Mercury - the elusive planet
Mercury - the elusive planetFrom Earth, the planet never appears more than 270 45' from the Sun, so that it is best seen when it is at its maximum elongation East or West of the Sun. [ See Figure 1 ] It appears either as an 'evening star' shortly after sunset or as a 'morning star' just before dawn. The ancient Greeks called it Hermes when it appeared in the evening sky and Apollo in the dawn sky; the Egyptians named it Set or Horus. It was eventually recognised, around 350 BC that the two 'stars' were one and the same. The Romans named the planet 'Mercury'. Mercury is only 1/50th as bright as Venus, and it is always low down in the sky where its light has to pass through a greater thickness of atmosphere, which makes it twinkle more than the other planets but rather less than a star. In the glare of twilight ( and increasing light pollution ) it is difficult to spot Mercury at all and most people have never seen it - it is said that Copernicus never saw the planet, because of the swirling mists rising from the river Vistula in his hometown of Krakow.
Before the Mariner 10 fly-by, very little was known about its surface.
The first astronomer to notice that it showed phases like the Moon and
Venus [ Fig 1 ] was Giovanni Zupus who, in 1639, used a telescope only
slightly more powerful than Galileo's. The best time to view the planet
through a telescope is between inferior and superior conjunction, when
it is hidden by the Sun, but great care must be taken not to look at the
Sun through the instrument or eye-damage will result. Transits of Mercury,
across the face of the Sun, are relatively rare, about 15 per century (
the next will be on November 15th this year at about 09:00 UT) because
the orbit is inclined at 70 to the plane of the ecliptic and the planet
usually passes above or below the solar disc.
[Transits can only occur in November and in May when the Earth's orbit
crosses the nodes of Mercury's orbit.]
It is exceedingly difficult, even under the best observing conditions, to make out anything other than vague shadings on the mercurial disc. The planet is only 1/3 of the diameter of the Earth and its disc never appears larger than 13 seconds of arc. Mercury's orbit is the second most elliptical after that of Pluto - its distance from the Sun varies from 46 to 70 million kilometres and its distance from the Earth from 79 to 218 million. Between 1881 and 1889, the Italian astronomer Giovanni Schiaparelli ( who first sketched the Martian 'canals' ) noted that certain markings showed a period of 88 days, the same as the planet's orbital period around the Sun. This suggested that the planet rotated once about its axis in the same time it took to revolve around the Sun. This 'captured rotation' would mean that Mercury always presented the same face to Sun ( in exactly the same way that our Moon always faces the same way towards the Earth ) making Mercury simultaneously the hottest and the coldest planet in the Solar System. This observation was confirmed by several other astronomers between 1889 and 1962 including Eugene Antoniadi who made detailed maps of the planet's surface using the 83cm telescope at the Meudon observatory near Paris. His maps showed dusky greyish markings which he named after Greek and Roman mythological figures - as fanciful as his imagination.
However, in 1965, radio astronomers at the 305 metre Arecibo radio telecope
in Puerto Rico, succeeded in measuring the true rotation period using the
Doppler shift in radar pulses bounced off the planet. It turned out to
be 58.65 days - almost exactly 2/3 of its orbital period. This is an example
of spin-orbit coupling and it means that Mercury rotates exactly three
times on its axis in every two revolutions around the Sun. The gravitational
pull of the Sun has removed angular momentum ( spin ) from the planet and
slowed down its rotational period from about eight hours to its present
value. [ Evidence that 'tidal friction' between the Earth and Moon has
reduced the length of the day from 21.9 hours 370 million years ago is
provided by studying fossil corals; it continues to reduce at the rate
of 1.8 milliseconds per century. At the same time the Moon is being accelerated
in its orbit; it is moving away from the Earth at the rate of 3.2 cm per
year, as measured by laser beams bounced off the surface of the Moon.]
