Water on Mars
In the late 1800s, astronomer Percival Lowell envisioned Mars as a planet laced with engineered waterways and canals. He derived his guess from observing narrow streaks on the surface of Mars through telescopes. Since straight lines don't typically appear in nature, he surmised, then
they must have been made by intelligent life. We now know, of course, that Lowell's canals didn't exist.
Using his telescopic views, Percival Lowell mapped the straight lines he saw on the surface of Mars, which he believed to be a system of canals. (Image credit: NASA)
Although liquid water does not currently exist on the surface of Mars, evidence from the Mars Global Surveyor, Odyssey, and Exploration Rover missions suggests that water once flowed in canyons and persisted in shallow lakes billions of years ago. Some scientists believe that a vast ocean may even have existed at one time.
This canyon on the Martian surface, known as Nanedi Vallis, may have been formed by flowing water. (Image credit: MGS Project, JPL, NASA)
Evidence for Past Liquid Water
Previous and current Mars missions have sent back numerous images that show evidence of features such as dry riverbeds, flood plains, and gullies, all of which indicate that liquid water did exist at some point in Mars' history. Hundreds of channels and valley networks that date to several billion years ago suggest an era when Mars had more hospitable, Earth-like conditions. While some channels seem to have been carved by rainfall, others appear to be the result of severe flooding.
Communities such as Hastings, Minnesota, are periodically flooded when the Mississippi River overflows its banks. Scientists believe that flows in Mars' Marte Vallis may have been up to 100 times the flow rate of the Mississippi. (Image credit: NOAA)
Images from the Viking mission, and more recently from the Mars Global Surveyor (MGS), revealed a large outflow channel system, called Marte Vallis, in the Elysium Plains region. Scientists have estimated that water flow rates in the main channel of the system could have been up to a hundred times the average flow rate of the Mississippi River. While most outflow channels on Mars date to about 2 billion years ago, the Marte Vallis channels cut through very young lava flows, indicating that liquid water may have flowed there within the last 20 million years. Prominent erosion features, such as streamlined islands that formed when flowing water cut around craters, are clearly visible in MGS photos, but the source of Marte Vallis' recent flows is still a mystery.
Another indicator of past liquid water on Mars is the presence of gullies - steep-walled trenches probably created by erosion. Gullies typically form on poorly cemented sediments where short, heavy rainstorms routinely remove the vegetation. On Earth, a classic example of gully formation can be found in South Dakota's Badlands National Park, an area of severe erosion. Scientists are uncertain about the age of the gullies on Mars, but relative to the rest of the planet's surface, they seem to be very young. Some scientists believe that periodic outbursts of water and debris, similar to flash floods, may have come from underground and remained liquid long enough to erode the gullies.
This Mars Orbiter Camera image shows a streamlined island in the Mangala Valles region of Mars. The presence of the high mesa at the south end of the form may have acted as an obstacle to the flow of floodwaters, causing erosion and deposition of sediment to create the teardrop-shaped form. (Image credit: NASA/JPL/Malin Space Science Systems)
The Mancos Shale badlands, located in Utah's Capitol Reef National Park, are a good example of gullies on Earth. (Image credit: U.S. Geological Survey)
Certain mineral deposits can also indicate the past presence of liquid water. Instruments onboard the MGS detected a large section of ground in Terra Meridiani that have high concentrations of the mineral hematite. On Earth, hematite is often associated with liquid water and often found in hot springs areas. Then in 2004, Mars Rover Opportunity sent photos back to Earth that showed the presence of round, pebble-sized hematite spherules, which came to be known as the Martian "blueberries".
Salt deposits are also associated with liquid water. As water runs over the ground, it dissolves salts and other minerals from the soil. When the water later evaporates from lakes or seas, the salts are left behind as white deposits. In the 1970s, Viking lander instruments analyzed portions of the Martian soil and found that it contained 10 to 20 percent salts.
Grey hematite, a component of the Martian "blueberries" found in Terra Meridiani, is a mineral that is often associated with water. (Image credit: NASA)
Shown are gullies on the wall of an impact crater on Mars. Scientists believe these may have formed when groundwater was released to the surface. (Image: NASA/JPL/Malin Space Science Systems)
At its landing site in the northern arctic plains, Phoenix will probe the history of liquid water that may have existed in the region as recently as 100,000 years ago. Analysis of the chemistry and mineralogy of the soil and ice using specialized instruments could lead to new understanding of the history of water on Mars. If fine sediments of mud and silt are found at the landing site, this may support the hypothesis of an ancient ocean on Mars. Alternatively, coarse sediments of sand might indicate past flowing water, especially if the grains are rounded and well-sorted - a sign of water erosion.
This salt pan, located in Death Valley, California, is a natural depression where water accumulated, evaporated, and left behind salt deposits. (Image credit: John W. Holt, Institute for Geophysics, University of Texas)
Analyzing the size and distribution of sand grains can help researchers determine whether sediments were created through water erosion. (Image credit: NASA/JPL/Cornell)