Radio Science and Venus

Radio Emissions from Venus

During the planning of Venera-1, M.V. Keldysh asked radio astronomers at the Pushchino Observatory to focus on determining the surface temperature of Venus. In 1956, American astronomers C. Mayer et al. from the Naval Research Laboratory, had measured 3.15 cm microwave emissions that suggested a surprisingly high 600° K source temperature. A.D. Kuz'min and A.E. Salomonovich used a precisely shaped 22-meter telescope, able to focus millimeter-band radiation. They observed at wavelengths of 0.4 to 10.3 centimeters, contributing to an international effort spanning 0.1 to 70 cm by the mid 1960s. The resulting spectrum indicated 600° K at centimeter-band wavelengths, but only about 300° K in the millimeter band (close to Urey's prediction).

Two explanations were proposed. The planet's surface could be extremely hot, with a cold atmosphere absorbing some wavelengths. Or the planet could have a cooler surface and a hot atmosphere that was emitting the excess centimeter-band radiation. Carl Sagan championed the hot-surface model, suggesting in his 1960 doctoral dissertation that heat was trapped by the greenhouse effect. However, the amount of millimeter-band absorption by the atmosphere seemed problematic at that time, requiring a surface pressure of 20 - 100 atmospheres. JPL scientist D.E. Jones suggested a cold-surface model in which the centimeter-band microwaves came from the ionosphere, energized by solar proton flux. Both the greenhouse and ionospheric models could fit the spectral data.

Based on the scientific uncertainty of the early 1960s, the first Venera probes hedged their bets, with descent capsules capable of floating and heat-resistant parachutes.

During close orbital approach in October and November 1962, D.V. Korol'kov, Iu.N. Pariiskii and S.E. Khaikin performed the first such test, They observing limb darkening at 3.02 cm wavelength, with the 130-meter knife-edge-resolution telescope at Pulkovo Observatory, and advocated a hot-surface model. Seen above, the telescope is the long fence-like antenna (not the small dish in the foreground), a precursor to Korol'lov's massive 600-meter telescope (RATAN-600). A month later, the Mariner-2 spacecraft detected limb darkening at 1.9 cm, during its historic planetary flyby. However, Caltech's interferometry experiment at 10 cm detected limb brightening.

A more conclusive experiment came in 1964, in an American/Soviet collaboration. B.G. Clark and A.D. Kuz'min, using interferometry with two 26-meter dishes at Owens Valley Observatory, demonstrated that emissions from the edges of Venus were partially polarized. This could not result from an isotropic gaseous source. It was strong evidence that the radiation was emitted from the edge of a hard surface. The first measurement of the radius of Venus was calculated in their experiment, giving an value of 6057 ± 55 km. Later more accurate radar measurements of radius were made, in the USSR and USA.

After Venera-4 analyzed the chemistry of the atmosphere, Kuz'min recalculated the temperature estimates from radio emission, correcting the 600° K to 700±100° K. Later direct measurements by Venera landers found a surface temperature of about 735° K. Venera-4's discovery, that the atmosphere was almost pure carbon dioxide at high pressure, finally explained the unusual radio spectrum of Venus.

Earth-Based Active Radar

The team at Evpatoriia included V.A. Kotel'nikov and O.N. Rzhiga. Radar experiments used 39 cm, the same wavelength used to transmit telecommands to spacecraft. The transmitting antenna initially output 10 kilowatts continuous power, upgraded to 40 and then 80 kW before the Venera-2 mission in 1965. In pulsed mode, a focused beam was produced with a peak radiant intensity of 250 megawatts per steradian. The total power reaching the surface of Venus was 15 watts.

In 1961, the receiver was an uncooled parametric amplifier, and the results were hampled by noise. By 1962, the sensitivity was increased six times, by using a liquid helium cooled ruby MASER. Cryogenic cooling reduces the thermal noise of the pre-amplifier. The radar receiver was installed in the Pluton transmitter antenna; but after 1964, the radar system was operated in a bistatic mode, using one of the more sensitive Pluton receivers a few kilometers from the transmitter.

