Water is an essential ingredient for life on Earth. If we want to find life on another world, we look for the presence of liquid water. We know there is liquid water on other worlds, but the question remains whether our planetary sibling, Mars, has liquid water. 

We have long known that Mars was once a wet world. Isotope measurements show Mars was an ocean world in its youth. Rain fell from the sky, and rivers flowed. But today Mars is cold and dry. Ice can be found on its surface, but long gone are the Martian rivers and seas. With weak gravity and little magnetic field, the red planet simply couldn’t hold a rich atmosphere, and so the liquid water evaporated and froze.

But it’s possible that some water might exist beneath its surface, and ice in its soil might be able to liquify under the right conditions. There is evidence to support this idea, such as through recurring slope lineae. These dark lines along the sides of hills look similar to what you would see if you scooped your hand through wet sand. Rather than a rich flow of water, it is a damp seeping through the sand. Looking at the images, it certainly looks like a liquid flow, but astronomers know the risk of simply assuming something to be true based upon appearances. If it looks like liquid water it’s worth a further look, but appearances are not proof.

In 2015 there was a great deal of excitement about these lineae when astronomers found evidence of perchlorate salts within them. The salts could mix with water to created a brine, and this brine could remain liquid even at the temperature and pressure of Mars’ summer weather. In the winter of Mars it would freeze, and this would explain why the recurring slope lineae seem to appear seasonally in the summer.

But while liquid water is a good explanation, it isn’t the only possible explanation. This is why the discovery was met with cautious optimism. Now it seems it was good to be cautious. New observations from the Mars Reconnaissance Orbiter point to a dry answer. Careful analysis shows that the lineae occur along steeper slopes, but stop when the slope reaches a shallow enough angle of incline. This critical incline is consistent with the critical incline seen in flows of dust and sand. It’s similar making a hill out of sand. It’s easy to build a shallow hill by piling on ever more sand, but if you try to make a steeper hill the sand topples down to the shallow angle.

New observations of recurring slope lineae show they behave more like dust than water. Credit: NASA/JPL/University of Arizona/USGS

Liquid water doesn’t have such a critical angle. If the lineae were due to liquid water, you might see it stop at a similar distance from its source, but water can still flow down shallow hills. So it seems this is a dry flow of dust and sand rather than a flow of water brine. If that’s the case, it could mean that Mars is very dry indeed. Perhaps too dry to support life.

As with earlier evidence, we should still be a bit cautious. There might still be liquid water on Mars in some places. The question of liquid water on Mars is so important that we want to be careful about both the evidence and our conclusions. It’s also the best way to do science if you want to get it right.

Paper: Colin M. Dundas, et al. Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water. Nature Geoscience doi:10.1038/s41561-017-0012-5 (2017)

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Mars is a cold, arid world with a thin atmosphere, but that wasn’t always so. In its youth Mars was wet with rivers and oceans. Presumably that also means young Mars had rainfall. But how much did it rain on Mars? New research proves not only did it rain on Mars, the rains were strong enough to shape the planet’s surface. 

To study Martian rain, two geologists used the same rainfall models used for Earth. These are well studied, and are proven to work well. But there are differences between Earth and early Mars. One big difference is that gravity on Mars is about a third that on Earth, meaning that water droplets would fall more slowly and strike the surface with less energy. Then there is the fact that the atmosphere of Mars has changed significantly. Young Mars had an atmosphere four times thicker than modern Earth.

Because of these factors, there wasn’t much rain on Mars despite plenty of liquid water. Instead water vapor would tend to coalesce into small droplets to form a thick fog. This fog could make the surface of mars wet, but wouldn’t alter the terrain much. As the atmosphere of Mars thinned to a pressure similar to Earth’s, larger water droplets could form. Given the low gravity, could merge into quite large rain drops. On Earth large raindrops tend to break apart as they fall faster, which limits their size to about 6 millimeters in diameter. Falling at a slower speed, Martian water droplets could grow to about 7.5 millimeters. Torrential rains with large rain droplets created surface runoff that cut paths on the surface of Mars. These “gully washers” can be seen today, such as in the image above.

