Sublimation in bright spots on (1) Ceres.

The dwarf planet (1) Ceres, the largest object in the main asteroid belt with a mean diameter of about 950 kilometres, is located at a mean distance from the Sun of about 2.8 astronomical units (one astronomical unit is the Earth-Sun distance). Thermal evolution models suggest that it is a differentiated body with potential geological activity. Unlike on the icy satellites of Jupiter and Saturn, where tidal forces are responsible for spewing briny water into space, no tidal forces are acting on Ceres. In the absence of such forces, most objects in the main asteroid belt are expected to be geologically inert. The recent discovery of water vapour absorption near Ceres and previous detection of bound water and OH near and on Ceres (refs 5-7) have raised interest in the possible presence of surface ice. Here we report the presence of localized bright areas on Ceres from an orbiting imager. These unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible. Of particular interest is a bright pit on the floor of crater Occator that exhibits probable sublimation of water ice, producing haze clouds inside the crater that appear and disappear with a diurnal rhythm. Slow-moving condensed-ice or dust particles may explain this haze. We conclude that Ceres must have accreted material from beyond the 'snow line', which is the distance from the Sun at which water molecules condense.

Dark material on Vesta from the infall of carbonaceous volatile-rich material.

Localized dark and bright materials, often with extremely different albedos, were recently found on Vesta's surface. The range of albedos is among the largest observed on Solar System rocky bodies. These dark materials, often associated with craters, appear in ejecta and crater walls, and their pyroxene absorption strengths are correlated with material brightness. It was tentatively suggested that the dark material on Vesta could be either exogenic, from carbon-rich, low-velocity impactors, or endogenic, from freshly exposed mafic material or impact melt, created or exposed by impacts. Here we report Vesta spectra and images and use them to derive and interpret the properties of the 'pure' dark and bright materials. We argue that the dark material is mainly from infall of hydrated carbonaceous material (like that found in a major class of meteorites and some comet surfaces), whereas the bright material is the uncontaminated indigenous Vesta basaltic soil. Dark material from low-albedo impactors is diffused over time through the Vestan regolith by impact mixing, creating broader, diffuse darker regions and finally Vesta's background surface material. This is consistent with howardite-eucrite-diogenite meteorites coming from Vesta.

Morphology and dynamics of the upper cloud layer of Venus.

Venus is completely covered by a thick cloud layer, of which the upper part is composed of sulphuric acid and some unknown aerosols. The cloud tops are in fast retrograde rotation (super-rotation), but the factors responsible for this super-rotation are unknown. Here we report observations of Venus with the Venus Monitoring Camera on board the Venus Express spacecraft. We investigate both global and small-scale properties of the clouds, their temporal and latitudinal variations, and derive wind velocities. The southern polar region is highly variable and can change dramatically on timescales as short as one day, perhaps arising from the injection of SO2 into the mesosphere. The convective cells in the vicinity of the subsolar point are much smaller than previously inferred, which we interpret as indicating that they are confined to the upper cloud layer, contrary to previous conclusions, but consistent with more recent study.

Rain, winds and haze during the Huygens probe's descent to Titan's surface.

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.

The spectrum of comet Hale-Bopp (C/1995 O1) observed with the Infrared Space Observatory at 2.9 astronomical units from the sun.

Comet Hale-Bopp (C/1995 O1) was observed at wavelengths from 2.4 to 195 micrometers with the Infrared Space Observatory when the comet was about 2.9 astronomical units (AU) from the sun. The main observed volatiles that sublimated from the nucleus ices were water, carbon monoxide, and carbon dioxide in a ratio (by number) of 10:6:2. These species are also the main observed constituents of ices in dense interstellar molecular clouds; this observation strengthens the links between cometary and interstellar material. Several broad emission features observed in the 7- to 45-micrometer region suggest the presence of silicates, particularly magnesium-rich crystalline olivine. These features are similar to those observed in the dust envelopes of Vega-type stars.

Results from the Mars Pathfinder camera.