The results from these studies indicated an unusually slow rotation rate of 200 to 400 days, but the retrograde direction of the rotation was not detected. The Soviet experiment detected two Doppler spreads, one equivalent to an 11 day rotation and the other equivalent to > 100 days. During the 1962 conjunction, Goldstone and Evpatoriia results narrowed the rotation to 250±40 days, and the retrograde direction of the rotation was detected. By 1964, the 305-meter radio telescope in Arecibo was operational. The best results that year were 230±25 days (Evpatoriia), 249±6 days (Goldstone), and 247±5 days (Arecibo).

The modern estimate of Venus' rotational period is 243.0187 days. It is close to lock step with its synodic cycle, so Venus almost shows the same face toward the Earth at each inferior conjunction. Early radar studies also refined our knowledge of the planets' orbits and the distance of the Earth from the Sun. In 1961, the position of Venus was only known to within a hundred thousand kilometers. Planetary radar and spacecraft ranging measurements vastly improved this ephemeris data.

Images of Venus were produced by a technique called range-doppler radar. Since the planet is a sphere, the reflected signal first returns from the center of its face. By separating the echo by delay, the reflections from different concentric rings can be distinguished. Venus is also rotating, which means the right half of its face is moving toward us and the left half is moving away. By separating the echo by doppler-effect frequency shift, the reflections from different vertical bands can be distinguished. North-South ambiguity is resolved by spatial separation, using a large antenna or interferometry between two antennas.

Range-Doppler radar was limited to seeing only about 1/3 of the surface of Venus, on the side that faces Earth during inferior conjunction. Image quality was poor at the equator, because the lines of constant delay and constant Doppler became parallel and because north-south ambiguity was difficult to resolve.

Spacecraft Doppler Tracking

Highly accurate spacecraft position and velocity were calculated from closed-loop Doppler ranging, retransmission of a frequency standard generated on Earth. The landing capsule had no receiver and therefore depended on its own stable frequency source. Its signal was generated by a crystal oscillator, thermally stabilized in a Dewar flask (OXCO). This was calibrated during flight by comparison against the closed-loop Doppler.

Doppler shift measures the component of descent capsule velocity in the direction of the Earth-based receiver. Aerodynamic drag calculations and the temperature/pressure measurements provided an estimate of a second velocity component, perpendicular to the surface of Venus. As the Soviet model of the Venusian atmosphere improved, those measurements became more accurate. Horizontal velocity was derived from the difference of these two components.

The retrograde super rotation of Venus' atmosphere was a significant result of this work. Since the early Venera capsules landed almost in the center of the planets disk, horizontal wind speed was not very accurate. Kerzhanovich stuggled to convince mission planners to drop the descent vehicles a little off center, to get better wind-velocity measurements. The best reading was obtained from Venera-8, which gave a complete wind profile, from the super-rotating cloud layer down to the calm surface.

Venera-8 Altimeter Experiment

Terrain Elevation from Venera-8 Altimeter

Venera-8 carried a pulse-modulated radar altimeter, which returned 35 readings during its descent. The capsule's trajectory was estimated by Doppler radio readings and aerodynamic calculations, and by subtracting this from the absolute radar altitude readings, a ground profile could be measured. Readings span a vertical range of 45.5 km down to 0.9 km, during which time the capsule drifted horizontally for a distance of about 60 km. Analysis of the return pulses yielded estimates of the local ground density.

From high-resolution radar maps, Venera-8 is known to have landed in Vasilia Regio. Russian geologists Abdrakhimov and Bazilevskii believe it probably landed in a region of small volcanoes formed from viscous lava. This is consistent with Venera-8's gamma-ray spectrometer measurements of rock which may be less like basalt and more like granite.

Venera-9,10 Bistatic Radar Mapping

Local Slopes from Venera-9 Bistatic Radar

A.G. Pavel'ev, who originally proposed the bistatic radar experiment, devised improved analysis algorithms for the data in the early 1980s. On five regions from Venera-10's data, he and his team constructed 2-dimensional information about local topography at a resolution of 5-20 km.

