This image of the “surface” of Venus is composed from a mosaic of radar images taken by the Magellan spacecraft. It is a “false colour” image, which means that scientists have told a computer to use different colours for different surface elevations. Blue areas show smooth surfaces, or possibly surfaces that are covered with dust. Brown areas represent rough terrain.
The Magellan spacecraft carried out a radar-mapping mission from 1989-1994, orbiting Venus from 1990-1994. It created the first and so far, the best, near-photographic quality, ultra-high resolution mapping of the planet's surface features. Prior Venus missions had created low resolution radar globes of general, continent-sized formations. Magellan, however, finally allowed detailed imaging and analysis of craters, hills, ridges, and other geologic formations, to a degree comparable to visible-light photographic mapping of other planets.
Magellan was the first planetary probe to be launched by a space shuttle when it was carried by the shuttle Atlantis on May 4, 1989 (mission STS-30). Atlantis took Magellan into low Earth orbit, where it was released from the shuttle's cargo bay.
Venus – An Alternative Earth?
Over the last couple of years two Mars oppositions, continuing exploration of the red planet and the Cassini mission to Saturn have provided planetary scientists with plenty of excitement. 2006 promises to be equally interesting with the European Space Agency’s (ESA) Venus Express, which was launched from Baikonur in November 2005 after a short delay. By taking advantage of a favourable launch window the spacecraft is due to reach Venus in April 2006 with its scientific programme starting three months later.
The vehicle is based on Mars Express technology and from a 24-hour polar operational orbit measuring 250 x 66000 km its 500-day mission will investigate the planet’s atmosphere, the plasma environment (interaction of the upper atmosphere with the solar wind) and surface characteristics to the extent that this can be done from orbit. It will be interesting to see what ingenious approaches ESA will deploy to this task.
The first issue when discussing Venus is the choice of an appropriate adjective. It appears that all the words with a classical pedigree have connotations far removed from planetary science. Accordingly, I use the etymologically incorrect ‘venusian’. I also hope that I may be forgiven the heresy of employing the prefix ‘geo-’outside the terrestrial domain.
Although our nearest planetary neighbour, Venus has always been a challenge to Earth-bound observers. When closest its dark side is presented to us, and otherwise its surface is shielded by a dense opaque atmosphere. Fortunately the atmosphere has two windows, one in the ultraviolet and the other in the infrared. However, even observers using penetrating radar and radio signals are disadvantaged because of an unexplained resonance between its very slow rotation period and the Earth’s orbital period. As a result the length of the period between oppositions (583.92 days - the synodic period) is almost exactly equal to five times the length of the venusian day (116.75 Earth days). Consequently, for any given orbital orientation, Venus presents the same face to the Earth, subject to a small mount of libration. So, if Venus is observed from the Earth at successive inferior conjunctions, we examine essentially the same part of the planet each time.
Whereas most of the planets in the Solar System rotate from west to east and their equators are tilted between 0° and 30° from their orbital planes (e.g. Earth ~ 23.5°), the equivalent angle (or obliquity) for Venus is only 2.7°. But because Venus rotates from east to west (i.e. retrograde) the convention which defines the North Pole means that the obliquity is given as 177.3°. In other words Venus is upside down. With the aid of some fearsome mathematics Correia et al (2003) and earlier investigators suggest that both the obliquity and the spin rate can, respectively, be accounted for by a chaotic interaction between axial precession, orbital perturbations and initial rotational characteristics and by an equilibrium between the (mainly) solar gravitational tidal forces (which drive the planet to synchronous rotation) and the tidal forces generated by the dense atmosphere (which speed up the spin rate). It is an interesting thought to consider whether, in the case of the Earth, the Moon might have anchored our planet into its more modest obliquity.
