Month: October 2016

Uranus May Have Two Undiscovered Moons

NASA’s Voyager 2 spacecraft flew by Uranus 30 years ago, but researchers are still making discoveries from the data it gathered then. A new study led by University of Idaho researchers suggests there could be two tiny, previously undiscovered moonlets orbiting near two of the planet’s rings.

uranus

Rob Chancia, a University of Idaho doctoral student, spotted key patterns in the rings while examining decades-old images of Uranus’ icy rings taken by Voyager 2 in 1986. He noticed the amount of ring material on the edge of the alpha ring — one of the brightest of Uranus’ multiple rings — varied periodically. A similar, even more promising pattern occurred in the same part of the neighboring beta ring.

“When you look at this pattern in different places around the ring, the wavelength is different — that points to something changing as you go around the ring. There’s something breaking the symmetry,” said Matt Hedman, an assistant professor of physics at the University of Idaho, who worked with Chancia to investigate the finding. Their results will be published in The Astronomical Journal and have been posted to the pre-press site arXiv.

Chancia and Hedman are well-versed in the physics of planetary rings: both study Saturn’s rings using data from NASA’s Cassini spacecraft, which is currently orbiting Saturn. Data from Cassini have yielded new ideas about how rings behave, and a grant from NASA allowed Chancia and Hedman to examine Uranus data gathered by Voyager 2 in a new light. Specifically, they analyzed radio occultations — made when Voyager 2 sent radio waves through the rings to be detected back on Earth — and stellar occultations, made when the spacecraft measured the light of background stars shining through the rings, which helps reveal how much material they contain.

They found the pattern in Uranus’ rings was similar to moon-related structures in Saturn’s rings called moonlet wakes.

The researchers estimate the hypothesized moonlets in Uranus’ rings would be 2 to 9 miles (4 to 14 kilometers) in diameter — as small as some identified moons of Saturn, but smaller than any of Uranus’ known moons. Uranian moons are especially hard to spot because their surfaces are covered in dark material.

“We haven’t seen the moons yet, but the idea is the size of the moons needed to make these features is quite small, and they could have easily been missed,” Hedman said. “The Voyager images weren’t sensitive enough to easily see these moons.”

Hedman said their findings could help explain some characteristics of Uranus’ rings, which are strangely narrow compared to Saturn’s. The moonlets, if they exist, may be acting as “shepherd” moons, helping to keep the rings from spreading out. Two of Uranus’ 27 known moons, Ophelia and Cordelia, act as shepherds to Uranus’ epsilon ring.

“The problem of keeping rings narrow has been around since the discovery of the Uranian ring system in 1977 and has been worked on by many dynamicists over the years,” Chancia said. “I would be very pleased if these proposed moonlets turn out to be real and we can use them to approach a solution.”

Confirming whether or not the moonlets actually exist using telescope or spacecraft images will be left to other researchers, Chancia and Hedman said. They will continue examining patterns and structures in Uranus’ rings, helping uncover more of the planet’s many secrets.

“It’s exciting to see Voyager 2’s historic Uranus exploration still contributing new knowledge about the planets,” said Ed Stone, project scientist for Voyager, based at Caltech, Pasadena, California.

Voyager 2 and its twin, Voyager 1, were launched 16 days apart in 1977. Both spacecraft flew by Jupiter and Saturn, and Voyager 2 also flew by Uranus and Neptune. Voyager 2 is the longest continuously operated spacecraft. It is expected to enter interstellar space in a few years, joining Voyager 1, which crossed over in 2012. Though far past the planets, the mission continues to send back unprecedented observations of the space environment in the solar system, providing crucial information on the environment our spacecraft travel through as we explore farther and farther from home.

NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, built the twin Voyager spacecraft and operates them for the Heliophysics Division within NASA’s Science Mission Directorate in Washington.

Preferentially Earth-Sized Planets With Lots Of Water

Computer simulations by astrophysicists at the University of Bern of the formation of planets orbiting in the habitable zone of low mass stars such as Proxima Centauri show that these planets are most likely to be roughly the size of Earth and to contain large amounts of water.

star

In August 2016, the announcement of the discovery of a terrestrial exoplanet orbiting in the habitable zone of Proxima Centauri stimulated the imagination of the experts and the general public. After all this star is the nearest star to our sun even though it is ten times less massive and 500 times less luminous. This discovery together with the one in May 2016 of a similar planet orbiting an even lower mass star (Trappist-1) convinced astronomers that such red dwarfs (as these low mass stars are called) might be hosts to a large population of Earth-like planets.

How could these objects look like? What could they be made of? Yann Alibert and Willy Benz at the Swiss NCCR PlanetS and the Center of Space and Habitability (CSH) at the University of Bern carried out the first computer simulations of the formation of the population of planets expected to orbit stars ten times less massive than the sun.

“Our models succeed in reproducing planets that are similar in terms of mass and period to the ones observed recently,” Yann Alibert explains the result of the study that has been accepted for publication as a Letter in the journal “Astronomy and Astrophysics.” “Interestingly, we find that planets in close-in orbits around these type of stars are of small sizes. Typically, they range between 0.5 and 1.5 Earth radii with a peak at about 1.0 Earth radius. Future discoveries will tell if we are correct!” the researcher adds.

Ice at the bottom of the global ocean

In addition, the astrophysicists determined the water content of the planets orbiting their small host star in the habitable zone. They found that considering all the cases, around 90% of the planets are harbouring more than 10% of water. For comparison: Earth has a fraction of water of only about 0,02%. So most of these alien planets are literally water worlds in comparison! The situation could be even more extreme if the protoplanetary disks in which these planets form live longer than assumed in the models. In any case, these planets would be covered by very deep oceans at the bottom of which, owing to the huge pressure, water would be in form of ice.

