Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Solar System”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 487 records · Page 27Linked to original sources
PubMed2001

The four final rotation states of Venus.

Venus rotates very slowly on its axis in a retrograde direction, opposite to that of most other bodies in the Solar System. To explain this peculiar observation, it has been generally believed that in the past its rotational axis was itself rotated to 180 degrees as a result of core-mantle friction inside the planet, together with atmospheric tides. But such a change has to assume a high initial obliquity (the angle between the planet's equator and the plane of the orbital motion). Chaotic evolution, however, allows the spin axis to flip for a large set of initial conditions. Here we show that independent of uncertainties in the models, terrestrial planets with dense atmosphere like Venus can evolve into one of only four possible rotation states. Moreover, we find that most initial conditions will drive the planet towards the configuration at present seen at Venus, albeit through two very different evolutionary paths. The first is the generally accepted view whereby the spin axis flips direction. But we have also found that it is possible for Venus to begin with prograde rotation (the same direction as the other planets) yet then develop retrograde rotation while the obliquity goes towards zero: a rotation of the spin axis is not necessary in this case.

Journal Article↗
PubMed2001

Discovery of water vapour around IRC+10216 as evidence for comets orbiting another star.

Since 1995, planets with masses comparable to that of Jupiter have been discovered around approximately 60 stars. These planets have not been seen directly, but their presence has been inferred from the small reflex motions that they gravitationally induce on the star they orbit; these motions result in small periodic wavelength shifts in the stellar spectrum. The presence of analogues of the smaller bodies in our Solar System cannot, however, be determined using this technique, because the induced reflex motions are too small-so an alternative approach is needed. Here we report the observation of circumstellar water vapour around the ageing carbon star IRC+10216; water is not expected in measurable quantities around such a star. The only plausible explanation for this water is that the recent evolution of IRC+10216, which has been accompanied by a prodigious increase in its luminosity, is causing the vaporization of a collection of orbiting icy bodies-a process considered in an earlier theoretical study.

Astronomical Phenomena↗
PubMed2001

History of trace gases in presolar diamonds inferred from ion-implantation experiments.

Diamond grains are the most abundant presolar grains found in primitive meteorites. They formed before the Solar System, and therefore provide a record of nuclear and chemical processes in stars and in the interstellar medium. Their origins are inferred from the unusual isotopic compositions of trace elements-mainly xenon-which suggest that they came from supernovae. But the exact nature of the sources has been enigmatic, as has the method by which noble gases were incorporated into the grains. One observation is that different isotopic components are released at different temperatures when the grains are heated, and it has been suggested that these components have different origins. Here we report results of a laboratory study that shows that ion implantation (previously suggested on other grounds) is a viable mechanism for trapping noble gases. Moreover, we find that ion implantation of a single isotopic composition can produce both low- and high-temperature release peaks from the same grains. We conclude that both isotopically normal and anomalous gases may have been implanted by multiple events separated in space and/or time, with thermal processing producing an apparent enrichment of the anomalous component in the high-temperature release peak. The previous assumption that the low- and high-temperature components were not correlated may therefore have led to an overestimate of the abundance of anomalous argon and krypton, while obscuring an enhancement of the light-in addition to the heavy-krypton isotopes.

Journal Article↗
PubMed1997

External supply of oxygen to the atmospheres of the giant planets.

The atmospheres of the giant planets are reducing, being mainly composed of hydrogen, helium and methane. But the rings and icy satellites that surround these planets, together with the flux of interplanetary dust, could act as important sources of oxygen, which would be delivered to the atmospheres mainly in the form of water ice or silicate dust. Here we report the detection, by infrared spectroscopy, of gaseous H2O in the upper atmospheres of Saturn, Uranus and Neptune. The implied H2O column densities are 1.5 x 10(15), 9 x 10(13) and 3 x 10(14) molecules cm(-2) respectively. CO2 in comparable amounts was also detected in the atmospheres of Saturn and Neptune. These observations can be accounted for by external fluxes of 10(5)-10(7) H2O molecules cm(-2) s(-1) and subsequent chemical processing in the atmospheres. The presence of gaseous water and infalling dust will affect the photochemistry, energy budget and ionospheric properties of these atmospheres. Moreover, our findings may help to constrain the injection rate and possible activity of distant icy objects in the Solar System.

Carbon Dioxide↗
PubMed1996

Detection of acetylene in the infrared spectrum of comet Hyakutake.

