Artists’ renderings show the VERITAS spacecraft (left) and DAVINCI+ probe (right) as they arrive at Venus (Lockheed Martin).

For NASA scientists, Venus missions must feel like buses. You wait thirty years for one, and then two come along at once.

Last week, NASA selected two Venus missions for the space agency’s Discovery Program; solar system exploration missions that can tuck under a lower cost cap than candidates for NASA’s New Horizons or Flagship categories. The first of these is DAVINCI+, which is an orbiter equipped with a descending probe that will take a big whiff of Venus’s stifling atmosphere. The second is the VERITAS orbiter that plans to peer through the clouds to scrutinise the Venusian surface.

While Europe and Japan have both visited Venus more recently than NASA (in fact, the Japanese orbiter is still there), there is little doubt that our inner neighbor is dramatically under-explored compared to Mars. But why the past neglect, and why go twice now?

The answer to the first question is perhaps the easiest.

Venus is hell.

The planet is wrapped in a thick atmosphere consisting of carbon dioxide and clouds of sulfuric acid that beat down on the Venusian surface with pressures nearly one hundred times higher than on Earth and create temperatures sufficient to melt lead.

These conditions have made it difficult to follow the usual pattern of planetary exploration from fly-bys and orbiters to landers and rovers. The Venusian surface is so inhospitable that a rover like NASA’s Mars Perseverance would become rover goop. Although recent engineering combined with high-temperature electronics means that the surface is no longer impossible, it does greatly add to the challenge (and therefore cost) of a lander mission.

Professor Stephen Kane, University of California, Riverside.

Hell-scape conditions have also resulted in Venus being overlooked for any astrobiological studies compared to (the still rather nasty but at least you can stand a rover on the surface) Mars. This makes the urgency to explore Venus now particularly surprising. The missions are a quest to understand habitability. The bottom line is that the hell world of Venus is essential to understanding how a planet becomes habitable and to discovering other habitable worlds outside our solar system.

“Imagine you live in a small town full of life,” explains Professor Stephen Kane from the DAVINCI+ team. “The nearest town is the same size and seems it was once identical. But now, it’s burned to the ground with no sign of life. What happened to this other town? How long ago did it happen? Nobody in your town knows. You ask, if we don’t know when or how it happened, then could the same fate befall your town? Again, silence.”

The two towns in Kane’s tale are the two Earth-sized planets in our solar system. Venus has nearly the same mass and radius as the Earth, but has evolved down a wildly different path. Was this because the planet formed doomed, with initial conditions that could never support a temperate environment? Or was it once like the Earth and something broke within the planet that made it unable to regulate the surface temperature?

The answer depends on the possible ways in which a freshly formed rocky planet can evolve. Understanding the options is necessary to identify the properties of planets around other stars that might indicate the world is on the ‘habitable’ pathway. Without this knowledge, the best we can do is examine the atmospheres of distant planets at random, and it is unlikely we will ever be certain that what we find is a signature of life. But unravel the divergence of Venus and Earth, and you have a wealth of data on the conditions to look out for on planets around other stars.

As an example of this, Europe’s Venus Express and Japan’s Akatsuki orbiter missions focussed on long-term observations of Venus’s upper atmosphere. One discovery by Akatsuki was that rapid winds on Venus are likely driven by sunlight. The existence of such a simple mechanism may save closely-orbiting planets such as Proxima Centauri-b and the TRAPPIST-1 worlds. These planets are thought to be in tidal lock, with one side permanently facing the star and the other the frozen darkness of space. The planets need a way to redistribute the heat between the day and night side to prevent atmospheric collapse. Venus’s rapid winds may provide evidence for a solution, as well as data on how the planet is affected.

DAVINCI+ and VERITAS will complement the discoveries of the climate orbiters by probing deeper into Venus.

The DAVINCI+ probe will descend through the atmosphere of Venus, taking an inventory of the composition, pressure, temperature and chemistry at different depths. It short, DAVINCI+ will return a top-to-bottom profile of Venus’s atmosphere. The importance of this would be difficult to overstate.

Nightside of Venus captured with the IR2 (infrared) camera on JAXA’s Akatsuki climate orbiter (JAXA).

One of the primary techniques that will be used to explore rocky exoplanets is to identify which wavelengths of starlight are absorbed by the planet’s atmosphere. The technique is known as “transmission spectroscopy” and it can provide the first real look at geological, chemical or even biological processes on the planet. However, transmission spectroscopy only gives a view of the upper atmosphere. Mapping that down to conditions on the planet surface is the task of models. Data on how Venus’s deep atmosphere close to its surface is related to its upper cloud layers is therefore one of the few comparison points we can collect to test the accuracy of these models.

