Category: Neptune

Neptune’s largest moon, Triton. Image credit: NASA. Click to enlarge
Neptune’s moon Triton is unique in the Solar System because it’s the only large moon that orbits in the opposite direction to its planet’s rotation. Researchers have developed a computer model that explains how Neptune could have captured Triton from another planet during a close approach. Under this scenario, Triton was originally part of a binary system with another planet. They got too close to Neptune and Triton was torn away.

Neptune’s large moon Triton may have abandoned an earlier partner to arrive in its unusual orbit around Neptune. Triton is unique among all the large moons in the solar system because it orbits Neptune in a direction opposite to the planet’s rotation (a “retrograde” orbit). It is unlikely to have formed in this configuration and was probably captured from elsewhere.

In the May 11 issue of the journal Nature, planetary scientists Craig Agnor of the University of California, Santa Cruz, and Douglas Hamilton of the University of Maryland describe a new model for the capture of planetary satellites involving a three-body gravitational encounter between a binary and a planet. According to this scenario, Triton was originally a member of a binary pair of objects orbiting the Sun. Gravitational interactions during a close approach to Neptune then pulled Triton away from its binary companion to become a satellite of Neptune.

“We’ve found a likely solution to the long-standing problem of how Triton arrived in its peculiar orbit. In addition, this mechanism introduces a new pathway for the capture of satellites by planets that may be relevant to other objects in the solar system,” said Agnor, a researcher in UCSC’s Center for the Origin, Dynamics, and Evolution of Planets.

With properties similar to the planet Pluto and about 40 percent more massive, Triton has an inclined, circular orbit that lies between a group of small inner moons with prograde orbits and an outer group of small satellites with both prograde and retrograde orbits. There are other retrograde moons in the solar system, including the small outer moons of Jupiter and Saturn, but all are tiny compared to Triton (less than a few thousandths of its mass) and have much larger and more eccentric orbits about their parent planets.

Triton may have come from a binary very similar to Pluto and its moon Charon, Agnor said. Charon is relatively massive, about one-eighth the mass of Pluto, he explained.

“It’s not so much that Charon orbits Pluto, but rather both move around their mutual center of mass, which lies between the two objects,” Agnor said.

In a close encounter with a giant planet like Neptune, such a system can be pulled apart by the planet’s gravitational forces. The orbital motion of the binary usually causes one member to move more slowly than the other. Disruption of the binary leaves each object with residual motions that can result in a permanent change of orbital companions. This mechanism, known as an exchange reaction, could have delivered Triton to any of a variety of different orbits around Neptune, Agnor said.

An earlier scenario proposed for Triton is that it may have collided with another satellite near Neptune. But this mechanism requires the object involved in the collision to be large enough to slow Triton down, but small enough not to destroy it. The probability of such a collision is extremely small, Agnor said.

Another suggestion was that aerodynamic drag from a disk of gas around Neptune slowed Triton down enough for it to be captured. But this scenario puts constraints on the timing of the capture event, which would have to occur early in Neptune’s history when the planet was surrounded by a gas disk, but late enough that the gas would disperse before it slowed Triton’s orbit enough to send the moon crashing into the planet.

In the past decade, many binaries have been discovered in the Kuiper belt and elsewhere in the solar system. Recent surveys indicate that about 11 percent of Kuiper belt objects are binaries, as are about 16 percent of near-Earth asteroids.

“These discoveries pointed the way to our new explanation of Triton’s capture,” Hamilton said. “Binaries appear to be a ubiquitous feature of small-body populations.”

The binary Pluto and its moon Charon and the other binaries in the Kuiper belt are especially relevant for Triton, as their orbits abut Neptune’s, he said.

“Similar objects have probably been around for billions of years, and their prevalence indicates that the binary-planet encounter that we propose for Triton’s capture is not particularly restrictive,” Hamilton said.

The exchange reaction described by Agnor and Hamilton may have broad applications in understanding the evolution of the solar system, which contains many irregular satellites. The researchers plan to explore the implications of their findings for other satellite systems.

This research was supported by grants from NASA’s Planetary Geology and Geophysics, Outer Planet Research, and Origins of Solar Systems programs.

Original Source: UC Santa Cruz

Blue-green Neptune and its satellites. Image credit: NASA/ESA Click to enlarge
New NASA Hubble Space Telescope images of the distant planet Neptune show a dynamic atmosphere and capture the fleeting orbits of its satellites. The images have been assembled into a time-lapse movie revealing the orbital motion of the satellites.

Images were taken in 14 different colored filters probing various altitudes in Neptune’s deep atmosphere so that scientists can study the haze and clouds in detail.

These are several snapshots from the Neptune movie.

The natural-color view of Neptune (to left), common to naked eye telescopic views by amateur astronomers, reveals a cyan colored planet. Methane gas in Neptune’s atmosphere absorbs most of the red sunlight hitting the planet, making it look blue-green. The image was created by combining images in red, green, and blue light.

