•Cassini UVIS solar occultations by Saturn’s F ring are presented.•Diffraction signatures detected in UVIS data correspond with collisional events observed in ISS images.•A smaller effective particle ...size best reproduces the occultation light curve when diffraction signatures are detected.•Collisions in F ring replenish micron-sized dust population in Saturn’s F ring.
We present an analysis of eleven solar occultations by Saturn’s F ring observed by the Ultraviolet Imaging Spectrograph (UVIS) on the Cassini spacecraft. In four of the solar occultations we detect an unambiguous signal from diffracted sunlight that adds to the direct solar signal just before or after the occultations occur. The strongest detection was a 10% increase over the direct signal that was enabled by the accidental misalignment of the instrument’s pointing. We compare the UVIS data with images of the F ring obtained by the Cassini Imaging Science Subsystem (ISS) and find that in each instance of an unambiguous diffraction signature in the UVIS data, the ISS data shows that there was a recent disturbance in that region of the F ring. Similarly, the ISS images show a quiescent region of the F ring for all solar occultations in which no diffraction signature was detected. We therefore conclude that collisions in the F ring produce a population of small ring particles that can produce a detectable diffraction signal immediately interior or exterior to the F ring. The clearest example of this connection comes from the strong detection of diffracted light in the 2007 solar occultation, when the portion of the F ring that occulted the Sun had suffered a large collisional event, likely with S/2004 S 6, several months prior. This collision was observed in a series of ISS images (Murray et al., 2008).
Our spectral analysis of the data shows no significant spectral features in the F ring, indicating that the particles must be at least 0.2 µm in radius. We apply a forward model of the solar occultations, accounting for the effects of diffracted light and the attenuated direct solar signal, to model the observed solar occultation light curves. These models constrain the optical depth, radial width, and particle size distribution of the F ring. We find that when the diffraction signature is present, we can best reproduce the occultation data using a particle population with an average effective particle size of less than 300 µm, while occultations without clear diffraction signals are best modeled using a population with an effective particle size larger than 400 µm.
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•We report the discovery of a sub-km object orbiting near the edge of Saturn’s A ring.•The object’s perturbing effect on the rings has been detectable since May 2012.•There is ...evidence of a changing orbit and possible disruption in early 2013.•The object could have formed in situ due to the Janus resonance near the ring edge.
We report on the serendipitous discovery of an unresolved, evolving, sub-km-radius object with a semimajor axis <10km inside that of the edge of Saturn’s main rings. The object has been detectable in Cassini images since at least May 2012 and its changing orbit shows evidence of a possible disruption in early 2013.
Martian araneiform terrain, located in the Southern polar regions, consists of features with central pits and radial troughs which are thought to be associated with the solid state greenhouse effect ...under a CO2 ice sheet. Sublimation at the base of this ice leads to gas buildup, fracturing of the ice and the flow of gas and entrained regolith out of vents and onto the surface. There are two possible pathways for the gas: through the gap between the ice slab and the underlying regolith, as proposed by Kieffer (2007), or through the pores of a permeable regolith layer, which would imply that regolith properties can control the spacing between adjacent spiders, as suggested by Hao et al. (2019). We test this hypothesis quantitatively in order to place constraints on the regolith properties. Based on previously estimated flow rates and thermophysical arguments, we suggest that there is insufficient depth of porous regolith to support the full gas flow through the regolith. By contrast, free gas flow through a regolith–ice gap is capable of supplying the likely flow rates for gap sizes on the order of a centimetre. This size of gap can be opened in the centre of a spider feature by gas pressure bending the overlying ice slab upwards, or by levitating it entirely as suggested in the original Kieffer (2007) model. Our calculations therefore support at least some of the gas flowing through a gap opened between the regolith and ice. Regolith properties most likely still play a role in the evolution of spider morphology, by regolith cohesion controlling the erosion of the central pit and troughs, for example.
•Spiders are Martian polar landforms associated with CO2 sublimation.•Sublimating gas flows under the CO2 ice, excavating the features.•We investigate numerically how much of this flow can be through the porous regolith.•This has implications for the regolith erosion and the spacing between spiders.