Paper: Robert A. Craddock and Ralph D. Lorenz. The changing nature of rainfall during the early history of Mars. Icarus, volume 293, (September 2017).

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SpaceX has announced it’s Interplanetary Transport System (ITS), with the goal of sending humans to Mars. While there remains many questions about how such a mission will be achieved, one thing that’s very clear is that the ITS will be the biggest rocket ever constructed. It has to be. Basic physics requires it. 

The ITS is designed to have more than 13 million Newtons of thrust at sea level, compared to the 3.5 million Newtons of the Saturn V rockets used to send Americans to the Moon. All this while having only about 10% heavier. Such a big increase in thrust vs weight is necessary, because it determines not only how much mass you can lift into Earth orbit, but whether you can get that mass all the way to Mars.

Delta-V needed to reach Mars. Credit: Wikipedia user Wolfkeeper

It all comes down to delta-V, or how much you can change the velocity of your rocket. When it comes to reaching Earth orbit, bigger is better. The SpaceX ITS should be capable of lifting up to 550 tonnes of payload into low Earth orbit, compared to the 140 tonnes of the Saturn V. This is necessary because a trip to Mars isn’t a few-day trip to the Moon. It will require a larger crew and significantly more food and resources.

Once in Earth orbit, getting to Mars will require even more rocket power to overcome what is known as delta-V. This is the amount of speed a spacecraft needs to gain or lose to reach your destination. It takes much more delta-v to reach the surface of Mars than it does the surface of the Moon. To reach Mars you not only have to overcome Earth’s gravity, you have to overcome the Sun’s pull as you travel toward Mars. You also have to account for the fact that the orbital speed of Mars is slower than the orbital speed of Earth. Finally you have to overcome the gravity of Mars to land softly on its surface. All of this adds to the total amount of needed delta-V. To meet this need the SpaceX plans to refuel the ITS in Earth orbit with a second launch.

There are ways to minimize your delta-V requirements for an interplanetary mission. One way is to make a close flyby of a different planet. Basically, if you approach a planet in the direction of its orbit (coming up from behind, if you will), then the gravity between the planet and your spacecraft will cause the spacecraft to speed up at the cost of slowing down the planet by a tiny, tiny amount. Making a flyby in the opposite direction can cause your spacecraft to slow down. This costs you nothing in terms of fuel, but takes time because you need to orbit the Sun in just the right way. It’s a common trick used for robotic spacecraft, where we use a flyby of Earth to reach Mars or Jupiter, or a flyby of Jupiter to reach the outer solar system.

A Hohmann orbit between Earth and Mars. Image by the author.

Flybys are cheap and easy for space probes, but they can add years to the time it takes to reach your destination. That’s a big problem for a crewed mission. So the alternative is to look at optimized orbital trajectories. For example, about every two years the positions of Earth and Mars are ideally suited so that a trip needs much less delta-V. This was actually discovered in 1925 by Walter Hohmann, who proposed a trajectory now known as the Hohmann transfer orbit. You could, for example, build a large spacecraft in such an orbit and use it as a shuttle between Earth and Mars. Such an idea was used in the book and movie The Martian.

There are other useful tricks, such as using a planets atmosphere to “aerobrake” a spacecraft, significantly reducing its delta-v once it reaches the planet. Since both Earth and Mars have atmospheres this can be used for landing spacecraft. You can also modify the flyby method by thrusting your spacecraft just as it makes its closest approach, in what is known as an Oberth maneuver (another trick used in The Martian). But these will only take you so far. To reach the surface of Mars in a reasonable time, any rocket will require more delta-v than we’ve ever had, which is why the ITS has to be so big.

