A day on Neptune lasts precisely 15 hours, 57 minutes and 59 seconds, according to the first accurate measurement of its rotational period made by University of Arizona planetary scientist Erich Karkoschka.
His result is one of the largest improvements in determining the rotational period of a gas planet in almost 350 years since Italian astronomer Giovanni Cassini made the first observations of Jupiter's Red Spot.
"The rotational period of a planet is one of its fundamental properties," said Karkoschka, a senior staff scientist at the UA's Lunar and Planetary Laboratory. "Neptune has two features observable with the Hubble Space Telescope that seem to track the interior rotation of the planet. Nothing similar has been seen before on any of the four giant planets."
The discovery is published in Icarus, the official scientific publication of the Division for Planetary Sciences of the American Astronomical Society.
Unlike the rocky planets – Mercury, Venus, Earth and Mars – which behave like solid balls spinning in a rather straightforward manner, the giant gas planets – Jupiter, Saturn, Uranus and Neptune – rotate more like giant blobs of liquid. Since they are believed to consist of mainly ice and gas around a relatively small solid core, their rotation involves a lot of sloshing, swirling and roiling, which has made it difficult for astronomers to get an accurate grip on exactly how fast they spin around.
"If you looked at Earth from space, you'd see mountains and other features on the ground rotating with great regularity, but if you looked at the clouds, they wouldn't because the winds change all the time," Karkoschka explained. "If you look at the giant planets, you don't see a surface, just a thick cloudy atmosphere."
"On Neptune, all you see is moving clouds and features in the planet's atmosphere. Some move faster, some move slower, some accelerate, but you really don't know what the rotational period is, if there even is some solid inner core that is rotating."
In the 1950s, when astronomers built the first radio telescopes, they discovered that Jupiter sends out pulsating radio beams, like a lighthouse in space. Those signals originate from a magnetic field generated by the rotation of the planet's inner core.
No clues about the rotation of the other gas giants, however, were available because any radio signals they may emit are being swept out into space by the solar wind and never reach Earth.
"The only way to measure radio waves is to send spacecraft to those planets," Karkoschka said. "When Voyager 1 and 2 flew past Saturn, they found radio signals and clocked them at exactly 10.66 hours, and they found radio signals for Uranus and Neptune as well. So based on those radio signals, we thought we knew the rotation periods of those planets."
But when the Cassini probe arrived at Saturn 15 years later, its sensors detected its radio period had changed by about 1 percent. Karkoschka explained that because of its large mass, it was impossible for Saturn to incur that much change in its rotation over such a short time.
"Because the gas planets are so big, they have enough angular momentum to keep them spinning at pretty much the same rate for billions of years," he said. "So something strange was going on."
Even more puzzling was Cassini's later discovery that Saturn's northern and southern hemispheres appear to be rotating at different speeds.
"That's when we realized the magnetic field is not like clockwork but slipping," Karkoschka said. "The interior is rotating and drags the magnetic field along, but because of the solar wind or other, unknown influences, the magnetic field cannot keep up with respect to the planet's core and lags behind."
Instead of spacecraft powered by billions of dollars, Karkoschka took advantage of what one might call the scraps of space science: publicly available images of Neptune from the Hubble Space Telescope archive. With unwavering determination and unmatched patience, he then pored over hundreds of images, recording every detail and tracking distinctive features over long periods of time.
Other scientists before him had observed Neptune and analyzed images, but nobody had sleuthed through 500 of them.
"When I looked at the images, I found Neptune's rotation to be faster than what Voyager observed," Karkoschka said. "I think the accuracy of my data is about 1,000 times better than what we had based on the Voyager measurements – a huge improvement in determining the exact rotational period of Neptune, which hasn't happened for any of the giant planets for the last three centuries."
Two features in Neptune's atmosphere, Karkoschka discovered, stand out in that they rotate about five times more steadily than even Saturn's hexagon, the most regularly rotating feature known on any of the gas giants.
Named the South Polar Feature and the South Polar Wave, the features are likely vortices swirling in the atmosphere, similar to Jupiter's famous Red Spot, which can last for a long time due to negligible friction. Karkoschka was able to track them over the course of more than 20 years.
An observer watching the massive planet turn from a fixed spot in space would see both features appear exactly every 15.9663 hours, with less than a few seconds of variation.
"The regularity suggests those features are connected to Neptune's interior in some way," Karkoschka said. "How they are connected is up to speculation."
One possible scenario involves convection driven by warmer and cooler areas within the planet's thick atmosphere, analogous to hot spots within the Earth's mantle, giant circular flows of molten material that stay in the same location over millions of years.
"I thought the extraordinary regularity of Neptune's rotation indicated by the two features was something really special," Karkoschka said.
"So I dug up the images of Neptune that Voyager took in 1989, which have better resolution than the Hubble images, to see whether I could find anything else in the vicinity of those two features. I discovered six more features that rotate with the same speed, but they were too faint to be visible with the Hubble Space Telescope, and visible to Voyager only for a few months, so we wouldn't know if the rotational period was accurate to the six digits. But they were really connected. So now we have eight features that are locked together on one planet, and that is really exciting."
In addition to getting a better grip on Neptune's rotational period, the study could lead to a better understanding of the giant gas planets in general.
"We know Neptune's total mass but we don't know how it is distributed," Karkoschka explained. "If the planet rotates faster than we thought, it means the mass has to be closer to the center than we thought. These results might change the models of the planets' interior and could have many other implications."
LINK:Neptune’s Rotational Period Suggested by the Extraordinary Stability of Two Features, Icarus, article in press (accepted manuscript), doi:10.1016/j.icarus.2011.05.013
Daniel Stolte | University of Arizona
Studying fundamental particles in materials
17.01.2017 | Max-Planck-Institut für Struktur und Dynamik der Materie
Seeing the quantum future... literally
16.01.2017 | University of Sydney
Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.
While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...
Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales
Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
At TU Wien, an alternative for resource intensive formwork for the construction of concrete domes was developed. It is now used in a test dome for the Austrian Federal Railways Infrastructure (ÖBB Infrastruktur).
Concrete shells are efficient structures, but not very resource efficient. The formwork for the construction of concrete domes alone requires a high amount of...
Many pathogens use certain sugar compounds from their host to help conceal themselves against the immune system. Scientists at the University of Bonn have now, in cooperation with researchers at the University of York in the United Kingdom, analyzed the dynamics of a bacterial molecule that is involved in this process. They demonstrate that the protein grabs onto the sugar molecule with a Pac Man-like chewing motion and holds it until it can be used. Their results could help design therapeutics that could make the protein poorer at grabbing and holding and hence compromise the pathogen in the host. The study has now been published in “Biophysical Journal”.
The cells of the mouth, nose and intestinal mucosa produce large quantities of a chemical called sialic acid. Many bacteria possess a special transport system...
10.01.2017 | Event News
09.01.2017 | Event News
05.01.2017 | Event News
17.01.2017 | Earth Sciences
17.01.2017 | Materials Sciences
17.01.2017 | Architecture and Construction