Solar eclipses of Phobos and Deimos observed from the surface of Mars.

The small martian satellites Phobos and Deimos orbit in synchronous rotation with inclinations of only 0.01 degrees and 0.92 degrees , respectively, relative to the planet's equatorial plane. Thus, an observer at near-equatorial latitudes on Mars could occasionally observe solar eclipses by these satellites (see ref. 1, for example). Because the apparent angular diameter of the satellites is much smaller than that of the Sun, however, such events are more appropriately referred to as transits. Transit data can be used for correcting and refining the orbital ephemerides of the moons. For example, Phobos is known to exhibit a secular acceleration that is caused by tidal dissipation within Mars. Long-term, accurate measurements are needed to refine the magnitude and origin of this dissipation within the martian interior as well as to refine the predicted orbital evolution of both satellites. Here we present observations of six transits of Phobos and Deimos across the solar disk from cameras on Mars aboard the Mars Exploration Rovers Spirit and Opportunity. These are the first direct imaging observations of satellites transiting the Sun from the surface of another planet.

The global topography of Mars and implications for surface evolution.

Elevations measured by the Mars Orbiter Laser Altimeter have yielded a high-accuracy global map of the topography of Mars. Dominant features include the low northern hemisphere, the Tharsis province, and the Hellas impact basin. The northern hemisphere depression is primarily a long-wavelength effect that has been shaped by an internal mechanism. The topography of Tharsis consists of two broad rises. Material excavated from Hellas contributes to the high elevation of the southern hemisphere and to the scarp along the hemispheric boundary. The present topography has three major drainage centers, with the northern lowlands being the largest. The two polar cap volumes yield an upper limit of the present surface water inventory of 3.2 to 4.7 million cubic kilometers.

Observations of the north polar region of Mars from the Mars orbiter laser altimeter.

Elevations from the Mars Orbiter Laser Altimeter (MOLA) have been used to construct a precise topographic map of the martian north polar region. The northern ice cap has a maximum elevation of 3 kilometers above its surroundings but lies within a 5-kilometer-deep hemispheric depression that is contiguous with the area into which most outflow channels emptied. Polar cap topography displays evidence of modification by ablation, flow, and wind and is consistent with a primarily H2O composition. Correlation of topography with images suggests that the cap was more spatially extensive in the past. The cap volume of 1.2 x 10(6) to 1.7 x 10(6) cubic kilometers is about half that of the Greenland ice cap. Clouds observed over the polar cap are likely composed of CO2 that condensed out of the atmosphere during northern hemisphere winter. Many clouds exhibit dynamical structure likely caused by the interaction of propagating wave fronts with surface topography.

Topography of the northern hemisphere of Mars from the Mars Orbiter Laser Altimeter.

The first 18 tracks of laser altimeter data across the northern hemisphere of Mars from the Mars Global Surveyor spacecraft show that the planet at latitudes north of 50 degrees is exceptionally flat; slopes and surface roughness increase toward the equator. The polar layered terrain appears to be a thick ice-rich formation with a non-equilibrium planform indicative of ablation near the periphery. Slope relations suggest that the northern Tharsis province was uplifted in the past. A profile across Ares Vallis channel suggests that the discharge through the channel was much greater than previously estimated. The martian atmosphere shows significant 1-micrometer atmospheric opacities, particularly in low-lying areas such as Valles Marineris.

The clementine mission to the moon: scientific overview.

In the course of 71 days in lunar orbit, from 19 February to 3 May 1994, the Clementine spacecraft acquired just under two million digital images of the moon at visible and infrared wavelengths. These data are enabling the global mapping of the rock types of the lunar crust and the first detailed investigation of the geology of the lunar polar regions and the lunar far side. In addition, laser-ranging measurements provided the first view of the global topographic figure of the moon. The topography of many ancient impact basins has been measured, and a global map of the thickness of the lunar crust has been derived from the topography and gravity.

Clementine observations of the aristarchus region of the moon.

