Planet Mercury as seen from the MESSENGER spacecraft in 2008. Credit: NASA/JPL
Virtually every planet in the Solar System has moons. Earth has The Moon, Mars has Phobos and Deimos, and Jupiter and Saturn have 67 and 62 officially named moons, respectively. Heck, even the recently-demoted dwarf planet Pluto has five confirmed moons – Charon, Nix, Hydra, Kerberos and Styx. And even asteroids like 243 Ida may have satellites orbiting them (in this case, Dactyl). But what about Mercury?
If moons are such a common feature in the Solar System, why is it that Mercury has none? Yes, if one were to ask how many satellites the planet closest to our Sun has, that would be the short answer. But answering it more thoroughly requires that we examine the process through which other planets acquired their moons, and seeing how these apply (or fail to apply) to Mercury.
Special Guest: Dr. Carolyn Porco is the leader of the Cassini Imaging Science team and the Director of the Cassini Imaging Central Laboratory for Operations (CICLOPS) at the Space Science Institute in Boulder, Colorado.
Artist's impression of Pluto and its moons. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute
Over the course of the past decade, many amazing discoveries have been made at the edge of the Solar System. Thanks to the work of astronomers working out of Earth-based observatories, with the Hubble Space Telescope, and those behind the recent New Horizons mission, not only have new objects been discovered, but additional discoveries have been made about the ones we already knew about.
For example, in 2005, two additional satellites were discovered in orbit of Pluto – Hydra and Nix. The discovery of these moons (which has since been followed by the discovery of two more) has taught astronomers much about the far-flung system of Pluto, and helped to advance our understanding of the Kuiper Belt.
Discovery and Naming:
Nix was discovered in June of 2005 by the Hubble Space Telescope Pluto Companion Search Team, using discovery images that were taken on May 15th and 18th, 2005. The team was composed of Hal A. Weaver, Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot F. Young, and Leslie A. Young.
The discovery images of Nix (and Hydra) obtained by the Hubble Space Telescope. Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI)
Nix and Hydra were also independently discovered by Max J. Mutchler on June 15th, 2005, and by Andrew J. Steffl on August 15th, 2005. At the time, Nix was given the provisional designation of S/2005 P 2 and casually referred to as “P2”. Once pre-recovery images from 2002 were confirmed, the discoveries were announced on October 31st, 2005.
In accordance with IAU guidelines concerning the naming of satellites in the Solar System, the moon was named Nix. Derived from Greek mythology, Nix is the goddess of darkness and night, the mother of Charon and the ferryman of Hades (the Greek equivalent of Pluto) who brought the souls of the dead to the underworld.
The name was officially announced on June 21st, 2006, in an IAU Circular, where the designation “Pluto II” is also given. The initials N and H (for Nix and Hydra) were also a deliberate reference to the New Horizons mission, which would be conducting a flyby of the Pluto system in less than ten years time after the announcement was made.
Images acquired by the New Horizon’s probe of Nix (left) and Hydra (right) on July 14th, 2015. Credit: NASA/JHUAPL/SWRI
Size, Mass and Orbit: Based on observations with the Hubble Space Telescope of Nix’s geometric albedo and shape, the satellite was estimated to measure 56.3 km (35 mi) along its longest axis and 25.7 km (16 mi) wide. However, images provided by the New Horizons’ Ralph instrument on July 14th, 2015, indicated that Nix measures 42 km (26 mi) in length and 36 km (22 mi) wide.
Nix follows a circular orbit with very little eccentricity (0.0020) and a low inclination of approximately 0.13°. It is in the same orbital plane as Charon, is in a 3:2 orbital resonance with Hydra, and a 9:11 resonance with Styx. Its orbital period is roughly 24.9 days, meaning it takes about 25 days to complete a single orbit of Pluto.
