Phobos and Deimos, the Moons of Mars

A Doomed Martian Moon Has A Story To Tell

By Judith E Braffman-Miller

Discovered in 1877 by the American astronomer Asaph Hall, the enigmatic duo of Martian moons, dubbed Phobos and Deimos, have both bewitched and bewildered planetary scientists trying to understand their many mysteries. The Martian moons are small and misshapen little objects, frequently considered to be captured asteroids, that were snared by the Red Planet’s gravity long ago–after they had both escaped from the Main Asteroid Belt between Mars and Jupiter. Phobos is both the larger of the two moons, as well as the one that is closest to its rust-colored parent-planet, and it orbits a mere 3,700 miles from the Martian surface, closer to its parent body than any other known moon in our Solar System. Indeed, Phobos is so close that it orbits its planet much faster than Mars rotates, finishing an orbit in only 7 hours and 39 minutes. In October 2017, astronomers announced that NASA’s longest-lived mission to the Red Planet has obtained its first peek at Phobos, in its quest to gain a deeper understanding by exploring it in infrared wavelengths.

The Thermal Emission Imaging System (THEMIS) camera aboard NASA’s Mars Odyssey Orbiter observed the little potato-shaped moon on September 29, 2017. Planetary scientists have now combined visible-wavelength and infrared data to create an image color-coded for surface temperatures on this intriguing tiny moon. Phobos has been considered for a potential future human-mission outpost.

“Part of the observed face of Phobos was in pre-dawn darkness, part in morning daylight, commented THEMIS Deputy Principal Investigator Dr. Victoria Hamilton in an October 4, 2017 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Hamilton is of the Southwest Research Institute, headquartered in San Antonio, Texas. The JPL is in Pasadena, California.

Studying the new image from left to right presents a sequence of times of day on the mysterious Martian moon, ranging from before dawn, to sunrise, to increasing quantities of time following the Martian dawn. This provides valuable information about how rapidly the ground on the Red Planet warms up, which is related to the texture of its rust-colored surface. Sand warms or cools much more rapidly than rocks or pavement.

“Including a predawn area in the observation is useful because all the heating from the previous day’s sunshine has reached its minimum there. As you go from predawn area to morning area you get to watch the heating behavior. If it heats up very quickly, it’s likely not very rocky but dusty instead,” Dr. Hamilton continued to explain in the JPL Press Release.

Ring Around The Red Planet

Phobos is an irregularly shaped moon with a mean radius of a mere 7 miles, and it is seven times as massive as its sibling moon, Deimos. Phobos is named after the Greek god Phobos, a son of Ares (Mars) and Aphrodite (Venus). The mythological Phobos also serves as the personification of horror–hence, the word phobia.

Because Phobos orbits Mars much faster than Mars rotates, some strange things happen in the Martian sky. From the surface of the Red Planet, Phobos seems to rise in the west, move across the sky in 4 hours and 15 minutes or less, and set in the east–twice each Martian day.

As one of the least reflective bodies in our Solar System, Phobos has an albedo of only 0.071. Surface temperatures range from approximately 25 degrees Fahrenheit on the sunlit side to -170 degrees Fahrenheit on the side that is shadowed. The most remarkable surface feature on this tiny moon is the large impact crater, dubbed Stickney, which scars a very substantial region of Phobos’s surface.

Rather than being a solid body, images and models suggest that Phobos may really be a rubble pile held together by only a slender crust. In addition, the little moon is being ripped apart by tidal interactions. Phobos travels closer to its parent-planet by approximately 2 meters every one hundred years, and it has been predicted that within about 30 to 50 million years it will either collide with Mars, or fragment into a myriad of pieces, thus forming a ring around its planet.

Asaph Hall discovered Phobos on August 18, 1877, at the United States Naval Observatory in Washington D.C..Hall had already discovered Deimos a few days earlier on August 12, 1877.

Phobos possesses too little mass to be rounded under the influence of its own gravitational pull, and it also has no atmosphere as a result of its very low mass and low gravity. Spectroscopically, it seems to be similar to the D-type asteroids, and it has a composition similar to that of carbonaceous chondrites. Phobos’s density is too low for it to be made of solid rock, and it is known to be very porous. These are the results that led to a proposal that Phobos might possibly harbor a substantial reservoir of ice. Spectral observations suggest that the surface regolith layer is devoid of water. However, ice lurking beneath the regolith has not been ruled out.

This surface of this tiny Martian moon is also heavily pockmarked with craters. Its most prominent crater, Stickney, is named after Hall’s wife, Angeline Stickney Hall. Stickney is a large impact crater that is about 5.6 miles in diameter, and it takes up a substantial proportion of the little moon’s surface area. The impact that excavated Stickney must have come very close to blasting Phobos apart.

