In this picture of Cydonia by Viking 1 (taken from this article), there are a bunch of black spots scattered everywhere. Are they an artifact of the image or are they an actual feature on the surface of Mars?
What are the dark spots in the “face of Mars” picture?
They are called "bit errors" in the NASA JPL image catalog for PIA01141. The explanation seems a bit handy-wavy to me; so I think we'll need to dig deeper to see if they are received noise on Earth or some electrical or imaging problem on the spacecraft.
Read more about the Voyagers' cameras in What is the brown band in which we find the pale blue dot that is Earth?
By the 1980's NASA spacecraft used solid state CCD detectors and end-to-end digital transmission. See Galileo (spacecraft) Solid State Imager
Original Caption Released with Image:
NASA's Viking 1 Orbiter spacecraft photographed this region in the northern latitudes of Mars on July 25, 1976 while searching for a landing site for the Viking 2 Lander. The speckled appearance of the image is due to missing data, called bit errors, caused by problems in transmission of the photographic data from Mars to Earth. Bit errors comprise part of one of the 'eyes' and 'nostrils' on the eroded rock that resembles a human face near the center of the image. Shadows in the rock formation give the illusion of a nose and mouth. Planetary geologists attribute the origin of the formation to purely natural processes. The feature is 1.5 kilometers (one mile) across, with the sun angle at approximately 20 degrees. The picture was taken from a range of 1,873 kilometers (1,162 miles).
Below is a closer-to-raw-data image from Views of the Solar System: Raw Viking Orbiter Image 035a72 of the Face. Notice that the dots are bright, not dark in the raw data. Voyager's cameras are 1970's technology; they are based on vidicon tubes not solid state detectors.
Prior to the design and construction of the Galileo probe to Jupiter in the late 1970s to early 1980s, NASA used vidicon cameras on nearly all the unmanned deep space probes equipped with the remote sensing ability. Vidicon tubes were also used aboard the first three Landsat earth imaging satellites launched in 1972, as part of each spacecraft's Return Beam Vidicon (RBV) imaging system. The Uvicon, a UV-variant Vidicon was also used by NASA for UV duties
From Arizona State University's Space Exploration Resources Control Network:
The majority of Mariner 10 high resolution images were sent back in real time (117.6 Kbits/s) with an attendant increase in bit errors during transmission [Danielson et al., 1975; Soha et al., 1975]. This resulted in severe salt and pepper random noise [Soha et al., 1975] that hampers the automatic identification of reseaux, especially in areas with low signal such as terminator regions and space. Due to the generally noisy nature of the data [Danielson et al., 1975; Soha et al., 1975] tolerances were set high (0.90 correlation) to help avoid spurious reseau identifications. Failure to find an actual reseau at a given position resulted in default to the nominal position for a given camera/encounter combination.
- Danielson, G.E., K.P. Klaasen, and J.L. Anderson, Acquisition and description of Mariner 10 television science data at Mercury, J. Geophys. Res., 80, 2357-2393, 1975.
- Soha et al., 1975 Soha, J.M., D.J. Lynn, J.J. Lorre, J.A. Mosher, N.N. Thayer, D.A. Elliot, W.D. Benton, and R.E. Dewar, IPL Processing of the Mariner images of Mercury, J. Geophys. Res., 80, 2394-2414, 1975.
From Digital Elevation Model of the Discovery Region on Mercury(Lunar and Planetary Science XXIX, 1849.pdf):
- Excessive salt and pepper noise in vidicon images, can cause the mis-identification of reseaus necessary for accurate image rectification.
From book.google.com Yap et al. (2010) Adaptive Image Processing: A Computational Intelligence Perspective, Second Edition (also here):
9.4.2 Models of Impulse Noise
When an image is coded and transmitted over a noisy channel, or degraded by electrical sensor noise, as in a vidicon TV camera, degradation appears as salt-and-pepper noise (i.e. positive and negative impulses).
Emily Lakdawalla discusses the Voyagers' vidicons and problems with the old "treasure trove" of images