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Spacecraft imaging of Phobos and Deimos
Vistas in Astronomy, Voi.22, pp.149-161. ©
Pergamon Press Ltd. 1978.
0083-6656/78/0701-0149 $05.00/0
Printed in Great Britain
SPACECRAFT IMAGING OF PHOBOS AND DEIMOS Thomas C. Duxbury Jet Propulsion Laboratory, CaliforniaInstituteof Technology, Pasadena, California 91103
INTRODUCTION Spacecraft imaging data play a major role in determining physical and dynamical properties of Phobos and Deimos.
These same data also yield information on physical and dynamical pro-
perties of Mars since the orbits of the two satellites and the observing spacecraft are dynamically tied to the Mars gravitational field.
Accurate knowledge of these physical and dynami-
cal properties are needed to determine the origins, evolutions and futures of the two moons of Mars. Physical properties of Phobos and Deimos which can be determined from imaging data include size, shape, topography, volume, cartographic and geologic maps, photometric function, albedo, color, regolith depth and distribution and crater morphology, slze-frequency and distribution. Dynamical properties include rotation, llbration, ephemerides, moments of inertia and mass. Additionally, the Mars gravity field, spin axis direction and internal structure can be determined from the dynamics of the Mars/satellite/spacecraft
system.
Other applications of satellite related imaging for improving the knowledge of Mars involves transits, occultatlons and eclipses.
The satellites and their shadows are imaged
against Mars to give geodetic control points for Mars surface features. eclipses give information on the size, shape and atmosphere of Mars.
Occultations and
Viewing the passing of
a satellite shadow over a lander simultaneously from the lander and an orbiting spacecraft can be used to locate the lander on Mars.
Also, viewing a satellite which is partially illu-
minated by Marshine can give information on the surface on Mars. Normally, Mariner and Viking spacecraft fly by Phobos and Deimos at distances greater than 4000 km.
Occasionally, flybys as close as a few thousand kilometer occur.
On Viking, the
orbits of the two spacecraft were changed to allow flybys within i00 km of the surfaces.
Fly-
bys at distances greater than 4000 km yielded lower resolution pictures where features larger than 200 meters were detected.
For the closer flybys, higher resolution pictures were obtained
where features as small as i0 to 200 meters were detected. The lower resolution pictures are well suited for determining the sizes, shapes, photometric functions, rotational properties, libration amplitudes, libration periods and ephemerides of the Martian satellites as well as the gravity field, spin axis direction and internal structure of Mars.
Higher resolution pictures are additionally valuable for determining topography,
geologic and cartographic maps, albedo, color, regollth depth and distribution and crater morphology, size-frequency and distribution.
The higher resolution pictures are also impor-
tant for detecting small scale features such as surface fractures, crater chains, wall slumping of craters, base-surge deposits, pool melting, and boulders either swept up by the orbiting satellites or ejected from craters. 149
150
T.C. Euxbury
LOWER RESOLUTION
IMAGING
The pictures in Figure i, showing Phobos from different lower resolution pictures used to determine
perspectlves,
simple elllpsoided model is used to define the mean surfaces. of the ellipsoid,
a best match is obtained
actual surface topography variations tion pictures having resolutions sizes and shapes
are typical of the
the sizes and shapes of the two Martian Moons. By adjusting
to the satellite outlines
in all pictures.
of up to 3 km from the elllpsoidal model,
of a few hundred meters are sufficient
With
lower resolu-
to determine
the
(mean surface or "sea level") of Phobos and Deimos.
The pictures in Figure 2, showing Phobos at a wide range of phase angles, the lower resolution pictures used to determine The integrated disk brightness properties
A
the three axes
Additionally,
the scattering properties
dence and emission angles. scattering properties
are typical of
functions of the satellites.
as a function of phase angle is used to determine
of the surface material.
are used to evaluate
the photometric
isophotes
in individual
scattering
pictures
Analysis of these lower resolution
or Lambertian
Law.
3)
pictures have showed that the
of both Phobos and Delmos are best modeled by the Hapke-Irvine
rather than a Minnaert
(Fig.
at a smaller scale and at a variety of inci-
These scatter properties
Law
suggest that both Phobos
and Deimos are covered with regollths. Phobos and Deimos are both in synchronous Mars as our Moon faces Earth. longest axes pointing intermediate
oblate satellites a significant
stable orientations
with their
toward Mars, their shortest axes normal to their orbit planes and their
axes in their orbit planes.
nous configuration
rotation about Mars with one side always facing
They are both in dynamically
Librations
(small oscillations)
are expected because of orbit eccentricities, and possible recent impacts.
about this synchro-
tidal forces acting on the
Orbit eccentricities
and tidal forces cause
libration about the shortest axis of Phobos with an amplitude
degrees and a period the same as the orbital period of Phobos.
