Since OSIRIS started acquiring high-resolution observations of the surface of the nucleus of comet 67P/Churyumov-Gerasimenko, over one hundred meter-sized bright spots have been identified in ...numerous types of geomorphologic regions, but mostly located in areas receiving low insolation. The bright spots are either clustered, in debris fields close to decameter-high cliffs, or isolated without structural relation to the surrounding terrain. They can be up to ten times brighter than the average surface of the comet at visible wavelengths and display a significantly bluer spectrum. They do not exhibit significant changes over a period of a few weeks. All these observations are consistent with exposure of water ice at the surface of boulders produced by dislocation of the weakly consolidated layers that cover large areas of the nucleus. Laboratory experiments show that under simulated comet surface conditions, analog samples acquire a vertical stratification with an uppermost porous mantle of refractory dust overlaying a layer of hard ice formed by recondensation or sintering under the insulating dust mantle. The evolution of the visible spectrophotometric properties of samples during sublimation is consistent with the contrasts of brightness and color seen at the surface of the nucleus. Clustered bright spots are formed by the collapse of overhangs that is triggered by mass wasting of deeper layers. Isolated spots might be the result of the emission of boulders at low velocity that are redepositioned in other regions.
Many technologies are available to produce solar cooling. The most widespread options are solar thermal driven sorption and photovoltaic driven compression chillers. Sorption chillers can be single ...or double effect LiBr–H2O water cooled or H2O–NH3 air cooled. The requested temperatures are respectively around 90 °C for the first engine and 160 °C for the other two. The solar section can be realised with flat plate or evacuated tube collectors at a fixed tilt or parabolic trough collectors with a tracking system. The possible solar cooling systems are evaluated during a sunny day and compared with the PV driven system first with respect to the overall system efficiency. Finally the comparison regards also the investment cost for the various systems.
Whereas in the past the PV driven system was considered too expensive with respect to the solar thermal, now it is quite comparable above all if the systems are air cooled.
•Solar cooling is obtained with solar thermal or PV with easy available equipment.•In the past PV driven systems for solar cooling were not considered as too expensive.•The investment cost of some thermal and PV solar cooling systems are comparable.•An energy/economic comparison is carried out for the various solar cooling systems.
We present OSIRIS/NAC observations of decimetre-sized, likely ice-containing aggregates ejected from a confined region on the surface of comet 67P/Churyumov-Gerasimenko. The images were obtained in ...January 2016 when the comet was at 2 AU from the Sun out-bound from perihelion. We measure the acceleration of individual aggregates through a two-hour image series. Approximately 50% of the aggregates are accelerated away from the nucleus, and 50% towards it, and likewise towards either horizontal direction. The accelerations are up to one order of magnitude stronger than local gravity, and are most simply explained by the combined effect of gas drag accelerating all aggregates upwards, and the recoil force from asymmetric outgassing, either from rotating aggregates with randomly oriented spin axes and sufficient thermal inertia to shift the temperature maximum away from an aggregate's subsolar region, or from aggregates with variable ice content. At least 10% of the aggregates will escape the gravity field of the nucleus and feed the comet's debris trail, while others may fall back to the surface and contribute to the deposits covering parts of the northern hemisphere. The rocket force plays a crucial role in pushing these aggregates back towards the surface. Our observations show the future back fall material in the process of ejection, and provide the first direct measurement of the acceleration of aggregates in the innermost coma (<2km) of a comet, where gas drag is still significant.
We directly measured twenty overhanging cliffs on the surface of comet 67P/Churyumov–Gerasimenko extracted from the latest shape model and estimated the minimum tensile strengths needed to support ...them against collapse under the comet’s gravity. We find extremely low strengths of around 1 Pa or less (1 to 5 Pa, when scaled to a metre length). The presence of eroded material at the base of most overhangs, as well as the observed collapse of two features andthe implied previous collapse of another, suggests that they are prone to failure and that the true material strengths are close to these lower limits (although we only consider static stresses and not dynamic stress from, for example, cometary activity). Thus, a tensile strength of a few pascals is a good approximation for the tensile strength of the 67P nucleus material, which is in agreement with previous work. We find no particular trends in overhang properties either with size over the ~10–100 m range studied here or location on the nucleus. There are no obvious differences, in terms of strength, height or evidence of collapse, between the populations of overhangs on the two cometary lobes, suggesting that 67P is relatively homogenous in terms of tensile strength. Low material strengths are supportive of cometary formation as a primordial rubble pile or by collisional fragmentation of a small body (tens of km).
Abstract
On 2016 July 03, several instruments onboard ESA's Rosetta spacecraft detected signs of an outburst event on comet 67P, at a heliocentric distance of 3.32 au from the Sun, outbound from ...perihelion. We here report on the inferred properties of the ejected dust and the surface change at the site of the outburst. The activity coincided with the local sunrise and continued over a time interval of 14–68 min. It left a 10-m-sized icy patch on the surface. The ejected material comprised refractory grains of several hundred microns in size, and sub-micron-sized water ice grains. The high dust mass production rate is incompatible with the free sublimation of crystalline water ice under solar illumination as the only acceleration process. Additional energy stored near the surface must have increased the gas density. We suggest a pressurized sub-surface gas reservoir, or the crystallization of amorphous water ice as possible causes.