Comets are rich in volatile materials, of which roughly 80% (by number) are water molecules. Considerable progress is being made in identifying the other volatile species, the abundances of which should enable us to determine whether comets formed primarily from ice-covered interstellar grains, or from material that was chemically processed in the early solar nebula. Here we report the detection of acetylene (C2H2) in the infrared spectrum of comet C/1996 B2 (Hyakutake). The estimated abundance is 0.3-0.9%, relative to water, which is comparable to the predicted solid-phase abundance in cold interstellar clouds. This suggests that the volatiles in comet Hyakotake may have come from ice-covered interstellar grains, rather than material processed in the accretion disk out of which the Solar System formed.

Acetylene↗
PubMed2002

Interstellar scintillation as the origin of the rapid radio variability of the quasar J1819+3845.

The liberation of gravitational energy as matter falls onto a supermassive black hole at the centre of a galaxy is believed to explain the high luminosity of quasars. The variability of this emission from quasars and other types of active galactic nuclei can provide information on the size of the emitting regions and the physical process of fuelling the black hole. Some active galactic nuclei are variable at optical (and shorter) wavelengths, and display radio outbursts over years and decades. These active galactic nuclei often also show faster intraday variability at radio wavelengths. The origin of this rapid variability has been extensively debated, but a correlation between optical and radio variations in some sources suggests that both are intrinsic. This would, however, require radiation brightness temperatures that seem physically implausible, leading to the suggestion that the rapid variations are caused by scattering of the emission by the interstellar medium inside our Galaxy. Here we show that the rapid variations in the extreme case of quasar J1819+3845 (ref. 10) indeed arise from interstellar scintillation. The transverse velocity of the scattering material reveals the presence of plasma with a surprisingly high velocity close to the Solar System.

Journal Article↗
PubMed2002

Transient aurora on Jupiter from injections of magnetospheric electrons.

Energetic electrons and ions that are trapped in Earth's magnetosphere can suddenly be accelerated towards the planet. Some dynamic features of Earth's aurora (the northern and southern lights) are created by the fraction of these injected particles that travels along magnetic field lines and hits the upper atmosphere. Jupiter's aurora appears similar to Earth's in some respects; both appear as large ovals circling the poles and both show transient events. But the magnetospheres of Jupiter and Earth are so different---particularly in the way they are powered---that it is not known whether the magnetospheric drivers of Earth's aurora also cause them on Jupiter. Here we show a direct relationship between Earth-like injections of electrons in Jupiter's magnetosphere and a transient auroral feature in Jupiter's polar region. This relationship is remarkably similar to what happens at Earth, and therefore suggests that despite the large differences between planetary magnetospheres, some processes that generate aurorae are the same throughout the Solar System.

Journal Article↗
PubMed2002

Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues.

The delivery of extraterrestrial organic molecules to Earth by meteorites may have been important for the origin and early evolution of life. Indigenous amino acids have been found in meteorites-over 70 in the Murchison meteorite alone. Although it has been generally accepted that the meteoritic amino acids formed in liquid water on a parent body, the water in the Murchison meteorite is depleted in deuterium relative to the indigenous organic acids. Moreover, the meteoritical evidence for an excess of laevo-rotatory amino acids is hard to understand in the context of liquid-water reactions on meteorite parent bodies. Here we report a laboratory demonstration that glycine, alanine and serine naturally form from ultraviolet photolysis of the analogues of icy interstellar grains. Such amino acids would naturally have a deuterium excess similar to that seen in interstellar molecular clouds, and the formation process could also result in enantiomeric excesses if the incident radiation is circularly polarized. These results suggest that at least some meteoritic amino acids are the result of interstellar photochemistry, rather than formation in liquid water on an early Solar System body.

Alanine↗
PubMed2002

The binary Kuiper-belt object 1998 WW31.

The recent discovery of a binary asteroid during a spacecraft fly-by generated keen interest, because the orbital parameters of binaries can provide measures of the masses, and mutual eclipses could allow us to determine individual sizes and bulk densities. Several binary near-Earth, main-belt and Trojan asteroids have subsequently been discovered. The Kuiper belt-the region of space extending from Neptune (at 30 astronomical units) to well over 100 AU and believed to be the source of new short-period comets-has become a fascinating new window onto the formation of our Solar System since the first member object, not counting Pluto, was discovered in 1992 (ref. 13). Here we report that the Kuiper-belt object 1998 WW31 is binary with a highly eccentric orbit (eccentricity e approximately 0.8) and a long period (about 570 days), very different from the Pluto/Charon system, which was hitherto the only previously known binary in the Kuiper belt. Assuming a density in the range of 1 to 2 g cm-3, the albedo of the binary components is between 0.05 and 0.08, close to the value of 0.04 generally assumed for Kuiper-belt objects.

