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Uranus: Time to Boldly Go


Uranus is a shiny blue-white orb, slightly off center in this image. Its rings are seen nearly face on, and six bright blue dots show the six brightest moons of the planet. There are several reddish galaxies in the background.

Otherworldly Oceans

Cover of the October 2023 issue of Eos

Uranus is a perplexing planet within our solar system with many intriguing features. Despite appearing as a plain blue-green sphere without any visible clouds in photographs, its distinct tilt, uncommon rings, misaligned magnetosphere, and diverse collection of moons hint at a complex past. By studying Uranus, we may gain insight into the evolution of our solar system and the characteristics of other planets.

Scientists are left with numerous unanswered questions about the planet after a brief visit by the Voyager 2 spacecraft decades ago and recent remote observations.

The opportunity for astronomers to send a spacecraft to Uranus is rapidly approaching. This mission is highly desired by astronomers and is currently their top priority. Due to the great potential for scientific discovery and minimal technological obstacles, experts believe that the present moment is ideal for revisiting Uranus and conducting an extended stay.

“The Uranian system presents challenges that challenge our fundamental understanding of planetary mechanics.”

Mark Hofstadter, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, stated that every aspect of the Uranian system poses a challenge to our fundamental knowledge of planetary workings. Despite observations from Voyager and Earth, there are anomalies found in the planet, its rings, its smaller moons, its larger satellites, and its magnetosphere that defy our preconceived notions.

Hofstadter emphasized the utmost significance of fully capitalizing on the rare chance to travel to Uranus and thoroughly investigate every aspect of its system. It is essential to equip the mission with a variety of instruments to effectively respond to any unexpected discoveries.

Voyager

In January of 1986, the Uranian system was visited by the first and, to date, only spacecraft from Earth. Over the course of 32 days, NASA’s Voyager 2 flew past the planet, utilizing gravitational energy to alter its course as it exited the solar system.

During its time near the planet, Voyager 2 captured the initial detailed pictures of Uranus’s upper atmosphere, the pockmarked and irregular exteriors of multiple moons, and its extremely thin rings. The spacecraft also recorded data on Uranus’s magnetic field, gathered samples of the radiation surrounding the planet, and detected lightning on Uranus.

There are still many unanswered questions regarding this topic. For instance, what is the reason for the planet’s extreme tilt of over 90 degrees? What type of chemical processes occur within its core? How does its magnetic field interact with the solar wind over the course of a Uranian year? What kind of weather patterns are observed during seasonal changes? Is there a possibility of recent or ongoing cryovolcanism or hidden oceans on the planet’s moons?

During its journey, Voyager 2 passed by Uranus during the summer solstice in the southern hemisphere, while the northern hemisphere was experiencing complete darkness. The moons of Uranus, which follow a tilted equatorial orbit, were also only partially lit. According to Hofstadter, this had an impact on our observations of the satellites, magnetosphere, and atmospheric weather patterns.

During the Voyager mission, Uranus was known for its uninteresting appearance. According to Hofstadter, it was often described as a plain blue sphere, with very few visible atmospheric characteristics. The atmospheric science team even considered sacrificing some of their allotted observation time to allow other teams to study different aspects of the planet.

According to Hofstadter, Uranus experiences a full orbit and seasonal cycle every 84 years on Earth, and as of 2007, it had reached its equinox. Recent observations from both ground and space have revealed that the planet’s atmosphere is more active and energetic during seasons other than summer. Hofstadter believes that launching a mission to Uranus during a different season is imperative.

The Next Generation

Numerous significant NASA missions are currently in progress or in advanced stages of planning, such as the James Webb Space Telescope (JWST), Perseverance rover, Parker Solar Probe, Nancy Grace Roman Space Telescope, Europa Clipper, and a Mars Sample Return mission. The top priority for astronomers after these missions is a project to Uranus, which would involve an orbiter and an atmospheric probe. These types of missions are referred to as flagship-class, representing NASA’s most extensive and costly endeavors capable of addressing multiple critical scientific inquiries.

“Among the planets in our solar system, Uranus and Neptune are still uncharted territories. They both possess fascinating arrays of moons, rings, intricate magnetospheres, and ever-changing atmospheres.”

According to Amy Simon, a planetary scientist at NASA Goddard Space Flight Center in Greenbelt, Md, Uranus and Neptune are the only two planets in our solar system that have not been explored yet. Despite this, they both have their own fascinating moons, rings, intricate magnetic fields, and ever-changing atmospheres. Out of the two, Uranus is the closer one and therefore more accessible for exploration.

In previous surveys of planetary science, there has been a proposal for a prominent mission to Uranus, which serves as a guide for NASA’s financial focus. Over time, this request has moved up in importance and is now the top priority in 2022. According to Simon, a mission to the Uranian system has great potential for diverse and interdisciplinary scientific discoveries.