It is known that near-Earth objects (NEOs) during their orbital evolution may often undergo close approaches to the Sun. Indeed it is estimated that up to ∼70 per cent of them end their orbital ...evolution colliding with the Sun. Starting from the present orbital properties, it is possible to compute the most likely past evolution for every NEO, and to trace its distance from the Sun. We find that a large fraction of the population may have experienced in the past frequent close approaches, and thus, as a consequence, a considerable Sun-driven heating, not trivially correlated to the present orbits. The detailed dynamical behaviour, the rotational and the thermal properties of NEOs determine the exact amount of the resulting heating due to the Sun. In the present paper, we discuss the general features of the process, providing estimates of the surface temperature reached by NEOs during their evolution. Moreover, we investigate the effects of this process on meteor-size bodies, analysing possible differences with the NEO population. We also discuss some possible effects of the heating which can be observed through remote sensing by ground-based surveys or space missions.
Context. Dust deposits or dust cover are a prevalent morphology in the northern hemi-nucleus of comet 67P/Churyumov-Gerasimenko (67P). The evolution of the dust deposits was captured by the OSIRIS ...camera system onboard the Rosetta spacecraft having escorted the comet for over two years. The observations shed light on the fundamental role of cometary activity in shaping and transforming the surface morphology. Aims. We aim to present OSIRIS observations of surface changes over the dust deposits before and after perihelion. The distribution of changes and a timeline of their occurrence are provided. We perform a data analysis to quantify the surface changes and investigate their correlation to water activity from the dust deposits. We further discuss how the results of our investigation are related to other findings from the Rosetta mission. Methods. Surface changes were detected via systematic comparison of images, and quantified using shape-from-shading technique. Thermal models were applied to estimate the erosion of water ice in response to the increasing insolation over the areas where surface changes occurred. Modeling results were used for the interpretation of the observed surface changes. Results. Surface changes discussed here were concentrated at mid-latitudes, between about 20◦N and 40◦N, marking a global transition from the dust-covered to rugged terrains. The changes were distributed in open areas exposed to ample solar illumination and likely subject to enhanced surface erosion before perihelion. The occurrence of changes followed the southward migration of the sub-solar point across the latitudes of their distribution. The erosion at locations of most changes was at least about 0.5 m, but most likely did not exceed several meters. The erosive features before perihelion had given way to a fresh, smooth cover of dust deposits after perihelion, suggesting that the dust deposits had been globally restored by at least about 1 m with ejecta from the intensely illuminated southern hemi-nucleus around perihelion, when the north was inactive during polar night. Conclusions. The erosion and restoration of the northern dust deposits are morphological expressions of seasonality on 67P. Based on observations and thermal modeling results, it is inferred that the dust deposits contained a few percent of water ice in mass on average. Local inhomogeneity in water abundance at spatial scales below tens of meters is likely. We suspect that dust ejected from the deposits may not have escaped the comet in bulk. That is, at least half of the ejected mass was afloat in the inner-coma or/and redeposited over other areas of the nucleus.
Context.
The near-Earth objects (NEOs), whose proximity makes them the most accessible bodies in the Solar System, allow us to sample asteroids from tens of kilometers down to objects of a few meters ...in size. However, while the physical properties for the largest bodies are mostly known, we have very little physical information regarding the small NEOs. These objects today represent the overall majority among the ~2500 new discoveries each year, but they are usually only bright enough to be observable during their close approaches.
Aims.
Our aim was to extend our survey that started in 2015 on the NEO population, using ground-based observations to characterize the fainter (and typically smaller) NEOs observable each night.
Methods.
We performed BVRIz photometry of NEOs, making use of the DOLORES instrument at the Telescopio Nazionale
Galileo
(TNG, La Palma, Spain) and the Asiago Schmidt telescope (Italy), in order to derive visible color indexes and the taxonomic classification for each target in our sample.
Results.
We taxonomically classified 51 new NEOs for the first time. Together with data obtained in our previous work and collected by other surveys available online, we analyzed an extended sample of 1081 individual NEOs. While the overall majority of them belong to the S-complex, our analysis of the taxonomic distribution found a larger contribution for dark bodies going toward larger H, suggesting that they could be more abundant among the fainter NEOs. Moreover, we find an interesting correlation between semi-major axis and diameter, which could be in part related to the Yarkovsky effect. Rapid characterization of the fainter NEO population shortly after their discovery will be crucial in the future, before those bodies become too faint to be observed, or lost forever.
The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) experiment onboard the Rosetta spacecraft currently orbiting comet 67P/Churyumov‐Gerasimenko has yielded unprecedented views of ...a comet's nucleus. We present here the first ever observations of meter‐scale fractures on the surface of a comet. Some of these fractures form polygonal networks. We present an initial assessment of their morphology, topology, and regional distribution. Fractures are ubiquitous on the surface of the comet's nucleus. Furthermore, they occur in various settings and show different topologies suggesting numerous formation mechanisms, which include thermal insulation weathering, orbital‐induced stresses, and possibly seasonal thermal contraction. However, we conclude that thermal insolation weathering is responsible for creating most of the observed fractures based on their morphology and setting in addition to thermal models that indicate diurnal temperature ranges exceeding 200 K and thermal gradients of ~15 K/min at perihelion are possible. Finally, we suggest that fractures could be a facilitator in surface evolution and long‐term erosion.
Key Points
Fractures are observed on the surface of a comet for the first time
Different fracture settings and topologies suggest various formation mechanisms
Fractures may be a key driver in the evolution of the surface
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