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The Rosetta spacecraft has investigated comet 67P/Churyumov-Gerasimenko from large heliocentric distances to its perihelion passage and beyond. We trace the seasonal and diurnal evolution of the colors of the 67P nucleus, finding changes driven by sublimation and recondensation of water ice. The whole nucleus became relatively bluer near perihelion, as increasing activity removed the surface dust, implying that water ice is widespread underneath the surface. We identified large (1500 square meters) ice-rich patches appearing and then vanishing in about 10 days, indicating small-scale heterogeneities on the nucleus. Thin frosts sublimating in a few minutes are observed close to receding shadows, and rapid variations in color are seen on extended areas close to the terminator. These cyclic processes are widespread and lead to continuously, slightly varying surface properties.

Images obtained by the Optical, Spectroscopie, and Infrared Remote Imaging System (OSIRIS) cameras onboard the Rosetta spacecraft reveal that asteroid 21 Lutetia has a complex geology and one of the highest asteroid densities measured so far, 3.4 ± 0.3 grams per cubic centimeter. The north pole region is covered by a thick layer of regolith, which is seen to flow in major landslides associated with albedo variation. Its geologically complex surface, ancient surface age, and high density suggest that Lutetia is most likely a primordial planetesimal. This contrasts with smaller asteroids visited by previous spacecraft, which are probably shattered bodies, fragments of larger parents, or reaccumulated rubble piles.

The European Space Agency's Rosetta mission encountered the main-belt asteroid (2867) Steins while on its way to rendezvous with comet 67P/Churyumov-Gerasimenko. Images taken with the OSIRIS (optical, spectroscopic, and infrared remote imaging system) cameras on board Rosetta show that Steins is an oblate body with an effective spherical diameter of 5.3 kilometers. Its surface does not show color variations. The morphology of Steins is dominated by linear faults and a large 2.1-kilometer-diameter crater near its south pole. Crater counts reveal a distinct lack of small craters. Steins is not solid rock but a rubble pile and has a conical appearance that is probably the result of reshaping due to Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) spin-up. The OSIRIS images constitute direct evidence for the YORP effect on a main-belt asteroid.

A bstract The transport behavior of strongly anisotropic systems is significantly richer compared to isotropic ones. The most dramatic spatial anisotropy at a critical point occurs at a Lifshitz transition, found in systems with merging Dirac or Weyl point or near the superconductor-insulator quantum phase transition. Previous work found that in these systems a famous conjecture on the existence of a lower bound for the ratio of a shear viscosity to entropy is violated, and proposed a generalization of this bound for anisotropic systems near charge neutrality involving the electric conductivities. The present study uses scaling arguments and the gauge-gravity duality to confirm the previous analysis of universal bounds in anisotropic Dirac systems. We investigate the strongly-coupled phase of quantum Lifshitz systems in a gravitational Einstein-Maxwell-dilaton model with a linear massless scalar which breaks translations in the boundary dual field theory and sources the anisotropy. The holographic computation demonstrates that some elements of the viscosity tensor can be related to the ratio of the electric conductivities through a simple geometric ratio of elements of the bulk metric evaluated at the horizon, and thus obey a generalized bound, while others violate it. From the IR critical geometry, we express the charge diffusion constants in terms of the square butterfly velocities. The proportionality factor turns out to be direction-independent, linear in the inverse temperature, and related to the critical exponents which parametrize the anisotropic scaling of the dual field theory.

The irreversible conversion of methane into higher hydrocarbons in Titan's stratosphere implies a surface or subsurface methane reservoir. Recent measurements from the cameras aboard the Cassini orbiter fail to see a global reservoir, but the methane and smog in Titan's atmosphere impedes the search for hydrocarbons on the surface. Here we report spectra and high-resolution images obtained by the Huygens Probe Descent Imager/Spectral Radiometer instrument in Titan's atmosphere. Although these images do not show liquid hydrocarbon pools on the surface, they do reveal the traces of once flowing liquid. Surprisingly like Earth, the brighter highland regions show complex systems draining into flat, dark lowlands. Images taken after landing are of a dry riverbed. The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent. The number density of haze particles increases by a factor of just a few from an altitude of 150 km to the surface, with no clear space below the tropopause. The methane relative humidity near the surface is 50 per cent. New views of Titan The Huygens probe landed on Titan on 14 January this year, and seven papers published in this issue record the encounter. They describe a world that resembles a primitive Earth, complete with weather systems and geological activity. The ‘Huygens on Titan’ section opens with an overview of the descent and landing and a News and Views piece. Tomasko et al . describe the dry riverbed and drainage channels seen during Huygens' descent, evidence that liquid methane falls as rain or erupts from cryovolcanoes, periodically flooding the surface. This paper includes the images used on the cover to the Huygens section. Niemann et al . measured the abundances of isotopes of argon, nitrogen and carbon in the atmosphere, and conclude that there is no evidence that Titan's methane comes from biological activity. Fulchignoni et al . obtained precise measurements of temperature and pressure from the upper atmosphere right down to the surface. On the way down Huygens recorded evidence for lightning. Zarnecki et al . report that the probe landed on a relatively smooth surface of icy grains with the consistency of wet clay or sand. Isräl et al . report that the aerosols in Titan's clouds have solid cores made from complex organic molecules containing carbon and nitrogen. And Bird et al . found that on average Titan's winds blow in the same direction as the moon rotates, and that close to the surface these winds are very weak, travelling at around walking speed.

