Voyager photometry of Rhea,Dione, Tethys,Enceladus and Mimas

Voyager imaging observations provide new photometric data on Saturn's satellites at large phase angles (up to 133° in the case of Mimas) not observable from Earth. Significant new results include the determination of phase integrals ranging from 0.7 in the case of Rhea to 0.9 for Enceladus. For Enceladus we find an average geometric albedo p_v = 1.04 ± 0.15 and Bond albedo of 0.9 ± 0.1. The data indicate an orbital lightcurve with an amplitude of 0.2 mag, the trailing side being the brighter. For Mimas, the lightcurve amplitude is probably less than 0.1 mag. The value of the geometric albedo of Mimas reported here, p_v = 0.77 ± 0.15 (corresponding to a mean opposition magnitude V₀ = +12.5) is definitely higher than the currently accepted value of about 0.5. For Dione, the Voyager data show a well-defined orbital lightcurve of amplitude about 0.6 mag, with the leading hemisphere brighter than the trailing one.     

Orbital dynamics of the planetary system HD 196885

The orbital dynamics of the single known planet in the binary star system HD 196885 has been considered. The Lyapunov characteristic exponents and Lyapunov time of the planetary system have been calculated for possible values of the planetary orbit parameters. It has been shown that the dynamics of the planetary system HD 196885 is regular with the Lyapunov time of more than 5 × 10⁴ years (the orbital period of the planet is approximately 3.7 years), if the motion occurs at a distance from the separatrix of the Lidov–Kozai resonance. The values of the planet’s orbital inclination to the plane of the sky and longitude of the ascending node lie either within ranges 30° < i _p < 90° and 30° < Ω_p < 90°, or 90° < i _p < 180° and 180° < Ω_p < 300°.     

Enceladus: Present internal structure and differentiation by early and long-term radiogenic heating

Pre-Cassini images of Saturn's small icy moon Enceladus provided the first indication that this satellite has undergone extensive resurfacing and tectonism. Data returned by the Cassini spacecraft have proven Enceladus to be one of the most geologically dynamic bodies in the Solar System. Given that the diameter of Enceladus is only about 500 km, this is a surprising discovery and has made Enceladus an object of much interest. Determining Enceladus' interior structure is key to understanding its current activity. Here we use the mean density of Enceladus (as determined by the Cassini mission to Saturn), Cassini observations of endogenic activity on Enceladus, and numerical simulations of Enceladus' thermal evolution to infer that this satellite is most likely a differentiated body with a large rock-metal core of radius about 150 to 170 km surrounded by a liquid water-ice shell. With a silicate mass fraction of 50% or more, long-term radiogenic heating alone might melt most of the ice in a homogeneous Enceladus after about 500 Myr assuming an initial accretion temperature of about 200 K, no subsolidus convection of the ice, and either a surface temperature higher than at present or a porous, insulating surface. Short-lived radioactivity, e.g., the decay of ²⁶Al, would melt all of the ice and differentiate Enceladus within a few million years of accretion assuming formation of Enceladus at a propitious time prior to the decay of ²⁶Al. Long-lived radioactivity facilitates tidal heating as a source of energy for differentiation by warming the ice in Enceladus so that tidal deformation can become effective. This could explain the difference between Enceladus and Mimas. Mimas, with only a small rock fraction, has experienced relatively little long-term radiogenic heating; it has remained cold and stiff and less susceptible to tidal heating despite its proximity to Saturn and larger eccentricity than Enceladus. It is shown that the shape of Enceladus is not that of a body in hydrostatic equilibrium at its present orbital location and rotation rate. The present shape could be an equilibrium shape corresponding to a time when Enceladus was closer to Saturn and spinning more rapidly, or more likely, to a time when Enceladus was spinning more rapidly at its present orbital location. A liquid water layer on Enceladus is a possible source for the plume in the south polar region assuming the survivability of such a layer to the present. These results could place Enceladus in a category similar to the large satellites of Jupiter, with the core having a rock-metal composition similar to Io, and with a deep overlying ice shell similar to Europa and Ganymede. Indeed, the moment of inertia factor of a differentiated Enceladus, C/MR², could be as small as that of Ganymede, about 0.31.     

