Editor’s Note: Meg Urry is the Israel Munson professor of physics and astronomy at Yale University and director of the Yale Center for Astronomy and Astrophysics. The opinions expressed in this commentary are solely those of the author.
NASA research shows definitive signs that liquid water is present on Mars today
Meg Urry: Water is a necessary ingredient to creating simple and eventually complex life over millions of years
NASA announced clear evidence that there is liquid water on Mars, in the present day.
Water is necessary for life as we know it. On Earth, the first single-celled organisms appeared at least 3.5 billion years ago, almost certainly in an aqueous environment that allowed them to maneuver to find food and energy. Eventually very complex life forms evolved, including humans, whom fossil evidence date back only a couple of hundred thousand years.
Biologists know that without water, we and all the other flora and fauna on Earth would not exist.
So finding water on Mars — the neighboring planet most like Earth in terms of size and temperature — is a very big deal.
For years, science missions to Mars had hinted that there might be liquid water. Images revealed features that look like dry riverbeds and gullies, and some of the water features appeared to change according to the season.
Dark streaks dubbed “recurring slope lineae” or RSL, were seen to grow during the summer months on Mars — that is, in the hemisphere of Mars whose pole is tipped closest to the sun – and vanish again in the winter.
It was tantalizing that summer temperatures are ideal for liquid water whereas at winter temperatures, any water would freeze. (Indeed, Mars has frozen polar caps that grow and shrink with the seasons.) That is, the temperature range during which the RSL features are seen on Mars would allow water to be liquid, rather than ice or steam.
Now, analysis of four years of imaging spectroscopic data from the Mars Reconnaissance Orbiter shows definitive signs that liquid water is present on Mars today.
Imaging refers to measuring the intensity of light as a function of its spatial position, while spectroscopy detects the dependence of intensity on the wavelength of the light. The early signs of water came from imaging cameras, and now key evidence has come from a spectroscopic instrument.
Specifically, during the Martian summer, when RSL features are most prominent in images of Mars, the light reflected from the Martian surface has characteristic dips at certain wavelengths of infrared light (1.9 and 2.1 microns), matching absorption bands from perchlorate salts. Such salts therefore must present on the Martian surface.
But perchlorate crystals are hydrated. That is, they contain molecules of water within their crystal structure. The CRISM measurement tells us liquid water – water that can be absorbed into mineral crystals – must be present on Mars. These crystals do not form without liquid water being present.
The absorption of light at characteristic wavelengths is actually a very familiar phenomenon. Such absorption and reflection explains why trees have green leaves: the sunlight falling on the leaf is white, but the chlorophyll molecules in the leaf selectively absorb red and blue light (this is the energy that helps the tree grow) and reflect the green light, which is what we see.
So the absorption of light by perchlorates on the Martian surface is definitive evidence of present-day water on Mars. It’s very briny water, way saltier than terrestrial oceans. It’s not fit for human consumption and unfit for most fish in the sea. But we know that on Earth, wherever liquid water molecules and a source of energy are present – even in acid baths or in oxygen-free or high-pressure depths of the ocean – some form of life has evolved. So the possibility of life on Mars is very real.
Interestingly, perchlorates can keep water in a liquid state even below normal freezing temperatures, much as the salt you spread on your driveway in the winter turns ice into liquid water. So in the presence of perchlorates, liquid water might exist as cold as minus 94 degrees Fahrenheit (or minus 70 degrees Celsius). Even though Mars is cold and dry, it’s not too harsh for life.
Scientific investigation of Mars continues.
Future missions planned by NASA and the European Space Agency will drill down into the soil looking for signs of life and one day will return samples from the Martian surface to Earth for detailed laboratory analysis. The Mars Curiosity rover is atop Mount Sharp, investigating surface conditions there. The Mars Reconnaissance Orbiter continues its time-dependent observations of RSL and other surface features. There is a huge amount still to learn.
But today, we applaud the ingenuity that creates instruments of such sophistication and precision that we can identify the conditions for life on the red planet. And we look forward to growing evidence for life elsewhere in the universe, with the ultimate goal of knowing whether there is intelligent Earth-like life somewhere out there.
There can be no more profound scientific discovery than that.