Mars

Mars

The planet Mars is named after the Roman god of war because of its red colour. It orbits the sun once in nearly two years at an average distance of about 142 million miles. It is brightest and most easily observed when it is in opposition, or in a direction opposite to that of the sun, but even then it can come no closer than about 35 million miles. The large distance, coupled with the planet’s comparatively small diameter of 4,200 miles, makes all but the coarser surface markings difficult to detect with earth-based telescopes.

During the Mars night and especially in the polar regions, the ground temperature falls well below the freezing point of water.

Mars. Photo by NASA in public domain

The red color of Mars is thought to be due to the existence of large deserts of orange-red dust. Any oxygen the planet once had in its atmosphere is now probably imprisoned in the surface rocks in the form of iron oxides. Carbon dioxide and water vapour have been detected in the Martian atmosphere, but the amount of water vapour is so small that it is were all turned into water and would cover the planet to a depth of only three-thousandths of an inch. White, high-altitude clouds, presumably composed of ice crystals, occasionally drift over the surface or appear beyond the planet’s edge or limb. A yellow haze, thought to be swirling dust clouds, sometimes obscures large areas for days or even weeks.

Mars has two white polar caps and various permanent dark areas. The caps change considerably in size with the Martian seasons, being largest in winter and smallest in summer. The dark areas also change with the seasons: a few change hardly at all, but the rest tend to darken as the polar cap in their particular hemisphere gets smaller. The effect is as if a wave of darkening were moving from the polar cap toward the equator.

A Parched World

The gravitational pull of Mars is much less than that of the Earth: an astronaut who weighed 140 pounds of the Earth would weigh only 56 pound on Mars. Consequently the Martian atmosphere is thin – so thin, perhaps, as to rule out all known forms of life. We have good reasons for thinking that the general climate on Mars would be similar to that on a dry, cold desert some 11 miles above the Earth’s surface.

The combined length of day and night on Mars is slightly longer than on the Earth. But during the day, at the planet’s equator, the temperature reaches a high of only 30 degrees centigrade.

Canals on Mars?

These changes encouraged the idea that Mars possessed some form of vegetation partly nourished by flows of moist air from the shrinking polar caps. One astronomer, Percival Lowell, suggested that the moisture took the form of water which moved towards the equator through artificial waterways. He made an intensive study of Mars during its close approach or opposition of 1894-1895, and concluded that the certain dusky streaks, first seen in 1877 by the Italian astronomer Giovanni Schiaparelli, were canals designed and built by intelligent Martians.

Although he drew them as forming a highly geometrical network of straight lines, he did not claim to see the canals themselves, but merely the vegetation growing on their banks.

Lowell’s conclusions have no foundation in the light of modern observations, but the precise nature of the dark areas still remains a mystery. One modern view, based largely on radar studies, is that they are high plateaux, comparatively free from dust. Another is that they are areas where alternate freezing and thawing on the ground has produced a highly porous surface. The polar caps certainly cannot produce large quantities of free water, for the atmospheric pressure on Mars is so low that ice and snow would not melt but sublime, or pass directly from the solid to the vapour state. The caps are undoubtedly quit thin, but whether they are deposits of snow, hoar frost, solid carbon dioxide, or a combination of all three, is still an open question.

Mariner IV. Photo by NASA in public domain

Mariner IV

If canals bordered by fringes of vegetation exist on Mars they should have shown up on the close-up photographs taken in July, 1965, by Mariner IV. As this little planetary probe passed within 9,000 miles of Mars its camera photographed sections of a long, narrow strip of the planet’s surface. Much to everyone’s surprise, the photographs showed dozens of ring mountains similar in appearance to some of the craters on the moon. Nearly 12 per cent of the well-defined rings have central peaks, but several others are ill-defined, as if almost eroded away by dust-laden winds. Studies of the photographs have led some experts to interpret certain linear features as mountain ridges similar to those found on the beds of oceans on the Earth. But any oceans Mars might have possessed dried up long ago.