Aims. We use four observational data sets, mainly from the Rosetta mission, to constrain the activity pattern of the nucleus of comet 67P/Churyumov-Gerasimenko (67P). Methods. We developed a ...numerical model that computes the production rate and non-gravitational acceleration of the nucleus of comet 67P as a function of time, taking into account its complex shape with a shape model reconstructed from OSIRIS imagery. We used this model to fit three observational data sets: the trajectory data from flight dynamics; the rotation state as reconstructed from OSIRIS imagery; and the water production measurements from ROSINA of 67P. The two key parameters of our model, adjusted to fit the three data sets all together, are the activity pattern and the momentum transfer efficiency (i.e., the so-called η parameter of the non-gravitational forces). Results. We find an activity pattern that can successfully reproduce the three data sets simultaneously. The fitted activity pattern exhibits two main features: a higher effective active fraction in two southern super-regions (~10%) outside perihelion compared to the northern regions (<4%), and a drastic rise in effective active fraction of the southern regions (~25−35%) around perihelion. We interpret the time-varying southern effective active fraction by cyclic formation and removal of a dust mantle in these regions. Our analysis supports moderate values of the momentum transfer coefficient η in the range 0.6–0.7; values η ≤ 0.5 or η ≥ 0.8 significantly degrade the fit to the three data sets. Our conclusions reinforce the idea that seasonal effects linked to the orientation of the spin axis play a key role in the formation and evolution of dust mantles, and in turn, they largely control the temporal variations of the gas flux.
Nongravitational forces induced by sublimation of volatiles affect the rotational and orbital dynamics of comets. In addition to contributing to the improvement of ephemerides and of rotational ...models, nongravitational effects can help constrain the regional distribution and temporal evolution of cometary activity, which in turn provides input for the development of thermophysical and dust transport models. We review the progress that has been made in this field thanks to the Rosetta mission and we highlight the open questions.
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).
The Comet Interceptor Mission Snodgrass, Colin; Lara, Luisa M.; Agarwal, Jessica ...
Space science reviews,
2024, Letnik:
220, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first ...time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum
Δ
V capability of
600
ms
−
1
. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.
•We catalogue 889 irregular features seen emanating from the F ring in Cassini images•They average ∼15 in the ring at any one time and are distributed randomly•∼1m/s Collisions with a population of ...∼100 local objects are proposed•Objects may form and evolve due to Prometheus perturbations•Those that survive may evolve to S/2004 S 6 like objects which create the larger jets
Small (∼50km scale), irregular features seen in Cassini images to be emanating from Saturn’s F ring have been termed mini-jets by Attree et al. (2012). One particular mini-jet was tracked over half an orbital period, revealing its evolution with time and suggesting a collision with a local moonlet as its origin. In addition to these data we present here a much more detailed analysis of the full catalogue of over 800 F ring mini-jets, examining their distribution, morphology and lifetimes in order to place constraints on the underlying moonlet population. We find mini-jets randomly located in longitude around the ring, with little correlation to the moon Prometheus, and randomly distributed in time, over the full Cassini tour to date. They have a tendency to cluster together, forming complicated ‘multiple’ structures, and have typical lifetimes of ∼1d. Repeated observations of some features show significant evolution, including the creation of new mini-jets, implying repeated collisions by the same object. This suggests a population of ≲1km radius objects with some internal strength and orbits spread over ±100km in semi-major axis relative to the F ring but with the majority within 20km. These objects likely formed in the ring under, and were subsequently scattered onto differing orbits by, the perturbing action of Prometheus. This reinforces the idea of the F ring as a region with a complex balance between collisions, disruption and accretion.
Abstract
Although ice fracturing and deformation is key to understanding some of the landforms encountered in the high‐latitude regions on Mars and on other icy bodies in the solar system, little is ...known about the mechanical characteristics of CO
ice. We have measured the hardness of solid CO
ice directly in the laboratory with a Leeb hardness tester and calculated the corresponding yield strength. We have also measured the hardness of water ice by the same method, confirming previous work. Our results indicate that CO
ice is slightly weaker, ranging between Leeb
200 and 400 (
10 and 30 MPa yield strength, assuming only plastic deformation and no strain hardening during the experiment), for typical Martian temperatures. Our results can be used for models of CO
ice rupture (depending on the deformation timescales) explaining surface processes on Mars and solar system icy bodies.
Plain Language Summary
We have measured the hardness and calculated the strength of dry (carbon dioxide) ice. It is slightly weaker than water ice in the temperature range that can be found on Mars. Our results can be used in models of how surface features on Mars and on other icy surfaces in the solar system are formed.
Key Points
We have measured the hardness of CO
ice in Martian temperatures
We derived the corresponding yield strength values
Under Martian conditions CO
ice is slightly weaker than H
O ice
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