On the stability of hypothetical satellites coorbital to Mimas or Enceladus

In the present work, we study the stability of hypothetical satellites that are coorbital with Enceladus and Mimas. We performed numerical simulations of 50 particles around the triangular Lagrangian equilibrium points of Enceladus and Mimas taking into account the perturbation of Mimas, Enceladus, Tethys, Dione, Titan and the oblateness of Saturn. All particles remain on tadpole orbits after 10 000 yr of integration. Since in the past the orbit of Enceladus and Mimas expanded due to the tidal perturbation, we also simulated the system with Enceladus and Mimas at several different values of semimajor axes. The results show that in general the particles remain on tadpole orbits. The exceptions occur when Enceladus is at semimajor axes that correspond to 6:7, 5:6 and 4:5 resonances with Mimas. Therefore, if Enceladus and Mimas had satellites librating around their Lagrangian triangular points in the past, they would have been removed if Enceladus crossed one of these first-order resonances with Mimas.     

Voyager disk resolved photometry of the Saturnian satellites

Photometric analysis of Voyager images of the medium-sized icy satellites of Saturn shows that their surfaces exhibit a wide range of scattering properties. At low phase angles, Rhea and Dione closely follow lunar behavior with almost no limb darkening. Mimas, Tethys, and especially Enceladus shiw significant limb darkening at low phase angles, which suggests multiple scattering is important for their surfaces. A simple photometric function of the form I/F = f(α)Aμ₀/(μ + μ₀) + (1 ? A)μ₀ has been fit to the observations. For normal reflectances <0.6, we find lunar-like scattering properties (A = 1). No satellite's surface can be described by Lambert's Law (A = 0). Dione exhibits the widest albedo variations (about 50%). A longitudinal dark stripe which represents a 15% decrease in albedo is situated near the center of the trailing side of Tethys. A correlation is found between the albedo and color of the satellites: the darker objects are redder. Similarly, darker areas of each satellite are redder. Spectral reflectances of Mimas and Enceladus can be derived for the first time. After the proper calibrations to the Voyager color images are made, it is found that both satellites have remarkably flat spectra into the ultraviolet.     

The long term stability of coorbital moons of the satellites of Saturn: I. Conservative case

The aim of this work is to understand the absence of objects along the orbits of Mimas and Enceladus in contrast to their presence at the orbits of neighbouring Tethys and Dione from the point of view of dynamical stability. Large scale numerical simulations of 360 test particles within the coorbital regions of these four saturnian satellites were carried out for 4×¹⁰⁵ yr or 1.6×¹⁰⁸ revolutions of the innermost moon Mimas. The tidal forcing of the satellites' orbits was not taken into account in these simulations. We have quantitatively reproduced the Mimas-Tethys 4:2 and Enceladus-Dione 2:1 mean motion resonances in the system and devised a scheme by which the parameter space of the coorbital resonance is sampled uniformly by our test particles. We observe that 6 out of the 36 integrated horseshoe particles of Enceladus escaped the coorbital region. All 54 tadpole particles remained stable. The main cause of instability for Enceladus coorbitals appears to be the overlap between the coorbital resonance and the 2:1 mean motion resonance between the particle and Dione. This leads particles with starting semimajor axes near the horseshoe-tadpole separatrix to be ejected from the resonance, as proposed by Morais [Morais, M.H.M., 2000. The effect of secular perturbations and mean motion resonances on trojan dynamics. Ph.D. thesis, Univ. of London], over timescales of ∼8×¹⁰⁷ revolutions of Enceladus. For Mimas we observe a larger number of coorbital escapes overall, both of tadpole (7/54) and horseshoe (29/36) librators. An analysis of the observed dynamical evolution suggests a two-stage process at work: The semimajor axis of particles with starting conditions near the horseshoe-tadpole separatrix undergoes a slow random walk over timescales of ¹⁰⁵ yr through a mechanism similar to that at Enceladus but involving the 4:2 inclination resonance with Tethys. These particles are eventually injected into a region of short-term (?¹⁰⁴ yr) instability just inside the nominal boundary of stable, symmetric horseshoe motion. The presence of the 4:2 eccentricity triplet at that location is the most likely culprit for the instability. In both the cases of Mimas and Enceladus small-amplitude tadpoles remain stable until the end of the integration. The existence of fast escapers at Mimas provides a dynamical avenue for the short-term survival of impact ejecta in horseshoe orbits within Mimas' coorbital region.     