Images of the martian surface returned by the Imager for Mars Pathfinder (IMP) show a complex surface of ridges and troughs covered by rocks that have been transported and modified by fluvial, aeolian, and impact processes. Analysis of the spectral signatures in the scene (at 440- to 1000-nanometer wavelength) reveal three types of rock and four classes of soil. Upward-looking IMP images of the predawn sky show thin, bluish clouds that probably represent water ice forming on local atmospheric haze (opacity approximately 0.5). Haze particles are about 1 micrometer in radius and the water vapor column abundance is about 10 precipitable micrometers.

Surface changes on comet 67P/Churyumov-Gerasimenko suggest a more active past.

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.

Rosetta's comet 67P/Churyumov-Gerasimenko sheds its dusty mantle to reveal its icy nature.

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.

Selective responses (actin polymerization, shape changes, locomotion, pinocytosis) to the PKC inhibitor Ro 31-8220 suggest that PKC discriminately regulates functions of human blood lymphocytes.

The results suggest that protein kinase C (PKC) plays a pivotal role in the control of F-actin levels, locomotion, pinocytosis, and cell shape in lymphocytes. The PKC inhibitor Ro 31-8220 elicits a high proportion of polarized (ED50 = 1.5 x 10(-6) M) and locomoting cells and reduces the relative amount of F-actin (by 29% at 10(-5) M) in initially resting cells. Phorbol myristate acetate (PMA) counterbalances the polarizing effect of Ro 31-8220. This indicates that the spherical shape and the F-actin content of resting cells are maintained by constitutive PKC activity. PMA-induced increases in fluid pinocytosis, F-actin content, and formation of nonpolar cells with surface protrusion are suppressed by Ro 31-8220 (IC50 = 2-4 x 10(-7) M). Spherical cells and, at higher concentrations (ED50 = 3.3 x 10(-6) M), polarized cells are formed instead. As a result, lymphocyte function switches from fluid pinocytosis to cell polarity and locomotion. The data indicate that PKC is instrumental in selectively switching lymphocyte function between resting state, locomotor activity, and fluid pinocytosis. Ro 31-8220 is extremely potent in stimulating lymphocyte polarity and locomotion (B and T cells). It acts faster and/or produces a higher proportion of polarized lymphocytes than other available agonists. It may thus be used as a tool in further experiments requiring locomoting lymphocytes.

Relationship between light scattering in flow cytometry and changes in shape, volume, and actin polymerization in human polymorphonuclear leukocytes.

Using two different cytometers, an Epics Profile II and a FACScan, we determined the extent to which changes in forward and right angle scatter are a reliable measure for changes in polymorphonuclear leukocyte (PMN) shape, volume, and actin polymerization and whether distinct types of shape changes in PMNs can be recognized. PMN stimulation can substantially change the positions of PMNs in the scatter diagram of the FACScan but not of the Epics Profile II. Within the limits of the experiments, it has been possible to determine whether or not a shape change has taken place using the FACScan but not using the Epics Profile II. However, using either cytometer, it has not been possible to determine which type of shape change (e.g., spherical vs. polarized vs. nonpolar cells) has taken place. Furthermore, forward or right angle scatter changes are not a reliable measure for changes in cell volume or actin polymerization of human PMNs.

Cometary science. Time variability and heterogeneity in the coma of 67P/Churyumov-Gerasimenko.

Comets contain the best-preserved material from the beginning of our planetary system. Their nuclei and comae composition reveal clues about physical and chemical conditions during the early solar system when comets formed. ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) onboard the Rosetta spacecraft has measured the coma composition of comet 67P/Churyumov-Gerasimenko with well-sampled time resolution per rotation. Measurements were made over many comet rotation periods and a wide range of latitudes. These measurements show large fluctuations in composition in a heterogeneous coma that has diurnal and possibly seasonal variations in the major outgassing species: water, carbon monoxide, and carbon dioxide. These results indicate a complex coma-nucleus relationship where seasonal variations may be driven by temperature differences just below the comet surface.

Heavy water (D2O)-induced shape changes, movements and F-actin redistribution in human neutrophil granulocytes.