The combination of Mercury's high orbital eccentricity and spin-orbit
coupling leads to a uniquely peculiar apparent motion of the Sun in the
night-black sky of this airless world. The planet makes a complete turn
relative to the distant stars ( a 'sidereal day' ) in 56.65 Earth-days
but, relative to the Sun, a solar 'day' is actually 176 Earth-days long
- the planet turns three times and orbits the Sun twice between successive
overhead passages of the solar disc. [ See Figure 3 ] The eccentricity
of the orbit also means that the apparent size of the Sun varies from 1.10
to 1.60 from aphelion to perihelion, that is from twice to three times
the diameter of the Sun as seen from Earth.
Mercury always presents one of two faces to the Sun at each perihelion passage so that two longitudes, taken as 00 and 1800, are at the sub-solar point at each perihelion. Mercury receives more than twice as much sunlight when it is close to the Sun than when it is furthest from it. This produces intense heating at the 'hot-poles' and less intense heating at longitudes 900 and 2700 which are called 'warm-poles' because they face the Sun when Mercury is at aphelion. Temperatures on the surface can vary from 700 Kelvin, which is hot enough to melt lead and tin, to 100 Kelvin at night when the heat is radiated away into space.
Another peculiar effect is due to the variation in Mercury's orbital speed. It is moving twice as fast at perihelion as at aphelion and this causes the Sun to perform a loop in the sky. At the 'hot-poles' it appears to stop in mid-heaven and then move backwards for about 4 Earth-days before it resumes its westward journey again. From the 'warm-poles' it would appear to perform either a double sunrise or a double sunset. An observer would see a large Sun rise slowly, stop at the horizon, and ponderously set again. It would then rise in earnest, shrinking in angular size and increasing in angular speed. A small Sun would climb through the zenith, swelling as it set, and very slowly sink below the horizon, only to rise again slowly and finally set, not to rise again for another Mercurian year of 88 Earth-days !
Surface features
The most striking feature of the Mariner 10 images is how similar the surface of Mercury is to that of the Moon. Most of the planet is covered by old, heavily cratered terrain. About 1/5th is considerably smoother, mostly in the region around the Caloris Basin, a multi-ringed impact structure which was only partially visible to the spacecraft. It is 1300 km across and covers more than ¼ of the planet's diameter. It was probably formed by an impact with a large asteroid 3.6 billion years ago. Half a planet away, at the antipodal point, the terrain is chaotic and features highland areas with filled basins, similar to those on the Moon. The smoother plains may have resulted from lava flows triggered by the impact. ( The Moon has a strikingly similar feature on its far side called Mare Orientalis - it is tempting to conjecture that shock waves from this major impact, travelling through the dense core of the Moon, could have led to the volcanic flooding of the large basins we see on the side facing Earth.)
Little has changed on Mercury since the end of the period of heavy bombardment in the Solar System 3.5 billion years ago. However, the surface shows signs of shrinkage - linear faults and scarps that run from North to South and from North-East to South-West and North-West to South-East. These are known as the Mercurian Grid and they may have been formed because the planet was rotating much faster than it is today as the crust solidified, perhaps in 20 hours. The planet would have had a slight equatorial bulge but after it slowed to its present rate, gravity would have pulled the crust into a more spherical shape creating the wrinkles. These do not cut across the Caloris impact structure which gives some idea of their relative ages. Once the crust had solidified, the outer core may have continued to contract, causing the crust to collapse in a network of curved scarps or cliffs and reducing the surface area of the planet by perhaps a million square kilometres.
The craters on the Moon, Mars and Mercury show a similar distribution
of sizes, with the exception that those on Mercury tend to be somewhat
larger. Most likely the objects striking Mercury had higher velocities
than those hitting the other planets. This would be expected if their orbits
were elliptical, bringing them closer to the Sun and so faster in the region
of Mercury than when farther out. These rocks may have been from the same
family, one that originated in the asteroid belt. In contrast, the moons
of Jupiter have a different distribution of crater sizes, indicating that
they collided with a different group of bodies.