Mars has two tiny satellites, discovered by the American astronomer Asaph Hall in 1877 and named Phobos and Deimos. Strangely enough, the existence of these flyweight moons was mentioned by Jonathan Swift in his Gulliver’s Travels some 150 years before Hall’s discovery. Phobos, 10 miles in diameter, flies around Mars in seven hours, 39 minutes at a distance of 5,800 miles from the planet’s center. It therefore revolves roughly three times faster than Mars rotates and consequently travels across the Martian sky in a west-to-east direction. Deimos, five miles across, has a period of revolution of 30 hours, 18 minutes and is 14,600 miles from the planet’s center. Since the period is only slighter longer than the Martian day. Deimos moves slowly across the Martian sky in an east-to-west direction.

Meteors and Meteorites

Meteors and Meteorites

Meteors

No one can watch the night sky for long on a clear, moonless night without noticing one or more meteors, or sudden streaks of light popularly called “shooting stars”. These events are not stars at all but tiny pieces of stone and iron generally no larger than a grain of sand. Millions of these grains bombard the earth every day, but we are protected from them by the blanket of the Earth’s atmosphere. The grains travel so fast that they are entirely vaporized at heights of about from 80 to 25 miles by the heat of friction with the thin upper air. The great majority are too small to give rise to any visible effect, or for that matter, to undergo appreciable heating, and even the bright ones are no larger than small pebbles.

Meteorites

On rare occasions, quite sizable chunks of meteoritic materials penetrate the earth’s atmosphere to bury themselves in the ground or to explode into a number of fragments above the ground. The former are called meteorites, the latter, fireballs. The larger known meteorite weighs about 79 tonnes and still lies partly buried where it fell near Grootfontein, Namibia. But this is only a tiny fragment compared with others that had excavated great hollows or craters. One of the best preserved formations of the kind is the Barrenger crater near Winslow, Arizona. It has a diameter of about four-fifths of a mile and a floor near 1000 feet below the level of the plain. An immense number of iron fragments have been collected in the area, but the main mass, if it still exists as one whole piece, must be deeply buried.

Meteoritic craters in Canada

Several large craters in Canada are thought to be meteoritic in origin. They are extremely old and have suffered much more than the Barringer crater from the effects of erosion over geological time. Notworthy among them is Pingualuit (formerly Chubb) crater in Northern Quebec, a tremendous circular hollow nearly two miles wide and nearly one quarter of a mile deep. The crater was first investigated in 1950 by an expedition organized by the Royal Ontario Museum in partnership with the Globe and Mail Limited, and later with the National Geographic Society. Since then the Dominion Observatory, Ottawa, has done extensive research on other larger hollows, and the Holleford and Brent craters in Ontario and the Deep Bay crater in Northern Saskatchewan are now generally considered to be meteoritic.

Aerial view of Pingualuit Crater (formerly Chubb Crater), Ungava, Quebec. Most of the time floating ice covers its surface.

Meteor showers

The most beautiful and arresting aspect of meteors occurs when the Earth sweeps into the path of a really dense swarm of meteorites. Shooting stars then streak across the night sky like snowflakes driven by a cosmic wind.

Some of these “showers” are associated with comets. One of the most interesting of these associations is that between Comet Tempel-Tuttle 1866, a faint comet with a period of just over 33 years, and an annual shower known as November Leonids. The latter are so named because they appear to originate in a point in the constellation of Leo. They gave particularly magnificent displays in 1833, 1866, and as recently as 1966. The fact that the shower is an annual event indicates that the meteors are strewn along the orbit of the comet. But whether they always have had an independent existence or represent material shed by the comet remains an open question.

The Leonid shower of 1833. This depiction of the meteor storm was produced in 1889 for the Seventh-day Adventist book Bible Readings for the Home Circle

Comets

Comets

Although no longer regarded as omens of death and destruction, comets have a fascination all of their own. They appear to come from nowhere, usually move across parts of the sky well removed from the zodiac, and then gradually fade from view. Yet despite this strange behavior most comets, if not all, seem to be regular members of the solar system. They move in elliptical path with the sun at one focus, and therefore accompany the sun and its family of planets on their journey through space.