Saturn's nonaxisymmetric ring edges at 1.95 Rs and 2.27 Rs

The outer edges of Saturn's A and B rings, at 2.27 R_s and 1.95 R_s, have been examined using data acquired by four Voyager experiments. The shapes and kinematics of these features are influenced by their proximity to strong low-order Lindblad resonances. The data for the A-ring edge are consistent with a seven-loded radial distortion of amplitude 6.7 ± 1.5 km which rotates with the mass-weighted mean angular velocity of the coorbital satellite system. The B-ring edge has essentially a double-lobed figure of radial amplitude 74 ± 9 km which rotates with the mean motion of Mimas, though there is an indication that it is not completely described withe a simple Saturn-centered ellipse. An upper limit of 10 m has been placed on the vertical thickness in the unperturbed region of the B ring.     

Tidal evolution of Mimas, Enceladus, and Dione

The tidal evolution through several resonances involving Mimas, Enceladus, and/or Dione is studied numerically with an averaged resonance model. We find that, in the Enceladus-Dione 2:1 e-Enceladus type resonance, Enceladus evolves chaotically in the future for some values of k₂/Q. Past evolution of the system is marked by temporary capture into the Enceladus-Dione 4:2 ee^′-mixed resonance. We find that the free libration of the Enceladus-Dione 2:1 e-Enceladus resonance angle of 1.5° can be explained by a recent passage of the system through a secondary resonance. In simulations with passage through the secondary resonance, the system enters the current Enceladus-Dione resonance close to tidal equilibrium and thus the equilibrium value of tidal heating of 1.1(18,000/QS) GW applies. We find that the current anomalously large eccentricity of Mimas can be explained by passage through several past resonances. In all cases, escape from the resonance occurs by unstable growth of the libration angle, sometimes with the help of a secondary resonance. Explanation of the current eccentricity of Mimas by evolution through these resonances implies that the Q of Saturn is below 100,000. Though the eccentricity of Enceladus can be excited to moderate values by capture in the Mimas-Enceladus 3:2 e-Enceladus resonance, the libration amplitude damps and the system does not escape. Thus past occupancy of this resonance and consequent tidal heating of Enceladus is excluded. The construction of a coherent history places constraints on the allowed values of k₂/Q for the satellites.     

Mapping of the icy Saturnian satellites: First results from Cassini-ISS

Images of the icy Saturnian satellites Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, and Phoebe, derived by the Voyager and Cassini cameras are used to produce new local high-resolution image mosaics as well as global mosaics [http://ciclops.org, http://photojournal.jpl.nasa.gov]. These global mosaics are valuable both for scientific interpretation and for the planning of future flybys later in the ongoing Cassini orbital tour. Furthermore, these global mosaics can be extended to standard cartographic products.     

Saturn’s wayward shepherds: the peregrinations of Prometheus and Pandora

Saturn’s narrow F ring is flanked by two nearby small satellites, Prometheus and Pandora, discovered in Voyager images taken in 1980 and 1981 (Synnott et al., 1983, Icarus 53, 156-158). Observations with the Hubble Space Telescope (HST) during the ring plane crossings (RPX) of 1995 led to the unexpected finding that Prometheus was ∼19° behind its predicted orbital longitude, based on the Synnott et al. (1983) Voyager ephemeris (Bosh and Rivkin, 1996 Science 272, 518-521; Nicholson et al., 1996, Science 272, 509-515). Whereas Pandora was at its predicted location in August 1995, McGhee (2000, Ph.D. thesis, Cornell University) found from the May and November 1995 RPX data that Pandora also deviates from the Synnott et al. (1983) Voyager ephemeris. Using archival HST data from 1994, previously unexamined RPX images, and a large series of targeted WFPC2 observations between 1996 and 2002, we have determined highly accurate sky-plane positions for Prometheus, Pandora, and nine other satellites found in our images. We compare the Prometheus and Pandora measurements to the predictions of substantially revised and improved ephemerides for the two satellites based on an extensive analysis of a large set of Voyager images (Murray et al., 2000, Bull. Am. Astron. Soc. 32, 1090; Evans, 2001 Ph.D. thesis, Queen Mary College). From December 1994 to December 2000, Prometheus’ orbital longitude lag was changing by −0.71° year⁻¹ relative to the new Voyager ephemeris. In contrast, Pandora is ahead of the revised Voyager prediction. From 1994 to 2000, its longitude offset changed by +0.44° year⁻¹, showing in addition an ∼585 day oscillatory component with amplitude Δλ_CR0 = 0.65 ± 0.07° whose phase matches the expected perturbation due to the nearby 3:2 corotation resonance with Mimas, modulated by the 71-year libration in the longitude of Mimas due to its 4:2 resonance with Tethys. We determine orbital elements for freely precessing equatorial orbits from fits to the 1994-2000 HST observations, from which we conclude that Prometheus’ semimajor axis was 0.31 km larger, and Pandora’s was 0.20 km smaller, than during the Voyager epoch. Subsequent observations in 2001-2002 reveal a new twist in the meanderings of these satellites: Prometheus’ mean motion changed suddenly by an additional −0.77° year⁻¹, equivalent to a further increase in semimajor axis of 0.33 km, at the same time that Pandora’s mean motion changed by +0.92° year⁻¹, corresponding to a change of −0.42 km in its semimajor axis. There is an apparent anticorrelation of the motions of these two moons seen in the 2001-2002 observations, as well as over the 20-year interval since the Voyager epoch. This suggests a common origin for their wanderings, perhaps through direct exchange of energy between the satellites as the result of resonances, possibly involving the F ring.     