Atmosphere and interior
Planets as hot as Mercury do not retain an atmosphere - any volatile material on the surface would soon be evaporated by the Sun's fierce rays and the highly energetic molecules would be moving fast enough to escape from the gravitational pull of the planet. However, the ultra-violet spectrometer on board Mariner 10 detected small amounts of hydrogen, helium and oxygen and Earth-based observations have also found traces of sodium and potassium. Much of this extremely thin atmosphere is probably created by the solar wind - a fast stream of energetic protons emitted by the Sun's corona - which reaches down to the surface when Mercury is closest to the Sun or when the Sun is particularly active. Atoms 'sputtered' off the surface will be trapped by the planet's magnetic field ( magnetosphere ) and will add to its tenuous 'atmosphere'. Recently, the polarization of radar waves reflected from the polar regions has suggested that there may be solid water ice, perhaps brought by comets, and which remains shaded from the Sun's heat due to the small axial tilt ( 2 0 ) of the planet. If this is the case it means that Mercury's axis has been incredibly stable for millions of years.
With a diameter of 4878 km, Mercury is only 40% larger than the Moon ( 3476 km ) and yet it is much denser. The Moon's density is 3.34 g/cm3 whereas that of Mercury is 5.4 g/cm3. This is comparable with the densities of the Earth ( 5.5 ) and Venus ( 5.2 ) and greater than Mars ( 3.9 ). In fact, if the compression of the cores of Earth and Venus are allowed for, Mercury would actually be denser. This suggests that Mercury may have a substantial iron core, perhaps up to 70% of the planet's mass and taking up ¾ of the planet's diameter. This is proportionately much larger than that of the Earth and it raises the question of how Mercury got such a large amount of iron. It may be that the heavier elements condensed out of the solar nebula in greater concentrations nearer to the Sun or it could be that Mercury has lost a greater proportion of its volatile elements due to its proximity to the Sun's heat.
A further challenge is the origin of Mercury's magnetic field which is 1% of the Earth's at its equator. For such a small planet this represents an appreciably large field and it suggests that, if it is produced by the dynamo effect, a proportion of the outer core must remain molten. Given Mercury's small size, the core should have cooled and solidified eons ago. Perhaps it contains a small proportion of sulphur mixed with the iron which would reduce the freezing point, allowing the core to remain liquid and so carry electric currents which drive the dyanmo. Did a giant impact similar to that which created the Moon also affect Mercury, robbing it of a large chunk of its mantle? Or perhaps, Mercury is what was left of another planetoid which collided with Venus, reversing its spin and entering a highly elliptical orbit around the Sun. Clearly there are still many questions to be answered and a new generation of spacecraft is needed to probe Mercury's remaining mysteries.
Mercury Polar Flyby
As a result of renewed interest in Mercury, there are two related proposals being developed as potential Discovery class missions. Discovery is NASA's new "cheaper, better, faster" line of solar system exploration spacecraft. These missions are capped at $150 million total mission costs. The two Mercury proposals are the Mercury Polar Flyby (MPF) and Hermes (Mercury orbiter). MPF's instruments include a neutron spectrometer (water detection), dual polarization radar (subsurface ice mapping), camera (imaging polar region and hemisphere not imaged by Mariner 10). We believe a flyby is cheaper and more technically feasible. MPF is designed to have multiple Mercury encounters at aphelion only. At aphelion a spacecraft only has to endure the equivalent of four times the Earth solar flux. The orbit of Mercury is eccentric such that at perihelion there is eleven times Earth solar flux. An orbiter would have to endure such conditions requiring elaborate (and expensive) cooling and thermal shielding systems.
Venus - Earth's Twisted Sister ?
Venus is the 'evening star' and the 'morning star' ; Hesperus and
Phosphorus of antiquity. It can reach a magnitude of -4.4, making it the
brightest object in the sky other than the Sun and Moon and it is our nearest
planetary neighbour in the Solar System, approaching to within 40 million
kilometres of the Earth. Its phases can be followed with binoculars or
a small telescope; it was Galileo's observations of the phases of Venus
which provided him with compelling evidence in support of the Copernican
theory.
Venus is similar in size, mass and density to the Earth and it used to be known as our "sister planet", a heavenly body associated with Peace and Love, but recent spacecraft observations have painted a picture of Venus which is closer to Hell than to Heaven. Its clouds, which reflect 76% of the sunlight falling on them,( giving an albedo of 0.76 ) are made of concentrated sulphuric acid and they obscure from human eyes a surface over which rivers of liquid lava flow at a temperature higher than the melting point of lead.
