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New look at Voyager data indicates that Uranus may have two more moons

October 26, 2016 Leave a comment
uranus-with-rings-and-moons-hubble-space-telescope-courtesy-wikimedia
This Hubble Space Telescope image of Uranus shows the planet surrounded by its four major rings and by 10 of its 27 known satellites. The false-color image was generated by Erich Karkoschka using data obtained by Hubble’s Near Infrared Camera and Multi-Object Spectrometer. Image courtesy NASA, Jet Propulsion Laboratory, Space Telescope Science Institute

Researchers at the University of Idaho say that Uranus, the distant gas giant known for being tipped by 90 degrees, might have two more moons than has previously been thought.

UI physics graduate student Robert O. Chancia and an assistant professor of physics at the university, Dr. Matthew M. Hedman, analyzed data obtained when Voyager 2 transmitted radio waves through Uranus’ rings.

They also looked at changes in the amount of light from distant stars that moves through the planet’s ring system.

Chancia and Hedman found that patterns in the distribution of ring material near the edges of Uranus’ alpha and beta rings vary over time, indicating that small moons may be present.

Dr. Richard G. French, a professor of astrophysics and director of the Whitin Observatory at Wellesley College in Massachusetts, said in an email message that the two UI scientists essentially used a meticulous process of comparison to determine that the pattern was caused by moons.

“Chancia and Hedman compared the wavelike properties from ring profiles taken at slightly different times and different geometries to work backwards to infer the properties of a nearby moon that might produce the satellite wakes,” he wrote.

Hedman pointed out that the results obtained when ground-based receiving stations interpreted the patterns of radio waves after they passed through the two rings shows that the waves were diffracted to varying degrees.

“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,” Hedman said. “There’s something breaking the symmetry.”

The researchers concluded that two possible small moonlets close to Uranus may be the culprits.

“We find for both rings that a moonlet located about 100 [kilometers] exterior to each ring could cause the optical depth variations seen in their occultation scans,” they write in the paper.

French explained that the proposed moons would be quite close to the alpha and beta rings.

“In this case, both moons are slightly exterior to the rings, so they orbit slightly more slowly than the rings themselves,” he wrote in an email message. “As the ring particles pass the moon, their orbits are slightly perturbed, resulting in a ripple pattern within the ring that is detectable as a periodic wavelike structure.”

Those “moonlet wakes” would help to maintain the structure of the Uranian rings, keeping them narrow. The rings are composed of a huge number of tiny particles, which  eventually spread out as collisions between them occur.

Moons near the rings can limit that effect.

French used the example of the planet that may be the solar system’s most famous example of a ring system to explain that a phenomenon called resonance could account for the confining  impact of small moons.

“If you are orbiting Saturn, for example, and you are a little ring particle and you orbit Saturn seven times and the little moon Prometheus six times in that same interval, that’s like getting pushed by the little finger on the swing,” he said. “That little push might be teeny but collectively is powerful. That’s kind of the notion that Matt and his student came up with. Those moons are in the right place to produce this wave pattern inside the rings.”

Chancia said in an email message that he and Hedman are not certain that such moonlet wakes occur in Uranus’ alpha and beta rings.

“We really just wanted to point it out as a possibility, because no one has come up with a universally accepted solution to how these rings are confined,” he wrote. “Anyway, the structures we found look like moonlet wakes.”

If they exist, the two moons would be Uranus’ smallest known and would have a diameter of four to 14 kilometers.

uranus-ring-scheme-courtesy-wikimedia
This diagram shows the Uranian ring scheme. Solid lines depict the rings, while dashed lines indicate the orbital paths of satellites. Graphic courtesy Wikimedia.
uranus-rings-image-courtesy-nasa
This silhouetted image of the rings of Uranus was taken by the Voyager 2 spacecraft on Jan. 24, 1986. A half-second exposure was made with the wide-angle camera at a distance of 63,300 kilometers (39,300 miles). This image shows the nine originally known rings appearing as dark lines against the brighter clouds of the planet. The most prominent ring, called epsilon, appears at the right; barely visible at the left are the three rings known simply as 4, 5 and 6. The resolution of the image is about nine kilometers (five miles). Image courtesy NASA, Jet Propulsion Laboratory.