The venusian atmosphere was discovered during the 1761 transit. When, in the 1930s, CO2 was found to be an important constituent, consideration soon turned to a possible greenhouse effect which would lead to a high surface temperature. However, in the first part of the twentieth century there endured a rather romantic theory about the evolution of the inner planets which was hard to dislodge. Under this theory the ages of the terrestrial planets were believed to be related to their distance from the Sun. Consequently, Mars was thought to be the oldest, occupied by an advanced civilisation but desperately short of water (yes, the canals again!). The younger Earth was just right whilst Venus represented a Carboniferous arcadia. Meanwhile Mercury was still a land of fire. It took the Space Age to dispel these ideas for good.
Further information pointing towards a high surface temperature was obtained from the investigation of thermal emissions in the radio wavelengths. In 1962 Carl Sagan put all these clues together and concluded that the data were consistent with a surface temperature of at least 700K and a surface atmospheric pressure of around 100 atmospheres. Crucially, the radio investigations did not reveal any water signature, suggesting that Venus had very little water vapour, if any, in its atmosphere. In 1967, Sagan’s predictions were brutally confirmed by the Soviet Venera and Vega Landers before they fried on the surface.
Current estimates give a surface temperature of ~750K (477°C) and a pressure of 92 atm. To put this into perspective, Gas Mark 8 on a domestic oven is 232°C. The venusian surface is hot enough to support lakes and maybe seas of molten lead and zinc and, perhaps significantly, the temperature is above the boiling point of sulphur, whilst the pressure is equivalent to that under a kilometre of ocean. On the positive side a suitably insulated submersible might be able to leisurely explore the surface whilst sedately cruising over it.
Despite their short survival the seven Russian landers delivered valuable information about surface conditions and local rock composition. Other space missions focussed on the radar mapping of the planet, culminating in the Magellan survey which was completed in the early 1990s. Magellan had two radar detectors. One was pointed at an angle to the surface so that a signal to a flat surface would reflect away from the spacecraft but those striking elevated edges, hillsides or uneven terrain would be reflected back in varying degrees. Thus the brightness of the imaged area is a function of the roughness of the surface. The other radar measured topographical variation with a resolution of about 50m. In addition, observations of the night side of Venus utilising the infrared window have been made both from the Earth and by the Galileo and Cassini spacecraft as they flew past Venus en route to Jupiter and Saturn. A Mercury bound spacecraft should also add data during its venusian flyby later in 2006.
Since the Earth and Venus are similar in size and density, and occupy adjacent orbits in the Solar System, they might also be expected to have a similar composition and exhibit common geological and geophysical processes. Furthermore, since the same laws of physics and chemistry must apply, the investigation of the differences between the two planets is fascinating and potentially very revealing and from which we might learn much about our own planet. There are three major observed differences between the Earth and Venus. Venus has no detectable magnetic field, there is an apparent absence of active volcanism at the present epoch and it has an intense greenhouse atmosphere. It seems quite likely that all three are interconnected. In principle, with the help of data from Venus Express, it should be possible to start resolving these mysteries. However, it is worth bearing in mind that not only is there an absence of crucial data for Venus, but many of the Earth’s own internal and atmospheric processes have not been fully worked out.
Before considering Venus, it is worth mentioning that the Earth’s magnetic field is far from stable. There is evidence of polar wandering (see December 2005 edition of What’s Up?) and periodic field reversals have been deduced from the ocean crust. This suggests that the Earth’s magnetic field varies over time and there may even have been periods during reversals, unresolvable on geological time scales, when the field strength was negligible. We know this from remanent magnetism in the ferro-magnetic minerals contained in igneous rocks. Palaeomagnetic data can be extracted from remanent magnetism because the field orientation is ‘frozen’ when magma cools below a characteristic temperature known as the Curie temperature which, for iron, is 770°C. Consequently, if future exploration of Venus detects remanent magnetism it may be evidence that the planet did have a magnetic field in the past.