Pioneer to Venus
Between 1978 and 1992, Venus was probed by the Pioneer Venus spacecraft,
consisting of an orbiter and a multiprobe containing four craft designed
to plunge into the thick Venusian atmosphere and send back data on conditions
along the way. The orbiter was launched in May 1978 and in December it
entered a highly elliptical orbit which looped to within 150-200 km from
the planet's surface and out to 66 900 km. During its closest approaches,
its instruments directly sampled the ionosphere and upper atmosphere while
12 hours later, it had receded far enough to obtain global images and sample
the planet's near-space environment. During its 14-year mission the orbiter
circled Venus 5,055 times until, its fuel exhausted, it burned up in the
Venusian atmosphere in October 1992.
The Venusian atmosphere
The largest atmospheric probe carried a mass spectrometer and a gas chromatograph to measure the exact composition of Venus's atmosphere which is extremely dry; it contains only 1/100 000th as much water as the Earth's oceans. If all the water in Venus's atmosphere was condensed on the surface it would make a global puddle only a few centimetres deep. The surface of Venus bakes under a dense atmosphere composed mainly of carbon dioxide ( CO2 ) at a pressure 93 times the atmospheric pressure at sea-level on Earth and at temperatures in excess of 4600C. Earth's atmosphere consists of 78% nitrogen and 21% oxygen while that of Venus is over 97% CO2 with nitrogen as the next abundant gas ( 3.5% by number of molecules). Both planets have about the same total amount of gaseous nitrogen but Venus has 30 000 times as much CO2 . Earth has a similar amount locked-up in carbonate rocks, but conditions on Venus have conspired to release this vast resevoir of CO2 into the atmosphere, with drastic consequences.
Without its atmosphere to trap the Sun's heat, Venus would have a surface temperature of around 540C, lower than that of Mercury and only slightly higher than the Earth's average temperature of about 160C. The mechanism by which sunlight is trapped inside a planet's atmosphere is known as the Greenhouse effect. It is caused mainly by gases such as carbon dioxide and water vapour, and to a lesser extent by sulphur dioxide and carbon monoxide, which absorb thermal radiation from the surface at virtually all wavelengths, preventing it from being radiated back into space. Venus is an example of what happens when the Greenhouse effect runs out of control on a massive scale and it gives us warning of what could occur on Earth unless we are careful.
There is little, if any, molecular oxygen in the lower atmosphere although
there is evidence that it is being lost from the upper atmosphere. Earth's
oxygen is produced by the biological action of plants (photosynthesis)
and without this, most of our oxygen would be locked up in the surface
rocks. Venus's atmosphere is far richer in sulphur, especially in the form
of sulphur dioxide ( SO2 ) which is removed efficiently from Earth's atmosphere
by the action of water - as acid rain !
Venus's clouds are composed largely of concentrated solutions of sulphuric
acid and water. The clouds are the product of a chemical cycle in which
sulphur, from surface rocks, reacts with CO2 and carbon monoxide
( CO ) to produce carbonyl sulphide ( COS ). This in turn reacts with
oxygen-rich gases to produce SO2. The action of ultra-violet rays in the
region above the cloud-tops, causes SO2 to react with water to form sulphuric
acid ( H2SO4 )droplets which sink slowly through the condensation zone
where they grow larger by drawing sulphuric acid vapour and water from
the air. Near the ground the drops vaporize and dissociate into SO2 and
water vapour.
Venus rotates extremely slowly, and in the opposite direction to the other planets, turning only once relative to the distant stars in 243 Earth-days. The surface temperature is very nearly constant from the equator to the poles because of the dense atmosphere, which is too sluggish for convection currents to efficiently remove the intense heat from the surface. Wind speeds near the surface are slow, rising to a maximum of about 100 m/s at a height of 90 km, which is 60 times faster than the rotation of the planet's surface. On Earth, winds at low latitudes move more slowly than the rotation of the planet, whereas at high latitudes they move faster - a state known as super-rotation. Venus's atmosphere super-rotates at all latitudes.