Neptune’s subtle features are more visible in the enhanced-color view (top right). Images taken in special methane filters show details not visible to the human eye (bottom right). The features seen in this enhanced image must be above most of the sunlight-absorbing methane to be detectable through these special filters.

The planet is so dark at the methane wavelengths that long exposures can be taken, revealing some of Neptune’s smaller moons. Clockwise from the top (in composite image at left), these moons are Proteus (the brightest), Larissa, Despina, and Galatea. Neptune had 13 moons at last count.

Neptune is the most distant giant planet in our Solar System, orbiting the Sun every 165 years. It is so large tht nearly 60 Earths could fit inside it. A day on Neptune is between 14 hours and 19 hours. The inner two thirds of Neptune is composed of a mixture of molten rock, water, liquid ammonia and methane. The outer third is a mixture of heated gases comprised of hydrogen, helium, water and methane.

On April 29 and 30, 2005, Hubble images were taken every 4-5 hours, spaced at about a quarter of Neptune’s rotational period. These where combined to create a time-lapse movie of the dynamic planet.

Original Source: Hubble News Release

In 30 years, a nuclear-powered space exploration mission to Neptune and its moons may begin to reveal some of our solar system’s most elusive secrets about the formation of its planets — and recently discovered ones that developed around other stars.

This vision of the future is the focus of a 12-month planning study conducted by a diverse team of experts led by Boeing Satellite Systems and funded by NASA. It is one of 15 “Vision Mission” studies intended to develop concepts in the United States’ long-term space exploration plans. Neptune team member and radio scientist Professor Paul Steffes of the Georgia Institute of Technology’s School of Electrical and Computer Engineering calls the mission “the ultimate in deep space exploration.”

NASA has flown extensive missions to Jupiter and Saturn, referred to as the “gas giants” because they are predominantly made up of hydrogen and helium. By 2012, these investigations will have yielded significant information on the chemical and physical properties of these planets. Less is known about Neptune and Uranus — the “ice giants.”

“Because they are farther out, Neptune and Uranus represent something that contains more of the original – to use a ‘Carl Saganism’ – ‘solar stuff’ or the nebula that condensed to form planets,” Steffes said. “Neptune is a rawer planet. It is less influenced by near-sun materials, and it’s had fewer collisions with comets and asteroids. It’s more representative of the primordial solar system than Jupiter or Saturn.”

Also, because Neptune is so cold, its structure is different from Jupiter and Saturn. A mission to investigate the origin and structure of Neptune — expected to launch between 2016 and 2018 and arrive around 2035 — will increase scientists’ understanding of diverse planetary formation in our solar system and in others, Steffes noted.

The mission team is also interested in exploring Neptune’s moons, especially Triton, which planetary scientists believe to be a Kuiper belt object. Such balls of ice are micro planets that can be up to 1,000 kilometers in diameter and are generally found in the outermost regions of our solar system. Based on studies to date, scientists believe Triton was not formed from Neptune materials, like most moons orbiting planets in our solar system. Instead, Triton is likely a Kuiper belt object that was accidentally pulled into Neptune’s orbit.

“Triton was formed way out in space,” Steffes said. “It is not even a close relative of Neptune. It’s an adopted child?. We believe Kuiper belt objects like Triton were key to the development of our solar system, so there’s a lot of interest in visiting Triton.”

Though they face a number of technical challenges — including entry probe design, and telecommunications and scientific instrument development — the Neptune Vision Mission team has developed an initial plan. Team members, including Steffes, have been presenting it this fall at a variety of scientific meetings to encourage feedback from other experts. On Dec. 17, they will present it again at the annual meeting of the American Geophysical Union. Their final recommendations are due to NASA in July 2005.

The plan is based on the availability of nuclear-electric propulsion technology under development in NASA’s Project Prometheus. A traditional chemical rocket would launch the spacecraft out of Earth orbit. Then an electric propulsion system powered by a small nuclear fission reactor – a modified submarine-type technology — would propel the spacecraft to its deep-space target. The propulsion system would generate thrust by expelling electrically charged particles called ions from its engines.

Because of the large scientific payload a nuclear-electric propelled spacecraft can carry and power, the Neptune mission holds great promise for scientific discovery, Steffes said.

The mission will employ electrical and optical sensors aboard the orbiter and three probes for sensing the nature of Neptune’s atmosphere, said Steffes, an expert in remote radio sensing of planetary atmospheres. Specifically, the mission will gather data on Neptune’s atmospheric elemental ratios relative to hydrogen and key isotopic ratios, as well as the planet’s gravity and magnetic fields. It will investigate global atmospheric circulation dynamics, meteorology and chemistry. On Triton, two landers will gather atmospheric and geochemical information near geysers on the surface.