Libratlons
of about five about the other
two axes of Phobos as well as all three axes of Deimos which would be attributed eccentricities
and tidal forces are less than a few tenths of a degree.
tlon for Phobos was observed by Mariner 9 (Fig. 4).
More detailed
able when the reduction of the Viking data is complete.
to orbital
The five deg libra-
information will be avail-
No librations
due to recent impacts
have been observed. Accurate ephemerides imaging data.
of Phobos and Delmos are vital for interpreting
Additionally,
obtain satellite
images.
accurate ephemerides
Ephemerldes
ner 9 and a majority of Viking. cies of 3 to 5 km.
Ephemeris
accurate
However,
improvements
pictures.
data since observations
the TV cameras to for all of Mariephemeris
to these levels were made by observing throughout
First, position measurements
even from the lower resolution spacecraft
are needed for pointing
to i0 - 20 km were sufficient
the close flybys on Viking required
tions to Phobos and Deimos from the spacecract craft data is twofold.
a majority of the
Secondly,
their orbits.
accura-
the direc-
The power of space-
to a few hundred meters are possible, significant
parallax is obtained
from
can be made from above and below a satellite orbit as well
as inside or outside of its orbit. Spacecraft-centered
direction m e a s u r e m e n t s
positions
of the satellites
taneously
image a satellite and stars in one picture
can he made of a satellite second camera
(Fig. 5.B).
detail on the overexposed technique yields excellent
against
of Phobos and Delmos are made by observing
star backgrounds.
Long exposures (Fig. 5.A).
the
can be used to simul-
On Viking,
a normal exposure
in one camera and a long exposure for stars can be made in a The simultaneous satellite;
satellite/star
however,
picture does not give any surface
only one picture is required.
surface detail but requires
two pictures.
is normally used on Viking unless other mission constraints
The two camera
The two camera approach
do not allow the extra pictures.
Spacecraft Imaging of Phobos and Deimos
Figure I.
Areas on Phobos are viewed from many directions to determine the size and shape of the Martian moon (Viking pictures IIOA01, 081B33 and I01A77).
Figure 2.
Phobos is shown at phase angles ranging from 35 to 123 deg (Viking pictures ILIA03, 055A32 and 077B03).
151
152
T.C.
Figure 3.
Duxbury
A raw and enhanced picture of Phobos is shown which are used to determine the scattering properties of surface particles (Viking picture 390A16).
Figure 4.
The predicted positions of surface features (ends of arrows) were used to identify an in-plane rotational amplitude of about 5 deg (Mariner 9 picture DAS04790460).
Spacecraft Imaging of Phobos and Deimos
Figure 5.
Figure 6.
153
A. Phobos is viewed simultaneously with two stars of 7.4 and 7.7 visual magnitude (Viking picture 235A01). B. Delmos is viewed by one Viking camera while stars are viewed the other camera (picture 428B21 and 428B22).
The south polar region of Phobos is viewed at different perspectives to determine the locations and elevations of surface features such as A thru I (Viking pictures 252A61, 62 and 63 and 315AII, 12 and 14).
154
T.C.
Duxbury
Figure 7. A. High resolution picture of Deimos showing flat areas and bright albedo patches (Viking pictures 428B36 and 38). B. High resolution picture of Phobos showing grooves, and large flat-bottomed craters with dark material (Viking picture 246A07).
Spacecraft Imaging of Phobos and Deimos
Figure 8.
A high resolution picture of Delmos which covers an area of only ~2x2 km. The surface is heavily cratered and covered by regolith and boulders (Viking picture 423B63).
155
156
T.C.
Figure 9.
Duxbury
Phobos was in transit over the Tharsis region of Mars. The complete outline of Phobos is seen (Viking pictures 304B74304B78).
Spacecraft Imaging of Phobos and Deimos
Figure i0.
The shadow of Phobos was observed in ~ e Tharsis region of Mars together with three large volcanoes and chaotic terrain (Viking pictures 496Agl-496A96).
157
158
T . C . Duxbury
Figure ii.
The shadow of Phobos is seen in the Chryse Basin region of Mars and is directly over Viking Lander-I (Viking picture 463A21).
Figure 12.
The complete outline of Phobos is visible from direct sunlight and sunlight reflected by Mars (Viking picture ILIA01).
PENU
SUN Figure 13.
A long exposure was used to detect Fhobos in the penumbra of Mars (Viking picture I18A71).
Spacecraft Imaging of Phobos and Deimos
159
Improvements to the ephemerides of Phobos and Deimos involve the spacecraft orbit and Mars gravity field and spin axis direction which are dynamically coupled through the equations of motion.
