Tag Archives: Metrology

Jupiter’s Supersonic Stratospheric Winds Make it a Unique Beast

Using the aftermath of a comet collision in 1994 astronomers have measured the winds blowing across Jupiter‘s stratosphere for the first time. The team has discovered that these winds raging around the middle atmosphere of the solar system’s largest planet are incredibly powerful–reaching speeds of up to 400 metres per second at the poles.

The team’s findings represent a significant breakthrough in planetary metrology and mark the gas giant out as what the team are describing as a ‘unique metrological beast in the solar system.’

This image shows an artist’s impression of winds in Jupiter’s stratosphere near the planet’s south pole, with the blue lines representing wind speeds. These lines are superimposed on a real image of Jupiter, taken by the JunoCam imager aboard NASA’s Juno spacecraft. (ESO/L. Calçada & NASA/JPL-Caltech/SwRI/MSSS)

To conduct the research the astronomers diverged from the usual methods used to measure the winds of Jupiter. Previous attempts to measure the gas giant’s winds have hinged on measuring swirling clouds of gas–seen as the planet’s distinctive red and white bands–but this method is only effective in measuring winds in the lower atmosphere. Whereas, by using aurorae at Jupiter’s poles researchers have been able to model winds in the upper atmosphere. But, both of these methods, even when used in conjunction, have left the winds in the middle section of the gas giant’s atmosphere–the stratosphere– something of a mystery.

That is until now. This team of astronomers used the Atacama Large Millimetre Array (ALMA) to track molecules left in Jupiter’s atmosphere by the collision with the comet Shoemaker-Levy 9 in 1994.

“We had to use ALMA’s ability to quickly map Jupiter’s spectral emission at very high spatial and spectral resolution in the submillimeter and observe the Doppler shifts induced by the winds on the spectral line we targeted,” team leader Thibault Cavalié,  Laboratoire d’Astrophysique de Bordeaux, France, exclusively tells ZME Science. “We could deduce the wind speeds just like you could deduce the speed of a passing fire engine by the change in frequency of its siren. This spectral line is formed in the stratosphere, giving us access to the winds at this altitude.

“It is the first time we achieve measuring directly winds in the stratosphere of Jupiter, which lacks visual tracers such as clouds.”

Thibault Cavalié,  Laboratoire d’Astrophysique de Bordeaux, France.

Cavalié explains that the team had to use ALMA’s ability to quickly map Jupiter’s spectral emission at very high spatial and spectral resolution in the submillimeter and observe the Doppler shifts induced by the winds on the spectral line they targeted.

“We could deduce the wind speeds just like you could deduce the speed of a passing fire engine by the change in frequency of its siren,” the researcher continues. “This spectral line is formed in the stratosphere, giving us access to the winds at this altitude.”

What the astronomers discovered was powerful winds in the middle atmosphere of Jupiter in two different locations. One set of winds conformed to expectations, but the other came as a surprise.

Jupiter’s ‘Supersonic Jet’ Winds

Cavalié explains that the team first found a 200 metres per second eastward jet just north of the equator in ‘super-rotation–meaning that the wind rotates faster around the planet than the planet rotates itself. “Winds at such latitudes were expected from models and previous temperature measurements at these low latitudes,” the astronomer adds.

But, not everything observed by the team conformed to expectations.

“Most surprisingly, we identified winds located under the main UV auroral emission near Jupiter’s poles. These winds have velocities of 300 to 400 meters per second,” Cavalié says. “While the equatorial winds were kind of anticipated, the auroral winds and their high speed were absolutely unexpected.”

To put this into perspective, the fastest winds ever recorded on earth reached a speed of just 103 metres per second–measured at the Mount Washington Observatory in 1931. These auroral winds even beat the winds recorded in Jupiter’s Great Red Spot–an ongoing raging storm on the surface of the gas giant–which have been clocked at around 120 metres per second.

The speed of these jets isn’t their only intimidating quality, however. The jets seem to behave like a giant vortex with a diameter around four times that of our entire planet, reaching a height of around 900 kilometres.

“A vortex of this size would be a unique meteorological beast in our Solar System.”

Thibault Cavalié,  Laboratoire d’Astrophysique de Bordeaux, France.

