NASA’s New Horizons spacecraft spied several features on Pluto that offer evidence of a time millions or billions of years ago when – thanks to much higher pressure in Pluto’s atmosphere and warmer conditions on the surface – liquids might have flowed across and pooled on the surface of the distant world.
Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute“In addition to this possible former lake, we also see evidence of channels that may also have carried liquids in Pluto’s past,” said Alan Stern, Southwest Research Institute, Boulder, Colorado—principal investigator of New Horizons and lead author of a scientific paper on the topic submitted to the journal Icarus.
This feature appears to be a frozen, former lake of liquid nitrogen, located in a mountain range just north of Pluto’s informally named Sputnik Planum. Captured by the New Horizons’ Long Range Reconnaissance Imager (LORRI) as the spacecraft flew past Pluto on July 14, 2015, the image shows details as small as about 430 feet (130 meters). At its widest point the possible lake appears to be about 20 miles (30 kilometers) across.
Scientists from NASA’s Dawn mission unveiled new images from the spacecraft’s lowest orbit at Ceres, including highly-anticipated views of Occator Crater, at the 47th annual Lunar and Planetary Science Conference in The Woodlands, Texas, on Tuesday.
Occator Crater, measuring 57 miles (92 kilometers) across and 2.5 miles (4 kilometers) deep, contains the brightest area on Ceres. Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI. Full Image and caption.Occator Crater, measuring 57 miles (92 kilometers) across and 2.5 miles (4 kilometers) deep, contains the brightest area on Ceres, the dwarf planet that Dawn has explored since early 2015. The latest images, taken from 240 miles (385 kilometers) above the surface of Ceres, reveal a dome in a smooth-walled pit in the bright center of the crater. Numerous linear features and fractures crisscross the top and flanks of this dome. Prominent fractures also surround the dome and run through smaller, bright regions found within the crater.
“Before Dawn began its intensive observations of Ceres last year, Occator Crater looked to be one large bright area. Now, with the latest close views, we can see complex features that provide new mysteries to investigate,” said Ralf Jaumann, planetary scientist and Dawn co-investigator at the German Aerospace Center (DLR) in Berlin. “The intricate geometry of the crater interior suggests geologic activity in the recent past, but we will need to complete detailed geologic mapping of the crater in order to test hypotheses for its formation.”
Ceres’ Haulani Crater (21 miles, 34 kilometers wide) is shown in these views from the visible and infrared mapping spectrometer (VIR) aboard NASA’s Dawn spacecraft. These views reveal variations in the region’s brightness, mineralogy and temperature at infrared wavelengths. The image at far left shows brightness variations in Haulani. Light with a wavelength of 1200 nanometers is shown in blue, 1900 nanometers in green and 2300 nanometers in red. The view at center is a false color image, highlighting differences in the types of rock and ejected material around the crater. Scientists see this as evidence that the material in this area is not uniform, and that the crater’s interior has a different composition than its surroundings. This is what scientists call a color ratio image (blue: 3200/3300 nanometers, green: 2900/3100 nanometers, red: 2600/2700 nanometers). The image at right shows information related to temperature. Bluer regions are colder zones and redder regions are warmer. The colors demonstrate that the interior of Haulani appears colder than its surroundings. Light with a wavelength of 2700 nanometers is shown in blue, 2000 nanometers in green and 5000 nanometers in red.Color Differences
The team also released an enhanced color map of the surface of Ceres, highlighting the diversity of surface materials and their relationships to surface morphology. Scientists have been studying the shapes of craters and their distribution with great interest. Ceres does not have as many large impact basins as scientists expected, but the number of smaller craters generally matches their predictions. The blue material highlighted in the color map is related to flows, smooth plains and mountains, which appear to be very young surface features.