A team of space experts created the Uranus Orbiter and Probe (UOP) as a potential mission and has submitted it to NASA for consideration. The mission would involve launching a probe into the atmosphere upon arrival, exploring multiple moons, and ultimately entering a 4.5-year orbit around Uranus. This proposed mission was evaluated by the latest planetary science and astrobiology survey, which deemed it to be a low-risk, cost-effective, and beneficial endeavor.

According to Athena Coustenis, a planetary scientist and research director at France’s Centre National de la Recherche Scientifique, traveling to Uranus requires significant time and effort. To maximize efficiency, a spacecraft could utilize a gravitational slingshot by passing Jupiter on its journey. The ideal launch window for this mission would be in the early 2030s, but there may be other opportunities in the following years.

The decadal survey clearly stated that delaying exploration of this planet is preventing important questions from being answered in the field of planetary science.

The spacecraft design by UOP will utilize proven technologies from previous missions and provide NASA with potential for international cooperation in instrumentation. According to Coustenis, the technology currently available is significantly advanced compared to what was used in Voyager 2’s flyby.

According to Kathy Mandt, a planetary scientist at NASA Goddard Space Flight Center, while new technology is thrilling, the community emphasized in the decadal survey that postponing exploration of this planet is hindering important inquiries in planetary science. Therefore, time is crucial and this mission must utilize current technology.

Side-by-side images of Uranus taken by the Hubble Space Telescope. The left-hand image was taken in 2014 and shows the planet as a bright blue orb with white stripes, several storms in the right-hand (northern) hemisphere, and a gray north pole. The right-hand image shows Uranus in 2022. Its north pole glows in white, while the remainder of the planet is a cool blue with no stripes or storms.

Due to its 92° axial tilt, the seasons on Uranus differ greatly from those on any other planet in our solar system and significantly change throughout its orbital year. In 2014, seven years after Uranus experienced its equinox, the Hubble Space Telescope captured an image of the planet showing multiple equatorial storms with clouds composed of methane ice crystals. In 2022, six years before its northern solstice, the north pole of Uranus emitted a bright glow and the number of storms on the planet decreased. Image credit: NASA, ESA, STScI, Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley), Joseph DePasquale (STScI), Public Domain.

Richard Anderson, a systems engineer at the Johns Hopkins University Applied Physics Laboratory, stated that the main obstacles for the UOP concept are related to cost and schedule due to its reliance on current technology. He also mentioned that efforts are being made to lower expenses and preserve valuable resources like plutonium, which is necessary for the spacecraft’s radioisotope thermoelectric power generators.

The proposed mission is less expensive and more compact compared to previous flagship ventures like Galileo (which studied Jupiter) and Cassini (which studied Saturn), but would still have comparable scientific goals. According to Simon, who was a co-leader for the UOP concept, the mission would include an atmospheric probe and scientific instruments capable of observing all aspects of the Uranian system.

According to Simon, even without any new technology, it still takes a considerable amount of time to design, construct, and test large missions.

While UOP may be comparable in size to previous flagship missions, Mandt emphasized the significance of its design and use of modern technology. By carefully controlling the project’s scope, there is potential to establish a new standard for cost-efficient large-scale missions.

According to Anderson, a trip to the Uranus system can be accomplished at a lower cost compared to a trip to Europa.

Strange New Worlds

Astronomers are eagerly anticipating the opportunity to study Uranus’s five largest moons: Oberon, Titania, Ariel, Umbriel, and Miranda. These moons are believed to consist of equal parts rock and water ice. (Uranus has a total of 27 moons, with some being as small as 15 kilometers and having a darker appearance than asphalt.)

Due to the lighting, Voyager 2 was only able to map approximately 40% of the moons’ surfaces. Despite this limitation, the mapping revealed significant variations in their surface geologies and levels of cratering.

The Uranus system was mapped in 3D using images captured by Voyager 2 during its flyby. One of its moons, Miranda, has a unique surface resembling a jigsaw puzzle. You can interact with the map by clicking and dragging. The credit for this visualization goes to NASA’s Visualization Technology Applications and Development (VTAD) team and it is available in the public domain.

In 2049, when the next equinox arrives, it will provide an opportunity to illuminate the previously unseen hemispheres of the moons. This will allow scientists to complete the mapping of their surfaces. According to Simon, close flybys that allow for imaging or spectroscopy will provide valuable information about the moons’ surface geology, composition, and potentially the age of observed features.

All five large satellites may have subsurface liquid water oceans at the boundary between their rocky interiors and icy surfaces. “Looking for oceans will be a very high priority and should be achievable at least for the inner moons,” said Francis Nimmo, who studies planetary evolution at the University of California, Santa Cruz and was a science colead for the UOP concept.

