Uranus and Neptune in the News

The blue planets beyond Saturn
made headlines recently

 

Uranus and Neptune have not been visited by a spacecraft since 1986 and 1989, when Voyager 2 encountered them in once-in-a-lifetime flybys. Though long neglected by NASA spacecraft, these two watery planets and their moons have been studied by planetary scientists poring through the Voyager data and observing with ground-based equipment and space telescopes. What’s new about those far out planets?

Introduction

NASA’s Hubble Celebrates Decade of Tracking Outer Planets (9 Dec 2024, NASA Goddard Spaceflight Center). This article provides a good introduction to  outer planet science, giving information, images and videos about the history of scientific observations of the gas giants. The Hubble Space Telescope has been monitoring Jupiter, Saturn, Uranus and Neptune since it began its mission. With its OPAL project (Outer Planet Atmospheres Legacy), Hubble has been able to follow up on mysteries discovered by the Voyagers and track changes over time.

Encountering Neptune in 1989, NASA’s Voyager mission completed humankind’s first close-up exploration of the four giant outer planets of our solar system. Collectively, since their launch in 1977, the twin Voyager 1 and Voyager 2 spacecraft discovered that Jupiter, Saturn, Uranus, and Neptune were far more complex than scientists had imagined. There was a lot more to be learned.

Having resolution comparable to the Voyager cameras, the Hubble telescope has been monitoring the north polar cap of Uranus as it grows brighter. It also found that the large “great dark spot” found on Neptune by Voyager 2 was transitory, as was another dark storm that appeared subsequently. Here was another surprise:

Hubble observations uncovered a link between Neptune’s shifting cloud abundance and the 11-year solar cycle. The connection between Neptune and solar activity is surprising to planetary scientists because Neptune is our solar system’s farthest major planet. It receives only about 1/1000th as much sunlight as Earth receives. Yet Neptune’s global cloudy weather seems to be influenced by solar activity. Do the planet’s seasons also play a role?

Voyager 2 also surprised scientists with data that Neptune, despite its distance, has the strongest winds in the solar system: 1,200 mph. That’s almost 5 times the fastest wind ever measured on Earth, and 1.5 times the speed of sound in Neptune’s outer atmosphere.

Formation

Planetary scientists appeal to the stock answer to every event in the solar system: collisions!

Accretion of Uranus and Neptune: Confronting different giant impact scenarios (10 Dec 2024: Esteves, Izodoro and Winter, Icarus). Assuming that Uranus and Neptune emerged from collisions, the trio of storytellers tries to get one scenario to work (large impactors) over another (small impactors). Each one has problems. Here are some of the challenges of starting with collision-ology:

The origins of Uranus and Neptune are not fully understood. Their inclined rotation axes – obliquities – suggest that they experienced giant impacts during their formation histories. Simulations modeling their accretion from giant impacts among 5 Earth masses planetary embryos – with roughly unity impactors’ mass ratios – have been able to broadly match their current masses, final mass ratio, and obliquity. However, due to angular momentum conservation, planets produced in these impacts tend to rotate too fast, compared to Uranus and Neptune. One potential solution for this problem consists of invoking instead collisions of objects with large mass ratios….

Magnetic Fields

The tilted, off-center magnetic fields of both Uranus and Neptune were among Voyager 2’s big surprises.

Neptune and Uranus have a magnetic mystery — but the case may finally be cracked (25 Nov 2024, Space.com). Conor Feehly reports on efforts by Burkhard Militzer, a planetary scientist at the University of California at Berkeley, to understand how these small gas giants can generate magnetic fields. Still wedded to dynamo theory, today’s planetary scientists believe that a separation of layers in the interior of planets is essential for creating a dynamo.

Militzer’s model predicts that underneath the 3,000-mile-thick (just over 4,800 kilometers) Uranian atmosphere sits a vast water-rich layer 5,000 miles deep (just over 8,000 kilometers). Below that, a 5,000 mile thick hydrocarbon-rich layer also exists, with a rocky core about the size of Mercury at its center.

Neptune may have a similar separation of layers, Militzer thinks. But he is ready for bouncing the idea off other planetary scientists.