America's Pioneer-12

Using an S-band pulse-modulation altimeter with a 38 cm antenna, Pioneer-12 returned over 127,000 altimetry measurements at a nominal spacing of 150 km. Earth-based radar was much higher resolution at that time (10 km), but it failed to give information about altitude and topographical structure. The Pioneer radar, designed by P.H. Pettengill's team at MIT, also recorded radio thermal emission and surface roughness.

Pioneer-12 Radar Altimeter Map

The Pioneer data was the first complete map of Venus that illustrated its overall structure. Seen above are stereographic and mercator projections of the data, hill shaded and hypsometrically colored. Pioneer revealed the continent-sized polar highland of Ishtar Terra with the giant tectonic mountain, Maxwell Montes. It also showed the equatorial continent of Aphrodite Terra. Both were more impressive than Alpha Regio and Beta Regio, seen on the side of Venus that faced the Earth during conjunction. Phoebe Regio was where most of the Soviet Venera probes landed.

To some, the topography suggested that Venus was a one-plate planet, unlike Earth with its many drifting and subducting plates. This question plate tectonics remained unanswered until higher resolution images could be obtained.

Pioneer-12's Side-Looking-Radar Data

During the closest approaches of the highly eccentric orbit, the radar altimeter could be angled to operate in a side-looking radar mode. Range-Doppler analysis measured 64 positions from each reflection of the beam. This filled in a narrow equatorial strip (where range-doppler methods are blind) with a resolution of about 30 km.

Venera-15 and Venera-16

Venus was found to be surprisingly different from Earth, largely covered with lava fields and shield volcanoes, networked with linear ridges and circular coronae, with patches of a more ancient wrinkled terrain called tessera.

Two identical spacecrafts were designed to orbit Venus and map the northern 25 percent of the planet at a resolution of 1 to 2 km. Venera-15 was launched on June 2, 1983 and arrive on October 10. Venera-16 was launched on June 7 and arrived October 11.

• Polyus-V Synthetic Aperture Radar
• Omega Radar Altimeter
• Infrared Fourier Spectrometer
• Cosmic-Ray Detectors (6 sensors)
• Solar-Plasma Detectors

Venera-15 Radar Antennas

The radar systems were designed by O.N. Rzhiga's team at IRE, The Institute of Radio Engineering and Electronics. The 1-meter parabolic altimeter antenna was pointed parallel to the axis of the spacecraft. The synthetic aperture radar antenna was a 6 × 1.4 meter parabolic cylinder, and it was angled at 10° from the spacecraft axis. The antennas were alternately switched to an 80-watt traveling wave tube oscillator, transmitting on 8-centimeter wavelength. Rather than pulse or chirp modulation, Venera used a continuous transmission modulated by a coded sequence of 180° phase shifts.

On Earth, a 2540-byte radar look was analyzed by computer. Digital filters separated the signal into time delays, dividing the radar spot into 127 ranges across the track of the radar beam. Fast Fourier transforms separated doppler shifts caused by the spacecraft motion, dividing the spot into 31 ranges along the track. Resolution was greatly increased by properly processing the signal over the 3.9 millisecond interval, as the moving spacecraft swept out a 70-meter virtual antenna. This processing is called coherent, because operating on complex values can simulate wave interference effects. The final result from processing a radar look was a 31 × 127 pixel image.

Computer System For Processing Venera Radar Data

3200 radar images from an orbital pass were combined into a survey strip of 195 × 10000 pixels. Typically 6 to 14 images would overlap at each element on the surface. Now only the real-valued image magnitudes were being averaged together, not coherent combination, and this served to reduce speckle noise. Speckling was a natural result of viewing the surface with coherent microwave radiation, just as objects illuminated by laser light appear speckled and grainy. Processing was performed at Rzhiga's institute using a pair of SM-4 minicomputers and a 20-mips special Fourier Transform Processor (SPF-SM).

The SPF-SM was developed at the Institute of Electronic Control Computers, under the leadership of Iu.N. Alexandrov. It appears to be closely related to their PS-2000 super computer, consisting of an SM host processor (a Soviet variant of the PDP-11) and a SIMD parallel math co-processor. PS-2000 co-processors could be configured for up to 200 mips.

