Shadow Boxing with ‘Fear’

Categories: Feature Stories Mars

Shadow Boxing with ‘Fear’

Solar Eclipse by Martian Moon, Phobos

Martian moon, Phobos, reduces sky illumination at Viking landing site. The colors have been distorted purposely to enhance detail. The blue and white horizontal stripes correspond to test chart patches. The brownish stripes in the middle represent the martian surface visible above the spacecraft. Note a decrease in light levels in the sky midway through the imaging event. The darkening, caused by the passage of the penumbral shadow of Phobos, is present over approximately 100 vertical lines. Moving at about 2 km sec, the shadow took about 20 sec to pass over the Lander. It should be obvious that successful acquisition of this picture requires extremely accurate calculations regarding the orbit of Phobos about Mars. Not only was the passage of the small shadow across the Lander correctly calculated, the time of the event also was predicted within a few seconds.
Credit: JPL/NASA

There is something comforting about finding the familiar in an alien landscape. Stars, moons, planets, and constellations — those are the tie-points used by navigators when they are lost.

Today’s unfamiliar panoramas of a desolate, martian landscape may offer few reference tie-points, except for the occasional hill beckoning on the horizon. The crew of Apollo 8 witnessed the first earthrise over the moon, a familiar Christmas greeting from Terra Firma in 1968.

Recreating an Apollo-like earthrise on Mars is difficult. As with all inner planets, a world’s speck of brightness gets overshadowed by the Sun’s illumination. Our planet when seen as a martian ‘morning star’, can barely escape the glare from its own star.

In fact, because of Mars’ one-percent atmospheric density compared to Earth’s, the entire night-sky as seen from Mars would not even show a familiar twinkling starfield. Fortunately because the stars are so distant, the standard constellation patterns would be surprisingly familiar, like Orion and the Pleiades.

The prospects for the rover Spirit to witness some familiar local weather, like tornadoes, may offer at least a few tiny twisters commonly seen by those used to traveling in the American Southwest. But such dust devils are somewhat exotic on Earth, much less when photographed on an alien planet.

Sunrise and sunset on Mars may give a glimpse of the familiar. Dawn and dusk governs the longer martian day, or Sol (24 hours, 39 minutes long), while providing a tie-point for referencing interplanetary clocks. But a martian observer on the surface may long for something more spectacular, but still familiar, than a pinkish sunset.

One such ‘edgey’ snapshot may be captured when a moon literally edges out the Sun itself. Sometime during its three-month primary mission on another planet, the Spirit rover may bear witness to an exotic solar eclipse.

"It will be the first eclipse observed from the surface of another planet," said Jim Bell, lead scientist for the rover’s panoramic camera, which is providing three to ten times clearer images than possible for any previous martian skyview. "These are images that would look good on an IMAX screen," echoed principal investigator, Steve Squyres, referring to the panorama’s clarity.

Moon Shadows

Mars has two natural satellites, or moons, called Phobos ( Greek for "Fear") and Deimos ("Terror"). Phobos is one of the darkest objects in our solar system, a mostly colorless (dark gray) satellite, except for a faint reddish-orange hue cast by reflection of sunlight off Mars. Because of its visual darkness, a full moon on Mars would do little to brighten the night.

To a person standing on the surface of Phobos, the red planet overhead would fill most of the sky. But to a person standing on the surface of Mars, Phobos would appear about one-third to half the size of our own moon, not because it is big, but because it is so close.

Shadow cast on cratered martian surface nera western Xanthe Terra on August 26, 1999, about 2 p.m. local time, by moon Phobos. Compared to surface missions returning 10,000 or so images, the orbiters continue to provide over ten times that many. Credit: NASA JPL/MSSS/MOC

Somewhere near the martian equator, Phobos eclipses the sun nearly every day.

Such a solar eclipse occurs about one hundred times more frequently than terrestrially observed, because Phobos orbits Mars rapidly– about 3 times a day at a distance of about 6,000 km (3,728 miles). In contrast, our moon circles Earth at a distance of more than 200,000 miles.

Martian solar eclipses last only about 30 seconds because Phobos is in such a low orbit — and it races so quickly across the sky as seen from Mars. But Phobos is also very small. It covers only a fraction of the sun’s disk and these martian eclipses are never total.

Mars Phobos crater, 1998. The large crater Stickney, so called because it was the maiden name of the astronomer’s wife who encouraged his continued search for such moons. In 1912 Edgar Rice Burroughs published a story entitled "Under the Moons of Mars" (printed in book form in 1917 as ‘A Princess of Mars’) in which he referred to the "hurtling moons of Barsoom" (Barsoom being the "native" word for Mars in the fictional account). Burroughs was inspired by the fact that Phobos, having an orbital period of slightly less than 8 hours, would appear from Mars to rise in the west and set in the east only five and a half hours later. (Despite Burroughs’ phrase, the outer moon, Deimos, does not "hurtle" — it takes nearly 60 hours to cross the sky from east to west, rising on one day and not setting again for over two more.)

On Mars, Phobos covers only about a quarter of the solar disc.