Analysis of the orbital evolution of Phobos, requiring both earth-based and
spacecraft-based data, also yields information on the interior of Mars by determining the rate of decay of the orbit of Phobos due to tidal forces.
With the accuracy of the space-
craft-based data limited to the ability to find the center of the highly irregularly shaped satellites, higher resolution pictures currently offer no advantage over lower resolution pictures for ephemeris improvement.
Higher resolution picture would be advantageous if land-
marks were used instead of the satellite center. HIGHER RESOLUTION PICTURES The two Viking Orbiter spacecraft (VO-I and VO-2) were successfully targeted to give close encounters of both Phobos and Deimos. these encounters.
Table i lists the dates and closest flyby distances for
High resolution imaging obtained from these flybys were spectacular.
It
was fitting that the greatest increase in knowledge of the two Martian moons occurred in 1977, i00 years after their discovery. Table i. Satellite
Phobos
Viking Close Encounters Phobos
Deimos
Deimos
Deimos
Spacecraft
VO-I
VO-I
VO-I
VO-2
VO-2
Date
Feb 1977
May 1977
Oct 1977
Oct 1977
May 1978
Minimum Attitude, km
88
290
1500
28*
?
*Closest flyby of any body in our solar system. Imaging data obtained from these closer encounters have higher surface resolution and are ideal for geologic and cartographic analyses.
Overlapping and stereo surface coverage at
these higher resolutions are extremely valuable for determining elevations of features relative to the mean elllpsoidal surface. at ranges less than 600 km.
Figure 6 shows the southern hemisphere of Phobos viewed
Common features in both pictures are labeled A through I.
The
parallax obtained from these pictures not only yields elevation information but also yields the body-fixed coordinates of features needed to produce cartographic maps.
An elevation
difference of over 1.5 km exists between points A and B. Small scale albedo differences are detected from the higher resolution images. ure 7.A the northern hemisphere of Deimos is viewed at a range of i000 km.
In Fig-
Bright albedo
patches are seen near the left llmb around what appears to be the intersecting of two large, flat areas. size.
Albedo differences could indicate a different material or different particle
For Phobos and Deimos whose shapes are highly irregular, determining whether a varia-
tion in surface brightness is due to an albedo or topographic variation can be difficult. Higher resolution pictures can be used to determine regollth thickness.
The height of
the surface above the floors of large, flat-bottomed craters has been used to determine regolith thickness on the Moon.
Thicknesses of over a few hundred meters are inferred from high
resolution pictures containing flat-bottomed craters on Phobos (Fig. 7.B). crater
Analysis of these
depths over the complete surface would yield regolith distribution.
Pictures such as Figure 7 can be used to analyze crater morphology, size-frequency and distribution.
Additionally,
small scale features such as surface fractures, crater chains,
wall slumping, base-surge deposits, pool melting and boulders can be studied.
The picture
in Figure 8, one of the highest resolution pictures ever taken of any body in our solar system, is an additional example of high resolution imaging showing features as small as a few meters across.
160
T.C.
Duxbury
MARS APPLICATIONS Viking provided unique opportunities to view Phobos, Deimos and their shadows against Mars.
These observations are valuable for determining the sizes and shapes of the satellites
Additionally,
the satellite or shadow can be used as a geodetic control point for mapping the
positions of the surrounding features on Mars.
Examples of these types of pictures are shown
in Figures 9 and i0. The shadow of Phobos was viewed passing over the Viking Lander-i (VL-I) simultaneously from the lander and an orbiter.
Using the time of shadow passage observed by the lander, the
position of the shadow (and, therefore, the lander) was located in the orbiter pictures (Fig. Ii).
Passage of the Phobos shadow over VL-I was successfully observed on three separ-
ate occasions in late September of 1977. Other unique satellite observations obtained from Viking involved eclipses and Marsshine. Phobos was viewed while being partially illuminated by sunlight reflected from the surface of Mars (Fig. 12).
Also, Phobos was viewed in the penumbra of Mars (Fig. 13) to determine
scattering properties of the Martian atmosphere. SUMMARY Imaging data obtained from spacecraft orbiting Mars has significantly increased our knowledge of the two Martian moons during the last few years. mation can be found in the references.
A major portion of this infor-
The two asteroid sized bodies may indeed have origi-
nated in the asteroid belt and may be comprised of primordial material from the solar nebula. Sufficient knowledge now exists on the two moons of Mars to seriously consider Phobos and Deimos as candidate targets for future sample/return mission or for locating bases to monitor Mars. ACKNOWLEDGEMENT I thank Drs. D. Elliot and L. Shigg of the JPL Image Processing Laboratory for enhancing the Viking television pictures.
