By Stephen Kane
What exactly is planetary habitability? This is a question that is currently both difficult and divisive to answer. The problematic nature of answering a seemingly simple question threatens to truncate our ability to effectively search for habitable environments beyond the Solar System. It may be considered by some that a major source for the stigma surrounding the question is the notion that we only have a single data point from which to construct our response. However, that is not the case. It never has been.
The truth of the matter is that we focus with such scrutiny on what habitability is that we forget to ask what it isn’t. Furthermore, we frequently neglect the axis of time, and how planetary evolution shapes the environments we observe at the present epoch. For better or worse, the anthropic nature of our guiding questions regarding life in the universe means that the search for other worlds will always target Earth-size planets. So what available data do we really have regarding the evolution of Earth-size planets, and how representative might that be for similar sized planets of arbitrary age around other stars?
We are exceptionally fortunate that we do indeed have another example of an Earth-sized planet right here in our planetary system: Venus. That need not have been the case. After all, Mars has no similar sized analogs within the Solar System, nor does Mercury. Venus and Earth, not just at the present epoch, but through their entire history, are the keys to understanding the detailed system (interior, surface, and atmosphere) evolution of temperate surface environments. The reason this is so crucial is because of the sobering fact that we will never have in-situ data for an exoplanet. We will never have a lander for TRAPPIST-1e, nor an orbiter for Proxima Centauri b, by definition the nearest exoplanet. Indeed, even a direct image for a terrestrial exoplanet remains decades out of our reach. That means that all of our inferences of surface conditions will be based upon models that have been constructed from the data available to us in our Solar System. Therefore, if our Solar System data is either incorrect or incomplete, that has the potential to translate into catastrophically incorrect conclusions regarding planets that we cannot see. If we are to have confidence in our remote sensing capabilities, then we must get it right for the nearest Earth-sized planet.
The newly announced missions to Venus – DAVINCI+, VERITAS, and EnVision – are the first steps in correcting the embarrassing holes in our knowledge for a sibling whom we barely recognize. The missions are complementary: DAVINCI+ will study the atmosphere, whilst VERITAS will investigate the surface and interior. Meanwhile, EnVision will map the surface, with particular attention to the “tesserae”, revealing basaltic versus granitic compositions, indicative of past surface water. All of these aspects are critical pieces of the habitability and evolution narrative. Venus is a harsh environment and presents numerous technical challenges. However, it is exactly because of that harsh environment that we must understand why, how, and when that environment emerged, not just for opening our understanding of planetary habitability and evolution, but for our discernment of Earth’s past and future pathway.