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Weather stations for Venus

Weather stations for Venus Abstract Ian Strangeways looks at the difficulty of operating meteorological instruments on Venus due to its harsh environment, and suggests some possible solutions In Greco-Roman mythology, Venus is the goddess of love, while Mars is the god of war. In reality, it might be argued (with our better astronomical knowledge) that the reverse is closer to reality. Venus is a harsh, dangerous place, while Mars is much more peaceful. Humans may be walking on Mars soon, while we are unlikely ever to be able to walk on Venus. Venus is easier to get to than Mars because it is nearer (40 million km compared to 62 million km for Mars – when at their closest to Earth). It is also closer to the Sun than Earth, allowing easier trajectories for spacecraft, reducing fuel demands and travel time (3.5 months compared to 5 months for Mars). But here Venus's advantages end, for while it is almost the same size and density as Earth, its atmosphere is extremely different to that of Mars and Earth (and Mercury, which has virtually no atmosphere). In contrast, the atmosphere on Venus is extremely hostile, being dense, with a surface barometric pressure around 90 times that of Earth. The atmosphere is composed mostly of carbon dioxide, causing an immensely strong greenhouse effect, leading to a surface temperature of around 450°C. This makes the design of surface-based automatic weather stations (AWSs) for Venus much more problematic than AWSs for operation on Earth or Mars (Strangeways & Smith 1985, Strangeways 2014, 2021). As a saving grace, however, high above the surface of Venus the atmospheric temperature is sufficiently low to cause condensation and clouds to form, although the clouds are of sulphuric acid rather than water. The cloud layers extend from around 45 km to 75 km above the surface, completely cloaking the planet. At an altitude of 55 km the temperature is only 20°C and the pressure about 0.5 bar (figure 2), representing an environment conducive to instruments and robotic vehicles such as balloons and airplanes. This offers a potential solution to operating AWSs on Venus and this article looks at some designs that could withstand that harsh environment. Measurements of the weather on Venus by space probes to date are summarized in table 1. Figure 1 shows an artist's impression of the Russian Venera 13 lander on Venus, which successfully measured surface temperature and barometric pressure in 1981. 1 Open in new tabDownload slide A 3D rendering of the Venera 13 spacecraft on the surface of Venus. (Russian Academy of Sciences/M Malmer, CC BY 3.0) 1 Open in new tabDownload slide A 3D rendering of the Venera 13 spacecraft on the surface of Venus. (Russian Academy of Sciences/M Malmer, CC BY 3.0) 2 Open in new tabDownload slide A cross-section of Venus's atmosphere, showing the levels of clouds and the variation of temperature and barometric pressure with altitude. (A Gunn) 2 Open in new tabDownload slide A cross-section of Venus's atmosphere, showing the levels of clouds and the variation of temperature and barometric pressure with altitude. (A Gunn) Table 1 Venus weather missions mission . operator . operating . type . instruments and measurements . Venera 1–16 USSR/Roscosmos 1961–83 landers landing probes successful from Venera 8 onwards operated for only an hour or so due to high temperature and pressure measured temperature around 450°C, and pressure of 90 Earth atmospheres Pioneer Venus 1 & 2 USA/NASA 1978–92 orbiter landers Pioneer Venus Orbiter mapped the surface by radar, measured clouds, solar wind and magnetic field Pioneer Venus Multiprobe inserted four probes that measured temperature and pressures as they descended not designed to survive landing; confirmed the earlier Russian values Vega 1 & 2 USSR/Roscosmos 1985 landers balloons same as Venera Landers Teflon 3.4 m super-pressure balloons stabilized at 54 km altitude, measured temperature of 27°C and pressure of 535 hPa windspeeds of 100 m s−1 measured, compared with 5 m s−1 at the surface Magellan USA/NASA 1990–94 orbiter known as the Venus Radar Mapper mapped the surface in great detail Venus Express Europe/ESA 2006–14 orbiter made extensive measurements of the atmosphere Akatsuki Japan/JAXA 2015– orbiter five cameras working at different wavelengths remote study of the atmosphere mission . operator . operating . type . instruments and measurements . Venera 1–16 USSR/Roscosmos 1961–83 landers landing probes successful from Venera 8 onwards operated for only an hour or so due to high temperature and pressure measured temperature around 450°C, and pressure of 90 Earth atmospheres Pioneer Venus 1 & 2 USA/NASA 1978–92 orbiter landers Pioneer Venus Orbiter mapped the surface by radar, measured clouds, solar wind and magnetic field Pioneer Venus Multiprobe inserted four probes that measured temperature and pressures as they descended not designed to survive landing; confirmed the earlier Russian values Vega 1 & 2 USSR/Roscosmos 1985 landers balloons same as Venera Landers Teflon 3.4 m super-pressure balloons stabilized at 54 km altitude, measured temperature of 27°C and pressure of 535 hPa windspeeds of 100 m s−1 measured, compared with 5 m s−1 at the surface Magellan USA/NASA 1990–94 orbiter known as the Venus Radar Mapper mapped the surface in great detail Venus Express Europe/ESA 2006–14 orbiter made extensive measurements of the atmosphere Akatsuki Japan/JAXA 2015– orbiter five cameras working at different wavelengths remote study of the atmosphere The efforts made so far to measure the weather on Venus. There have been many missions to Venus, starting in 1961: 29 by the USSR (Roscosmos), 11 by the USA (NASA), three by Europe (ESA), and three by Japan (JAXA/UNISEC). Of these 46 missions, 29 were successful. The Soviet Venera programme, 1961–84, is one of the largest efforts ever undertaken to study another planet. Much of what is known today about Venus was discovered by these missions. But the most recent data to be returned from within the atmosphere date back to 1985. Since then, there have been no further Russian missions. Five of the 11 American missions were made post-1985, although four were simply gravity-assist fly-bys, just one being an orbiter (Magellan). Of ESA's three missions, two were gravity-assist, one, Venus Express, being an orbiter, making measurements of the atmosphere remotely. Unlike on Mars (Strangeways 2021), most measurements within Venus's atmosphere have, so far, been restricted to single brief descents, with just a short time at the surface; in-all, just a few hours of data-collection. The only exception was Vega 1 and 2, which in 1984 deployed balloons that operated within the cloud layers, but only for a few days (until their batteries ran out). At the time of writing, there are 12 proposals for the future, including a feasibility study for a rover by NASA in 2039. Under development is an orbiter for 2023 by ISRO (Indian Space Research Organisation), and an orbiter (and lander?) by the Russian Roscosmos for 2026. Open in new tab Table 1 Venus weather missions mission . operator . operating . type . instruments and measurements . Venera 1–16 USSR/Roscosmos 1961–83 landers landing probes successful from Venera 8 onwards operated for only an hour or so due to high temperature and pressure measured temperature around 450°C, and pressure of 90 Earth atmospheres Pioneer Venus 1 & 2 USA/NASA 1978–92 orbiter landers Pioneer Venus Orbiter mapped the surface by radar, measured clouds, solar wind and magnetic field Pioneer Venus Multiprobe inserted four probes that measured temperature and pressures as they descended not designed to survive landing; confirmed the earlier Russian values Vega 1 & 2 USSR/Roscosmos 1985 landers balloons same as Venera Landers Teflon 3.4 m super-pressure balloons stabilized at 54 km altitude, measured temperature of 27°C and pressure