The one up-side of all this is that once SpaceX, Blue Origin, or NASA builds a rocket with enough power to send humans to Mars, lots of other destinations open up as well. The delta-V requirements to reach the asteroids, Jupiter or Saturn aren’t significantly different. If we can land on Mars, we can reach the moons of Jupiter, or even start mining asteroids.

Mars is not only an awesome destination, it is also a gateway to the solar system.

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Mars has two small moons, Deimos and Phobos. Deimos is smaller and more distant from Mars, while Phobos is quite close to the red planet. Its orbital radius only 2.8 Mars radii, compared to our Moon at 60 Earth radii. Phobos is so close to Mars that it orbits the planet three times a day. It’s also so close that the small moon is doomed.

We’ve known for a while that Phobos’ time was limited. Tidal forces between Phobos and Mars cause the moon to move ever closer to the planet. Measurements of its orbit since the 1950s have found its orbit is decaying at a rate of about 1.8 centimeters per year. This and the fact that early observations of the moon had a rubble-pile look to them led some astronomers to speculate that Phobos could be artificial. More recent observations show that it is natural in origin, and (along with Deimos) was likely captured from the asteroid belt.

The inward spiral of Phobos means that it will only be around for about 30 million years. By then it will either be broken up by the tidal forces of Mars, or it will remain solid and impact Mars. Recent observations of the moon point to fragmentation. In fact it may have already begun. Notable on its surface are long grooves. If Phobos has a rubble like interior with a thick outer layer of dust, then these grooves are what you’d expect from tidal forces. If that’s the case, Phobos will gradually break apart, and may even form a ring system around Mars.

There has been talk about sending a mission to Phobos to land on the moon and study its interior. If that happens we may find out just how much time Phobos has left.

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NASA has an Office of Planetary Protection. Its purpose is not to protect Earth from invasion by some green skinned monsters, but rather to protect the rest of the Universe from us. With the recent evidence of liquid water on Mars, the possibility of life on the red planet has become a bit more possible, and protecting potential martians from an alien threat is a real challenge.

While Earth is the only planet for which the presence of life is certain, we know that living things can be extraordinarily hardy. We’ve seen organisms thrive in areas of extreme temperature and acid environments. Tardigrades (also known as water bears) have survived the cold vacuum of space, and bacteria have been found within nuclear waste. So it’s not unreasonable to imagine a terrestrial organism hitching a ride on a spacecraft. If that spacecraft comes into contact with a potentially habitable environment they could become an invasive species on another world.

To protect against such an event, NASA has defined different levels of risk, each requiring a different level of spacecraft decontamination. These categories are part of the Outer Space Treaty, which is in international agreement of space faring nations. Under the agreement there are four broad categories:

I – No risk of contamination. This includes orbiters and landers on Mercury and Jupiter’s moon Io, where the chance of life is (as far as we know) zero.

II – Negligible risk. Landers on the Moon and Venus, for example, where indigenous life is not likely to be found.

III – Slight risk. This typically applies to flybys and orbiters for worlds that might have life, such as Mars, Jupiter’s moon Europa and Saturn’s moon Enceladus. These worlds are known to have liquid water and conditions that could sustain life, but the mission is not scheduled to come into direct contact with them.

IV – Possible risk. Landing on potentially habitable worlds such as Mars, Europa and Enceladus.

A Viking lander being prepared for dry heat sterilization, which is currently the most stringent method. Credit: NASA

The last category is further differentiated into three subcategories, based upon whether a lander mission will contain experiments seeking to detect life, or whether it will enter a “special region” such as the areas of Mars where liquid water could be present.

Because of these subcategories the Curiosity rover currently active on Mars is forbidden to study a region with liquid water, even though it’s less than 30 miles away from one of them. NASA’s next generation of Mars rovers scheduled to launch in 2020 cannot study the regions, since they won’t meet the most stringent of sterilization levels.