This paper is JPL Planetology Publication Number 314-78-14
and presents the results of one phase of research carried out under Contract NAS 7-100, sponsored by the Viking Program Office and Planetary Geology Program Office, Office of Space Science, National Aeronautics and Space Administration. REFERENCES i. Born, G. H. and Duxbury, T. C. (1975) The Motions of Phobos and Deimos from Mariner 9 TV Data. Celestial Mechanics, 12, 77-88. 2. Duxbury, T. C. (1974) Phobos:
Control Network Analysis.
Icarus, 23, 290-297.
3. Duxbury, T. C. (1977) Phobos and Deimos Geodesy. In Planetary Satellites, Ed. J. Burns, pp. 346-362. University of Arizona Press, Tucson. 4. Duxbury, T. C. and Veverka, J. (1977) Viking Imaging of Phobos and Deimos: An Overview of the Primary Mission. J. of Geophysical Research, 82, No. 28, 4203-4211. 5. Duxbury, T. C. (1978) Phobos Transit of Mars as Viewed by Viking.
Science, In press.
6. Duxbury, T. C. and Veverka, J. (1978) Deimos Encounter by Viking.
Science, In press.
7. Gougin, J., Veverka, J. and Duxbury, T. (1978) Phobos Illuminated by Marshine. In press.
Icarus,
8. Noland, M. and Veverka, J. (1977) The Photometric Functions of Phobos and Deimos. II. Surface Photometry of Phobos. Icarus, 30, 200-211. 9. Noland, M. and Veverka, J. (1977) The Photometric Functions of Phobos and Deimos. III. Surface Photometry of Deimos. Icarus, 30, 212-223.
Spacecraft Imaging of Phobos and Deimo;
161
i0. Pollack, J. B., et al., (1973) Mariner 9 Television Observations of Phobos and Delmos, 2. J. of Geophysical Research, 78, No. 20, 4313-4326. ii. Pollack, J. B. (1977) Phobos and Deimos. In Planetary Satellites, Ed. J. Burns, pp. 319345. University of Arizona Press, Tucson. 12. Smith, J. C. and Born, G. H. (1976) Secular Acceleration of Phobos and Q of Mars. Icarus, 2!, 51-53. 13. Thomas, P., Veverka, J. and Duxbury, T. (1978) The Origin of the Grooves on Phobos. Nature, In press. 14. Tolson, R. H., et al., (1978) Viking First Encounter of Phobos: Science, 199, No. 64, 61-64. 15. Veverka, J., et al., (1974)
Preliminary Results.
A Mariner 9 Atlas of the Moons of Mars.
Icarus, 23, 206-289.
16. Veverka, J. and Duxbury, T. C. (1977) Viking Observations of Phobos and Deimos: nary Results. J. of Geophysical Research, 82, No. 28, 4213-4223.
Prelimi-
DISCUSSION Question: Answer:
You say regolith means the layer of surface dust on Phobos? Yes, that is right.
Question:
You say the regolith is much thicker on Phobos than on the Moon?
Answer: Yes, that's right. If we make the same interpretation of crater data on Phobos as we do on the Moon, which seems reasonable, we find the regollth thickness on Phobos to be a few hundred meters thick. The regolith is only tens of meters thick on the Moon. Question:
Is that because it is a smaller body than the Moon?
Answer: Yes. The lower gravity supports a much thicker, more loosely packed regollth than for the Moon. Question:
If a man landed on Phobos, would he drown in the dust?
Answer: I think that was a concern even with the Moon. We think the regolith is less dense and much deeper for Phobos than the Moon. If people were concerned about landing on the Moon, they should also be concerned about landing on Phobos. However, with the mass of Phobos so small, one should be more concerned about staying on the surface rather than landing and then bouncing off. The low gravity is such that one might have to use Elmer's glue on the foot pads to hold onto the surface. Question: You talked about the Mariner and the Viking data, can you determine the secular acceleration of Phobos from this data? Answer: Determining the secular acceleration just from spacecraft data is very difficult. Even though the individual position fixes are on the order of I to i0 kilometers, our time line is too small to determine an acceleration. The combination of Earth based data and spacecraft data complement each other very well for determining the secular acceleration by giving high accuracy and a long time base. Question: You seem to be using radar ranging data to determine the distance to the satellite. These values vary irradically, do you pick minimum distance, maximum distance, or what? Answer: Question:
We actually do the radar tracking of the spacecraft, not of Phobos. You really don't know how far you are then?
Answer: We don't get a range to Phobos, that's true. We observe the direction to the moon and by taking a series of pictures, above and below, inside and outside the orbit, we obtain range through the parallax.
