Peering through the clouds of Venus
Click on images to enlarge.
What do you know about the planet Venus? It's inaccurately nicknamed "the morning star" or "the evening star" because of its prominence at different times of year. It's not a star at all. Some may know it is the second planet from the sun and about the same size as Earth. But what else? Scientists have attempted to send probes to Venus since 1961, including several landers, some probes gathering information as they descend through the atmosphere, and a few orbiters or flybys. NASA is now planning a new mission called DAVINCI to fly by and to drop another atmospheric probe.
Sumerians have probably the first recorded knowledge of Venus with their writings in the seventeenth century B.C.E. Regardless of the culture that observed it, the names given to Venus always reflected its brightness, and to no surprise since it is the third brightest object seen from Earth, next to the sun and moon. In 1610, Galileo noticed that it showed phases like the moon does and therefore proved it revolves around the sun not the Earth.
A suggestion that it has an atmosphere was first shown by Russian scientist Mikhail Lomonosov in 1761. He noticed a luminous band around it as it passed in front of the sun. His results were controversial, though, and better proof was shown in 1769 by others. Nevertheless, it was surrounded by some kind of atmosphere. What was under the atmosphere, though? Lots of speculation arose in the 16th and 17th centuries about the possibility of mountains, some even higher than the cloud cover which was already obvious.
Not much of significance was learned about Venus after that until the 20th century. What was known or suspected up to then was so incorrect that movies were made and stories were written as if humans could safely and comfortably land there.
In 1946, World War II was over, and surplus radar equipment was put into use by various scientists. Hey and Stewart, for example, used it to track the speed of meteors entering the Earth's atmosphere in 1946. The U.S. Army's Signal Corps Engineering Laboratories later bounced the first radar signal off the Moon, confirming that it was possible to receive radar echoes from an object in space and map its surface geography. Physicist Gordon H. Pettengill began observing Venus in 1958 with radar astronomy techniques and mapped what he called its “relatively smooth rocky surface” in 1961. Others used the same technique and in 1958 estimated the planet's temperature at around 300°C (600°F). Not exactly fit for humans.
Pettengill and others reported in 1964 that radar data showed Venus rotates backwards. That is, if viewed from the North Pole, it spins clockwise, unlike all other planets except Uranus. Radar signals were necessary because telescopes could not see through Venus' dense atmosphere to show any ground features to use as a references.
Pettengill also determined that one Venusian day (one full rotation on its axis) is longer than its year around the sun. One day (rotation) takes 243 Earth days, and one revolution around the sun takes only 225 days.
NASA developed the Pioneer 5 probe to investigate magnetic fields, solar flare particles, and cosmic radiation between Earth and Venus in 1959 and to test digital telemetry advances since Explorer 1, the U.S.'s first satellite in 1958. However, technical problems delayed the Pioneer 5 launch schedule to 1960, which didn't match the timing when Earth and Venus are close enough for a flyby.
The Soviet Union's Sputnik 7 was launched on Feb. 4, 1961 to probe Venus, but it failed to leave Earth orbit and crashed in Siberia. The next attempt was Venera 1 (also called Sputnik 8) launched 8 days later by the Soviets. It was the first interplanetary spacecraft and did manage to fly by Venus as intended, but communications were lost, and it ended up orbiting the sun without transmitting data.
NASA made an attempt with its Mariner 1 on July 22, 1962, and the Soviets again with Venera 2 on August 25 that year, but both suffered launch failures. Finally, 2 days later, NASA succeeded in sending Mariner 2 as a flyby probe. It flew within 34,854 km (21,660 miles), and its 7 instruments took readings for 42 minutes before it entered orbit around the sun. It found no difference in temperature between dark and lit side, a heavy atmosphere 56-80 km (35-50 miles) above the surface, and no measurabe magnetic field.
From September 1, 1962 to November 12, 1965, six Soviet crafts were sent and failed. The seventh, Venera 3, also failed but was the first craft to enter the atmosphere and impact on another planet. On October 18, 1967, Venera 4 succeeded in entering the atmosphere and sending back data. It parachuted down for 93 minutes and recorded the chemistry of the atmosphere as 90-93% carbon dioxide, 0.4-0.8% oxygen, 7% nitrogen and 0.1-1.6% water vapor. Before it lost the ability to transmit, it measured a temperature of 262°C (504°F) and a pressure at 55 km (34 miles) above the ground that was 22 times heavier than Earth's.
Two day later, the NASA probe Mariner 5 performed a flyby. Its instruments showed a surface temperature of 527°C (981°F)--that's hot enough to melt lead (328°C)--and surface pressure of 75-100 atmospheres. Pressures that strong are the equivalent of being under a kilometer (half a mile) of water.