Comets differ not only in size in appearance, but also in the time they take to orbit the sun. Some have comparatively short periods of revolution. Comet Encke, for example, a small comet discovered in 1818 by the German astronomer Johann F. Encke, orbits the sun once every 3 ½ years. Comet Halley, last seen in 1986, has a much longer period of about 77 years, while comet Mrkos 1957 is thought to travel 28 times farther from the sun than Pluto and to have a period of 13,000 years.

Composition of a comet

A comet is made mostly of ice – water H2o ice, with a little methane (CH4) ice, and some ammonia (NH3) ice. Striking the Earth’s atmosphere, a modest cometary fragment would produce a great radiant fireball and a mighty blast wave, which would burn trees, level forests and be heard around the world. But it might not make much of the crater in the ground. The ices would all be melted during entry. There would be few recognizable pieces of the comet left, perhaps only a smattering of small-grains from the non-icy parts of the cometary nucleus. The Soviet scientist E. Sobotovich has identified a large number of tiny diamonds strewn over the Tunguska site. Such diamonds are already known to exist in meteorites that have survived impact, and that may originate ultimately from comets.

On many a clear night, if you look patiently up at the sky, you will see a solitary meteor blazing briefly overhead. On some nights you can see a shower of meteors, always on the same few days of every year – a natural fireworks display, an entertainment in the heavens. These meteors are made by tiny grains, smaller than a mustard seed. They are less shooting stars than falling fluff. Momentarily brilliant as they enter the Earth’s atmosphere, they are heated and destroyed by friction at a height of about 100 kilometers.

Meteors are the remnants of comets. That meteors and meteorites are connected with the comets was first proposed by Alexander von Humboldt in his broad-gauge popularisation of all of science, published in the years 1845 to 1862, a work called Kosmos. It was reading Humboldt’s earlier work that fired the young Charles Darwin to embark on a career combining geographical exploration and natural history. Shortly thereafter he accepted a position as naturalist aboard the ship H.M.S. Beagle, the event that led to The Origin of Species.

Old comets, heated by repeated passages near the Sun, break up, evaporate and disintegrate. The debris spreads to fill the full cometary orbit. Where that orbit intersects the orbit of the Earth, there is a swarm of meteors waiting for us. Some part of the swarm is always at the same position in the Earth’s orbit, so the meteor shower is always observed on the same day of every year. June 30,, 1908 was the day of the Beta Taurid meteor shower, connected with the orbit of Comet Encke. The Tunguska Event seems to have been caused by a chunk of Comet Encke, a piece substantially larger than the tine fragments that cause those glittering, harmless meteor showers.

Comet tails

Comets now are being discovered at the rate to five or six a year, but most of them are faint, telescopic objects. A really large and bright one is a comparatively rare but most spectacular addition to the night sky. As it moves towards the sun it groves from a faint, fuzzy patch into an object of great glory, throwing out one or most tails that stretch far across the sky as if in honour of the sun. As its swings away from the sun the tail or tails lead the way but shorten, and eventually the entire object fades in obscurity. A comet’s tail can stream to a distance of millions of miles from the brightest part of “head” but always points more or less directly away from the sun.

A large comet has been described as a “bag full of nothing”. The aptness of this was well shown in 1910 and 1986, when the earth passed through the tail of comet Halley. Nothing unusual appeared in the sky, stars shone undimmed through the comet’s tail, and when the head passed between the earth and the sun, not a trace of it could be seen on the sun’s disk.