While robotic missions can be sterilized to high standards, crewed missions cannot. Currently any landing of a crewed mission must first be visited by a robotic mission to verify the region is inhospitable, and humans wouldn’t be allowed to enter special regions. But will that be enough? Our dreams of a human voyage to Mars will be an invasion of terrestrial organisms, and that could put our dreams of finding alien life at risk.

This post first appeared on Forbes

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In The Martian, journeys to Mars are made possible through a large spacecraft known as the Hermes. Unlike the Apollo program, where each trip to the Moon required a separate spacecraft, the fictional Ares program uses the Hermes as a taxi to between Earth and Mars. Individual missions dock with the Hermes, but the Hermes simply makes the rounds between Earth and Mars over and over. While the Hermes is a work of fiction, it’s based in well established science.

The first ideas a spacecraft traveling between Earth and Mars are nearly a century old. In 1925 Walter Hohmann proposed an elliptical orbit between the two worlds. The Hohmann transfer orbit, as it came to be known, relies on Earth and Mars to be in the right positions relative to each other so that a spacecraft in a Hohmann orbit. This occurs about every 26 months, and a low delta-v trajectory. While it has its advantages, the one big disadvantage is that each Hohmann orbit has a different orientation each time. Another problem is that the orbits of Earth and Mars are not quite in the same plane, so things aren’t quite as simple as Hohmann proposed.

To have a large spacecraft that passes Earth and Mars with each orbit, you need some kind of thrust to adjust your orbit. In principle, chemical rockets could do the job, but they aren’t well suited for it. Chemical rockets are great for producing a large thrust in a short time, but a craft like the Hermes would need gradual thrust over longer periods. This can be done with ion drives, which accelerate charged particles at high speeds. In the novel, ion drives accelerate the Hermes at a constant 2 mm/s², which is enough to continually adjust the orbit to match Earth and Mars. While we don’t yet have drives powerful enough for a craft like Hermes, ion drives are being used in missions such as the Dawn spacecraft currently at Ceres.

The only real disadvantage of ion drives is calculating their trajectories. If a spacecraft is continuously accelerating, its trajectory has to be determined computationally. This posed a real challenge for Andy Weir as he was writing the book. To get realistic trajectories for the Hermes he had to write a program to calculate them, and fiddle with parameters until he got a set of trajectories that worked. You can see the resulting trajectories here.

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At the dawn of the 20th century, Percival Lowell saw canals on Mars. This network of dark bands across the martian surface must surely be signs of open water. Their geometric patterns hinting that they were built by alien life. Subsequent observations proved Lowell wrong, and it serves as a cautionary tale about making unsubstantiated claims. With that in mind, let’s look at yesterday’s NASA announcement of liquid water on Mars.

Lowell’s sketches of the martian canals.

The history of our search for water on Mars is a bit convoluted, and like many discoveries in science it has happened in small steps rather than a revolutionary discovery. We’ve known that water exists on Mars for some time. First hinted by the discovery of martian polar caps, and then later when detailed observations of the northern ice cap confirmed that it was largely water ice. We’ve also found surface on other parts of Mars, and ice in the martian soil.

Water ice on the surface of Mars.

But it’s always been frozen water or water vapor, and that has to do with the temperature and atmospheric pressure on Mars. The martian surface can vary in temperature from -80 to 10 Celsius  (-110 to 50 F), and atmospheric pressure is 0.06% that of Earth at sea level. As a result, pure water ice sublimes directly to vapor, just as dry ice does on Earth. Just as our atmosphere isn’t thick enough to allow frozen carbon dioxide to melt, the atmosphere of Mars makes it impossible for liquid water to form. So we know that even a small lake of water on Mars was impossible. Mars was once a wet world, but it is now a desert planet.

Recurring slope lineae. Credit: NASA / JPL / UA / Emily Lakdawalla

But there has been some hints of liquid water appearing occasionally on the surface. The most tantalizing bit of evidence has been recurring slope lineae (RSL). They look similar to what you might see if you scoop your hand in wet sand. Observations of RSL have found that they are seasonal, and are most present when the surface temperature is warmest. This screams liquid water, but we don’t want to fall into Lowell’s trap of over extrapolating. “Warm” is a relative term here, and the atmospheric pressure is still far to low for RSLs to be caused by pure water.