Pergamon Press Ltd. 1978.
0083-6656/78/0701-0149 $05.00/0
Printed in Great Britain
SPACECRAFT IMAGING OF PHOBOS AND DEIMOS Thomas C. Duxbury Jet Propulsion Laboratory, CaliforniaInstituteof Technology, Pasadena, California 91103
INTRODUCTION Spacecraft imaging data play a major role in determining physical and dynamical properties of Phobos and Deimos.
These same data also yield information on physical and dynamical pro-
perties of Mars since the orbits of the two satellites and the observing spacecraft are dynamically tied to the Mars gravitational field.
Accurate knowledge of these physical and dynami-
cal properties are needed to determine the origins, evolutions and futures of the two moons of Mars. Physical properties of Phobos and Deimos which can be determined from imaging data include size, shape, topography, volume, cartographic and geologic maps, photometric function, albedo, color, regolith depth and distribution and crater morphology, slze-frequency and distribution. Dynamical properties include rotation, llbration, ephemerides, moments of inertia and mass. Additionally, the Mars gravity field, spin axis direction and internal structure can be determined from the dynamics of the Mars/satellite/spacecraft
system.
Other applications of satellite related imaging for improving the knowledge of Mars involves transits, occultatlons and eclipses.
The satellites and their shadows are imaged
against Mars to give geodetic control points for Mars surface features. eclipses give information on the size, shape and atmosphere of Mars.
Occultations and
Viewing the passing of
a satellite shadow over a lander simultaneously from the lander and an orbiting spacecraft can be used to locate the lander on Mars.
Also, viewing a satellite which is partially illu-
minated by Marshine can give information on the surface on Mars. Normally, Mariner and Viking spacecraft fly by Phobos and Deimos at distances greater than 4000 km.
Occasionally, flybys as close as a few thousand kilometer occur.
On Viking, the
orbits of the two spacecraft were changed to allow flybys within i00 km of the surfaces.
Fly-
bys at distances greater than 4000 km yielded lower resolution pictures where features larger than 200 meters were detected.
For the closer flybys, higher resolution pictures were obtained
where features as small as i0 to 200 meters were detected. The lower resolution pictures are well suited for determining the sizes, shapes, photometric functions, rotational properties, libration amplitudes, libration periods and ephemerides of the Martian satellites as well as the gravity field, spin axis direction and internal structure of Mars.
Higher resolution pictures are additionally valuable for determining topography,
geologic and cartographic maps, albedo, color, regollth depth and distribution and crater morphology, size-frequency and distribution.
The higher resolution pictures are also impor-
tant for detecting small scale features such as surface fractures, crater chains, wall slumping of craters, base-surge deposits, pool melting, and boulders either swept up by the orbiting satellites or ejected from craters. 149
150
T.C. Euxbury
LOWER RESOLUTION
IMAGING
The pictures in Figure i, showing Phobos from different lower resolution pictures used to determine
perspectlves,
simple elllpsoided model is used to define the mean surfaces. of the ellipsoid,
a best match is obtained
actual surface topography variations tion pictures having resolutions sizes and shapes
are typical of the
the sizes and shapes of the two Martian Moons. By adjusting
to the satellite outlines
in all pictures.
of up to 3 km from the elllpsoidal model,
of a few hundred meters are sufficient
With
lower resolu-
to determine
the
(mean surface or "sea level") of Phobos and Deimos.
The pictures in Figure 2, showing Phobos at a wide range of phase angles, the lower resolution pictures used to determine The integrated disk brightness properties
A
the three axes
Additionally,
the scattering properties
dence and emission angles. scattering properties
are typical of
functions of the satellites.
as a function of phase angle is used to determine
of the surface material.
are used to evaluate
the photometric
isophotes
in individual
scattering
pictures
Analysis of these lower resolution
or Lambertian
Law.
3)
pictures have showed that the
of both Phobos and Delmos are best modeled by the Hapke-Irvine
rather than a Minnaert
(Fig.
at a smaller scale and at a variety of inci-
These scatter properties
Law
suggest that both Phobos
and Deimos are covered with regollths. Phobos and Deimos are both in synchronous Mars as our Moon faces Earth. longest axes pointing intermediate
oblate satellites a significant
stable orientations
with their
toward Mars, their shortest axes normal to their orbit planes and their
axes in their orbit planes.
nous configuration
rotation about Mars with one side always facing
They are both in dynamically
Librations
(small oscillations)
are expected because of orbit eccentricities, and possible recent impacts.
about this synchro-
tidal forces acting on the
Orbit eccentricities
and tidal forces cause
libration about the shortest axis of Phobos with an amplitude
degrees and a period the same as the orbital period of Phobos.