On the damping of the bending waves in Saturn’s ring

We study numerically the motion of a single particle in the bending wave of finite thickness in Saturn’s ring. We include the forcing due to the planet, a moon, the coriolis force and the self gravity of the ring. In particular, we compute the variation of the velocity arising due to the variation of the amplitude and the phase of the epicyclic motion across the local vertical height of the ring. We suggest that the dissipation of energy due to the collision of ring particles in this shear layer damps out the bending wave of Saturn’s ring at the 5:3 vertical resonance of Mimas within a distance of 150 km from the site of its launching as is observed in Voyager data.     

Ring torque on janus and the melting of Enceladus

Voyager 2 images show parts of Enceladus' surface to be very smooth, lacking craters down to the resolution limit of 4 km. This absence of craters indicates geologically recent resurfacing, probably due to internal melting. However, calculations of current heating mechanisms, including radioactive decay and tidal heating due to Enceladus' resonance with Dione, yield heating rates too small to cause melting. The orbital mean motion of Janus (1980S1) is slightly less than twice that of Enceladus and, according to theoretical calculations, is currently decreasing as Janus' orbit evolves outward due to resonant torques from Saturn's rings. If Janus were ever locked into a stable 2:1 orbital commensurability with Enceladus, the resulting angular momentum transfer could have sufficiently enhanced the eccentricity of Enceladus' orbit for the ensuing tidal heating to have melted Enceladus' interior. The existence of a Laplace-like three-body resonance including Dione, although unlikely, would have increased heating. If Janus were indeed held in resonance with Enceladus until recently (10⁷–10⁸ years B.P.) when the lock was disrupted by an unspecified event (possibly a catastrophic collision which simultaneously created the coorbital pair, or by the influence of Dione) both the recent internal activity of Enceladus and the proximity of Janus to Saturn's rings may be explained. However, the predicted rapid time scale for ring evolution due to resonant torques from Saturn's inner moons remains a major problem.     

Eccentric features in Saturn's outer C ring

A systematic search has been made for as yet unrecognized eccentric and inclined features in Saturn's outer C ring. The radii of all sharp-edged features in the outer C ring were measured in Voyager data consisting of six high-resolution images, the Photopolarimeter occultation data, and the Radio Science λ3.6-cm occultation data corrected for the effects of diffraction. Besides the well-known Maxwell ringlet at 87,491 km (1.450R_s), whose eccentric shape and kinematics have already been studied, two other narrow ringlets at 88,716 km (1.470R_S), and 90,171 km (1.495R_S) have been found to be demonstrably eccentric. The former has a mean width of ∼16 km and is located within a gap ∼30 km wide. The latter has a mean width of ∼62 km and is only partially isolated: its outer edge is defined by a gap ∼15 km wide. Though a coincidence of these two gaps with the Mimas 3:1 inner vertical and inner Lindblad resonances has been noted by previous workers, we find that neither ringlet shows conclusive evidence for the anticipated resonantly forced distortions. The 1.495R_S ringlet is best fitted by a model describing a freely precessing Keplerian ellipse with a radial amplitude of 2.8 ± 0.5 km. Neither a resonant forcing nor a free precession model fitted to the 1.470R_S ringlet provides conclusive results, though the latter is marginally better, yielding an amplitude no larger than ∼2.2 km. These two newly identified eccentric ringlets are compared with the previously studied Titan and Maxwell ringlets (C. Porco, P. D. Nicholson, N. Borderies, G. E. Danielson, P. Goldreich, J. B. Holberg, and A. L. Lane, Icarus 60 (1984), 1–16) and with the Uranian α, β, and ϵ ring.     