‘Most Unusual of all Viking Images’

During its multi-year mission, the Viking lander captured what appeared to be such an exotic eclipse–a twenty-second phase of daylight darkening that matched up with predictions for when Phobos’ orbit should have passed in front of the Sun. Passage of the penumbral shadow caused a general drop in light level that was instrumentally detected. The shadow swept over the lander at nearly five hundred miles per hour.

This was the first eclipse of our sun recorded instrumentally from another planet.

NASA historical documents have nominated this event as "possibly the most unusual of all Viking Lander pictures record[ing] the passage of Phobos’ shadow during a solar eclipse."

As exotic as such phenomenon might sound at first, this theme echoes the inscription, "Two Worlds, One Sun", etched on the Spirit rover’s color calibration target. There are a few obvious solar differences when viewed between the two worlds, namely that the sun appears to be about two-thirds the size it looks from Earth because of its distance from Mars.

Shadow Boxing

During such solar eclipses, mission scientists not only get rare pictures, but also collect extra data useful for triangulating the coordinates of a particular surface spot. The effect is not unlike that used by global positioning satellites (GPS). For instance, one way to measure the precise location of a surface rover is to time the movement of this tiny moon’s eclipse shadow as it passes over the lander. The shadow of Phobos was seen during the Viking missions in the late 1970s, and in fact one day the shadow was observed to pass right over the Viking 1 lander–a good test of orbital mechanics and triangulation on Viking’s precise location. Even today, the orbit of Phobos has an unknown accuracy to within 5 to 10 kilometers.

How these martian solar eclipses might appear from the surface depends on exactly where they are viewed, just like their more familiar terrestrial counterparts occur regionally.

Mars Moon, Phobos, potato-shaped satellite, 2003, On June 1, 2003, the Mars Global Surveyor (MGS) spacecraft was slewed eastward to capture these views of the inner moon, Phobos, shortly before it set over the afternoon limb. The image resolution is about 36 meters (118 ft.) per pixel; the maximum dimension of Phobos as seen in this image (the diagonal from lower left to upper right) is just over 24 km (15 mi). This is the "trailing" hemisphere, the part of Phobos that faces opposite the direction that the moon orbits Mars.

"In February 2004 [near where the European Beagle lander was targeting], the Sun will have a partial eclipse by Phobos," said Tom Duxbury from the Jet Propulsion Laboratory, Pasadena, California, who was helping to characterize the Beagle landing site at Isidis Planitia using Mars Global Surveyor (MGS) data. "By timing the eclipse, we [would have been] able to pinpoint the lander’s position quite accurately." Due to the changing geometry of the MGS orbit relative to that of Phobos, the shadow is actually seen in MOC global map images about a dozen times a month.

Moon or Captured Meteor?

Even when not eclipsing the Sun, Phobos has an unusual, oblong appearance already. Phobos is a potato-shaped object about 0.006 times the size of Earth’s Moon–with dimensions approximately the size of medium-sized city, or 27 by 22 by 18 kilometers (about 17 by 14 by 11 miles). The rows of grooves and aligned pits on Phobos are related to, and were probably caused by, a large meteor impact that occurred on the side of Phobos.

The surface of Phobos itself was first imaged by Mariner 9 in 1971, and global coverage was obtained by the Viking orbiters in 1976-80. Phobos was the target of the ill-fated Phobos 1 and Phobos 2 spacecraft, launched by the Soviet Union in 1988. Phobos 2 actually reached Mars in 1989 and obtained a few pictures of the satellite—it also captured the shadow of Phobos cast upon the martian surface using its thermal infrared imager, Termoskan.

Viking images of Phobos proved very valuable in determining the opacity of the night sky. Color and infrared reflectance values of the integrated disc were clues to the satellite’s chemical composition. The spectral data best fit that of carbonaceous chondrite, a carbon-rich variety of meteorite believed to represent primitive solar system material.

Pinkish sunset on Mars, Pathfinder mission
Credit: NASA/JPL

In the equivalent of a lunar eclipse on Earth, Mars can also eclipse Phobos. Like the earthshine reflection this gives our own moon, depending on atmospheric scattering, the shadow cast back on Phobos gives information about the dust conditions on Mars. In 1997, the Pathfinder mission used such a lunar eclipse of Phobos to measure martian dust. In fact, almost every night offers such a lunar eclipse of Phobos.

Under the Martian Moon Shadow

Phobos and the smaller, more distant satellite, Deimos, were discovered in 1877 by Asaph Hall, an astronomer at the United States Naval Observatory in Washington, D.C. Hall had been hunting for martian satellites for some time, and was about to abandon the search when he was encouraged by his wife to continue. In honor of her role in the discovery, the largest crater on Phobos was named Stickney, her maiden name. In a strange case of prescience, author Jonathan Swift wrote a century and a half earlier in his Gulliver’s Travels (1726) that " [The astronomers]..have likewise discovered two lesser stars, or satellites, which revolve about Mars ".

If all goes well with the rover missions, both Spirit and Opportunity cameras are anticipated to triple the number of returned images compared to the 16,000 filed by the 1997 Pathfinder camera.

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