Halley’s Comet. Photo: Space.com

How the tails are formed

A comet’s trail consists of extremely thin gases (mostly compounds of carbon) and fine dust. It shines by luminescence induced by solar radiation energizing the gases, and also by sunlight reflected by the dust. The material is released by the nucleus, presumably under the heating action of the sun. It then is driven away in the form of a trail, partly by the pressure of the sun’s radiation and more forcibly by the combined action of the “solar wind” and the interplanetary magnetic field. Only the nucleus seems to be at all substantial, and even this, according to one theory, is no more than a collection of metallic and stony-iron particles embedded in the spongy mass of frozen gases.

Since comets are such light affairs they can easily be deflected from their paths by the planets. Massive Jupiter and Saturn are most effective in this respect, and in the past they have caused the period of Comet Halley to vary between 75.5 years and 79.4 years. Preliminary investigations indicate that it should again pass close to the Sun in 2063.

Comet Biela and the Bielids

Some comets actually have been seen to break up: in 1846, for instance, comet Biela broke into two distinct comets which then gradually separated. The twins returned in 1852, but did not reappear on subsequent occasions. Instead, in 1872, when the earth passed through the track of the lost comet, a fine shower of meteors occured. This was repeated at later returns, indicating that the comet had disintegrated. The shower, known as the Andromedes or Bielids, is now an annual event, but the hourly number of meteors has been consistently small for many years.

Comets have always evoked fear and awe and superstition. Image: Black Dark Blue Mosaic Art by © Megan Jorgensen (Elena)

Pluto

Pluto

Astronomers soon found that the gravitational effect of Neptune failed to account for all the irregularities in the motion of Uranus. Two American astronomers, Percival Lowell and William H. Pickering, undertook the formidable task of calculating the likely position of a hypothetical planet, and in 1930 Clyde Tombaugh, guided by Pickering’s predictions, discovered Pluto, the most distant of the Sun’s family of planets.

Pluto’s average distance from the Sun is nearly forty times that of the Earth. Yet because its orbit is quite elliptical – more so, in fact, than that of any other planet, it can come closer to the Sun than can Neptune. The two planets could not collide, however, since the planes of their orbits are inclined by about 15 degrees to each other. In fact, the inclination of the plane of Pluto’s orbit to the plane of the earth’s orbit (and therefore to the general plane of the Solar System) is just over 17 degrees. Pluto, unlike the other planets, can therefore wander outside the zodiac. Its orbital period is 248 years, and it will next be closest to the sun (and hence to the Earth) in 2237. Its distance will then be about 2,800 million miles, or less than that of Neptune.

 Orbit of Pluto

Very little is known about Pluto itself, but this is not surprising in view of its immense distance. Its diameter is thought to be less than 4,225 miles. If we assume for the planet an average density of four times that of water, its mass would be less than one-tenth that of the earth. At all events, it must be a barren and frigid world, the coldest and darkest planet in the solar system.

Neptune

Neptune

During the first half of the 19th century astronomers noticed that the actual position of Uranus was gradually falling behind its predicted position. The discrepancy indicated that a disturbing body in the form of an unseen planet was pulling Uranus “off course”, and two mathematicians, J. Couch Adams in England and Urbain J.J. Leverrier in France, independently calculated its most likely position. Their investigations ended in 1846 with the discovery of Neptune, found in a position close to the predicted places.

Neptune takes nearly 165 years to orbit the Sun, a period so long that the planet has not yet made one complete revolution since it was discovered. Its average distance from the Sun is 2,794 million miles, or about 30 times the corresponding distance for the Earth. In large telescopes is shows a definite disk, and measurements of this indicate an actual diameter of 27,800 miles, or slightly less than the Diameter of Uranus.

Neptune. Photo in public domain

Analysis of the sunlight reflected by Neptune shows the planet’s atmosphere is rich in methane, but since the temperature is thought to be as low as -270 degrees centigrade, any ammonia would be frozen solid. No belts can be seen, but there are two satellites – Triton, discovered by William Lassell in 1846, and Nereid, first seen on photographs taken by Kuiper in 1949. Triton has a diameter of about 2,300 miles and is therefore slighter larger than our moon. Nereid can be seen only on photographs taken with large telescopes, and is probably no more than about 200 miles across.