of 535 hPa windspeeds of 100 m s−1 measured, compared with 5 m s−1 at the surface Magellan USA/NASA 1990–94 orbiter known as the Venus Radar Mapper mapped the surface in great detail Venus Express Europe/ESA 2006–14 orbiter made extensive measurements of the atmosphere Akatsuki Japan/JAXA 2015– orbiter five cameras working at different wavelengths remote study of the atmosphere mission . operator . operating . type . instruments and measurements . Venera 1–16 USSR/Roscosmos 1961–83 landers landing probes successful from Venera 8 onwards operated for only an hour or so due to high temperature and pressure measured temperature around 450°C, and pressure of 90 Earth atmospheres Pioneer Venus 1 & 2 USA/NASA 1978–92 orbiter landers Pioneer Venus Orbiter mapped the surface by radar, measured clouds, solar wind and magnetic field Pioneer Venus Multiprobe inserted four probes that measured temperature and pressures as they descended not designed to survive landing; confirmed the earlier Russian values Vega 1 & 2 USSR/Roscosmos 1985 landers balloons same as Venera Landers Teflon 3.4 m super-pressure balloons stabilized at 54 km altitude, measured temperature of 27°C and pressure of 535 hPa windspeeds of 100 m s−1 measured, compared with 5 m s−1 at the surface Magellan USA/NASA 1990–94 orbiter known as the Venus Radar Mapper mapped the surface in great detail Venus Express Europe/ESA 2006–14 orbiter made extensive measurements of the atmosphere Akatsuki Japan/JAXA 2015– orbiter five cameras working at different wavelengths remote study of the atmosphere The efforts made so far to measure the weather on Venus. There have been many missions to Venus, starting in 1961: 29 by the USSR (Roscosmos), 11 by the USA (NASA), three by Europe (ESA), and three by Japan (JAXA/UNISEC). Of these 46 missions, 29 were successful. The Soviet Venera programme, 1961–84, is one of the largest efforts ever undertaken to study another planet. Much of what is known today about Venus was discovered by these missions. But the most recent data to be returned from within the atmosphere date back to 1985. Since then, there have been no further Russian missions. Five of the 11 American missions were made post-1985, although four were simply gravity-assist fly-bys, just one being an orbiter (Magellan). Of ESA's three missions, two were gravity-assist, one, Venus Express, being an orbiter, making measurements of the atmosphere remotely. Unlike on Mars (Strangeways 2021), most measurements within Venus's atmosphere have, so far, been restricted to single brief descents, with just a short time at the surface; in-all, just a few hours of data-collection. The only exception was Vega 1 and 2, which in 1984 deployed balloons that operated within the cloud layers, but only for a few days (until their batteries ran out). At the time of writing, there are 12 proposals for the future, including a feasibility study for a rover by NASA in 2039. Under development is an orbiter for 2023 by ISRO (Indian Space Research Organisation), and an orbiter (and lander?) by the Russian Roscosmos for 2026. Open in new tab Phosphine It was suggested recently (Greaves et al. 2021), following the analysis of data from Earth-based radio telescopes, that traces of phosphine may be present in the atmosphere of Venus. Phosphine, or phosphorus trihydride, PH3, is a colourless, flammable, very toxic gas and the only known natural source of the gas (on Earth) is from bacteria (Morton et al. 2005). Phosphine is also present in the atmospheres of Saturn (Fletcher et al. 2007) and Jupiter (Irwin et al. 2004). The presence of this gas in Venus's clouds suggests that it might indicate the presence of microbiological life in the atmosphere of Venus. However, its presence has since been questioned. The original detection may be statistically unreliable or contaminated by a close-by line of SO2 (Villanueva et al. 2021, Snellen et al. 2020, Akins et al. 2021). In order to confirm or refute the claims of phosphine on Venus, an in situ measurement is needed. It therefore seems reasonable to include a sensor to detect the presence and concentration of phosphine in the design of future surface probes. However, even detecting its presence in the clouds of Venus, for certain, would not prove it was the product of biological processes; it could have a purely chemical or geochemical origin in the hot, high-pressure environment at the surface of the planet. Weather stations On Earth, AWSs consist of a collection of sensors (figure 3) and associated electronics such as a datalogger. These are strictly surface-based stations (Strangeways & Smith 1985). But measurements up through the atmosphere are also important. On Earth, radiosondes (figure 4) carried aloft by simple balloons (figure 5), make measurements of temperature, humidity, barometric pressure and wind up to the outer reaches of the atmosphere. Radiosondes are launched daily from many sites around the globe and are invaluable in weather forecasting. They are cheap and dispensable; their balloons burst upon reaching high altitude and they are rarely recovered. On Mars, there is so little atmosphere that most meteorological measurements are made just at the surface, although profiles up through the atmosphere are also measured by remote sensing (Gröller et al. 2016). 3 Open in new tabDownload slide A typical AWS on Earth (here shown operating in the Cairngorm mountains, Scotland). It has sensors for wind speed and wind direction, temperature and humidity, solar and net radiation. A rain gauge is out of shot. The antenna telemeters the measurements via Meteosat to a distant base. (I Strangeways) 3 Open in new tabDownload slide A typical AWS on Earth (here shown operating in the Cairngorm mountains, Scotland). It has sensors for wind speed and wind direction, temperature and humidity, solar and net radiation. A rain gauge is out of shot. The antenna telemeters the measurements via Meteosat to a distant base. (I Strangeways) 4 Open in new tabDownload slide A radiosonde, as used on Earth. The miniature temperature and humidity sensors are deployed on the small arm, upper right. Barometric pressure is measured internally. (I Strangeways) 4 Open in new tabDownload slide A radiosonde, as used on Earth. The miniature temperature and humidity sensors are deployed on the small arm, upper right. Barometric pressure is measured internally. (I Strangeways) 5 Open in new tabDownload slide A typical balloon used to carry radiosondes aloft. The balloons are zero-pressure, rubber, helium-filled and burst upon reaching a certain height. The one here is being launched at RAF Cardington, Bedfordshire. (I Strangeways) 5 Open in new tabDownload slide A typical balloon used to carry radiosondes aloft. The balloons are zero-pressure, rubber, helium-filled and burst upon reaching a certain height. The one here is being launched at RAF Cardington, Bedfordshire. (I Strangeways) On Venus, however, because of its highly dominant atmosphere, the full depth of the atmosphere becomes important in meteorological terms and measurements need to be made throughout its considerable depth, down to, and including, the surface. AWSs for Venus thus need to take a completely different form to those on Earth or Mars, with hybrid stations being more appropriate. These adopt a conventional set of ground-based AWS sensors, but carry them aloft by balloon, like a radiosonde, making measurements both at the surface and up through the full height of the atmosphere and in the clouds. Because of the very high temperature and barometric pressure at the surface, it is much more difficult to design instruments to operate continually at the surface on Venus. Indeed, it has so far proved impossible to get anything to operate there for more than about an hour before failing. Silicon-based electronics cannot function at Venus's surface temperatures, but silicon-carbide electronics can. Currently, however, only individual components are