On the 28th November 1659 Christiaan Huygens observed Mars with the relatively new invention of the telescope and became the first person in history to document a feature on another planet’s surface. He did not have sufficient magnification to discern much detail but he could track and sketch a dark triangular shape which appeared on each rotation of Mars. As telescope technology improved cartographers raced to name the area, resulting in the dark patch ending up with at least four different names. This confusion came to an end with Giovanni Schiaparelli’s classic map of Mars produced in 1877. In his work Schiaparelli called the feature Syrtis Major and today we call the region Syrtis Major Planum (planum meaning a highland plain).
Huygens could use Syrtis Major Planum as a marker to work out how fast the planet was rotating. Calculating day length this way is known as the sidereal day, Huygens value of 24.5 hours is impressively close to the value we have today (24 h 37 m 22 s). However, the sidereal day is not very suitable for timekeeping as it doesn’t account for the movement of the planet around the sun. A day length calculated by the time between when the sun is directly north or south from your position (depending on which hemisphere you are in) is called the solar day and ensures the sun is always directly above at noon. Solar day is often abbreviated to sol, a Mars sol is 24h 39m 35s, and it is the standard way of measuring day length on Mars. The current convention is for a Mars mission’s life to be counted by sols from the landing date, known as Sol 0, for example Curiosity Rover is currently on Sol 520 and Opportunity Rover Sol 3553.
Accounting for the time difference between an Earth day and a Mars day is a real headache for the engineers and scientists working on a Mars mission. One approach is for people to be given an extra watch that runs at the slower rate of a Mars sol. You can read a fantastic article about the engineering of these watches by clicking here.
The evolution of telescopes allowed Syrtis Major Planum to be studied in greater detail. Variations in the size of the dark coloured region could be seen between different observations. This led some to believe it could be evidence of water or vegetation on the planet and the changes in the size of dark patch were perhaps due to seasonal effects. However, the first flyby missions in the 60s revealed that Mars was definitely not an environment where abundant liquid water and life could currently exist on the surface. This left two possibilities; the dark patch was most likely either an impact crater or a volcanic province. There was also the question of why the region appeared darker relative to its surroundings.
As technology advanced even further and flyby missions evolved to orbiting spacecraft and landers high resolution maps and ground truths could be established. One way an orbiter can examine the planet below is to measure the gravitational pull it experiences as it passes over different regions. Large impact craters on Mars are distinguished by a positive anomaly in the gravitational pull; Syrtis Major Planum has no such anomaly and has a gravitational signature similar to more obvious volcanic provinces seen elsewhere on Mars.
Syrtis Major Planum is in fact an enormous but very flat shield volcano that extends 1,300 km across. Shield volcanoes are typically the largest types of volcanoes we see on Earth and Mars and are so called as their extensive and gentle slopes look like a shield laid flat against the ground. The slopes of Syrtis Major are extremely gentle and slope at less than 1° and its layered lavas are only around 1 km thick. Its slopes are exposed to the strong Martian winds, which strip away the red dust that covers most of the planet and constantly exposes fresh darker rock. This doesn’t happen on other volcanic provinces as they are generally much taller and the atmosphere is too thin for the winds to be strong enough to displace the dust that settles down on them. Movement of the Martian dust on the edge of Syrtis Major Planum would make its extent appear to vary in size for Earth based observers. Two volcanic craters were discovered in the centre of the province, Nili Patera and Meroe Patera. Patera is the terminology Mars scientists use for caldera, a caldera is a large volcanic crater that forms over emptying magma chambers.
The story of how Syrtis Major Planum was finally understood is a good analogy for our exploration of Mars. The shifting dust across the Martian surface confuses and obscures the interesting geology underneath. Vicious dust storms have even swallowed a few lander missions which were never seen again. Curiosity Rover is equipped with a little brush so that it can access the rock underneath (in NASA speak this is a Dust Removal Tool). Mars is not an easy planet to explore but with each hurdle that is overcome we uncover more about our diverse and fascinating neighbour.