Libratlons
of about five about the other
two axes of Phobos as well as all three axes of Deimos which would be attributed eccentricities
and tidal forces are less than a few tenths of a degree.
tlon for Phobos was observed by Mariner 9 (Fig. 4).
More detailed
able when the reduction of the Viking data is complete.
to orbital
The five deg libra-
information will be avail-
No librations
due to recent impacts
have been observed. Accurate ephemerides imaging data.
of Phobos and Delmos are vital for interpreting
Additionally,
obtain satellite
images.
accurate ephemerides
Ephemerldes
ner 9 and a majority of Viking. cies of 3 to 5 km.
Ephemeris
accurate
However,
improvements
pictures.
data since observations
the TV cameras to for all of Mariephemeris
to these levels were made by observing throughout
First, position measurements
even from the lower resolution spacecraft
are needed for pointing
to i0 - 20 km were sufficient
the close flybys on Viking required
tions to Phobos and Deimos from the spacecract craft data is twofold.
a majority of the
Secondly,
their orbits.
accura-
the direc-
The power of space-
to a few hundred meters are possible, significant
parallax is obtained
from
can be made from above and below a satellite orbit as well
as inside or outside of its orbit. Spacecraft-centered
direction m e a s u r e m e n t s
positions
of the satellites
taneously
image a satellite and stars in one picture
can he made of a satellite second camera
(Fig. 5.B).
detail on the overexposed technique yields excellent
against
of Phobos and Delmos are made by observing
star backgrounds.
Long exposures (Fig. 5.A).
the
can be used to simul-
On Viking,
a normal exposure
in one camera and a long exposure for stars can be made in a The simultaneous satellite;
satellite/star
however,
picture does not give any surface
only one picture is required.
surface detail but requires
two pictures.
is normally used on Viking unless other mission constraints
The two camera
The two camera approach
do not allow the extra pictures.
Spacecraft Imaging of Phobos and Deimos
Figure I.
Areas on Phobos are viewed from many directions to determine the size and shape of the Martian moon (Viking pictures IIOA01, 081B33 and I01A77).
Figure 2.
Phobos is shown at phase angles ranging from 35 to 123 deg (Viking pictures ILIA03, 055A32 and 077B03).
151
152
T.C.
Figure 3.
Duxbury
A raw and enhanced picture of Phobos is shown which are used to determine the scattering properties of surface particles (Viking picture 390A16).
Figure 4.
The predicted positions of surface features (ends of arrows) were used to identify an in-plane rotational amplitude of about 5 deg (Mariner 9 picture DAS04790460).
Spacecraft Imaging of Phobos and Deimos
Figure 5.
Figure 6.
153
A. Phobos is viewed simultaneously with two stars of 7.4 and 7.7 visual magnitude (Viking picture 235A01). B. Delmos is viewed by one Viking camera while stars are viewed the other camera (picture 428B21 and 428B22).
The south polar region of Phobos is viewed at different perspectives to determine the locations and elevations of surface features such as A thru I (Viking pictures 252A61, 62 and 63 and 315AII, 12 and 14).
154
T.C.
Duxbury
Figure 7. A. High resolution picture of Deimos showing flat areas and bright albedo patches (Viking pictures 428B36 and 38). B. High resolution picture of Phobos showing grooves, and large flat-bottomed craters with dark material (Viking picture 246A07).
Spacecraft Imaging of Phobos and Deimos
Figure 8.
A high resolution picture of Delmos which covers an area of only ~2x2 km. The surface is heavily cratered and covered by regolith and boulders (Viking picture 423B63).
155
156
T.C.
Figure 9.
Duxbury
Phobos was in transit over the Tharsis region of Mars. The complete outline of Phobos is seen (Viking pictures 304B74304B78).
Spacecraft Imaging of Phobos and Deimos
Figure i0.
The shadow of Phobos was observed in ~ e Tharsis region of Mars together with three large volcanoes and chaotic terrain (Viking pictures 496Agl-496A96).
157
158
T . C . Duxbury
Figure ii.
The shadow of Phobos is seen in the Chryse Basin region of Mars and is directly over Viking Lander-I (Viking picture 463A21).
Figure 12.
The complete outline of Phobos is visible from direct sunlight and sunlight reflected by Mars (Viking picture ILIA01).
PENU
SUN Figure 13.
A long exposure was used to detect Fhobos in the penumbra of Mars (Viking picture I18A71).
Spacecraft Imaging of Phobos and Deimos
159
Improvements to the ephemerides of Phobos and Deimos involve the spacecraft orbit and Mars gravity field and spin axis direction which are dynamically coupled through the equations of motion.
