History of planetary science. The Pic du Midi Planetary Observation Project : 1941–1971 (original) (raw)

Introduction

Planetary observations were pioneered at Pic du Midi when, in 1941, B. Lyot realized that the site offered unusually good conditions for powerful magnification. The planets could be observed at the telescope with high angular resolution.

With a provisional and modest 30 cm diameter refractor, Bernard Lyot, with Henri Camichel and Marcel Gentili, recorded photographs of Mars, Mercury and the Moon which were among the best produced in the world at the time.

The high altitude of the site (2877 m), and its location at the exact top of a mountain which is snow covered and isolated ahead of the range, were understood to be reasons for this success.

Bernard Lyot carefully analyzed the physical processes involved in high resolution telescopic imaging. Thermal and dynamical effects of the dome, heat exchanges and convections in the telescope, were rather unexplored problems at the time. With this experience, Bernard Lyot convinced Jules Baillaud, the Pic du Midi director, to instal a larger refractor specifically designed to produce high angular resolution.

A 60 cm objective was available at Observatoire de Paris. Its focal length of 18.24 m was very long. The dome had an internal diameter of 8 m but the beam was folded by using two flat mirrors, polished specially by André Couder. Controlled mechanical stresses on one of these mirrors was used to carefully optimize, in real time, the shape of the wavefront and the figure of the diffraction pattern before an observation.

The new instrument came into operation in 1943. It was a total success. Very often, the planetary surfaces were seen at the diffraction limited resolution of 0.2 arcsec, which requires an unusual magnification approaching ×1000. This unprecedented capability offered a completely new field of research.

At the time, there was not yet a deep concern for planetary investigations. Planets were still the stepchildren of astronomy. The Pic du Midi initiative to develop knowledge about planetary bodies with high magnification optical observations received a warm welcome from prominent scientists at the time. In particular, Gerard Kuiper, from Yerkes observatory and University of Arizona, paid great attention to these developments. The International Astronomical Union was instrumental in fostering the project at a high level.

From 1945 to 1965, the Pic du Midi planetary analysis program was essentially conducted by Henri Camichel, Jean Focas and Audouin Dollfus, with students and workers from the Observatoire de Paris at Meudon.

In 1962, a new reflecting telescope, 107 cm in diameter, was installed by Jean Rösch, the Pic du Midi director. It offered a still better resolution, more light and a larger spectral range, which allowed UV work.

For the survey work, the observation techniques were essentially photography, visual inspection, micrometry and polarimetric sensing. A specific characteristic was the simultaneous use of all these four approaches in sequence, or the choice of those techniques adapted for the observing circumstances and the seeing conditions.

The instruments were constructed light weight and moderate in size. A single observer was able to exchange them at the focus in a few minutes, allowing several changes during a night.

Typically, with Mars for example, to benefit from the slow rotation of the planet around its polar axis and explore optically the surface in longitude, we usually started a run by a sequence of photographic images, then removed the camera and used the eyepiece for visual inspection, then the polarimeter for polarization cartography. The complete sequence took less than one and half hours. We next started a similar run, and so on, as long as the observing conditions remained favorable.

When we had two observers at work, the plates for a run were immediately processed in the dark room, in order to be inspected before the next sequence. Around the date of exact opposition, micrometric measurements had a priority.

Another characteristic of the project was its long-term basis. Each of the four approaches was exploited until its capacity for producing new knowledge was exhausted. For each of them, this took more than twenty years. But these limits happened to be reached nearly at the time when planetary exploration with spacecraft began to revitalize the field.

In addition to this basic survey-type activity, there was specific research work. Additional instruments in use included the long boom sun-screen for day-time observation, the fringe photometer for albedo mapping on planetary surfaces, the spectrograph for reflectance analysis, the disk meter for matching and measuring the satellites diameters, and the focal coronographic instrument for Saturn rings and planetary satellites.

Visual inspection, by its sharpness, provided an invaluable potential for planetary observations. Pioneering views of their surfaces were produced. At the time, rare were the glimpses of planetary features observable with a high magnification, due to the scarcity of proper seeing conditions. At the Pic, high resolution was frequent enough to enable effective and coordinated research, with a high potential for discoveries.

The smallest details which appear visually on planetary disks were sharper than those detected by photography ; the eye is exploring the image with an automated processing which does not exist with photography.