Venus's atmosphere appears to have retained higher concentrations of inert gases such as neon and argon than does that of the Earth, which may have lost its original primitive atmosphere as a result of the collision in which the Moon was formed. How did Venus become so hot and dry compared with the Earth, given that they both formed in the same part of the Solar System and therefore presumably started with similar compositions?
Where did all the water go ?
Both planets grew by colliding with and absorbing smaller bodies and scattering some of them into orbits which intersected the path of the other. Hence both the Earth and Venus should have accumulated comparable quantities of water-rich bodies - the roughly equal total amounts of nitrogen and CO2 support this notion. Both quickly developed thick atmospheres from gases expelled from their interiors and from the vaporized remains of comets and other impactors. On Earth, water condensed into lakes and oceans and CO2 was incorporated into carbonate rocks by chemical weathering; CO2 dissolved in water forms a weak acid called carbonic acid which dissolves soluble minerals such as calcium to form carbonates e.g. stalactites).
Venus may also have had oceans; perhaps from 4 to 570 metres deep. The
newborn Sun was 30% less luminous than it is today, so temperatures on
Venus could have been well below the boiling point of water
( around 300C ?) but, as the Sun brightened the surface temperature
rose and the oceans evaporated. CO2 released from volcanoes could no longer
be removed from the air by chemical weathering of surface rocks and , as
it accumulated in the atmosphere, the Greenhouse effect became ever more
intense. But where did the water go ? Pioneer found evidence that Venus
is continuing to lose water as a result of solar radiation at the cloud
tops.
Water molecules are dissociated by ultra-violet radiation into hydrogen and oxygen atoms. The lighter hydrogen atoms may escape into space as a result of collisions with energetic atoms or the solar wind, a stream of charged particles which blasts the upper atmosphere because of the absence of a magnetic field around the planet. Left-over oxygen may combine with surface minerals or it too may be carried off by the solar wind. A few billion years ago, the atmosphere contained more water vapour than it does now, and the yound Sun emitted more energetic UV-rays. Both of these factors hastened the rate at which water was destroyed and carried off into space. Calculations indicate that over 4.5 billion years, Venus could have lost an amount of water equal to that in the Earth's oceans. The reason Earth retained its water is because it remained on the ground, where it was not subjected to the energetic process which drove off Venus's.
Pioneer found that some 5 x 10 25 atoms of hydrogen are still being lost per second from Venus's atmosphere. This rate of loss could have removed the entire amount of water in the atmosphere in about 200 million years. Are we seeing the last trickle of Venus's oceans only now, in which case, did some catastrophic event trigger the process in the relatively recent past, and what was that event ?
Venus unveiled
No human eye has ever gazed on the surface of Venus, at least not directly. In the 1970s and 80s, Soviet Venera and American Pioneer craft made coarse radar maps but it was not until the Magellan mission began mapping the surface with a resolution of 120 metres in September 1990 that planetary scientists were able to see the fine details needed to develop theories of the planet's evolutionary history. Radio waves a few centimetres long can penetrate Venus's dense clouds allowing earthbound radio telescopes and spacecraft to 'see' surface details. Magellan began mapping the surface on September 15 and in its first three days it returned more data than the entire Mariner 9 mission to Mars. Before it was terminated due to lack of funds in October 1994, Magellan returned over a trillion bytes of data and mapped 99% of the planet's surface in greater detail than we know our own.
The Venusian surface appears to be 100 million to one billion years old - ancient by terrestrial standards but still young enough for the planet's outer layers to have been significantly reworked relatively recently. Terraced volcanic calderas, extensive lava flows, folded mountain ranges and intricate networks of faults are evidence of internal activity. Some features, such as shield volcanoes are familiar, but some, like the peculiar circular features called coronae, are unique in the solar system. Of particular significance is the relative scarcity of impact features - craters. Magellan images show just over 900 impact craters distributed more or less randomly over the entire surface, far fewer than on the Moon or Mars. On Mars, the age of various surface features can be estimated from the density of cratering - older, less active areas have more craters than younger more active ones; the northern plains of Mars are more sparsely cratered than the ancient southern highlands. The surface of Venus, as deduced from the number of observed craters, has an average age of about 500 million years ( give or take 100 million ) making it the youngest planetary surface in the inner solar system.
