NASA/JPL/University of Arizona
NASA/JPL/University of Arizona

There's Liquid Water on Mars

NASA/JPL/University of Arizona
NASA/JPL/University of Arizona

In the image above, do you see those dark, narrow streaks flowing downhill on the steep slopes at Mars's Horowitz crater? Scientists say they provide definitive evidence of water flowing on the red planet. The findings were published today in the journal Nature Geoscience. NASA also held a press conference today to discuss the discovery.

That they might represent water flow has been suspected for several years. In 2011, a team of researchers working on the University of Arizona's HiRISE (High Resolution Imaging Science Experiment), an imaging system aboard the Mars Reconnaissance Orbiter (MRO), hypothesized that these streaks, known as recurring slope lineae, or RSLs, might be evidence of intermittent salty water flows that change with the seasons: 

The current research team (which includes planetary scientists from HiRISE, a few U.S. universities, NASA, and a French research center) combined the HiRISE documentation of RSLs—which were subsequently found at dozens of sites—with spectral data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), an instrument also onboard the MRO.

RSLs have low reflectance compared to the surrounding terrain, and they appear to get larger during warm seasons. The team analyzed at what wavelengths these RSLs absorb light, and then compared their abilities to absorb different wavelengths to those of minerals on Earth. The closest matches were magnesium perchlorate, magnesium chlorate, and sodium perchlorate—hydrated salts, which were detected at four locations during the seasons when RSLs are most extensive.

"Our findings strongly support the hypothesis that recurring slope lineae form as a result of contemporary water activity on Mars," the researchers write. They don't know where the water originates, or how it formed; the favored theory is that it's the result of deliquescence, in which salts absorb moisture from the atmosphere to create liquid water. This water is likely much saltier than our oceans.

Here are two views of slopes where hydrated salts were detected.

Dark narrow streaks known as recurring slope lineae emanating out of the walls of Garni crater on Mars. The dark streaks here are up to few hundred meters in length. Image credit: NASA/JPL/University of Arizona

Planetary scientists have detected hydrated salts on these slopes at Hale crater. The blue color seen upslope of the dark streaks are thought not to be related to their formation, but instead are from the presence of the mineral pyroxene. This is a false-color image. Image credit: NASA/JPL/University of Arizona

What are the implications of this discovery? Potential life on Mars, of course; either native life—which if it does exist is likely microbial and subsurface—or human life, in the future, as part of a manned Mars mission. 

As for Martian life, "I think it’s likely there’s life in the crust of Mars—microbes," said University of Arizona planetary geologist and study co-author Alfred McEwen, speaking at the press conference from Nantes, France. "To me, the chances of life being in the subsurface of Mars has always been very high."

But as Mars Exploration Program lead scientist Michael Meyer noted, “We have only one example of life, and that is us. We don’t know how it started, and so one of the things we found at Mars is that it could have supported life. But we don’t know how life started here, so we don’t know if it’s possible for life to start on Mars.”

As for the possibility of human life on Mars, "these observations are giving us a much better view that Mars has resources that are useful to future travels," said John Grunsfeld, astronaut and associate administrator of NASA’s Science Mission Directorate. For one thing, he noted, there's the potential to make rocket fuel (which is commonly made from liquid hydrogen and liquid oxygen). "The exciting thing is that we'll send humans to Mars in the near future," Grunsfeld said.

Before humans ever set foot on the red planet, there are several unmanned missions to Mars on the horizon. Next year, NASA will send the InSight lander to Mars to peer into its interior for the first time. The European Space Agency is launching two ExoMars missions—one in 2016 and the other, in collaboration with the Russian Federal Space Agency, in 2018. And in 2020 NASA's Mars Exploration Program continues with the launch of another rover, which will collect samples and bring them back to Earth.

Because the slopes featuring these briny water flows are steep, they're not good landing places for rovers. Nimble-footed astronauts, on the other hand, might one day be able to make the climb for a closer look.