Although the Earth’s magnetic field is generally modelled on the lines of the dipole field of a common bar magnet, the geophysical mechanism that produces it is poorly understood. Fortunately, the space age has enabled researchers to examine in detail the magnetic fields of a number of planetary bodies and according to Stevenson (2003) three criteria need to be satisfied to generate the dynamo thought to be responsible. These are an electrically conducting core, a mechanism to liquefy and cause convection in at least some of the core material and rotation to generate a sufficiently large Coriolis force. Furthermore, Stevenson suggests that convection can only occur if the rate of heat flow exceeds that which can be attributed to conduction. This is an important reservation for terrestrial planets with thermally conductive metallic cores. In the case of the Earth we have seismic evidence of a liquid outer core which is believed to consist mainly of an iron alloy. A number of candidates have been suggested for the alloy’s lighter constituent. Stevenson suggests sulphur because it would reduce the melting temperature of the resulting compound to below the temperature of the overlying mantle, thus accounting for its liquid state. The cause of convection in the outer core is attributed to the release of latent heat as the solid inner core grows in response to the overall outward heat flow through the crust, enhanced by plate margin and other volcanics. In the case of Venus, which is assumed to be geophysically similar to the Earth, Stevenson argues that the dynamo process is absent not because of the slow spin rate but because the outer core does not convect. This is attributed to a reduced heat flow as a result of the high surface temperature and the apparent absence of a plate tectonic process. Of course, different processes might take place during a resurfacing phase (see below) when dynamo conditions might occur – hence the importance of detecting remanent magnetism.
Although igneous rocks dominate the venusian landscape and over 1000 large volcanoes together with other volcanic structures have been identified, no ongoing volcanic activity has been detected. The search for so-called ‘hot-spots’ using infrared detectors will be part of the Venus Express programme. A paper by Hashimoto & Imamura (2001) presents an interesting overview of the logistics concerning the detection of volcanic hot-spots from orbit. The data transmitted by the landers suggest that most of the lower-lying terrain is covered by basaltic rocks of a composition which on Earth might be associated with the ocean crust or mantle. However, two landers detected rocks of more evolved composition, including radioactive element proportions commonly found in continental crust on the Earth, and Magellan imaged a number of steep-sided volcanic domes which have been interpreted as resulting from lavas that were more viscous than basaltic lava. In relation to igneous rocks ‘evolved’ means derived from magma with a greater proportion of silicon than mantle (or mafic) magma. Evolution occurs because magmas are an amalgam of different minerals, each of which has a different melting point. Accordingly (and generally) as magma cools it undergoes a process called fractional crystallisation by virtue of which the minerals crystallise out in order of their melting temperatures. Since silicate minerals have the lowest melting temperature they remain longest in the melt so the more evolved the magma is the greater is its silica content. Radiogenic isotopes also remain in the melt because, in general, they are too large to fit into the structure of the crystallising minerals. On the Earth’s surface the relatively low ambient temperature ensures that in general erupting magmas quickly solidify without undergoing further fractionation. With Venus’s surface temperature being much higher it seems possible that sometimes magma might have cooled so slowly that it evolved as it flowed.
Obviously geophysical data for Venus is patchy and tentative but altimetry data indicate that the vertical distance between the highest mountain top and the deepest basin is less on Venus than on the Earth. To some extent this can be explained by the higher surface temperature allowing lava more time to find its own level. However, on the Earth, the two different types of crust result in a bimodal distribution of topographical heights whereas on Venus the distribution is unimodal. This, together with the apparently random distribution of volcanoes is evidence that Venus lacks an active plate tectonic system. Another clue to venusian interior processes is the stronger correlation between topography and gravity data* than on the Earth, which also leads to the conclusion that the venusian crust is more homogenous, another pointer to the absence of active plate tectonics. Crustal thickness is dependent on the geothermal gradient (a plot of temperature against depth). Although this might be expected to be similar to the Earth’s the much higher surface temperature coupled with the knowledge that the lower crust will be molten at around 1100°C, leads to the conclusion that the venusian crust should be thinner than the Earth’s. This could have important implications in terms of the physical properties of the crust and is perhaps evidenced by the ridge and rift zones which suggest it may not be as susceptible to brittle fracture as the Earth’s. A mechanism for venusian tectonics, based on a number of assumptions, has been proposed by Sandwell et al (1997). These researchers suggest that ridges represent compression zones whilst rifting reflects regions of extension -not too dissimilar to terrestrial collision and rift zones, accounted for by swell-push forces acting on the base of the crust.
