Researchers have determined the size of carbon dioxide snow particles on Mars, depicted in this artist’s rendering as a mist or fog that eventually settles to the surface as carbon dioxide snow. Image: NASA, Christine Daniloff/MIT News |
In the dead of a
Martian winter, clouds of snow blanket the Red Planet’s poles—but unlike our
water-based snow, the particles on Mars are frozen crystals of carbon dioxide.
Most of the Martian atmosphere is composed of carbon dioxide, and in the
winter, the poles get so cold—cold enough to freeze alcohol—that the gas
condenses, forming tiny particles of snow.
Now researchers at Massachusetts
Institute of Technology (MIT) have calculated the size of snow particles in
clouds at both Martian poles from data gathered by orbiting spacecraft. From
their calculations, the group found snow particles in the south are slightly
smaller than snow in the north—but particles at both poles are about the size
of a red blood cell.
“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.”
Cahoy and graduate
student Renyu Hu worked with Maria Zuber, the E.A. Griswold Professor of
Geophysics at MIT, to analyze vast libraries of data gathered from instruments
onboard the Mars Global Surveyor (MGS) and Mars Reconnaissance Orbiter (MRO).
From the data, they determined the size of carbon dioxide snow particles in
clouds, using measurements of the maximum buildup of surface snow at both
poles. The buildup is about 50% larger at Mars’ south pole than its north pole.
Over the course of a
Martian year (a protracted 687 days, versus Earth’s 365), the researchers
observed that as it gets colder and darker from fall to winter, snow clouds
expand from the planet’s poles toward its equator. The snow reaches halfway to
the equator before shrinking back toward the poles as winter turns to spring,
much like on Earth.
“For the first time,
using only spacecraft data, we really revealed this phenomenon on Mars,” says
Hu, lead author of a paper published in the Journal of Geophysical Research,
which details the group’s results.
Diving through data
To get an accurate picture of carbon dioxide condensation on Mars, Hu analyzed
an immense amount of data, including temperature and pressure profiles taken by
the MRO every 30 sec 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 team also sifted
through 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.
Hu analyzed these
cloud returns, looking for additional evidence to confirm carbon dioxide
condensation. He looked at every case where a cloud was detected, 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. Hu then analyzed
the opacity of each cloud—the amount of light reflected—and through
calculations, determined the density of carbon dioxide in each cloud.
To estimate the total
mass of carbon dioxide snow deposited at both poles, Hu used earlier
measurements of seasonal variations in the Martian gravitational field done by
Zuber’s group: 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 data sets,” Cahoy says. “If you put different pieces of them
together, you can learn something new just from the data.”
What can the size of
snow tell us?
Hu says knowing the size of carbon dioxide snow cloud particles on Mars may
help researchers understand the properties and behavior of dust in the planet’s
atmosphere. For snow to form, carbon dioxide requires something around which to
condense—for instance, a small silicate 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?”
Just as snow on Earth
affects the way heat is distributed around the planet, Hu says snow particles
on Mars may have a similar effect, reflecting sunlight in various ways,
depending on the size of each 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.”
