Mars's satellites

The two satellites of Mars, Phobos and Deimos, almost certainly originated as captured asteroids. These two small satellites could serve as future way stations for human exploration of Mars. They may provide future astronauts visiting Mars with an orbital base and even essential resources.

Overview

In early August 1877, astronomerAsaph Hall began his search for Martian satellites at the US Naval Observatory in Washington, DC. His search was initiated for two primary reasons. He found that many astronomy texts and ephemerides of the day contained serious errors and incorrect statements. One contention was that Mars had no satellites; because none had yet been identified, that statement was correct for its time. However, Hall knew from consulting Frederick Kaiser’s summary of Martian observations in the Annals of the Leiden Observatory (1872) that few astronomers had even looked for such bodies. While Mars made its close approach to the Earth (called “opposition”) in 1877, Hall used the Naval Observatory’s twenty-six-inch Clark refractor telescope to search for potential Martian satellites.

Even as he began his search, Hall knew that the probability of finding a Martian satellite was slim. Any object even a fraction of the size of Earth’s moon would have been discovered long before. Any smaller object could not even exist at any great distance from Mars, as the Sun’s gravitational influence would snatch it away. Hall, therefore, began his search looking for a very small satellite orbiting very close to the planet, one that therefore might be very close to the visible disk of Mars as seen through the telescopic lens and thus obscured in the planet’s glare. In view of these discouraging considerations, Hall said, “I might have abandoned the search had it not been for the encouragement of my wife.” This statement would become indelibly etched on the discovery. On August 12, 1877, Hall first glimpsed one of Mars’s two satellites, which he confirmed on August 16. The next evening, he discovered a second satellite. The announcement was made several days later by Navy admiral John Rodgers, the observatory’s superintendent. These bodies were named Deimos (meaning “flight” or “panic”) and Phobos (“fear”) from Homer’s Iliad: “He [Mars] spake, and summoned Fear and Flight to yoke his steed, and put his glorious armor on.”

The first clear images of the satellites were made by the National Aeronautics and Space Administration’s (NASA’s) Mariner 9 orbiter in 1972. Five years later, even more dramatic and detailed photographs were obtained by the Viking orbiters. Those photographs revealed that the two satellites are among the darkest bodies in the solar system. Because of their density, size, and curious orbital characteristics, they are widely thought to be asteroids captured by Mars’s gravitational field. They are not circular in shape. Some have even described their appearance as potato-shaped; their modest size allows for the weakest of gravitational fields, which will not permit the body to collapse into a spherical shape like that of planets and much larger satellites.

Phobos, the larger of the two Martian satellites, has a size of twenty-eight by twenty-three by twenty kilometers. Phobos’s orbit is exceptionally low—directly above the equator, only 9,378 kilometers over the planet’s surface—so low that it cannot be observed from the surface at latitudes greater than seventy degrees north or south. The orbit of Phobos is just barely outside the Roche limit, where tidal forces would tear it apart. It probably will be torn apart and crash into the surface of Mars in the next thirty-eight million years. Phobos’s orbital period is very fast. It circles the planet in only seven hours and thirty-nine minutes, making the satellite appear to rise in the west and set in the east.

Deimos measures sixteen by twelve by ten kilometers. Its orbit is considerably higher than that of Phobos: 23,459 kilometers above the equator, with a period of thirty hours and eighteen minutes. Deimos’s orbit is high enough that it should eventually escape Mars’s gravitational field and fly off into an independent orbit around the Sun.

Initial observations of the two satellites showed them to be heavily cratered, suggesting that their surfaces are very old—nearly as old as the solar system itself, some 4.6 billion years. There are two very large craters (relative to the body’s size) on Phobos. The largest crater, dominating Phobos’s northern hemisphere, is named Stickney, for discoverer Hall’s wife. The astronomer himself has been remembered by a lesser surface marking: Crater Hall is a six-kilometer depression on the satellite’s southern pole. Crater Roche lies near Phobos’s north pole, a reminder of the satellite’s eventual fate as a result of Mars’s dominant gravitational influence. As a Viking orbiter flew to within eighty-eight kilometers of Phobos’s surface, it photographed what appeared to be cracks, as wide as several hundred meters and up to ten kilometers in length, emanating from Crater Stickney. These cracks in the satellite’s surface were almost certainly caused by the impacting body that formed the crater itself.