The bright central spots near the center of Occator Crater are shown in enhanced color in this view from NASA’s Dawn spacecraft. Such views can be used to highlight subtle color differences on Ceres’ surface. Lower resolution color data have been overlaid onto a higher resolution view (see PIA20350) of the crater. The view was produced by combining the highest resolution images of Occator obtained in February 2016 (at image scales of 35 meters, or 115 feet, per pixel) with color images obtained in September 2015 (at image scales of 135 meters, or about 440 feet, per pixel). The three images used to produce the color were taken using spectral filters centered at 438, 550 and 965 nanometers (the latter being slightly beyond the range of human vision, in the near-infrared). The crater measures 57 miles (92 kilometers) across and 2.5 miles (4 kilometers) deep. Dawn’s close-up view reveals a dome in a smooth-walled pit in the bright center of the crater. Numerous linear features and fractures crisscross the top and flanks of this dome.“Although impact processes dominate the surface geology on Ceres, we have identified specific color variations on the surface indicating material alterations that are due to a complex interaction of the impact process and the subsurface composition,” Jaumann said. “Additionally, this gives evidence for a subsurface layer enriched in ice and volatiles.”
This global map shows the surface of Ceres in enhanced color, encompassing infrared wavelengths beyond human visual range. Images taken using infrared (965 nanometers), green (555 nanometers) and blue (438 nanometers) spectral filters were combined to create this view. This type of map is known as an elliptical, or Mollweide, projection and has a resolution of 460 feet (140 meters) per pixel. Some areas near the poles are black where Dawn’s color imaging coverage is incomplete. The images used to make this map were taken from Dawn’s high-altitude mapping orbit (HAMO), at a distance of 915 miles (1,470 kilometers) from Ceres.Counting Neutrons
Data relevant to the possibility of subsurface ice is also emerging from Dawn’s Gamma Ray and Neutron Detector (GRaND), which began acquiring its primary data set in December. Neutrons and gamma rays produced by cosmic ray interactions with surface materials provide a fingerprint of Ceres’ chemical makeup. The measurements are sensitive to elemental composition of the topmost yard (meter) of the regolith.
This map shows a portion of the northern hemisphere of Ceres with neutron counting data acquired by the gamma ray and neutron detector (GRaND) instrument aboard NASA’s Dawn spacecraft. These data reflect the concentration of hydrogen in the upper yard (or meter) of regolith, the loose surface material on Ceres. The color information is based on the number of neutrons detected per second by GRaND. Counts decrease with increasing hydrogen concentration. The color scale of the map is from blue (lowest neutron count) to red (highest neutron count). Lower neutron counts near the pole suggest the presence of water ice within about a yard (meter) of the surface at high latitudes. The GRaND data were acquired from Dawn’s low-altitude mapping orbit (LAMO) at Ceres, a distance of 240 miles (385 kilometers) from the dwarf planet. Ceres’ north pole is marked with a white line. The longitude is centered on Occator Crater.In Dawn’s lowest-altitude orbit, the instrument has detected fewer neutrons near the poles of Ceres than at the equator, which indicates increased hydrogen concentration at high latitudes. As hydrogen is a principal constituent of water, water ice could be present close to the surface in polar regions.
“Our analyses will test a longstanding prediction that water ice can survive just beneath Ceres’ cold, high-latitude surface for billions of years,” said Tom Prettyman, the lead for GRaND and Dawn co-investigator at the Planetary Science Institute, Tucson, Arizona.
The outer reaches of our Solar System are home to hundreds of thousands of small icy worlds. Their present orbits are a sculpted signature of the early migrations of the giant planets, particularly Neptune. Yet the faintness and highly eccentric orbits of most of these worlds mean only a tiny fraction of them have yet been discovered.
With the Outer Solar System Origins Survey on CFHT, we are discovering up to five hundred new outer Solar System objects, with exquisitely well-determined orbital parameters. Our complementary Large Program on Gemini North is observing the brightest of our discoveries in the optical and infrared with unprecedented precision, providing information on the ices, silicates and organic compounds on the surfaces of these small worlds.
This colourful map of the structure of the outer Solar System is providing new constraints on Neptune’s migration.
A year ago, Pluto was just a bright speck in the cameras of NASA’s approaching New Horizons spacecraft, not much different than its appearances in telescopes since Clyde Tombaugh discovered the ninth planet in 1930.