The team’s measurements and stunning discovery, documented in a paper published in the latest edition of Astronomy & Astrophysics, wouldn’t have been possible without a violent incident in Jupiter’s recent history.

Shoemaker-Levy 9 Still has Impact

The impact of Shoemaker-Levy 9 upon the surface of Jupiter was an event–or more precisely a series of events– that had already made history before its effects made this research possible.

The comet broke up in the planet’s atmosphere resulting in a series of impacts that had never been studied prior to 1994, and its somewhat ironic that thanks to this study, Shoemaker-Levy 9 is still having an impact today. The comet left traces of hydrogen cyanide swirling in Jupiter’s atmosphere which the team was able to track.

This image, taken with the MPG/ESO 2.2-metre telescope and the IRAC instrument, shows comet Shoemaker–Levy 9 impacting Jupiter in July 1994. (ESO)

“The team measured the Doppler shift of hydrogen cyanide molecules — tiny changes in the frequency of radiation emitted by the molecules — caused by their motion driven by stratospheric winds on Jupiter,” says Thomas K Greathouse, Senior Research Scientist at Southwest Research Institute (SwRI), responsible for the development of the study and analysis of the observational results. “

“The high spectral and spatial resolution and the exquisite sensitivity of the observations at the wavelengths covered by ALMA allowed us to map such small Doppler shifts caused by the winds in the stratosphere all along the limb of Jupiter.”

Thomas K Greathouse, Senior Research Scientist at Southwest Research Institute (SwRI).

The fact that the team was able to obtain all the measurements they did with just 30 minutes of operating time with ALMA is a striking testament to the power and precision of the 66 antennas that make up the telescope array located in the Atacama Desert of Nothern Chile, currently the most powerful radio telescope on Earth.

“It was the availability of ALMA that made these measurements possible.  Previous radio observatory facilities did not have the combination of spectral and spatial resolution along with the high sensitivity needed to measure the winds as was done in this study,” Greathouse tells ZME Science. “Making further observations using ALMA to capture Jupiter at different orientations will allow us to study these winds in more detail and allow us to look for temporal variability in them as well. 

“Additionally, more extensive measurements will be possible from the JUICE mission and its Submillimetre Wave Instrument slated for launch in 2022.”

The Future of Jupiter Investigations

JUICE or JUpiter ICy moons Explorer is the first large-class mission in the European Space Agency’s (ESA) Cosmic Vision program and will arrive at Jupiter in 2029 when it will begin a three-year mission observing the gas giant in intense detail.

“This is why science is so much fun.  We have worked hard to understand a system–Jupiter’s stratosphere in this case–as best we can, we make our predictions about something–stratospheric wind behaviour–and then go test those predictions. If we are right, fantastic, we move on to the next problem, but if we are wrong we have learned something new and unique and can then continue making further studies to come to a more complete understanding of the system.”

Thomas K Greathouse, Senior Research Scientist at Southwest Research Institute (SwRI).
Amazing image of Jupiter taken in infrared light on the night of 17 August 2008 with the Multi-Conjugate Adaptive Optics Demonstrator (MAD) prototype instrument mounted on ESO’s Very Large Telescope. This false colour photo is the combination of a series of images taken over a time span of about 20 minutes, through three different filters. 9ESO/F. Marchis, M. Wong, E. Marchetti, P. Amico, S. Tordo)

For Cavalié, who has been involved with the measurement of Jupiter’s winds since 2009, the future is bright for such investigations and what they can tell us about the solar system’s largest planet and gas giants in general. “We now want to use ALMA again to characterize the temporal variability of the equatorial winds,” the astronomer says. “It is expected from temperature measurements and models that the direction of the equatorial winds should oscillate from eastward to westward with a period of about 4 years.”

The scientist is also clear, just because he and his colleagues have achieved a first, that doesn’t mean they are prepared to rest on their laurels. There are a lot of exciting developments on the way, and thus a lot of work to be done.

“We also want to observe the auroral winds during a Juno perijove pass to compare our data with observations of the poles by the spacecraft to better understand their origin and what maintains them,” he explains. “In addition, this study is a stepping stone for future investigations to be conducted using the same technique with JUICE and its Submillitre Wave Instrument.”

In addition to these missions, the ESO’s Extremely Large Telescope (ELT)–due to start operations later this decade–will also join investigations of Jupiter and should be capable of providing highly detailed investigations of the gas giant’s atmosphere.