The mission’s three entry probes will be dropped into Neptune’s atmosphere at three different latitudes – the equatorial zone, a mid-latitude and a polar region. Mission designers face the challenge of transmitting data from the probes through Neptune’s radiowave-absorbing atmosphere. Steffes’ lab at Georgia Tech has conducted extensive research and gained a thorough understanding of how to address this problem, he noted.

The mission team is still discussing how deep the probes should be deployed into Neptune’s atmosphere to get meaningful scientific data. “If we pick a low enough frequency of radio signals, we can go down to 500 to 1,000 Earth atmospheres, which is 7,500 pounds of pressure per square inch (PSI),” Steffes explained. “That pressure is similar to what a submarine experiences in the deep ocean.”

However, that depth will probably not be required, according to the mission team’s atmospheric modelers, Steffes said. The probes will be able to obtain most information at only 100 Earth atmospheres, or 1,500 PSI.

Original Source: Georgia Tech News Release

Image credit: NASA

Researchers from the Southwest Research Institute believe they have a theory that could help explain why there are so few objects in the Kuiper belt – a band of objects outside the orbit of Neptune. According to theories of how planetary systems form, there should be 100 times more material in the Kuiper belt than astronomers have observed. The researchers believe that the gas giants, including Neptune, formed closer to the Sun, and have slowly drifted further out over time. As Neptune migrated out, it could have pushed the Kuiper objects out of the solar system.

A new study by researchers at Southwest Research Institute (SwRI) and the Observatoire de la C?te d’Azur provides an explanation for one of the more mysterious aspects of the population of objects beyond Neptune. In doing so, it provides a unique glimpse into the proto-planetary disk from which the Solar System’s planets formed. Results will be published in the November 27 issue of Nature.

The Kuiper belt is a region of the Solar System that extends outward from Neptune’s orbit, containing billions of icy objects from kilometers to thousands of kilometers across. It was discovered in 1992 and, since that time nearly 1,000 objects have been cataloged. Some of these objects are very large – the largest having a diameter of more than 1,000 kilometers.

As astronomers have studied this structure, a mystery has unfolded. Like most of the planets in the Solar System, the large Kuiper belt objects are believed to have been formed from smaller objects that stuck together when they collided. For this process to have worked in the distant regions beyond Neptune, the Kuiper belt would have to contain more than 10 times the amount of material than is in the Earth. However, telescopic surveys of this region show that it currently contains roughly one-tenth the mass of the Earth, or less.

To solve the puzzle, researchers have been searching for several years for a way to remove more than 99 percent of the Kuiper belt’s material. However, Dr. Harold Levison (SwRI) and Dr. Alessandro Morbidelli (Observatoire de la C?te d’Azur of Nice, France) describe in their article, “Forming the Kuiper Belt by the Outerward Transport of Objects During Neptune’s Migration,” that the Kuiper belt may not have lost much mass at all.

“The mass depletion problem has been sticking in our throat for some time,” says Levison, a staff scientist in the SwRI Space Studies Department. “It looks like we may finally have a possible answer.”

Levison and Morbidelli argue that the proto-planetary disk from which the planets, asteroids and comets all formed had a heretofore unanticipated edge at the current location of Neptune, which is at 30 astronomical units (AU, the average distance between the Sun and Earth), and that the region now occupied by the Kuiper belt was empty. All the Kuiper belt objects we see beyond Neptune formed much closer to the Sun and were transported outward during the final stages of planet formation.

Researchers have known for 20 years that the orbits of the giant planets moved around as they formed. In particular, Uranus and Neptune formed closer to the Sun and migrated outward. Levison and Morbidelli show that Neptune could have pushed all the observed Kuiper belt objects outward as it migrated.

“We really didn’t solve the mass depletion problem, we circumvented it,” says Levison. “According to our work, the void beyond Neptune was probably devoid of objects.”

However, in this model, the region interior to 30 AU contained enough material for the Kuiper belt objects to form. The mechanisms employed by Neptune to push out the Kuiper belt only affected a small fraction of the objects. These became the objects seen by astronomers; the rest were scattered out of the Solar System by Neptune. This new theory explains many of the observable features of the outer Solar System, including the characteristics of the orbits of the Kuiper belt objects and the location of Neptune.

“One of the puzzling aspects of Neptune’s migration is why it stopped where it did,” says Morbidelli. “Our new model explains this as well. Neptune migrated until it hit the edge of the proto-planetary disk, at which point it abruptly stopped.”

NASA, the National Science Foundation and the Centre National de la Recherche Scientifique in Paris funded this research.

Original Source: SwRI News Release

Image credit: NASA

A team of astronomers from the Harvard-Smithsonian Center for Astrophysics have discovered three previously unknown moons orbiting the planet Neptune. Since they’re only 30-40km across, the moons were a challenge to spot. The team had to digitally merge multiple exposures of the planet moving across a background of stars. Over time, the planets and their motions were picked up as points of light. This brings the gas giant’s total to 11 known moons.