"We are on a journey to Mars, and science is leading the way. Each time we learn something new about Mars, Mars becomes more and more interesting," Grunsfeld said. "I think it's going to provide us with a great sense of our place in the universe and our solar system in particular."

Dean Mouhtaropoulos/Getty Images
Essential Science
What Is a Scientific Theory?
Dean Mouhtaropoulos/Getty Images
Dean Mouhtaropoulos/Getty Images

In casual conversation, people often use the word theory to mean "hunch" or "guess": If you see the same man riding the northbound bus every morning, you might theorize that he has a job in the north end of the city; if you forget to put the bread in the breadbox and discover chunks have been taken out of it the next morning, you might theorize that you have mice in your kitchen.

In science, a theory is a stronger assertion. Typically, it's a claim about the relationship between various facts; a way of providing a concise explanation for what's been observed. The American Museum of Natural History puts it this way: "A theory is a well-substantiated explanation of an aspect of the natural world that can incorporate laws, hypotheses and facts."

For example, Newton's theory of gravity—also known as his law of universal gravitation—says that every object, anywhere in the universe, responds to the force of gravity in the same way. Observational data from the Moon's motion around the Earth, the motion of Jupiter's moons around Jupiter, and the downward fall of a dropped hammer are all consistent with Newton's theory. So Newton's theory provides a concise way of summarizing what we know about the motion of these objects—indeed, of any object responding to the force of gravity.

A scientific theory "organizes experience," James Robert Brown, a philosopher of science at the University of Toronto, tells Mental Floss. "It puts it into some kind of systematic form."


A theory's ability to account for already known facts lays a solid foundation for its acceptance. Let's take a closer look at Newton's theory of gravity as an example.

In the late 17th century, the planets were known to move in elliptical orbits around the Sun, but no one had a clear idea of why the orbits had to be shaped like ellipses. Similarly, the movement of falling objects had been well understood since the work of Galileo a half-century earlier; the Italian scientist had worked out a mathematical formula that describes how the speed of a falling object increases over time. Newton's great breakthrough was to tie all of this together. According to legend, his moment of insight came as he gazed upon a falling apple in his native Lincolnshire.

In Newton's theory, every object is attracted to every other object with a force that’s proportional to the masses of the objects, but inversely proportional to the square of the distance between them. This is known as an “inverse square” law. For example, if the distance between the Sun and the Earth were doubled, the gravitational attraction between the Earth and the Sun would be cut to one-quarter of its current strength. Newton, using his theories and a bit of calculus, was able to show that the gravitational force between the Sun and the planets as they move through space meant that orbits had to be elliptical.

Newton's theory is powerful because it explains so much: the falling apple, the motion of the Moon around the Earth, and the motion of all of the planets—and even comets—around the Sun. All of it now made sense.


A theory gains even more support if it predicts new, observable phenomena. The English astronomer Edmond Halley used Newton's theory of gravity to calculate the orbit of the comet that now bears his name. Taking into account the gravitational pull of the Sun, Jupiter, and Saturn, in 1705, he predicted that the comet, which had last been seen in 1682, would return in 1758. Sure enough, it did, reappearing in December of that year. (Unfortunately, Halley didn't live to see it; he died in 1742.) The predicted return of Halley's Comet, Brown says, was "a spectacular triumph" of Newton's theory.

In the early 20th century, Newton's theory of gravity would itself be superseded—as physicists put it—by Einstein's, known as general relativity. (Where Newton envisioned gravity as a force acting between objects, Einstein described gravity as the result of a curving or warping of space itself.) General relativity was able to explain certain phenomena that Newton's theory couldn't account for, such as an anomaly in the orbit of Mercury, which slowly rotates—the technical term for this is "precession"—so that while each loop the planet takes around the Sun is an ellipse, over the years Mercury traces out a spiral path similar to one you may have made as a kid on a Spirograph.