Heavy water (D2O) induces characteristic shape changes and a distinct type of movement in human neutrophil granulocytes. In contrast to front-tail polarity as evoked by chemotactic peptides and microtubule-disassembling agents, D2O-based media produce non-polar neutrophils with many small or long surface projections. This phenotype is similar to that elicited by both phorbol myristate acetate and diacylglycerols, but the surface projections are smaller and more densely placed and are often associated with a single large projection. D2O-induced non-polar cells with surface projections perform continuous shape changes without front-tail polarity and without the unidirectional movement and cytoplasmic streaming seen in cells with front-tail polarity. Some of the cells show circus movements of a large projection indicating circular polarity. In neutrophils suspended in D2O, F-actin is shifted to the cell periphery, mainly into the surface projections of activated cells. The D2O-induced effects are reversed in H2O-based medium. D2O is dominant over the chemotactic peptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), colchicine and taxol in that the combined action of D2O with any of these agents results in the D2O-induced phenotype. In contrast, cytochalasin B alone and in combination with fMLP induces a considerable decrease of non-polar cells and an increase of spherical cells similar to non-stimulated cells in H2O-based medium. Earlier studies indicated that D2O acts on microtubules. Our results suggest that D2O may act on the microfilament system. Neutrophils suspended in D2O-based medium may represent a useful model to study the relationship between shapes, movements, and particular functions of these cells.

Phenotype modulation in non-adherent and adherent sublines of Walker carcinosarcoma cells: the role of cell-substratum contacts and microtubules in controlling cell shape, locomotion and cytoskeletal structure.

We characterised two sublines of Walker carcinosarcoma cells generated by epigenetic changes. Subline 1 cells were mostly polarised and made no or only non-adhesive cell-substratum contacts. Subline 2 cells were spread, adhesive and mainly non-polar. Subline 1 cells migrate in a non-adhesive mode which is very efficient but operates only in a 3D environment, whereas subline 2 cells migrate in an adhesive mode, which is less efficient but works on 2D and 3D substrata. Nocodazole had little or no effect on shape, polarity and locomotion of subline 1 cells. In glass-adherent subline 2 cells, 10(-6)M nocodazole increased the proportion of polarised cells migrating in an adhesive mode and decreased adhesion to the substratum, whereas 10(-5)M nocodazole further reduced the contacts and the cells reverted to a non-adhesive mode of locomotion. When non-polar subline 2 cells were detached mechanically or by nocodazole, they became polarised and morphologically indistinguishable from non-adherent subline 1 cells. On more adhesive plastic substrata, subline 2 cells produced heterogeneous responses to nocodazole including loss of polarity. The phenotypes of Walker carcinosarcoma sublines have similarities with a broad range of cell types ranging from leucocytes to fibroblast-like cells, suggesting that these phenotypic differences can be controlled by the adhesive and contractile state rather than the cell type. Adhesion modulates contractility (isometric or isotonic contraction) and vice versa and this determines morphology (shape, F-actin, myosin and alpha-actinin), locomotion and responses to microtubule-disassembly. The model may be applied to analyse the mechanisms controlling the phenotype of cells in general.

Physiology of chemotaxis and random motility.

The following aspects of random and directional migration of leukocytes have been considered: (1) basic cellular mechanisms involved in leukocyte locomotion; (2) the mechanisms of formation and the nature of various factors such as cytotaxins, cytotaxin-inactivators and inhibitors participating in the regulation of the chemotactic response, and (3) the relationship between chemotaxis and leukocyte emigration. The need for adequate techniques for measuring chemotaxis has been emphasized with special reference to clinical investigation.

Analysis of lymphocyte shape by visual classification, calculated measures of shape or light scattering.

Direct visual assays, calculated measures of shape (Dunn and Brown, 1986) and light scattering were applied to detect shape changes and to identify human peripheral blood lymphocytes displaying different shapes (e.g., spherical cells, polarized cells, non-polar cells with surface projections). Spherical cells could be distinguished from polarized cells and from non-polar cells with surface projections by means of calculated measures of shape obtained from outline drawings, the dispersion parameter being more efficient than extension or elongation parameters. Median values for elongation and extension of polarized cells vs. non-polar cells with surface projections differed significantly provided the cell populations consisted entirely of one particular morphological class. However, if the proportion of cells changing shape is small, this may not be detectable by a significant change in the median values. Therefore, we attempted to apply calculated shape factors to identify individual cells showing a particular type of shape change. Spherical cells could be distinguished relatively easily from non-spherical cells on the basis of the dispersion values. However, it was not possible to distinguish unequivocally between polarized cells and non-polar cells with surface projections because the values overlap to a great extent. For this purpose the visual classification was found to be more reliable. Preliminary experiments with light scattering showed that median values for right angle scatter or forward angle scatter are not sensitive enough to permit the detection of a small proportion of cells changing shape.