This image of haze layers above Pluto’s limb was taken by the Ralph/Multispectral Visible Imaging Camera (MVIC) on NASA’s New Horizons spacecraft. About 20 haze layers are seen; the layers have been found to typically extend horizontally over hundreds of kilometers, but are not strictly parallel to the surface. For example, white arrows indicate a haze layer about three miles (five kilometers) above the surface on the left, which has descended to the surface at the right. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/AAAS/Science
But this week, in the journal Science, New Horizons scientists published the first comprehensive set of papers describing results from last summer’s Pluto system flyby. “These five detailed papers completely transform our view of Pluto – revealing the former ‘astronomer’s planet’ to be a real world with diverse and active geology, exotic surface chemistry, a complex atmosphere, puzzling interaction with the sun and an intriguing system of small moons,” said Alan Stern, New Horizons principal investigator from Southwest Research Institute, Boulder, Colorado.
After a 9.5-year, 3-billion-mile journey – launching faster and traveling farther than any spacecraft to reach its primary target – New Horizons zipped by Pluto on July 14, 2015. New Horizons’ seven science instruments collected about 50 gigabits of data on the spacecraft’s digital recorders, most of it coming over nine busy days surrounding the encounter. About half of that data has now been transmitted home – from distances where radio signals at light speed need nearly five hours to reach Earth – with all of it expected back by October.
New Horizons views of the informally named Sputnik Planum on Pluto (top) and the informally named Vulcan Planum on Charon (bottom). Both scale bars measure 20 miles (32 kilometers) long; illumination is from the left in both instances. The Sputnik Planum view is centered at 11°N, 180°E, and covers the bright, icy, geologically cellular plains. Here, the cells are defined by a network of interconnected troughs that crisscross these nitrogen-ice plains. At right, in the upper image, the cellular plains yield to pitted plains of southern Sputnik Planum. This observation was obtained by the Ralph/Multispectral Visible Imaging Camera (MVIC) at a resolution of 1,050 feet (320 meters) per pixel. The Vulcan Planum view in the bottom panel is centered at 4°S, 4°E, and includes the “moated mountain” Clarke Mons just above the center of the image. As well as featuring impact craters and sinuous troughs, the water ice-rich plains display a range of surface textures, from smooth and grooved at left, to pitted and hummocky at right. This observation was obtained by the Long Range Reconnaissance Imager (LORRI) at a resolution of 525 feet (160 meters) per pixel. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research InstituteThe first close-up pictures revealed a large heart-shaped feature carved into Pluto’s surface, telling scientists that this “new” type of planet – the largest, brightest and first-explored in the mysterious, distant “third zone” of our solar system known as the Kuiper Belt – would be even more interesting and puzzling than models predicted.
“Observing Pluto and Charon up close has caused us to completely reassess thinking on what sort of geological activity can be sustained on isolated planetary bodies in this distant region of the solar system, worlds that formerly had been thought to be relics little changed since the Kuiper Belt’s formation,” said Jeff Moore, from NASA Ames Research Center, California, and lead author of the paper covering geology.
This enhanced color view of Pluto’s surface diversity was created by merging Ralph/Multispectral Visible Imaging Camera (MVIC) color imagery (650 meters per pixel) with Long Range Reconnaissance Imager panchromatic imagery (230 meters per pixel). At lower right, ancient, heavily cratered terrain is coated with dark, reddish tholins. At upper right, volatile ices filling the informally named Sputnik Planum have modified the surface, creating a chaos-like array of blocky mountains. Volatile ice also occupies a few nearby deep craters, and in some areas the volatile ice is pocked with arrays of small sublimation pits. At left, and across the bottom of the scene, gray-white CH4 ice deposits modify tectonic ridges, the rims of craters, and north-facing slopes. The scene in this image is 260 miles (420 kilometers) wide and 140 miles (225 kilometers) from top to bottom; north is to the upper left. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research InstituteScientists studying Pluto’s composition say the diversity of the planet’s landscape stems from eons of interaction between highly volatile and mobile methane, nitrogen and carbon monoxide ices with inert and sturdy water ice. “We see variations in the distribution of Pluto’s volatile ices that point to fascinating cycles of evaporation and condensation,” said Will Grundy, from Lowell Observatory in Flagstaff, Arizona, and lead author of the composition paper. “These cycles are a lot richer than on Earth, where there’s really only one material that condenses and evaporates – water. On Pluto, there are least three materials, and while they interact in ways we don’t yet fully understand, we definitely see their effects all across Pluto’s surface.”