Commentators often refer to the venusian surface as being ‘young’. This is by reference to impact crater density. But this is only the case when it is compared to truly ancient surfaces such as the Moon’s. In fact the Earth’s ocean crust, which is recycled by plate tectonics, may be much younger. Basilevsky & Head (1998) have tentatively proposed an average age of 600 ± 200 Million years (Ma) for the venusian surface with the oldest lithological unit 840 ± 200 Ma old. In all, these researchers identify five stratigraphical groups above the bottom unit and interpret them as the product of successive episodes of crustal compression and extension. The oldest stratigraphical layer is associated with the tessera structures whose genesis has been attributed to various catastrophic mantle processes acting on a planet-wide scale. Above this, four tectonic cycles are considered to be responsible for lava flows covering 85 – 95% of the surface, all believed to have been erupted over a period of 180 ± 60 Ma. Basilevsky & Head consider that the most recent period of major volcanism might have lasted until some 60 ± 20 Ma (about the time of the demise of the dinosaurs) but may even be continuing on a small scale at the present time.
Although the solar flux at the orbit of Venus is twice that received by the Earth, 80% is reflected back and the residual amount reaching the surface is only about 54% of that received at the surface of the less cloudy Earth. Interestingly, there was still enough daylight for the Venera landers to image in natural light. The venusian albedo is so high that without the greenhouse effect the surface temperature of Venus (termed the equilibrium temperature) would be less than the equivalent temperature at the Earth’s surface, despite Venus being closer to the Sun. But with CO2 comprising 96% of the venusian atmosphere the surface temperature is boosted by 400° to 500°C (sources vary!). On the Earth this greenhouse effect adds 33°C to the equilibrium temperature (changing it, very conveniently, from a frigid -18°C to a tolerable 15°C). It is often assumed that the venusian greenhouse is entirely due to atmospheric CO2. This is not the case because quantum mechanics determines that CO2 only absorbs energy at the wavelengths of its absorption bands. Other gases are involved, including H2O, HCl (hydrogen chloride) and SO2 (sulphur dioxide), as well as aerosols of sulphuric acid (H2SO4), all of which are likely to have been of volcanic origin. HCl, SO2 and HF (hydrogen fluoride) are present in greater concentrations than in the Earth’s atmosphere where the heavier volcanic gases tend to be rained out. In addition, on Venus the high surface temperature may constrain the precipitation of sulphur, which would remain gaseous, particularly near the surface. The high level of atmospheric CO2 probably results from Venus not having a mechanism for removing it. On the Earth atmospheric CO2 levels are moderated through a complex carbon cycle which results in most of the Earth’s carbon being incorporated into crustal rocks or subducted into the mantle as part of a long term geological sub-cycle. This is triggered by shorter term chemical, hydrological and biological cycles. Of course, Homo sapiens is currently working the rock reservoir and putting the CO2 back into the atmosphere. Both chemical and biological mechanisms depend on the presence of liquid water and it is a moot point as to whether or not Venus had surface water in the past.
A clue to past water content is found in the fact that in the venusian atmosphere the proportion of deuterium (2H) to hydrogen (1H) is 100 times greater than on the Earth. Water vapour dissociates under UV radiation. The hydrogen escapes into space and the oxygen reacts with other elements to form oxides. Since deuterium is heavier than hydrogen it escapes less readily so its concentration increases. Unfortunately this process only works efficiently when there is liquid water available so it is not possible to extrapolate backwards to establish when the deuterium/hydrogen ratio was similar to the Earth’s. It is possible that in the early Solar System, when the Sun was cooler, liquid water might have existed on Venus and boiled away when the solar flux increased.
