January 15, 2021 | Laboratory experiments demonstrate promising new approach
Back to Venus armed with laboratory findings
Venus – the 'opaque neighbour'
Venus, Earth's neighbour in space, hides its surface under a dense atmosphere. Venus's clouds of sulphuric acid prevent telescopes and instruments on board spacecraft having a direct view of the planet’s surface in visible wavelengths of light. These images of Venus were acquired in 2019 by the Japanese Venus orbiter Akatsuki from different distances in ultraviolet wavelengths. The extraordinary dynamics of the planet's atmospheric processes are clearer in ultraviolet light than in visible light. Bands of cloud exhibit 'super-rotation' as they move around Venus faster than the planet turns around its axis of rotation.
In the direction of the Sun, Venus is the closest planet to Earth. It orbits on average 40 million kilometres closer to the Sun. The closer proximity is only one reason why Venus's surface is hotter than Earth's. A much greater influence on the temperature is an extreme greenhouse effect that heats the planet to a consistent 470 degrees Celsius at its surface. The high temperatures and the high atmospheric pressure of 90 bar make landing on Venus with spacecraft extremely difficult. The Soviet Union succeeded in landing on Venus eight times between 1970 and 1983 with landers from its Venera programme. Many of these landers transmitted data and images for up to two hours. These images show the landscape at the landing sites of Venera 9 and Venera 13, in a field of volcanic basalt rocks.
Venus is similar to Earth in many ways. The planets are almost the same size, but they have developed very differently since their formation 4.5 billion years ago. Almost nothing is known about the early days of Venus because observations made by spacecraft reveal a surprise – about half a billion years ago, there was a global cataclysm on the planet as tens of thousands of volcanoes erupted and reshaped the entire planetary surface. Traces of anything that existed before were almost completely erased. One tenth of the surface has structures, reminiscent of Earth's continents, that may have survived the global volcanic catastrophe. This perspective image created using radar data from NASA's Magellan mission shows the eight-kilometre-high Maat Mons volcano, with solidified lava flows in the foreground.
Scientists are simulating the extreme surface conditions of Venus in a DLR furnace.
Emission spectra measured in the laboratory of ferrous rocks at temperatures of 440–480°C can be used to identify similar rocks on Venus.
Laboratory measurements provide new results from data acquired by Venus landers 46 years ago.
The mineralogy and geochemistry of Venusian rocks can now be mapped with new instruments.
Focus: Space, Solar System exploration, Venus
Venus's impenetrable atmosphere has long made it difficult to conduct a thorough investigation of our neighbouring planet. In a step forward, by conducting laboratory experiments scientists from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have now developed a way of determining the nature of the planet's surface using new instruments from orbit. The entire surface of Venus can now be mapped mineralogically for the first time, addressing a large gap in planetary research.
Venus and Earth – two very different siblings
Venus is Earth's sister planet. It is almost exactly the same size and orbits on average only 40 million kilometres closer to the Sun. However, the two planets developed in very different ways. On Earth, continents formed, separated by oceans. Then some three and a half billion years ago, life emerged under its atmosphere and evolved into the vast variety of organisms that we know today. Things happened very differently on Venus, which is surrounded by an atmosphere of gas a hundred times thicker than that of Earth. Within it, the extreme greenhouse effect results in a constant surface temperature of 470 degrees Celsius – a temperature at which water would instantly evaporate and even lead would melt. The planet is permanently enveloped in thick clouds of sulphuric acid, making it impossible for telescopes on Earth or instruments on board spacecraft to acquire even a glimpse of the surface. Scientists have managed, however, to map its landscape using radar. And now, through a series of laboratory experiments, DLR researchers have developed a new method for determining the nature of the planet's surface from orbit.
"For a good ten years, we have been using a unique laboratory facility to measure the emission properties of various rocks of the kind we might expect to find on Venus under the same extreme conditions that prevail on the planet," says Jörn Helbert, Head of DLR's Planetary Spectroscopy Laboratory (PSL) and lead author of a research paper that has now been published in Science Advances magazine. "The reflectance and emissivity of rocks change when they are exposed to the high temperatures that you find on Venus. As a result, spectral profiles measured at terrestrial temperatures cannot simply be applied there. But now, we have a tool that we can use as the basis for new instruments on the next planned missions to Venus that will finally allow us to determine which types of rock exist there."