Which brings us to yesterday’s announcement. Spectroscopic observations of RSLs from the Mars Reconnaissance Orbiter have found evidence of perchlorate salts in those regions. These are water soluble, and could create a briny water that could (temporarily) maintain a liquid state. This wouldn’t be a flow of water, but would be enough to wet the soil like a damp sponge. It’s possible that the salts would attract water moisture out of the air and dissolve to create a brine.

This is the best evidence we have of liquid water on Mars. If by liquid you mean damp soil. But even that is enough to get some folks excited about the possibility of life currently existing on Mars. Damp sand and soil on Earth is filled with life, and if we want to look for life on Mars, visiting a region of RSLs would be a great place to look. But we don’t want to assume too much. The possibility of life is very different from the confirmation of life.

Paper: Lujendra Ojha, et al. Spectral evidence for hydrated salts in recurring slope lineae on Mars. Nature Geoscience doi:10.1038/ngeo2546 (2015)

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Let’s drive across country, Dad said. It will be fun, Dad said. Two days in you’re squirming in your seat and your siblings are driving you nuts. A week-long road trip is bad enough, but imagine taking a trip for 18 months. That’s how long a round-trip mission to Mars could take. The challenge of making such a mission succeed isn’t just about engineering, it’s also about psychology.

To study the psychological challenges of such a mission, NASA has begun an isolation study with a crew of 6 who will spend a year in a dome not 12 meters wide. If that isn’t bad enough, the team will have no fresh food and no fresh air. If they want to step outside for a bit to calm down, they’ll have to do it in a space suit. Nothing like a bit of stir crazy in the name of science.

From earlier experiments we know conflicts will arise. The purpose of the study is not to spend a year without conflicts, but rather to study what causes them and how they can best be resolved. What we learn with the study will play a vital role in the success of a future Mars mission. We already know how to build spacecraft capable of traveling to Mars, and we know how to build habitats in space and in remote regions like Antarctica.

But the road trip to Mars is very long, and there aren’t any rest areas along the way.

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Mars had a continental crust similar to early Earth’s. That’s the finding of a new paper published in Nature Geoscience.

The results come from Gale crater, where the Curiosity rover found igneous rocks that are light in color. Analysis of these rocks found that they were rich in feldspar. They are similar to terrestrial rocks found in Earth’s continental crust about 2.5 billion years ago. The rocks are very different from the dark and dense basalt rocks that make up other portions of Mars’ surface. Combined with earlier evidence of a wet young Mars, it seems the planet’s history is more similar to Earth’s than we originally thought.

What’s interesting about this research is that it could only be done on site. This is why we send probes and rovers to other worlds. Sometimes just looking from a distance isn’t enough.

Paper: V. Sautter et al. In situ evidence for continental crust on early Mars. Nature Geoscience. doi: 10.1038/ngeo2474 (2015)

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Mars is now a dry, dusty world, but we know it had significant surface water in its past. The predominant view is that early Mars was likely warm and wet, with a large ocean in its northern hemisphere. In addition to the geographic features, we also have things such as isotopic evidence to support the theory of early Martian oceans. The idea of a warm and wet Mars is exciting because it might have provided an opportunity for Martian life to appear. But an alternative model is that Mars was always cold. Early water on Mars was largely ice, much as it is today. Recent work published in the Journal of Geophysical Research supports this “cold Mars” model.

The team used climate models and the orbital history of Mars to simulate both a warm/wet and cold/icy scenario. What they found was that the cold/icy model arose more naturally, given the distance of Mars from the Sun and the young planet’s axial tilt. A more tilted Mars, for example, would have caused ice to form more along the equator of the planet. This could explain some of the erosion features seen in the equatorial region.