Section of Venera-15 SAR Survey Strip

The Venera satellites were placed in elliptical polar orbits, with a minimum altitude of 1000 km above 62° N latitude. During close approach, 16 minutes were spent gathering data from near the north pole down to about 30° latitude. The survey strips were about 120 km wide and 7500 km long. Later in the 24-hour orbit, the 3200 gathered radar looks were transmitted to Earth, a total of 8.1 megabytes plus altimeter and IR-spectrometer data. The Soviet Union did not have a global deep-space communications network, so the spacecrafts had to maintain a 24-hour orbit, synchronized to be in radio line of sight with Evpatoriia every day. Several course corrections were made to maintain the shape of the orbits.

The plane of an orbit remains fixed in space, but Venus rotates 1.48° every 24 hours, allowing the entire polar cap to be scanned during the mission, from November 11 to July 10. Initially the orbital planes of the two spacecraft's orbits was 4° apart. While Venus was out of radio contact behind the Sun, a portion of the surface was not mapped. Later, Venera-16's orbital plane was changed by 20° to scan the missed section.

Venera Mosaic of Lakshmi Planum & Maxwell Montes

The individual survey strips were joined into mosaics with image processing to form a complete map of the northern cap of the planet. A catalog of 27 mosaics were assembled, each about 4000 to 5000 pixels wide, in Lambert-Gauss conformal projection. The slopes of Maxwell Montes are seen on the upper right, with the giant crater Cleopatra. Maxwell was initially believed to be a volcano. Russian geologists, using Venera-15 images, asserted that it was actually a mountain pushed up by tectonic pressure, with a large meteorite impact crater near its summit. After years of controversy, Magellan images proved them right.

Stereographic Map of Venera-15,16 Altimetry Data

The radar altimeter pointed directly down at the surface beneath the spacecraft and produced an altitude estimate with an accuracy of 50 meters. The footprint of the altimeter beam was about 70×40 km wide on the surface (along track × cross track). For the first few weeks (up to November 28), a low-resolution mode was used, with the 127-element phase modulation. This yielded a long ambiguity distance of 29.3 km.

After the orbits of the satellites were accurately determined, the altimeters were switched to high-resolution mode. A 31-element phase modulation was used, with an ambiguity of 7.15 km, and Doppler frequency analysis narrowed the effective footprint to 10×40 km.

Over 415,000 altimeter readings were gathered during the lifetime of the mission. This data is displayed above, with some systematic error in orbital data that the author will attempt to correct someday. The high (yellow) mountain to the left of the north pole is Maxwell Montes, a giant volcano and the highest feature on Venus.

Cartographic Synthesis of Venera Data

The combination of altimeter data and radar imaging allowed Russian cartographers and geologists to produce the first maps of Venus that showed its surface with enough detail to accurately analyze the planet's geomorphology.

Seen above is the singular structure of Lakshmi Planum, a high plateau of lava surrounded by mountains, including Maxwell Montes on its eastern side. Russian geologist A.A. Pronin proposed that Lakshmi is a giant corona, formed by an upwelling of the underlying mantle of Venus. Brown University geologist J.W. Head, a guest scientist on the Venera mission, proposed an alternative hypothesis, that Lakshmi is formed by a confluence of compression forces in the surrounding crust.

Volcanoes like Colette on Venus are most often broad and gently sloping, built from basaltic lava of low viscosity, like the shield volcanoes of Hawaii. The high atmospheric pressure may also muffled the explosive pyroclastic release of volcanic gas during eruptions, which might explain the rarity of tall conical volcanos. Altimeter data is particularly useful when large features have undramatic radar relief.

Collapsed domes with radial cracks were called arachnoids, because of their spider-like appearance. 80 were discovered by Venera. Coronae, arachnoids and domes were believed to result from hot spots in the underlying mantle. Unlike the Earth, Venus does not seem to have moving tectonic plates. Rather than producing chains of volcanoes, like Hawaii, hot spots on Venus remain stationary and generate volcanic blisters in the planet's crust.

