4 July 2011

Neptune’s day measured to the second

The first accurate measurement of Neptune's rotational period has been determined by tracking specific atmospheric features. A day on Neptune lasts precisely 15 hours, 57 minutes and 59 seconds.

The colours in this image were modified to emphasize the planet’s atmospheric features. Neptune’s Great Dark Spot stands out as the most prominent feature on the left. The two key features Karkoschka used can also be seen: the fainter Dark Spot 2 and the South Polar Feature, which are locked to the planet’s rotation. Credit: Erich Karkoschka

TUCSON: The first accurate measurement of Neptune’s rotational period has been determined by tracking specific atmospheric features. A day on Neptune lasts precisely 15 hours, 57 minutes and 59 seconds.

This finding adds to our knowledge of the fundamental properties of Neptune and also provides a mechanism for understanding how Neptune’s mass is distributed. The study could lead to a better understanding of the giant gas planets in general.

“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,” said lead author Erich Karkoschka, a senior staff scientist at the University of Arizona’s Lunar and Planetary Laboratory.

Earlier measurements

As early as the 1950s, astronomers knew that Jupiter emitted radio frequencies, produced by the magnetic field generated by the rotation of the planet’s inner core.

In contrast the radio emission of the other gas giants, Saturn, Uranus and Neptune, are swept away by the solar wind, while only Jupiter’s emissions are detected on Earth.

Much later the Voyager 1 and 2 spacecraft probed the interior rotation of the other gas planets on their fly-bys, and estimates of the rotational periods of these gas giants, including Neptune, were based on data from these missions.

Difficult gas planets

More than a decade after Voyager 2 the Cassini mission discovered that Saturn’s northern and southern hemispheres were rotating at different speeds.

As a result scientists realised that using radio emissions was an inaccurate method of measuring spin as the magnetic field was slipping due to the solar wind and other factors.

Unlike the solid planets, gas planets rotate more like giant blobs of liquid gas, which has made it difficult for astronomers to pinpoint exactly how fast the core is spinning.

Tracking visible features

Instead of using mega-expensive spacecraft missions, Karkoschka took advantage of what some refer to as discarded pieces of space science: publicly available images of Neptune from the Hubble Space Telescope archive.

The study, published in the Icarus, the official scientific publication of the Division for Planetary Sciences of the American Astronomical Society, involved examining hundreds of images painstakingly recording every detail.

Karkoschka measured Neptune’s spin by tracking two visible features and measuring their longitude on every image, and then determining the time interval between the observations provided the spin period. Since Neptune has rotated about 10,000 times in 20 years, one can get the spin period very accurately by tracking these features over that time frame.

How does it work?

Using only images Hubble’s archive images, Karkoschka determined the two key observational features that had been recorded some years before; namely, Dark Spot 2 and the South Polar Feature.

The result of this study 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.

“This shows it’s still possible to make significant contributions by niche players, using trash science,” said Stephen Hughes, a physicist at Brisbane’s Queensland University of Technology.

There seems to be a general consensus that Karkoschka’s findings are indeed correct. Craig O’Neill, a planetary scientist at Sydney’s Macquarie University, confirmed the results of the study, “The author shows the periodicity of these features is fairly robust. He also shows that in the Polar Regions, the wind speeds are much milder than in the equator. The big question now is how that works.”


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