What Happened to Mars's Atmosphere?

According to NASA's Curiosity rover data, Mars has an atmosphere that is only a fraction of the size that it could have once been. As of today, Mars's thin atmosphere is known to be enriched with heavy carbon and oxygen isotopes, measured by the rover's SAM suite, Sample Analysis at Mars, in several samples of the Martian atmosphere. How did Mars's atmosphere decrease in size? The exact answer is unknown but there is concrete evidence that said atmosphere was once much thicker: water. Because the Martian atmosphere is currently too thin, liquid water can't be present on the Red Planet. However, there is evidence that there once was liquid water on our neighboring planet and thus gives us the certainty that at one given moment the atmosphere was much thicker. Meteorites were also analyzed and indicated that the loss of Mar's atmosphere could have began around the first billion years of the planet's 4.6 billion year existence. Although the Curiosity rover can't measure the exact loss of atmospheric presence, a future mission to Mars will be able to do so. The MAVEN, Mars Atmosphere and Volatile Evolution Mission, will bring data to understand the changes Mars's has undergone through the centuries and perhaps allow us to know our neighboring planet a little better.