Significantly, Einstein’s theory also made predictions that differed from Newton's. One was the idea that gravity can bend starlight, which was spectacularly confirmed during a solar eclipse in 1919 (and made Einstein an overnight celebrity). Nearly 100 years later, in 2016, the discovery of gravitational waves confirmed yet another prediction. In the century between, at least eight predictions of Einstein's theory have been confirmed.


And yet physicists believe that Einstein's theory will one day give way to a new, more complete theory. It already seems to conflict with quantum mechanics, the theory that provides our best description of the subatomic world. The way the two theories describe the world is very different. General relativity describes the universe as containing particles with definite positions and speeds, moving about in response to gravitational fields that permeate all of space. Quantum mechanics, in contrast, yields only the probability that each particle will be found in some particular location at some particular time.

What would a "unified theory of physics"—one that combines quantum mechanics and Einstein's theory of gravity—look like? Presumably it would combine the explanatory power of both theories, allowing scientists to make sense of both the very large and the very small in the universe.


Let's shift from physics to biology for a moment. It is precisely because of its vast explanatory power that biologists hold Darwin's theory of evolution—which allows scientists to make sense of data from genetics, physiology, biochemistry, paleontology, biogeography, and many other fields—in such high esteem. As the biologist Theodosius Dobzhansky put it in an influential essay in 1973, "Nothing in biology makes sense except in the light of evolution."

Interestingly, the word evolution can be used to refer to both a theory and a fact—something Darwin himself realized. "Darwin, when he was talking about evolution, distinguished between the fact of evolution and the theory of evolution," Brown says. "The fact of evolution was that species had, in fact, evolved [i.e. changed over time]—and he had all sorts of evidence for this. The theory of evolution is an attempt to explain this evolutionary process." The explanation that Darwin eventually came up with was the idea of natural selection—roughly, the idea that an organism's offspring will vary, and that those offspring with more favorable traits will be more likely to survive, thus passing those traits on to the next generation.


Many theories are rock-solid: Scientists have just as much confidence in the theories of relativity, quantum mechanics, evolution, plate tectonics, and thermodynamics as they do in the statement that the Earth revolves around the Sun.

Other theories, closer to the cutting-edge of current research, are more tentative, like string theory (the idea that everything in the universe is made up of tiny, vibrating strings or loops of pure energy) or the various multiverse theories (the idea that our entire universe is just one of many). String theory and multiverse theories remain controversial because of the lack of direct experimental evidence for them, and some critics claim that multiverse theories aren't even testable in principle. They argue that there's no conceivable experiment that one could perform that would reveal the existence of these other universes.

Sometimes more than one theory is put forward to explain observations of natural phenomena; these theories might be said to "compete," with scientists judging which one provides the best explanation for the observations.

"That's how it should ideally work," Brown says. "You put forward your theory, I put forward my theory; we accumulate a lot of evidence. Eventually, one of our theories might prove to obviously be better than the other, over some period of time. At that point, the losing theory sort of falls away. And the winning theory will probably fight battles in the future."

The Northern Lights May Be Visible Over Parts of America Tonight

The Northern Lights are rarely visible in the continental U.S., but Americans living in the Upper Midwest and New England occasionally catch a glimpse. Tonight, March 14, may mark one such occasion, according to Thrillist.

Thanks to a mild geomagnetic storm on March 14 and 15, the aurora borealis could be visible as far south as Montana, North Dakota, Minnesota, Wisconsin, Michigan, and Maine, the National Space Weather Prediction Center (SPWC) reports. The storm has been rated as a G1 geomagnetic storm, the weakest rating on a scale from G1 to G5, meaning it probably won’t disrupt power grids or satellites.

If you don’t live within the U.S.’s higher latitudes, you’ll have to be content with watching videos of the spectacular phenomenon.

If you do live along the country’s northern tier near the Canadian border, you can check the SPWC’s 30-minute aurora forecast to get a better sense of where the Northern Lights might appear in the sky in the near future.

[h/t Thrillist]


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