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Дата изменения: Thu Oct 16 01:00:27 1997
Дата индексирования: Tue Oct 2 06:32:55 2012
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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.
Background
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)
Bottom Line
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).
Mars
Face