All About Space: Why Explore the Ice Giants?

In June 2019 I was asked to provide an entry for All About Space magazine's "Ask Space" section, with the following topic:

Why is it important to study the ice giants, Uranus and Neptune?
Dr. Leigh Fletcher
Associate Professor in Planetary Science, University of Leicester

Uranus and Neptune have never had a dedicated spacecraft mission, having been visited only once by the brief flyby mission of Voyager 2, three decades ago.  These distant Ice Giants are the least-explored type of planet in our Solar System, intermediate between the big hydrogen-rich Gas Giants (Jupiter and Saturn) and the smaller rocky planets.  And yet Neptune-sized worlds appear to be commonplace in our galaxy, a natural outcome of the chaotic process of planet formation.  A mission to these icy worlds is the logical next step in humanity’s exploration of our planetary system, to understand how Uranus and Neptune formed, to explore their deep water-rich interiors and exotic hot ices, their stormy atmospheres, and their complex magnetic fields that are totally unlike anything witnessed at Jupiter and Saturn.  The two worlds are superb examples of how planets with shared origins can go down different evolutionary paths:  Neptune as the archetype for Ice Giants, with its seasonal tilt and powerful winds; Uranus as the oddball, with its extreme tilted inclination and sluggish atmosphere.  And both worlds harbour diverse satellite systems, from Uranus’ collection of natural icy satellites with evidence of extreme geological activity, to Neptune’s captured satellite Triton, a visitor from the more distant Kuiper Belt, which may harbour a sub-surface ocean and exhibits erupting geysers from its surface.  For all these reasons and more, scientists across the globe are urging their space agencies to mount an ambitious robotic mission to explore these worlds in the coming decade.

First Thermal Detection of the Rings of Uranus

The rings of #Uranus! We combined thermal-infrared imaging from the @ESO Very Large Telescope with millimetre imaging from ALMA to make the first ever detection of thermal emission from the rings: Molter et al. https://t.co/VzFp5XMb1Khttps://t.co/0NLDU7xJOI pic.twitter.com/UQ1gKr3U3T

— Leigh Fletcher (@LeighFletcher) June 20, 2019

The image above is a composite image of Uranus’s atmosphere and rings at radio wavelengths, taken with the ALMA array in December 2017. The image shows thermal emission, or heat, from the rings of Uranus for the first time, enabling scientists to determine their temperature: a frigid 77 Kelvin (-320 F). Dark bands in Uranus’s atmosphere at these wavelengths show the presence of molecules that absorb radio waves, in particular hydrogen sulfide gas. Bright regions like the north polar spot (yellow spot at right, because Uranus is tipped on its side) contain very few of these molecules. (UC Berkeley image by Edward Molter and Imke de Pater)

The rings of Uranus are invisible to all but the largest telescopes — they weren’t even discovered until 1977 — but they’re surprisingly bright in new heat images of the planet taken by two large telescopes in the high deserts of Chile.

The thermal glow gives astronomers another window onto the rings, which have been seen only because they reflect a little light in the visible, or optical, range and in the near-infrared. The new images taken by the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Telescope (VLT) allowed the team for the first time to measure the temperature of the rings: a cool 77 Kelvin, or 77 degrees above absolute zero — the boiling temperature of liquid nitrogen and equivalent to 320 degrees below zero Fahrenheit.

The observations also confirm that Uranus’s brightest and densest ring, called the epsilon ring, differs from the other known ring systems within our solar system, in particular the spectacularly beautiful rings of Saturn.

“Saturn’s mainly icy rings are broad, bright and have a range of particle sizes, from micron-sized dust in the innermost D ring, to tens of meters in size in the main rings,” said Imke de Pater, a UC Berkeley professor of astronomy. “The small end is missing in the main rings of Uranus; the brightest ring, epsilon, is composed of golf ball-sized and larger rocks.”

By comparison, Jupiter’s rings contain mostly small, micron-sized particles (a micron is a thousandth of a millimeter). Neptune’s rings are also mostly dust, and even Uranus has broad sheets of dust between its narrow main rings.

“We already know that the epsilon ring is a bit weird, because we don’t see the smaller stuff,” said graduate student Edward Molter. “Something has been sweeping the smaller stuff out, or it’s all glomming together. We just don’t know. This is a step toward understanding their composition and whether all of the rings came from the same source material, or are different for each ring.”

Rings could be former asteroids captured by the planet’s gravity, remnants of moons that crashed into one another and shattered, the remains of moons torn apart when they got too close to Uranus, or debris remaining from the time of formation 4.5 billion years ago.