As with Hydra and perhaps the other small Plutonian moons, Nix rotates chaotically, which is due mainly to its oblong shape. This means that the moon’s axial tilt and day length vary greatly over short timescales, to the point that it regularly flips over.
Composition: Early observations conducted by Marc W. Buie and William M. Grundy at the Lowell Observatory appeared to show that Nix has a reddish color like Pluto, but unlike any of its other moons. However, more-recent studies conducted by S. Alan Stern et al. using the Hubble Space Telescope’s Advanced Camera for Surveys (ACS), have indicated that it is likely as grey as the remaining satellites.
From these observations, it is likely that the surface of Nix is composed primarily of water ice (like Hydra) and may or may also have trace amount of methane ice on its surface. If true, then the exposure of these deposits of methane ice to ultra-violet radiation from the Sun would result in the presence of tholins, which would give it a reddish hue.
However, when the New Horizons space probe photographed Hydra and Nix during its flyby of the Pluto system, it spotted a large region with a distinctive red tint, probably a crater. The appearance of this surface region – a spot of red against an otherwise grey landscape – may explain these conflicting results.
Exploration: Thus far, only one mission has been performed to the Pluto system that resulted in close-up and detailed photographs of Nix. This would be the New Horizons mission, which flew through the Pluto-Charon system on July 14th, 2015 and photographed Hydra and Nix from an approximate distance of 640,000 km (400,000 mi).
Until July 13th, 2015, when NASA’s Long Range Reconnaissance Imager (LORRI) on board New Horizons determined Nix’s dimensions, its size was unknown. More images and information will be downloaded from the spacecraft between now and late 2016.
Prior to the discovery of Hydra and Nix in 2005, Pluto was believed to share its orbit with only the satellite of Charon – hence why astronomers often refer to it as the “Pluto-Charon system”. However, since the discovery of these two additional satellites in 2005, two more have been discovered – Kerberos in July of 2011 and Styx in July of 2012.
This raises the number of bodes in the Pluto-Charon system to one primary and five satellites. And thanks to the recent New Horizons flyby, we got to see all of them up close for the first time!
Like most large bodies in the Kuiper Belt (not to mention their satellites) much remains to be learned about Nix and its companions. In time, and with more missions to the outer Solar System, we are sure to address many of the mysteries surrounding this particular satellite, and will probably find many more waiting for us!
We have written many interesting articles on Pluto, its system of moons and the Kuiper Belt here at Universe Today.
A portrait from the final approach. Pluto and Charon display striking color and brightness contrast in this composite image from July 11, showing high-resolution black-and-white LORRI images colorized with Ralph data collected from the last rotation of Pluto. Color data being returned by the spacecraft now will update these images, bringing color contrast into sharper focus. Credits: NASA-JHUAPL-SWRI
Measurements by New Horizons gathered just in the past few days as the spacecraft barrels towards the Pluto planetary system now confirm that Pluto is indeed the biggest object in the vast region beyond the orbit of Neptune known as the Kuiper Belt.
Pluto is thus the undisputed King of the Kuiper Belt!
Pluto measures 1,473 miles (2,370 kilometers) in diameter, which is at the higher end of the range of previous estimates.
The big news was announced today, by New Horizons principal investigator Alan Stern of the Southwest Research Institute, Boulder, Colorado, during a live media briefing at Pluto mission control at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.
“This settles the debate about the largest object in the Kuiper Belt,” Stern noted.
New Horizons will swoop to within about 12,500 kilometers (nearly 7,750 miles) of Pluto’s surface and about 17,900 miles (28,800 kilometers) from Charon during closest approach at approximately 7:49 a.m. EDT (11:49 UTC) on July 14.
The new and definitive measurement of Pluto’s size is based on images taken by the high resolution Long Range Reconnaissance Imager (LORRI) to make this determination.
“The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest,” said mission scientist Bill McKinnon, Washington University, St. Louis.
Pluto was the first planet discovered by an American, Clyde Tombaugh.
