Interstellar Dust in the Solar System: Model versus In-Situ Spacecraft Data

In the early 1990s, contemporary interstellar dust penetrating deep into the heliosphere was identified with the in-situ dust detector on board the Ulysses spacecraft. Later on, interstellar dust was also identified in the data sets measured with dust instruments on board Galileo, Cassini and Helios. Ulysses monitored the interstellar dust stream at high ecliptic latitudes for about 16 years. The three other spacecraft data sets were obtained in the ecliptic plane and cover much shorter time intervals.We compare in-situ interstellar dust measurements obtained with these four spacecrafts, published in the literature, with predictions of a state-of-the-art model for the dynamics of interstellar dust in the inner solar system (Interplanetary Meteoroid environment for EXploration, IMEX), in order to test the reliability of the model predictions. Micrometer and sub-micrometer sized dust particles are subject to solar gravity and radiation pressure as well as to the Lorentz force on a charged dust particle moving through the Interplanetary Magnetic Field. The IMEX model was calibrated with the Ulysses interstellar dust measurements and includes these relevant forces. We study the time-resolved flux and mass distribution of interstellar dust in the solar system. The IMEX model agrees with the spacecraft measurements within a factor of 2 to 3, also for time intervals and spatial regions not covered by the original model calibration with the Ulysses data set. It usually underestimates the dust fluxes measured by the space missions which were not used for the model calibration, i.e. Galileo, Cassini and Helios. IMEX is a unique time-dependent model for the prediction of interstellar dust fluxes and mass distributions for the inner and outer solar system. The model is suited to study dust detection conditions for past and future space missions.Comment: 24 pages, 7 figures, 1 tabl

Monitoring of the Interstellar Dust Stream in the Inner Solar System Using Data of Different Spacecraft

Interstellar dust plays a key role in many astrophysical processes. However, until recently, astronomers could infer some properties of interstellar dust only through observations of starlight extinction and infrared emission. Therefore, the in-situ detection of interstellar dust, based on detectors carried by spacecraft, was a major step for dust astronomy. This method showed that one can learn a lot about the interstellar medium, if one decodes the information carried by the dust grains. Goal of this work is to analyse data sets for interstellar dust, that have been collected by the interplanetary probes Helios, Galileo, Ulysses and Cassini. The analysis of the in-situ data allows us to identify interstellar dust between 0.3 AU and 5 AU. In addition, the data provide in-situ evidence for the interaction of the interstellar dust stream with the heliospheric environment. In particular, the influence of the radiation pressure and of the gravitation focusing on the interstellar dust size distribution could be measured as function of the heliocentric distance

Organic matter in interstellar dust lost at the approach to the heliosphere: Exothermic chemical reactions of free radicals ignited by the Sun

Aims. We tackle the conundrums of organic materials missing from interstellar dust when measured inside the Solar System, while undoubtedly existing in the local interstellar cloud (LIC), which surrounds the Solar System. Methods. We present a theoretical argument that organic compounds sublimate almost instantaneously by exothermic reactions, when solar insolation triggers the recombination of free radicals or the rearrangement of carbon bonds in the compounds. Results. It turns out that the triggering temperature lies in the range of 20$-$50 K by considering that sublimation of organic materials takes place beyond the so-called filtration region of interstellar neutral atoms. We find that in-situ measurements of LIC dust in the Solar System result in an overestimate for the gas-to-dust mass ratio of the LIC, unless the sublimation of organic materials is taken into account. We also find that previous measurements of interstellar pickup ions have determined the total elemental abundances of gas and organic materials, instead of interstellar gas alone. Conclusions. We conclude that LIC organic matter suffers from sublimation en route to the heliosphere, implying that our understanding of LIC dust from space missions is incomplete. Since space missions inside the orbit of Saturn cannot give any information on the organic substances of LIC dust, one must await a future exploration mission to the inner edge of the Oort cloud for a thorough understanding of organic substances in the LIC. Once our model for the sublimation of interstellar organic matter by exothermic chemical reactions of free radicals is confirmed, the hypothesis of panspermia from the diffuse interstellar medium is ruled out.Comment: 9 pages, 6 figures, to appear in Astronomy & Astrophysic

Micrometeoroid infall onto Saturn’s rings constrains their age to no more than a few hundred million years

There is ongoing debate as to whether Saturn’s main rings are relatively young or ancient— having been formed shortly after Saturn or during the Late Heavy Bombardment. The rings are mostly water-ice but are polluted by non-icy material with a volume fraction ranging from ∼0.1 to 2%. Continuous bombardment by micrometeoroids exogenic to the Saturnian system is a source of this non-icy material. Knowledge of the incoming mass flux of these pollutants allows estimation of the rings’ exposure time, providing a limit on their age. Here we report the final measurements by Cassini’s Cosmic Dust Analyzer of the micrometeoroid flux into the Saturnian system. Several populations are present, but the flux is dominated by low-relative velocity objects such as from the Kuiper belt. We find a mass flux between 6.9 · 10−17 and 2.7 · 10−16 kg m−2s−1 from which we infer a ring exposure time ≲100 to 400 million years in support of recent ring formation scenarios

Helios spacecraft data revisited: Detection of cometary meteoroid trails by in-situ dust impacts