Analysis of the orbital evolution of Phobos, requiring both earth-based and
spacecraft-based data, also yields information on the interior of Mars by determining the rate of decay of the orbit of Phobos due to tidal forces.
With the accuracy of the space-
craft-based data limited to the ability to find the center of the highly irregularly shaped satellites, higher resolution pictures currently offer no advantage over lower resolution pictures for ephemeris improvement.
Higher resolution picture would be advantageous if land-
marks were used instead of the satellite center. HIGHER RESOLUTION PICTURES The two Viking Orbiter spacecraft (VO-I and VO-2) were successfully targeted to give close encounters of both Phobos and Deimos. these encounters.
Table i lists the dates and closest flyby distances for
High resolution imaging obtained from these flybys were spectacular.
It
was fitting that the greatest increase in knowledge of the two Martian moons occurred in 1977, i00 years after their discovery. Table i. Satellite
Phobos
Viking Close Encounters Phobos
Deimos
Deimos
Deimos
Spacecraft
VO-I
VO-I
VO-I
VO-2
VO-2
Date
Feb 1977
May 1977
Oct 1977
Oct 1977
May 1978
Minimum Attitude, km
88
290
1500
28*
?
*Closest flyby of any body in our solar system. Imaging data obtained from these closer encounters have higher surface resolution and are ideal for geologic and cartographic analyses.
Overlapping and stereo surface coverage at
these higher resolutions are extremely valuable for determining elevations of features relative to the mean elllpsoidal surface. at ranges less than 600 km.
Figure 6 shows the southern hemisphere of Phobos viewed
Common features in both pictures are labeled A through I.
The
parallax obtained from these pictures not only yields elevation information but also yields the body-fixed coordinates of features needed to produce cartographic maps.
An elevation
difference of over 1.5 km exists between points A and B. Small scale albedo differences are detected from the higher resolution images. ure 7.A the northern hemisphere of Deimos is viewed at a range of i000 km.
In Fig-
Bright albedo
patches are seen near the left llmb around what appears to be the intersecting of two large, flat areas. size.
Albedo differences could indicate a different material or different particle
For Phobos and Deimos whose shapes are highly irregular, determining whether a varia-
tion in surface brightness is due to an albedo or topographic variation can be difficult. Higher resolution pictures can be used to determine regollth thickness.
The height of
the surface above the floors of large, flat-bottomed craters has been used to determine regolith thickness on the Moon.
Thicknesses of over a few hundred meters are inferred from high
resolution pictures containing flat-bottomed craters on Phobos (Fig. 7.B). crater
Analysis of these
depths over the complete surface would yield regolith distribution.
Pictures such as Figure 7 can be used to analyze crater morphology, size-frequency and distribution.
Additionally,
small scale features such as surface fractures, crater chains,
wall slumping, base-surge deposits, pool melting and boulders can be studied.
The picture
in Figure 8, one of the highest resolution pictures ever taken of any body in our solar system, is an additional example of high resolution imaging showing features as small as a few meters across.
160
T.C.
Duxbury
MARS APPLICATIONS Viking provided unique opportunities to view Phobos, Deimos and their shadows against Mars.
These observations are valuable for determining the sizes and shapes of the satellites
Additionally,
the satellite or shadow can be used as a geodetic control point for mapping the
positions of the surrounding features on Mars.
Examples of these types of pictures are shown
in Figures 9 and i0. The shadow of Phobos was viewed passing over the Viking Lander-i (VL-I) simultaneously from the lander and an orbiter.
Using the time of shadow passage observed by the lander, the
position of the shadow (and, therefore, the lander) was located in the orbiter pictures (Fig. Ii).
Passage of the Phobos shadow over VL-I was successfully observed on three separ-
ate occasions in late September of 1977. Other unique satellite observations obtained from Viking involved eclipses and Marsshine. Phobos was viewed while being partially illuminated by sunlight reflected from the surface of Mars (Fig. 12).
Also, Phobos was viewed in the penumbra of Mars (Fig. 13) to determine
scattering properties of the Martian atmosphere. SUMMARY Imaging data obtained from spacecraft orbiting Mars has significantly increased our knowledge of the two Martian moons during the last few years. mation can be found in the references.
A major portion of this infor-
The two asteroid sized bodies may indeed have origi-
nated in the asteroid belt and may be comprised of primordial material from the solar nebula. Sufficient knowledge now exists on the two moons of Mars to seriously consider Phobos and Deimos as candidate targets for future sample/return mission or for locating bases to monitor Mars. ACKNOWLEDGEMENT I thank Drs. D. Elliot and L. Shigg of the JPL Image Processing Laboratory for enhancing the Viking television pictures.
