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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.

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Astronomers discover moon orbiting dwarf planet Makemake

June 17, 2016 Leave a comment
makemake's moon

This image obtained by the Hubble Space Telescope in April 2015 shows a moon – the first discovered – in orbit around the dwarf planet Makemake. The tiny satellite, located just above Makemake in this image, is barely visible because it is almost lost in the glare of the very bright dwarf planet. Image courtesy NASA, European Space Agency, Southwest Research Institute.

Makemake, a dwarf planet far beyond Pluto in the Kuiper Belt, has a moon.

Imagery obtained by the Hubble Space Telescope detected the satellite last April, according to a recently published paper.

The moon, which has been temporarily catalogued as S/2015 (136472) 1 and nicknamed MK2, is about 100 miles in diameter. It was observed orbiting Makemake from a distance of about 13,000 miles.

MK2 probably orbits Makemake in an orientation that makes it difficult for astronomers to observe the moon.

“Our preliminary estimates show that the moon’s orbit seems to be edge-on, and that means that often when you look at the system you are going to miss the moon because it gets lost in the bright glare of Makemake,” Dr. Alex H. Parker, a planetary scientist at Southwest Research Institute in Boulder, Colo., said.

Parker led the team that analyzed the Hubble image of MK2.

One implication of the Makemakean moon’s discovery is an increased ability to measure Makemake’s mass and density.

To measure the dwarf planet’s mass, scientists will need to overcome the challenges inherent in observing MK2’s orbit. If they succeed in doing so, and because the distance from Earth to Makemake is already known, it would be possible to use the orbital period and the mean distance of MK2 from Makemake to determine Makemake’s mass.

To determine Makemake’s density, scientists must know its volume, which in turn requires knowledge of the dwarf planet’s diameter. Makemake’s diameter is about 1,500 kilometers.

Once the volume is determined, density can be calculated by dividing Makemake’s mass by its volume.

makemake-artist-view

Early artist’s impression of Makemake. Courtesy NASA.

Astronomers used a camera on the Hubble Space Telescope to find MK2. That instrument, called Widefield Camera 3, is able to capture images across the radiation spectrum.

Widefield Camera 3 has a 16 megapixel capacity and can capture an image as large as 160 arcseconds by 160 arcseconds in size.

Makemake is the second-brightest object, after Pluto, in the Kuiper Belt and is, like Pluto, covered with methane ice.

The dwarf planet, which was discovered in 2005, is named for a deity that is worshiped by native people of Easter Island.

In addition to Makemake, there are four other dwarf planets in the solar system: Pluto, Eris, Haumea, and Ceres.

With the discovery of MK2, astronomers now know that all of the solar system’s dwarf planets have moons.

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.

Looking back at Cassini: A last look at Dione

February 23, 2016 Leave a comment

NASA’s Cassini probe is in its last year of operation, continuing to explore the Saturn system, and it seems appropriate to look back at some of the amazing images the spacecraft has sent home to Earth.

This one was taken on August 17, 2015, during the probe’s last flyby of Saturn’s moon Dione.

Dione, Aug. 2016

Icy Dione, as seen by Cassini last August. You can see some of the rings of Saturn, as well as the solar system’s second largest planet, in the background. Courtesy NASA.

Dione (Di-OH-nee) is a small moon, with a diameter of only about 1,120 kilometers, and it orbits Saturn at about the same distance as the Moon orbits Earth. Covered by ice for which one of Saturn’s rings is the main source, Dione has many craters. Some are as large as 100 kilometers across.

Like Earth’s sole satellite, Dione does not rotate. Only one side of the moon ever faces Saturn.

Giovanni Cassini discovered Dione in 1864. The moon takes its name from the mythological Greek goddess who was said by Homer to be the mother of Aphrodite.

NASA: Charon, Pluto’s moon, may once have had ocean

February 21, 2016 1 comment

In recent years humanity has learned that oceans are not unique to Earth. Discoveries of them on Saturn’s moons Enceladus and Titan and Jupiter’s moon Europa indicate that oceans may actually be ubiquitous in our solar system.

Now, it seems, it is possible that an ocean once existed on a satellite of the dwarf planet Pluto.

NASA said Friday that images obtained by the New Horizons probe suggest that Charon once had a subsurface sea that expanded outward as it froze, cracking the surface of the moon.

The photographs show tectonic faults, including some valleys more than 6.5 kilometers deep, that indicate an expansion of Charon sometime in the past.

Here’s how it likely happened:

The outer-most layer of Charon, including its surface, is now water ice. But the satellite’s internal heat would have caused that ice to melt deep under the surface, which would have resulted in a subsurface ocean. As the decay of radioactive elements that constitute Charon’s structure subsided, and as the rocky object cooled as the time since its formation passed, that ocean would have frozen. Just as the change of water’s state from liquid to solid on Earth causes an increase in the the compound’s volume, ice formation under the surface of Charon would have pushed the surface outward, resulting in the valleys and also numerous ridges and scarps.

Charon images, Feb. 2016

This image shows canyons on Charon. The blow-up photo at top right shows Serenity Chasma, one of many chasms near Charon’s equator. Those chasms constitute a system that is at least 1,800 kilometers long. The blow-up image at bottom right shows the topography of the same area and indicates that ice melted and re-froze there. Images courtesy NASA, Johns Hopkins University Applied Physics Laboratory, Southwest Research Institute.

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