First Pictures of the Surface of Venus (original) (raw)
America's Mariner-10
America launched the 474 kilogram Mariner-10 on November 3, 1973. This mission to Mercury used Venus for a gravity assist, an innovative maneuver. Because of the enormous difference in orbital velocity, reaching Mercury by a direct Hohmann transfer requires 10 times the energy of reaching Venus. "Falling toward the Sun" seems easy, but canceling orbital velocity makes it more costly to approach the Sun than to escape the solar system.
Mariner-10 provided the best images of Venus, whose subtle features were irresolvable from ground-based telescopes. Viewed in ultraviolet, details of the atmospheric circulation were visible, due to a still-unknown UV-absorbing substance. The series of images clearly demonstrated the four-day superrotation of the planet's atmosphere, first seen in the Doppler-effect velocity measurements of Venera-4 through Venera-8.
At its closest approach of 5768 kilometers, Mariner-10 photographed layers of haze extending 6 km above the opaque cloud deck. By the time of Mariner-10, vidicon tube technology had reached its peak, coming a long ways since the small, noisy images of Mariner-4. Similar technology was used in Voyager, but after that, CCD technology would make camera tubes obsolete. Nevertheless, these are the best images of Venus that have ever been taken.
Images were sent at up to 117,600 bits/sec without error coding. This resulted in bit-error rates of 2 to 7 percent, as the haze image demonstrates. High quality images could be sent at 7350 bits/sec. The images above have not been processed to remove transmission noise. Science data was sent coded at 490 to 2450 bits/sec to achieve 0.01 percent bit-error rates.
It would not be valid to compare Mariner-10's uncoded 117,600 bits/sec mode to Venera-9's error-coded 3072 bits/sec. Bit rates are only commensurate at the same error rate and coding redundancy. Convolution codes to correct errors typically use two bits of signal for one bit of data. By the late 1970s, American telemetry technology was the most advanced in the world, but not by a factor of 40. Cosmic background noise presents a fundamental constraint on the information capacity of interplanetary radio channels.
Venera-9 and Venera-10
Mysteriously, Soviet scientists skipped the 1973 launch opportunity for Venus. The reason became clear in 1975, when a radically new spacecraft was unveiled. Fully fueled, Venera-9 weighted 4936 kg, and Venera-10 5033 kg. They were topped with a 2.4 meter spherical reentry vehicle weighing 1560 kg, and within that was the 660 kg lander.
Conditions on Venus are extraordinarily harsh: ~100 atmospheres of pressure, temperatures of 475° C (890° F) and the chemical actions of sulphuric acid. Under these conditions, common materials like aluminum and glass soften or melt, titanium and magnesium can combust, and organic compounds can pyrolyze or dissolve in the supercritical carbon dioxide. Later American probes would be partially disabled by chemical corrosion, short circuits caused by condensation of unknown conductive substances and mysterious mid-air electrical shocks. There is still much that is not known about this environment.
Venera-9 Lander
The core of the descent vehicle was a spherical titanium hull about 80 cm in diameter. It was formed in several sections, bolted and sealed with gold-wire gaskets. That was covered in a 12 cm layer of thermal insulation (a composite honeycomb material) and a thin outer skin of titanium. The pressure hull was lined inside with insulation, possibly layers of fiberglass and metal foil. A large thermal accumulator of lithium nitrate trihydrate and a circulating fan distributed and absorbed excess heat. This lithium salt has a high specific heat of fusion, like ice, but melting at 30° C.
Assembly of Venera-9 Lander
The pressure hull housed the transmitters, control sequencer, electrical battery and scientific instruments designed to function for an extended time after landing. The two pipes seen on the left carried thermal regulation gas to a heat exchanger in the lander. It was cooled to -10° C before separating from the bus, and the interior temperature rose to 60° C after an hour on the surface. Mission lifetime was limited by loss of radio contact, not thermal failure.
Several new experiments were designed to study the cloud layer in more detail. M.Ia Marov's experiment measured cloud density by nephelometers, which sent rapidly pulsing light through a segment of atmosphere, measuring scattered reflection at angles of 4°, 15°, 45° and 180°. From these measurements and Mie's theory of scattering, it would be possible to measure the density of droplets and some information about the their distribution of sizes. Sensors for the nephelometer and spectrometers were outside the pressure hull, with their own insulation and phase change materials for thermal protection.
Sunlight was measured by a battery of spectrometers, at altitudes from 63 km down to the surface. The instrument diagrammed above measured light levels at green, yellow, red and two infrared wavelengths. Another instrument measured three narrow IR wavelengths corresponding to absorption bands of carbon dioxide, water vapor, and an unabsorbed reference wavelength. The spectrometer can be seen to the left of the two pipes. It was connected by fiber optics to sensors placed above and below the aerobrake.
Venera-9 Gamma-Ray Densitometer
A new surface experiment was a gamma-ray densitometer. Rays from a radioactive isotope would be emitted into the surface rock and the scattered reflection detected at several angles. This is the "paint-roller" shaped device seen to the right of the two pipes and in the image above. The cylindrical densitometer is 36.2 cm long and 4 cm in diameter. Three Geiger-tube detectors measure the distribution of reflected gamma rays from a radioactive cesium-137 source in the end of the densitometer. A thick lead shield between the source and the detectors blocked unscattered rays. The device was operated during the descent, measuring atmospheric-scattering, and then deployed onto the surface to measure the additional scattering due to the much denser rock.