Above the surface, scientists discovered Pluto’s atmosphere contains layered hazes, and is both cooler and more compact than expected. This affects how Pluto’s upper atmosphere is lost to space, and how it interacts with the stream of charged particles from the sun known as the solar wind. “We’ve discovered that pre-New Horizons estimates wildly overestimated the loss of material from Pluto’s atmosphere,” said Fran Bagenal, from the University of Colorado, Boulder, and lead author on the particles and plasma paper. “The thought was that Pluto’s atmosphere was escaping like a comet, but it is actually escaping at a rate much more like Earth’s atmosphere.”
Randy Gladstone, of SwRI, San Antonio, and the lead author of the Science paper on atmospheric findings, added, “We’ve also discovered that methane, rather than nitrogen, is Pluto’s primary escaping gas. This is pretty surprising, since near Pluto’s surface the atmosphere is more than 99-percent nitrogen.”
Scientists are also analyzing the first close-up images of Pluto’s small moons Styx, Nix, Kerberos, and Hydra. Discovered between 2005 and 2012, the four moons range in diameter from about 25 miles (40 kilometers) for Nix and Hydra to about six miles (10 kilometers) for Styx and Kerberos. Mission scientists further observed that the small satellites have highly anomalous rotation rates and uniformly unusual pole orientations, as well as icy surfaces with brightness and colors distinctly different from those of Pluto and Charon.
They’ve also found evidence that some of the moons resulted from mergers of even smaller bodies, and that their surface ages date back at least 4 billion years. “These latter two results reinforce the hypothesis that the small moons formed in the aftermath of a collision that produced the Pluto-Charon binary system,” said Hal Weaver, New Horizons project scientist from the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, and lead author of the Science paper on Pluto’s small moons.
“This is why we explore,” said Curt Niebur, New Horizons program scientist at NASA Headquarters in Washington. “The many discoveries from New Horizons represent the best of humankind and inspire us to continue the journey of exploration to the solar system and beyond.”
New Horizons is part of NASA’s New Frontiers Program, managed by the agency’s Marshall Space Flight Center in Huntsville, Alabama. The Johns Hopkins Applied Physics Laboratory designed, built, and operates the New Horizons spacecraft and manages the mission for NASA’s Science Mission Directorate. The Southwest Research Institute leads the science mission, payload operations, and encounter science planning.
Observations made using the HARPS spectrograph at ESO’s La Silla Observatory in Chile have revealed unexpected changes in the bright spots on the dwarf planet Ceres. Although Ceres appears as little more than a point of light from the Earth, very careful study of its light shows not only the changes expected as Ceres rotates, but also that the spots brighten during the day and also show other variations. These observations suggest that the material of the spots is volatile and evaporates in the warm glow of sunlight.
This artist’s impression is based on a detailed map of the surface compiled from images taken from NASA’s Dawn spacecraft in orbit around the dwarf planet Ceres. It shows the very bright patches of material in the crater Occator and elsewhere. New observations using the HARPS spectrograph on the ESO 3.6-metre telescope at La Silla in Chile have revealed unexpected daily changes on these spots, suggesting that they change under the influence of sunlight as Ceres rotates.
Ceres is the largest body in the asteroid belt between Mars and Jupiter and the only such object classed as a dwarf planet. NASA’s Dawn spacecraft has been in orbit around Ceres for more than a year and has mapped its surface in great detail. One of the biggest surprises has been the discovery of very bright spots, which reflect far more light than their much darker surroundings [1]. The most prominent of these spots lie inside the crater Occator and suggest that Ceres may be a much more active world than most of its asteroid neighbours.