This paper is JPL Planetology Publication Number 314-78-14
and presents the results of one phase of research carried out under Contract NAS 7-100, sponsored by the Viking Program Office and Planetary Geology Program Office, Office of Space Science, National Aeronautics and Space Administration. REFERENCES i. Born, G. H. and Duxbury, T. C. (1975) The Motions of Phobos and Deimos from Mariner 9 TV Data. Celestial Mechanics, 12, 77-88. 2. Duxbury, T. C. (1974) Phobos:
Control Network Analysis.
Icarus, 23, 290-297.
3. Duxbury, T. C. (1977) Phobos and Deimos Geodesy. In Planetary Satellites, Ed. J. Burns, pp. 346-362. University of Arizona Press, Tucson. 4. Duxbury, T. C. and Veverka, J. (1977) Viking Imaging of Phobos and Deimos: An Overview of the Primary Mission. J. of Geophysical Research, 82, No. 28, 4203-4211. 5. Duxbury, T. C. (1978) Phobos Transit of Mars as Viewed by Viking.
Science, In press.
6. Duxbury, T. C. and Veverka, J. (1978) Deimos Encounter by Viking.
Science, In press.
7. Gougin, J., Veverka, J. and Duxbury, T. (1978) Phobos Illuminated by Marshine. In press.
Icarus,
8. Noland, M. and Veverka, J. (1977) The Photometric Functions of Phobos and Deimos. II. Surface Photometry of Phobos. Icarus, 30, 200-211. 9. Noland, M. and Veverka, J. (1977) The Photometric Functions of Phobos and Deimos. III. Surface Photometry of Deimos. Icarus, 30, 212-223.
Spacecraft Imaging of Phobos and Deimo;
161
i0. Pollack, J. B., et al., (1973) Mariner 9 Television Observations of Phobos and Delmos, 2. J. of Geophysical Research, 78, No. 20, 4313-4326. ii. Pollack, J. B. (1977) Phobos and Deimos. In Planetary Satellites, Ed. J. Burns, pp. 319345. University of Arizona Press, Tucson. 12. Smith, J. C. and Born, G. H. (1976) Secular Acceleration of Phobos and Q of Mars. Icarus, 2!, 51-53. 13. Thomas, P., Veverka, J. and Duxbury, T. (1978) The Origin of the Grooves on Phobos. Nature, In press. 14. Tolson, R. H., et al., (1978) Viking First Encounter of Phobos: Science, 199, No. 64, 61-64. 15. Veverka, J., et al., (1974)
Preliminary Results.
A Mariner 9 Atlas of the Moons of Mars.
Icarus, 23, 206-289.
16. Veverka, J. and Duxbury, T. C. (1977) Viking Observations of Phobos and Deimos: nary Results. J. of Geophysical Research, 82, No. 28, 4213-4223.
Prelimi-
DISCUSSION Question: Answer:
You say regolith means the layer of surface dust on Phobos? Yes, that is right.
Question:
You say the regolith is much thicker on Phobos than on the Moon?
Answer: Yes, that's right. If we make the same interpretation of crater data on Phobos as we do on the Moon, which seems reasonable, we find the regollth thickness on Phobos to be a few hundred meters thick. The regolith is only tens of meters thick on the Moon. Question:
Is that because it is a smaller body than the Moon?
Answer: Yes. The lower gravity supports a much thicker, more loosely packed regollth than for the Moon. Question:
If a man landed on Phobos, would he drown in the dust?
Answer: I think that was a concern even with the Moon. We think the regolith is less dense and much deeper for Phobos than the Moon. If people were concerned about landing on the Moon, they should also be concerned about landing on Phobos. However, with the mass of Phobos so small, one should be more concerned about staying on the surface rather than landing and then bouncing off. The low gravity is such that one might have to use Elmer's glue on the foot pads to hold onto the surface. Question: You talked about the Mariner and the Viking data, can you determine the secular acceleration of Phobos from this data? Answer: Determining the secular acceleration just from spacecraft data is very difficult. Even though the individual position fixes are on the order of I to i0 kilometers, our time line is too small to determine an acceleration. The combination of Earth based data and spacecraft data complement each other very well for determining the secular acceleration by giving high accuracy and a long time base. Question: You seem to be using radar ranging data to determine the distance to the satellite. These values vary irradically, do you pick minimum distance, maximum distance, or what? Answer: Question:
We actually do the radar tracking of the spacecraft, not of Phobos. You really don't know how far you are then?
Answer: We don't get a range to Phobos, that's true. We observe the direction to the moon and by taking a series of pictures, above and below, inside and outside the orbit, we obtain range through the parallax.