Penetration of nearby supernova dust in the inner solar system

We investigate the method by which nearby supernovae – within a few tens of pc of the solar system – can penetrate the solar system and deposit live radioactivities on earth. The radioactive isotopic signatures that could potentially leave an observable geological imprint are in the form of refractory metals; consequently, it is likely they would arrive in the form of supernova-produced dust grains. Such grains can penetrate into the solar system more easily than the bulk supernova plasma, which gets stalled and deflected near the solar system due to the solar wind plasma pressure. We therefore examine the motion of charged grains as they decouple from the supernova plasma and are influenced by the solar magnetic, radiation, and gravitational fields. We characterize the dust trajectories with analytical approximations which display the roles of grain size, initial velocity, and surface voltage. These results are verified with full numerical simulations for wide ranges of dust properties. We find that supernova dust grains traverse the inner solar system nearly undeflected, if the incoming grain velocity – which we take to be that of the incident supernova remnant – is comparable to the solar wind speeds and much larger than the escape velocity at 1 AU. Consequently, the dust penetration to 1 AU has essentially 100% transmission probability and the dust capture onto the earth should have a geometric cross section. Our results cast in a new light the terrestrial deposition of radioisotopes from nearby supernovae in the geological past. For explosions beyond ～10 pc from earth, dust grains can still deliver supernova ejecta to earth, and thus the amount of supernova material deposited is set by the efficiency of dust condensation and survival in supernovae. Turning the problem around, we use observations of live ⁶⁰Fe in both deep-ocean and lunar samples to infer a conservative lower bound iron condensation efficiency of M_dust,Fe/M_tot,Fe ? 4  × 10^?4 for the supernova which apparently produced these species 2–3 Myr ago.     

Comparative studies of meteoroid-planet interaction in the inner solar system

We review the current state of studies in planet–meteoroid interactions, a relatively new discipline in planetary science. Recent observations of phenomena such as meteor trails in the atmosphere of Mars and impact flashes on the Moon have prompted new theoretical work in the field. However, our ability to test these new models and advance our understanding of the processes involved is being inhibited by the lack of systematic long-term observations with instruments dedicated to the task. Here we consider the different types of meteoroid effects on a planetary environment. The current state of knowledge leads us to expect signatures detectable by existing instrumentation, either serendipitously or, in a more targeted fashion, by employing such apparatus in innovative ways and making use of already available model predictions. These will result in near-term advances in the field, to be used towards incorporating meteoroid-effect-detecting capabilities explicitly into future planetary instrumentation or building dedicated instruments.     

The inner solar system cratering record and the evolution of impactor populations

We review previously published and newly obtained crater size-frequency distributions in the inner solar system. These data indicate that the Moon and the terrestrial planets have been bombarded by two populations of objects. Population 1,dominating at early times, had nearly the same size distribution as the present-day asteroid belt, and produced heavily cratered surfaces with a complex, multi-sloped crater size-frequency distribution. Population 2, dominating since about 3.8–3.7 Gyr,had the same size distribution as near-Earth objects(NEOs) and a much lower impact flux, and produced a crater size distribution characterized by a differential –3single-slope power law in the crater diameter range 0.02 km to 100 km. Taken together with the results from a large body of work on age-dating of lunar and meteorite samples and theoretical work in solar system dynamics, a plausible interpretation of these data is as follows. The NEO population is the source of Population 2 and it has been in near-steady state over the past ～ 3.7–3.8 Gyr; these objects are derived from the main asteroid belt by size-dependent non-gravitational effects that favor the ejection of smaller asteroids. However, Population 1 was composed of main belt asteroids ejected from their source region in a size-independent manner, possibly by means of gravitational resonance sweeping during orbit migration of giant planets;this caused the so-called Late Heavy Bombardment(LHB). The LHB began some time before ～3.9 Gyr, peaked and declined rapidly over the next ～ 100 to 300 Myr,and possibly more slowly from about 3.8–3.7 Gyr to ～2 Gyr. A third crater population(Population S) consisted of secondary impact craters that can dominate the cratering record at small diameters.     