Former studies using the chlorophyll fluorescence technique on evergreen Ilex aquifolium L. showed that its photosynthetic potential for electron transport in winter recovers quickly when exposed to more favorable conditions. Since little is known, however, about its photosynthetic carbon gain in winter, we investigated its leaf gas exchange over an entire winter and spring period. Measurements were made rotationally in the field and in the laboratory to also investigate if I. aquifolium profits from warmer phases during winter in terms of net carbon gain. From the end of autumn until the end of spring, three different climate-driven phases of photosynthetic responses could be distinguished: first, an acclimation phase which lasted until February and was characterized by a gradually decreasing light-saturated gross photosynthesis (Amax(gross)), decreasing apparent quantum yield of CO2-assimilation (ΦΦi) and a decreasing ability of these parameters to recover overnight inside the laboratory. At the same time, maximal quantum yield of PSII (Fv/Fm) could fully regenerate. In this phase, single warmer days had a positive effect on carbon assimilation. Second, a phase of relatively constant but low photosynthesis which was virtually unaffected by temperature, lasting for almost two months occurred. Here, Amax(gross) and Φi had lost their ability to recover from winter conditions in the field, while Fv/Fm was much less affected. I. aquifolium was still able to conduct positive light-saturated net photosynthesis at a leaf temperature of -0.5 °C, but during this time it could not profit from milder temperatures in terms of carbon gain. Third, a phase of increasing photosynthesis (spring recovery) occurred, starting in March when the 5-day average temperature was above 5 °C and radiation in the field increased, and where all parameters slowly recovered from winter depressions. Our findings show that I. aquifolium is photosynthetically active over the whole winter, even at temperatures around 0 °C. In terms of carbon gain, however, I. aquifolium does not profit from warmer phases during winter, despite the fast recovery seen in chlorophyll fluorescence measurements.

The Rosetta spacecraft spent ~2 years orbiting comet 67P/Churyumov-Gerasimenko, most of it at distances that allowed surface characterization and monitoring at submeter scales. From December 2014 to June 2016, numerous localized changes were observed, which we attribute to cometary-specific weathering, erosion, and transient events driven by exposure to sunlight and other processes. While the localized changes suggest compositional or physical heterogeneity, their scale has not resulted in substantial alterations to the comet’s landscape. This suggests that most of the major landforms were created early in the comet’s current orbital configuration. They may even date from earlier if the comet had a larger volatile inventory, particularly of CO or CO₂ ices, or contained amorphous ice, which could have triggered activity at greater distances from the Sun.

