Mariner 10 was launched on November 3, 1973, 12:45 am PST, from Cape Canaveral on an Atlas/Centaur rocket (a reconditioned Intercontinental Ballistic Missile - ICBM).
Within 12 hours of launch the twin cameras were turned on and several hundred pictures of both the Earth and the Moon were acquired over the following days. Twenty images acquired looking down on the Moon's North Pole have been mosaicked (below) as part of the current calibration effort.
In this unusual view eastern Mare Frigoris is near the center of the disc, while Mare Crisium is the large circular feature near the lower right limb. The heavily cratered region shown in the top of the mosaic shows portions of the Moon not seen from the Earth. Click here for a larger image.
This mosaic is composed of 22 frames acquired in orange (15), clear (4), UV (2), and UV-polarized (1) wavelengths by the Mariner 10 Spacecraft. [Robinson et al., 1992; Robinson and Edwards, 1995].
The Earth and Moon were imaged by Mariner 10 from 2.6 million km while completing the first ever Earth-Moon encounter by a spacecraft capable of returning high resolution digital color image data. These images have been combined at right to illustrate the relative sizes of the two bodies. From this particular viewpoint the Earth appears to be a water planet!
On February 5th Venus first came into view and Mariner 10 acquired this dramatic image (Feb 5 1974, 9:49 AM PDT) of the lighted cusp of Venus' North Pole. Click here for a larger image.
Over the next week, Mariner 10 acquired over 4000 images of Venus showing the capabilities of the imaging systems and revealing for the first time a high resolution look at the cloud-shrouded planet.
Mercury Encounter I
Mariner 10's first image of Mercury was acquired on March 24, 1974, from a distance of 3,340,000 miles (5,380,000 km). Mariner 10's trajectory brought its closest approach behind the lighted hemisphere of Mercury in order to acquire important measurements with other instruments (March 29). Thus, the images were acquired in two steps, an inbound leg (images acquired before passing into Mercury's shadow), and an outbound leg (after exiting from Mercury's shadow).
More than 2300 useful images of Mercury were taken, both moderate resolution (3-20 km/pixel) color and high resolution (better than 1 km/pixel) black and white coverage. For an animated sequence of the whole encounter
The current calibration and mosaicking effort have led to substantial improvements in the Mariner 10 color image data. Analysis of these exciting new mosaics is underway and some preliminary results follow.
Very little is known about the regolith or the bulk composition of the mercurian crust [c.f. Chapman, 1988]. Color images exist for only half of Mercury's surface [Danielson et al., 1975], and its close proximity to the Sun makes terrestrial observations problematical at best [Vilas, 1988].
From Earth-based spectral measurements it is proposed that Mercury has no gross hemispherical mineralogical differences and that a weak Fe absorption feature might exist near 0.9 µm [Vilas, 1988]. Due to Mercury's high bulk density a high bulk Fe content has been assumed [c.f. Chapman, 1988]. However, there is no direct evidence of a high Fe content in the crust.
Earlier analysis of Mariner 10 orange and UV images led to the startling inference that Mercury's regolith, and thus crust, may actually be deficient in Fe2+ and Ti4+ relative to the Moon [c.f. Rava and Hapke, 1987]. However, this apparent contradiction may not be real.
An average impact event on Mercury should produce about twice the melt as a similar event on the Moon [Cintala, 1992]. Agglutinate glasses formed from such impacts alter the spectral properties of the regolith [Adams and Jones, 1970; Conel and Nash, 1970; Adams and McCord, 1973; Hapke, 1973; Charette et al., 1976]. Increased agglutinate content results in reduced overall spectral contrast and a general reddening of the visible slope. Cintala  has proposed that abundant agglutinates in the mercurian regolith are masking the 0.9 µm Fe absorption feature, thus the regolith and crust may not be deficient in Fe.
Many questions such as this can be addressed with digital calibrated Mariner 10 image data. We are extending the original Mariner 10 color and albedo analyses [Hapke et al., 1980; Rava and Hapke, 1987] by refining the calibration for all four bandpasses (UV-polarized, UV, blue, orange), as well as the broadband visible filter [Robinson et al., 1992; Robinson and Edwards, 1995], and by creating a high resolution (1 km) albedo map for all portions of Mercury imaged by Mariner 10.
The color data will allow for mapping of the distribution and abundance of opaque minerals, such as ilmenite (titanium bearing). In particular these color and albedo data will be compared to the Moon so that basin ejecta deposits, intercrater plains, smooth plains, and heavily cratered terrain can be examined (see also NASA Planetary Science Research Discoveries article, Mercury Unveiled (January 1997)).
Mercury Encounter II
After passing Mercury the first time and making a trip around the Sun, Mariner 10 again flew by Mercury on September 21 at 1:59 PM PDT. This encounter brought the spacecraft in front of Mercury in the southern hemisphere.
Despite failure of the onboard tape recorder, over 750 useful images of Mercury were acquired that greatly enhance the earlier data. A subset of these images is currently being mosaicked into a southern hemisphere view of the planet that bridges the coverage between the incoming and outgoing views of the first encounter.
Mercury Encounter III
As an added bonus, a limping spacecraft was coaxed into a third and final encounter with Mercury in March of 1975. Due to several problems with the aging spacecraft, only ~450 useful images of the planet were acquired, though many are at significantly higher resolution than previous encounters.
Space Exploration Resources ~ School of Earth and Space Exploration ~ Arizona State University