Observations of the "Face on Mars" and similar features by the Mars Global Surveyor Orbiter Camera
Michael C. Malin, Principal Investigator, Mars Global Surveyor Orbiter Camera
There is some interest concerning whether or not the Mars Global Surveyor Orbiter Camera (MOC) will observe the "Face on Mars" and other features in the Cydonia region on Mars. This page will describe why there is interest and what the MOC plans are for photographing the features described below.
For those not familiar with the topic, several Viking images show features on the surface of Mars that, in the eyes of some people, resemble "faces," "pyramids," and other such "artifacts." The most famous of these is the "Face on Mars" and associated features "The City," "The Fortress," "The Cliff," "The Tholus," and "The D&M Pyramid." A fairly substantial "cottage" industry has sprung up around these features, with several books having been written about them, newsletters published, public presentations, press conferences, and, of course, "supermarket tabloid" published reports. The basic premise of these people is that the features are artificial, and are messages to us from alien beings. Their tack is to say, "These should be rephotographed by Mars Global Surveyor, since with high resolution we should be able to prove that they are artificial. If they are in fact artificial, this would rank as one of the greatest discoveries in history and thus every effort should be made to acquire images." Evidence cited as presently "proving" these are unnatural landforms include measurements of angles and distances that define "precise" mathematical relationships. One of the most popular is that "The D&M Pyramid" is located at 40.868 degrees North Latitude, relative to the control network established by Merton Davies (the RAND scientist who has been more or less singularly responsible for establishing the longitude/latitude grids on the planets) to an accuracy (actually, a precision) of order 0.017 degrees. They point out that 40.868 equals arctan (e / pi); alternatively, one of the advocates notes that the ratio of the surface area of a tetrahedron to its circumscribing sphere is 2.72069 (e = 2.71828), which, if substituted for e in the above arctan equation gives 40.893 degrees, which is both within the physical perimeter of the "Pyramid" and within the above stated precision. Other mathematical relationships abound. The advocates of this view argue that "no scientific study of these features has been conducted under NASA auspices" and that NASA and the conservative science community are conspiring to keep the "real" story from the American public.
The conventional view is that this is all nonsense. The Cydonia region lies on the boundary between ancient upland topography and low-lying plains, with the isolated hills representing remnants of the uplands that once covered the low-lying area. The features seen in these mesas and buttes (to bring terrestrial terminology from the desert southwest to bear on the problem) result from differential weathering and erosion of layers within the rock materials. The area is of considerable importance to geologists because it does provide insight into the sub-surface of Mars, and to its surface processes. The measurement of angles and distances seems so much numerology, especially when one understands the actual limitations in the control network (of order 5-10 km, or 0.1-0.2 degrees) and the imprecision of our corrections of the images (neglecting, for example, topography when reprojecting data for maps) on which people are trying to measure precise angles and distances. For example, using the latest Mars Digital Image Mosaic and the U. S. Geological Survey control network, the aforementioned "Pyramid" is located at 40.67 N, 9.62W. Using the Viking spacecraft tracking and engineering telemetry, the position is 40.71 N, 9.99 W. The difference, 0.04 deg latitude and 0.37 deg longitude, represents nearly 17 km on the ground, or 7X the size of the Pyramid. These positions differ from the e / pi position by a similar number. Even given accurate data, however, most science does not depend solely on planimetric measurements, even when using photographs. There are many other attributes used to examine features, especially those suspected of being artificial, and the martian features do not display such attributes. No one in the planetary science community (at least to my knowledge) would waste their time doing "a scientific study" of the nature advocated by those who believe that the "Face on Mars" artificial.
Things Limiting MOC Observations
Before discussing the observations MOC will attempt to make of "The Face" and other such features, some facts about the camera and its ability to look at specific locations are needed.
The MOC is body-fixed to the spacecraft
It has no independent pointing capability. It makes pictures the same way a fax machine does (i.e., the scene is moved past the single line detector).
The MOC has a limited cross-track Field of View (FOV)
The MOC has a very small field of view (0.44 degrees), which is about 3 km from the 400 km orbital altitude. It typically takes very small images at very high resolution (lots of data). Anything wider than 3 km cannot be imaged in its entirety.
