Today marks Mars’ passage through aphelion, its furthest point in its orbit from the Sun. At aphelion, Mars is moving slowest in its orbit while at perihelion, Mars is closest to the Sun and moving at its fastest velocity.  Mars has a more eccentric or elliptical orbit than Earth, and has the second highest eccentricity out of the 8 planets in our Solar System.

Plot of the inner solar system orbits (yellow dots are asteroid positions). You can see how eccentric Mars’ orbit is compared to the other inner Solar System planets – image Credit: JPL Solar System Dynamics/P. Chodas/ NASA/JPL http://ssd.jpl.nasa.gov/?ss_inner

The Red Planet’s orbital eccentricity may actually be an important factor in  Mars’ climate. The Southern hemisphere right now is pointed away from the Sun and the carbon dioxide ice sheet is growing. Compared to the Northern hemisphere, Southern summers are  shorter (because Mars is at perihelion during that time) and the solar insolation is more intense while the Southern winters are colder and longer.  This dichotomy  may be responsible for why seasonal fans and blotches are abundant during the thawing of the carbon dioxide ice sheet in Southern Spring and Summer, but fans and blotches are spotted far and in between in the same seasons in the  Northern Hemisphere.

Image credit: http://www-mars.lmd.jussieu.fr/mars/time/solar_longitude.html

So far seasonal fans and blotches have mainly been spotted on the slopes of dunes at the Martian North Pole and tend to be smaller than their Southern hemisphere counterparts. One of the goals of Planet Four is to better study this. With your measurements of the frequencies, locations, and sizes of fans we’ll eventually compare Northern hemisphere fans to the occurrence and sizes of fans in the Southern hemisphere.

We’ve mentioned before in the live chats, and Talk about how the science team would like to test whether the fan directions and the blotch measurements we ask you to make are indicators of the prevailing direction of surface winds on the South Polar regions. We think that the wind is the mostly likely culprit for the sculpting of the fans and that blotches occur when the wind was not strong enough to blow the material into the fans you see in other Planet Four images.

How will we check this? Unlike on Earth, there aren’t weather stations covering most of the globe of Mars taking pressure, temperature, and wind measurements. We’ll have to rely on computer simulations, on the output from a Global Circulation Model/Global Climate Model (GCM) of Mars. A GCM is a computer model of Mars’ atmosphere and climate that simulates all the conditions of the planet’s atmosphere and evolve it over time. PlanetWRF and Ames Mars General Circulation Model are two examples of Mars GCMs.

A GCM attempts to contain in it all the physics and chemistry that planetary scientists know and believe are acting in the planet’s atmosphere. This includes dealing with winds, pressure, atmospheric temperature, chemical reactions, impact of dust and particulate transport in the atmosphere on the whole planet and smaller scales. The simulations also have to take into account how solar insolation varies on the surface of the planet over time accounting for Mars’ orbital parameters changing as well. In addition the climate and weather impact the  surface conditions which evolve on the planet over time and feedback into the climate. If you’re interested in reading more about the processes that impact the Martian climate check out the NASA AMES Mars Climate Modeling Group Climate page

Martian GCMs are trying to represent and replicate complicated processes that are happening in the Martian atmosphere. Sometimes things are parametrized to make it possible to program into a computer and may not fully represent the reaction but it is the best scientists can do. The models are continuously improved by comparing to what limited weather and atmospheric  measurements we do have of Mars. Mars is the best studied and detailed climate besides Earth in the Solar System.  There have been weather stations on the Mars rovers (Curiosity, Spirit, and Opportunity) and landers (including Viking, Pathfinder, Phoenix etc) measuring the wind direction, temperature, pressure etc on the ground. Both Opportunity and Curiosity are continuing to measure the current conditions at their landing sites. Also the fleet of US and European spacecraft (and two new Mars orbiters – one from  India and one from the  US  are currently on their way to Mars) are monitoring the Martian atmosphere providing  rich data sets of the upper and middle atmosphere. For example  the vertical distribution of dust in the Martian atmosphere and how it changes over the Martian year is obtained from observations from the Mars Climate Sounder aboard the Mars Reconnaissance Orbiter (which also is the home for the HiRISE camera that takes the images you classify).

Once we have the fan pointings  and the locations of the blotches over several Martian seasons we’ll use GCMs to predict wind directions and speeds at different times and dates on the ground at the locations the images were taken and see how they compare.

A year on Earth is 365.25 days, but it takes Mars nearly twice as long to complete one revolution around the Sun. With 668.6 sols (a sol is a Martian day), how do scientists keep track of the Martian time and date?

A Martian sol happens to be 39 minutes and 35 seconds  longer than Earth’s. Dating back to the days of the Viking landers, sols are kept on a 24 hour clock. With the extra 40 minutes, Martian hours, minutes and seconds are slightly longer than their Earthly counterparts. That can make it a bit difficult to have a Mars time wrist watch.

The Martian calendar is not broken up into months like we have on Earth. Instead planetary scientists use the position of Mars in its orbit to tell time and mark seasons. They use the solar longitude L_s (pronounced “L sub S”) which is the Mars-Sun angle to keep track of the year.  L_s=0 degrees is when Mars is at the northern vernal (Spring) equinox.  L_s=90 degrees when at the Northern Summer Solstice) 180 degrees at the  Northern Autumnal Equinox and 270 degrees at the Northern Winter Solstice. Just think the opposite to get the specific season for Mars’s Southern Hemisphere.

Image credit: http://www-mars.lmd.jussieu.fr/mars/time/solar_longitude.html

This upcoming February 23rd will mark the Winter solstice in the Northern hemisphere (Summer solstice in the Southern hemisphere). So on the South Pole there should be some seasonal fans, like the ones you’re mapping on the main classification interface, currently visible to HiRES where there is still thawing carbon dioxide ice.

You can find out more about the Martian Calendar and the dates of future solstices with this  post by the Planetary Society.