available, not the full complexity of a computer, although NASA is investigating the technology at the Glenn Research Centre (Neudeck et al. 2016). Future missions to Venus are unlikely to provide more than a brief period on the surface, so the best method of getting a successful longer-duration mission is to look upward, where cooler temperatures are to be found. Balloons for planetary exploration Balloons (and aircraft) have received much attention for planetary exploration in recent years, with NASA even operating a small helicopter on Mars. There are two types of basic balloon: the zero-pressure and super-pressure. In the former, the elastic envelope (generally rubber) is partly filled with a lifting gas (typically helium). At launch from the surface, the pressure inside and outside the balloon are equal. As the balloon rises it expands, the pressure difference remaining zero. Eventually, it can expand no further and the pressure inside the balloon starts to exceed that outside, and sooner or later it bursts, giving it a limited lifetime of just a few hours. This is the type of balloon used with radiosondes on Earth (figure 5). The super-pressure balloon has a tough inelastic envelope and is filled with a light gas to a pressure greater than that outside, as used by the two Vega missions in 1985 (figure 6). It cannot expand and settles at an altitude where lift equals weight and maintains this altitude, drifting with the wind. In the case of Vega, the balloons settled at 54 km altitude. Such balloons have a lifetime dependent mainly on the rate of gas leakage and can operate for months. In the case of Vega, the system operated for just a few days, limited not by balloon leakage but by battery life. 6 Open in new tabDownload slide A super-pressure balloon, as used on the Russian Vega 1 and 2 missions. It is the only balloon used so far on another planet. (G A Landis, CC BY 4.0) 6 Open in new tabDownload slide A super-pressure balloon, as used on the Russian Vega 1 and 2 missions. It is the only balloon used so far on another planet. (G A Landis, CC BY 4.0) However, such balloons are limited, in that the zero-pressure type has a lifetime limited to how long it survives before it bursts as it rises, and the super-pressure balloon to the height at which it can operate – the level at which balance is reached. Its lifetime is also limited by slow gas leakage. To explore Venus's atmosphere fully and over an extended time, it is necessary to be able to change altitude, up and down, not once but repeatedly, making measurements at a wide range of heights, including the surface. Balloons that can do this exist and have received attention of late by NASA. • The pumped-helium balloon employs two interconnected balloons (either one inside the other, or as two separate balloons), both filled with helium. Their combined buoyancy is varied by pumping helium from the super-pressure balloon (which acts as a constant-volume reservoir) into the main lifting (zero-pressure) balloon, changing its volume, and so its lift. The reverse process reverts to the earlier state. This allows the combined balloon arrangement to rise and fall as required (Hall et al. 2019). • The air-ballast balloon achieves changes in buoyancy by changing the overall weight of the system, unlike the pumped-helium combination, which changes its overall volume. This is achieved by combining a zero-pressure helium balloon as the lifting element, with a separate constant-volume super-pressure balloon. Air (the local atmosphere) is pumped into the super-pressure balloon or released to alter the weight (Hall et al. 2019). • The mechanical-compression balloon compresses the balloon, making it smaller and thereby reducing its lift. Releasing the compression increases the volume again, and so the lift. The balloon is made of super-pressure helium segments, connected together with a cable that squeezes them (Hall et al. 2019). These three types of balloon are illustrated in figure 7. They all have one disadvantage in common: they require power to pump or squeeze them. However, if the overall system, including the electronics and sensors, is powered by a radioisotope thermoelectric generator, this would not be a problem (see the section below on the gondola). 7 Open in new tabDownload slide (a) Pumped-helium balloon. (b) Pumped-air ballast balloon. (c) Mechanical-compression balloon. See text for details. (I Strangeways) 7 Open in new tabDownload slide (a) Pumped-helium balloon. (b) Pumped-air ballast balloon. (c) Mechanical-compression balloon. See text for details. (I Strangeways) • The reversible-fluid balloon (also known as a “phase-change” balloon) overcomes the need for power by deriving its energy (to change and control the altitude) from the temperature gradient of the atmosphere itself, from the hot surface to the cool upper reaches. The design of such a balloon can take several forms, but the basic principle is the same for them all (figure 8). In this basic design, the balloon is freely expandable (zero-pressure), filled with helium. The same balloon is also connected to a reservoir of fluid that vaporizes and condenses at the different temperatures and pressures of the venusian atmosphere. The reversible fluids that have been considered include water, freon, ethanol, methanol, acetone and pentane. In the case illustrated (figure 8), water is the choice. 8 Open in new tabDownload slide A phase-change balloon, illustrated here in its simplest design, using just one balloon. Other designs are possible, in which the phase-change vapour is contained in a second, separate balloon; the principle is the same. See text for details. (I Strangeways) 8 Open in new tabDownload slide A phase-change balloon, illustrated here in its simplest design, using just one balloon. Other designs are possible, in which the phase-change vapour is contained in a second, separate balloon; the principle is the same. See text for details. (I Strangeways) The system functions as follows. At the surface and in the lower atmosphere, water occurs in the vapour state and when injected into the balloon causes it to rise. When the balloon reaches a certain height (and so a certain temperature and pressure), the water vapour condenses and the balloon starts to fall. This balance point occurs at around 55 km altitude for water. As the balloon falls, the surrounding atmospheric temperature increases and the water re-evaporates, so the balloon rises again. By appropriate choice of fluid, with different boiling points, the altitude at which this changeover occurs can be controlled. If, however, the condensed liquid is fed into a thermally insulated container, evaporation does not automatically reoccur immediately as the balloon descends. By controlling the flow of the vapour back into the balloon, by means of the valve and the evaporator vessel (figure 8), the system can be adjusted to any required altitude, including the planet's surface. Other designs use a second balloon for the reversible fluid, the main lifting balloon being separate, containing just helium. But the principle is exactly the same. The two-balloon design might be a safer option, however, as the helium balloon remains permanently sealed, reducing the possibility of leakage. Although not yet attempted on other planets, reversible-fluid balloons have received much attention by NASA. The Alice project (Altitude Control Experiment) used a reversible-fluid balloon that was tested in Earth's atmosphere (Wu & Jones 1995, Jones 1995). The Valor project (Venus Atmospheric Long-duration Observatory for in-situ Research), was a study of what needs to be achieved in future Venus missions and the technology that will be required (Balint & Baines 2008). The use of balloons for planetary exploration is illustrated well and discussed in the book by Carroll (2011). Dorrington (2010) also explains the principles in good detail. The payload or gondola With an altitude-controlled