The proposed moons, if they are there, were not seen by Voyager 2’s cameras. One reason is that the moons are likely so small that the 1970s-vintage equipment could not detect them.

“[G]iven the small predicted sizes of the ∝ and ß moonlets, a convincing detection may not be possible in the Voyager 2 images,” wrote Chancia and Hedman in their paper.

The two newly-hypothesized Uranian moons may also have a very low albedo,  which would make imaging of them difficult. Like the rings to which they are adjacent, they would not reflect much sunlight because the material from which they are constructed is not especially reflective.

“We know that the Uranian rings are dark because we can compare the amount of  light they block during a stellar occultation – a measure of how much material there is in the rings – with how bright they are in reflected sunlight,” French wrote in an email.

“The answer is that they are quite dark – they are not composed of pure water ice, and it’s likely that they are darkened by dust contamination and perhaps by charged particles in the Uranian environment.”

French explained that the two moons proposed by Chancia and Hedman are likely to exhibit the same characteristic.

“If the satellites are dark, too, then they are stealth objects,” he said. “They are also bloody far away.”

Uranus has 27 known moons, all named for literary characters in William Shakespeare’s plays, and 13 rings that have widths between one and 100 kilometers.

The planet’s ring system was discovered in 1977 by ground-based observers using the Kuiper Airborne Observatory.

Voyager 2 was launched in 1977. Voyager 1, a twin outer solar system probe, was sent into space the same year. The latter has now left the solar system and Voyager 2 is likely to do so within the next few years.

The Chancia and Hedman paper is to be published in The Astronomical Journal and appears online at ArXive.

Update, Oct. 26, 2017, 2:42 pm MDT: The word “part” was changed to the word “particle” in a quote by Professor Richard G. French in order to reflect the correct quotation.

NASA says Hubble data indicates possible water plumes on Europa

September 27, 2016 Leave a comment
europa-with-water-vapor-at-7-oclock-position-jan-26-2014-composite-image-image-of-europa-superimposed-on-hubble-data-courtesy-nasa-esa-w-sparks-stsci-usgs-astrogeology-science-center
This composite image shows suspected plumes of water vapor erupting at the 7 o’clock position off the limb of Jupiter’s moon Europa. The plumes were photographed by the Hubble Space Telescope Imaging Spectrograph and were seen in silhouette as the moon passed in front of Jupiter. Hubble’s ultraviolet sensitivity allowed for the suspected plumes, which rise at least 160 kilometers above Europa’s icy surface, to be observed. The Hubble STIS data was obtained on Jan. 26, 2014. The image of Europa is superimposed on the Hubble STIS data and was assembled from data gathered during the Galileo and Voyager missions. Image courtesy NASA, European Space Agency, Space Telescope Science Institute/W. Sparks, U.S. Geological Survey Astrogeology Science Center.

NASA announced Monday that scientists using the Hubble Space Telescope have found evidence of water plumes on Europa, which means that spacecraft may be able to explore the moon’s ocean without the need to penetrate its icy surface.

A research team led by astronomer William Sparks of the Space Telescope Science Institute in Baltimore used a technique that has often been instrumental in discovering exoplanets to identify the plumes.

As an exoplanet moves in front of its star, the light from that star changes as it passes through the exoplanet’s atmosphere. This happens because the light encounters a variety of molecules.

europa-transit-illustration-courtesy-a-field-stsci
This image from a STScI animation shows Europa as it transits Jupiter. Animation courtesy Space Telescope Science Institute, animation by A. Field.

On Europa, Sparks and his team noticed that the molecules in the moon’s atmosphere included water vapor. That led them to embark on a quest to determine whether water from Europa’s subsurface ocean is being expelled into space.

The scientists observed Europa pass in front of Jupiter, from which the Sun’s light would be reflected through the atmosphere of the Jovian moon, ten times in 15 months. On three of the occasions water vapor was detected.