Cometary meteoroid trails exist in the vicinity of comets, forming fine structure of the interplanetary dust cloud. The trails consist predominantly of cometary particles with sizes of approximately 0.1 mm to 1 cm which are ejected at low speeds and remain very close to the comet orbit for several revolutions around the Sun. When re-analysing the Helios dust data measured in the 1970s, Altobelli et al. (2006) recognized a clustering of seven impacts, detected in a very narrow region of space at a true anomaly angle of 135 deg, which the authors considered as potential cometary trail particles. We re-analyse these candidate cometary trail particles to investigate the possibility that some or all of them indeed originate from cometary trails and we constrain their source comets. The Interplanetary Meteoroid Environment for eXploration (IMEX) dust streams in space model is a new universal model for cometary meteoroid streams in the inner solar system, developed by Soja et al. (2015). Using IMEX we study cometary trail traverses by Helios. During ten revolutions around the Sun, and in the narrow region of space where Helios detected the candidate dust particles, the spacecraft repeatedly traversed the trails of comets 45P/Honda-Mrkos-Pajduvsakova and 72P/Denning-Fujikawa. Based on the detection times and particle impact directions, four detected particles are compatible with an origin from these two comets. We find a dust spatial density in these trails of about 10^-8 to 10^-7 m^-3. The in-situ detection and analysis of meteoroid trail particles which can be traced back to their source bodies by spacecraft-based dust analysers opens a new window to remote compositional analysis of comets and asteroids without the necessity to fly a spacecraft to or even land on those celestial bodies. This provides new science opportunities for future missions like Destiny+, Europa Clipper and IMAP.Comment: 13 pages, 9 Figures, 2 Tables, accepted for pubication by Astronomy and Astrophysic

Modelling DESTINY+ interplanetary and interstellar dust measurements en route to the active asteroid (3200) Phaethon

The JAXA/ISAS spacecraft DESTINY$^+$ will be launched to the active asteroid (3200) Phaethon in 2022. Among the proposed core payload is the DESTINY+ Dust Analyzer (DDA) which is an upgrade of the Cosmic Dust Analyzer flown on the Cassini spacecraft to Saturn (Srama et al. 2011). We use two up-to-date computer models, the ESA Interplanetary Meteoroid Engineering Model (IMEM, Dikarev et al. 2005), and the interstellar dust module of the Interplanetary Meteoroid environment for EXploration model (IMEX; Sterken2013 et al., Strub et al. 2019) to study the detection conditions and fluences of interplanetary and interstellar dust with DDA. Our results show that a statistically significant number of interplanetary and interstellar dust particles will be detectable with DDA during the 4-years interplanetary cruise of DESTINY+. The particle impact direction and speed can be used to descriminate between interstellar and interplanetary particles and likely also to distinguish between cometary and asteroidal particles.Comment: 40 pages, 18 Figures, accepted for Planetary and Space Scienc

Sample return of interstellar matter (SARIM)

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Thermal Transport In Saturn\u27S B Ring Inferred From Cassini Cirs

We examine the heat budget of Saturn\u27s B ring using all of the data from Cassini\u27s Composite Infra Red Spectrometer (CIRS) taken during one Saturn season, together with a detailed numerical calculation of the incident flux. We find that at all times 30-40% of the energy incident on the sunlit side finds its way through the ring to be emitted on the unlit side, and that the specific fraction of heat throughput from the lit to unlit side of the ring varies inversely with the normal optical thickness as f~0.41-0.024τ. From this we derive a high effective conductivity of the ring, on the order of 0.5Wm-1K-1, together with a heat flux through the midplane of the rings that is about 1.5Wm-2 at high solar elevations. The derived conductivity is at the high end of plausible values for the ring, but the flux rate can easily be met by particle diffusion across the midplane of the rings.While the integrated normal flux from the B ring varies linearly with sinB\u27 as expected, an important finding is that it is dominated for by isotropic emission both on the lit and unlit sides. On the lit side there is an additional emission from a low-phase hot spot with an angular width of 50° that accounts for approximately 2-10% of the total emission. Accounting for wake orientation affects only the inner B Ring, decreasing the absorbed incident radiation by 5-10% there. The numerical model uses only a scalar albedo to parameterize the absorption. It requires a low albedo of A~0.3 to be brought into agreement with ring emission, which is expected if there is enhanced radiance absorption over the single scattered estimates, due to multiple scattering within the ring

Penetration of the heliosphere by the interstellar dust stream during solar maximum

We analyze the Ulysses in situ measurements of interstellar dust in the solar system with respect to the temporal variation of the flux density. The data set that is used covers the time from after Ulysses' fly-by of Jupiter up to the most recent data taken end of November 2002. The decrease in interstellar dust flux observed in 1996 can be explained by the interaction of the small, electrostatically charged grains with the solar wind magnetic field during the solar minimum with the polarity prevailing since 1991, as was reported earlier. Now with the new data, taken especially between 2000 and the end of 2002, we find that the amplitude of the decrease and the timing of the rebound with the 2000/2001 solar maximum is such that grains with relatively high charge to mass ratio of 1.33 C kg-1 and effective radii of 0.2 µm cannot account for the observed profile, as was concluded earlier. The simulation of the interaction of charged dust grains with the solar wind magnetic field shows that the best fit to the observations is achieved when the interstellar dust stream is dominated by grains with a charge to mass ratio of 0.59 C kg-1 and a radiation pressure efficiency factor of β = 1.1, which corresponds to an effective radius of 0.3 µm. We predict that the cumulative interstellar dust flux measured by Ulysses will level off at a constant value greater than 1 × 10-4 m-2 s-1 until the end of 2004