Pluto’s “Heart” is seen in this new image from New Horizons’ Long Range Reconnaissance Imager (LORRI) received on July 8, 2015 after normal science operations resumed following the scary July 4 safe mode anomaly that briefing shut down all science operations. The LORRI image has been combined with lower-resolution color information from the Ralph instrument. Credits: NASA-JHUAPL-SWRI
Pluto is bigger than Eris, another big Kuiper Belt object discovered in 2005 by Mike Brown of Caltech, which is much further out from the Sun than Pluto. The discovery of Eris further fueled the controversial debate about the status of Pluto’s planethood.
Eris comes in second in size in the Kuiper Belt at only 1,445 miles (2,326 km) in diameter.
On July 11, 2015, New Horizons captured a world that is growing more fascinating by the day. For the first time on Pluto, this view reveals linear features that may be cliffs, as well as a circular feature that could be an impact crater. Rotating into view is the bright heart-shaped feature that will be seen in more detail during New Horizons’ closest approach on July 14. The annotated version includes a diagram indicating Pluto’s north pole, equator, and central meridian. Credits: NASA/JHUAPL/SWRI
Stern also noted that because Pluto is slight bigger than the average of previous estimates, its density is slightly lower than previously thought. Therefore the fraction of ice in its interior is slightly higher and the fraction of rock is slightly lower. But further data is required to pin the density down more precisely.
The uncertainty in Pluto’s size has persisted for decades and was due to the fact that Pluto has a very tenuous atmosphere composed of nitrogen.
Furthermore Pluto’s lowest atmospheric layer called the troposphere, is shallower than previously believed.
On the other hand, its largest moon Charon with which it forms a double planet, lacks a substantial atmosphere and its size was known with near certainty based on ground-based telescopic observation.
New Horizons LORRI imagery has confirmed that Charon measures 751 miles (1208 km) kilometers) across.
Stern also confirmed that frigid Pluto also has a polar cap composed of methane and nitrogen ices based on measurements from the Alice instrument.
LORRI has also zoomed in on two of Pluto’s smaller moons, Nix and Hydra.
“We knew from the time we designed our flyby that we would only be able to study the small moons in detail for just a few days before closest approach,” said Stern. “Now, deep inside Pluto’s sphere of influence, that time has come.”
The approximate sizes of Pluto’s moons Nix and Hydra compared to Denver, Colorado. While Nix and Hydra are illustrated as circles in this diagram, mission scientists anticipate that future observations by New Horizons will show that they are irregular in shape. Credits: JHUAPL/Google
But because they are so small, accurate measurement with LORRI could only be made in the final week prior to the July 14 flyby.
Nix is estimated to be about 20 miles (about 35 kilometers) across, while Hydra is roughly 30 miles (roughly 45 kilometers) across. These sizes lead mission scientists to conclude that their surfaces are quite bright, possibly due to the presence of ice.
Determinations about Pluto’s two smallest moons, Kerberos and Styx, will be made later at some point during the 16-month long playback of data after the July 14 encounter.
It has been three decades since we last visited planetary bodies at the outer reaches of our solar system when Voyager 2 flew past Uranus and Neptune in 1986 and 1989.
New Horizons’ last look at Pluto’s Charon-facing hemisphere reveals the highest resolution view of four intriguing darks spots for decades to come. This image, taken early the morning of July 11, 2015, shows newly-resolved linear features above the equatorial region that intersect, suggestive of polygonal shapes. This image was captured when the spacecraft was 2.5 million miles (4 million kilometers) from Pluto. Credit: NASA/JHUAPL/SWRI
New Horizons is closing in fast on its quarry at a whopping 31,000 mph (49,600 kph) after a nine year interplanetary voyage and is now less than half a million miles away, in the final hours before closest approach.
The New Frontiers spacecraft was built by a team led by Stern and included researchers from SwRI and the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. APL also operates the New Horizons spacecraft and manages the mission.