Uranian rings:  Near-infrared image of the Uranian ring system taken with the adaptive optics system on the 10-meter Keck telescope in Hawaii in July 2004. The image shows reflected sunlight. In between the main rings, which are composed of centimeter-sized or larger particles, sheets of dust can be seen. The epsilon ring seen in new thermal images is at the bottom. (UC Berkeley image by Imke de Pater, Seran Gibbard and Heidi Hammel, 2006)

The new data were published this week in The Astronomical Journal. De Pater and Molter led the ALMA observations, while Michael Roman and Leigh Fletcher from the University of Leicester in the United Kingdom led the VLT observations.

“The rings of Uranus are compositionally different from Saturn’s main ring, in the sense that in optical and infrared, the albedo is much lower: they are really dark, like charcoal,” Molter said. “They are also extremely narrow compared to the rings of Saturn. The widest, the epsilon ring, varies from 20 to 100 kilometers wide, whereas Saturn’s are 100’s or tens of thousands of kilometers wide.”

The lack of dust-sized particles in Uranus’s main rings was first noted when Voyager 2 flew by the planet in 1986 and photographed them. The spacecraft was unable to measure the temperature of the rings, however.

To date, astronomers have counted a total of 13 rings around the planet, with some bands of dust between the rings. The rings differ in other ways from those of Saturn.

“It’s cool that we can even do this with the instruments we have,” he said. “I was just trying to image the planet as best I could and I saw the rings. It was amazing.”

Both the VLT and ALMA observations were designed to explore the temperature structure of Uranus’ atmosphere, with VLT probing shorter wavelengths than ALMA.

“We were astonished to see the rings jump out clearly when we reduced the data for the first time,” Fletcher said.

What's amazing about this is that the Uranus observations were targeting the thermal structure of the atmosphere.  When I first processed the data, I was astonished to see the rings.  That's never happened before.  I actually thought I'd messed up and created artefacts in the mid-IR imaging... (heh, wouldn't be the first time). It was only when I zoomed out that I realised what I was seeing.  The Uranus thermal data was not supposed to be taken... the panel rejected us. But our Jupiter programme was getting harder and harder to finish (it was sinking lower in the sky), so they offered to let me change target and.... hey-presto... discovery! 

This presents an exciting opportunity for the upcoming James Webb Space Telescope, which will be able provide vastly improved spectroscopic constraints on the Uranian rings in the coming decade.

Images of the Uranian ring system captured at different wavelengths by the ALMA and VLT telescopes. The planet itself is masked since it is very bright compared to the rings. (Images by Edward Molter, Imke de Pater, Michael Roman and Leigh Fletcher, 2019)

The Berkeley research was funded by the National Aeronautics and Space Administration (NNX16AK14G). Work at the University of Leicester was supported by the European Research Council (GIANTCLIMES) under the European Union’s Horizon 2020 research and innovation program (723890)..

NASA's Webb Telescope Will Survey Saturn and its Moon Titan

This image shows a giant Saturnian storm that was observed at mid-infrared wavelengths by the European Southern Observatory’s Very Large Telescope in 2011. The warm gases powering the storm make it glow brightly compared to the rest of the planet.

Credits: L. Fletcher (University of Leicester) and ESO

Christine Pullen writes about our plans to use the James Webb Space Telescope to explore Saturn's atmospheres, rings, and satellites, continuing the legacy of the Cassini mission:

https://www.nasa.gov/feature/goddard/2019/nasa-s-webb-telescope-will-survey-saturn-and-its-moon-titan

If you stop a random person on the sidewalk and ask them what their favorite planet is, chances are their answer will be Saturn. Saturn’s stunning rings are a memorable sight in any backyard telescope. But there is still a lot to learn about Saturn, especially about the planet’s unique weather and chemistry, as well as the origin of its opulent ring system. After its launch in 2021, NASA’s James Webb Space Telescope will observe Saturn, its rings, and family of moons as part of a comprehensive solar system program.

This study will be conducted through a Guaranteed Time Observations program headed up by Heidi Hammel, a planetary astronomer and executive vice president of the Association of Universities for Research in Astronomy (AURA) in Washington, D.C. Hammel was selected by NASA as a Webb Interdisciplinary Scientist in 2002.

“The purpose of this program is to demonstrate the capabilities of Webb for solar system observations, including observing bright objects, tracking moving objects, and spotting faint targets next to bright ones,” Hammel explained. “The data will be made available to the solar system community as soon as possible to show them that Webb can do what we’ve promised them.”

Webb will pick up where NASA’s Cassini spacecraft left off. Cassini orbited Saturn for 13 years, from 2004 until the mission ended in 2017 when the spacecraft plunged into Saturn’s atmosphere. Since then, programs like the Hubble Space Telescope’s Outer Planet Atmospheres Legacy program and ground-based measurements have been the only way to monitor Saturn.