Water is required for life as we know it. So could these planets be habitable indeed? “While liquid water is generally thought to be an essential ingredient, too much of a good thing may be bad,” says Willy Benz. In previous studies the scientists in Bern showed that too much water may prevent the regulation of the surface temperature and destabilizes the climate. “But this is the case for Earth, here we deal with considerably more exotic planets which might be subjected to a much harsher radiation environment, and/or be in synchronous ” he adds.

Following the growth of planetary embryos

To start their calculations, the scientists considered a series of a few hundreds to thousands of identical, low mass stars and around each of them a protoplanetary disk of dust and gas. Planets are formed by accretion of this material. Alibert and Benz assumed that at the beginning, in each disk there were 10 planetary embryos with an initial mass equal to the mass of the Moon. In a few day’s computer time for each system the model calculated how these randomly located embryos grew and migrated. What kind of planets are formed depends on the structure and evolution of the protoplanetary disks.

“Habitable or not, the study of planets orbiting very low mass stars will likely bring exciting new results, improving our knowledge of planet formation, evolution, and potential habitability,” summarizes Willy Benz. Because these stars are considerably less luminous than the sun, planets can be much closer to their star before their surface temperature becomes too high for liquid water to exist. If one considers that these type of stars also represent the overwhelming majority of stars in the solar neighbourhood and that close-in planets are presently easier to detect and study, one understands why the existence of this population of Earth-like planets is really of importance.

Ice Shelf Vibrations Cause Unusual Waves In Antarctic Atmosphere

Low-frequency vibrations of the Ross Ice Shelf are likely causing ripples and undulations in the air above Antarctica, a new study finds. Using mathematical models of the ice shelf, the study’s authors show how vibrations in the ice match those seen in the atmosphere, and are likely causing these mysterious atmospheric waves.

ice

Scientists at McMurdo Station detected unusual atmospheric waves with an altitude between 30 to 115 kilometers (20 to 70 miles) above Antarctica in 2011. The waves, which have a long period and take hours to cycle, were observed for several years. Scientists routinely observe atmospheric waves around the world, but the persistence of these waves made them unusual, and scientists didn’t know what was causing them.

The new research solves this mystery by connecting the atmospheric waves to vibrations of the Ross Ice Shelf — the largest ice shelf in the world with an area of almost half a million square kilometers (188,000 miles), roughly the size of France. Imperceptible vibrations of the ice shelf, caused by ocean waves and other forces, are transferred and amplified in the atmosphere, according to the new study.

If the study’s predictions are correct, scientists could use these atmospheric waves to measure properties of the ice shelf that are normally difficult to track, such as the amount of stress the ice shelf is under from ocean waves.

“If atmospheric waves are generated by ice vibrations, by rhythmic vibrations of ice — then that carries a lot of information of the ice shelf itself,” said Oleg Godin, a professor at the Naval Postgraduate School in Monterey, California, and lead author of the new study, published in the Journal of Geophysical Research: Space Physics, a journal of the American Geophysical Union.

This information could help scientists better understand the status and stability of ice shelves, which are permanently floating sheets of ice connected to landmasses. Scientists closely monitor the size and movement of ice shelves because when they break up, they indirectly contribute to sea level rise through their impact on land ice.

“Ice shelves buffer or restrain land ice from reaching the ocean,” said Peter Bromirski, a research oceanographer at Scripps Institute of Oceanography in La Jolla, California, who was not involved in the new study. “The long term evolution of an ice shelf — whether or not it breaks up and disintegrates — is an important factor in how fast sea level will rise.”

Explaining the vibrations

In the new study, Godin and his co-author, Nikolay Zabotin, used two theoretical models of the Ross Ice Shelf to show vibrations within the ice could create the atmospheric waves.

One model approximates the ice shelf as a smooth rectangular slab of ice, while the other approximates the ice as a layered fluid. The authors incorporated known properties of the ice sheet such as elasticity, density, and thickness into each model to calculate the time it would take vibrations in the ice to complete one cycle.

They found both models predict that the ice shelf produces vibrations within a 3- to 10-hour period, which matches the duration of vibrations seen in the atmosphere. These ice shelf vibrations would likely also produce atmospheric waves with a vertical wavelength of 20 to 30 kilometers (12 to 18 miles), another feature of the observed waves.

“Even in this simplified description [of the ice], it readily explains the most prominent features of the observations,” Godin said. “That’s why it goes beyond hypothesis. I would say it’s now a theory.”

The vibrations are transferred from the ice shelf to the atmosphere through direct contact with the air above the ice shelf, according to the new study. While the vibrations of the ice sheet are small, the atmospheric disturbances they create can be large because of reduced air pressure high in the atmosphere. For example, an ice shelf vibration one centimeter in amplitude pushes on the air directly above it. As the vibration cascades upward, it can grow in amplitude to move air hundreds of meters up and down when the wave reaches the less-dense air high in the atmosphere, Godin said.

The size of these atmospheric waves makes them easy to observe with radar and Lidar, a radar-like system using laser light to scan the atmosphere. Godin and Zabotin plan to use an advanced research radar to study the atmospheric waves in more detail to better understand the behavior of the Ross Ice Shelf.

“There are suggestions in the literature that accelerated breakup of ice shelves will lead to rise of sea level by several meters by the end of the century,” Godin said. “Anything we can do to quantify what is going on with these large ice shelves is of huge importance.