Atmospheric physics and dynamics are always complex and at present those of Venus are poorly understood. Researchers are only beginning to formulate ideas as to what might be happening there. In principle the issues are simplified because with an almost circular orbit, an axis nearly perpendicular to the orbital plane and a slow rotation, seasonal variation seems unlikely to be significant and Coriolis forces relatively subdued. This is possibly why the vertical (Hadley) circulation of the atmosphere is from equator to high latitudes rather than forming the more complicated zonal flows that drive terrestrial weather patterns.
Superimposed on this relatively straight forward scenario there are long lived structures at the poles with large temperature differentials between adjacent streams and a mid-level atmosphere in super-rotation (i.e. moving faster than the surface rotation velocity). The cause of this is unknown but modelling has suggested a possible link between the vertical circulation and momentum transferred from the solid surface. Wind speeds above and below the super-rotating level decline to zero at 100 km altitude, and at the surface where the absence of significant aeolian erosion leaves craters in pristine condition. Surface evidence of higher level winds is found in downwind streaks of ejecta that were propelled into the high velocity regions of the atmosphere after impact.
With physical parameters so similar to the Earth’s it is natural to wonder why Venus is so different. Is it all due to Venus being nearer the Sun? There seems to be an obvious connection between the high surface temperature and the absence of both a magnetic field and plate tectonics. But was the greenhouse triggered by the loss of surface water when the solar flux increased, or did greenhouse heating boil the oceans because a carbon cycle could not develop once plate tectonics had ceased due to a reduction in the geothermal gradient? And did periodic resurfacing events prevent the development of a biosphere? We seem indeed fortunate with the Earth – if nothing else let us hope that the outcome from Venus Express teaches us to respect our own planet and recognise the importance of the planetary cycles. Otherwise Venus may really be the Earth to come.
Roy Sturmy, February 2006
*Gravity highs and lows (anomalies) can be compared to hills and depressions on a contour map. Measurements of the acceleration due to gravity (g) over a planetary surface are compared to a datum surface on which the value of g is equal to a standard for that body. The actual measurements are then converted into linear distances above or below the standard according to whether the measured value of g is greater (gravity highs) or lower (gravity lows) compared to the datum. This represents the vertical distance above or below the datum at which the value of g is equal to that of the datum at that point, just as topographical heights are measured by reference to mean sea-level. The resulting ‘contoured’ surface is known as the planetary geoid with the highs and lows reflecting local mass concentrations – as might be the case with different types of rock. For the Earth the datum is the surface at which gravity is equal to the amount calculated by the International Gravity Formula which takes into account the Earth’s rotation and polar flattening. On Venus the datum is simply the surface of a sphere at a specified radius from the centre. Gravity data for Venus and other extra-terrestrial bodies is obtained from minute changes in the velocities of orbiting spacecraft, which are picked up through the Doppler shifts on the incoming radio signals. Usually the differences are very small and highs and lows are measured in metres, or minigals (where 1 Gal = 1cms-2).
- The New Solar System 4th edition, Beatty, Petersen and Chaikin (1999) Cambridge University Press.