A team of astronomers led by Matthew Holman (Harvard-Smithsonian Center for Astrophysics) and JJ Kavelaars (National Research Council of Canada) has discovered three previously unknown moons of Neptune. This boosts the number of known satellites of the gas giant to eleven. These moons are the first to be discovered orbiting Neptune since the Voyager II flyby in 1989, and the first discovered from a ground-based telescope since 1949.

It now appears that each giant planet’s irregular satellite population is the result of an ancient collision between a former moon and a passing comet or asteroid. “These collisional encounters result in the ejection of parts of the original parent moon and the production of families of satellites. Those families are exactly what we’re finding,” said Kavelaars.

The team that discovered these new satellites of Neptune includes Holman and Kavelaars, graduate student Tommy Grav (University of Oslo & Harvard-Smithsonian Center for Astrophysics), and undergraduate students Wesley Fraser and Dan Milisavljevic (McMaster University, Hamilton, Ontario, Canada).

Needle in a Haystack

The new satellites were a challenge to detect because they are only about 30-40 kilometers (18-24 miles) in size. Their small size and distance from the Sun prevent the satellites from shining any brighter than 25th magnitude, about 100 million times fainter than can be seen with the unaided eye.

To locate these new moons, Holman and Kavelaars utilized an innovative technique. Using the 4.0-meter Blanco telescope at the Cerro Tololo Inter-American Observatory, Chile, and the 3.6-meter Canada-France-Hawaii Telescope, Hawaii, they took multiple exposures of the sky surrounding the planet Neptune. After digitally tracking the motion of the planet as it moved across the sky, they then added many frames together to boost the signal of any faint objects. Since they tracked the planet’s motion, stars showed up in the final combined image as streaks of light, while the moons accompanying the planet appeared as points of light.

Prior to this find, two irregular satellites and six regular satellites of Neptune were known. The two irregular satellites are Triton, discovered in 1846 by William Lassell, and Nereid, discovered in 1949 by Gerard Kuiper. Triton is considered irregular because it orbits the planet in a direction opposite to the planet’s rotation, indicating that Triton is likely a captured Kuiper Belt Object. (The Kuiper Belt is a disk-shaped collection of icy objects that circle the Sun beyond the orbit of Neptune.) Nereid is considered irregular because it has a highly elliptical orbit around Neptune. In fact, its orbit is the most elliptical of any satellite in the solar system. Many scientists believe that Nereid once was a regular satellite whose orbit was disrupted when Triton was gravitationally captured. The six regular satellites were discovered by the Voyager probe during its encounter with Neptune. The three new satellites were missed by Voyager II because of their faintness and great distance from Neptune. According to Holman, “The discovery of these moons has opened a window through which we can observe the conditions in the solar system at the time the planets were forming.”

Tracking Faint Blips

The researchers are currently conducting follow-up observations to better define the orbits of the newfound moons using orbital predictions supplied by Brian Marsden (Director of the Minor Planet Center in Cambridge, Mass.) and Robert Jacobson (Jet Propulsion Laboratory).

To follow up the initial find, team members Brett Gladman (University of British Columbia, Canada); Jean-Marc Petit, Philippe Rousselot, and Olivier Mousis (Observatoire de Besancon, France); and Philip Nicholson and Valerio Carruba (Cornell University) conducted additional observations using the Hale 5-meter telescope on Mount Palomar and one of the four 8.2-meter telescopes of the European Southern Observatory’s Very Large Telescope at Paranal Observatory, Chile. Grav made additional tracking observations using the 2.6-meter Nordic Optical Telescope on La Palma, Spain.

Holman says, “Tracking these moons is an enormous, international undertaking involving the efforts of many people. Without teamwork, such faint objects could be easily lost.”

Based in La Serena, Chile, the Cerro Tololo Inter-American Observatory is part of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation.

The Canada-France-Hawaii Telescope is operated by the CFHT Corporation under a joint agreement between the National Research Council of Canada, the Centre National de la Recherche Scientifique of France, and the University of Hawaii.

The European Southern Observatory is an intergovernmental, European organization for astronomical research. It has ten member countries. ESO operates astronomical observatories in Chile and has its headquarters in Garching, near Munich, Germany.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists organized into six research divisions study the origin, evolution, and ultimate fate of the universe.

Original Source: CfA News Release

Astronomers have discovered a new object which shares a very similar orbit with Neptune. Part of a classification of objects called Trojans, 2001 QR322 is 230 km across and requires 166 years to orbit the Sun. Although clusters of Trojans have been found following Jupiter’s orbit, none have ever been found to share an orbit with any other giant planet; although, they’ve been predicted for years.