Multispectral and topographic data acquired by the Clementine spacecraft provide information on the composition and geologic history of the Aristarchus region of the moon. Altimetry profiles show the Aristarchus plateau dipping about 1 degrees to the north-northwest and rising about 2 kilometers above the surrounding lavas of Oceanus Procellarum to the south. Dark, reddish pyroclastic glass covers the plateau to average depths of 10 to 30 meters, as determined from the estimated excavation depths of 100- to 1000-meter-diameter craters that have exposed materials below the pyroclastics. These craters and the walls of sinuous rilles also show that mare basalts underlie the pyroclastics across much of the plateau. Near-infrared images of Aristarchus crater reveal olivine-rich materials and two kilometer-sized outcrops of anorthosite in the central peaks. The anorthosite could be either a derivative of local magnesium-suite magmatism or a remnant of the ferroan anorthosite crust that formed over the primordial magma ocean.

Deimos encounter by viking: preliminary imaging results.

Recent close flybys of Deimos by Viking revealed a smooth-appearing surface void of grooves. Higher-resolution pictures showed that the surface was actually covered with craters but that a regolith filled the smaller craters, giving the smooth appearance. The surface was also covered with boulders and bright streak-like markings analogous to base-surge or ejecta cloud deposits.

Phobos transit of Mars as viewed by the viking cameras.

A Viking orbiting spacecraft successfully obtained pictures of the martian satellite Phobos with Mars in the background. This is the first time that a single picture was obtained from a spacecraft which contained both a planet and a moon and had significant surface detail visible on both. The region of Mars below Phobos included volcanoes in the Tharsis Montes region. These pictures showed Phobos to be smaller than previously thought. The image of Phobos can be used as a control point to determine the map coordinates of surface features on Mars.

Viking first encounter of phobos: preliminary results.

During the last 2 weeks of February 1977, an intensive scientific investigation of the martian satellite Phobos was conducted by the Viking Orbiter-1 (VO-1) spacecraft. More than 125 television pictures were obtained during this period and infrared observations were made. About 80 percent of the illuminated hemisphere was imaged at a resolution of about 30 meters. Higher resolution images of limited areas were also obtained. Flyby distances within 80 kilometers of the surface were achieved. An estimate of the mass of Phobos (GM) was obtained by observing the effect of Phobos's gravity on the orbit of VO-1 as sensed by Earth-based radiometric tracking. Preliminary results indicate a value of GM of 0.00066 +/- 0.00012 cubic kilometer per second squared (standard deviation of 3) and a mean density of about 1.9 +/- 0.6 gram per cubic centimeter (standard deviation of 3). This low density, together with the low albedo and the recently determined spectral reflectance, suggest that Phobos is compositionally similar to type I carbonaceous chondrites. Thus, either this object formed in the outer part of the asteroid belt or Lewis's theory that such material cannot condense at 1.5 astronomical units is incorrect. The data on Phobos obtained during this first encounter period are comparable in quantity to all of the data on Mars returned by Mariner flights 4, 6, and 7.

Mariner 9 television reconnaissance of Mars and its satellites: preliminary results.

At orbit insertion on 14 November 1971 the Martian surface was largely obscured by a dust haze with an extinction optical depth that ranged from near unity in the south polar region to probably greater than 2 over most of the planet. The only features clearly visible were the south polar cap, one dark, spot in Nix Olympica, and three dark spots in the Tharsis region. During the third week the atmosphere began to clear and surface visibility improved, but contrasts remained a fraction of their normal value. Each of the dark spots that apparently protrude through most of the dust-filled atmosphere has a crater or crater complex in its center. The craters are rimless and have featureless floors that, in the crater complexes, are at different levels. The largest crater within the southernmost spot is approximately 100 kilometers wide. The craters apparently were formed by subsidence and resemble terrestrial calderas. The south polar cap has a regular margin, suggsting very flat topography. Two craters outside the cap have frost on their floors; an apparent crater rim within the cap is frost free, indicating preferentia loss of frost from elevated ground. If this is so then the curvilinear streaks, which were frost covered in 1969 and are now clear of frost, may be low-relief ridges. Closeup pictures of Phobos and Deimos show that Phobos is about 25 +/-5 by 21 +/-1 kilometers and Deimos is about 13.5 +/- 2 by 12.0 +/-0.5 kilometers. Both have irregular shapes and are highly cratered, with some craters showing raised rims. The satellites are dark objects with geometric albedos of 0.05.