Journal Article↗
PubMed2002

CO and H(3)(+) in the protoplanetary disk around the star HD141569.

Massive planets have now been found orbiting about 80 stars. A long outstanding question critical to theories of planet formation has been the timescale on which gas-giant planets form; in particular, stars more massive than the Sun may blow away the surrounding gas associated with their formation more quickly than it can be accumulated by the protoplanetary cores. Evidence for a protoplanet around a Herbig AeBe star (such stars are 2 3 times more massive than the Sun) would constrain the timescale of planet formation. Here we report the detection of CO and H(3)(+) emission from the 5-10-million-year-old Herbig AeBe star HD141569. We interpret the CO data as indicating that the inner disk surrounding the star is past the early phase of accretion and planetesimal formation, and that most of the gas has been cleared out to a distance of more than 17 astronomical units. CO effectively destroys H(3)(+) (ref. 2), so their presence in the same source is surprising. Moreover, H(3)(+) line emission has previously been detected only from the atmospheres of the giant planets in the Solar System. The H(3)(+) and CO may therefore be distributed in the disk at different circumstellar distances, or, alternatively, H(3)(+) may be located in the extended envelope of a protoplanet.

Journal Article↗
PubMed2002

A wind origin for Titan's haze structure.

Titan, the largest moon of Saturn, is the only satellite in the Solar System with a dense atmosphere. Titan's atmosphere is mainly nitrogen with a surface pressure of 1.5 atmospheres and a temperature of 95 K (ref. 1). A seasonally varying haze, which appears to be the main source of heating and cooling that drives atmospheric circulation, shrouds the moon. The haze has numerous features that have remained unexplained. There are several layers, including a 'polar hood', and a pronounced hemispheric asymmetry. The upper atmosphere rotates much faster than the surface of the moon, and there is a significant latitudinal temperature asymmetry at the equinoxes. Here we describe a numerical simulation of Titan's atmosphere, which appears to explain the observed features of the haze. The critical new factor in our model is the coupling of haze formation with atmospheric dynamics, which includes a component of strong positive feedback between the haze and the winds.

Journal Article↗
PubMed2002

Spin vector alignment of Koronis family asteroids.

Studies of asteroid families--groups of asteroids that formed from the fragmentation of larger bodies--are of broad interest to solar system researchers because they can provide insights into collisional processes, as well as the interior structures, strengths, and compositions of asteroids. It is generally accepted that members of the Koronis family were created by collisional disruption of a homogeneous parent body and therefore share the same formation age and subsequent collisional history. The temporal variations in observed brightness of the Koronis family members (a consequence of their rotation) are, however, larger than expected. Preferential alignment of spin vectors had been proposed as a possible explanation, but recent modelling predicted that family formation yields random spin vectors among the resulting fragments. Both hypotheses have been untested by observations. Here I show that the actual distribution of spin vectors among the largest members of the Koronis family falls within markedly nonrandom 'spin clusters'. Reconciling models of family formation and evolution with the unexpected alignments of spin obliquity and correlations with spin rates presents a new challenge in understanding asteroid collisional processes.

Journal Article↗
PubMed2002

Formation of Kuiper-belt binaries by dynamical friction and three-body encounters.

The Kuiper belt is a disk of icy bodies that orbit the Sun beyond Neptune; the largest known members are Pluto and its companion Charon. A few per cent of Kuiper-belt bodies have recently been found to be binaries with wide separations and mass ratios of the order of unity. Collisions were too infrequent to account for the observed number of binaries, implying that these binaries formed through collisionless interactions mediated by gravity. These interactions are likely to have been most effective during the period of runaway accretion, early in the Solar System's history. Here we show that a transient binary forms when two large bodies penetrate one another's Hill sphere (the region where their mutual forces are larger than the tidal force of the Sun). The loss of energy needed to stabilize the binary orbit can then occur either through dynamical friction from surrounding small bodies, or through the gravitational scattering of a third large body. Our estimates slightly favour the former mechanism. We predict that five per cent of Kuiper-belt objects are binaries with apparent separations greater than 0.2 arcsec, and that most are in tighter binaries or systems of higher multiplicity.

Journal Article↗
PubMed2003

Core formation in planetesimals triggered by permeable flow.

