David E. Kaufmann

Capture of the Sun's Oort Cloud from Stars in Its Birth Cluster

Out of the Oort Cloud Long-period comets originate from the Oort cloud, a vast reservoir of icy bodies that surrounds the solar system. These bodies are thought to be remnants from the formation of the solar system. But did they all form in the Suns protoplanetary disk, or could they have been generated in the protoplanetary disks of other stars in the cluster where the Sun probably formed? Levison et al. (p. 187, published online 10 June) used detailed numerical simulations to investigate what fraction of comets might transfer from the outer reaches of one stellar system to another. The simulations suggest that a substantial number of comets can be captured through this mechanism, which may explain why the number of bodies in the Oort cloud is larger than models predict. Some Oort cloud comets may have originated in the protoplanetary disk of stars other than the Sun. Oort cloud comets are currently believed to have formed in the Suns protoplanetary disk and to have been ejected to large heliocentric orbits by the giant planets. Detailed models of this process fail to reproduce all of the available observational constraints, however. In particular, the Oort cloud appears to be substantially more populous than the models predict. Here we present numerical simulations that show that the Sun captured comets from other stars while it was in its birth cluster. Our results imply that a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary disks of other stars.

LRO-LAMP observations of the LCROSS impact plume.

Watering the Moon About a year ago, a spent upper stage of an Atlas rocket was deliberately crashed into a crater at the south pole of the Moon, ejecting a plume of debris, dust, and vapor. The goal of this event, the Lunar Crater Observation and Sensing Satellite (LCROSS) experiment, was to search for water and other volatiles in the soil of one of the coldest places on the Moon: the permanently shadowed region within the Cabeus crater. Using ultraviolet, visible, and near-infrared spectroscopy data from accompanying craft, Colaprete et al. (p. 463; see the news story by Kerr; see the cover) found evidence for the presence of water and other volatiles within the ejecta cloud. Schultz et al. (p. 468) monitored the different stages of the impact and the resulting plume. Gladstone et al. (p. 472), using an ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO), detected H2, CO, Ca, Hg, and Mg in the impact plume, and Hayne et al. (p. 477) measured the thermal signature of the impact and discovered that it had heated a 30 to 200 square-meter region from ∼40 kelvin to at least 950 kelvin. Paige et al. (p. 479) mapped cryogenic zones predictive of volatile entrapment, and Mitrofanov et al. (p. 483) used LRO instruments to confirm that surface temperatures in the south polar region persist even in sunlight. In all, about 155 kilograms of water vapor was emitted during the impact; meanwhile, the LRO continues to orbit the Moon, sending back a stream of data to help us understand the evolution of its complex surface structures. A controlled spacecraft impact into a crater in the lunar south pole plunged through the lunar soil, revealing water and other volatiles. On 9 October 2009, the Lunar Crater Observation and Sensing Satellite (LCROSS) sent a kinetic impactor to strike Cabeus crater, on a mission to search for water ice and other volatiles expected to be trapped in lunar polar soils. The Lyman Alpha Mapping Project (LAMP) ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO) observed the plume generated by the LCROSS impact as far-ultraviolet emissions from the fluorescence of sunlight by molecular hydrogen and carbon monoxide, plus resonantly scattered sunlight from atomic mercury, with contributions from calcium and magnesium. The observed light curve is well simulated by the expansion of a vapor cloud at a temperature of ~1000 kelvin, containing ~570 kilograms (kg) of carbon monoxide, ~140 kg of molecular hydrogen, ~160 kg of calcium, ~120 kg of mercury, and ~40 kg of magnesium.