Saturn experiences auroras, also known as northern and southern lights, just like Earth. Here, Hubble ultraviolet-light observations of an aurora are superposed on a visible-light image of the planet.

Credits: NASA, ESA, J. Clarke (Boston University), and Z. Levay (STScI)

Saturn’s Seasons

Saturn is tilted on its axis just like the Earth, and as a result, it also experiences seasons as it orbits the Sun. However, since the Saturnian year is 30 Earth-years long, each season lasts about 7-1/2 years. Cassini arrived during the southern hemisphere’s summer (winter in the northern hemisphere). Now it is summer in the northern hemisphere. Astronomers are eager to look for seasonal changes in Saturn’s atmosphere.

“These observations will give us a full assay of the Saturnian system to see what’s changed, to see how the seasons have evolved since Cassini’s last glimpses and to harness capabilities Webb has that Cassini never did,” said Leigh Fletcher of the University of Leicester, England, a principal investigator on the program.

In late 2010, a monster storm erupted in Saturn’s northern hemisphere. It began as a tiny spot but grew rapidly, until by the end of January 2011 it encircled the planet. Astronomers were surprised because such storms normally don’t form until after the summer solstice, which occurred in 2017. They will be watching for more storms as Saturn’s northern hemisphere moves from summer into fall over the course of Webb’s mission.

Storms aren’t the only atmospheric phenomena that Saturn and Earth share. Saturn also experiences auroras, or northern and southern lights. Those auroras trigger chemical changes in Saturn’s atmosphere, breaking apart some molecules and allowing new ones to form. Webb will look for signatures of that unusual chemistry glowing at mid-infrared wavelengths, particularly in the north polar region.

Hazy layers of hydrocarbons enshroud Saturn’s moon, Titan. On its surface, methane rivers flow into tar-edged seas.

Credits: NASA/JPL-Caltech/Space Science Institute

Titan, Saturn’s Largest Moon

Saturn’s largest moon, Titan, also will fall under Webb’s powerful gaze. Titan is unique because it is the only moon in our solar system with a substantial atmosphere. In fact, it’s bigger than the planet Mercury. The atmospheric pressure on Titan is about 50% greater than on Earth. Like Earth, that atmosphere is mostly nitrogen, but Titan also has vaporous hydrocarbons like methane. Titan also is much colder than Earth, with a surface temperature around minus 290° Fahrenheit (minus 180° Celsius).

Within Titan’s atmosphere, chemical reactions are constantly churning its composition. Molecules are broken up into their constituents like carbon, hydrogen, oxygen and nitrogen. Those atoms form new molecules, which percolate through the air and settle at whichever pole is currently experiencing winter.

“Titan’s atmosphere is like a big chemistry lab,” said Conor Nixon of NASA’s Goddard Space Flight Center, Greenbelt, Maryland., a principal investigator on the program. Nixon and his colleagues will use Webb’s Near-Infrared Spectrograph (NIRSpec) and Mid Infrared Imager (MIRI) to study these molecules in much greater detail than Cassini’s instruments allowed.

Titan also is the only object in our solar system besides Earth with liquid seas and lakes on its surface. While Earth has a water cycle in which water evaporates, falls as rain, and flows down rivers to the ocean, Titan experiences a similar cycle with methane. On Titan, methane rain carves river beds through rock-hard water ice before flowing into tar-edged seas. Cassini and its Huygens probe from the European Space Agency, which landed on Titan in 2004, made remarkable discoveries about this Saturnian moon. Webb will study Titan’s seasonal climate cycles to compare them to astronomers’ models.

“Titan has clouds and weather that we can see changing in real time. Its chemistry is very different from Earth’s, but it’s still organic, carbon-based chemistry,” said Stefanie Milam of NASA Goddard, a co-investigator on the program.

Jupiter’s moons Io and Europa, and Saturn’s moons Enceladus and Titan, show remarkable geological activity for their small size, with features ranging from volcanoes and water plumes to possible subsurface oceans.

Credits: J Olmsted (STScI)

While Webb’s mission lifetime after launch is designed to be at least 5-1/2 years, it could potentially last 10 years or more. As a result, it could watch Saturn go from northern summer through the autumnal equinox and back to southern spring. That would nearly “complete the circle” begun when Cassini arrived during southern summer.

“We will genuinely have covered an entire Saturnian year. That would be quite an eye-opening experience,” said Fletcher.

The James Webb Space Telescope will be the world's premier space science observatory when it launches in 2021. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international project led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

For more information about Webb, visit www.nasa.gov/webb.

By Christine Pulliam
Space Telescope Science Institute, Baltimore, Md.