- Physics and Chemistry of the Solar System, Lewis, J. S. (1997) Academic Press
- Open University, S267 Course Manuals (1997)
- Encyclopedia of Astronomy & Astrophysics (2006), IOP Publishing
- Sandwell, D.T. Johnson, C.L., Bilotti, F., Suppe, J. Driving Forces for Limited Plate Tectonics on Venus, Icarus 129 pp 232-244 (1997)
- Stevenson, D.J., Planetary Magnetic Fields, Earth & Planetary Science Letters 208 (2003) 1-11
- Hughes, D.W., Planetary Spin Planetary & Space Science 51 (2003), pp 517-523
- Correia, A.C.M., Laskar, J., Neron de Surgy, O., Long-term evolution of the spin of Venus I. Theory, Icarus 163 (2003) pp 1-23
- Hashimoto,G.L. & Imamura, T. Elucidating the Rate of Volcanism on Venus: detection of Lava eruptions using Near-Infrared Observations Icarus 154 (2001) pp239-243
- Geology of Venus, Wikipedia (Dec 2005)
- ESA website
The recurrent nova RS Ophiuchi is currently in outburst according to an International Astronomical Union email (CBET 399) received this morning (Monday 13th). Normally at about mag 11, it is now mag 4.8 and was seen as bright as 4.5 yesterday morning by Japanese observers. Its last outburst was in 1985, and these events are eagerly awaited by variable star observers. It is a morning object at the moment, and is visible low in the south-east between about 5.15 and 6.15 am to the north-west of the third magnitude star Nu Ophiuchi. Full details of this star are available at: http://popastro.c.topica.com/maaeudiaboghObb49C1b/ and a comparison chart can be found at http://popastro.c.topica.com/maaeudiaboghPbb49C1b/
By Robin Scagell
FAINTEST ANDROMEDA SATELLITE GALAXY
The Sloan Digital Sky Survey has turned up the faintest satellite galaxy yet found around the Andromeda Nebula. No doubt other dim galaxies remain undetected. Certain theories would like there to be about 100 times more dwarf galaxies than are actually observed – a discrepancy known as the missing-satellite problem -- and astronomers have been scrabbling around trying to explain the difference. But in the last two years astronomers have discovered two particularly faint galaxies -- UMaj around our Milky Way and Andromeda IX around our nearest large galactic neighbour, Andromeda. Those finds suggest that the missing-satellite problem may actually be less dire than originally thought. The new discovery is called Andromeda X and is the faintest known satellite of Andromeda. It appears to lie about 280,000 to 450,000 light-years from Andromeda, which itself lies 2.5 million light-years from us. All three of the new finds have been made from the Sloan Digital Sky Survey, which is systematically mapping one-quarter of the sky.
Astronomers using the VLA radio telescope have studied motions within a disc of material that is orbiting a still-forming star some 500 light-years from Earth in the direction of Ophiuchus, and have found a curious result -the inner part of the disc seems to be orbiting the proto-star in the opposite direction from the outer part. Any planetary system that forms around the star will therefore presumably include planets orbiting in different directions, unlike our own Solar System in which all the planets orbit the Sun in the same direction. The system may have acquired material from two clouds of pre-stellar material instead of the usual single one. It is in a large star-forming region where chaotic motions and eddies in the gas and dust result in smaller cloudlets that could rotate in different directions. Though this is the first time such a phenomenon has been seen in a disc around a young star, it has previously been reported in the discs of certain galaxies.
ROTATING RADIO TRANSIENTS
Astronomers at Jodrell Bank have discovered a new type of star, Rotating Radio Transients (RRATs). A survey of the Milky Way for pulsars, with the Parkes radio telescope in Australia, revealed 11 sources of very short radio flashes, each around one hundredth of a second long and typically separated by three or four hours. The time a RRAT may be visible to telescopes is tiny, a total of something like a tenth of a second per day. It is thought that RRATs, like pulsars, are a form of rotating neutron star. Whereas radio signals from pulsars have a regular period concurrent with their rotation, the RRATs at first seemed to pulse at random. Further investigation showed that the long silences were always multiples of a shorter time, which the researchers believe to be the period of the RRAT’s rotation. The period of more than half the known RRATs is over four seconds, much longer than for the vast majority of known pulsars. However, the four-second period is similar to that of ‘magnetars’, which emit only X-rays or gamma radiation. Astronomers speculate that RRATs may represent an evolutionary stage of neutron stars to or from magnetars and that the new stellar class probably outnumbers pulsars.
