From Earth to Mars--Carbon Dioxide
Crystals Help Interplanetary Studies
Four views of Mars in northern summer as seen on March 30, 1997, from NASA's
Hubble space telescope. Each shows a quarter turn in rotation.
"If life was once sustainable on Mars, it is important to know what
caused Mars to evolve into the cold and lifeless planet it is today. With this
knowledge, we can terraform Mars by reversing the process."--Carl
Technology from the Agricultural Research
Service often shows up on farms, in food and industrial plants, and in the
home. Soon, ARS technology could find a new theater of operations: the Red
Mars, with its mountains, valleys, polar ice caps, dry river beds,
atmosphere, and relatively moderate climate, is more like Earth than any other
body in our solar system. Many scientists believe Mars holds a key to answering
questions about the geologic and climatic history of the Earth.
Over the next decade, the National Aeronautic and Space Administration
(NASA) will investigate Mars with a number of spacecraft and landers. Several
missions are already under way. The Mars Pathfinder began exploring the Martian
surface in July of 1997. The Mars Global Surveyor is being readied for its
mission of sensing the Martian atmosphere. And discoveries by ARS scientists
may play a role in future Mars missions that will look at the planet's polar
It started a few years ago. Cytologist William P. Wergin and botanist Eric
F. Erbe, who are with the ARS Nematology Laboratory at the Beltsville
(Maryland) Agricultural Research Center (BARC), developed the first technique
for viewing and photographing snowflake crystals with a scanning electron
microscope (SEM). Their technique relied on a new procedure for collecting and
preserving snow crystals. (See "Anatomy of a Snowflake,"
Agricultural Research, April 1995, pp. 18-21.)
Wergin and Erbe were interested in the crystalline structure of snowflakes
as a source of new clues about the potential available water in mountain
snowpacks--information vital to irrigated lands in much of the West.
"This was the first time the SEM was used to obtain highly magnified
images that clearly show the details of intact and well-focused snow
crystals," says Wergin. "And because the viewing stage allows a
sample to be viewed at different angles, we were able to record true,
three-dimensional images of the crystals."
Unlike conventional microscopes, the SEM does not use light passed through a
glass lens to form images of a specimen. Rather, the images are formed by
electrons that pass through a magnetic field that serves as a lens. The images
can be stored digitally and displayed on a cathode ray tube similar to a TV
The scientists' procedure uses liquid nitrogen to instantly chill snow
crystals to -320oF. This keeps them from melting while the SEM
images are obtained.
Carbon dioxide crystals, magnified about 600x.
Now the scientists have adapted the technique to capture the structure of
dry ice--frozen crystals of carbon dioxide (CO2). Gaseous
CO2 is a minor constituent of our atmosphere, making up less than 1
percent. There is no evidence that frozen CO2 exists in nature on
our planet's surface. On Mars, frozen CO2 makes up most of the
planet's polar ice caps.
"We used a special low-temperature SEM to visualize the structure of
CO2 crystals," says Erbe. "The microscope's stage, on
which the specimens are placed for viewing, can be cooled to
The ARS scientists say that CO2 crystals are as small as
1/200,000 of an inch. "They're considerably smaller than snowflakes--some
only 1/100 the size of snow crystals," says Erbe.
When magnified up to 20,000 times, the CO2 crystals generally
look like eight-sided structures compared to six-sided ones for snowflakes.
"Carbon dioxide crystals often appear as two attached four-sided pyramids,
called octahedrons," says Erbe.
Carbon dioxide gas freezes to a solid at extremely cold temperatures, around
-240oF compared to 32oF for water. Earth and Mars are the
only two planets in our solar system with ice caps that expand and contract in
response to changing seasons. These two facts make the ARS scientists' research
of great interest to colleagues at NASA's Goddard Space Flight Center in nearby
ARS and NASA scientists are collaborating to adapt the Beltsville
scientists' SEM technology to interplanetary studies of the Martian surface.
Collaborators include climatologist James L. Foster, physicist Al T.C. Chang,
and hydrologist Dorothy K. Hall at NASA's Laboratory of Hydrospheric Processes,
and geologist J. Barton of General Sciences Corp. in Greenbelt.
Foster says that the "SEM technology, together with modeling studies
and experiments using microwave radiometers, may allow us to assess, for the
first time, the thickness of the seasonal Martian ice caps. On Mars, as on
Earth, ice plays a role in large-scale climate processes. However, unlike
Earth, Mars has polar ice caps made up of both frozen water and frozen
CO2," says Foster.
"Knowing the size and shape and scattering properties of both types of
frozen crystals will tell us how these crystals behave and scatter energy in
different parts of the microwave portion of the electromagnetic spectrum,"
This information should lead to more accurate estimates of the thickness and
extent of frozen material covering the Martian surface.
What It's Like on Mars
More than 20 years ago, Viking I and II spacecraft recorded various spectral
data on this Martian CO2 mantle. Future missions will be making
measurements of Mars in the microwave region of the spectrum. In order to model
how CO2 crystals scatter and absorb microwave radiation, NASA
scientists needed to know the exact shape and size range of CO2
Foster says, "The ARS studies have enabled us to look at the
CO2 crystals in much more detail than ever before." The ARS
scientists think that other gases that freeze at ultralow temperatures--such as
ammonia and methane--could also be observed using this technique.
Back on Earth, seeing the crystalline structure of CO2 will give
ARS and other scientists clues that may help them learn more about how frozen
CO2 can cause rain to fall, as in cloud seeding.
"Dry ice crystals are among the most effective materials used for cloud
seeding," says Wergin.
He believes that CO2 crystals serve as nuclei around which water
vapor freezes, forming snowflakes. Eventually, the flakes melt to form
raindrops if the temperature at the ground is above freezing.
Studies of CO2 crystals may help scientists identify more
efficient or economical materials for cloud seeding. More importantly, the SEM
technology may help them learn how gaseous CO2 contributes to the
"Carbon dioxide ranks first among the greenhouse gases--along with
nitrous oxide and methane--that contribute to global warming as they increase
in the Earth's atmosphere," says Gary R. Evans. Formerly chief scientist
to the Secretary of Agriculture on global change issues, he is director of the
ARS Natural Resources Institute at BARC.
"Knowing the crystalline structure of solid CO2 may give us
clues as to the capacity of the gas to absorb and re-radiate energy," says
"Carbon dioxide can be sequestered by the way we manage farming
practices," he says. "Plants take up CO2 through
photosynthesis. After harvest, their residue gets returned to the soil and
becomes organic carbon. So farmers have a chance to reduce CO2 by
improving their management of agricultural systems to increase soil organic
The spinoff benefits from increased soil organic carbon, Evans says, are
reduced soil erosion and better soil tilth--an indicator of soil health.--By
Hank Becker, Agricultural
Research Service Information Staff.
William P. Wergin and Eric F. Erbe
are at the USDA-ARS Nematology Laboratory,
10300 Baltimore Ave., Beltsville, MD, 20705-2350; phone (301) 504-9027, fax
James L. Foster is at the
NASA Laboratory of Hydrospheric Processes, Goddard Space Flight Center,
Greenbelt, MD, 20771; phone (301) 286-7096, fax (301) 286-1758.
To view snow and CO2 crystals under ultrahigh
"FROM EARTH TO MARS--Carbon Dioxide Crystals Help Interplanetary
Studies" was published in the October 1998 issue of Agricultural
Research magazine. Click here to see this
issue's table of contents.