The MOC has a large but not "infinite" along-track Field of View
The MOC's downtrack field of view is limited by the amount of data that will fit in its buffer (about 10 MB). If one uses the entire buffer (which is not likely to be completely empty unless it's planned to be) and 2:1 realtime predictive compression, this translates to a downtrack image length of about 15 km. The camera has been designed to be able to average pixels together to synthesize poorer resolution, which frees up data. Under the best case buffer availability, an 8X summed image would be 3 km wide (but only 256 pixels across) by about 78,000 pixels long which, at 12 m/pxl (8 X 1.5) would be over 800 km long. One of the big uncertainties in taking pictures of specific places on Mars is the uncertainty in when the spacecraft will pass over that place: the timing uncertainty of 40-120 seconds translates to 120 to 360 km uncertainty in position.
The spacecraft has limited pointing control
The spacecraft uses infra-red horizon sensors for in-orbit pointing control. Owing to variations in the IR flux of the horizon with latitude, season, surface topography, atmospheric dust content, cloudiness, and other meteorological and climatological conditions, the control capability is about 10 mrad (0.6 degrees = 4 km), which is larger than the MOC field of view.
There will be a substantial uncertainty in the predicted inertial position of the spacecraft (and hence, the camera)
The position of the spacecraft is determined by radio tracking for 8 hours (roughly 4.5 hours of actually seeing the spacecraft) a day, and by computing the position of the Earth, Mars, and the spacecraft in an inertial coordinate system. It takes a few days to do this, and to use it to determine where the spacecraft will be a few days later. By that time, gravity perturbations, atmospheric drag, and autonomous momentum unloadings will have changed the orbit. Error studies suggest that the uncertainty seven days after the end of a given period of tracking can be represented as (at best)a 40 second uncertainty in the time the spacecraft will be at a specific point in its orbit. This translates (at the orbital rate of the spacecraft projected on the ground of 3 km/s) to 120 km downtrack and (because Mars rotates at 0.24 km/s at the equator) 9.6 km crosstrack. At 40 degrees latitude, the crosstrack uncertainty is 7.4 km, over twice the size of the MOC field of view. At some times in the mission, when the orbit geometry is unfavorable, predictions will be worse.
The non-inertial position of the spacecraft will also be uncertain
The position of the spacecraft is determined inertially. As noted above, the position of the longitude/latitude grid is also uncertain to about 5-10 km.
The spacing of orbits will be uncertain
If, in spite of the preceding, orbits were equally spaced, then the average spacing of orbits at the equator for the 687 day mission would be about 2.5 km, which means that each spot on the equator would fall within the MOC field of view in (possibly) two images. In fact, the repeat distance is just over 3.1 km, again assuming equal spacing, and it is more than likely that each spot on the equator will only be seen once. At 40 degrees latitude, the spacing is roughly 2.4 km, and any location will be seen, at most, twice. Given Items 1-6 (above), it is most likely that some places will be overflown twice, and others not at all, and that our ability to predict this is very limited.
The MOC team is attempting to address some of these issues with, for example, optical navigation. This could reduce the spacecraft position uncertainty by perhaps a factor of five or more. An attempt will be made to generate a new control grid with higher precision (perhaps as good as 1 km). But nothing can be done about the orbit spacing or the pointing control or the width of the MOC field of view. Thus, hitting anything as small as a specific 3 km piece of the planet is going to be very difficult.
This discussion doesn't address the variability of the martian atmosphere, which is very dynamic. Given the occurrence of dust storms during some seasons, and polar clouds during others, there is no guarantee that, even when the spacecraft flies over a specific area, the ground will actually be visible.
Plans for Observing the "Face on Mars"
Despite providing a number of people involved with the "private" studies of the "Face of Mars" with exactly the same information presented above, there appears to be a continuing view that MOC will purposefully avoid taking pictures of the "Face" and other features. Much of their focus is on "conspiracies" they feel exist to keep information from the public. This, of course, isn't the case: if an image of the "Face" is acquired, it will most definitely be released. The "Face on Mars," "City," "Fortress," "Cliff," "Tholus," "D&M Pyramid," etc. are in the MOC target database. Image acquisitions will be scheduled each time the spacecraft is predicted to pass over each target. This is done automatically. Given the factors noted above, however, there is no certainty that the images will actually include the features of interest.
Output from planning software showing Cydonia "targets" (GIF = 252 KBytes)
It is planned to try to acquire images of the "Face" and other features in Cydonia. Contrary to what some people have said and written, this has been the plan for some time. This plan was not established in response to outside pressure; rather, there are two reasons for acquiring these images. First, given the interest in the general public about the "Face," it is appropriate to acquire such images for public relations purposes, especially since the public interest has been generated in no small way by the people who claim there is a conspiracy at NASA to withhold information from the public. Second, there are valid scientific reasons to examine landforms in the area (which, after all, is why the Viking spacecraft were photographing the area in the first place).