|
In many cases astronomical phenomena viewed from the planet Mars are the same or similar to those seen from Earth but sometimes (as with the view of Earth as an evening/morning star) they can be quite different. For example, because the atmosphere of Mars does not contain an ozone layer, it is also possible to make UV observations from the surface of Mars. ==Seasons== Mars has an axial tilt of 25.19°, quite close to the value of 23.44° for Earth, and thus Mars has seasons of spring, summer, autumn, winter as Earth does. As on Earth, the southern and northern hemispheres have summer and winter at opposing times. However, the orbit of Mars has significantly greater eccentricity than that of Earth. Therefore the seasons are of unequal length, much more so than on Earth: In practical terms, this means that summers and winters have different lengths and intensities in the northern and southern hemispheres. Winters in the north are warm and short (because Mars is moving fast near its perihelion), while winters in the south are long and cold (Mars is moving slowly near aphelion). Similarly, summers in the north are long and cool, while summers in the south are short and hot. Therefore extremes of temperature are considerably wider in the southern hemisphere than in the north. The seasonal lag on Mars is no more than a couple of days,〔(radiative time constant )〕 due to its lack of large bodies of water and similar factors that would provide a buffering effect. Thus, for temperatures on Mars, "spring" is approximately the mirror image of "summer" and "autumn" is approximately the mirror image of "winter" (if you consider the solstices and equinoxes to be the beginnings of their respective seasons), and if Mars had a circular orbit the maximum and minimum temperatures would occur a couple of days after the summer and winter solstices rather than about one month after as on Earth. The only difference between spring temperatures and summer temperatures is due to the relatively high eccentricity of Mars's orbit: in northern spring Mars is farther from the Sun than during northern summer, and therefore by coincidence spring is slightly cooler than summer and autumn is slightly warmer than winter. However, in the southern hemisphere the opposite is true. Of course, the temperature variations between spring and summer are much less than the very sharp variations that occur within a single Martian sol (solar day). On a daily basis, temperature peak at local solar noon and reach a minimum at local midnight. This is similar to the effect in Earth's deserts, only much more pronounced. It is interesting to note that the axial tilt and eccentricity of Earth (or Mars) are by no means fixed, but rather vary due to gravitational perturbations from other planets in the solar system on a timescale of tens of thousands or hundreds of thousands of years. Thus, for example Earth's eccentricity of about 1% regularly fluctuates and can increase up to 6%, and at some point in the distant future the Earth will also have to deal with the calendrical implications of seasons of widely differing length (and the major climate disruptions that go along with it). Aside from the eccentricity, the Earth's axial tilt can also vary from 21.5° to 24.5°, and the length of this "obliquity cycle" is 41000 years. These and other similar cyclical changes are thought to be responsible for ice ages (see Milankovitch cycles). By contrast, the obliquity cycle for Mars is much more extreme: from 15° to 35° over a 124,000-year cycle. Some recent studies even suggest that over tens of millions of years, the swing may be as much as 0° to 60°.〔(The Obliquity of Mars )〕 Earth's large Moon apparently plays an important role in keeping Earth's axial tilt within reasonable bounds; Mars has no such stabilizing influence and its axial tilt can vary more chaotically. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Astronomy on Mars」の詳細全文を読む スポンサード リンク
|