There is a compelling motivation of great global concern for exploring Venus: As we discover how climate and geology work on a world similar to our own, we gain a deeper understanding of the processes at work in our own environment.
With the realization that the Earth’s climate system is not sufficiently well understood, and the threat of accelerating anthropogenic changes to the atmosphere, comes a valid concern about the natural vulnerability of the world in which we have thrived. What are the limits of stability of the global system under the influence of human consumption and effluent? Could rapid or irreversible changes be triggered by the current unprecedented pace of greenhouse gas input to the atmosphere? Are there climate tipping points beyond which there is no return? To this last question, because of planetary exploration, we know the answer. Yes. Venus’ oceans boiled away in a dramatic runaway greenhouse and were eventually lost to space. If this happened to Venus, could it happen to Earth? Again, the answer is yes. Earth will someday pass the tipping point, its oceans will boil, and a desiccated, hot Earth will be like Venus today. We know this because main sequence stars like the Sun slowly increase in luminosity as their fuel is used up. Subtler discontinuities in climate, with real consequences for society, are certainly possible and climate feedbacks that might be difficult to discern in the Earth system might be illuminated by the deeper understanding of planetary climate gained by studying the climate of Venus.
Our great progress in exploring Mars illustrates how in-depth exploration of a nearby terrestrial planet can successfully illuminate Earth processes. Mars’s dynamic surface, accessible to our eyes for centuries and comparatively benign as an environment for spacecraft exploration, has revealed how physics and chemistry have shaped another rocky world. This cold, dry planet has a history of water, climate, and potential habitability starkly different from our own. Other planets will, of course, offer radically different comparisons. Venus, too, we believe, had early oceans but lost this habitable environment for completely different reasons. Verifying and quantifying this story will immensely improve our understanding of how Earth-like worlds come to be and how they might evolve to either encourage life or extinguish it. More immediately, the nature of climate feedbacks that might ultimately determine the physical safety and economic security of society must be understood. Some of the most revealing secrets to the formation of the solar system, the evolution of climate on our own planet, and the habitability of terrestrial planets around other stars can be found only on Venus. But the searching is difficult: Venus’ obscuring cloud layer and hostile environment have made it a challenging planet to explore.
Nevertheless, many of the scientific investigations that should be done to understand the Venus system and relate those results to our own world can be achieved by a flagship mission to Venus.