Review of the population of impactors and the impact cratering rate in the inner solar system

Abstract— All terrestrial planets, the Moon, and small bodies of the inner solar system are subjected to impacts on their surface. The best witness of these events is the lunar surface, which kept the memory of the impacts that it underwent during the last 3.8 Gyr. In this paper, we review the recent studies at the origin of a reliable model of the impactor population in the inner solar system, namely the near‐Earth object (NEO) population. Then we briefly expose the scaling laws used to relate a crater diameter to body size. The model of the NEO population and its impact frequency on terrestrial planets is consistent with the crater distribution on the lunar surface when appropriate scaling laws are used. Concerning the early phases of our solar system's history, a scenario has recently been proposed that explains the origin of the Late Heavy Bombardment (LHB) and some other properties of our solar system. In this scenario, the four giant planets had initially circular orbits, were much closer to each other, and were surrounded by a massive disk of planetesimals. Dynamical interactions with this disk destabilized the planetary system after 500–600 Myr. Consequently, a large portion of the planetesimal disk, as well as 95% of the Main Belt asteroids, were sent into the inner solar system, causing the LHB while the planets reached their current orbits. Our knowledge of solar system evolution has thus improved in the last decade despite our still‐poor understanding of the complex cratering process.     

VLBI observations of turbulence in the inner solar wind

I discuss the use of Very Long Baseline Interferometer (VLBI) phase scintillations to probe the conditions of plasma turbulence in the solar wind. Specific results from 5.0 and 8.4 GHz observations with the Very Long Baseline Array (VLBA) are shown. There are several advantages of phase scintillation measurements. They are sensitive to fluctuations on scales of hundreds to thousands of kilometers, much larger than those probed by IPS intensity scintillations. In addition, with the frequency versatility of the VLBA one can measure turbulence from the outer corona 5–10R to well past the perihelion approach of the Helios spacecraft. This permits tests of the consistency of radio propagation and direct in-situ measurements of turbulence. Such a comparison is made in the present paper. Special attention is dedicated to measuring the dependence of the normalization coefficient of the density power spectrum,C _N ² on distance from the sun. Our results are consistent with the contention published several years ago by Aaron Roberts, that there is insufficient turbulence close to the sun to account for the heating and acceleration of the solar wind. In addition, an accurate determination of theC _N ² (R) relationship could aid the detection of transients in the solar wind.     

A statistical study of MHD discontinuities in the inner solar system: Helios 1 and 2

A statistical study is made of the magnetic field directional discontinuities observed in early 1976 onboard Helios 1 and 2. Strong day-to-day variations of occurrence rates are found on either tangential (TD) or rotational (RD) discontinuities. No large variation (if any) is found versus either heliocentric distance or heliographic latitude. This contradicts previous findings obtained by the same technique on Pioneer 8 data in 1968–69; however, reasons are given to expect different results, under different solar conditions. The most interesting results come from the study of the morphology of discontinuities: first of all, the orientation of TD's and RD's normals (identified by a minimum variance technique) are strongly organized by the average magnetic field, following their progressive directional change when approaching the Sun. The inclination (θ _n ) and azimuthal (? _n ) distributions are gaussian and strongly peaked along the field lines for RD's; as regards TD's the normals are perpendicular to the average field and follow its progressive variation; the θ _n distribution is isotropic in solid angle, which is interpreted as evidence of crossing of flux tubes on the order of one/hour. Implications of this interpretation in contrast with a turbulent approach are also discussed.     

THERMAP: a mid-infrared spectro-imager for space missions to small bodies in the inner solar system

We present THERMAP, a mid-infrared spectro-imager for space missions to small bodies in the inner solar system, developed in the framework of the MarcoPolo-R asteroid sample return mission. THERMAP is very well suited to characterize the surface thermal environment of a NEO and to map its surface composition. The instrument has two channels, one for imaging and one for spectroscopy: it is both a thermal camera with full 2D imaging capabilities and a slit spectrometer. THERMAP takes advantage of the recent technological developments of uncooled microbolometer arrays, sensitive in the mid-infrared spectral range. THERMAP can acquire thermal images (8–18 μm) of the surface and perform absolute temperature measurements with a precision better than 3.5 K above 200 K. THERMAP can acquire mid-infrared spectra (8–16 μm) of the surface with a spectral resolution Δλ of 0.3 μm. For surface temperatures above 350 K, spectra have a signal-to-noise ratio >60 in the spectral range 9–13 μm where most emission features occur.     

The kinematics of solar inner coronal transients

Solar Physics - The kinematic properties of a dozen ‘loop-like’ coronal transients have been examined over the range 1.2–2.4 R⊙ from Sun center. Values and trends of...     

Organics in the solar system

Complex organics are now commonly found in meteorites, comets, asteroids, planetary satellites and interplanetary dust particles. The chemical composition and possible origin of these organics are presented. Specifically, we discuss the possible link between Solar System organics and the complex organics synthesized during the late stages of stellar evolution. Implications of extraterrestrial organics on the origin of life on Earth and the possibility of existence of primordial organics on Earth are also discussed.     