One possible reason for the absence of small craters - there are none smaller than 3 km in diameter - is that the dense atmosphere causes smaller objects to burn up or explode before they reach the ground. A large number of asymmetric craters suggest that the atmosphere alters the trajectory of large meteors or the way in which ejecta are distributed. Some craters are surrounded by smooth dark areas, possibly caused by shock waves from an air-bursting meteor. All the craters look unnaturally pristine - we would expect to see some in various stages of degradation due to weathering or partial flooding by lava flows or by cracking and faulting due to tectonic disruption. Only 4% are partially covered and almost none are mostly buried. Some process has removed almost all traces of older craters leaving the most recent planted on top of the volcanic plains which cannot be more than a billion years old.
To explain these observations, planetary scientists have come to the conclusion, in some cases reluctantly, that a catastrophic global resurfacing of the planet must have occurred some 500-600 million years ago which resulted in streams of lava covering most of the planet with vast plains of basalt to a depth of from 5 to 10 kilometres, covering up the pre-existing craters in the process. The activity died down quickly, resulting in a surface that is the same age nearly everywhere, with the exception of the rugged, radar bright, tesserae - deformed highland mountains - which predate the smooth plains which lap up to them. A lower rate of volcanic activity has continued however, as evidenced by the high proportion of SO2 which suggests active volcanoes, and this has resulted in the small number of flooded and disrupted craters. Fresh lava flows can be seen on some large shield volcanoes, although the absence of erosion could mean that they are in fact a lot older. Some models of Venus's evolution predict that it gets rid of its internal heat in periodic spasms of planet-wide resurfacing rather than the steady cycling of lithospheric plates that occurs on Earth. This would amount to the planet turning itself inside out at intervals of several hundred million years.
An alternative theory might be that we are seeing the results of yet another giant impact in Venus's recent past, one which gave the planet enough thermal energy to melt a substantial fraction of its crust and possibly one which slowed down and reversed its rotation in the process - or are we seeing giant impacts around every corner now ?
Rivers of lava
Earth has a thin layer of crust, a thick mantle, a molten outer core ( which produces its magnetic field ) and a solid inner core. Venus has a thicker crust than scientists thought, a thick mantle and perhaps a core but the ways the planets work are quite different. Venus has no plate tectonics, which is a horizontal process which recycles the crust at plate boundaries in subduction zones and by sea-floor spreading. On Venus, the activity is vertical, with upwelling of internal heat ,creating domes and shield volcanoes and down-welling, which could produce features such as the coronae and tesserae.
Some features bear a striking resemblance to erosional features on Earth, resembling riverbeds, but water cannot remain liquid at a scorching 4600C. Meandering lava channels ( called sinuous rilles ) on the Moon can be more than 100 km long and over 2 km wide. Hadley Rille, explored by the Apollo 15 astronauts, was formed by lava that contained large amounts of titanium and iron but little silicon making it less viscous and therefore more free-flowing than lava with a higher silicon content. Lunar rilles typically become narrower and shallower downstream, eventually fading away completely. On Earth, the surfaces of lava rivers harden quickly, but the lava continues to flow in a tunnel which empties, leaving a lava tube. The roof of this tube usually collapses, leaving a boulder filled channel typically 5m wide and rarely longer than 10km.
Venus has a different type of channel, called a canale ( plural : canali ) which can be 100s to 1000s of kilometres in length and 1 to 2 km wide.Baltis Vallis is the longest known channel of any type anywhere in the Solar System - it is 6,800 kilometres long. Canali are much shallower and longer than lunar rilles and do not fade downstream. The meander characteristics of some canali are identical to those of many rivers on Earth. Radar echoes indicate that they lack boulders and are very smooth on scales from centimetres to hundreds of metres. Compound channels ( also known as outflow channels ) have streamlined 'islands' within them ( as on Mars ) and braided channels and lava deltas at their mouths. Such outpourings can only occur for very low viscosity lava. Venus may have had shallow lava 'acquifers' from which lava springs fed flows which eroded valleys similar to those formed by water on Earth. In some cases meteorite impact may have triggered sudden outpourings of lava from these acquifers. Lava deltas rivalling the Mississipi and longer than the Nile were formed by eruptions of lava that raced over the surface like water and pooled in the lowlands where they created temporary lava seas soon after the climax of global resurfacing.
