Observing the moons of Mars


By Tom Burns - Stargazing



A recent study tracing the origins of Deimos and Phobos, the two moons of Mars, brought to mind a memorable Mars-observing session more than three decades ago.

The time — 2 a.m. or thereabouts. The date — sometime in late September 1988. Bob Bunge, my constant observing buddy at the time, and I were 30 feet above the ground on a hydraulic scissors jack. Before us, barely visible and illuminated only by starlight, jutted the eyepiece of the Warren Rupp Observatory’s gigantic telescope near Mansfield, Ohio.

The night was moonless. The darkness was palpable, an inky syrup that enfolded us. Above our heads, the stars shimmered through the narrow slit of the telescope dome. The only sound was the telescope’s insistent whir as it slowly followed the brilliant orange dot of light that was the planet Mars.

Mars shone brilliantly that night because we were in the midst of one of its rare close approaches to Earth. At only 36 million miles away, it was a prime target for any telescope, and we were suspended next to one of the largest amateur telescopes in the world.

Intellectually, I had full confidence in the stability of the scissors jack. But without a clear frame of visual reference, I could not shake the impression that it was slowly swaying in a nonexistent breeze. To put it another way, I was scared out of my gourd.

But I also felt an electric excitement. Were the thunderbolts that passed up my spine caused by the cool night air or my anticipation of a rare astronomical experience?

A scene from an old Monty Python movie flashed into my head.

“What is your quest?” I muttered.

“I seek the Holy Grail,” I replied.

Bob and I were waiting for the exact moment when Deimos, one of the two diminutive satellites of Mars, would be briefly visible.

Mars is that bright red “star” now setting slowly in the west in the early evening. Point a telescope at it, and you will experience the same disappointment that many planetary observers have suffered since the invention of the astronomical telescope in the 1620s.

Mars is, after all, only 4,000 miles wide, only twice the diameter of Earth’s moon. Luna is a scant 260,000 miles away. Mars rarely gets closer than 34 million miles away.

Stargazers look forward to the rare times every 15 years that Mars and Earth are close enough to each other so that they can get a decent look.

The October 2020 close approach is now over. You’ll have to wait until 2035 to see it at its best again. Given my age, I have seen the last of Mars.

Right now, the Red Planet is about to disappear into the sun’s glare. At 140 million miles away, its orbital motion has taken it almost to the other side of the sun from us. Mars looks like a speck now, but then again, Mars looks like a speck almost all the time.

Close approaches afford amateur astronomers a chance to see various features on the Martian surface — its mysterious green markings and its bright polar ice caps.

They also give telescopists a chance to pursue one of the most difficult of stargazing’s holy grails. Even the most dedicated amateur has never observed Mars’ mysterious moons, Deimos and Phobos.

The Martian moons are supremely difficult to observe. For one thing, they are tiny. Our moon is about 2,000 miles wide. Phobos, the largest of the two, is about 14 miles wide. Irregularly shaped Deimos more aptly fits the potato description. It measures 9-by-7 miles.

Both Deimos and Phobos would fit with room to spare inside the I-270 outer belt that surrounds Columbus.

Also, they are exceedingly non-reflective. Earth’s moon reflects about 12% of the sunlight that hits it back out into space. That’s about the reflectivity of asphalt. By comparison, a fresh patch of green summer grass reflects about 25%. The full moon seems bright to you because of its proximity and not its reflectivity.

Deimos and Phobos reflect only about 7% of the sun’s light. They are among the dimmest objects in our solar system. And remember, even at their best, you are trying to observe them when they are 34 million miles away.

Most critically, Phobos orbits only 3,700 miles above the Martian surface. It zips around the planet in a startling seven hours. Dimmer and smaller Deimos orbits at about 14,500 miles away from Mars and takes a more leisurely 30 hours to make an orbit.

Consequently, they are easily lost in the bright Martian glare. A telescopist must catch them when they appear farthest from Mars. Even in the best circumstances, Phobos is far too close to see.

Despite its size and dimness, Deimos is an amateur telescopist’s only hope. Since its orbit takes 30 hours, it will be farthest from Mars every 15 hours when it is on opposite sides of the Red Planet.

So, I waited for years until the rare opportunity arose. When it came, I realized that Deimos was too dim to see in my small telescope.

And so it was that Bob and I made the long drive to the biggest telescope available to an amateur astronomer in Ohio.

All the pieces of the observing puzzle had fallen into place. Fifteen minutes seemed like 15 years as we waited for that precise moment when mysterious Deimos would emerge briefly from the Martian glare.

While we waited, I contemplated the most puzzling of the Martian moons’ mysteries. Astronomers can’t quite figure out how Mars got its moons.

The most accepted speculation is that the Martian moons are asteroids captured into orbit by the gravity of Mars.

Deimos and Phobos certainly look like asteroids. The common cliché about them is that, like asteroids, they look like potatoes, but that’s “an insult to potatoes,” as George Dvorsky remarked.

They certainly possess the irregular shape of potatoes. Like asteroids, they are pockmarked by hundreds of collisions with smaller objects, as we might expect over billions of years from asteroids originating in the relatively crowded asteroid belt. Those potatoes have been sitting on the counter for a really long time.

With its tens of thousands of so-called “minor planets,” the asteroid belt hovers just outside the orbit of Mars. As the asteroids in the belt stray close to each other over time, asteroids are occasionally ejected. In the Martian case, Demos and Phobos were ejected at separate times in the general direction of Mars.

If they had passed Mars a bit farther away, they would have continued in the general direction of the Earth and sun. If they had come a bit closer to Mars, Martian gravity would have sucked them into an impact with the Red Planet.

Instead, their rather unlikely path cut within the small margin in between those other possibilities, and they were captured into orbit.

Here the Goldilocks principle applied. Most paths were too far. Some paths were too close. The paths of Deimos and Phobos were just right.

As unlikely as the captured-asteroid conjecture seems, the phenomenon has happened at least once before in our solar system. Neptune’s moon Triton is almost surely a captured object.

However, computer simulations by researchers from the Institute of Geophysics at ETH Zurich and the Physics Institute at the University of Zurich suggests an even more unlikely and controversial origin. Using data from NASA’s InSight Mars Lander, they conclude that Deimos and Phobos are the shattered remnants of a larger moon that collided with a small asteroid 1 to 2.7 billion years ago.

Their computer simulations more reliably predict a time when Mars will have no moons at all. Phobos is so close to Mars that Martian gravity gradually spirals the moon inward. Also, Martian gravity bends and twists the moon’s rocky structure, slowly tearing it apart.

As a result, Phobos may be just a pile of rocky rubble held together loosely by its weak gravity. When Phobos gets close enough to the Martian surface, Martian gravity will completely rend it asunder. The pieces will rain down on the planet. Phobos will be no more.

Because of its relative distance from Mars, Deimos is slowly moving away from the planet. Demos will drift out into space until the Martian gravity loses its grip. An asteroid Demos was, and an asteroid Demos will become again.

Back at the Warren Rupp telescope on that chill September night, “Demos time” finally arrived. Bob was taking his turn at the telescope.

“Got it!” he said. After a moment, he yielded his place. For a few precious seconds, the tiny speck of light that was Deimos struggled fitfully through the Martian glare.

Bob looked again for a moment, and then it was my turn again. But by the time I got another chance, Deimos had receded from the orbit of my life forever.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.