Chaos in the solar system

We have accumulated thousands of orbits of test particles in the Solar System from the asteroid belt to beyond the orbit of Neptune. We find that the time for an orbit to make a close encounter with a perturbing planet, T _c ,is a function of the Lyapunov time, T _ty .The relation is log (T _c /T _o )= a + b log (T _ly T _o )where T _o is a fiducial period which we have taken as the period of the principal perturber or the period of the asteroid. There are exceptions to this rule interior to the 2/3 resonance with Jupiter. There, at least in the restricted problem, for sufficiently small Jupiter mass, orbits may have a positive Lyapunov exponent and still be blocked from having a close approach to Jupiter by a zero velocity curve. Of more serious concern is whether the relation holds for purely secular resonances, and if it does, how to choose T _o .This is the case of interest for the planets in the solar system.     

Orbital and physical characteristics of micrometeoroids in the inner solar system as observed by Helios 1

The Helios 1 spacecraft was launched in December 1974 into a heliocentric orbit of 0.3 AU perihelion distance. Helios 2 followed one year later on a similar orbit. Both spaceprobes carry on board micrometeoroid experiments each of which contains two sensors with a total sensitive area of 121 cm². To date, only preliminary data are available from Helios 2. Therefore the results presented here mainly apply to data from Helios 1. The ecliptic sensor of Helios 1 measures dust particles which have trajectories with elevations from ?45° to + 55° with respect to the ecliptic plane. The south sensor detects dust particles with trajectory elevations from ?90° (ecliptic south-pole) to ?4°. The ecliptic sensor is covered by a thin film (3000 Å parylene coated with 750 Å aluminium) as protection against solar radiation. The other sensor is shielded by the spacecraft rim from direct sunlight and has an open aperture. Micrometeoroids are detected by the electric charge produced upon impact. During the first 6 orbits of Helios 1 around the sun the experiment registered a total of 168 meteoroids, 52 particles were detected by the ecliptic sensor and 116 particles by the south sensor. This excess of impacts on the south sensor with regard to the impacts on the ecliptic sensor is due predominantly to small impacts which are characterized by small pulse heights of the charge signals. But also large impacts were statistically significantly more abundant on the south sensor than on the ecliptic sensor. Most impacts on the ecliptic sensor were observed when it was pointing in the direction of motion of Helios (apex direction). In contrast to that the south sensor detected most impacts when it was facing in between the solar and antapex direction. Orbit analysis showed that the “apex” particles which are predominantly detected by the ecliptic sensor have eccentricities e < 0.4 or semi-major axes a ? 0.5 AU. From a comparison with corresponding data from the south sensor it is concluded that the average inclination f of “apex” particles is -i < 30°. The excess of impacts on the south sensor, called “eccentric” particles, have orbit eccentricities e > 0.4 and semimajor axes a > 0.5AU. β-meteoroids leaving the solar system on hyperbolic orbits are directly identified by the observed imbalance of outgoing (away from the sun) and ingoing particles. It is shown that “eccentric” particles, due to their orbital characteristics, should be observable also by the ecliptic sensor. Since they have not been detected by this sensor it is concluded that the only instrumental difference between both sensors, i.e. the entrance film in front of the ecliptic sensor, prevented them from entering it. A comparison with penetration studies proved that particles which do not penetrate the entrance film must have bulk densities ρ(g/cm³) below an upper density limit ρ_max. It is shown that approximately 30% of the “eccentric” particles have densities below ρ_max = 1 g/cm³.     

Bright threads in the inner wing of solar Ca II K line

On spectrograms of the K line at quiet regions of the Sun, bright threads visible in the real continuum due to the granulations are also seen in the outer wing as far as ¦¦ 3 Å from the line centre. At the inner wing (3 Å ¦¦ 0.5 Å) bright threads are also seen, but their spatial distribution is different from the former ones. The threads at the inner wing appear at intergranular regions, and many of them are seen inside the supergranulation. Their size and number density are about the same as those of the granulation. These facts reflect that the penetration of the granular high temperature layer stops at a certain height in the photosphere, and that the intergranular bright threads at the inner wing are due to a hotter temperature layer, located at a considerably higher photospheric layer than the granulation.     

Deuterium in the early solar system

An attempt is made to construct a self-consistent picture of the deuterium abundance in the early Solar System based on the assumption of chemical equilibrium in the solar nebula. A recent determination of the $\text{D}\text{H}$ ratio for the atmosphere of Jupiter is consistent with a previous estimate of the $\text{D}\text{H}$ ratio for the proto-Sun. The high (> 1.5 × 10^?4) $\text{D}\text{H}$ ratios determined from analyses of carbonaceous meteorites imply an equilibrium temperature < 270°K, in marked disagreement with the equilibrium temperature determined for the same material by oxygen isotope cosmothermometry.     