The surface of Deimos is not as spectacular as that of the larger satellite. A Viking orbiter flew to within twenty-three kilometers of Deimos and resolved a relatively quiescent surface, with smaller craters (the largest discovered is only 2.3 kilometers across), no visible cracks, and a lack of a single spectacular formation.

The theory of asteroid origins was bolstered by these visual images. An extraordinarily dark surface and a density only twice that of water (half that of Mars) lent credibility to the theory that the satellites were composed of carbon and carbon compounds, such as are conjectured for type C asteroids, which populate the outermost regions of the asteroid belt. These bodies may contain up to 20 percent water by weight. Why their orbits are so nearly perfectly circular and equatorial has no easy explanation. In terms of capture probability, wayward asteroids would not necessarily slip into such neat orbits; the resulting orbit would most likely be inclined and considerably eccentric. Hence, for confirmation, such theories will have to await both the physical exploration of their surfaces and a firsthand look at carbonaceous asteroids themselves.

The Soviet Union’s probes to the Martian satellite’s surface were collectively called the Phobos mission. Two craft, Phobos 1 and Phobos 2, were launched in the summer of 1988; their mission was to send a lander to Phobos so that it could analyze the satellite’s surface. Communication with Phobos 1 was lost before it reached Mars’s orbit. Phobos 2 successfully attained orbit around Mars in 1989 and began returning photographs of Phobos and Mars. On March 27, 1989, merely days before the probe’s planned close encounter with Phobos and release of its landers, the spacecraft spun irretrievably out of control, and the mission was lost.

Intense interest in the tiny satellites of Mars has been generated for several reasons. They provide a natural “space station” for future Mars explorers. Their tiny gravitational fields require very little energy to overcome, but they offer a stable platform for the staging of expeditionary landing and observation teams. They may also contain substantial quantities of water that may one day be mined to provide hydrogen and oxygen for space travelers, thus reducing the necessary burden of transporting it from Earth to the surface of Mars. Finally, less energy is required for a mission to the satellites of Mars than to and from the surface of Earth’s Moon because of their weak gravitational fields and, hence, their low escape velocities. Such a mission, which would require a minimal round-trip travel time of two to three years, was seriously discussed by both the Soviet government and certain interests in the United States near the time when the Soviet Union collapsed in 1991. Two decades then passed with little interest in Phobos or Deimos on the part of the two major spacefaring nations.

Methods of Study

NASA spacecraft that encountered Phobos and Deimos produced photographic studies from onboard cameras and mass studies based on flyby navigational data. The most detailed photographic studies were conducted with Viking orbiter cameras. Viking 1 flew by Phobos on February 12, 1977. Viking 2 flew by Deimos on September 25, 1977. The photographic system on the orbiters was called the Viking Imaging System. It returned a total of 51,539 images of Mars and its satellites to Earth. The masses of Phobos and Deimos were estimated by determining how much the Viking spacecraft were deflected in their orbits around Mars by the gravitational fields produced by the satellites. Until the Viking encounters, the masses of Phobos and Deimos were not precisely known. Through observation of their orbital motion, coupled with a Martian mass determination, these encounters provided good estimates of the masses of Phobos and Deimos.

Photographic studies alone produced a wealth of information. A technique known as reflectivity—measuring the amount of light reflected from the surface of the satellites—enabled planetary scientists to speculate that the satellites may have been captured asteroids. It has long been speculated that a class of asteroids made of carbon and carbon compounds would be exceedingly dark, as Phobos and Deimos proved to be.

The surfaces of both satellites are saturated with craters, which indicates that their surfaces are very old. This finding enabled the dating of the asteroids. In addition, the peculiar ten-kilometer crater on Phobos named Stickney displayed very large cracks down the surface of the satellite, which hinted to some planetary geologists that the composition of the body may contain substantial amounts of water ice. The cracks also indicated other subsurface structural features as well as the depth of the regolith, or top layer of soil overlying the bedrock.

