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Color, composition, and thermal environment of Kuiper Belt object (486958) Arrokoth
The New Horizons spacecraft flew past the Kuiper Belt object (486958) Arrokoth (also known as 2014 MU69) in January 2019. Because of the great distance to the outer Solar System and limited bandwidth, it will take until late 2020 to downlink all the spacecraft's observations back to Earth. Three papers in this issue analyze recently downlinked data, including the highest-resolution images taken during the encounter (see the Perspective by Jewitt). Spencer et al. examined Arrokoth's geology and geophysics using stereo imaging, dated the surface using impact craters, and produced a geomorphological map. Grundy et al. investigated the composition of the surface using color imaging and spectroscopic data and assessed Arrokoth's thermal emission using microwave radiometry. McKinnon et al. used simulations to determine how Arrokoth formed: Two gravitationally bound objects gently spiraled together during the formation of the Solar System. Together, these papers determine the age, composition, and formation process of the most pristine object yet visited by a spacecraft.
Journal Article
The solar nebula origin of (486958) Arrokoth, a primordial contact binary in the Kuiper Belt
The New Horizons spacecraft’s encounter with the cold classical Kuiper Belt object (486958) Arrokoth (provisional designation 2014 MU₆₉) revealed a contact-binary planetesimal. We investigated how Arrokoth formed and found that it is the product of a gentle, low-speed merger in the early Solar System. Its two lenticular lobes suggest low-velocity accumulation of numerous smaller planetesimals within a gravitationally collapsing cloud of solid particles. The geometric alignment of the lobes indicates that they were a co-orbiting binary that experienced angular momentum loss and subsequent merger, possibly because of dynamical friction and collisions within the cloud or later gas drag. Arrokoth’s contact-binary shape was preserved by the benign dynamical and collisional environment of the cold classical Kuiper Belt and therefore informs the accretion processes that operated in the early Solar System.
Journal Article
Impact craters on Pluto and Charon indicate a deficit of small Kuiper belt objects
The flyby of Pluto and Charon by the New Horizons spacecraft provided high-resolution images of cratered surfaces embedded in the Kuiper belt, an extensive region of bodies orbiting beyond Neptune. Impact craters on Pluto and Charon were formed by collisions with other Kuiper belt objects (KBOs) with diameters from ∼40 kilometers to ∼300 meters, smaller than most KBOs observed directly by telescopes. We find a relative paucity of small craters ≲13 kilometers in diameter, which cannot be explained solely by geological resurfacing. This implies a deficit of small KBOs (≲1 to 2 kilometers in diameter). Some surfaces on Pluto and Charon are likely ≳4 billion years old, thus their crater records provide information on the size-frequency distribution of KBOs in the early Solar System.
Journal Article
Observation of the Kibble–Zurek scaling law for defect formation in ion crystals
Traversal of a symmetry-breaking phase transition at finite rates can lead to causally separated regions with incompatible symmetries and the formation of defects at their boundaries, which has a crucial role in quantum and statistical mechanics, cosmology and condensed matter physics. This mechanism is conjectured to follow universal scaling laws prescribed by the Kibble–Zurek mechanism. Here we determine the scaling law for defect formation in a crystal of 16 laser-cooled trapped ions, which are conducive to the precise control of structural phases and the detection of defects. The experiment reveals an exponential scaling of defect formation
γ
β
, where
γ
is the rate of traversal of the critical point and
β
=2.68±0.06. This supports the prediction of
β
=8/3≈2.67 for finite inhomogeneous systems. Our result demonstrates that the scaling laws also apply in the mesoscopic regime and emphasizes the potential for further tests of non-equilibrium thermodynamics with ion crystals.
The formation of defects in numerous systems is believed to follow universal scaling laws arising from the Kibble–Zurek mechanism. Ulm
et al.
measure this scaling law for defects created in ion Coulomb crystals, confirming the predicted behaviour for a system of 16 ions.
Journal Article
Surface compositions across Pluto and Charon
The Kuiper Belt hosts a swarm of distant, icy objects ranging in size from small, primordial planetesimals to much larger, highly evolved objects, representing a whole new class of previously unexplored cryogenic worlds. Pluto, the largest among them, along with its system of five satellites, has been revealed by NASAs New Horizons spacecraft flight through the system in July 2015, nearly a decade after its launch.