“If you ask my colleagues, ‘What do you think explains the fields of Uranus and Neptune?’ they may say, ‘Well, maybe it’s this diamond rain, but maybe it’s this water property which we call superionic,'” Militzer said. “From my perspective, this is not plausible. But if we have this separation into two separate layers, that should explain it.”

He would only be able to test his model my mimicking the interiors in the lab, or by another mission to Uranus and Neptune.

A clue to what lies beneath the bland surfaces of Uranus and Neptune (25 Nov 2024, UC Berkeley News). This is the official press release from UCB with more detail about Militzer’s model.

Hidden Oceans Under the Moons

Uranus’s Swaying Moons will Help Spacecraft Seek Out Hidden Oceans (25 November 2024, Univ of Texas at Austin). This press release announces another mission to Uranus by NASA. One goal will be to determine if any of the moons, like Ariel, Oberon or Miranda, have liquid oceans under their crusts. This is a popular idea invoked for many of the bodies in the outer solar system, like Europa at Jupiter and Enceladus at Saturn.

The mission is still in an early planning stage. But researchers at the University of Texas Institute for Geophysics (UTIG) are preparing for it by building a new computer model that could be used to detect oceans beneath the ice using just the spacecraft’s cameras.

The research is important because scientists don’t know which ocean detection method will work best at Uranus. Scientists want to know if there’s liquid water there because it’s a key ingredient for life.

The yearning for Uranus: A far-out world with a tale to tell (6 Dec 2024, Space.com). Reporter Leonard David tells more about the proposed Uranus Orbiter and Probe (UOP) and its goals. Astrobiologists are heavily involved, but the mission would also seek to understand the planet, its atmosphere and magnetic field, its faint rings and its six moons.

Hydrobioscopy Bias

The assumption that water might mean life has appeared under Uranian moons, a belief we call hydrobioscopy, also made news.

New research prompts rethink on chances of life on Uranus moons (11 Nov 2024, BBC News). Reporter Pallab Ghosh always latches onto claims of water on or in planets or moons with hopes that life is thriving there. Such life, he believes of course, would have “emerged” by mindless processes of chemical evolution. In the tradition of the BBC’s one-sentence paragraphs, he writes, turning up the perhapsimaybecouldness dial:

The planet Uranus and its five biggest moons may not be the dead sterile worlds that scientists have long thought.

Instead, they may have oceans, and the moons may even be capable of supporting life, scientists say.

Much of what we know about them was gathered by Nasa’s Voyager 2 spacecraft which visited nearly 40 years ago.

Ghosh points out that the weird magnetic field at Uranus may have been due to a powerful solar storm that was squeezing Uranus’ magnetosphere at the time Voyager 2 flew by. At the time, they thought the moons were outside the planet’s protective magnetic field. Now that they believe the moons were inside the field, this gives him and evolutionary planetary scientists hope that the moons are not sterile.

With unscientific imaginations totally out of control, he quotes Dr William Dunn of University College London:

“These results suggest that the Uranian system could be much more exciting than previously thought. There could be moons there that could have the conditions that are necessary for life, they might have oceans below the surface that could be teeming with fish!”.

Linda Spilker, previously a key planetary scientist for the Voyager missions, joined in the hydrobioscopy party. “The results are fascinating,” she said, “and I am really excited to see that there is potential for life in the Uranian system.”

When you hear the L-word life, that means it’s funding time at NASA again. “Nasa [sic] has plans to launch a new mission, the Uranus Orbiter and Probe, to go back for a closer look in 10 years’ time.”

We think space exploration is worth it for other reasons. The more we learn about other planetary bodies, the more special Earth appears, with all its finely-tuned parameters that make its created life abundant. There’s no need to keep titillating the public with imaginary life. Don’t assume people are stupid enough to believe the Stuff Happens Law. Not all of them like the Darwin Party clown acts at the Origin of Life Circus.

I knew Linda Spilker, a sweet and intelligent scientist, when she was on the Cassini Mission. She deserves better than to encourage the hydrobioscopy hype. Maybe she is just being polite not to criticize a colleague, but I think she knows better. That may be true of others who go along to get along, or who mistakenly think that the only way to drive public interest in space exploration is to talk about life. It’s a self-perpetuating myth that deserves to be exposed as folly, and scientists like Dr James Tour are beginning to challenge it.