NASA spacecraft reveals dramatic changes in Mars' atmosphere
As the tilt of the Red Planet’s axis increases, frozen carbon dioxide enters the atmosphere and swells its mass.
April 22, 2011
NASA's Mars Reconnaissance Orbiter (MRO) has discovered that the total amount of atmosphere on Mars changes dramatically as the tilt of the planet's axis varies. This process can affect the stability of liquid water if it exists on the martian surface and increase the frequency and severity of martian dust storms.
A newly found, buried deposit of frozen carbon dioxide — dry ice — near the south pole of Mars contains about 30 times more carbon dioxide than previously estimated to be frozen near the pole. NASA/JPL-Caltech/Sapienza University of Rome/Southwest Research Institute
Researchers using MRO's ground-penetrating radar identified a large, buried deposit of frozen carbon dioxide, or dry ice, at the Red Planet's south pole. The scientists suspect that much of this carbon dioxide enters the planet's atmosphere and swells the atmosphere's mass when Mars' tilt increases.
The newly found deposit has a volume similar to Lake Superior's nearly 3,000 cubic miles (12,500 cubic kilometers). The deposit holds up to 80 percent as much carbon dioxide as today's martian atmosphere. Collapse pits caused by dry ice sublimation and other clues suggest the deposit is in a dissipating phase, adding gas to the atmosphere each year. Mars' atmosphere is about 95 percent carbon dioxide, in contrast to Earth's much thicker atmosphere, which is less than .04 percent carbon dioxide.
"We already knew there is a small perennial cap of carbon-dioxide ice on top of the water ice there, but this buried deposit has about 30 times more dry ice than previously estimated," said Roger Phillips from the Southwest Research Institute in Boulder, Colorado.
This cross-section view of underground layers near Mars' south pole is a radargram based on data from the Shallow Subsurface Radar (SHARAD) instrument on NASA's Mars Reconnaissance Orbiter. NASA/JPL-Caltech/Sapienza University of Rome/Southwest Research Institute
"We identified the deposit as dry ice by determining the radar signature fit the radio-wave transmission characteristics of frozen carbon dioxide far better than the characteristics of frozen water,” said Roberto Seu from the Sapienza University of Rome. Additional evidence came from correlating the deposit to visible sublimation features typical of dry ice.
"When you include this buried deposit, martian carbon dioxide right now is roughly half frozen and half in the atmosphere, but at other times it can be nearly all frozen or nearly all in the atmosphere," Phillips said.
An occasional increase in the atmosphere would strengthen winds, lofting more dust and leading to frequent and intense dust storms. Another result is an expanded area on the planet's surface where liquid water could persist without boiling. Modeling based on known variation in the tilt of Mars' axis suggests several-fold changes in the total mass of the planet's atmosphere can happen on time frames of 100,000 years or less.
These images from orbit show an area near Mars' south pole where coalescing or elongated pits are interpreted as signs that an underlying deposit of frozen carbon dioxide, or "dry ice," has been shrinking by sublimation. NASA/JPL-Caltech/Univ. of Arizona
The changes in atmospheric density caused by the carbon-dioxide increase also would amplify some effects of the changes caused by the tilt. Researchers plugged the mass of the buried carbon-dioxide deposit into climate models for the period when Mars' tilt and orbital properties maximize the amount of summer sunshine hitting the south pole. They found at such times that global year-round average air pressure is approximately 75 percent greater than the current level.
"A tilted Mars with a thicker carbon-dioxide atmosphere causes a greenhouse effect that tries to warm the martian surface, while thicker and longer-lived polar ice caps try to cool it," said Robert Haberle from NASA's Ames Research Center in Moffett Field, California. "Our simulations show the polar caps cool more than the greenhouse warms. Unlike Earth, which has a thick, moist atmosphere that produces a strong greenhouse effect, Mars' atmosphere is too thin and dry to produce as strong a greenhouse effect as Earth's, even when you double its carbon-dioxide content."