Why is Venus so hot? Mercury is closer to the sun: 47-70 million km (29-43 million miles) away. Venus is 107-109 million km (67-68 million miles) away from it. But Mercury's daytime temperature is cooler at 430°C (800°F). And, it rotates much faster (176 Earth days long). Is the shorter day and subsequent time in darkness enough to counteract its closeness to the sun? Mercury has no atmosphere to speak of, only a surface-level thin layer of gases, some from the solar wind and some arising from its own crust. But Venus has a very thick atmosphere. That's the key.
Heat from the sun can be reflected or absorbed. On Earth, most is absorbed, but infrared radiation from the warm surface is released. Some of it escapes the planet, but some stays trapped because of "greenhouse gas" molecules and continues to warm the planet. Venus has far more carbon dioxide in its atmosphere (90-93%) compared to 0.04% on Earth, so its capability to heat up is greatly enhanced.
In 1940, astronomer Rupert Wildt thought Venus' atmosphere would create surface temperatures around the boiling point of water, nothing like what it really is. When the 1958 radar astronomy data suggested 300°C (600°F), a young Carl Sagan made calculations in 1960 to confirm a hellish greenhouse effect. He incorrectly felt the opaque clouds were caused by water vapor that boiled off the surface. Venera 4 showed the lack of vapor, though. Spectroscopic measurements from Earth in the last 1960s and early 1970s estimated <1% water vapor in the atmosphere.
Sulfur was detected in the clouds then, probably able to mix with any water vapor there due to the lower temperature, and then create sulfuric acid. But sulfur reflects infrared radiation and therefore can't be a source of greenhouse warming. The mystery as to the complete cause of the greenhouse effect remains to this day. However, some people felt opacity of the clouds was caused by the sulfuric acid clouds, and that the source of sulfur would be volcanoes. These would be discovered later.
Venera 5 and Venera 6 arrived at Venus on May 16 and 17, 1969, respectively. This time, both landed after almost an hour of parachuting. They were 3.5 m (11.5 ft) tall.
Beginning with the Soviet Venera 7, the 1970s saw a great deal of exploration of Venus. Venera 7 performed the first successful landing on December 15, 1970. Unlike 5 and 6, this one was designed to withstand the temperature and pressure, but a faulty parachute caused it to land roughly. It transmitted data for 33 minutes during its descent and another 20 minutes from the ground.
Venera 8 was designed to be a lander but also an atmospheric probe, which arrived on July 22, 1972. Its built-in refrigeration unit kept it functional longer (50 min, 11 sec). One of its measurements confirmed a 1-km (0.6-mile) visibility like a cloudy day on Earth, which meant future probes could take pictures. Its spectrometer measured mineral ratios of the surface rock, showing similarity to alkali basalt.
On November 3, 1973, NASA sent Mariner 10 to be the first to study Mercury, and on its way it would get a gravitational assist from Venus on January 21, 1974. The UV filters allowed its cameras to collect great detail on cloud shapes on Venus. It spent 8 days recording images, ultimately sending 4,165 pictures to Earth. Notably, it monitored clouds moving around the planet rapidly in 4 days. It also detected several layers of temperatures and chemical composition.
Mariner 10 also showed a 6-km (3.7-mile) thick haze of several layers above the main atmosphere, which is 50-70 km (31-43 miles) above the planet.
Venera 9 entered orbit on October 20, 1975, and its orbiter separated from a 2-m (6-ft) high spherical lander on October 22. Among other measurements, this was the first time images were sent back from another planet. Non-eroded rocks and no airborne dust were seen. Data was sent back via a link to the orbiter.
Venera 10 landed on October 25, 1975 and collected similar images and data. It may have also detected lightning. Venera 11 landed on December 25, 1978 and found sulfur and chlorine in the cloud layers, but its cameras failed. Venera 12 cameras failed, too, but it collected more data (hints of lightning and thunder, plus evidence of sulfur and chlorine in the atmosphere) from its lander on December 21, 1978. Both 11 and 12 suffered failures in soil collection or analysis devices.
Meanwhile, NASA sent two orbiters, Pioneer Venus 1 (Pioneer 12) and 2 (Pioneer Venus Multiprobe, Pioneer 13), arriving on December 4, 1978 and December 9, 1978, respectively. Pioneer 12 was an orbiter which mapped 93% of the surface and provided 10 years of detailed information from 17 experiments on Venus' ionospheric reactions to the solar wind. Pioneer 13 consisted of a squat cylindrical orbiter, one large probe, and 3 smaller ones. The probes fell through the atmosphere and took readings on gas composition, cloud particle size and shape, solar flux penetration, temperature, pressure, and infrared radiation.