The three-dimensional structure of Saturn’s E ring

Saturn’s diffuse E ring consists of many tiny (micron and sub-micron) grains of water ice distributed between the orbits of Mimas and Titan. Various gravitational and non-gravitational forces perturb these particles’ orbits, causing the ring’s local particle density to vary noticeably with distance from the planet, height above the ring-plane, hour angle and time. Using remote-sensing data obtained by the Cassini spacecraft in 2005 and 2006, we investigate the E-ring’s three-dimensional structure during a time when the Sun illuminated the rings from the south at high elevation angles (>15°). These observations show that the ring’s vertical thickness grows with distance from Enceladus’ orbit and its peak brightness density shifts from south to north of Saturn’s equator plane with increasing distance from the planet. These data also reveal a localized depletion in particle density near Saturn’s equatorial plane around Enceladus’ semi-major axis. Finally, variations are detected in the radial brightness profile and the vertical thickness of the ring as a function of longitude relative to the Sun. Possible physical mechanisms and processes that may be responsible for some of these structures include solar radiation pressure, variations in the ambient plasma, and electromagnetic perturbations associated with Saturn’s shadow.     

A seasonal climate model of the atmospheres of the giant planets at the voyager encounter time: I. Saturn's stratosphere

A radiative seasonal model which incorporates a multilayer radiative transfer treatment at wave-lengths longward of 7 μm is presented and applied to Saturn's stratosphere. Opacities due to H₂-He, CH₄, C₂H₂, and C₂H₆ are included. Season-dependent insolation is shown to produce a strong hemispheric asymmetry decreasing with depth at the Voyager encounter times, and seasonal amplitudes of 30°K at the poles are predicted in the high stratosphere. The ring-modulated dependence of the insolation and the orbital eccentricity are shown to have a significant effect. Calculations agree closely with the Voyager 1 and 2 radio occultation ingress profiles recorded at 76°S and 36.5°S for CH₄/H₂ = 3.5 + 1.4/? 1.0 × 10^?3;the estimated errors include modeling systematic errors and uncertainties in the occultations profiles. The possible role of aerosols in the stratospheric heating is analyzed. The Voyager 2 egress profile recorded at 31°S cannot be reproduced by calculations. Some constraints on the C₂H₂ and C₂H₆ abundances are derived. The upper portion of the occultation profiles (p < 3mbar) can be matched for C₂H₂/H₂ = 1.0 + 1.3/?0.6 × 10^?7, C₂H₆/H₂ = 1.5 + 1.8/?0.9 × 10^?6 at 76°S and C₂H₂/H₂ = 4 + 6/?4 × 10^?8, C₂H₆/H₂ = 6 + 9/?6 × 10^?7 at 36.5°N. At the northern occultation latitude, the discrepancy with the concentrations derived from analysis of IRIS spectra by R. Courtin, D. Gautier, A. Marten, B. Bézard, and R. Hanel (1984, Astrophys. J.287) can be explained by a sharp variation of the mixing ratios of these gases with altitude in the upper stratosphere. Other interpretations are discussed.     

The shape and structure of Mimas

The Voyager images have shown that Mimas and Enceladus have regular shapes, with topography of the order of 1% of the diameter. Therefore, we can compare the global shapes of these satellites with the corresponding figures of gravitational equilibrium. In the case of Mimas, this comparison rules out a homogeneous interior, but implies the existence of a denser, presumably rocky core within this small icy satellite.     