balloon (of any of the types outlined above), an instrument package can be developed that would be suspended below the balloon as the payload or gondola. It could be “parked” at around 55 km altitude, in the benign cloud layers, making periodic excursions down to the surface, taking readings during its descent. It would then spend a brief time (an hour or less) at the surface making measurements and taking photographs and then rise back to its parking altitude to cool down and relay its recorded data to an orbiting spacecraft for transmission to Earth. At the surface, and during descent and ascent, measurements would be made of the major meteorological variables, notably temperature, wind speed and direction, barometric pressure, humidity, and solar and terrestrial radiation. Measurements could also include phosphine. Cameras would obtain images of the surface. Remaining on the surface only briefly, all the electronics, batteries and cameras would be protected from the high temperature and pressure by housing them in an insulated, pressure-resistant gondola, which would be made as small as possible. A suggested gondola design is illustrated in figure 9. Sensors that can withstand the high atmospheric pressure and temperature at the surface are housed on the outside of the shell, while those that cannot survive exposure are housed in the cool, insulated inner cavity, with appropriate links of cables and optical fibres to the outside environment. The battery could be solar-powered (not shown in the figure), recharged at the parking altitude where solar radiation levels are high, but it would be necessary to establish that the panels would survive exposure to the high surface temperature, which seems unlikely. It would be preferable, probably essential, to use a small radioisotope thermoelectric generator. In this case, it would also be possible to use a pumped-helium, air-ballast, or mechanical-compression balloon, rather than the phase-change design. It could be argued that this might be preferable, as it probably gives a more positive control to the height. But as the lifetime of any balloon system is likely to be in months, not years (as for Mars rovers), long-lasting radioisotope generators might not be appropriate, although perhaps inevitable and in most ways preferable. Such a mission is technically complex and no doubt expensive, but no more so than martian rovers. 9 Open in new tabDownload slide Design for a sensor gondola, to be carried by a balloon (figures 7 & 8) able to change altitude, repeatedly, from cloud level to the surface. Only sensors that have to be in direct contact with what they are measuring will be outside of the sphere (wind, temperature, phosphine and pressure). Those that can sense variables “remotely”, via optical links, can be housed in the cool interior (radiation, humidity and cameras). The mechanical wind sensors would not be adversely affected by the high temperature for short intervals, nor would the platinum resistance thermometer (within a radiation shield). The pressure sensor could be of a rugged mechanical (aneroid) design. While shown here as a flat disc, the sensors would be distributed over the whole surface of the sphere. (I Strangeways) 9 Open in new tabDownload slide Design for a sensor gondola, to be carried by a balloon (figures 7 & 8) able to change altitude, repeatedly, from cloud level to the surface. Only sensors that have to be in direct contact with what they are measuring will be outside of the sphere (wind, temperature, phosphine and pressure). Those that can sense variables “remotely”, via optical links, can be housed in the cool interior (radiation, humidity and cameras). The mechanical wind sensors would not be adversely affected by the high temperature for short intervals, nor would the platinum resistance thermometer (within a radiation shield). The pressure sensor could be of a rugged mechanical (aneroid) design. While shown here as a flat disc, the sensors would be distributed over the whole surface of the sphere. (I Strangeways) Concluding remarks Because of the harsh environment, weather measurements at the surface of Venus have so far been limited to one-off, brief visits of an hour or two, at just a few locations. Looking at the possible options currently available for the future-monitoring of Venus's weather and atmosphere, a hybrid design of station is suggested here, deploying a conventional set of ground-based AWS sensors on a gondola, suspended from a balloon that can raise and lower the station, repeatedly, through the full depth of the atmosphere, from cloud height to the surface. The balloon could be of the phase-change type or, if sufficient power is available, a pumped-helium, air-ballast or mechanical-compression design. This approach offers the possibility of missions lasting weeks or months, circumnavigating the planet many times and covering the full depth of the atmosphere, including brief descents to the surface at many points around the planet. Coverage would be greater than could be achieved by simpler balloon designs or by purely surface-based systems. Balloons are under consideration for Saturn's moon Titan (Paulken & Hall 2014), which has a thick atmosphere of nitrogen. The large gas planets with atmospheres of hydrogen would need different balloons – a Montgolfier “hot air” type (Jones & Heun 1997). Similar designs of station are, therefore, relevant to other planets and moons with substantial atmospheres, not just Venus. Certainly, balloon missions will be shorter-lived than surface-based rovers, but they will compensate for this by having a much larger geographical coverage and extend through the full depth of the atmosphere. Measuring the weather on Venus and Mars in greater detail than has been achieved so far would help us to understand more about Earth's own climate, by providing data from three very different, real-world environments, on three adjacent rocky planets of similar size to Earth, all orbiting close to the same star, each having extremely different atmospheres, weathers and climates. Arguably, a study of Venus's climate is more likely to yield practical and useful results than a single-minded focus on searching for signs of life beyond Earth, as important and as interesting as that undoubtedly is, especially in human emotional terms. The significance of understanding Earth's climate, however, is of real practical importance to humanity, and underlines the need for a “climate mission” to Venus. AUTHOR Open in new tabDownload slide Open in new tabDownload slide Dr Ian Strangeways is an environmental instrumentation consultant, director of TerraData and former head of applied physics at the Institute of Hydrology REFERENCES Akins AB et al. 2021 Astrophys. J. Lett. 907 L27 Crossref Search ADS Balint TS & Baines KH 2008 AGU Fall Meeting abstract id.P33A-1439 Carroll M 2011 Drifting on Alien Worlds ( Springer ) Google Scholar Crossref Search ADS Google Preview WorldCat COPAC Dorrington GE 2010 Adv. Space Res. 46 310 Crossref Search ADS Fletcher LN et al. 2007 Icarus 188 72 Crossref Search ADS Greaves JS et al. 2021 Nature Astron. 5 655 Crossref Search ADS Gröller H et al. 2016 47th Lunar and Planetary Science Conf., Houston 1811 Hall JL et al. 2019 AIAA Aviation Forum doi: 10.2514/6.2019-3194 Irwin PGJ et al. 2004 Icarus 172 37 Crossref Search ADS Jones JA 1995 11th Lighter-than-Air Systems Technology Conference 83 doi: 10.2514/6.1995-1621 Crossref Jones JA & Heun MK 1997 AIAA International Balloon Technology Conference 1 doi: 10.2514/6.1997-1445 Crossref Morton SC et al. 2005 Environ. Sci. Technol. 39 4369 Crossref Search ADS PubMed Neudeck PG et al. 2016 AIP Advances 6 125119 doi: 10.1063/1.4973429 Crossref Search ADS Paulken MT & Hall JL 2014 11th International Planetary Probe Workshop 8006 Snellen IAG et al. 2020 Astron. & Astrophys. 644 L2 Crossref Search ADS Strangeways IC & Smith SW 1985 Weather 40 277 Crossref Search ADS Strangeways IC 2014 Weather 69 8 Crossref Search ADS Strangeways IC 2021 Astron. & Geophys. 62 3.20 Crossref Search ADS Villanueva G et al. 2021 Nature Astron. 5 631 Crossref Search ADS Wu J-J & Jones JA 1995 11th Lighter-than-Air Systems Technology Conference 31 hdl.handle.net/2014/30075 © 2021 Royal Astronomical Society This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Astronomy & Geophysics Oxford University Press