Venera 13 landed on March 1, 1982; Venera 14 landed March 5, 1982. The former ran experiments for 127 minutes on soil samples. Venera 13 also transmitted the first sound from the surface (wind, the lander hitting the ground, lens cap explosive removal and its impact on regolith, and noise of the regolith drilling apparatus). Venera 14 stayed active for 57 minutes and analyzed rock samples. It determined them to be a type of magma basalt.
Venera 15 and 16 were identical orbiters that arrived October 10 and 11, 1983 and did radar mapping of the surface of northern and southern hemispheres, respectively, for 8 months. Here is a sample:
The Soviet Vega 1 and Vega 2 probes landed a module on June 11 and 15, 1985. Vega 1 took readings of the atmosphere on the way down. Vega 2 also analyzed soil. Both also released an inflated balloon 3.5 meters (11.5 feet) in diameter which floated for 47 hours at 50 km (31 miles) from the day to night side and took readings of atmospheric dynamics, pressure, temperature, lightning, illumination levels, and cloud properties.
NASA launched Magellan from the Space Shuttle (a first feat), and it arrived to orbit Venus on August 10, 1990. It used radar similar to Venera 15 and 16, but operated far longer, ending up mapping of 98% of the surface on Sept. 13, 1992. It learned that 85% or more of the surface is covered with volcanic flows; lack of erosion by water maintains surface features for hundreds of millions of years, and there was no sign of tectonic plate activity.
Magellan's equipment provided much better resolution than Venera 15 or 16. See comparisons below.
On April 11, 2006, the European Space Agency (ESA) parked an orbiter around Venus as a joint mission called Venus Express. This craft was the same as the Mars Express sent to investigate Mars and its moon Phobos in June 2003. The Venus Express planned 500-day exploration lasted until December 14, 2014 when it was allowed to enter the atmosphere and crash. It measured the ionosphere, wind patterns, rotation of Venus (which had slowed down by 6.5 minutes since the Magellan probe), and geographical features. Part of its atmospheric data suggest that Venus was once covered in oceans, the best proof so far of earlier hypotheses.
Data from the Venus Express have also shown some wind patterns and light signatures describing in more detail Venus' runaway greenhouse effect.
Most recently, Japan's space agency JAXA sent a craft called Akatsuki to Venus. It made orbit on December 7, 2015. This Venus Climate Orbiter (VCO) is still operating to investigate stratification of the atmosphere, atmospheric dynamics, and cloud physics as well as to confirm the presence of lightning and to determine how much volcanism currently occurs there.
Enter DAVINCI, the next planned NASA mission to Venus. The name stands for Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging. It will involve a flyby craft called CRIS (carrier, relay, and imaging spacecraft) plus a descent probe that will take measurements as it parachutes down.
Noble gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). They are non-reactive (inert), but they are useful in learning more about greenhouse effects and volcano activity.
While instruments on CRIS record >60 gigabytes of data from the atmosphere with UV and infrared cameras, the 1-m (3-ft) diameter lander probe (Venus Analytic Laboratory, VAL) will ingest atmospheric samples as it drifts down and perform chemistry analyses as well as take photos before the VAL settles on the surface to lie dormant. Actually, it may be able to survive for 17-20 minutes longer. The point is to conduct the most extensive chemistry than ever before and examine the terrain relief in more detail, using more modern technology like that from the Curiosity rover on Mars. Its primary aim is to learn how the atmosphere formed and evolved. Plans are to launch DAVINCI in 2029 and reach Venus in June 2031.
Cameras are intended to capture high-resolution images of a highland area called Maxwell Montes, which contains terrain called tesserae, deformed by the intersection of tectonic plates. Tesserae cover 7.3% of Venus' surface.
DAVINCI will examine poorly understood areas on Venus' surface as well as more of the atmosphere. It will blaze a trail for NASA's VERITAS and ESA's EnVision missions that plan to study the planetary heat evolution and crust.
In 2020, some scientists claimed to have detected a chemical called phosphine in the upper atmosphere, and in 2023 the same people found it in the lower atmosphere. Other researchers have been unable to confirm these results. Phosphine is normally found in Jupiter and Saturn, where there is a lot of hydrogen to couple to the phosphorus, and there is a little in Earth's atmosphere. Here in our own atmosphere, it is sometimes a sign of decaying biological material, so this finding on Venus suggests that perhaps there is a form of life in the clouds. Maybe. It may also be formed naturally in acidic environments (remember the sulfuric acid in Venus' clouds?) when iron-rich compounds interact with small amounts of phosphorus. Therefore, future studies on the clouds may help understand what is really going on there and potentially point toward life on our sister planet.
More images from Magellan can be found here.
Many Soviet pictures of the surface of Venus (mentallandscape.com)
Here is a short (<3-minute) YouTube video explaining DAVINCI's mission with some great CGI.