Context

Phobos and Deimos may someday become two of the most important way stations in the solar system. As the Earth’s focus of exploration turns to Mars as the next most logical frontier of exploration and colonization beyond the Moon, Phobos and Deimos will serve as stepping-off points to the surface of Mars. They could provide a base for operations to and from the planet. They could also provide a communications base for ground-to-space and -Earth information exchanges. Finally, these two tiny satellites might be able to supply water, fuel, and oxygen to future space explorers.

Plans have been considered for sending a crewed mission to Phobos as a dress rehearsal for a later mission to the surface of Mars. This mission would test the critical life-support and medical issues that currently limit a Mars mission at a fraction of the energy that would be required to land on the surface of the planet itself.

In the aftermath of the Columbia accident (February 1, 2003), the Bush administration reconsidered NASA’s mission. In early 2004 the White House directed NASA to complete the International Space Station and retire the space shuttle fleet by 2010 and then proceed with human exploration beyond low-Earth orbit. This plan called for a return to the Moon and then an evolutionary approach leading to a crewed expedition to Mars. Though the United States changed those plans in the early 2020s with the new space shuttle Constellation Orion, no plans to study Phobos and Deimos were announced. Phobos and Deimos will undoubtedly play a prominent role in eventual plans for the human exploration of Mars. In November 2011, the Russian Space Agency embarked on a sample return mission (the Phobos-Grunt mission) to Phobos; the mission was ultimately unsuccessful and the probe failed to leave Earth's orbit, eventually crashing in 2012. In 2023, scientists from the United Arab Emirates announced that their orbiter, Hope, has taken exciting new pictures of the moons, suggesting a different theory of planetary origin. Further, the Japanese Aerospace Exploration Agency (JAXA) announced plans to launch a solar-powered spacecraft in 2024, which would reach Mars’s orbit in 2025 and scoop samples from Phobos in 2026.

Bibliography

Andrews, George. “Why the 'Super Weird' Moons of Mars Fascinate Scientists (Published 2020).” The New York Times, 25 July 2020, www.nytimes.com/2020/07/25/science/mars-moons-phobos-deimos.html. Accessed 15 Sept. 2023.

Clark, Stuart. "Phobos: Our Next Outpost in Space?" New Scientist 30 Jan. 2010: 28–31.

Collins, Michael. Mission to Mars. New York: Grove Weidenfeld, 1990.

Encrenaz, Thérèse, et al. The Solar System. New York: Springer, 2004.

Ezell, Edward, and Linda Ezell. On Mars: Exploration of the Red Planet, 1958–1978. NASA SP-4212. Washington: Govt. Printing Office, 1984.

Hartmann, William K. Moons and Planets. 5th ed. Belmont: Thomson Brooks/Cole, 2005.

Hartmann, William K., et al. Out of the Cradle: Exploring the Frontiers Beyond Earth. New York: Workman, 1984.

Higgins, Nick, and Jonathan O'Callaghan. “Where Did Mars's Moons Come From?” Scientific American, 2 May 2023, www.scientificamerican.com/article/where-did-marss-moons-come-from. Accessed 15 Sept. 2023.

Jöels, Kerry Mark. The Mars One Crew Manual. New York: Ballantine, 1985.

Kargel, Jeffrey S. Mars: A Warmer, Wetter Planet. New York: Springer, 2004.

McKee, Maggie. "Life to Blast Off for Mars Moon Phobos." New Scientist 5 Nov. 2011: 15.

Mutch, Thomas A., et al. The Geology of Mars. Princeton: Princeton UP, 1976.

National Aeronautics and Space Administration. The Case for Mars: Concept Development for a Mars Research Station. San Francisco: UP of the Pacific, 2002.

Perkins, Sid. "Martian Moon Is Probably Porous." Science News 5 June 2010: 11.

Sheehan, William, and Stephen James O’Meara. Mars: The Lure of the Red Planet. Amherst: Prometheus, 2001.

Zubrin, Robert. Entering Space: Creating a Spacefaring Civilization. New York: Tarcher, 2000.