Weather stations for Venus

Strangeways, Ian
Astronomy & Geophysics , Volume 62 (5) – Oct 1, 2021
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Oxford University Press
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Copyright © 2021 The Royal Astronomical Society
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1366-8781
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10.1093/astrogeo/atab088
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Abstract

Abstract Ian Strangeways looks at the difficulty of operating meteorological instruments on Venus due to its harsh environment, and suggests some possible solutions In Greco-Roman mythology, Venus is the goddess of love, while Mars is the god of war. In reality, it might be argued (with our better astronomical knowledge) that the reverse is closer to reality. Venus is a harsh, dangerous place, while Mars is much more peaceful. Humans may be walking on Mars soon, while we are unlikely ever to be able to walk on Venus. Venus is easier to get to than Mars because it is nearer (40 million km compared to 62 million km for Mars – when at their closest to Earth). It is also closer to the Sun than Earth, allowing easier trajectories for spacecraft, reducing fuel demands and travel time (3.5 months compared to 5 months for Mars). But here Venus's advantages end, for while it is almost the same size and density as Earth, its atmosphere is extremely different to that of Mars and Earth (and Mercury, which has virtually no atmosphere). In contrast, the atmosphere on Venus is extremely hostile, being dense, with a surface barometric pressure around 90 times that of Earth. The atmosphere is composed mostly of carbon dioxide, causing an immensely strong greenhouse effect, leading to a surface temperature of around 450°C. This makes the design of surface-based automatic weather stations (AWSs) for Venus much more problematic than AWSs for operation on Earth or Mars (Strangeways & Smith 1985, Strangeways 2014, 2021). As a saving grace, however, high above the surface of Venus the atmospheric temperature is sufficiently low to cause condensation and clouds to form, although the clouds are of sulphuric acid rather than water. The cloud layers extend from around 45 km to 75 km above the surface, completely cloaking the planet. At an altitude of 55 km the temperature is only 20°C and the pressure about 0.5 bar (figure 2), representing an environment conducive to instruments and robotic vehicles such as balloons and airplanes. This offers a potential solution to operating AWSs on Venus and this article looks at some designs that could withstand that harsh environment. Measurements of the weather on Venus by space probes to date are summarized in table 1. Figure 1 shows an artist's impression of the Russian Venera 13 lander on Venus, which successfully measured surface temperature and barometric pressure in 1981. 1 Open in new tabDownload slide A 3D rendering of the Venera 13 spacecraft on the surface of Venus. (Russian Academy of Sciences/M Malmer, CC BY 3.0) 1 Open in new tabDownload slide A 3D rendering of the Venera 13 spacecraft on the surface of Venus. (Russian Academy of Sciences/M Malmer, CC BY 3.0) 2 Open in new tabDownload slide A cross-section of Venus's atmosphere, showing the levels of clouds and the variation of temperature and barometric pressure with altitude. (A Gunn) 2 Open in new tabDownload slide A cross-section of Venus's atmosphere, showing the levels of clouds and the variation of temperature and barometric pressure with altitude. (A Gunn) Table 1 Venus weather missions mission . operator . operating . type . instruments and measurements . Venera 1–16 USSR/Roscosmos 1961–83 landers landing probes successful from Venera 8 onwards operated for only an hour or so due to high temperature and pressure measured temperature around 450°C, and pressure of 90 Earth atmospheres Pioneer Venus 1 & 2 USA/NASA 1978–92 orbiter landers Pioneer Venus Orbiter mapped the surface by radar, measured clouds, solar wind and magnetic field Pioneer Venus Multiprobe inserted four probes that measured temperature and pressures as they descended not designed to survive landing; confirmed the earlier Russian values Vega 1 & 2 USSR/Roscosmos 1985 landers balloons same as Venera Landers Teflon 3.4 m super-pressure balloons stabilized at 54 km altitude, measured temperature of 27°C and pressure of 535 hPa windspeeds of 100 m s−1 measured, compared with 5 m s−1 at the surface Magellan USA/NASA 1990–94 orbiter known as the Venus Radar Mapper mapped the surface in great detail Venus Express Europe/ESA 2006–14 orbiter made extensive measurements of the atmosphere Akatsuki Japan/JAXA 2015– orbiter five cameras working at different wavelengths remote study of the atmosphere mission . operator . operating . type . instruments and measurements . Venera 1–16 USSR/Roscosmos 1961–83 landers landing probes successful from Venera 8 onwards operated for only an hour or so due to high temperature and pressure measured temperature around 450°C, and pressure of 90 Earth atmospheres Pioneer Venus 1 & 2 USA/NASA 1978–92 orbiter landers Pioneer Venus Orbiter mapped the surface by radar, measured clouds, solar wind and magnetic field Pioneer Venus Multiprobe inserted four probes that measured temperature and pressures as they descended not designed to survive landing; confirmed the earlier Russian values Vega 1 & 2 USSR/Roscosmos 1985 landers balloons same as Venera Landers Teflon 3.4 m super-pressure balloons stabilized at 54 km altitude, measured temperature of 27°C and pressure of 535 hPa windspeeds of 100 m s−1 measured, compared with 5 m s−1 at the surface Magellan USA/NASA 1990–94 orbiter known as the Venus Radar Mapper mapped the surface in great detail Venus Express Europe/ESA 2006–14 orbiter made extensive measurements of the atmosphere Akatsuki Japan/JAXA 2015– orbiter five cameras working at different wavelengths remote study of the atmosphere The efforts made so far to measure the weather on Venus. There have been many missions to Venus, starting in 1961: 29 by the USSR (Roscosmos), 11 by the USA (NASA), three by Europe (ESA), and three by Japan (JAXA/UNISEC). Of these 46 missions, 29 were successful. The Soviet Venera programme, 1961–84, is one of the largest efforts ever undertaken to study another planet. Much of what is known today about Venus was discovered by these missions. But the most recent data to be returned from within the atmosphere date back to 1985. Since then, there have been no further Russian missions. Five of the 11 American missions were made post-1985, although four were simply gravity-assist fly-bys, just one being an orbiter (Magellan). Of ESA's three missions, two were gravity-assist, one, Venus Express, being an orbiter, making measurements of the atmosphere remotely. Unlike on Mars (Strangeways 2021), most measurements within Venus's atmosphere have, so far, been restricted to single brief descents, with just a short time at the surface; in-all, just a few hours of data-collection. The only exception was Vega 1 and 2, which in 1984 deployed balloons that operated within the cloud layers, but only for a few days (until their batteries ran out). At the time of writing, there are 12 proposals for the future, including a feasibility study for a rover by NASA in 2039. Under development is an orbiter for 2023 by ISRO (Indian Space Research Organisation), and an orbiter (and lander?) by the Russian Roscosmos for 2026. Open in new tab Table 1 Venus weather missions mission . operator . operating . type . instruments and measurements . Venera 1–16 USSR/Roscosmos 1961–83 landers landing probes successful from Venera 8 onwards operated for only an hour or so due to high temperature and pressure measured temperature around 450°C, and pressure of 90 Earth atmospheres Pioneer Venus 1 & 2 USA/NASA 1978–92 orbiter landers Pioneer Venus Orbiter mapped the surface by radar, measured clouds, solar wind and magnetic field Pioneer Venus Multiprobe inserted four probes that measured temperature and pressures as they descended not designed to survive landing; confirmed the earlier Russian values Vega 1 & 2 USSR/Roscosmos 1985 landers balloons same as Venera Landers Teflon 3.4 m super-pressure balloons stabilized at 54 km altitude, measured temperature of 27°C and pressure of 535 hPa windspeeds of 100 m s−1 measured, compared with 5 m s−1 at the surface Magellan USA/NASA 1990–94 orbiter known as the Venus Radar Mapper mapped the surface in great detail Venus Express Europe/ESA 2006–14 orbiter made extensive measurements of the atmosphere Akatsuki Japan/JAXA 2015– orbiter five cameras working at different wavelengths remote study of the atmosphere mission . operator . operating . type . instruments and measurements . Venera 1–16 USSR/Roscosmos 1961–83 landers landing probes successful from Venera 8 onwards operated for only an hour or so due to high temperature and pressure measured temperature around 450°C, and pressure of 90 Earth atmospheres Pioneer Venus 1 & 2 USA/NASA 1978–92 orbiter landers Pioneer Venus Orbiter mapped the surface by radar, measured clouds, solar wind and magnetic field Pioneer Venus Multiprobe inserted four probes that measured temperature and pressures as they descended not designed to survive landing; confirmed the earlier Russian values Vega 1 & 2 USSR/Roscosmos 1985 landers balloons same as Venera Landers Teflon 3.4 m super-pressure balloons stabilized at 54 km altitude, measured temperature of 27°C and pressure of 535 hPa windspeeds of 100 m s−1 measured, compared with 5 m s−1 at the surface Magellan USA/NASA 1990–94 orbiter known as the Venus Radar Mapper mapped the surface in great detail Venus Express Europe/ESA 2006–14 orbiter made extensive measurements of the atmosphere Akatsuki Japan/JAXA 2015– orbiter five cameras working at different wavelengths remote study of the atmosphere The efforts made so far to measure the weather on Venus. There have been many missions to Venus, starting in 1961: 29 by the USSR (Roscosmos), 11 by the USA (NASA), three by Europe (ESA), and three by Japan (JAXA/UNISEC). Of these 46 missions, 29 were successful. The Soviet Venera programme, 1961–84, is one of the largest efforts ever undertaken to study another planet. Much of what is known today about Venus was discovered by these missions. But the most recent data to be returned from within the atmosphere date back to 1985. Since then, there have been no further Russian missions. Five of the 11 American missions were made post-1985, although four were simply gravity-assist fly-bys, just one being an orbiter (Magellan). Of ESA's three missions, two were gravity-assist, one, Venus Express, being an orbiter, making measurements of the atmosphere remotely. Unlike on Mars (Strangeways 2021), most measurements within Venus's atmosphere have, so far, been restricted to single brief descents, with just a short time at the surface; in-all, just a few hours of data-collection. The only exception was Vega 1 and 2, which in 1984 deployed balloons that operated within the cloud layers, but only for a few days (until their batteries ran out). At the time of writing, there are 12 proposals for the future, including a feasibility study for a rover by NASA in 2039. Under development is an orbiter for 2023 by ISRO (Indian Space Research Organisation), and an orbiter (and lander?) by the Russian Roscosmos for 2026. Open in new tab Phosphine It was suggested recently (Greaves et al. 2021), following the analysis of data from Earth-based radio telescopes, that traces of phosphine may be present in the atmosphere of Venus. Phosphine, or phosphorus trihydride, PH3, is a colourless, flammable, very toxic gas and the only known natural source of the gas (on Earth) is from bacteria (Morton et al. 2005). Phosphine is also present in the atmospheres of Saturn (Fletcher et al. 2007) and Jupiter (Irwin et al. 2004). The presence of this gas in Venus's clouds suggests that it might indicate the presence of microbiological life in the atmosphere of Venus. However, its presence has since been questioned. The original detection may be statistically unreliable or contaminated by a close-by line of SO2 (Villanueva et al. 2021, Snellen et al. 2020, Akins et al. 2021). In order to confirm or refute the claims of phosphine on Venus, an in situ measurement is needed. It therefore seems reasonable to include a sensor to detect the presence and concentration of phosphine in the design of future surface probes. However, even detecting its presence in the clouds of Venus, for certain, would not prove it was the product of biological processes; it could have a purely chemical or geochemical origin in the hot, high-pressure environment at the surface of the planet. Weather stations On Earth, AWSs consist of a collection of sensors (figure 3) and associated electronics such as a datalogger. These are strictly surface-based stations (Strangeways & Smith 1985). But measurements up through the atmosphere are also important. On Earth, radiosondes (figure 4) carried aloft by simple balloons (figure 5), make measurements of temperature, humidity, barometric pressure and wind up to the outer reaches of the atmosphere. Radiosondes are launched daily from many sites around the globe and are invaluable in weather forecasting. They are cheap and dispensable; their balloons burst upon reaching high altitude and they are rarely recovered. On Mars, there is so little atmosphere that most meteorological measurements are made just at the surface, although profiles up through the atmosphere are also measured by remote sensing (Gröller et al. 2016). 3 Open in new tabDownload slide A typical AWS on Earth (here shown operating in the Cairngorm mountains, Scotland). It has sensors for wind speed and wind direction, temperature and humidity, solar and net radiation. A rain gauge is out of shot. The antenna telemeters the measurements via Meteosat to a distant base. (I Strangeways) 3 Open in new tabDownload slide A typical AWS on Earth (here shown operating in the Cairngorm mountains, Scotland). It has sensors for wind speed and wind direction, temperature and humidity, solar and net radiation. A rain gauge is out of shot. The antenna telemeters the measurements via Meteosat to a distant base. (I Strangeways) 4 Open in new tabDownload slide A radiosonde, as used on Earth. The miniature temperature and humidity sensors are deployed on the small arm, upper right. Barometric pressure is measured internally. (I Strangeways) 4 Open in new tabDownload slide A radiosonde, as used on Earth. The miniature temperature and humidity sensors are deployed on the small arm, upper right. Barometric pressure is measured internally. (I Strangeways) 5 Open in new tabDownload slide A typical balloon used to carry radiosondes aloft. The balloons are zero-pressure, rubber, helium-filled and burst upon reaching a certain height. The one here is being launched at RAF Cardington, Bedfordshire. (I Strangeways) 5 Open in new tabDownload slide A typical balloon used to carry radiosondes aloft. The balloons are zero-pressure, rubber, helium-filled and burst upon reaching a certain height. The one here is being launched at RAF Cardington, Bedfordshire. (I Strangeways) On Venus, however, because of its highly dominant atmosphere, the full depth of the atmosphere becomes important in meteorological terms and measurements need to be made throughout its considerable depth, down to, and including, the surface. AWSs for Venus thus need