Cytotaxin-induced cAMP peak in granulocytes: its relationship to crawling movements, chemokinesis and chemotaxis.

The relationship between the short transient intracellular increase in cAMP levels on the one hand and chemotaxis or crawling movements on the other hand was investigated using human and equine granulocytes. C5ades arg, f-met-leu-phe, human serum albumin and immunoglobulin were used as stimulating agents. There was no strict correlation between the induction of crawling movements or of chemokinesis in general and the generation of the cAMP peak. But there was so far a strict parallelism between the occurrence of the chemotactic response and the cAMP peak. However, the magnitude of the peak was not representative for the extent of directional locomotion. It seems possible that cAMP is an essential step in the transduction of the chemotactic response, but there is no proof for a causal relationship as yet.

The post-natal development of lymph nodes in mice.

A total of ten major lymph node stations have been studied in ICR-strain mice at various time intervals between birth and the age of four weeks. Comparison of lymph node volumes and of volume changes during the observation period revealed that the mesenteric lymph node (MLN) complex was larger at birth and grew at a faster rate than all other nodes. A distinct numerical increase of cortical lymphocytes in MLN at the end of the first week of life was interpreted to be the result of both immigration of T cells and moderate local proliferation, while the quantitatively more important second phase of growth at the age of between two and three weeks might be the consequence of local lymphocyte production. The comparatively modest changes in lymph nodes removed from the gut (axillary and popliteal nodes) could be explained as being due in large measure to immigration of lymphocytes originating in gut-associated lymphoid tissues (GALT).

Shape, movement and function of neutrophil granulocytes.

Evidence that distinct types of shapes and movement can be induced by different neutrophil-activating agents like the chemotactic peptides and PMA is reviewed. Front-tail polarity seems to be preferentially associated with locomotion, whereas non-polar cells with surface projections show preferentially pinocytosis. The following partially hypothetical concept on neutrophil shape changes has been derived from these experiments: Neutrophils are capable to perform distinct types of movements depending on the stimulus or stimuli. Distinct movements are preferentially associated with distinct functions such as locomotion, pinocytosis or others. The underlying cytoskeletal changes may differ accordingly.

Locomotion of tumor cells as an element of invasion and metastasis.

Malignant tumor cells are endowed with the ability to invade host tissues and to produce metastases. In this review, tumor cell locomotion as an important pathogenic mechanism in the invasive process is discussed. The traffic of neoplastic cells along preformed or newly created tissue pathways will not only depend on cellular cues and/or extracellular stimulatory or inhibitory soluble factors, but also on the interactions of tumor cells with the invaded tissue. Normal tissue exerts its influence on tumor cell migration mainly by providing contact surfaces, as both the growth and locomotor phenomena of tumor cells depend on the cells ability to adhere to structures present along the invasion front. Migrating tumor cells are morphologically characterized by a cell shape change typical for locomoting cells, i.e. polarization. This locomotor phenotype is associated with altered functions of the cytoskeleton induced by agents acting on cell surface receptors, on the signal cascade, or on the cytoskeletal apparatus itself, or evoked by mutations of cytoskeletal proteins. The search for stop signals of tumor cell locomotion is of particular interest, as this may represent an approach to exert an influence on the invasive process.

A short transient increase in cyclic adenosine 3', 5'-monophosphate levels of neutrophil granulocytes following exposure to chemotactic factors.

Stimuli which are chemotactic for the retrospective neutrophils always produce a transient increase in cAMP within the first minute whereas the chemokinetic or other non-chemotactic stimuli tested have no such effect. The question as to whether the transient increase of cAMP levels plays any direct role in the transduction of a chemotactic stimulus cannot be answered. Restimulation experiments indicate that the extent of the cAMP response is not a reliable indicator for the chemotactic responsiveness of neutrophils; there is no quantitative correlation between these two parameters. Further detailed kinetic and quantitative studies of the cAMP system would be needed to clarify this point.