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Dark storm on Neptune baffles astronomers after surviving its migration to the equator

A dark storm 4,600 miles across on the distant ice giant Neptune has left astronomers confused after it changed direction, a phenomenon never seen before.

Storm vortices like Neptune’s dark spot aren’t unusual, but nobody had previously witnessed one turning back towards the poles after migrating to the equator.  

Using images from the Hubble space telescope, University of California, Berkeley astronomers said the change of direction was ‘unexpected’ and surprising.

The storm is wider than the Atlantic Ocean on Earth and was first discovered by Hubble in 2018 where it was born in the northern hemisphere of Neptune.

A year after it was first observed it had started migrating towards the equator, but the latest observations from August 2020 show it moving back to the north.

Hubble has tracked similar dark spots over the past 30 years but this ‘unpredictable atmospheric behaviour’ is something never before witnessed by astronomers. 

Storm vortices like Neptune’s dark spot aren’t unusual, but nobody had previously witnessed one turning back towards the poles after migrating to the equator

This massive storm wasn’t alone, according to the UC team. Images captured by Hubble in January 2020 revealed a small, similar dark spot on Neptune.

It was short lived and may have been part of the large storm that ‘broke off’ and dissipated – as it wasn’t visible in follow up observations from August. 

‘We are excited about these observations because this smaller dark fragment is potentially part of the dark spot’s disruption process,’ said author Michael H. Wong.

‘This is a process that’s never been observed. We have seen some other dark spots fading away, and they’re gone, but we’ve never seen anything disrupt, even though it’s predicted in computer simulations.’ 

The large storm is 4,600 miles across and is the fourth dark spot Hubble has observed on Neptune since the first observations of the ice giant in 1993.

Voyager 2 – the spaceship that is now outside the solar system in interstellar space – also observed two dark storms in 1989 when it flew by the distant world.

These ‘dark vortices’ are high-pressure systems that can form at mid-latitudes and could then eventually migrate towards the equator on the blue planet.

They start stable – due to the Coriolis forces which cause northern hemisphere storms to rotate clockwise – but as it drifts towards the equator this effect gets weaker and the storm begins to disintegrate.

Computer simulations predicted these storms would follow a relatively straight path to the equator until there is no Coriolis effect to hold them together.

However, observations by Hubble revealed this didn’t happen, in fact, as the dark spot approached the equator it began to turn back towards the polar region.

‘It was really exciting to see this one act like it’s supposed to act and then all of a sudden it just stops and swings back,’ Wong said. ‘That was surprising.’ 

This massive storm wasn't alone, according to the UC team. Images captured by Hubble in January 2020 revealed a small, similar dark spot on Neptune

This massive storm wasn’t alone, according to the UC team. Images captured by Hubble in January 2020 revealed a small, similar dark spot on Neptune

The exact cause of the Dark Spot surviving the equatorial kill zone is unknown, but Wong believes it could be to do with the smaller dark spot seen in the images.

He dubbed it ‘Dark Spot Jr’ – it was about 3,900 miles across and near the side of the main spot that faces the equator – where simulations predicted it would fall apart.  

‘When I first saw the small spot, I thought the bigger one was being disrupted,’ Wong wrote in a blog post for NASA. 

‘I didn’t think another vortex was forming because the small one is farther towards the equator. So it’s within this unstable region. But we can’t prove the two are related. It remains a complete mystery.

‘It was also in January that the dark vortex stopped its motion and started moving northward again,’ Wong added. 

‘Maybe by shedding that fragment, that was enough to stop it from moving towards the equator.’

Another unusual feature of this dark spot is the absence of a lighter set of bright companion clouds surrounding it – that were visible in the 2018 Hubble images.

The Voyager 2 spacecraft sent back images of dark spots on Neptune when it did a fly by of the ice giant in 1989

The Voyager 2 spacecraft sent back images of dark spots on Neptune when it did a fly by of the ice giant in 1989

These clouds form when flow of air is perturbed and diverted upwards over the vortex and causing gasses to freeze into methane ice crystals.

Astronomers plan to study this phenomenon in more detail in the hope it could reveal information on how the dark spots first evolve. 

This doesn’t solve the larger mystery of how these giant storms form on the ice giant, but this latest spot is the most closely studied so far thanks to Hubble.  

‘We wouldn’t know anything about these latest dark spots if it wasn’t for Hubble,’ Amy Simon of NASA’s Goddard Space Flight Center wrote.

 ‘We can now follow the large storm for years and watch its complete life cycle. If we didn’t have Hubble, then we might think the Great Dark Spot seen by Voyager in 1989 is still there on Neptune, just like Jupiter’s Great Red Spot. 

‘And, we wouldn’t have known about the four other spots Hubble discovered.’ 

The team’s research was presented at the 2020 Fall Meeting of the American Geophysical Union. 

NASAs Hubble Space Telescope is still working and has made more than 1.3 million observations since its mission began in 1990

The Hubble telescope was launched on April 24, 1990, via the space shuttle Discovery from Kennedy Space Centre in Florida.

It is named after famed astronomer Edwin Hubble who was born in Missouri in 1889.

He is arguably most famous for discovering that the universe is expanding and the rate at which is does so – now coined the Hubble constant. 

The Hubble telescope is named after famed astronomer Edwin Hubble who was born in Missouri in 1889 (pictured)

The Hubble telescope is named after famed astronomer Edwin Hubble who was born in Missouri in 1889 (pictured)

Hubble has made more than 1.3 million observations since its mission began in 1990 and helped publish more than 15,000 scientific papers.

It orbits Earth at a speed of about 17,000mph (27,300kph) in low Earth orbit at about 340 miles in altitude.

Hubble has the pointing accuracy of .007 arc seconds, which is like being able to shine a laser beam focused on Franklin D. Roosevelt’s head on a dime roughly 200 miles (320km) away.

The Hubble telescope is named after Edwin Hubble who was responsible for coming up with the Hubble constant and is one of the greatest astronomers of all-time

The Hubble telescope is named after Edwin Hubble who was responsible for coming up with the Hubble constant and is one of the greatest astronomers of all-time

Hubble’s primary mirror is 2.4 meters (7 feet, 10.5 inches) across and in total is 13.3 meters (43.5 feet) long – the length of a large school bus.

Hubble’s launch and deployment in April 1990 marked the most significant advance in astronomy since Galileo’s telescope. 

Thanks to five servicing missions and more than 25 years of operation, our view of the universe and our place within it has never been the same. 

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