Putting together data from multiple NASA missions, we're getting a better understanding of how much water is out there.

Where did the water on Mars go? It's clear some of it is in the Martian polar ice caps and below the surface. We also think much of Mars' early atmosphere was stripped away by the wind of charged particles that streams from the sun, causing the planet to dry out. NASA's MAVEN mission is hard at work following this lead from its orbit around Mars. 

The story of how Mars dried out is intimately connected to how the Red Planet's atmosphere interacts with the solar wind. Data from the agency's solar missions -- including STEREO, Solar Dynamics Observatory and the planned Solar Probe Plus -- are vital to helping us better understand what happened.

Understanding the distribution of water in our solar system tells us a great deal about how the planets, moons, comets and other bodies formed 4.5 billion years ago from the disk of gas and dust that surrounded our sun. The space closer to the sun was hotter and drier than the space farther from the sun, which was cold enough for water to condense. The dividing line, called the "frost line," sat around Jupiter's present-day orbit. Even today, this is the approximate distance from the sun at which the ice on most comets begins to melt and become "active." Their brilliant spray releases water ice, vapor, dust and other chemicals, which are thought to form the bedrock of most worlds of the frigid outer solar system.

Scientists think it was too hot in the solar system's early days for water to condense into liquid or ice on the inner planets, so it had to be delivered -- possibly by comets and water-bearing asteroids. NASA's Dawn mission is currently studying Ceres, which is the largest body in the asteroid belt between Mars and Jupiter. Researchers think Ceres might have a water-rich composition similar to some of the bodies that brought water to the three rocky, inner planets, including Earth.

The amount of water in the giant planet Jupiter holds a critical missing piece to the puzzle of our solar system's formation. Jupiter was likely the first planet to form, and it contains most of the material that wasn't incorporated into the sun. The leading theories about its formation rest on the amount of water the planet soaked up. To help solve this mystery, NASA's Juno mission will measure this important quantity beginning in mid-2016.

From Singularity Hub:

When scientists looked at Mars through early telescopes, they saw a fuzzy, rust-colored globe scored by mysterious dark gashes some believed were alien canals. Later, armed with sharper images, we scoffed at such naiveté. Mars is obviously dry as a bone and uninhabited. Now, with a great deal more information from rovers and satellites, we believe Mars was once wet. As for life? The jury's still out.

It shows how much we still have to learn (and are learning) about our solar system. Not too long ago, we only suspected one ocean of liquid water beyond Earth (on Europa). Now, thanks to robotic explorers, like NASA's Dawn and Cassini missions, we're finding evidence of oceans throughout the solar system.

Why do astronomers care so much about water? As far as we know, water (especially liquid water) and life go hand in hand. This is one reason our hunt for exoplanets focuses on the "Goldilocks zone." This orbital area is at just the right distance from a host star to allow liquid oceans like those found on Earth.

But life, at least the simplest forms, may survive in more environments than once believed. Indeed, even on Earth, life is ubiquitous, from scalding volcanic vents to frozen wastes. And we may not, as it turns out, need to travel light years to find extraterrestrial oceans or observe our first alien life forms. The proverbial backyard may suffice.

That page also has a cool infographic that I'll include below.