Only carbonatite lavas made of molten carbonates and salts have low
enough viscosity at the temperature on the Venusian surface to travel for
1000s of km. Natural carbonates, which have melting points as low as 5000C
and viscosities similar to water, could have been formed in the primitive
Venusian oceans. When these evaporated the salts would have been melted
during the global resurfacing and formed the lava which created the canali,
adding CO2 and other gases to the atmosphere in the process. It has been
observed with irony that on the outer satellites, surface water behaves
like lava while on Venus lava behaves like water, but a similar fate could
be in store for the Earth. When our Sun becomes a red giant some 4 ½
billion years hence, the Earth's store of limestones and marbles will start
to decompose, releasing CO2 and water vapour into the atmosphere and causing
our climate to spiral into a super Greenhouse. Rivers of carbonatite lava
may then flow over Earth's barren surface, briefly creating the illusion
that water still flows on this arid, dying world before the expanding Sun
swallows it up and extinguishes forever.
Meanwhile . . . the weather on Mars will be warm and sunny . .
. for a few million years only !
References
Venus Unveiled David H Grinspoon Astronomy May 97
The Rivers of Venus Jeffrey S Kargel Sky&Tel Aug 97
Magellan Reveals Venus Cordula Robinson Astronomy Feb 95
Venus Peter Cattermole Astronomy Now Oct 94
The Pioneer Mission to Venus Luhmann/Pollack/Colin Scientific American Apr 94
The Surface of Venus R Stephen Saunders Scientific American Dec 90
Atlas of the Solar System P Moore & G Hunt Mitchell Beazely 1983
Mars - the Red Planet
Mars is the fourth planet from the Sun and the seventh largest;
it is about half the size of the Earth. Named after the Roman god of war,
probably due to its obviously red colour, Mars was also a god of agriculture
before becoming associated with the Greek god Aries. The first spacecraft
to visit the Red Planet was Mariner 4 in 1965 but it was the Mariner 9
mission in 1971 which gave us our first detailed views of the surface.
It revealed craters, sand dunes, channels eroded by water and the largest
rift valley in the solar system, named appropriately, Mariner Valley or
Valles Marineris. In 1976 two robotic Viking landers sampled the Martian
soil and tested for micro-organisms, with inconclusive results. The Viking
orbiters provided us with the best images yet of the Martian landscape
and its thin clouds of water vapour. In July 1997, Mars Pathfinder captured
the interest of the Internet community. It provided more pictures of rocks,
details of their composition and gave us information about the Martian
climate.
Mars is closest to the Earth on April 24th this year (1999) and it will appear bigger and brighter than it has done for a decade. The reason for this is Mars's orbit which is significantly non-circular. When Mars is at opposition ( see Figure 1 ) it is closest to the Earth when it is at perihelion ( its closest distance to the Sun ). This occurs on May 1st, when Mars will be 86 million kilometres from Earth and its disc will be 16.2 seconds of arc across. Oppositions occur every 26 months but at intervals of roughly 16 years, Mars's orbit brings it closest to the Earth at opposition - the next closest approach will be in 2003 when the disc will have its maximum apparent size of 25 seconds of arc. In June 1999, Mars is in the constellation of Virgo, close to the blue-white, first magnitude star, Spica. This means that the planet, which will reach a magnitude of - 1.1 in early April and -1.6 to -1.7 by the end of the month, will be easily visible and high enough above the horizon to be seen after 10 pm (11 pm BST !).