“Jupiter and the giant planets are fascinating worlds. Understanding how these planets formed and how they work is a source of daily motivation, especially when working with world-class observatories like ALMA and participating in space missions to explore Jupiter and its satellites.”

Thibault Cavalié,  Laboratoire d’Astrophysique de Bordeaux, France.



What is mass? Baby don’t weigh me – revamping the metrology of mass

The metric system is due for a mass makeover, as scientists are preparing to redefine four basic units by the end of 2018 in an effort to provide accurate measurements at all scales.

The shift will most notably affect the kilogram, the base measure of mass and the last member of the International System of Units still defined by a physical object. Current efforts are under way to check and fine-tune measurements of fundamental natural quantities — such as Avogadro’s number — for use in giving the kilogram a new mathematical definition.

The kilogram standard.
Image via itsoktobesmart

How do we define a kilogram, and how will this change?

Since 1889, the standard for mass has been a 1-kilogram cylinder of platinum and iridium metal at the Bureau International des Poids et Mesures in Sèvres, France. While this standard is handled carefully, it’s at risk of becoming dirty or damaged, says Michael Stock, a physicist at the French bureau.

“Any material object can change over time,” he says.

“It’s also hard to accurately scale this physical standard down to very small masses, like those of electrons,” added physicist David Newell of the National Institute of Standards and Technology (NIST) in Gaithersburg, Md.

Scientists aim to give the kilogram a new definition based on nature’s fundamental physical constants. This task requires a highly accurate measurement of Planck’s constant, which links energy and frequency. Planck’s constant can be used to measure and describe mass, as the two are mathematically linked through another natural constant, the speed of light.

The American kilogram standard.
image via nist.gov

Researchers are using the existing physical definition of a kilogram to measure Planck’s constant as accurately as possible. Then, this value can be set in stone and used to define mass in the future.

With devices known as watt balances, scientists can measure Planck’s constant directly using precisely known standards of mass and electrical current. Once Planck’s constant has been fixed, watt balances will then use Planck’s constant to calculate unknown mass.

A watt balance. And a dude.
Image credits Robert Rathe

In another approach, scientists count the number of atoms in extremely pure 1-kilogram silicon spheres. This method determines the number of atoms in a kilogram, which could be used to define the unit of mass. This technique also allows scientists to calculate a different fundamental value, the Avogadro constant (or Avogadro’s number). This constant describes the number, roughly 6.02 x 1023, of units per mole, the metric unit for amount of a substance. (A mole is the mass of a substance equal to its atomic or molecular weight expressed in grams.) A precise Avogadro constant can be used to calculate and confirm Planck’s constant.

When the new value and its uncertainty is averaged with previous calculations, the Avogadro constant comes out to 6.02214082 x 1023 per mole with an uncertainty of 18 parts in a billion, scientists report July 14 in the Journal of Physical and Chemical Reference Data. This number is just slightly smaller than the value of the constant currently described by NIST — 6.022140857 x 1023 per mole.

The watt balance and atom-counting techniques now give a nearly identical value of Planck’s constant, currently given by NIST as 6.6260704 x10-34 joule-seconds, with an uncertainty of under 20 parts in a billion, says metrologist Ian Robinson of the National Physical Laboratory in Teddington, England. Further measurements are still under way.

Where is the metric system headed?

In fall 2018, international delegates at a meeting of the General Conference on Weights and Measures will decide whether or not to approve the kilogram’s new definition. Based on existing plans, many believe the redefinition will happen at this time, Stock says, though nothing is guaranteed.

Because researchers’ careful calculations have accounted for the existing definition of mass, the redefinition should cause no perceptible shift in measurement.

“If we do our jobs right, nobody’s going to notice a thing,” Newell says. But future mass measurements should become stable, Robinson says.

While redefining the kilogram will be the most critical change ahead, Stock says, scientists also hope to redefine other units, including the mole and the kelvin, which measures temperature. These redefinitions will depend on fixing other constants, including the Avogadro constant. Making all of these changes at once will limit the number of times textbooks must be changed, Stock says.

The redefinitions won’t mark an end to the quest for a perfect metric system, Newell says.

“Metrologists are going to make the measurement exactly right. And the corollary is, they never finish their measurement.”