to take a completely different form to those on Earth or Mars, with hybrid stations being more appropriate. These adopt a conventional set of ground-based AWS sensors, but carry them aloft by balloon, like a radiosonde, making measurements both at the surface and up through the full height of the atmosphere and in the clouds. Because of the very high temperature and barometric pressure at the surface, it is much more difficult to design instruments to operate continually at the surface on Venus. Indeed, it has so far proved impossible to get anything to operate there for more than about an hour before failing. Silicon-based electronics cannot function at Venus's surface temperatures, but silicon-carbide electronics can. Currently, however, only individual components are available, not the full complexity of a computer, although NASA is investigating the technology at the Glenn Research Centre (Neudeck et al. 2016). Future missions to Venus are unlikely to provide more than a brief period on the surface, so the best method of getting a successful longer-duration mission is to look upward, where cooler temperatures are to be found. Balloons for planetary exploration Balloons (and aircraft) have received much attention for planetary exploration in recent years, with NASA even operating a small helicopter on Mars. There are two types of basic balloon: the zero-pressure and super-pressure. In the former, the elastic envelope (generally rubber) is partly filled with a lifting gas (typically helium). At launch from the surface, the pressure inside and outside the balloon are equal. As the balloon rises it expands, the pressure difference remaining zero. Eventually, it can expand no further and the pressure inside the balloon starts to exceed that outside, and sooner or later it bursts, giving it a limited lifetime of just a few hours. This is the type of balloon used with radiosondes on Earth (figure 5). The super-pressure balloon has a tough inelastic envelope and is filled with a light gas to a pressure greater than that outside, as used by the two Vega missions in 1985 (figure 6). It cannot expand and settles at an altitude where lift equals weight and maintains this altitude, drifting with the wind. In the case of Vega, the balloons settled at 54 km altitude. Such balloons have a lifetime dependent mainly on the rate of gas leakage and can operate for months. In the case of Vega, the system operated for just a few days, limited not by balloon leakage but by battery life. 6 Open in new tabDownload slide A super-pressure balloon, as used on the Russian Vega 1 and 2 missions. It is the only balloon used so far on another planet. (G A Landis, CC BY 4.0) 6 Open in new tabDownload slide A super-pressure balloon, as used on the Russian Vega 1 and 2 missions. It is the only balloon used so far on another planet. (G A Landis, CC BY 4.0) However, such balloons are limited, in that the zero-pressure type has a lifetime limited to how long it survives before it bursts as it rises, and the super-pressure balloon to the height at which it can operate – the level at which balance is reached. Its lifetime is also limited by slow gas leakage. To explore Venus's atmosphere fully and over an extended time, it is necessary to be able to change altitude, up and down, not once but repeatedly, making measurements at a wide range of heights, including the surface. Balloons that can do this exist and have received attention of late by NASA. • The pumped-helium balloon employs two interconnected balloons (either one inside the other, or as two separate balloons), both filled with helium. Their combined buoyancy is varied by pumping helium from the super-pressure balloon (which acts as a constant-volume reservoir) into the main lifting (zero-pressure) balloon, changing its volume, and so its lift. The reverse process reverts to the earlier state. This allows the combined balloon arrangement to rise and fall as required (Hall et al. 2019). • The air-ballast balloon achieves changes in buoyancy by changing the overall weight of the system, unlike the pumped-helium combination, which changes its overall volume. This is achieved by combining a zero-pressure helium balloon as the lifting element, with a separate constant-volume super-pressure balloon. Air (the local atmosphere) is pumped into the super-pressure balloon or released to alter the weight (Hall et al. 2019). • The mechanical-compression balloon compresses the balloon, making it smaller and thereby reducing its lift. Releasing the compression increases the volume again, and so the lift. The balloon is made of super-pressure helium segments, connected together with a cable that squeezes them (Hall et al. 2019). These three types of balloon are illustrated in figure 7. They all have one disadvantage in common: they require power to pump or squeeze them. However, if the overall system, including the electronics and sensors, is powered by a radioisotope thermoelectric generator, this would not be a problem (see the section below on the gondola). 7 Open in new tabDownload slide (a) Pumped-helium balloon. (b) Pumped-air ballast balloon. (c) Mechanical-compression balloon. See text for details. (I Strangeways) 7 Open in new tabDownload slide (a) Pumped-helium balloon. (b) Pumped-air ballast balloon. (c) Mechanical-compression balloon. See text for details. (I Strangeways) • The reversible-fluid balloon (also known as a “phase-change” balloon) overcomes the need for power by deriving its energy (to change and control the altitude) from the temperature gradient of the atmosphere itself, from the hot surface to the cool upper reaches. The design of such a balloon can take several forms, but the basic principle is the same for them all (figure 8). In this basic design, the balloon is freely expandable (zero-pressure), filled with helium. The same balloon is also connected to a reservoir of fluid that vaporizes and condenses at the different temperatures and pressures of the venusian atmosphere. The reversible fluids that have been considered include water, freon, ethanol, methanol, acetone and pentane. In the case illustrated (figure 8), water is the choice. 8 Open in new tabDownload slide A phase-change balloon, illustrated here in its simplest design, using just one balloon. Other designs are possible, in which the phase-change vapour is contained in a second, separate balloon; the principle is the same. See text for details. (I Strangeways) 8 Open in new tabDownload slide A phase-change balloon, illustrated here in its simplest design, using just one balloon. Other designs are possible, in which the phase-change vapour is contained in a second, separate balloon; the principle is the same. See text for details. (I Strangeways) The system functions as follows. At the surface and in the lower atmosphere, water occurs in the vapour state and when injected into the balloon causes it to rise. When the balloon reaches a certain height (and so a certain temperature and pressure), the water vapour condenses and the balloon starts to fall. This balance point occurs at around 55 km altitude for water. As the balloon falls, the surrounding atmospheric temperature increases and the water re-evaporates, so the balloon rises again. By appropriate choice of fluid, with different boiling points, the altitude at which this changeover occurs can be controlled. If, however, the condensed liquid is fed into a thermally insulated container, evaporation does not automatically reoccur immediately as the balloon descends. By controlling the flow of the vapour back into the balloon, by means of the valve and the evaporator vessel (figure 8), the system can be adjusted to any required altitude, including the planet's surface. Other designs use a second balloon for the reversible fluid, the main lifting balloon being separate, containing just helium. But the principle is exactly the same. The two-balloon design might be a safer option, however, as