This artist’s impression video [click here for higher resolution versions] is based on a detailed map of the surface compiled from images taken from NASA’s Dawn spacecraft in orbit around the dwarf planet Ceres. It shows the very bright patches of material in the crater Occator and elsewhere. New observations using the HARPS spectrograph on the ESO 3.6-metre telescope at La Silla in Chile have revealed unexpected daily changes on these spots, suggesting that they change under the influence of sunlight as Ceres rotates.
This illustration shows how the features in the spectrum of the light reflected from the bright spots is alternately red and blue shifted slightly compared to the average light of Ceres as it rotates. This very subtle effect has been measured from the ground using the HARPS spectrograph on the ESO 3.6-metre telescope at La Silla in Chile. The effect has been greatly exaggerated to make it visible and excludes the much brighter light coming from the rest of the disc of Ceres. Credit: ESO/L.Calçada/NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Steve Albers
New and very precise observations using the HARPS spectrograph at the ESO 3.6-metre telescope at La Silla, Chile, have now not only detected the motion of the spots due to the rotation of Ceres about its axis, but also found unexpected additional variations suggesting that the material of the spots is volatile and evaporates in sunlight.
This image taken from NASA’s Dawn spacecraft in orbit around the dwarf planet Ceres shows the very bright patches of material in the crater Occator and elsewhere. New observations using the HARPS spectrograph on the ESO 3.6-metre telescope at La Silla in Chile have revealed unexpected daily changes on these spots, suggesting that they change under the influence of sunlight. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDAThe lead author of the new study, Paolo Molaro, at the INAF–Trieste Astronomical Observatory, takes up the story:
“As soon as the Dawn spacecraft revealed the mysterious bright spots on the surface of Ceres, I immediately thought of the possible measurable effects from Earth. As Ceres rotates the spots approach the Earth and then recede again, which affects the spectrum of the reflected sunlight arriving at Earth.”
Ceres spins every nine hours and calculations showed that the effects due to the motion of the spots towards and away from the Earth caused by this rotation would be very small, of order 20 kilometres per hour. But this motion is big enough to be measurable via the Doppler effect with high-precision instruments such as HARPS.
This artist’s impression video is based on a detailed map of the surface compiled from images taken from NASA’s Dawn spacecraft in orbit around the dwarf planet Ceres. It shows the very bright patches of material in the crater Occator and elsewhere. New observations using the HARPS spectrograph on the ESO 3.6-metre telescope at La Silla in Chile have revealed unexpected daily changes on these spots, suggesting that they change under the influence of sunlight as Ceres rotates. Credit: ESO/L.Calçada/NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Steve Albers
The team observed Ceres with HARPS for a little over two nights in July and August 2015. “The result was a surprise,”adds Antonino Lanza, at the INAF–Catania Astrophysical Observatory and co-author of the study.
“We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night.”
The team concluded that the observed changes could be due to the presence of volatile substances that evaporate under the action of solar radiation [2]. When the spots inside the Occator crater are on the side illuminated by the Sun they form plumes that reflect sunlight very effectively. These plumes then evaporate quickly, lose reflectivity and produce the observed changes. This effect, however, changes from night to night, giving rise to additional random patterns, on both short and longer timescales.
If this interpretation is confirmed Ceres would seem to be very different from Vesta and the other main belt asteroids. Despite being relatively isolated, it seems to be internally active [3]. Ceres is known to be rich in water, but it is unclear whether this is related to the bright spots. The energy source that drives this continual leakage of material from the surface is also unknown.
Dawn is continuing to study Ceres and the behaviour of its mysterious spots. Observations from the ground with HARPS and other facilities will be able to continue even after the end of the space mission.
Notes
[1] Bright spots were also seen, with much less clarity, in earlier images of Ceres from the NASA/ESA Hubble Space Telescope taken in 2003 and 2004.
[2] It has been suggested that the highly reflective material in the spots on Ceres might be freshly exposed water ice or hydrated magnesium sulphates.
[3] Many of the most internally active bodies in the Solar System, such as the large satellites of Jupiter and Saturn, are subjected to strong tidal effects due to their proximity to the massive planets.
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