Watch for Ken’s continuing onsite coverage of the Pluto flyby on July 14 from the Johns Hopkins University Applied Physics Laboratory (APL).
Stay tuned here for Ken’s continuing Earth and planetary science and human spaceflight news.
Host: Fraser Cain (@fcain) Special Guest:Dr. Rhys Taylor, Former Arecibo Post Doc; Current research involves looking for galaxies in the 21cm waveband.
Pluto's solar system in a 2012 artist's conception. P4 and P5 are now called Kerberos and Styx, respectively. Credit: NASA/John Hopkins University Applied Physics Laboratory
A smash-up that created Pluto’s largest moon, Charon, likely sprayed debris four billion years ago that formed the genesis of the other moons scientists are spotting today, a new study concludes.
The find could explain why the satellites Styx, Nix, Kereberos and Hydra have orbital periods that are, respectively, just about exactly 3, 4, 5 and 6 times longer than Charon’s, scientists said.
“Any initially surviving satellites would likely be destroyed in collisions, but these shattered moons wouldn’t be lost; rather, their remains would stay in the Pluto/Charon system and become the starting point for building new satellites,” stated the Southwest Research Institute (SWRI), which led the study.
Artist’s impression of New Horizons’ encounter with Pluto and Charon. Credit: NASA/Thierry Lombry
“In modeling the destruction of the satellites, the SWRI study found that there may be a method for moving them, or their building blocks, outward, due to the competing effects of Charon’s gravitational kicks and collisions among the debris of the disrupted satellites.”
Given Charon’s large size relative to Pluto (it’s a tenth of the dwarf planet’s size, compared to the Earth-Moon 81: 1 ratio), its large mass could easily perturb these smaller moons if they got close. Also, collisions between the debris could alter the orbits “to keep things away from Charon”, the scientists said.
The findings were presented yesterday (Oct. 9) at the American Astronomical Association’s division of planetary sciences meeting in Denver; information on whether the results are peer-reviewed was not immediately available.
Pluto's solar system in a 2012 artist's conception. P4 and P5 are now called Kerberos and Styx, respectively. Credit: NASA/John Hopkins University Applied Physics Laboratory
It looks like Vulcan was not the logical choice for the International Astronomical Union when it came to naming Pluto’s new moons.
The internationally recognized body for astronomy names selected Kerberos and Styx as the new names for Plutonian moons P4 and P5, respectively. While these names were popular in a public vote last year concerning Pluto’s new moons, Vulcan — the overwhelming favorite, and backed by none other than Star Trek‘s Captain Kirk (William Shatner) — was not selected.
The Search For Extraterrestrial Intelligence (SETI) said Vulcan, which was first popularized in the 1960s as the home world of Star Trek character Spock, was considered.
“The IAU gave serious consideration to this name, which happens to be shared by the Roman god of volcanoes. However, because that name has already been used in astronomy, and because the Roman god is not closely associated with Pluto, this proposal was rejected,” a release stated.
Vulcan received the support of William Shatner, pictured here in his Star Trek role as Captain James Kirk. Credit: Paramount
There will be more about Styx and Kerberos in this SETI-hosted Google Hangout, which will be held live starting at noon Eastern (4 p.m. GMT).
Kerberos is a three-headed dog in Greek mythology and Styx a mythological river that is the boundary between the living world and that of the dead. These are fitting names given Pluto’s other moons: Charon, Nix and Hydra, all of which meet the IAU’s rules to name them after Greek and Roman underworld personas.
We’ll get a closer look at these strange new worlds in 2015, when the New Horizons spacecraft skims through the Pluto system. There may be other, tiny moons lurking around the dwarf planet that New Horizons could find.
Do you feel the IAU made the right choice? It’s not the first time it waded into tricky waters concerning Pluto; some in the public still complain today about the decision to demote Pluto to dwarf planet status in 2006.