the helium balloon remains permanently sealed, reducing the possibility of leakage. Although not yet attempted on other planets, reversible-fluid balloons have received much attention by NASA. The Alice project (Altitude Control Experiment) used a reversible-fluid balloon that was tested in Earth's atmosphere (Wu & Jones 1995, Jones 1995). The Valor project (Venus Atmospheric Long-duration Observatory for in-situ Research), was a study of what needs to be achieved in future Venus missions and the technology that will be required (Balint & Baines 2008). The use of balloons for planetary exploration is illustrated well and discussed in the book by Carroll (2011). Dorrington (2010) also explains the principles in good detail. The payload or gondola With an altitude-controlled balloon (of any of the types outlined above), an instrument package can be developed that would be suspended below the balloon as the payload or gondola. It could be “parked” at around 55 km altitude, in the benign cloud layers, making periodic excursions down to the surface, taking readings during its descent. It would then spend a brief time (an hour or less) at the surface making measurements and taking photographs and then rise back to its parking altitude to cool down and relay its recorded data to an orbiting spacecraft for transmission to Earth. At the surface, and during descent and ascent, measurements would be made of the major meteorological variables, notably temperature, wind speed and direction, barometric pressure, humidity, and solar and terrestrial radiation. Measurements could also include phosphine. Cameras would obtain images of the surface. Remaining on the surface only briefly, all the electronics, batteries and cameras would be protected from the high temperature and pressure by housing them in an insulated, pressure-resistant gondola, which would be made as small as possible. A suggested gondola design is illustrated in figure 9. Sensors that can withstand the high atmospheric pressure and temperature at the surface are housed on the outside of the shell, while those that cannot survive exposure are housed in the cool, insulated inner cavity, with appropriate links of cables and optical fibres to the outside environment. The battery could be solar-powered (not shown in the figure), recharged at the parking altitude where solar radiation levels are high, but it would be necessary to establish that the panels would survive exposure to the high surface temperature, which seems unlikely. It would be preferable, probably essential, to use a small radioisotope thermoelectric generator. In this case, it would also be possible to use a pumped-helium, air-ballast, or mechanical-compression balloon, rather than the phase-change design. It could be argued that this might be preferable, as it probably gives a more positive control to the height. But as the lifetime of any balloon system is likely to be in months, not years (as for Mars rovers), long-lasting radioisotope generators might not be appropriate, although perhaps inevitable and in most ways preferable. Such a mission is technically complex and no doubt expensive, but no more so than martian rovers. 9 Open in new tabDownload slide Design for a sensor gondola, to be carried by a balloon (figures 7 & 8) able to change altitude, repeatedly, from cloud level to the surface. Only sensors that have to be in direct contact with what they are measuring will be outside of the sphere (wind, temperature, phosphine and pressure). Those that can sense variables “remotely”, via optical links, can be housed in the cool interior (radiation, humidity and cameras). The mechanical wind sensors would not be adversely affected by the high temperature for short intervals, nor would the platinum resistance thermometer (within a radiation shield). The pressure sensor could be of a rugged mechanical (aneroid) design. While shown here as a flat disc, the sensors would be distributed over the whole surface of the sphere. (I Strangeways) 9 Open in new tabDownload slide Design for a sensor gondola, to be carried by a balloon (figures 7 & 8) able to change altitude, repeatedly, from cloud level to the surface. Only sensors that have to be in direct contact with what they are measuring will be outside of the sphere (wind, temperature, phosphine and pressure). Those that can sense variables “remotely”, via optical links, can be housed in the cool interior (radiation, humidity and cameras). The mechanical wind sensors would not be adversely affected by the high temperature for short intervals, nor would the platinum resistance thermometer (within a radiation shield). The pressure sensor could be of a rugged mechanical (aneroid) design. While shown here as a flat disc, the sensors would be distributed over the whole surface of the sphere. (I Strangeways) Concluding remarks Because of the harsh environment, weather measurements at the surface of Venus have so far been limited to one-off, brief visits of an hour or two, at just a few locations. Looking at the possible options currently available for the future-monitoring of Venus's weather and atmosphere, a hybrid design of station is suggested here, deploying a conventional set of ground-based AWS sensors on a gondola, suspended from a balloon that can raise and lower the station, repeatedly, through the full depth of the atmosphere, from cloud height to the surface. The balloon could be of the phase-change type or, if sufficient power is available, a pumped-helium, air-ballast or mechanical-compression design. This approach offers the possibility of missions lasting weeks or months, circumnavigating the planet many times and covering the full depth of the atmosphere, including brief descents to the surface at many points around the planet. Coverage would be greater than could be achieved by simpler balloon designs or by purely surface-based systems. Balloons are under consideration for Saturn's moon Titan (Paulken & Hall 2014), which has a thick atmosphere of nitrogen. The large gas planets with atmospheres of hydrogen would need different balloons – a Montgolfier “hot air” type (Jones & Heun 1997). Similar designs of station are, therefore, relevant to other planets and moons with substantial atmospheres, not just Venus. Certainly, balloon missions will be shorter-lived than surface-based rovers, but they will compensate for this by having a much larger geographical coverage and extend through the full depth of the atmosphere. Measuring the weather on Venus and Mars in greater detail than has been achieved so far would help us to understand more about Earth's own climate, by providing data from three very different, real-world environments, on three adjacent rocky planets of similar size to Earth, all orbiting close to the same star, each having extremely different atmospheres, weathers and climates. Arguably, a study of Venus's climate is more likely to yield practical and useful results than a single-minded focus on searching for signs of life beyond Earth, as important and as interesting as that undoubtedly is, especially in human emotional terms. The significance of understanding Earth's climate, however, is of real practical importance to humanity, and underlines the need for a “climate mission” to Venus. AUTHOR Open in new tabDownload slide Open in new tabDownload slide Dr Ian Strangeways is an environmental instrumentation consultant, director of TerraData and former head of applied physics at the Institute of Hydrology REFERENCES Akins AB et al. 2021 Astrophys. J. Lett. 907 L27 Crossref Search ADS Balint TS & Baines KH 2008 AGU Fall Meeting abstract id.P33A-1439 Carroll M 2011 Drifting on Alien Worlds ( Springer ) Google Scholar Crossref Search ADS Google Preview WorldCat COPAC Dorrington GE 2010 Adv. 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Astronomy & Geophysics – Oxford University Press

Published: Oct 1, 2021

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