Knowledge of the conditions through the atmosphere is also vital for understanding Venusian chemistry. The recent detection of phosphine in Venus’s atmosphere made a big stir, as the molecule is primarily produced biologically on Earth. As the missions were designed long before the phosphine detection and are running on the tight budget of the Discovery missions, neither DAVINCI+ nor VERITAS plan to detect phosphine directly. However, the atmospheric conditions recorded by DAVINCI+ will enable exploration of the chemical reactions that can occur in Venus’s alien environment through non-biological means. Understanding abiotic chemistry on non-Earth-like worlds will be an essential part of identifying when a signature, such as phosophine, must have a biological origin on another planet.

The DAVINCI+ probe will also measure noble gases, such as xenon and krypton. These are highly unreactive molecules, making it difficult to alter their abundances. As a result, the quantity of noble gases on Venus today is likely to be similar as to when the planet first formed. Comparing this with Earth is a way of deciphering how different the two planets were during their early formation.

Artist impression of the DAVINCI+ probe descending into the Venusian atmosphere. DAVINCI+ uses observations from both above and within the planetary atmosphere to answer major questions about how Venus was formed, has evolved, and possibly lost its habitability (and past surface oceans). (NASA GSFC visualization and CI Labs Michael Lentz and colleague)

Conversely to the long-lived elements, DAVINCI+ will additionally detect radioactive element forms of argon and also the hard-to-retain helium. Both of these elements are products of volcanism and they are both lost over time, as the radioactive argon decays into another element and the light helium can escape Venus’s atmosphere. This provides a method to measure Venus’s volcanic activity through its history.

Volcanism will also be explored by VERITAS, which will be able to create three-dimensional global maps of the surface from orbit. By examining the infrared wavelengths emitted from Venus’s rocky surface, VERITAS will be able to determine their composition (currently unknown) and identify erupted magma along with hotspots indicating active eruptions. The orbiter can also search for faults on the Venusian surface, indicating that Venus’s crust is not a static lid but broken into mobile plates like that on Earth.

Plate tectonics is one part of the Earth’s habitability. Our planet’s crust is divided into multiple plates that can overlap and push material from the surface into the planet’s mantle. This forms part of the carbon cycle, in which carbon dioxide in the atmosphere is removed by forming solid carbonates that are subducted into the mantle and then returned to the air via volcanism. Such mobility has helped to keep our planet habitable by adjusting the surface temperature. If Venus had the mechanisms for a similar system, it may once have been habitable. A break in this balance, such as excessive volcanism, may also have brought about Venus’s current state.

Artist concept of VERITAS using its onboard radar to produce high resolution maps of Venus’s topographic and geologic content (NASA/JPL-Caltech)

VERITAS will also probe what goes on beneath the surface of Venus by an accurate measure of the planet’s gravitational field. This gives clues to the planet’s internal structure, including the size of the core which may help to understand why Venus has no planetary magnetic field.

Both VERITAS and DAVINCI+ will also explore another key ingredient for habitability: water. VERITAS will search for evidence Venus may have water still below its surface in the form of outgassing from any recent volcanic activity. DAVINCI+ will measure the ratio of hydrogen to its heavier counterpart, deuterium, in the planet’s atmosphere. Water molecules consist of an oxygen atom and two hydrogen atoms, with a small fraction forming with deuterium rather than hydrogen. Over time, the lighter hydrogen in any evaporated water on Venus will have escaped more rapidly from Venus’s atmosphere than the deuterium. This alters the ratio between the two atoms and can reveal the rate at which water may have been lost from the planet.

DAVINCI+ is not designed to function on Venus’s surface. Similar to the Huygens probe on Cassini, the probe will die close-to or shortly after landing. High resolution photographs of the landing area captured by the probe along with those from the orbiter will compliment those from VERITAS.

Together with data from Venus Express and Akatsuki, DAVINCI+ and VERITAS form a trifecta of data about Venus’s climate dynamics, atmospheric profile and geological history. The result will explore a multitude of paths that the Earth could have taken but mercifully, did not.

“Habitability is complex,” says Kane. “It’s a jigsaw with many different pieces all interlinked together. But when you complete a jigsaw, you often start with the edges. You want the boundary to the problem. That’s what Venus and the Earth are. They are the edge pieces.”