Skip to content
Skip to main content

About this free course

Download this course

Share this free course

The search for water on Mars
The search for water on Mars

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

1 Observing Mars from Earth

Mars appears like a star when observed with the naked eye, but it is impossible to discern any details. The first person to take a closer look at Mars with a telescope was Galileo Galilei, in 1609. As astronomers developed better telescopes, they were able to see distinct features on Mars and used these to calculate key information such as its rotation period and axial tilt. Table 1 shows some of the key data we now have about Mars, updated from the 1600s as technology and our understanding of the Solar System has improved.

Table 1 Comparison between Earth and Mars based on current data
  Earth Mars
Average distance from Sun (km) 1.50 × 108 2.27 × 108
Orbital period (length of year, Earth days) 365 687
Rotation period (length of day, hours:min) 23:56 24:37
Axial tilt 23.44° 25.19°
Equatorial radius (km) 6378 3397
Average surface temperature (°C) 14 −63
Atmospheric pressure* 1013 mbar/1 atm 7.5 mbar/0.006 atm
Atmospheric composition

carbon dioxide 0.04%

nitrogen 78%

argon 0.9%

oxygen 21%


carbon dioxide 95%

nitrogen 2.6%

argon 1.9%

oxygen 0.16%

carbon monoxide 0.06%

Satellites The Moon Phobos and Deimos


*The SI unit of pressure is actually the Pascal (Pa) and 1 atmosphere is 101 325 Pa.

On 13 August 1672, Dutch astronomer Christiaan Huygens observed a white spot at Mars’ south pole (Figure 3). This was the first sign that ice might be present on the planet, and was a finding corroborated by Giacomo Miraldi in the early 18th century. It is now known that there is also an ice cap at Mars’ north pole, and that the ice caps are made of a mixture of water ice and dry ice (frozen carbon dioxide). You will hear more about this later.

This figure has two parts. The upper part is a sketch of Mars shown as a circle. It is divided in two by a horizontal line through its middle, marking the equator. Some light grey shading indicates two areas across and south of the equator. This sketch is labelled: Aug. 6, 1672, 11h 00m past moon, CM = 213 degrees. The lower part is another sketch of Mars shown as a circle. Darker grey shading is shown across the equator and southern hemisphere, with a more distinct shape than in the top sketch. The sketch is labelled: Aug. 13, 1672, 10h 30m past moon, CM=290 degrees. Both circles also have a small mark at their top point, which signifies the south pole.
Figure 3 Christiaan Huygens’ sketch of Mars made 13 August 1672. He observed dark patches on the planet’s surface and a white patch at the south pole. The south pole on his sketch appears to be the north pole because the lenses in some telescopes produce images that are upside down.

Improvements in the size and quality of telescopes throughout the 19th century allowed more detailed studies of the red planet. Among the most famous, and significant in the search for water – and life – on Mars, is that of Giovanni Schiaparelli. His global map of the planet’s surface (Figure 4) displayed a number of straight lines, which he called ‘canali’ (Italian for channels).

This figure shows a hand-drawn map, about double the width compared to its height. Its title reads ‘Mars 1890’. The map area is divided into a grid pattern by horizontal and vertical lines, with a thicker horizontal line in the middle marking the equator. The top quarter of the map is mostly featureless and is labelled “Mare Austraale”. There are three circles in that area, named Thyle I and Thyle II, and Argyre II. Towards the bottom of the map, the drawing becomes more complex. In the second quarter from the top, dark shading surrounds two larger circles, one labelled Hellas and one labelled Thaumasia. The bottom half of the map is very different. There many random criss-crossing lines, no large circles, and only one small, shaded area in the bottom quarter. There are many labels on the map, including Hesperia and Arabia.
Figure 4 Global Mars map by Giovanni Schiaparelli in 1890 after a period of Mars observations starting in 1877. The most striking feature of the map are the straight lines, which Schiaparelli called ‘canali’ meaning channels but were mistranslated to canals implying their creation by a martian civilisation.
  • Can you think of reasons why astronomers of Schiaparelli’s time thought they saw lines on Mars?

  • The telescopes available in the 19th century were unable to distinguish small features on Mars and they therefore appeared to be thin lines. These were then interpreted as straight lines. Figure 5, taken with a telescope comparable to the one Schiaparelli used (although much more recently) gives you an idea what Mars may have looked like for observers during the late 19th century.

    This figure is a photograph taken through a telescope. It shows a large black square, with a tiny orange feature that covers less than 1 % of the area of the image. The feature is oval with a tiny white mark at its southern pole.
    Figure 5 Mars, taken using a telescope comparable to that used by astronomers in the 1800s. Image credit: NASA/Ron Wayman

The word ‘canali’ used by Schiaparelli was mistakenly translated to ‘canals’, which implied the presence of significant amounts of water and a civilisation capable of shaping the planet. Naturally, excitement grew around the theory of a martian civilisation. American astronomer Percival Lowell was its most fervent advocate and believed that the canals were swaths of vegetation and that the intelligent beings inhabiting the planet would resort to irrigation to survive the otherwise harsh conditions on Mars. However, the ‘canali’ proved to be elusive to other skilled observers, who argued that the straight lines were an optical illusion. In the early 20th century, Eugène Antoniadi was able to show that the features on Mars were of irregular shapes rather than straight lines and that there were no canals, or civilisations. Despite this, the search for water has continued and the next section of this course will help you understand why it remains central to many missions to the red planet.