Mass distribution in the solar system

The present-day observed mass distribution in the solar system including the Sun is shown to be compatible with the idea of the splitting of a number of ring-shaped rotating clouds of particles in the equatorial plane of a single contracting nebula. The formation of such a nebula is discussed and it is inferred that during the course of contraction this nebula has remained a sphere of uniform density spinning with the Keplerian velocity of its surface layer. The mass of a planet is taken as the portion of this spherical solar nebula gained at the time of splitting by its gaseous ring of dimensions satisfying Roche and accretional limits.     

Angular momenta in the solar system

A statistical study of the orbital parameters of comets, asteroids and meteor streams shows that the vectors representing their angular momenta per mass unit (or the average angular momentum for meteor streams) are not arbitrarily distributed in the space: They are clustered around determinated values of angles . This synthesizes the eccentricities and inclinations of the orbital planes in a unique parameter adequated for the statistical purposes of the present work being defined by cos = cos (arc sin e) cos i.The discreteness of the obtained distribution N() and its relation with the components of the angular momenta per mass unit is analysed having this distribution common features for objects of different nature and located in different places in the solar potential well. Some hypotheses concerning to these effects are discussed.     

Resonant structures in the solar system

The resonance theory is discussed with respect to the Solar System with a view to show that every triad of successive planets in the Solar System follows Laplace's resonance relation. With rings now known to exist around three of the four major planets, scientists have begun to speculate about the possible existence of ring structure and one or two small planets going around the Sun itself. It is also believed that the ring systems may exist around the planets Neptune and Mars. In this paper an attempt is made to provide a basis to these beliefs using Laplace's resonance relation. The triads of successive innermost objects (rings and/or satellites) in the satellite — systems of Jupiter, Saturn and Uranus are also shown to follow Laplace's resonance relation.     

Interstellar dust in the solar system

Dust is an important component of galactic stucture and the cyclic processing of particulate matter leads to stellar and planetary formation. Though astronomical methods using analysis of dust-penetrating starlight can provide some limited information about the dust, the prospect of its in-situ sampling within the Solar System by spacecraft and its remote sensing by ground-based techniques open up a new field in galactic exploration.     

Observations from space of the solar corona/inner zodical light

Observations, from the Apollo 16 Spacecraft, in lunar orbit, of the total radiance of the K + F corona, from 3 R to 55 R are presented and discussed.

The logarithmic slope of the K + F coronal radiance, in the region r > 20 R, is found to be n = 1.93, slightly less steep than previous determinations. The photometric axis of the radiance is found to be displaced 3 ± 1° north of the ecliptic, for the region r > 20 R, and this displacement is interpreted as an annual variation due to non-coincidence of the ecliptic and the symmetry axis of the zodiacal cloud.     

The effects of nebula surface density profile and giant‐planet eccentricities on planetary accretion in the inner solar system

Abstract— We describe results of 32 N‐body planetary accretion simulations that investigate the dependence of terrestrial‐planet formation on nebula surface density profile σ and evolution of the eccentricities of Jupiter and Saturn e_j,s. Two surface density profiles are examined: a decaying profile with σ ∝ 1/a, where a is orbital semi‐major axis, and a peaked profile in which σ increases for a < 2 AU and decreases for a > 2 AU. The peaked profiles are generated by models of coagulation in an initially hot nebula. Models with initial e_j,s = 0.05 (the current value) and 0.1 are considered. Simulations using the decaying profile with e_j,s = 0.1 produce systems most like the observed planets in terms of mass‐weighted mean a and the absence of a planet in the asteroid belt. Simulations with doubled σ produce planets roughly twice as massive as the nominal case. Most initial embryos are removed in each simulation via ejection from the solar system or collision with the Sun. The asteroid belt is almost entirely cleared on a timescale of 10–100 Ma that depends sensitively on e_j,s. Most initial mass with a < 2 AU survives, with the degree of mass loss increasing with a. Mass loss from the terrestrial region occurs on a timescale that is long compared to the mass loss time for the asteroid belt. Substantial radial mixing of material occurs in all simulations, but is greater in simulations with initital e_j,s = 0.05. The degree of mixing is equivalent to a feeding zone of half width 1.5 and 0.9 AU for an Earth mass planet at 1 AU for the cases e_j,s = 0.05 and 0.1, respectively. In simulations with e_j,s = 0.05, roughly one‐third and 5–10% of the mass contained in final terrestrial planets originated in the region a > 2.5 AU for the decaying and peaked profiles, respectively. In the case e_j,s = 0.1, the median mass accreted from a > 2.5 AU is zero for both profiles.