Only 45 years later, after the advent of CCD detectors and image processing, can we now do with the help of computers what the eye was doing directly, and produce on CCD images the faint details that visual inspection showed directly during the pioneering years.

The photographic survey was conducted from 1941–1975, essentially by Henri Camichel. All the plates were scaled, oriented and calibrated for photometry. They were systematically processed by the composite method, a technique which was recently adapted for CCD images and renamed shift and add. Analysis, measurements and archiving were carried out at Toulouse and at Meudon observatories, where students and scientists conducted research works with the documents.

Micrometric measurements : In 1940s, planetary globe diameters were not known with an accuracy better than 1%. Our goal, due to the sharpness of the telescopic images available at the Pic, was to push the accuracy near 0.1%, in order to make meaningful planetary densities and investigate internal structures.

The principle was to duplicate the planetary image by birefringence techniques and to adjust the contact between the opposite limbs, in order to cancel the fuzziness of the edges.

With two types of birefringent micrometers, the measurement required almost twenty years, until the desired accuracy was reached for all objects, essentially between 1950 and 1970.

Polarimetry was a Pic du Midi speciality. It caused great excitement, in 1948, when the degree of polarization of the light was analyzed with an accuracy of 10−3, on areas of planetary surfaces as small as 2 arcsec in diameter, using a visual fringe polarimeter with the 60 cm refractor.

The potential was large, the field completely virgin. The method was exploited for all planetary disks.

The phase angle dependence of the polarization held most of the information ; to construct such curves requires a large number of observations, because the phase angle is slowly varying from night to night. The interpretation required theoretical developments and extensive laboratory work, which were conducted at Meudon observatory, with several scientists and students.

The polarimetric observations changed nature when, around 1960, photoelectric polarimeters were made available, together with the two Pic du Midi and Meudon one meter size new reflecting telescopes, allowing IR and UV work.

After 1970, planetary investigation with spacecraft developed intensively. A large community of scientists organized itself through the world to work in the field. Planetary science acquired the stature of a new discipline. At the time, the Pic du Midi program had almost reached its initial objectives.

Polarimetric sensing, however, still held potentialities. It experienced new developments, with the availability of the lunar samples, with a space mission launched to Mars with polarimeters, and with the new imaging-polarimetry technique producing full images of the polarization parameters at the surfaces of planetary bodies. These works, having been developed essentially at Meudon observatory, will not be included in the present review.

Altogether, the planetary science program conducted at the Pic du Midi and Meudon observatories, since 1941 and over thirty years, produced comprehensive sets of findings and knowledge. They were instrumental for the planning of the first spacecraft missions to planets and the choice of their goals. They gave training to the new generation of scientists which entered into the field of planetary science.

The knowledge which resulted from the Pic du Midi program is expressed in 9 doctorate theses and more than one hundred publications.

In what follows, the major contributions by the scientists which were involved in the Pic du Midi planetary project are summarized. The works on the Moon, lunar samples, asteroids and instrumentation are not included.

For the references, only the publications by the direct participants are given. These references are listed by topics. Works by other scientists but relevant to the Pic du Midi results are discussed in the paper. Their references can be found in the papers cited.

Review papers about other aspects of the Pic du Midi planetary work has already been solicitated in the past. They are also listed in the attached bibliography.

Section snippets

Mercury

When, in 1942, Bernard Lyot and Henri Camichel analyzed the planet Mercury for the first time at Pic du Midi, little reliable knowledge was available about the planet.

The new world which was offered to the telescopic analysis had a size estimated to be between Mars and the Moon. A solid surface was assumed, producing phases and crescents. The rotation period was wrong, the assumption of an atmosphere with clouds, erroneous.

In 1974, planet Mercury entered into the field of spacecraft

Venus

In the 1940s, speculations were numerous about the physical nature of Venus. Surfaces covered with oceans of oil were argued, as were lands of chalk. Some astronomers surmized a dense atmosphere, but nobody predicted a surface pressure as high as it is.

The spacecraft exploration period flourished essentially after 1970, when VENERA-4 and Mariner 5 produced their results.

Meanwhile, the Pic du Midi workers had analyzed the clouds of the upper atmosphere. They announced the 4 day global retrograde

Mars

Planet Mars was the prime goal, when the Pic du Midi planetary observation program was initiated in 1941. The body was almost unknown but puzzling. The canal controversy was still hot. The presence of life was vividly debated and often optimistically postulated.

