Much greater resolution was achieved when the two Viking Orbiters flew within a few hundred kilometers of the surfaces of the moons in the summer and fall of 1976, then as close as 100 to 300 kilometers in February and May 1977, detecting features as small as 10 to 200 meters. In October 1977 Viking Orbiter II flew less than 30 kilometers from Deimos. The low resolution photos were useful in determining the sizes, shapes and rotational properties of the Martian satellites, while the higher resolution photos revealed small scale features such as surface fractures, crater chains, and wall slumping of craters. It was during a close flyby of Phobos in late 1976 that numerous mysterious grooves were discovered on the surface. More study showed they were likely associated with the crater Stickney, and may come from deep fractures under the surface. Deimos has no such grooves. A major objective of Vikings observations of the moons was to determine their composition through color measurements and mass determination, a goal in which it was only partially successful. In 1977 scientists noted that the surface of Phobos was similar to Type I or II carbonaceous chondrites and to the asteroid Ceres. From this the researchers inferred that Phobos and Deimos may have originated in the asteroid belt and were captured by Mars. Almost exactly one century after Hall had discovered the moons, enough information had been gleaned to speculate on their origin.
A recent conference on the moons of Mars reminded me of the wonders that await us even in our own solar system. The two tiny moons, Phobos and Deimos, have a storied history. Long before their discovery in 1877, Johannes Kepler speculated that since the Earth had one moon and Jupiter had four known in his time, Mars might have two moons since it orbits between Earth and Jupiter. In 1726 Jonathan Swift, probably influenced by this speculation, wrote in Gulliver's Travels that the scientists of Laputa discovered "two lesser stars, or satellites, which revolve about Mars."
In real life, however, all searches failed until the American astronomer Asaph Hall discovered them using the giant 26-inch refractor of the U. S. Naval Observatory in Washington, D.C. The discovery was made because Hall came to doubt the conventional wisdom that Mars had no moon, and because he searched much closer to the planet than others had. In the early morning of August 17, 1877, having been urged on in the search by his wife Chloe Angeline Stickney, Hall made the remarkable discovery: not one, but two moons that traveled so close to Mars that they were within the glare of the planet. The inner moon was brighter, and zipped around Mars in less than 8 hours, faster than the planet's own rotation – instead of taking a leisurely month as does our Moon. To a hypothetical Martian, the inner moon would rise in the west and set in the east, and pass through all its phases in a few hours. The news took both the public and the astronomical world by storm. Hall chose to name the inner satellite Phobos (Flight) and the outer satellite Deimos (Fear), based on Homer's Illiad, where Phobos and Deimos are the attendants, or sons, of Mars.
The moons of Mars have often been featured in popular culture, beginning (aside from Jonathan Swift) with the Mars novels of Edgar Rice Burroughs at the beginning of the 20th century. In 1955 science fiction writer Donald Wollheim published his bracing adventure novel The Secret of the Martian Moons (I still have my copy!). In Kim Stanley Robinson's Mars trilogy the first colonists to Mars build a city within the crater Stickney. More recently novelist Tom Mallon set his story Two Moons in 1877 Washington, D.C., and Ty Drago wrote his debut novel Phobos. Some video games even feature Phobos. Meanwhile, NASA continues to study its options for exploration of the Mars system, including even a human mission to a near earth asteroid, which may resemble the moons of Mars.