Venera's radar images first permitted impact craters to be accurately distinguished from other circular features like coronae. The Russian team counted 150 impact craters in the area surveyed. This was a sparse density of craters, leading to the conclusion that the crust of Venus is only 750 ± 250 million years old. Some believe that Venus was catastrophically resurfaced, forming the tessera landscape, which was later covered by volcanic lava flows.

The thick atmosphere of Venus stops small meteorites, but larger objects have enough kinetic energy to punch through to the surface. In the dense atmosphere of Venus, a typical kilometer rock creates a linear explosion, releasing 600 megatons per kilometer along their path. B.A. Ivanov suggested that even if the meteorite burned up before impact, a rock-pulverizing shockwave could still result. He was no doubt mindful of the 1908 Tunguska impact, which flattened trees in Siberia but left no crater.

Venera-15's Infrared Fourier Spectrometer

The strong 15 µm carbon dioxide band was used to measure temperature profiles of the atmosphere, and the abundance of water vapor and sulphur dioxide were mapped. This provided detailed information about the thermal structure, dynamics and chemical variations in the atmosphere. Moroz discovered that the cold polar collar was composed of larger particles than the upper cloud layer, and a second jet stream was found at a lower latitude.

The clouds of Venus were known to contain droplets of sulphuric acid, but Moroz' experiment was the first to actually identify the chemical directly. Seen above, in comparison with the Earth's spectrum, a strong carbon dioxide band is seen, but the oxygen and organic-molecular signatures are absent. This cast further doubt on the remote possibility of living organisms within the clouds.

America's Magellan Mission

Launched on May 4, 1989, the normal trajectory to Venus in that time required enormous energy to decelerate into orbit. An unusual type-3 trajectory was chosen instead, taking the spacecraft around the Sun before entering orbit, over a year later, on August 10 1990. This also avoided scheduling conflicts with the Galileo mission, which was using a more conventional trajectory to Venus for a gravity assist to Jupiter. Like Pioneer-12, a large solid-fuel engine was used to decelerate the craft.

After participating as guests on the Venera mission, American scientists reciprocated by inviting some Soviet investigators on the Magellan mission. Data about Venus and Mars was exchanged under a treaty between the Soviet Union and the state of Rhode Island, and annual joint symposia were begun at The Vernadskii Institute and Brown University.

Magellan Image Mosaic of Crater Mead

Magellan made a 37-minute survey pass from the north pole to 75° south latitude. Images were transmitted to Earth at 115 - 268.9 kilobits/sec, during 2 hours of playback before the next orbital pass. The individual radar images were synthesized into survey strips of 20 × 17,000 km. SAR resolution varied from 100 to 250 meters depending on spacecraft altitude. As with Venera, the strips were composited into mosaics by image processing.

The image above is the impact crater, Mead. This early image demonstrates the mosaic of SAR survey strips, but later reprocessed images are usually seamless.

Another improvement from Magellan was its more complete coverage. From 1990 to 1994, it mapped 98% of the surface of Venus. Consequently, the Magellan survey confirmed that Venus has a unified crust without moving and subducting plates. While Soviet analysis of the Venera survey had found no compelling evidence for plate tectonics, it was still possible that they were seeing just the wrong 25% of the surface.

Soviet geologists had begun the long tedious process of classifying the relative age of different types of terrain, by meteorite-crater density, tectonic fracturing of features, and stratigraphic analysis of overlapping lava flows. Magellan's increased resolution and coverage both contributed to this important study, and the hypothesis that tessera landscape is the most ancient type of crust was confirmed.

Altimetry Map of Ishtar Terra

Magellan's radar altimeter took over 4.2 million readings, with a resolution of 30 meters and a range ambiguity of 10.2 km. The image above is assembled from Magellan data and the newly recalculated spacecraft ephemeris. Venera data was used to fill in missing sections.

Biographies

A.T. Bazilevskii

A.D. Kuz'min

O.N. Rzhiga

Acknowledgements

Special thanks to Aleksandr Bazilevskii, Vladimir Krasnopolskii, Arkadii Kuz'min and Oleg Rzhiga for their comments, and answers to questions and feedback on this page. I would also like to thank Peter Ford and Piotr Masek for their assistance in obtaining Venera and Magellan radar data.