The tungsten isotope composition of meteorites indicates that core formation in planetesimals occurred within a few million years of Solar System formation. But core formation requires a mechanism for segregating metal, and the 'wetting' properties of molten iron alloy in an olivine-rich matrix is thought to preclude segregation by permeable flow unless the silicate itself is partially molten. Excess liquid metal over a percolation threshold, however, can potentially create permeability in a solid matrix, thereby permitting segregation. Here we report the percolation threshold for molten iron-sulphur compounds of approximately 5 vol.% in solid olivine, based on electrical conductivity measurements made in situ at high pressure and temperature. We conclude that heating within planetesimals by decay of short-lived radionuclides can increase temperature sufficiently above the iron-sulphur melting point (approximately 1,000 degrees C) to trigger segregation of iron alloy by permeable flow within the short timeframe indicated by tungsten isotopes. We infer that planetesimals with radii greater than about 30 km and larger planetary embryos are expected to have formed cores very early, and these objects would have contained much of the mass in the terrestrial region of the protoplanetary nebula. The Earth and other terrestrial planets are likely therefore to have formed by accretion of previously differentiated planetesimals, and Earth's core may accordingly be viewed as a blended composite of pre-formed cores.

Journal Article↗
PubMed2003

Nanosecond radio bursts from strong plasma turbulence in the Crab pulsar.

The Crab pulsar was discovered by the occasional exceptionally bright radio pulses it emits, subsequently dubbed 'giant' pulses. Only two other pulsars are known to emit giant pulses. There is no satisfactory explanation for the occurrence of giant pulses, nor is there a complete theory of the pulsar emission mechanism in general. Competing models for the radio emission mechanism can be distinguished by the temporal structure of their coherent emission. Here we report the discovery of isolated, highly polarized, two-nanosecond subpulses within the giant radio pulses from the Crab pulsar. The plasma structures responsible for these emissions must be smaller than one metre in size, making them by far the smallest objects ever detected and resolved outside the Solar System, and the brightest transient radio sources in the sky. Only one of the current models--the collapse of plasma-turbulent wave packets in the pulsar magnetosphere--can account for the nanopulses we observe.

Journal Article↗
PubMed2003

Structure and thermal history of the H-chondrite parent asteroid revealed by thermochronometry.

Our Solar System formed approximately 4.6 billion years ago from the collapse of a dense core inside an interstellar molecular cloud. The subsequent formation of solid bodies took place rapidly. The period of &<10 million years over which planetesimals were assembled can be investigated through the study of meteorites. Although some planetesimals differentiated and formed metallic cores like the larger terrestrial planets, the parent bodies of undifferentiated chondritic meteorites experienced comparatively mild thermal metamorphism that was insufficient to separate metal from silicate. There is debate about the nature of the heat source as well as the structure and cooling history of the parent bodies. Here we report a study of 244Pu fission-track and 40Ar-39Ar thermochronologies of unshocked H chondrites, which are presumed to have a common, single, parent body. We show that, after fast accretion, an internal heating source (most probably 26Al decay) resulted in a layered parent body that cooled relatively undisturbed: rocks in the outer shells reached lower maximum metamorphic temperatures and cooled faster than the more recrystallized and chemically equilibrated rocks from the centre, which needed approximately 160 Myr to reach 390K.

Journal Article↗
PubMed2003

An abundant population of small irregular satellites around Jupiter.

Irregular satellites have eccentric orbits that can be highly inclined or even retrograde relative to the equatorial planes of their planets. These objects cannot have formed by circumplanetary accretion, unlike the regular satellites that follow uninclined, nearly circular and prograde orbits. Rather, they are probably products of early capture from heliocentric orbits. Although the capture mechanism remains uncertain, the study of irregular satellites provides a window on processes operating in the young Solar System. Families of irregular satellites recently have been discovered around Saturn (thirteen members, refs 6, 7), Uranus (six, ref. 8) and Neptune (three, ref. 9). Because Jupiter is closer than the other giant planets, searches for smaller and fainter irregular satellites can be made. Here we report the discovery of 23 new irregular satellites of Jupiter, so increasing the total known population to 32. There are five distinct satellite groups, each dominated by one relatively large body. The groups were most probably produced by collisional shattering of precursor objects after capture by Jupiter.

Journal Article↗
PubMed2003

The evolution of comets in the Oort cloud and Kuiper belt.

Comets are remnants from the time when the outer planets formed, approximately 4-4.5 billion years ago. They have been in storage since then in the Oort cloud and Kuiper belt-distant regions that are so cold and sparsely populated that it was long thought that comets approaching the Sun were pristine samples from the time of Solar System formation. It is now recognized, however, that a variety of subtle but important evolutionary mechanisms operate on comets during their long storage, so they can no longer be regarded as wholly pristine.

Evolution, Chemical↗