A device used to measure magnetic fields, known as a magnetometer, installed on a spacecraft, was able to detect the presence of a magnetic field emanating from an underground ocean. Scientists involved in the Galileo mission utilized this technique to validate the existence of oceans on Jupiter’s moons Europa, Callisto, and Ganymede.

Researchers are specifically intrigued by the potential of Ariel’s ocean, as its flat terrain may suggest resurfacing due to cryovolcanism. According to Simon, conducting thermal assessments and capturing images from a high perspective could reveal any ongoing cryovolcanic activity or geysers on the moon.

Pictures taken during Voyager 2’s observation of the Uranus system allowed researchers to construct a three-dimensional depiction of one of its satellites, Ariel, which could potentially possess a hidden ocean beneath its surface. Use your mouse to navigate and discover. Credit: NASA Visualization Technology Applications and Development (VTAD), Public Domain.

Similar to other planets with oceans, there exists the possibility of life on Uranus’s moons. While Enceladus and Europa are often considered the most promising targets for astrobiology, it is not impossible for the ocean moons of Uranus to also sustain life.

Coustenis expressed her enthusiasm for exploring the potential for life in the habitable environments around the ice giants. She stated that researchers can search for signs of complex organic molecules, subsurface warmth, and surface organics within Uranus’s moons.

Leigh Fletcher, an atmospheric scientist at the University of Leicester in the United Kingdom, suggested that if these planets are indeed ocean worlds with hidden water beneath their icy surfaces, it could impact our understanding of habitable environments in our solar system and potentially provide insight into potential habitats for life in the universe.

Discovery

It is astonishing that despite the abundance of ice giant planets in our galaxy, we still do not have a clear understanding of their composition.

Planetary scientists are highly curious about exploring the planet Uranus, specifically its mysterious moons. One of their top priorities is to gather information about the planet’s internal makeup and composition, a task that has proven difficult with current data. A probe launched into the planet’s atmosphere would be crucial in achieving this goal.

Hofstadter expressed his fascination with the numerous ice giant planets found throughout our galaxy and the mystery surrounding their composition. For over 30 years, he has been tirelessly researching Uranus’s makeup. Surprisingly, the planet’s upper cloud layers, observed by Voyager 2, contain significantly less ammonia and a higher amount of hydrogen sulfide compared to what was predicted by models.

The cause for this phenomenon could possibly be due to unfamiliar atmospheric reactions or the combination of different cloud layers. Gathering information on the chemistry, temperature, pressure, and movements beneath the clouds would require a probe to descend into the planet’s atmosphere.

According to Fletcher, Uranus presents unique challenges for atmospheric scientists due to its unusual seasons and surprisingly tranquil atmosphere. He is eager to observe and study these features in person, in order to better understand its climate, circulation, and meteorology, which differ from those of the more extensively studied planets Jupiter and Saturn.

Uranus’s tilted spin axis means that its seasons are quite different from those on Earth: At the height of summer, one hemisphere is always illuminated, and the other is entirely dark—just as at Earth’s North and South Poles, but lasting for decades. Only around the equinoxes do both hemispheres experience rapid day-night cycles and the temperature changes that come with them.

The James Webb Space Telescope captured this image of Uranus, the “blue billiard ball,” along with its narrow ring system and several of its moons, on 6 February 2023.

On February 6, 2023, the James Webb Space Telescope took a photo of Uranus, also known as the “blue billiard ball,” showing its thin ring system and multiple moons. This image was credited to NASA, ESA, CSA, STScl and is in the Public Domain.

In the summer of the southern hemisphere, Voyager 2 traveled through the system. However, a future mission launching in the mid-2030s is expected to reach Uranus in the early 2040s, right before the planet’s equinox. This shift in seasons could potentially uncover new weather patterns and shed light on the interaction between Uranus’ tilted and misaligned magnetosphere and solar wind.

Investigating the rings of Uranus could potentially uncover new insights about the planet’s characteristics. According to Nimmo, the rings could serve as a seismometer, allowing us to examine its internal makeup similar to what has been achieved with Saturn. The smoothness of a planet’s rings is influenced by its gravitational field, and any internal movement or disturbance can cause changes in this field. These changes would manifest as patterns in the rings, such as waves, ripples, or spirals, which could be observed by a spacecraft orbiting the planet.

There is a plethora of peculiarities on Uranus that are worth investigating. According to Hofstadter, I would also like to mention the small moon Mab. Measuring less than 5 kilometers in diameter, this moon is believed to be responsible for Uranus’s mu ring, composed of tiny particles in the micrometer range.