This world was known to exhibit a surface with variegations and these features have been noted to change with time. An atmosphere was postulated because occasional and transient clouds have been observed. Our goals were to decipher the

Jupiter satellites

When, in 1941, the Pic du Midi planetary observation project was initiated, only occasionally features of the Galilean satellite surfaces had been glimpsed. The sizes of these bodies were scarcely known, their albedo uncertain. There was no real science attached to their existence, other than celestial mechanics.

Saturn system

Between 1941 and 1980, knowledge about the Saturn system was enriched by many findings and discoveries, at Pic du Midi and Meudon observatories.

The rings were seen striated with gaps, subrings, ringlets. A value was given for the rings disk thickness.

Ring A disclosed enigmatic azimuthal brightness variations. Polarimetry indicated vigorous dynamical effects within the rings, producing particle associations.

Rare events are the bright clouds in the Saturn atmosphere. Observed twice, they produced

Saturns globe

Atmospheric bright disturbances are exceptional events in the Saturnian atmosphere. They were noted only six times during the past 150 years, following their first observation by William Herschel in 1793.

At Pic du Midi, Camichel (1956) observed a white spot in 1946 at latitude −12.3° with a rotation period of 10 h 21 m 04s. Dollfus, 1963 found a spot in 1960 at +57° with 10 h 39 m 54 s. From these observations, an extremely rapid eastward current around the equator was evidenced, three times

Structures of rings

In the past, gaps in Saturns rings had been suspected, in addition to the classical Cassini and Encke divisions ; but there was no clear description of these features. With the high resolution 60 cm Pic du Midi refractor, the gaps were shown as obvious features. In 1945, Lyot (1953) produced a detailed map (Fig. 33).

A photometric profile of the rings was derived (Fig. 34), in which the positions of the gaps and ringlets are indicated, with a nomenclature. Contrasts and profiles of these

Ring photometry

From 1941 and the following 15 years, Camichel, 1958 collected photographs of Saturn, calibrated for photometry. Almost the full ranges of the incidence, emergence and phase angles reachable from Earth were covered. In 1966, the rings were observed at low incidences and edge-on (Focas and Dollfus, 1969). Photographic photometry gave the luminance variation with angle B′, for both rings (Fig. 36).

Ring A exhibited azimuthal brightness variations, with maxima at NW and SE and minima at NE and SW (

Ring polarimetry

Fig. 38 shows an example of linear polarization distribution over the Saturn image, as viewed with the Meudon imaging polarimeter (Dollfus, 1979a,Dollfus, 1979b, Dollfus, 1984a,Dollfus, 1984b, Dollfus, 1996). The degree of polarization and its azimuth vary with the position on the ring, with phase angle and with wavelength.

Ring B was found to show two effects : (1) a polarization from direct solar illumination which has the phase angle dependence shown on Fig. 39, (2) a component with a

Ring thickness

Saturns rings were observed exactly edge-on, on December 18, 1966, at 07±02 h UT. With our focal mask design, a faint lineament remained visible. Assuming the ring to be plane parallel, flux measurements gave an estimate of the up to then unknown disk thickness of 2.4±1.3 km (Focas and Dollfus, 1969 ; Dollfus, 1979a,Dollfus, 1979b,Dollfus, 1979c). The rings may still be thinner if particles remain outside the exact plane, or in presence of warping.

For the subsequent edge-on presentations, which

The outer-ring E

In the past, visual observers questioned the presence of a tenuous ring extending outward from those already known. When the ring is seen near grazing, a lineament has been searched for. During the edge-on presentation of 1966, William Feibelman took at Allegany Observatory a long exposure photographic image in which such a lineament was suspected ; but its presence remained controversial.

On November 1st, 1979, the outer ring appeared as a clear feature on Pic du Midi plates exposed with a

The tenth satellite Janus

When attempting to explain the micrometric determinations of the gap positions on the rings by resonant perturbations from satellites, the presence of an unobserved satellite, very close to the outer edge of ring A, was postulated.

The edge-on presentation of the rings in 1966 offered an opportunity to search for the new object. A focal instrument was designed, based upon the principle of the solar coronograph.

On 15 December, 1966, at Pic du Midi, the satellite appeared on several plates (Fig. 42