Hi-Rise catches the start of "Avalanche Season" on Mars.
Below the remaining dry ice deposits, geysers are forming, spreading their tracery through the red sands:
Side by side images of sublimation spiders: (Left) a Hi-rise image shows the form of these interesting geomorphic features. (Right) An artist's (Ron Miller's) conception of what an erupting geyser at the nexus of these spiders would look like from the surface.
Above, the clouds and snows of summer will soon be here:
The arrival of clouds in Northern Hemisphere spring and summer are seen clearly in these two Viking images of the same region.
Will even the dunes start to move again, once their frozen overburden is removed?
Sand dunes in Mars' polar erg in the process of defrosting. The residual ice shows as blue in the colour-stretched Hi-Rise image.
The Phoenix Lander is revealed after weathering a Martian winter. Sadly the accumulated CO2 ice irreparably damaged the spacecraft.
What? This doesn't sound like the weather where you are on this September 13th*? Well it would be what you would feel and see if you were in the northern hemisphere of Mars! Mars has just passed solar longitude 0° as of today meaning that the vernal equinox is behind it and it's full speed ahead for the boreal solstice in another 200 days on March 30!
While there will be no flowers pushing their way up through the martian regolith, Mars does have crocus dates. But what do these mean? Aharonson et al defined CROCUS dates back in 2004 as being "Cap Recession Observations that reveal CO2 has Ultimately Sublimated" which is a nifty term for when the ground can once again breathe and interact with the atmosphere because the overlying CO2 seasonal ice cap has evaporated away. This meters-thick layer forms every year all the way down to 60° of latitude in some places:
View of the changing north polar cap of Mars from the planetary photojournal.The bright white region at left is the seasonal cap made up primarily of CO2, the remaining white at right is H2O ice. The dark band is the polar erg - a sand sea made up of dark dunes.
Despite the acronym, the timing is similar to the thawing of the ground in the terrestrial spring when our own meters-thick seasonal cap of water ice begins to evaporate. And for those of us who live in the middle to high latitudes, this event is one that is greeted by millions of impossibly hardy flowers called crocuses pushing their way up through the remaining snow:
Crocuses push up through recently defrosted northern latitude ground on the Author's way into work in 2009.
As on the Earth, this crocus date varies with latitude; the more northerly one travels, the later is the onset of thawed ground. At the lowest latitudes of the seasonal martian cap, around 60°N, the crocus date is about Ls=30° or mid November on Earth for this Martian year. From that time, the thawing will proceed northward, cluing up with the full reveal of the Northern Perennial Water-Ice cap a little after Ls=120° in early June, 2012 Earth time.
This delay in evaporation is just another way in which Mars is similar to the Earth. Here, we don't usually experience peak summer temperatures until late July and Mid August. Still, it is a surprising thing. After all, on our home planet, the delay is due to the time required to heat up the oceans and other bodies that can effectively store heat. But without much thermal inertia, how can the martian surface hope to hold out for long against a warming sun?
The solution can be found by examining the seasons on Mars. Since Mars' orbit is very eccentric (0.09 compared to 0.01 for the Earth), its distance to the sun is roughly as significant as its axial tilt. Both make about 30% worth of difference in terms of the amount of solar energy received at the surface. In northern spring, Mars is actually getting further and further from the sun and this moderates the rate at which the northern hemisphere warms:
The classic representation of Mars' orbit from Malin Space Science Systems.
You might also wonder: how can we tell when the ice all sublimates on Mars? Partly, we can tell from looking at the receding of the bright white seasonal cap from orbit. As well, we can see that same cap thin out in our measurements of elevation (or at least we could until the loss of MGS and the MOLA instrument). At the same time there is an increase in the number of neutrons emitted by the water ice below. But the most telltale sign of spring comes from looking at the nature of the ground itself. You see, much of the last ice to disappear is actually clear, just like the ice covering a pond. But it reacts differently when we look at it in thermal wavelengths. Solid ice rapidly conducts energy away from the surface, so it takes a long time to heat up in the day and a long time to cool at night. Therefore, when we look at the surface with an IR camera, the temperature of ice will be moderated.
However, dry Martian regolith is very fluffy stuff and once heated, the temperature rises quickly. At night this heat is easily radiated away. The result is more extreme temperatures for dry ground. How quickly heat is gained or lost to the environment from a surface is called "Thermal Inertia" and we can look for a change in the thermal inertia to determine when Spring has finally Sprung on Mars!
*I apologize to my Southern friends - I haven't forgotten about you. This year the southern spring corresponds with the north of Mars!