The Small Satellites of Pluto as Observed by New Horizons

New Horizons unveils the Pluto system In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanitys first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so far: Moore et al. present the complex surface features and geology of Pluto and its large moon Charon, including evidence of tectonics, glacial flow, and possible cryovolcanoes. Grundy et al. analyzed the colors and chemical compositions of their surfaces, with ices of H2O, CH4, CO, N2, and NH3 and a reddish material which may be tholins. Gladstone et al. investigated the atmosphere of Pluto, which is colder and more compact than expected and hosts numerous extensive layers of haze. Weaver et al. examined the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast-rotating, and have bright surfaces. Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these findings massively increase our understanding of the bodies in the outer solar system. They will underpin the analysis of New Horizons data, which will continue for years to come. Science, this issue pp. 1284, 10.1126/science.aad9189, 10.1126/science.aad8866, 10.1126/science.aae0030, & 10.1126/science.aad9045 Pluto’s rapidly rotating small moons have bright icy surfaces with impact craters. INTRODUCTION The Pluto system is surprisingly complex, comprising six objects that orbit their common center of mass in approximately a single plane and in nearly circular orbits. When the New Horizons mission was selected for flight by NASA in 2001, only the two largest objects were known: the binary dwarf planets Pluto and Charon. Two much smaller moons, Nix and Hydra, were discovered in May 2005, just 8 months before the launch of the New Horizons spacecraft, and two even smaller moons, Kerberos and Styx, were discovered in 2011 and 2012, respectively. The entire Pluto system was likely produced in the aftermath of a giant impact between two Pluto-sized bodies approximately 4 to 4.5 billion years ago, with the small moons forming within the resulting debris disk. But many details remain unconfirmed, and the New Horizons results on Pluto’s small moons help to elucidate the conditions under which the Pluto system formed and evolved. RATIONALE Pluto’s small moons are difficult to observe from Earth-based facilities, with only the most basic visible and near-infrared photometric measurements possible to date. The New Horizons flyby enabled a whole new category of measurements of Pluto’s small moons. The Long Range Reconnaissance Imager (LORRI) provided high–spatial resolution panchromatic imaging, with thousands of pixels across the surfaces of Nix and Hydra and the first resolved images of Kerberos and Styx. In addition, LORRI was used to conduct systematic monitoring of the brightness of all four small moons over several months, from which the detailed rotational properties could be deduced. The Multispectral Visible Imaging Camera (MVIC) provided resolved color measurements of the surfaces of Nix and Hydra. The Linear Etalon Imaging Spectral Array (LEISA) captured near-infrared spectra (in the wavelength range 1.25 to 2.5 μm) of all the small moons for compositional studies, but those data have not yet been sent to Earth. RESULTS All four of Pluto’s small moons are highly elongated objects with surprisingly high surface reflectances (albedos) suggestive of a water-ice surface composition. Kerberos appears to have a double-lobed shape, possibly formed by the merger of two smaller bodies. Crater counts for Nix and Hydra imply surface ages of at least 4 billion years. Nix and Hydra have mostly neutral (i.e., gray) colors, but an apparent crater on Nix’s surface is redder than the rest of the surface; this finding suggests either that the impacting body had a different composition or that material with a different composition was excavated from below Nix’s surface. All four small moons have rotational periods much shorter than their orbital periods, and their rotational poles are clustered nearly orthogonal to the direction of the common rotational poles of Pluto and Charon. CONCLUSION Pluto’s small moons exhibit rapid rotation and large rotational obliquities, indicating that tidal despinning has not played the dominant role in their rotational evolution. Collisional processes are implicated in determining the shapes of the small moons, but collisional evolution was probably limited to the first several hundred million years after the system’s formation. The bright surfaces of Pluto’s small moons suggest that if the Pluto-Charon binary was produced during a giant collision, the two precursor bodies were at least partially differentiated with icy surface layers. Pluto’s family of satellites. NASA’s New Horizons mission has resolved Pluto’s four small moons, shown in order of their orbital distance from Pluto (from left to right). Nix and Hydra have comparable sizes (with equivalent spherical diameters of ~40 km) and are much larger than Styx and Kerberos (both of which have equivalent spherical diameters of ~10 km). All four of these moons are highly elongated and are dwarfed in size by Charon, which is nearly spherical with a diameter of 1210 km. The scale bars apply to all images. The New Horizons mission has provided resolved measurements of Pluto’s moons Styx, Nix, Kerberos, and Hydra. All four are small, with equivalent spherical diameters of ~40 kilometers for Nix and Hydra and ~10 kilometers for Styx and Kerberos. They are also highly elongated, with maximum to minimum axis ratios of ~2. All four moons have high albedos (~50 to 90%) suggestive of a water-ice surface composition. Crater densities on Nix and Hydra imply surface ages of at least 4 billion years. The small moons rotate much faster than synchronous, with rotational poles clustered nearly orthogonal to the common pole directions of Pluto and Charon. These results reinforce the hypothesis that the small moons formed in the aftermath of a collision that produced the Pluto-Charon binary.

Temporal variability of lunar exospheric helium during January 2012 from LRO/LAMP

Abstract We report observations of the lunar helium exosphere made between December 29, 2011, and January 26, 2012, with the Lyman Alpha Mapping Project (LAMP) ultraviolet spectrograph on NASA’s Lunar Reconnaissance Orbiter Mission ( LRO ). The observations were made of resonantly scattered He i λ 584 from illuminated atmosphere against the dark lunar surface on the dawn side of the terminator. We find no or little variation of the derived surface He density with latitude but day-to-day variations that likely reflect variations in the solar wind alpha flux. The five-day passage of the Moon through the Earth’s magnetotail results in a factor of two decrease in surface density, which is well explained by model simulations.

Propeller Orbits in Barred Galaxy Models

The central, or x1, family of periodic orbits is the most important one in almost all two-dimensional numerical models of galactic bars in the literature. However, we present evidence that in two-dimensional models with sufficiently large bar axial ratios (a/c 6), stable orbits having propeller shapes play the dominant role. In our models this propeller family is in fact a distant relative of the x1 family. There are also intermediate cases in which both families are important. The dominance of one family over the other may have direct consequences on the morphological properties of the bars that can be constructed from them, properties such as face-on bar thinness and strength as well as the boxiness of the outer isophotes.

LRO-LAMP failsafe door-open performance: improving FUV measurements of dayside lunar hydration

The Lunar Reconnaissance Orbiter’s (LRO) Lyman Alpha Mapping Project (LAMP) is a lightweight (6.1 kg), lowpower (4.5 W), ultraviolet spectrograph based on the Alice instruments aboard the European Space Agency’s Rosetta spacecraft and NASA’s New Horizons spacecraft. Its primary job is to identify and localize exposed water frost in permanently shadowed regions (PSRs) near the Moon’s poles, and to characterize landforms and albedos in PSRs. LRO launched on June 18, 2009 and reached lunar orbit four days later. LAMP operated with its failsafe door closed for its first seven years in flight. The failsafe door was opened in October 2016 to increase light throughput during dayside operations at the expense of no longer having the capacity to take further dark observations and slightly more operational complexity to avoid saturating the instrument. This one-time irreversible operation was approved after extensive review, and was conducted flawlessly. The increased throughput allows measurement of dayside hydration in one orbit, instead of averaging multiple orbits together to reach enough signal-to-noise. The new measurement mode allows greater time resolution of dayside water migration for improved investigations into the source and loss processes on the lunar surface. LAMP performance and optical characteristics after the failsafe door opening are described herein, including the new effective area, wavelength solution, and resolution.