Application of a radiative transfer model to bright icy satellites

A radiative transfer model, derived largely from the work of B.W. Hapke (1981, J. Geophys. Res.86, 3039–3054) and J.D. Goguen (1981, Ph.D. thesis, Cornell University, Ithaca, N.Y.), is fit to Voyager imaging observations of Europa, Mimas, Enceladus, and Rhea. It is possible to place constraints on the single-scattering albedo, the porosity of the optically active upper regolith, the single-particle phase functions, and, in the cases of Europa and Mimas, the mean slope angle of macroscopic surface features. The texture of the surfaces of the Saturnian satellites appears to be similar to the Earth's moon. However, Europa is found to have a distinctly more compact regolith and a more forward-scattering single-particle phase function.     

Astrometry and dynamics of Anthe (S/2007 S 4), a new satellite of Saturn

We describe the astrometry and dynamics of Anthe (S/2007 S 4), a new satellite of Saturn discovered in images obtained using the Imaging Science Subsystem (ISS) of the Cassini spacecraft. Included are details of 63 observations, of which 28 were obtained with Cassini's narrow-angle camera (NAC) and 35 using its wide-angle camera (WAC), covering an observation time-span of approximately 3 years. We estimate the diameter of Anthe to be ∼1.8 km. Orbit modeling based on a numerical integration of the full equations of motion fitted to the observations show that Anthe is in a first-order 11:10 mean motion resonance with Mimas. Two resonant arguments are librating: ?₁=11λ^′−10λ−? and ?₂=11λ^′−10λ−?^′−Ω^′+Ω, where λ, ? and Ω refer to the mean longitude, longitude of pericenter and longitude of ascending node of Mimas and Anthe, with the primed quantities corresponding to Anthe. These resonances cause periodic variations in the orbital elements. The semi-major axis varies by ±26 km over a 913-day period. Anthe is also close to a second-order eccentricity-type mean motion resonant relationship of the form 77:75 with Methone. Since Methone is also in a first-order resonance with Mimas [Spitale, J.N., Jacobson, R.A., Porco, C.C., Owen, W.M., 2006. Astron. J. 132, 692-710], an additional indirect perturbation exists between Methone and Anthe via Mimas. Neither effect is detectable in the orbit fitting and the short-term dynamical evolution of Anthe is dominated by the Mimas-Anthe resonances alone. The expected modulation effect from the Mimas-Tethys 4:2 inclination resonance is also insignificant over this time period. By including Cassini ISS observations of Mimas in the numerical integration fit, we estimate the GM of Mimas to be , consistent with Jacobson et al. [Jacobson, R.A., Spitale, J., Porco, C.C., Owen, W.M., 2006. Astron. J. 132, 711-713].     

Tidal dissipation,orbital evolution,and the nature of Saturn's inner satellites

Estimates of tidal damping times of the orbital eccentricities of Saturn's inner satellites place constraints on some satellite rigidities and dissipation functions Q. These constraints favor rock-like rather than ice-like properties for Mimas and probably Dione. Photometric and other observational data are consistent with relatively higher densities for these two satellites, but require lower densities for Tethys, Enceladus, and Rhea. This leads to a nonmonotonic density distribution for Saturn's inner satellites, apparently determined by different mass fractions of rocky materials. In spite of the consequences of tidal dissipation for the orbital eccentricity decay and implications for satellite compositions, tidal heating is not an important contributor to the thermal history of any Saturnian satellite.     

On the distance to the Ophiuchus star‐forming region

The Ophiuchus molecular cloud complex has produced in Lynds 1688 the richest known embedded cluster within ∼300 pc of the Sun. Unfortunately, distance estimates to the Oph complex vary by nearly ∼40% (∼120–165 pc). Here I calculate a new independent distance estimate of 135±8 pc to this benchmark star‐forming region based on Hipparcos trigonometric parallaxes to stars illuminating reflection nebulosity in close proximity to Lynds 1688. Combining this value with recent distance estimates from reddening studies suggests a consensus distance of 139±6 pc (4% error), situating it within ∼11 pc of the centroid of the ∼5 Myr old Upper Sco OB subgroup of Sco OB2 (145 pc). The velocity vectors for Oph and Upper Sco are statistically indistinguishable within ∼1 km s^–1 in each vector component. Both Oph and Upper Sco have negligible motion (<1 km s^–1) in the Galactic vertical direction with respect to the Local Standard of Rest, which is inconsistent with the young stellar groups having formed via the high velocity cloud impact scenario. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)