IN STARRY SKIES

Evening Post, Volume CXVIII, Issue 53, 31 August 1934, Page 16

IN STARRY SKIES

(By "Omega Centauri.")

MARS

Although-ono of the smallest of the planets, Mars has attracted more popular interest than any celestial body except the sun and the moon. This is doubtless due to the fact that somo astronomers have considered' it, amongst all known bodies outside ' the earth, the most likely, homo of intolligent life. Even-in this respect its claims rest on rather an insecure foundation. We have already seen how astonishingly little we know about the.physical conditions on our neighbouring worlds Mercury and Venus. It is rather a strange fact that we have much, more certain knowltidgo about the distant stars than wo havo about any, planot 'in the solai' system. Tho reason is that most of our information comes to us in'vravos of light. Now tho stars omit the light by which we see them, but tho planets shine almost entiroly by reflected sunlight modified to. a certain extent, by tho earth's atmosphere and that' of tho planets themselves. If tho light is reflected by high,, clouds in. .a planet's atmosphoro it tells us nothing nf what lies below. ■"'.-■

Mars is, however, tho,planet whose surface it is easiest for man to observe.

The moon, Mercury, and Mars arc to some extent alike. Being so much smaller than the earth, they have had greater- difficulty, in retaining an atmosphere. The moon lias lost almost completely any that it originally had. Mercury also has little loft, and that of Mars is very thin in,, comparison with our air. The thinness of the atmosphere and the general absence of <-louds removes one great difficulty that appears insuperable in the case of Venus, and that stands in the way of the accurate observation of surface details of most other planets. In the vcase of Mercury astronomers encounter a different obstacle. The planet always nppoars inconveniently close 'to the sun, and when nearest to us it presents only its dark side. The moon is ideally situated for observation, and many of the features, on the side turned towards us, are almost as well, known as those on earth. Mars is nearly as favourably situated except for the fact that it is usually about COO, and never less than 147, times as far away. If wo. imagine J,he moon removed to this distance wo shall appreciate tho difficulty of interpreting rhe features of Martian scenery. Mars has a diameter of 4216 miles, nearly twice that of the moon, and 53 per cent, of that of tho earth. . Its surface, volume, mass, and surface gravitational force aro 28, 15> 11, and 38 per cent, respectively of tho > terrestrial values. Its albedo, or reflecting power, although double that of tho moon or Morcury, is only, one-quarter of that of Venus, or a third of that of the earth. Its surface must therofbro be much more like the lunar than the terrestrial one. -The small bodies of the'solar system would naturally cool much more rapidly than tho larger ones. Then, losing their atmospheres, they.:would bo more exposed to meteoric bombardment.. The surface of the moon thus gives us some idea of what those of Mercury and Mars must be. The bombardment would bo.more intense on Morcury, and less intense on Mars, owing" to the speed of the meteors increasing as they approach the' sun. - .

The mean distance of Mars from the sun: is ■ 141,500,000' miles, or 1.524 'astronomical units. The actual distance varies by 13,000,000 miles each way, ranging from about 128,000,000 to 154,000,000 miles. The distance from the earth varies much more widely. Tho best time to obscrvo Mars is when it is in opposition and so on ihe meridian at midnigTit. Besides being nearer to us. then than at other times, itgifulldise is illuminated. The actual' distance at ■opppsitipn-.range's fr0m.35,000,000-tb;6i;000,000 miles;. In August, 1924, Mars was nearer 'to vus than it had been for a century prithan it will be for anotlier" hundred years. The disc then appeared nearly,;twice; as large as at an unfavourable opposition j

and nearly seven times as large as at an unfavourable conjunction. Mars completes, oue revolution round the sun in just under 687 days, and the earth catches up one round in 780 'days'.- This is tho longest of the synodic periods. Tho orbit is inclined only one degree fifty-one minutes to the ecliptic, so that Mars seems to follow closely the path of tho sun against tho background of stars. The rotation period is'known with precision, and is usually stated to within, a fraction of a second as 24dcg' 37iniu 2i!.sSsec, which is astonishingly close to the lougth of oiu- day. Tho inclination of the planet's equator to the plane of its. orbit is not known so ■ accurately, but ' it' does not differ much' £ rom that of the earth. The estimates vary from 23} degrees ,to 25 degrees 10 minutes. When nearest to us the south pole of the planet- is turned towards us, so , the southern hemisphere is better known than the northern. Mars has seasons similar to ours, but longer, colder, and more accentuated. The atmosphere is known to contain small quantities of oxygen and water-vapour, and although there arc very few clouds the polar caps vary in size with the seasons. These must be very thin, for they sometimes dc-j velop in two. or- three days over an j area extending from 20 to 30 degrees from tli o pole. After showing brilliantly throughout the . winter; each polar cap diminishes slowly in sizo during tho spring' of that Martian hemisphere,* taking at least five or six of our months to disappear. Even assuming that the polar caps consist of snow, we cannot say that the tcin-

i porature is above the freezing, point i when the. caps disappear, for snow 1 evaporates' when the temperature is .below zero Fahrenheit if the vapour ; pressure'is sufficiently low. Tho.tem.peraturo' of the Martian surface is a subject on which there are still great 'differences of opinion, which "will be referred to later. In considering the evidence afforded by telescopic observation we nrnst remember that the apparent Martian diameter must be magnified from 75 to 540 times to make the ,disc appear as large as that of the moon to the naked eye, and the magnification must bo twice as great as that to enable us to see surface details as well as we do those of the raooii "without any optical aid. When we see drawings, of the surface, or enlargements of actual photographs, we must remember that the actual images studied, of the original photographs from which the enlargements were made were much smaller than a threepenny'piece. The direct focal image of Mars in the great'Yorkes refractor at the most favourable opposition was less than a tenth of an inch in diameter, and that with the Lowell telescope, whose revelations thrilled the world, was less than half as large. When an enlarging lens is placed in tke path of the rays it mak^s th* ira&ge fainter and necessitates n correspondingly longer exposure. . The planet Mars was directly associated with one of the greatest advances ever made in astronomy. From his study of tho observations of T-ycho Brahe, Kepler found that the Martian orbit is not circular but elliptical. An enormous amount of laborious resparch enabled him to deduce his celebrated laws of planetary motion, which became the sure foundation of gravitational astronomy. In 1877 a remarkable disco%rery was made by Asaph Hall, of Washington, lie found two extremely small satellites revolving round Mars in astonishingly small orbits lying approximately in tho plane of the planet's equator. Their size can be judged only-by the a-niount of light.they reflect, and most estimates give the diameters as only ton or fifteen miles. Phobos is unique in the solar system in revolving in a period shorter than that of the rotation of the planet, so that from Mars it would appear to rise in the west and set in 1 the east. Its distance from the surface 1 of the planet is only 3742 miles and < it would be invisible from latitudes *■ ■greater than G8 degrees 15 minutes. It ' revolves in less than 7hr 40min. Deimos is much fainter- and. probably smaller l than Phobos,' and revolves in 30hr lSmin at a distance 'of 14,C00' miles from tho centre; of the planet. : Owing to tho rotation of Mars Deimos would appear to take more than 131 of our f hours to return to the meridian; whilst s, Phobos, going the other1 way, takes c only 11. hours 7 minutes. ' r ..,' \