Concordance with existing surface measurements from Venus
The four-person research group made up of researchers from DLR, the Planetary Science Institute in Tucson, Arizona, and Mount Holyoke College in Massachusetts have used the results of their laboratory experiments to devise a new 'spectral library' for various types of rock. "Determining the emission spectra in this way has enabled us to reconstruct the iron oxide content at the landing site of the Soviet Union’s Venera 9 and Venera 10 landers for the very first time," says DLR Planetary Scientist Alessandro Maturilli. "In 1975, the two landers transmitted images from Venus and provided important measurements. However, they were not equipped with an instrument that could directly measure iron oxide content."
The two landers provided the only direct spectral measurements of Venusian rocks. The emission profiles determined in the laboratory and the spectra determined by the Venera missions are in very good agreement. "As such, we have demonstrated the accuracy of our new method, which represents a big step forward," says Jörn Helbert. Although further Venus lander missions are currently under discussion, global mapping of the planet can only be conducted from orbit. However, the atmosphere of Venus is impenetrable to wavelengths of visible light – those that the human eye can see. To make mapping of the surface possible despite the planet's atmosphere, scientists are focusing on what are known as 'atmospheric windows'. These are narrow bands of wavelengths at which the atmosphere of Venus is transparent, and thus allow a view of the surface. Five such 'windows' exist at wavelengths close to 1000 nanometres (one micrometre). These wavelengths are in the near-infrared, which is adjacent to the visible portion of the electromagnetic spectrum (approximately 400–700 nanometres).
Venus returns to the focus of planetary research
Through their experiment, the researchers have shown that for rock types measured in the laboratory, the spectra and their characteristic profiles can be used to reliably identify them from orbit. "Our aim is now to create the first global map of rocks on Venus," says Helbert. "That would be a major achievement! After all, we know far too little about Venus. In its infancy, the planet may have had water, like Earth, and may also have been less hostile to life." The scientists are looking to use the Venus Emissivity Mapper (VEM) instrument to implement their plan, mapping emissions in the few atmospheric windows available at near-infrared wavelengths. VEM could be installed on the EnVision mission of the European Space Agency (ESA) and on NASA's VERITAS orbiter later this decade.
Venus, Earth's celestial neighbour and often described as its sister planet, was the first target for interplanetary spacecraft. The Soviet mission Venera 1 launched in 1961 and the American Mariner 2 in 1962. Despite a number of spectacular successes, particularly the Soviet space programme's initial orbiters and then eight landings between 1970 and 1983, the planet named after the Roman goddess of love later fell somewhat out of favour in planetary research. Between 1990 and 1994, NASA's Magellan orbiter mapped Venus using radar, revealing myriad details of its multifaceted surface in high-resolution. From 2006 to 2015, ESA's Venus Express mission investigated the planet across a series of seven experiments.
However, we still know little about the nature of the planet's surface. For Earth's other immediate neighbours, the Moon and Mars, determining rock types, mineralogical composition and the abundance of chemical elements was far easier. As a result, nowadays we have a fairly thorough understanding of them. The astronauts involved in the Apollo missions and the robotic missions of the Soviet Union returned 400 kilograms of sample material from the Moon to Earth. Half a dozen landers have been able to analyse the rocks on Mars. A landing on Venus would be a heroic feat, as the lander descends into an increasingly hot oven. The high temperatures and atmospheric pressure – which reaches 92 bar on the ground (equivalent to the pressure at 900 metres underwater on Earth) – would put the electronics under immense strain. The Venera 13 lander was able to withstand these conditions for the greatest amount of time so far and, on 30 October 1981, succeeded in transmitting data from the furnace-like surface of Venus for almost two hours. The new method developed in this laboratory study would allow the surface of Venus to be studied systematically from orbit over a period of many years, without having to run the risk of a landing.