It should be emphasized that this is not definitive proof that young Mars was icy. Other factors such as meteor impacts could have triggered a warming period to produce a wet planet. But it does demonstrate that proponents of the warm/wet model of Mars still have things to account for, and the climate history of Mars remains a puzzle.

Paper: Robin D. Wordsworth, et al. Comparison of “warm and wet” and “cold and icy” scenarios for early Mars in a 3D climate model. Journal of Geophysical Research – Planets, DOI: 10.1002/2015JE004787 (2015)

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We’ve known for a while that Mars was a wet planet about 4 billion years ago, but less clear was where that water went. Did it freeze into the surface, or evaporate into the planet’s thin atmosphere? Now a new paper in Science answers a bit of that question, and shows that Mars once had even more water than we’ve thought.

The work looks at water evaporating from the polar ice caps of Mars. In particular, the authors measured the level of deuterium water in the atmosphere compared to regular water. Deuterium is an isotope of hydrogen that has a proton and neutron in its nucleus, rather than just a proton. Since deuterium is almost twice as heavy as regular hydrogen, deuterium water (HDO) is a bit heavier than regular water (H₂O). This means that when water evaporates, HDO is a bit more likely to be left behind. In Earth’s oceans deuterium isn’t very common compared to hydrogen, and exists at about 26 parts per million.

What they found was that the deuterium levels in Martian water is about seven times greater than that of Earth. Since the water of Earth and Mars would likely have had similar origins, the higher deuterium level on Mars means that much of the planet’s water must have evaporated into the atmosphere, and eventually into space. From the deuterium levels it was calculated that Mars must have had about 20 million cubic kilometers of water on its surface. Given the terrain of Mars, that would be an ocean covering most of the planet’s northern plains.

Of course that amount of water is calculated just from the evaporation of water. Mars could have had even more water that froze below the planet’s surface, which wouldn’t have changed the ratio of deuterium to hydrogen. So the level calculated is a lower bound on the amount of water Mars once had. It’s clear then that Mars, like Earth, was once a blue ocean world.

Paper: G. L. Villanueva, et al. Strong water isotopic anomalies in the martian atmosphere: Probing current and ancient reservoirs. Science DOI: 10.1126/science.aaa3630 (2015)

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Valles Marineris, or Mariner Valley, is one of the most prominent features on Mars. It’s often compared to the Grand Canyon, but is about 7 times wider, 4 times deeper, and 9 times longer. If such a feature existed on Earth it would stretch from New York to Los Angeles. But despite their similarities as great canyons, they have radically different histories.

The Grand Canyon likely began to form about 60 – 70 million years ago as different sections began to be carved by water erosion. About 6 million years ago the different regions merged to form the truly grand canyon with the Colorado river flowing through it. Although there is still debate about the exact age and history, it’s clear that the formation of the canyon was driven by water erosion.

That makes sense given that water flow is pretty common on Earth. But Mars is not a river world. While it may have had liquid surface water at some point in its early history, Mars has had its water frozen beneath the surface for much of its history. So it isn’t likely that Valles Marineris is a river canyon. In fact there is plenty of evidence to say that it’s not. What it appears to be is a rift canyon due to geologic activity.

A topographic map of Valles Marineris. Credit: NASA / JPL-Caltech / Arizona State University

Just to the west of the canyon is a volcanic plateau known as the Tharsis bulge. Driven by volcanic and tectonic processes, the bulge began to form about 3.5 billion years ago. As it swelled, fissures began to form in the valley. These fissures then exposed sub-surface water, which flowed through the canyon, eroding it further. As the valley continued to crack and widen, more water was released, which eventually flooded the region to the north and east of the canyon. In the image here you can see this effect, where the high regions of the canyon have distinct cracks where the crust fractured, while the lower regions of the east show more signs of water erosion.

Obviously things aren’t quite that simple, and there is still much debate over the details, but it’s clear that Valles Marineris shows you don’t need a river world to make a grand canyon.

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