8 kilometres tall; that is hardly one-third the height of Olympus Mons! You can guess looking at its dimensions that this volcano has a very gentle slope. With an average slope of just 0.5 degrees even at its steepest northern side, this “mountain” looks nothing like the ones we have here on the Earth!

In reality, until 2007 Alba Mons (Latin for “white mountain”) used to be called Alba Patera (Latin for “white saucer”) because of the white clouds found around the volcanic craters of the mountain. Today, Alba Patera refers only to the two calderas (hollowed out regions found on top of volcanoes after an eruption), the larger of which is more than 100 kilometres wide. They are relatively shallow though - only about one kilometre deep! Curved fault lines, called Tantalus Fossae and Alba Fossae, run on the eastern and western sides of the volcano. We can also find evidence of lava flows and surface water run-off around it.

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If you focus a telescope on the Red Planet on a clear Martian “sol,” you will see near its centre, a dark scar stretching over nearly half its face. This is Valles Marineris, the largest canyon system on Mars.

In Latin its name means “Mariner valley.” But Valles Marineris is quite unlike any of the river valleys we see on the Earth. Nearly seven kilometres deep, it surpasses the world’s deepest gorge, the Yarlung Tsangpo Grand Canyon in Tibet, China, by close to a kilometre. At certain points it may be as deep as 10 kilometres, and as wide as 200 kilometres! It is also more than 4,000 kilometres long; this is nearly double the distance between Kochi and Delhi! For comparison, the famous Grand Canyon in Arizona, USA, is only 446 kilometres long, 29 kilometres at its widest and 1.9 kilometres at its deepest.

Valles Marineris runs below the Martian equator on the eastern side of the Tharsis bulge and Chryse Planitia, a circular plain where the U.S. National Aeronautics and Space Administration’s (NASA) Viking 1 spacecraft landed in 1976. This fascinating canyon system is made up of a number of chasmata (its singular form is chasma), or deep and steep-sided fractures on the Martian surface. It starts with a chaotic system (a rough terrain with cracks, ridges and plains mixed together) on the east, and ends in a region of crisscrossing valleys, called Noctis Labyrinthus, on its west.

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