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A planet for all seasons

William Herschel (1738-1822) had many opportunities to observe Mars in 1783 with the aid of his unrivaled reflecting telescopes. Below is a picture of Herschel's grand telescope:

Encyclopaedia Britannica Vol 18, "telescope", 1797

This is an approximate view of what Mars could have looked like during ideal conditions. Notice anything interesting?

This view shows what Mars looks like through the eyepiece of an 8" telescope. Image: Ron Wayman

Herschel noticed both of Mars’ polar caps, as well as the slight tilt in the axis of Mars’ rotation. With the optical power of his telescopes, Herschel was able to give the most detailed evidence that Mars, like Earth, experiences seasonal changes.  Herschel noted and measured that Mars has an axial tilt of 30 degrees. Today we know that the actual value of this axial tilt  is 25.19 degrees.

Herschel's figures . Image: University of California Irvine

Herschel showed that the tilt of Mars’ axis was very similar to Earth’s (23.4 degrees), and that Mars’ seasons must occur for the same reasons. However, due to the known orbital period (687 days), Herschel suggested that these seasons should occur at twice the length of those on Earth. Remember, it's the tilt of the Earth which causes us to experience distinct seasons. 

Notice the temperature change based on the angle at which sunlight is hitting the Earth. Image: Peter Collingridge

Need more information on seasons?  Check out Sal's playlist here

In 1784, Herschel reported to the Royal Society:

"The analogy between Mars and the earth is, perhaps, by far the greatest in the whole solar system.  The diurnal motion is nearly the same; the obliquity of their respective ecliptics, on which the seasons depend, not very different … the bright polar spots are owing to the vivid reflection of light from frozen regions; and that the reduction of those spots is to be ascribed to their being exposed to the sun."

Mars began to look more and more like our twin planet…

Mars Mania

A good time to get a close-up look of Mars is during opposition. This celestial event occurs about every 26 months when Mars and the Sun are on directly opposite sides of Earth (you could draw a straight line from the Sun to Earth to Mars when they are in this orbital configuration). However, the very best opposition occurs when Mars is also at its closest point to the Sun (perihelic opposition). This special alignment happens every 15 - 17 years. Even then, there are variations in distances. 

Example of two perihelic oppositions. Image: Peter collingridgeAncient observers eagerly anticipated perihelic oppositions, because they were ideal times to test out the best telescopes available. On September 5, 1877, Mars came to a perihelic opposition in the constellation Aquarius, approaching to within 35 million miles (56 million km) of the Earth. During this event Italian astronomer Giovanni Virginio Schiaparelli used a powerful telescope to produce the first detailed map of Mars.

Giovanni Virginio Shiaparelli, Meyers Konversations-Lexikon, 1888

In this picture, we can see that he identified and named several features known today.  Hellas (Hellas = Greece), for example, is the largest visible impact crater in the solar system. As he tried to resolve and describe more and more detail, Schiaparelli hypothesized about the features he observed.  Schiaparelli used the Italian word for channels ("canali") to describe streaks he recorded on the Martian surface. Others mistranslated this description into English as "canals," which implies structures constructed by intelligent life, in this case supposed “Martians.” His observations lead to a Mars mania which swept through Europe and America.

Percival Lowell observing from his refracting telescope, installed in the summer of 1896 at the Lowell Observatory. Image: Joe Haythornthwaite

Driven by these new mappings of Mars and his own enthusiasm for the idea of intelligent life on Mars,  Percival Lowell built the Lowell Observatory at Flagstaff, Arizona, making his first observations of Mars in 1894. His notebooks show that he observed bright and dark areas, a hint of polar caps, and canals. He assumed that an intelligent society of Martians constructed them. Below is an actual photo of Mars taken in 1907 from Lowell’s Observatory:

Actual photo of Mars taken in 1907. Image: Lowell Observatory

Lowell imagined this world as an arid desert, similar to the Earth’s desert. As he put it:

“Mars has an atmosphere; we have reason to believe this atmosphere to be very thin,--thinner at least by half than the air upon the summit of the Himalayas,--and in constitution not to differ greatly from our own.”

And of water he wrote:

“After air, water. If Mars be capable of supporting life, there must be water upon his surface; for, to all forms of life, water is as vital a matter as air. On the question of habitability, therefore, it becomes all-important to know whether there be water on Mars.”

Early telescopic observations of Mars showed dark surface features that changed over time. Some interpreted these changing patterns as vegetation growing and dying with the seasons on Mars.

Ground-based, telescopic image of Mars. Image: Lowell Observatory

This animation shows actual images of the surface of Mars during the period from August 3, 2003 to September 5, 2003.

Surface of mars over one month. Image: Tom ruen

Myths abound, what was needed was scientific evidence. Observations which can be repeated and confirmed by others. First we simply wanted to see our twin in detail, then take its temperature, reach out and touch it, smell it and determine what it’s made of and who or what, if anything lived there. To do this extensive study, we needed to extend our reach in ways that would have been unimaginable before the space age.