“This is an exciting find because it potentially gives us access to the ocean below,” Sparks said at a NASA teleconference on Monday.

Scientists are interested in sampling Europa’s ocean because it may provide indications of whether the moon is, or ever has been, hospitable to life.

“On Earth, life is found wherever there is energy, water, and nutrients, so we have a special interest in any place that has those characteristics,” Dr. Paul Hertz, director of NASA’s astrophysics division in Washington, D.C., said. “Europa might be such a place.”

Europa has a large sub-surface ocean that is thought to contain more water than all of the oceans on Earth. However, the satellite also has a thick icy crust atop that ocean.

The water plumes may rise as high as 200 kilometers off Europa’s surface.

“Europa’s ocean is considered to be one of the most promising places that could potentially harbor life in the solar system,” Geoff Yoder, the acting associate administrator for NASA’s Science Mission Directorate, said. “These plumes, if they do indeed exist, may provide another way to sample Europa’s subsurface.”

A team led by Lorenz Roth of the Southwest Research Institute in Austin, Tex. identified water plumes rising from Europa’s south pole once during 2012.

The Roth group used the Hubble Space Telescope’s Imaging Spectrograph to identify hydrogen and oxygen ions by the ultraviolet radiation they emit after particles accelerated by Jupiter’s magnetic field split water molecules in the Europan atmosphere.

The STScI group also used the STIS instrument, but instead obtained imagery of Europa’s atmosphere in ultraviolet light.

“It’s a technique that complements the Roth team’s,” Dr. Jennifer J. Wiseman, an astrophysicist at NASA’s Goddard Space Flight Center and the agency’s senior scientist assigned to the Hubble Space Telescope, said.

Wiseman explained that STIS’ ultraviolet imaging capacity was particularly helpful to the STScI researchers.

“In ultraviolet light, the surface of Jupiter looks more uniform in color than in visible light, so that allowed the Sparks team to more clearly see the silhouette image of the possible plumes on Europa as the moon passed in front of the smooth Jupiter background,” she wrote in an email message.

The Roth team also used STIS during their quest for Europa’s plumes in 2012.

Sparks said that, notwithstanding a different methodology of investigation, the STScI results are similar to those found by Roth and his colleagues.

“When we calculate in a completely different way the amount of material that would be needed to create these absorption features, it’s pretty similar to what Roth and his team found,” he explained. “The estimates for the mass are similar, the estimates for the height of the plumes are similar. The latitude of two of the plume candidates we see corresponds to their earlier work.”

comparison-of-2014-transit-and-2012-europa-aurora-observations-transit-on-left-lorenz-roth-team-image-on-right-courtesy-nasa-esa-w-sparks-left-image-l-roth-right-image
These images compare the 2014 transit observed by the STSI team (left) and the 2012 spectroscopy obtained by the Roth team (right). Images courtesy NASA, European Space Agency, W. Sparks (left), L. Roth (right).

The STScI and Roth teams have not seen plumes erupting from Europa at the same times. Sparks and his colleagues observed what they believe to be water plumes in January, March, and April, 2014.

Wiseman said that detection of Europa’s plumes is difficult.

“Such plumes would be faint, probably intermittent, and the ultraviolet wavelengths of light being observed are at the high frequency edge of what Hubble can detect,” she wrote in an email message.

Sparks explained that he and his team do not claim that their work proves the plumes’ existence, though he also said Monday that he does not believe that any other explanation for the findings his team made is likely.

“In a formal sense, we have a statistically significant result,” Sparks said. “The problem is that there may be something we don’t understand about the instrument or the scene. It’s more of a subjective uncertainty than a quantitative uncertainty.”

“I’m not aware of any other plausible natural explanation for the appearance of these patches of absorption,” he continued.

Two of the water plumes that were apparently observed by Sparks and his team occurred near the south pole of Europa and one was seen near the moon’s equator.

A paper detailing the findings by the STScI team will be published in the Sept. 29 edition of Astrophysical Journal.

Saturn’s moon Enceladus is the only body in the solar system known to eject water vapor to space.