“The only other ring in the solar system having similar properties is Saturn’s E ring, which is generated by the water plumes erupting on Saturn’s moon Enceladus. Mab is far too small to have active plumes, so how is the mu ring generated and maintained?” An onboard spectrometer could help pin down the exact size and composition of the ring particles, determine whether they’re a match for Mab, and, from there, narrow down how and why Mab sheds material—maybe even catch it in the act.

Reworded: The ninth planet in deep space.

A team of astronomers suggests that a lengthy expedition to the outer region of the solar system could lead to significant scientific breakthroughs that surpass our current knowledge of Uranus and its satellites. The most notable impact would be on our understanding of Neptune, the other ice giant, as well as other planets beyond our own solar system. Studies have revealed that a significant number of exoplanets share similarities in terms of size and mass with Uranus and Neptune.

Hofstadter expressed his surprise, stating that previous planetary formation models suggested that planets the size of Uranus would be uncommon. In order to improve these models and gain a deeper understanding of ice-giant-sized exoplanetary systems, further research on Uranus and Neptune is necessary.

“All knowledge gained about Uranus contributes to our understanding of this particular type of exoplanet.”

Mandt stated, “Gaining knowledge about Uranus is significant in comprehending other exoplanets in this category, including their internal composition, formation process, and potential appearance of their atmosphere through advanced telescopes.”

During its ten-year journey, the spacecraft may also contribute to scientific discoveries. Even though some instruments may not be active or at a low power during this time, the spacecraft will still need to regularly communicate with Earth to update its condition, position, and velocity. This data can potentially help identify any unknown large objects in our solar system, such as asteroids, comets, or even a new planet.

An ideal spacecraft orbiting Uranus would have instruments on board to observe the planet’s magnetic, gravitational, and plasma surroundings. The data collected by these instruments could also offer insights into the solar wind and cosmic ray activity in the outer regions of the solar system, which have not been studied since the Voyager missions.

According to Hofstadter, it may be feasible to identify gravitational waves while traveling. He mentioned that the Earth’s LIGO and the upcoming LISA observatory in space will be able to detect gravitational waves from various black hole mergers, but they may not be able to detect them from the merging of supermassive black holes. He suggested that with the right equipment, the Uranus mission could potentially make these observations during its 10-year journey to Uranus.

Enterprise

A potential mission to Uranus, whether it be the Uranus Orbiter and Probe or another proposed by different space agencies, remains speculative. The fact is that the most favorable launch opportunity for a mission to the outer solar system is less than a decade away and no official mission has been approved. Despite the fact that the primary mission idea does not necessitate any novel technologies, it could still take at least ten years to progress from the planning stage to actual launch.

Scientists are concerned that NASA may be giving higher priority to other missions over one to the outer solar system, despite a sense of urgency from the scientific community. The long-delayed and costly JWST project has already caused delays for other NASA astrophysics projects for over ten years. Now, astronomers are worried that the planned NASA-European Space Agency Mars Sample Return mission will have a similar impact on the timeline for planetary science missions.

In response to the proposal for a flagship mission to Uranus, NASA expressed initial approval but also recognized financial challenges. During a town hall meeting held by NASA’s Planetary Science Division (PSD) after the decadal survey was released, PSD director Lori Glaze announced that the agency intends to launch studies for an orbiter and probe mission by fiscal year 2024. However, Glaze also warned that starting a new flagship mission in the near future may be difficult due to budget limitations.

If we plan to launch a flagship mission to Uranus in the near future, we need to act quickly.

During a town hall meeting at AGU’s Fall Meeting 2022, she elaborated that the planetary budget is under considerable strain and has become increasingly delicate. Factors such as the COVID-19 pandemic, supply chain disruptions, rising inflation, and labor expenses have added strain to the budgets of ongoing projects, which have had to be accommodated within the PSD budget, Glaze clarified.

“The last hurdle to overcome, which can be addressed with funding, is the accessibility of radioisotope power systems for the mission,” elaborated Hofstadter. “Due to Uranus’ distance from the Sun, it is highly likely that we will need to utilize a plutonium-based power source, and at present, NASA does not possess enough for all the missions planned in the next ten years.”

In summary, in order for a flagship mission to Uranus to take place within the upcoming launch window, the entire project must accelerate its progress.

“The planetary science community is filled with anticipation,” stated Fletcher. “We cannot delay any longer as Jupiter will soon progress in its orbit, making the journey to Uranus more challenging,” he urged. “It is imperative that we begin promptly to take advantage of this opportunity.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Reference: Cartier, K.M.S. (2023). Uranus: Exploring New Frontiers. Eos, 104, https://doi.org/10.1029/2023EO230367. Retrieved from September 25, 2023.

Text © 2023. The authors. CC BY-NC-ND 3.0

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