Journal Article
Interstellar Pickup Ion Observations to 60 au
Prior to the Solar Wind Around Pluto (SWAP) observations from New Horizons, the only significant interstellar pickup ion (PUI) observations were taken inside 5.4 au from the Ulysses and 1 au missions. Here, we analyze SWAP data to provide the first comprehensive radial profile of interstellar PUIs in the outer heliosphere from ∼22 to 60 au. This study is the first to combine the low time resolution (one day) data inside 49.5 au with the high time resolution (half hour) data outside of that. We show that these two data sets can be reliably analyzed together in what is now a nearly continuous set of PUI observations. We derive radial gradients for all of the PUI parameters, as well as ratios to their solar wind counterparts from these combined data and extrapolate them out to a nominal termination shock distance at ∼90 au. We find that the fraction of PUIs to all ions in the solar wind continues to grow, reaching ∼18% by 60 au and extrapolated to be ∼30% at 90 au. The PUI internal pressure is ∼11% of the solar wind dynamic pressure at 60 au and expected to be ∼21% by 90 au. Such large values further highlight the importance of PUIs as the dominant driver of the physics of the outer heliosphere, as well as at the termination shock.
Journal Article
Fast thermometry for trapped ions using dark resonances
We experimentally demonstrate a method to determine the temperature of trapped ions which is suitable for monitoring fast thermalization processes. We show that observing and analyzing the lineshape of dark resonances in the fluorescence spectrum provides a temperature measurement which is accurate over a large dynamic range, applied to single ions and small ion crystals. Laser induced fluorescence is detected over a time of only , allowing for rapid determination of the ion temperature. In the measurement range of 10−1- mK we reach better than accuracy. Tuning the cooling laser to selected resonance features allows us to control the ion temperatures between mK and more than mK. Experimental work is supported by a solution of the eight-level optical Bloch equations when including the ions' classical motion. This technique paves the way for many experiments, including heat transport in ion strings, heat engines, non-equilibrium thermodynamics or thermometry of large ion crystals.
Journal Article
Self-organization of supramolecular helical dendrimers into complex electronic materials
The discovery of electrically conducting organic crystals
1
and polymers
1
,
2
,
3
,
4
has widened the range of potential optoelectronic materials
5
,
6
,
7
,
8
,
9
, provided these exhibit sufficiently high charge carrier mobilities
6
,
7
,
8
,
9
,
10
and are easy to make and process. Organic single crystals have high charge carrier mobilities but are usually impractical
11
, whereas polymers have good processability but low mobilities
1
,
12
. Liquid crystals exhibit mobilities approaching those of single crystals and are suitable for applications
13
,
14
,
15
,
16
,
17
,
18
, but demanding fabrication and processing methods limit their use. Here we show that the self-assembly of fluorinated tapered dendrons can drive the formation of supramolecular liquid crystals with promising optoelectronic properties from a wide range of organic materials. We find that attaching conducting organic donor or acceptor groups to the apex of the dendrons leads to supramolecular nanometre-scale columns that contain in their cores π-stacks of donors, acceptors or donor–acceptor complexes exhibiting high charge carrier mobilities. When we use functionalized dendrons and amorphous polymers carrying compatible side groups, these co-assemble so that the polymer is incorporated in the centre of the columns through donor–acceptor interactions and exhibits enhanced charge carrier mobilities. We anticipate that this simple and versatile strategy for producing conductive π-stacks of aromatic groups, surrounded by helical dendrons, will lead to a new class of supramolecular materials suitable for electronic and optoelectronic applications.
Journal Article
Formation of H+ PUI Tails Downstream of Distant Interplanetary Shocks
New Horizons’ Solar Wind Around Pluto (SWAP) instrument has observed interplanetary shocks from ∼20 to 60 au from the Sun. Different studies have analyzed the effects of these shocks on the solar wind ion (SWI) and interstellar pickup ion (PUI) distributions. A key finding includes the observation of PUI tails downstream of some shocks. However, PUI tails have only been observed when SWAP was measuring 24 hr averages of the count rate histograms. After a software change in 2021 February, SWAP’s measurement resolution was improved to ∼30 minutes averages, but no PUI tails have been observed since this change. In this study, we use a test particle code to simulate the downstream SWI and PUI distributions to study the possible existence and formation of PUI tails, using the shock compression ratio, shock thickness, and turbulence as free parameters. We find that in most cases, a suprathermal tail does not develop downstream of interplanetary shocks, but a persistent hump and PUI cutoff exists at energies above the PUI injection energy. The extent of the hump largely depends on the shock compression ratio. We find PUI tails do form when turbulence is included in the model. After we convert our distributions to count rates in the spacecraft frame, the results show similar structure to the data. We conclude that significant H+ PUI tails can form downstream of distant interplanetary shocks if there is a relatively large compression ratio (>2) and significant turbulence present.
Journal Article