Venus, Io, Triton: What we know about destinations of NASA’s Discovery finalists

As NASA’s four Discovery finalists look to prepare for the next round of solar system exploration, ThePrint highlights key facts about their destinations.

Artist concept of the solar system. | Photo: NASA

Bengaluru: Humankind has been exploring the solar system for decades now, but there have been few missions to Venus in the recent past, and none directly aimed to study Jupiter’s volcanic moon, Io, or Neptune’s moon, Triton.

On Thursday, US space agency NASA announced four finalists for the next round of solar system exploration missions under its Discovery programme. Two of these missions are planned for Venus, while a third is for Io, and the fourth for Triton. Of these, two missions will be selected to fly to their destinations.

As NASA prepares to continue its explorations with these missions, ThePrint highlights key facts known to humanity about these destinations.

Venus

Venus is the hottest planet in the solar system, hotter than Mercury despite being farther away from the Sun. Caused by an extreme greenhouse effect, its average surface temperature of nearly 450 C is hot enough to melt lead on the surface.

The second planet from the Sun is very cloudy and hazy, covered by a thick layer primarily of carbon dioxide, which makes up over 95 per cent of the atmosphere. There is also water vapour and sulphur dioxide, all of which are potent greenhouse gases.

The main chunk of Venus’s atmosphere, the troposphere, extends up to an estimated 65 km from the surface, as compared to Earth’s 12 km. Both its atmosphere’s mass as well as surface pressure are over 90 times that of Earth’s.

Higher up through the layers of atmosphere, the temperature falls rapidly. At about a height of 50 km, temperatures become a lot cooler and solar radiation falls within what is considered a habitable range. Pressures fall quite dramatically too, with all conditions reaching similar to that of Earth’s surface.

Studies have shown that these cloud tops could potentially be able to harbour microbes. The argument is further bolstered by the discovery of microbes in Earth’s atmosphere, 15 km from the surface. Researchers suggest that its clouds could hold biomass of 3.6 billion tonnes, or as much mass as all of life in Earth’s oceans.

Venus is also theorised to have had liquid water on its surface for about a billion years before a runaway greenhouse effect evaporated everything and made the surface hellish. It’s not known what caused it.

Earth was able to grow microbial life within a billion years. If Venus also had, all that life would have swept up by winds, leading to the possibility that microbes rode the wind just like on Earth and survived in the cloud layers.

The clouds of Venus are not well explored, and one of the finalists — the DAVINCI+, or Deep Atmosphere Venus Investigation of Noble gases, Chemistry and Imaging Plus — is hoping to fix it.

The mission will analyse the atmosphere to understand how it formed and evolved, and if these cloud layers show a signature of the planet having harboured oceans in the past. The mission consists of an orbiter and a descent sphere. The latter is intended to plunge into the atmosphere, sending data and images back as it falls.

Venus’ surface

Venus is similar to Earth in size and mass, and is thus often referred to as Earth’s twin planet. But the similarities end there.

Venus is actively and extensively volcanic, and has more than 160 volcanoes that are 100 km wide. The volcanism causes most of the surface to be covered by smooth volcanic plains. There are relatively fewer craters, evidencing the young surface.

However, it’s believed that Venus does not have plate tectonics. Despite the continuous volcanism, it’s understood that Venus undergoes a process where instead of tectonic plates moving around, the entire surface recycles by sinking into the mantle repeatedly over a 100 million year cycle.

The second NASA finalist for the programme — VERITAS, or Venus Emissivity, Radio Science, InSAR, Topography and Spectroscopy — will orbit Venus with the intention of mapping the entire surface. It will also gather data about how the planet’s surface evolved and why Venus is so different from Earth despite being similar in size and mass. The spacecraft will have synthetic aperture radar and orbit the entire planet over different elevations, creating a 3D topographical map.

Jupiter’s moon, Io

Io is the fourth largest moon, and the most geologically active body in the solar system. Its extreme geological activity results in extreme volcanism. This is caused by tidal forces from the friction created by the gravitational tug of Jupiter and the other three Galilean satellites — Europa, Ganymede, and Callisto.

Volcanic plumes on the moon can release sulphur dioxide as high as 500 km above Io’s surface. This geological activity has ensured that Io has the least amount of water of any known astronomical body in the solar system.

Io’s volcanism also makes its surface colourful — yellow, green, red, white, and black — because of the spread of different compounds of sulphur. The constant release of sulphur also doubles Jupiter’s magnetic field. The giant planet’s magnetosphere scrapes up material from Io’s wispy atmosphere at the rate of over 1000 kg of material per second. This material is primarily ionised sulphur and other volcanic gases and dust.

Io is only slightly larger and more massive than Earth’s moon. Its interior consists primarily of silicate rock and iron, and its composition makes it very similar to rocky planets.

Both the 1995 Galileo mission and the 2016 Juno mission to Jupiter discovered that Io exhibited magnetic properties which are possible only if a molten silicate magma ocean was present about 50 km below the surface. Additional analysis has also seemingly confirmed the presence of a magma ocean.

The third NASA finalist — IVO or Io Volcano Observer — will probe the presence of a magma ocean beneath the moon’s crust by performing close fly-bys of Io.

Neptune’s moon, Triton

Triton is the seventh largest moon in the solar system and Neptune’s largest. It orbits Neptune in a direction opposite to Neptune’s revolution, which is highly unusual for a natural satellite. Because of this, and its similarity in composition to Pluto, Triton is thought to have been a captured object from beyond Neptune, from the icy region of the solar system called the Kuiper belt.

Triton is an icy body, and its surface is mostly made up of frozen nitrogen and water ice. It also possesses a thick core of rock and metal.

Triton has been “visited” only by one spacecraft, Voyager-2, as it flew by Neptune on its way out of the solar system. It is also one of the few moons to be geologically active. But instead of spewing out large quantities of molten rock, Triton spits out ice.

Through a process called cryovolcanism, Triton’s icy plumes rise to a height of nearly 8 km. But eruptions from Triton’s geysers can last up to a year. Its atmosphere has an ionosphere, which can potentially harbour a water ocean under the icy surface as well.

Voyager-2 managed to image only about 40 per cent of Triton, so many processes on the moon remain a mystery.

The last NASA finalist, the Trident (not an acronym), will investigate all these processes, map the surface of Triton and determine the existence of a subsurface ocean — all with one fly-by.