In addition, a gamma-ray spectrometer, like the one in Venera-8, was contained within the pressure hull, for the measurement of potassium, uranium and thorium abundance. This sensor consisted of a large sodium iodide crystal scintillator and a photomultiplier tube. Both the densitometer and spectrometer were built by Iu.A. Surkov's team.
Flight plan of Venera-9 and Venera-10
Venera-9 and Venera-10 were launched in 1975 on June 8 and 14. Two days before reaching Venus, the descent vehicle and the spacecraft bus separated. The descent vehicle was placed on a trajectory to enter the atmosphere, and the buses entered elliptical orbits, becoming the first artificial satellites of Venus.
To transmit pictures, the lander needed radio bandwidth and light. This meant being on the sunlit side of Venus, out of radio line of sight with Earth. To solve these problems, the spacecraft bus "hovered" over the landing site, in a highly eccentric orbit, much like Earth-orbiting television satellites hovered over the Soviet Union. With a 512 bit/sec connection to the lander and a 3072 bit/sec connection to earth, the orbiter relayed data in real time and also recorded it on tape for several later playbacks.
Venera-9's lander reached Venus on October 22. At an angle of 20° from the horizon, its entry was shallower than previous missions and overloads of about 170g were experienced. When deceleration forces reached a predetermined level, parachute deployment began at an altitude of about 65 kilometers. A series of pilot, drogue, braking and main parachutes slowed the vehicle in easy stages, and the two halves of the spherical reentry pod were jettisoned. The vehicle spent about 20 minutes passing through the cloud layer.
Once through the clouds, there was no point in spending unnecessary time in the hot atmosphere. At an altitude of 50 kilometers, the parachutes were jettisoned, and the lander fell for 55 minutes, slowed only by the aerobrake. In the thick atmosphere, terminal velocity was 7 meters/sec at touchdown, equivalent to the impact of being dropped from 10 feet. Conditions were 90 atm pressure and 455° C (851° F). Venera-10, reached Venus three days later and carried out a similar landing. It found 91 atm of pressure and 464° C.
Image transmission from Venera-9
After landing, the gamma densitometer was deployed and, the protective camera covers were removed by pyro charges. Unfortunately, one camera cover failed to come off. The remaining camera scanned a complete 174° panorama, reversed direction, and scanned 124° of a second pass. The orbiter moved out of radio range, and the telemetry ended after 53 minutes on the surface, at which time the lander was still functioning normally.
Lines of static at the beginning of the transmission are bit-stream misalignments and not actual noise. Theoretically be corrected to reveal more of the image. The periodic vertical bars of static are transmissions of telemetry from other experiments. The densitometer measured 2.88 grams/cm3. The gamma-ray spectrometer found lower levels of radiation than Venera-8. The density and potassium/uranium/thorium levels were similar to basalt.
Venera-9 landed near Beta Regio, within a 150km radius of 31.01° N, 291.64° E. It landed on a steep (20°) slope covered with boulders, distinctly different from subsequent images. From high-resolution radar mapping of the region, it is suspected to be the slope of the tectonic rift valley, Aikhulu Chasma. A better view of the panorama can be seenhere.
Image transmission from Venera-10
Venera-10 landed on October 25, about 2200 kilometers from Venera-9. Again, one camera cover failed to come off, but the other camera returned a 63° forward scan, 184° in the reversed scanning mode, and 17° of forward scanning. Transmission ended after 65 minutes, when the orbiter went out of radio range.
Both Venera-9 and Venera-10 found the halogen lamps unnecessary. Venera-8's light readings, near the morning terminator, were much darker than the near noon-time conditions where the images were acquired. Subsequent missions did not include the lamps.
Analysis of rock density and gamma spectra again indicated basalt. This time, a level flat plain of rock and soil was observed, similar to what Venera-13 would later view. The site is somewhere within a 150 km radius of 15.42° N, 291.51° E, and high resolution radar indicates a flat lava bed highly fractured by tectonic compression. A better view ishere.
The images above are partially composed from existing 6-bit digital image telemetry, and partially from a photocopied print of the entire data set. Hopefully better versions of the whole data set will be found, but it may be lost due to the aging of magnetic tapes.
Nephelometer readings, Extinction coefficients, IR Spectrometer readings
From Nephelometer results, M.Ia Marov concluded that Venus had three major cloud layers, with a bottom around 49 kilometers above the surface. Above left are the four nephelometer angular scattering measurements for Venera-9. In the center are computed optical extinction for Venera-9 (1) and the thicker clouds where Venera-10 (2) penetrated.
Venera-9 and 10 found the clouds to be extremely tenuous, just a light fog with visibilities of a several kilometers. But their great depth does not allow seeing more than halfway through the upper layer. Earth-based spectral analysis of the upper surface of clouds had already shown it to be made up of very small droplets, about 1/10 the diameter of water droplets in terrestrial clouds. Marov's analysis of the multi-angle scattering indicated that the distribution of droplet sizes had two or more peaks (modes).
Optical spectrometers in three visible colors and two infrared wavelengths showed an increasingly orange color to the illumination with depth. These measurements continued to the surface level, and indicated a blue-absorbing chemical may be present, in addition to the expected effects of Rayleigh scattering. Above right, the narrow-band infrared spectrometer measured light levels at a 45° angle with a very narrow field of view. Remarkably, it showed 10-fold decreases and increases of light, as the vehicle descended through regions of dense and rarefied clouds.
Venera-9 and 10 carried the first mass spectrometers ever used in a planetary atmosphere, although both instruments had problems, possibly contamination by cloud material. Nitrogen and ammonia levels and the first detection of carbonyl sulphide (COS) were reported, with the caveat that the spectrometers were behaving erratically.