During most of April planetary orbits put the sun between Mars and Earth and cut off reliable communications with Curiosity, the NASA rolling laboratory on the Red Planet.
But before going incommunicado, data that Curiosity sent to Earth that may help answer the question: What happened to Mars’ atmosphere?
Today’s Martian atmosphere is sparse and thin, only about 1 percent the density of the Earth’s. The present Martian air pressure is too low for surface water — any exposed water on the surface would rapidly boil away — yet the mudstone rocks that Curiosity is studying show evidence of long-standing liquid water on the surface in the past. For there to have been surface water, Mars must have had a significantly greater atmosphere to produce greater air pressure.
Just before going behind the sun, Curiosity took a sample of the Martian atmosphere for analysis using its Sample Analysis at Mars (SAM) instrument. This instrument can concentrate selected gases in the atmosphere by removing other more dominant gases. This was the first use of this procedure on the mission.
SAM was looking for the ratio of two isotopes of the element argon in the atmosphere. Isotopes are different versions of the same element that have different weights, in this case argon-36, the lighter version, and argon-38, the heavier version. Curiosity found that there are 4.2 atoms of argon-36 for every one atom of argon-38.
It is believed that the solar system originally formed from a common gas cloud, meaning that the atmospheres of all bodies in the solar system should have started with the same ratio of these isotopes.
The sun and Jupiter are examples of this primordial argon; the ratios are the same for both, about 5.5 to 1. Even though the Earth’s atmosphere has undergone changes over time, the ratio is still about 5.5 to 1.
It is also important to note that argon is an inert gas. Because it does not react with any other elements, the only way for the ratio to change is to gain or lose argon. Therefore, the lower ratio on Mars means that the Martian atmosphere has lost a significant number of the lighter argon-36 atoms.
To have lost this amount of argon-36, a very large amount of the total atmosphere would have to have been lost, because argon has always been a small fraction of the Martian atmosphere. As much as 95 percent of Mars’ original atmosphere may have been lost.
Curiosity has also studied oxygen and carbon dioxide isotopes and water vapor, and the results are the same. All of the key components in the present atmosphere of Mars show a leaning toward heavier isotopes.
So where did the atmosphere go?
The most likely answer is that it was lost to space. Just as helium balloons rise in the heavier air here on Earth, the lighter argon isotopes are more likely to rise to the top of the Martian atmosphere. There they are exposed to the solar wind, a continuous outflow of ionized hydrogen from the sun.
The protons and electrons of the solar wind travel at high speeds, as the normal solar wind travels at more than 200 miles per second and can reach speeds greater than 600 miles per second. These speeds give particles in solar winds enough energy to knock the atoms in the Martian atmosphere free and into space.
Today this erosion rate is slow, but early Mars would have been hit by a solar wind estimated to have been 300 times denser.
Does this erosion also occur to the Earth’s atmosphere?
No, because the Earth has a magnetic field that shields it from the solar wind. The high-energy particles are deflected around the Earth and its atmosphere, whereas the Martian magnetic field has always been too weak to prevent the solar wind from penetrating its upper atmosphere.
In November NASA is planning to launch the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which will enter orbit around Mars in fall of 2014. MAVEN is the first mission devoted to understanding Mars’ upper atmosphere. It will determine how much of the Martian atmosphere has been lost over time by measuring the current erosion rates.
The loss of the Martian atmosphere has been a long-standing mystery. Curiosity and MAVEN are now helping us solve that mystery.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.