Imagery obtained by the Galileo spacecraft during the late 1990s indicated that Europa has an ocean. Observation of the moon’s magnetic fields confirmed its existence.

Wiseman said during Monday’s teleconference that NASA plans to use the James Webb Space Telescope, due to be launched in 2018, to further investigate the possible water plumes of Europa.

james-webb-space-telescope-to-launch-in-2018-courtesy-nasa
The James Webb Space Telescope will be launched in 2018. Artist’s conception courtesy NASA.

In addition, the European Space Agency’s Jupiter Icy Moon Mission (JUICE) and NASA’s planned Europa orbiter will have future opportunities to explore the Jovian satellite.

NOTE 1: This post was updated at 5:38 pm MDT on Sept. 27, 2016 to add a discussion of Dr. Jennifer Wiseman’s interview responses.

NOTE 2: This post was updated at 9:32 pm MDT on Sept. 27, 2016 to correct an inaccurate statement contained in the headline, correct several minor errors in the quotation of Dr. Jennifer Wiseman’s email communication, and correct the acronym applicable to the Space Telescope Science Institute.

NOTE 3: This post was updated at 9:37 pm MDT on Sept. 27, 2016 to clarify the difference between the Roth team’s use of the Hubble Space Telescope’s Imaging Spectrograph in 2012 and the STScI team’s use of that instrument in 2014.

Some interesting things to see in the night sky during April

April 4, 2016 Leave a comment

Video courtesy NASA

This month provides an opportunity to see a meteor shower and three planets in the night sky.

The highlight will continue to be the chance to see Jupiter, which will shine brightly after being at opposition on March 8. Look to the south-southeast in the early evening, just after twilight; the solar system’s largest planet is in the constellation Leo. On April 17 you can see it moving with the waxing gibbous moon.

Mercury will become visible on April 8 as a fairly bright object with a -1 magnitude. The closest planet to the Sun passed its perihelion on April 5 and can be seen low in the western sky about 30 or 40 minutes after sunset. By April 18 Mercury will become quite a bit less bright, though it will be about 20 degrees east of the Sun by then. That makes it easier to see because it will be higher in the sky and will take longer to set. Whenever you observe the meteor shower, try to find a location away from any artificial lights.

You can best see Mars on or about April 24, when it reaches a magnitude of about -1.2 to -1.4. It will appear below the waxing gibbous moon as a yellowish-orange object. Look to the constellation Opheuchus to find Mars.

The Lyrid meteor shower peaks before dawn on April 22, but its annual visit corresponds with a full moon. However, the meteor shower starts on April 16, which might allow for some visibility of objects falling through the atmosphere before the full moon arrives.

 

 

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Does Pluto have a frozen lake?

March 31, 2016 Leave a comment

NASA has released an image obtained by the New Horizons spacecraft that appears to show a frozen lake on Pluto.

The photograph is of a mountainous area north of Sputnik Planum and shows details as small as 130 meters in size.

The substance in the lake is not water. It is instead, in all likelihood, frozen nitrogen. If you look closely, you can also see features around the lake that may have once carried liquids.

Pluto must have had a thicker atmosphere that exerted greater pressure and conditions on the dwarf planet’s surface must have been warmer in the past if liquids flowed there.

NASA released the image on March 24.

Pluto's frozen bond, captured July 14, 2015

The frozen lake on Pluto, photographed by the New Horizons probe, may be 30 kilometers wide. Image courtesy NASA, Johns Hopkins University Advanced Physics Laboratory, Southwest Research Institute.

People in Singapore see total solar eclipse

March 9, 2016 Leave a comment

Earth experienced its first total solar eclipse since March 20, 2015 on Tuesday, with Indonesia being the locale on the planet where the spectacle could be seen.

The eclipse began at 8:38 pm EST and lasted for four minutes. It was visible to people in about half of Indonesia’s provinces. A report in the New York Times said that the town of Ternate, located in the Maluku Islands, was the “prime viewing location.”

