In the beginning, the most obvious answer to this question seems to be that water is frozen in the glacial ice caps. With the studies carried out by the previous probes it was found that the surface of the caps consisted of frozen carbon dioxide and not water. However, Mariner 9 determined that the caps have a stratified structure: at this point, nothing says that there can't be layers of water ice under the carbon dioxide surface, as measured by the instruments from the probe. The atmosphere contains absolutely minimum amounts of water, but this doesn't mean that Mariner 9 couldn't demonstrate other structures that are very interesting for an Earthling: clouds and fog. Astronauts also agree that the most outstanding feature of our planet is the immense variety of white forms that surround the Earth. We know how much the circulating atmosphere helps to support life on our planet, redistributing the solar heat and bringing beneficial rain in places that otherwise would be completely covered by the desert. To find clouds on another planet is very exciting because of the direct connection between clouds and life. Even on Mars, the clouds observed consist of water vapor and not deadly mixes of sulphuric acid as we found on Venus. But they are certainly too few and too thin to generate a significant amount of precipitation. In any case, each Martian year the two glacial ice caps increase or decrease in size according to the seasons. There is a huge layer of ice that evaporates and redeposits each time. Even if this phenomenon concerns almost exclusively frozen carbon dioxide, small quantities of water may be involved. And then, here and there on the planet, some drops of water or some snowflakes may appear. Thus, geologists have admitted that water was present on the planet in the far past and they have developed the following theory. One or two billion years ago, after the planet was formed, the increase in the internal heat expanded the Martian globe, just like on the Earth, producing the fractures of the great canyons and intense volcanism. The gases released from the interior had to form what was a much denser atmosphere than the one which is present today. At that time, it had to contain large amounts of water vapor, and so Mars probably went through a rainy period which lasted several millions of years and led to the formation of the rivers as seen today. Then, as the planet cooled, the water could not remain in the liquid state and it withdrew, frozen, to the polar caps and, perhaps, into the subsoil. With such a low force of gravity (a little more than a third of the Earth), Mars could not retain an atmosphere like ours, and so a large part of the primordial gases forming such an atmosphere were probably lost into space.
In the beginning, the most obvious answer to this question seems to be that water is frozen in the glacial ice caps. With the studies carried out by the previous probes it was found that the surface of the caps consisted of frozen carbon dioxide and not water. However, Mariner 9 determined that the caps have a stratified structure: at this point, nothing says that there can't be layers of water ice under the carbon dioxide surface, as measured by the instruments from the probe. The atmosphere contains absolutely minimum amounts of water, but this doesn't mean that Mariner 9 couldn't demonstrate other structures that are very interesting for an Earthling: clouds and fog. Astronauts also agree that the most outstanding feature of our planet is the immense variety of white forms that surround the Earth. We know how much the circulating atmosphere helps to support life on our planet, redistributing the solar heat and bringing beneficial rain in places that otherwise would be completely covered by the desert. To find clouds on another planet is very exciting because of the direct connection between clouds and life. Even on Mars, the clouds observed consist of water vapor and not deadly mixes of sulphuric acid as we found on Venus. But they are certainly too few and too thin to generate a significant amount of precipitation. In any case, each Martian year the two glacial ice caps increase or decrease in size according to the seasons. There is a huge layer of ice that evaporates and redeposits each time. Even if this phenomenon concerns almost exclusively frozen carbon dioxide, small quantities of water may be involved. And then, here and there on the planet, some drops of water or some snowflakes may appear. Thus, geologists have admitted that water was present on the planet in the far past and they have developed the following theory. One or two billion years ago, after the planet was formed, the increase in the internal heat expanded the Martian globe, just like on the Earth, producing the fractures of the great canyons and intense volcanism. The gases released from the interior had to form what was a much denser atmosphere than the one which is present today. At that time, it had to contain large amounts of water vapor, and so Mars probably went through a rainy period which lasted several millions of years and led to the formation of the rivers as seen today. Then, as the planet cooled, the water could not remain in the liquid state and it withdrew, frozen, to the polar caps and, perhaps, into the subsoil. With such a low force of gravity (a little more than a third of the Earth), Mars could not retain an atmosphere like ours, and so a large part of the primordial gases forming such an atmosphere were probably lost into space.