Missions to Mars
NASA very cleverly focussed the public's imagination on the possibility of life on Mars by its announcement in 1996 that several meteorites found in Antarctica, showed possible traces of fossilized bacteria and that these meteorites had been blasted from the surface of Mars by meteor impact millions of years ago. Whether the fossils are evidence of Martian life-forms or not, the search for extra-terrestrial life is one of the main factors in the current program of missions to Mars. During the next decade, Mars is due to be visited by more spacecraft than any other planet in the solar system. Spacecraft are scheduled to be launched at each close approach of the planet, that is roughly every two years, leading perhaps to a manned landing by the year 2014.
In 1996, NASA launched Mars Pathfinder and Mars Global Surveyor; the Russian Mars '96 probe unfortunately failed to leave Earth-orbit and crashed off the coast of Chile with 270 grams of plutonium onboard. The next group consists of NASA's Mars Climate Orbiter (MCO) and Mars Polar Lander (MPL) collectively known as Mars Surveyor '98 plus a Japanese spacecraft called Nozomi ( originally Planet B ) which, due to an engine malfunction, will not arrive until 2004 when it will be the first Japanese spacecraft to visit another planet.
MCO is due to arrive on 23 September 1999 when it will study the distribution of water on Mars using an infra-red radiometer and a camera which will take daily snapshots of the planet's surface with a resolution of 40 metres. In December, MPL will jettison two small probes which are designed to penetrate the surface to a depth of 2 metres in a search for water ice. The lander will descend through the thin atmsophere, taking pictures on the way down to a landing at the edge of the South Polar Cap where a robot arm will sample the soil to a depth of 0.5 to 1 metre.
A series of additional Surveyor missions is planned for 2001, 2003, and 2005, when a sample return mission will return soil samples for analysis on Earth. Strict control procedures are being put in place to ensure that no Martian microbes are released into the environment ( see H G Wells !)
Surface features
Although Mars is only half the size of Earth, its surface area is about the same as that of the land surface area of Earth. Much of the surface is very old and heavily cratered, but there are also much younger rift valleys, ridges, hills and plains. The southern hemisphere of Mars is predominantly ancient cratered highlands somewhat similar to the Moon. In contrast, most of the northern hemisphere consists of plains which are much younger, lower in elevation and have a much more complex history. An abrupt elevation change of several kilometers seems to occur at the boundary. The reasons for this global dichotomy and abrupt boundary are unknown (some speculate that they are due to a very large impact shortly after Mars's accretion). Certain features are worthy of note :
The interior of Mars is known only by inference from data about the surface and the bulk statistics of the planet. The most likely scenario is a dense core about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth's and a thin crust. The lack of a global magnetic field indicates that Mars's core is probably solid. Mars's relatively low density compared to the other terrestrial planets indicates that its core probably contains a relatively large fraction of sulfur in addition to iron (iron and iron sulfide). Like Mercury and the Moon, Mars appears to lack active plate tectonics; there is no evidence of horizontal motion of the surface such as the folded mountains so common on Earth. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This, along with the lower surface gravity, may account for the Tharis bulge and its enormous volcanoes.
There is very clear evidence of erosion in many places on Mars including large floods and small river systems. At some time in the past there was clearly water on the surface. There may have been large lakes or even oceans. But it seems that this occurred only briefly and very long ago; the age of the erosion channels is estimated at about nearly 4 billion years. (Valles Marineris was NOT created by running water. It was formed by the stretching and cracking of the crust associated with the creation of the Tharsis bulge.) Early in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth's plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun. Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for months. Although its atmosphere is mostly carbon dioxide (like Venus's), the greenhouse effect on Mars is strong enough to raise the surface temperature by only 5 degrees (K).
Mars has permanent ice caps at both poles composed mostly of solid carbon dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the northern summer the carbon dioxide completely sublimes, leaving a residual layer of water ice. It's not known if a similar layer of water ice exists below the southern cap since its carbon dioxide layer never completely disappears. The mechanism responsible for the layering is unknown but may be due to climatic changes related to long-term changes in the inclination of Mars's equator to the plane of its orbit. There may also be water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the polar caps changes the global atmospheric pressure by about 25% (as measured at the Viking lander sites).Recent observations with the Hubble Space Telescope have revealed that the conditions during the Viking missions may not have been typical. Mars's atmosphere now seems to be both colder and dryer than measured by the Viking landers.