The oddly shaped surface of Phobos is scarred by numerous grooves and streaks. The grooves are usually less than 98 feet deep, and 330 to 660 feet wide, and they can extend up to 12 miles in length. These strange, slashing grooves and streaks were at first considered to be the outcome of the same impact that excavated Stickney. However, a careful analysis derived from results obtained from the Mars Express spacecraft indicated that the grooves are not radial to Stickney, but are instead centered on the leading apex of Phobos in its orbit around Mars (which is not very far from Stickney). Astronomers currently suspect that they have been carved out as the result of material being shot out into space by impacts on the surface of the Red Planet itself. As a result, the grooves formed as crater chains, and all of them fade away as the trailing apex of Phobos is approached. Astronomers have grouped them into a dozen or more families of varying age, presumably excavated by at least 12 impacts on the Martian surface.

Very faint rings of dust created by Phobos and Deimos have been predicted for years. However, efforts to observe these faint rings have, so far, turned up empty-handed. Images obtained from the Mars Global Surveyor suggest that Phobos is covered by a blanket of fine-grained regolith that is at least 100 meters thick. Some astronomers have proposed that the regolith was produced by impacts from other bodies–but it is not known how the material stuck to an object like Phobos that possesses almost no gravity.

The Kaidun meteorite that crashed on a Soviet military base in Yemen, back in 1980, has been hypothesized to be a chunk of Phobos. However, this scenario has proven to be difficult to verify. This is because little is known about the exact composition of Phobos.

Mars hasn’t always looked the way it does today. The planet experienced an enormous, catastrophic tilt billions of years ago. Before this great tilt had occurred, the Martian poles were not where astronomers now see them.

Recently obtained data describing the Red Planet is derived from seven active probes that either roam the Martian surface or orbit the planet. The seven spacecraft include five orbiters and two rovers. This successful collection includes 2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, MAVEN, Opportunity, and Curiosity.

Many crewless spacecraft, including rovers, orbiters, and landers have been sent to the Red Planet by the United States, Soviet Union, Europe, and India to study the Martian climate, geology, and surface. Since 2000, cameras orbiting around Mars have dispatched back to Earth a treasure trove of revealing images. These images have shown that the Martian surface has been etched with small valleys and carved into slopes. These features are hauntingly similar in their shape to gullies carved by flowing water on Earth. The Martian gullies are believed to be less than a few million years old–a blink of the eye on geological time scales. Indeed, some of the gullies may even be younger than that!

A Doomed Martian Moon Has A Story To Tell

Cameras aboard Mars orbiters have previously taken higher-resolution images of Phobos. However, none of the earlier images were obtained with the infrared information available from THEMIS. Observations in multiple bands of thermal-infrared wavelengths can provide information about the mineral composition of the surface of the little moon, as well as its surface texture.

One intriguing question about Phobos, as well as its smaller sibling moon, Deimos, is whether they are really captured asteroids or, alternatively, chunks of Mars blasted into the sky by impacts. Compositional information from THEMIS might finally help astronomers determine their origin.

Since Odyssey began orbiting Mars back in 2001, THEMIS has provided scientists with information about both the compositional and thermal-properties existing all over the Red Planet–but never before from either of the duo of Martian moons. The observations made on September 29, 2017 were completed to demonstrate that the spacecraft could safely do so, as the beginning of a possible series of observations of Phobos and Deimos in coming months.

In normal operating mode, Odyssey keeps the THEMIS camera pointed directly down as the spacecraft circles the Red Planet. In 2014, the spacecraft team at Lockheed Martin Space Systems, in Denver, Colorado, along with NASA’s JPL, and the THEMIS team at Arizona State University in Tempe, Arizona, developed procedures to rotate the spacecraft for upward-looking imaging of a comet flying close to Mars. The teams have adapted those procedures for imaging the duo of potato-shaped Martian moons.

“We now have the capability of rotating the spacecraft for THEMIS observations. There is heightened interest in Phobos because of the possibility that future astronauts could use it as an outpost,” commented Dr. Jeffrey Plaut in the October 4, 2017 JPL Press Release. Dr. Plaut is an Odyssey Project Scientist at JPL.

With the first observation now completed, plans are being made for additional important observational opportunities at different illumination phases of Phobos and Deimos.

As Dr. Hamilton notes, “We want to get observations under all types of lighting–fully daylit, a small crescent, during eclipse. We hope this is the first of several observations that will help us understand Phobos and Deimos.”

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, newspapers, and magazines. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.

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