Venus

Venus is the nearest planet, it is nearly Earth’s twin in size and mass, and it is by a wide margin the most hostile surface humans have ever landed hardware on. It was intensively explored in the 1970s and 1980s, then largely abandoned for thirty years, and it is now the subject of three approved missions.

The surface environment

Ninety-two bar is the pressure at about 900 m depth in Earth’s oceans. A lander is therefore a submersible as much as a spacecraft, and its pressure vessel is the dominant mass item.

The temperature is the harder constraint. At 464 °C, conventional silicon semiconductors stop working: junction leakage rises exponentially and the devices simply cease to function well below that temperature. There is no shielding solution, because heat conducts in and the interior has nowhere to reject it to. The environment is hotter than the spacecraft, so passive thermal control cannot work.

The three approaches are all unsatisfying. Thermal inertia: build a heavily insulated, pre-chilled pressure vessel and accept that it will heat up, giving a mission measured in hours. Active refrigeration: pump heat from the interior to a hotter exterior, which is thermodynamically expensive and has never flown at Venus. High-temperature electronics: silicon carbide devices that operate at 500 °C have been demonstrated in laboratory chambers for weeks, and represent the only route to a long-lived surface mission.

The Venera record

The Soviet Venera programme remains the only successful surface exploration of Venus, and the results were hard won.

One hundred and twenty-seven minutes is the longest anything has operated on the Venusian surface, achieved more than forty years ago. That single number frames the difficulty better than any description.

The Vega balloons are the underappreciated result. Floating at about 54 km, where conditions are far milder, they operated for two days and demonstrated that the atmosphere, rather than the surface, is where a long-duration mission is practical.

Radar mapping

The permanent cloud cover makes optical mapping impossible, so Venus was mapped by radar. Magellan, between 1990 and 1994, imaged 98% of the surface at about 100 m resolution using synthetic aperture radar, and remains the basis of Venus geology. See Earth observation for how the technique works.

What it revealed was a surface with very few impact craters, and those it has are distributed almost randomly rather than clustered by age. The standard interpretation is that the entire surface is geologically young, perhaps 300 to 600 million years, and was resurfaced globally. Whether that happened in a single catastrophic event or through continuous volcanic activity is unresolved and is one of the main questions the new missions are meant to answer.

Recent reanalysis of Magellan data has identified surface changes between observation cycles that appear to be active volcanism, which would make Venus the second volcanically active body in the inner solar system.

The cloud layer

At 50 to 60 km altitude, conditions are remarkable:

Apart from the sulfuric acid, this is the most Earth-like environment anywhere off Earth. Breathable air is a lifting gas in a carbon dioxide atmosphere, so a habitat filled with ordinary air floats without any additional buoyancy system. This is the basis of the crewed aerial platform concepts that resurface periodically.

The acid is a real obstacle rather than a detail. Concentrated sulfuric acid droplets attack most polymers and many metals, so envelope materials are the limiting technology for any long-duration aerial platform.

The phosphine episode

In 2020 a team reported detection of phosphine in the Venusian cloud layer, suggesting a possible biological source since no known abiotic process on Venus was thought to produce it in quantity.

The claim has been substantially disputed. Reanalysis of the same data using different baseline-fitting produced weaker or absent signals, and the spectral feature is close to a sulfur dioxide line that is abundant in the atmosphere. Subsequent observations have been inconsistent.

The episode is worth recording for two reasons rather than one. It illustrates how much of an extraordinary claim rests on data reduction choices that are invisible in the headline. And regardless of whether it holds up, it demonstrably redirected attention and funding back to a planet that had been neglected for three decades.

The returning missions

The scientific case rests on a comparative planetology argument. Venus and Earth formed in similar places from similar material at similar sizes, and one became habitable while the other became an inferno. Understanding the divergence bears directly on which of the many rocky exoplanets now being catalogued are likely to be habitable, which connects Venus science to exoplanet research more than to anything else in the solar system.

The noble gas isotope measurements DAVINCI will make are the specific discriminator. Isotope ratios record whether Venus lost a substantial water inventory to space, and if so, when.

References

• Taylor, F. W. The Scientific Exploration of Venus, Cambridge University Press, 2014.
• Marov, M. Y. and Grinspoon, D. H. The Planet Venus, Yale University Press, 1998. Includes a detailed account of the Venera programme.
• Saunders, R. S. et al. “Magellan mission summary”, Journal of Geophysical Research, 97:13067, 1992.
• Greaves, J. S. et al. “Phosphine gas in the cloud decks of Venus”, Nature Astronomy, 5:655, 2021, together with the several published reanalyses disputing it.
• Herrick, R. R. and Hensley, S. “Surface changes observed on a Venusian volcano during the Magellan mission”, Science, 379:1205, 2023.
• Neveu, M. et al. “The DAVINCI mission to Venus”, and Smrekar, S. et al. on VERITAS, The Planetary Science Journal, 2022 onward.