Researchers have for the first time calculated the size of the snow particles in the clouds at the Martian poles, by using data from orbiting spacecraft. They found that the snow particles at the south pole are somewhat smaller than the snow particles at the north pole, but that they are around the size of a red blood cell at both poles.
During the Martian winter, clouds of snow fall at the planet’s poles — this snow is made from frozen crystals of carbon dioxide though, not water like on the Earth. This is because the Martian atmosphere is composed primarily of carbon dioxide, and the poles get so cold during the winter that the gas condenses, forming tiny particles of snow. During the Martian winters the poles get cold enough to freeze alcohol.
“These are very fine particles, not big flakes,” says Kerri Cahoy, the Boeing Career Development Assistant Professor of Aeronautics and Astronautics at MIT. If the carbon dioxide particles were eventually to fall and settle on the Martian surface, “you would probably see it as a fog, because they’re so small.”
The research was done by analyzing vast libraries of data that were gathered from the instruments onboard the Mars Global Surveyor (MGS) and Mars Reconnaissance Orbiter (MRO). Using the data, the size of the carbon dioxide snow particles in the clouds was determined by using “measurements of the maximum buildup of surface snow at both poles. The buildup is about 50 percent larger at Mars’ south pole than its north pole.”
The researchers observed that over the length of the 687-day-year, as it gets darker and colder during the transition of the fall to the winter, the snow clouds start expanding from the poles to the equator. The snow makes it halfway to the equator before retreating back to the poles as the winter returns to spring, similar to what happens on the Earth.
“To get an accurate picture of carbon dioxide condensation on Mars, the researchers analyzed an immense amount of data, including temperature and pressure profiles taken by the MRO every 30 seconds over the course of five Martian years (more than nine years on Earth). The researchers looked through the data to see where and when conditions would allow carbon dioxide cloud particles to form.”
The research team also analyzed measurements from the MGS’ laser altimeter, “which measured the topography of the planet by sending laser pulses to the surface, then timing how long it took for the beams to bounce back. Every once in a while, the instrument picked up a strange signal when the beam bounced back faster than anticipated, reflecting off an anomalously high point above the planet’s surface. Scientists figured these laser beams had encountered clouds in the atmosphere.”
Analyzing these ‘cloud’ returns, the researchers looked for additional evidence to confirm carbon dioxide condensation. Using every case where a cloud was detected, they then tried to “match the laser altimeter data with concurrent data on local temperature and pressure. In 11 instances, the laser altimeter detected clouds when temperature and pressure conditions were ripe for carbon dioxide to condense.” They then looked at the opacity of each cloud, that is, how much light is reflected; and then calculated the density of carbon dioxide in each cloud.
Using estimates of the seasonal variations in the Martian gravitational field, the researches estimated the total mass of the carbon dioxide snow deposited at both poles.
“As snow piles up at Mars’ poles each winter, the planet’s gravitational field changes by a tiny amount. By analyzing the gravitational difference through the seasons, the researchers determined the total mass of snow at the north and south poles. Using the total mass, Hu figured out the number of snow particles in a given volume of snow cover, and from that, determined the size of the particles. In the north, molecules of condensed carbon dioxide ranged from 8 to 22 microns, while particles in the south were a smaller 4 to 13 microns.”
“It’s neat to think that we’ve had spacecraft on or around Mars for over 10 years, and we have all these great datasets,” Cahoy says. “If you put different pieces of them together, you can learn something new just from the data.”
Since the Martian atmosphere is made up primarily of carbon dioxide, understanding its behavior on the planet will allow more understanding of Mars’ overall climate, says Paul Hayne, a postdoc in planetary sciences at the California Institute of Technology.
“The big-picture question this addresses is how the seasonal ice caps on Mars form,” says Hayne, who was not involved in the research. “The ice could be freezing directly at the surface, or forming as snow particles in the atmosphere and snowing down on the surface … this work seems to show that at least in some cases it’s snowfall rather than direct ice deposition. That’s been suspected for a long time, but this may be the strongest evidence.”
Knowing the size of the carbon dioxide snow cloud particles on Mars will help the researchers understand the properties and behavior of the dust in the planet’s atmosphere. The formation of snow requires something around which the carbon dioxide can condense, such as a small sand or dust particle.
“What kinds of dust do you need to have this kind of condensation?” Hu asks. “Do you need tiny dust particles? Do you need a water coating around that dust to facilitate cloud formation?”
Snow on the Earth affects the way heat distribution cycle around the whole planet. Snow particles on Mars might function in a similar way, increasing the reflection of sunlight in variety of ways, which would depend on the size of the particle.
“They could be completely different in their contribution to the energy budget of the planet,” Hu says. “These datasets could be used to study many problems.”
Image Credits: : NASA, Christine Daniloff/MIT News; ESA/ DLR/ FU Berlin (G. Neukum)