Venus : our Earth-sized neighbour

Despite the emphasis on Venus of early space exploration (Mariner, Venera, Vega, & Pioneer), and the more recent Magellan, Venus Express, and Akatsuki Missions, Venus remains a global mystery. Sitting in our own planetary back yard, Venus represents an unusual example of terrestrial planet formation and evolution that obviously differs from Earth and the other solid planets of the inner solar system. Many fundamental questions remain unanswered. For example, did Venus have oceans, how has that atmosphere evolved over time, and when and why did the runaway greenhouse begin ? How does Venus lose its heat, how volcanically and tectonically active has Venus been over the last billion years ? Has Venus always had a “stagnant-lid”, or was a plate tectonics regime ever present earlier in her history ? What is the composition of the highland tessera terrain, are these regions the oldest rocks exposed on the Venus surface, how oxidized are those rocks and do these surfaces retain evidence of an earlier time when water was more prevalent ?

The NASA Magellan, ESA Venus Express, and JAXA’s Akatsuki missions to Venus have revealed a complex, dynamic planet with surprising similarities as well as differences from Earth, and yet it remains the least understood of the terrestrial planets. The dramatic discovery of volcanic hotspots in Venus Express data suggest that the next stage of Venus exploration must focus on its surface : the geological interface between its dense, hostile atmosphere and its Earth-like but puzzling interior. Magellan data reveal an incredible number of volcanoes, as well as rift systems, mountain belts, and a range of features still poorly understood, on a world with a crater count indicative of mean surface age of only 500 Ma. 

The first two questions of ESA’s Cosmic Vision plan focus on the formation of planets and the emergence of life, and on the solar system itself. Central to both of these goals is the study of the unexplained variety of terrestrial planets in the inner Solar System. Earth, Mars, Mercury and Venus – shown here on scale – show a wide range of evolutionary pathways, and so are key to understanding planets and their habitability everywhere, including in other stellar systems. 

EnVision : Europe’s next mission to Venus

EnVision, a low-altitude polar orbiter, is the M5 mission candidate in the ESA Science Programme. It will carry 5 instruments and 1 experiment (an S-band Synthetic Aperture Radar, a Subsurface Radar, 3 spectrometers and a radio science experiment). EnVision will investigate Venus from its inner core to its atmosphere at an unprecedented scale of resolution, characterising in particular, core and mantle structure, signs of active and past geologic processes and looking for evidence of the past existence of oceans. EnVision will help understanding why the most Earth-like planet in the solar system has turned out so differently, opening a new era in the exploration of our closest neighbour.

Left : a 225 meter per pixel Magellan radar image mosaic of Venus, centered at 47 degrees south latitude, 25 degrees east longitude in Lada regio. The scene is approximately 550 kilometers (341 miles) east-west by 630 kilometers (391 miles) north-south. The mosaic shows a system of east-trending radar-bright and dark lava flows encountering and breaching a north-trending ridge belt (left of center). Upon breaching the ridge belt, the lavas pool in a vast, radar-bright deposit, covering approximately 100,000 square kilometers (right side of image). The source caldera for the lava flows, named Ammavaru, lies approximately 300 kilometers west of the scene (JPL/Caltech). Right : at latitude 6.6 south , and an east longitude 298.6 degrees, lies this 100.5 km wide volcano.

Role of Venus investigations in understanding the Earth

Further Venus exploration to find the causes of differences between Earth and Venus is crucial to understanding how the Solar System works and the conditions for planet formation and the emergence of life. This is of particular relevance in this era of discovery and characterisation of a variety of terrestrial planets in other stellar systems and discussion on their path toward habitability.  In our solar system, Venus is the most Earth-like planet, yet at some point in planetary history there was a bifurcation between the two: Earth has been continually habitable since the end-Hadean, whereas Venus became uninhabitable. Indeed, Venus is the type-planet for a world that has transitioned from habitable and Earth-like conditions, through the inner edge of the habitable zone ; thus it provides a natural laboratory to study the evolution of habitability.

Pathway to habitability and astrobiology

Venus accounts for 40% of the mass of terrestrial planets in our solar system, yet even fundamental parameters such as the relative size of its core to mantle are unknown. As we expand the scope of planetary science to include those planets around other stars, the lack of measurements for basic planetary properties such as moment of inertia, whether parts of its current surface was formed in the presence of water, core-size and density variations with depth, and thermal profile for Venus hinders our ability to compare the potential uniqueness of the Earth and our solar System to other planetary systems.

A false-color image using two ultraviolet channels from Akatsuki’s UVI camera at 283 nm and 365 nm distinguishes different components of the Venusian atmosphere (using 283 nm as a blue hue and 365nm as an orange hue). The upper cloud is mainly composed of sulphuric acid, but includes other minor constituents such as liquid water and an unknown UV absorber. This unknown UV absorber shows up particularly in the 365 nm images. In contrast, the other UV channel at 285 nm is also sensitive to sulphur dioxide, a gas which may originally have been emitted from volcanoes (volcanism is the main source of SO2 on Earth, this is likely to be the case on Venus as well).

The prime focus of astrobiology research is the search for life elsewhere in the universe, and this proceeds with the pragmatic methodology of looking for water and Earth-like conditions. If we seek to understand habitability, proper understanding of the boundaries of the habitable zone are necessary: further study and development of our understanding of the evolution of Venus’ environment is imperative.