Close to the Sun
Mars is slowly coming towards summer in the southern hemisphere and the same time to the perihelion on its orbit around the Sun.
This project is focused on fans in Martian southern polar areas. But do you know why southern? There are similar features in the northern polar areas, but they are much smaller. In fact, for quite some time scientists believed that the kind of activity that produces dark fans and blotches (cold CO2 jets) did not happen in the north. They thought so, because they could not see any signs of it in the north. Or better to say, they could not resolve it. At that time, from 1996 to 2005, the images of martian ground were coming from the Mars Orbiter Camera (MOC) and their highest ground resolution was 1 m/pix. It is enough to resolve large southern fans, but just not enough for the northern smaller ones! Only when HiRISE came around and imaged northern dunes, we saw that there are blotches and fans too. So, why the scales of them are so different? There might be several explanations. Below I will offer you one, which is probably the first to think about.
Martian seasons in southern and northern hemispheres are not equal.
Mars has elliptical orbit, its eccentricity is 0.0934 (e = 0 would be a circle). It is small, but in the planetary scale it takes Mars some 42 million kilometers closer to the Sun in its closest approach than in the furthest position on the orbit. The closest approach is called perihelion, from Greek “near the Sun”. It happens during summer in the southern hemisphere.
So, Mars is closer to Sun when it is summer in the south – this means during southern summer it gets more solar energy than during northern. Unlike our Earth, Mars does not have a huge water reservoir of the oceans to dampen temperature variations. These 2 facts together lead to that southern summer is hotter than the northern. But how does this affects what happens in spring? In two ways: first, the amplitude of change from cold dark winter to hot bright summer is larger in the south. And second: Mars is moving faster on its orbit when it is closer to perihelion. So the changes happen faster!
Every day in spring the amount and strength of sunshine increases. In the north this increase is steady but slow. It probably makes seasonal ice layer subliming steadily from the top faster than creating under-ice gas cavities that burst and create cold CO2 jets and associated fans. While in the south every day energy increase is more like a jump: getting these bursts makes for higher probability of under-ice explosions. And lets us observe beautiful fans!