A partial solar eclipse could be observed elsewhere in the south Pacific region, including in Hawaii.

Solar eclipses occur when the Moon passes between Earth and its star, temporarily blocking some or most of the sun’s light from reaching Earth. The event happens only about once per year because the plane of Moon’s orbit does not exactly match Earth’s orbit around the sun.

The Moon seems to block the sun because, while the sun is about 400 times larger than the Moon, it is also about 400 times farther away.

This image shows Tuesday’s eclipse in progress from South Tangerang, Indonesia:

2016_solar_eclipse_copyright_ridwan_arifiandi

Photo copyright Ridwan Arifiandi; Creative Commons license CC BY-NC 2.0

The March 2015 eclipse was visible in areas in the region of the North Atlantic Ocean, including the Faroe Islands and Norway’s Svalbard archipelago.

Cassini sends home picturesque view of Enceladus

February 26, 2016 Leave a comment

NASA’s Cassini spacecraft made its last fly-by of Enceladus on Dec. 19. Since then it has been transmitting to Earth images taken of the ocean moon, including this beauty received Feb. 15:

Enceladus - last fly-by, Dec. 19, 2015 - received at Earth Feb. 15, 2016 - courtesy NASA, JPL, SSI, Justin Cowart

This view of Enceladus is from about 83,000 kilometers. The moon is captured in winter. It’s north pole is to the upper left, while its south pole is obscured by darkness in the lower right. Courtesy NASA, Jet Propulsion Laboratory, Space Science Institute, Justin Cowart.

 

NASA releases Cassini image of three Saturnian moons

February 23, 2016 Leave a comment

As NASA’s Cassini probe continues its exploration of Saturn and its moons, it sends home some amazing images of that distant region of our solar system.

Yesterday, NASA released an image showing three of the gas giant’s moons: Tethys, Enceladus, and Mimas. Here it is:

Three moons, obtained Dec. 2015, released Feb. 22, 2016

Tethys (above the rings), Enceladus (just below center and below the rings), and Mimas (below and to the left of Enceladus) appear in this image obtained by Cassini in visible light on Dec. 3, 2015. Courtesy NASA, Jet Propulsion Laboratory – California Institute of Technology, Space Science Institute.

According to NASA:

The view was acquired at a distance of approximately 837,000 miles (1.35 million kilometers) from Enceladus, with an image scale of 5 miles (8 kilometers) per pixel. Tethys was approximately 1.2 million miles (1.9 million kilometers) away with an image scale of 7 miles (11 kilometers) per pixel. Mimas was approximately 1.1 million miles (1.7 million kilometers) away with an image scale of 6 miles (10 kilometers) per pixel.

Tethys’ diameter is about 1,066 kilometers. The cratered moon orbits Saturn from a distance of about 294,600 kilometers. That is about 20 percent farther from Saturn than the Moon is from Earth. Tethys is slightly less dense than liquid water, which suggests that its structure is mostly ice. This frigid moon is tidally locked to Saturn – it does not rotate and only one side of Tethys faces Saturn.

Enceladus is smaller than Tethys, with a diameter of about 500 kilometers. The host of a sub-surface liquid water ocean, Enceladus is covered by water ice and  reflects nearly all the sunlight that hits it. That makes it a very cold satellite. The surface temperature on the moon is about -201 degrees Celsius. It orbits Saturn from a mean distance of about 238,000 kilometers, which is about the same distance as Earth’s satellite is from our planet.

enceladus-small

This whimsical poster depicting Enceladus is part of NASA’s Visions of the Future project. You can see more at http://www.jpl.nasa.gov/visions-of-the-future/. Courtesy NASA, Jet Propulsion Laboratory – California Institute of Technology.

Mimas is the smallest of Saturn’s major moons. It is heavily cratered, with a giant crater called Herschel stretching across about one-third of its surface. That feature has led Mimas to sometimes be called the “Death Star moon” because it evokes the fictional planet-destroying spacecraft in the Star Wars films. Mimas’ diameter is about 400 kilometers; it orbits Saturn from a mean distance of about 200,000 kilometers.

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