POSSIBLE MARS AURORAE
Evidence is accumulating that aurorae occur over the night side of Mars, especially over areas of the surface where variations in the magnetic properties of the crust have been detected. Observations from Mars Express show structures (inverted-V features) of accelerated electrons and ions above the night side of Mars; almost identical to those that occur in connection with aurorae on Earth. On our planet, as well as on Jupiter, Saturn, Uranus and Neptune, aurorae occur at the feet of the planetary magnetic field lines near the poles, and are produced by charged particles -- electrons, protons or ions – moving along those lines. A few years ago it was suggested that auroral phenomena could exist on Mars too. That hypothesis was reinforced by the Mars Global Surveyor's discovery of crustal magnetic anomalies, most likely the remnants of a one-time planetary magnetic field. Scientists have now found that the structures of accelerated particles are indeed associated with the crustal magnetic anomalies at Mars, but that strong acceleration mainly occurs in a region close to local midnight. No identification of the auroral emission lines is possible at present, since even the composition of the upper atmosphere on the night side is unknown. Since we see Mars as always sunlit, the night side of Mars cannot be observed from the Earth.
STARDUST FINDINGS MAY ALTER VIEW OF COMET FORMATION
Particles from Comet Wild-2 that were collected and returned to Earth in a capsule from the Stardust spacecraft show dozens of minerals that form only in extreme heat -- a finding that complicates theories about how the Solar System formed. The grains include a titanium-vanadium-nitrogen mineral that forms only in temperatures higher than 1,100 degrees Celsius. The minerals must either have formed in the hottest, innermost region of the nebula that eventually became our Solar System, or have come from another star. If they were formed in our Solar System, then they would have had to be closer than Mercury to the Sun to form, and no mechanism has been identified that would transport them far outwards to reach the Kuiper Belt region. Researchers plan extensive studies to try to determine the chemical histories of the sample particles, which eventually will reveal whether the Sun or another star heated the grains. Between 150 and 200 samples from the comet are currently circulating among scientists and research labs around the world. Next month, work begins on another set of samples collected during the mission, interstellar dust grains. Unlike the comet samples, the bits of interstellar dust picked up during Stardust's travels are tiny. Scientists plan an Internet-based detection programme of volunteers who will use their home computers to scrutinise images and try to identify any particles.
PLUTO'S NEW MOONS PROBABLY BORN WITH CHARON
A team of U.S. scientists concludes that two newly discovered small moons of Pluto were very probably born in the same giant impact that gave birth to Pluto's much larger moon, Charon. The team also argues that many other large binary Kuiper Belt Objects (KBOs) may also possess small moons, and that the small moons orbiting Pluto may generate debris rings around Pluto. The evidence for the small satellites being born in the Charon-forming collision is strong; it is based on the supposed facts (not yet conclusively established) that the small moons are in circular orbits in the same orbital plane as Charon, and that they are also in, or very near, orbital resonances with Charon. There is a growing realisation that binary 'ice dwarf' pairs like Pluto + Charon are common in the Kuiper Belt, so it is natural to suppose that numerous multiple systems may be discovered there in years to come. Finding small satellites around KBOs is difficult because their large distances from the Sun make them appear very faint. One way to see how common it is for KBOs to have multiple satellites might be to search around objects that are thought to have been ejected from the Kuiper Belt into orbits that bring them much closer to the Sun. So far, about 160 such objects, called Centaurs, have been discovered. Astronomers hope to use Hubble to search for faint moons around some of them.
Bulletin compiled by Clive Down
© 2006 the Society for Popular Astronomy
Buzz Aldrin deploys a seismometer in the Sea of Tranquility
When astronauts establish any kind of base on the moon they may need quake-proof housing. This is the conclusion of Clive R. Neal, associate professor of civil engineering and geological of Notre Dame, after scientists re-examined Apollo data from 1970s. “The moon is seismically active,” he told a gathering of scientists at the NASA Lunar Exploration Analysis Group (LEAG) meeting in Texas, last October. Between 1969 and 1972, Apollo astronauts placed seismometers at their landing sites around the moon. The Apollo 12, 14, 15, and 16 instruments continued to return data to Earth until they were finally switched off in 1977.