Career situation of first and presenting authorStudent for a master or a PhD.IntroductionOsteoarthritis (OA) and osteoporosis are destructive bone diseases causing chronic pain and leading to disability. Mesenchymal stromal cells (MSC) are essential to bone health and tissue repair. Adipokines such as visfatin alter the osteogenic potential of MSCs and contribute to the loss of bone homeostasis. Long non-coding RNA H19 is one of the first lncRNAs discovered and relevant for distinct processes, e.g. during embryonic growth and tumor formation. lncRNAs interact directly with DNA, RNA as well as proteins, modulating transcription, protein expression and protein function. H19 upregulation was shown in osteogenic differentiation of MSCs.1 H19 increases the osteogenic potential via TGFβ1 and Wnt/β-catenin pathways.2 3 ObjectivesTo investigate the influence of visfatin on H19 expression during osteogenesis.MethodsHuman MSCs from healthy donors (hMSC) and primary human MSCs from osteoarthritis patients (phMSC) after knee replacement surgery were treated with differentiation medium to induce osteogenic differentiation (OD). Matrix mineralization (MM) was quantified after 21 days by Alizarin Red. H19 expression by realtime PCR and IL-6 production by ELISA were measured.ResultslncRNA H19 was up-regulated during OD. Although the H19 upregulation was not altered by co-stimulation with resistin, leptin or TNFα, visfatin co-stimulation during OD down-regulated H19 expression up to 10-fold as compared to unstimulated MSCs. The effect was significant in phMSCs at two of three measured time points (day 7 p=0.03; day 14 p=0.002, n=3) and in hMSCs at day 14 (p=0.0003, n=4). Visfatin co-stimulation of MSCs in OD increased MM, as well as IL-6 levels. However, TNF did not alter H19 expression or increase MM.ConclusionsVisfatin co-stimulation during osteogenesis down-regulated lncRNA H19 expression, indicating a loss of the growthpromoting effects of lncRNA H19 in affected areas of destructive bone disease. This regulatory effect was specific to visfatin and did not occur upon co-stimulation with other adipokines or inflammatory stimuli such as TNF supporting a TNF-independent effect of visfatin.ReferencesWang, L. Differential expression of long noncoding ribonucleic acids during osteogenic differentiation of human bone marrow mesenchymal stem cells. Int. Orthop. 2015;39:1013–1019.Huang, Y. Long Noncoding RNA H19 Promotes Osteoblast Differentiation Via TGF-?1/Smad3/HDAC Signaling Pathway by Deriving miR-675. Stem Cells 2015;33:3481–3492.Liang, W.-C. H19 activates Wnt signaling and promotes osteoblast differentiation by functioning as a competing endogenous RNA. Sci. Rep. 2012;6:20121.Disclosure of InterestNone declared.

Outbursts occur commonly on comets 1 with different frequencies and scales 2 , 3 . Despite multiple observations suggesting various triggering processes 4 , 5 , the driving mechanism of such outbursts is still poorly understood. Landslides have been invoked 6 to explain some outbursts on comet 103P/Hartley 2, although the process required a pre-existing dust layer on the verge of failure. The Rosetta mission observed several outbursts from its target comet 67P/Churyumov–Gerasimenko, which were attributed to dust generated by the crumbling of materials from collapsing cliffs 7 , 8 . However, none of the aforementioned works included definitive evidence that landslides occur on comets. Amongst the many features observed by Rosetta on the nucleus of the comet, one peculiar fracture, 70 m long and 1 m wide, was identified on images obtained in September 2014 at the edge of a cliff named Aswan 9 . On 10 July 2015, the Rosetta Navigation Camera captured a large plume of dust that could be traced back to an area encompassing the Aswan escarpment 7 . Five days later, the OSIRIS camera observed a fresh, sharp and bright edge on the Aswan cliff. Here we report the first unambiguous link between an outburst and a cliff collapse on a comet. We establish a new dust-plume formation mechanism that does not necessarily require the breakup of pressurized crust or the presence of supervolatile material, as suggested by previous studies 7 . Moreover, the collapse revealed the fresh icy interior of the comet, which is characterized by an albedo >0.4, and provided the opportunity to study how the crumbling wall settled down to form a new talus. A bright outburst of activity from the nucleus of comet 67P, observed by Rosetta in July 2015, is traced back to a cliff that partially collapsed at the same time as the outburst, establishing a link between the two events. The collapse has also exposed the fresh ice present under the surface.

We present preliminary results of the narrow-band CN observations of comet 8P/Tuttle from early January 2008, realized as part of a project consisting of near-simultaneous spectroscopic monitoring of HCN at millimeter-wavelengths and optical imaging of the coma. The mean-image subtraction method revealed low-contrast CN envelopes. Using the image cross-correlation technique we measured the projected velocity of these shells. For the sunward part we found it to be equal to 0.96 +/- 0.03 km s-1 on January 4 and 1.10 +/- 0.01 km s-1 on January 9, whereas the anti-sunward part reached 0.73 +/- 0.05 and 0.80 +/- 0.02 km s-1, respectively. The periodicity of gas emission was investigated using a repeatability of the shells, their kinematics, and an aperture photometry of the near nucleus region. We found a period of 5.70 +/- 0.07 h (along with multiples), consistent with previous findings by other authors. A toy Monte Carlo model was implemented to reproduce the time-series of the CN images. We show that emission of HCN into a relatively wide cone by a single active region on a rotating nucleus is the most probable scenario. [PUBLICATION ABSTRACT]