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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.

Spacecraft in solar orbit regains contact with Earth

August 24, 2016 Leave a comment
STEREO image of whole sun, Feb. 2, 2011 - courtesy NASA

This image shows our sun’s far side. It was obtained by the STEREO probes on Feb. 2, 2011 at 23:56 UT. Image courtesy NASA.

A probe launched by NASA in 2006 has resumed communication with the agency after nearly 23 months of silence.

The STEREO-B spacecraft, which orbits the sun, lost contact with Earth on Oct. 1, 2014.

The Deep Space Network reestablished the link with STEREO-B at 6:21 pm EDT on Sunday.

The long interruption in communication with the spacecraft was most likely the result of a series of events that began with a test of its command loss timer. The device is a kind of automated switch that allows the spacecraft to recover after a hardware failure. It functions by re-setting the hardware if no commands are received for a certain period of time. In STEREO-B’s case, that re-set time was three days.

NASA explained a likely scenario for the communications failure at a website posted shortly after the loss of contact event and still available here. An animated film that provides one possible model for the probe’s communication failure is also available.

STEREO-B’s navigation system probably failed because it was unable to detect guide stars. This caused the probe’s high gain antenna to be pointed away from Earth, which meant that it could not receive a signal.  The breakdown in the star tracking system was likely the result of a failed laser.

“Basically, we made a mistake in not accounting for one of those individual lasers failing,” Dr. Joseph Gurman, the STEREO project scientist at NASA’s Goddard Space Flight Center, said. “The data still looked good coming out of the unit as a whole even though one laser was bad. That got us into a situation where the spacecraft was getting bad navigational information.”

STEREO-B remains in an uncontrolled spin, a problem for which there is not currently enough power available to correct. The spacecraft obtains energy by means of solar arrays that extend out from its main section.

Gurman explained that NASA scientists are not sure how much power the probe’s batteries can produce or whether they can be fully re-charged.

“We don’t know if the batteries are damaged,” he said. “We know they can take some charge.”

Whether the spacecraft’s instrumentation is still functional is also unknown.

“I would say that we know nothing about the state of the instruments at this point,” Gurman said.

NASA will proceed cautiously to investigate the STEREO-B probe’s status. What Gurman and his colleagues want to avoid is any command that would return STEREO-B to an uncontrolled spin.

“We have an inertial problem that is giving bad information to the control system on the spacecraft,” he said. “We have to proceed in a step-by-step method.”

The first step will be to figure out the extent to which the probe has, to put it metaphorically, any gas in its tank.

“We have to heat up the probably frozen fuel in the fuel tank,” Gurman explained. “We can proceed from there to use the thrusters to right our attitude by using the autonomy software.”

Gurman is optimistic, though, about the prospects for obtaining more data about the sun from STEREO-B. NASA’s prior experience with another spacecraft that experienced a communication failure – the Solar and Heliospheric Observatory – indicates that instruments can survive with little or no damage, even in the cold of space, for quite some time.

“On SOHO, there were 12 principal investigator experiments,” Gurman said. “Only one mechanism in one telescope was damaged in such a way that we really couldn’t use the instrument. There was one instrument that suffered some degradation. That’s about it. We’re cautiously optimistic that we’ll be able to regain most of the scientific capability, if not all, that we had before.”

In any case, the STEREO mission formally ended eight years ago, so any data obtained from STEREO-B is beyond what was expected at the time of launch.

“Anything we get is gravy, to say the least,” Gurman said.

The probe’s twin, STEREO-A, also revolves around our closest star.

STEREO is an acronym that shortens the twin probes’ formal name – Solar Terrestrial Relations Observatories.

 

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.

ISS astronaut shares photo of moon

April 1, 2016 Leave a comment

Tim Kopra, a NASA astronaut on board the International Space Station, posted to his Twitter account Friday a photo of the moon. The photo is worthy of a share:

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.

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.

 

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