Lunar seismograms from the Apollo 16 station
The data showed that there are at least four different kinds of moonquake: (1) deep moonquakes about 700 km below the surface, probably caused by tides; (2) vibrations from the impact of meteorites; (3) thermal quakes caused by the expansion of the frigid crust when first illuminated by the morning sun after two weeks of deep-freeze lunar night; and (4) shallow moonquakes only 20 or 30 km below the surface.
The first three were generally mild and harmless. Shallow moonquakes on the other hand were extraordinary. Between 1972 and 1977, the Apollo seismographs registered twenty-eight of them; a few reaching up to 5.5 on the Richter scale. A magnitude 5 quake on Earth can crack plaster or move heavy furniture. Shallow moonquakes lasted a remarkably long time. Once started, they continued for more than 10 minutes. “The moon was ringing like a bell,” Neal says.
On Earth, vibrations from quakes usually die away in thirty seconds or so. The reason has to do with chemical weathering, Neal explains: “Water weakens stone, expanding the structure of different minerals. When energy propagates across such a compressible structure, it acts like a foam sponge -it deadens the vibrations.” Even the biggest earthquakes stop shaking in less than 2 minutes.
The moon, is dry, cool and mostly rigid, like a chunk of stone or iron. Moonquakes set it vibrating like a tuning fork. Even if a moonquake isn’t intense, it just keeps shaking. This persistence was first recognised when the abandoned Ascent Modules were crashed back on to the Moon to test the seismometers left on the surface. For a lunar habitat, the persistence could be more significant than a moonquake’s magnitude. Habitats will need to be constructed from fairly flexible materials, to prevent cracks developing, that could allow air to escape. They will also need to be resistant to the fatigue caused by repeated bending and shaking that could occur.
Neal and his team aren’t sure what causes the shallow moonquakes, or where they occur. The Apollo seismometers were all in one relatively small region on the front side of the moon, so it isn’t possible to pinpoint the exact locations of these quakes. However, Neal and his colleagues do have some good ideas, among them being, the rims of large and relatively young craters that may occasionally slump. “We’re especially ignorant of the lunar poles,” Neal says. That’s important, because one candidate location for a lunar base is on a permanently sunlit region on the rim of Shackleton Crater at the Moon’s south pole.
Neal and his colleagues are developing a proposal to deploy a network of 10 to 12 seismometers around the entire moon, to gather data for at least three to five years. This kind of work is necessary, Neal believes, to find the safest spots for permanent lunar bases. Other planets may be shaking, too: “The moon is a technology test bed for establishing such networks on Mars and beyond.”
- Tuesday 4 April Jodrell Bank Workshop
- Tuesday 18 April – Pre-telescopic Astronomy – Martin Lunn
- Tuesday 2 May Jodrell Bank Workshop
- Tuesday 16 March – Pulsars Prove Einstein Right – Sir Francis Graham-Smith
Contributions, if possible, by email to: dogden at ntlworld dot com
Remember to change the at and dot :)
The sky on March 31st 2006
In the late evening sky, Mars at magnitude 1.2 and angular diameter of 5.7" hangs low beneath Auriga and above Orion. Saturn at magnitude 0.1 and substantially greater angular diameter of 19.1", is below Gemini, in the constellation of Cancer.
Several hours later, in the dawn sky, Jupiter hangs low and bright at magnitude -2.4 with an angular diameter of 42.9". At its present distance, light takes about 45 minutes to travel from Jupiter to the Earth. Venus is just rising in the south-east and will illuminate the dawn horizon at a brightness of -4.3, even though its diameter appears to be substantially less than that of Jupiter. This is of course, because Venus is so close to the Earth. Light currently takes about six minutes to travel from Venus to Earth.