The enduring, and maybe endearing, mystery around Mars is what happened to its water? We can say with near-certainty now, thanks to the squad of Mars rovers and orbiters, that Mars was once much wetter. In fact that planet may have had an ocean that covered a third of the surface. But what happened to it all?
As it turns out, the global dust storms that envelop Mars, and in particular the most recent one that felled the Opportunity rover, may offer an explanation.
NASA has shared Opportunity’s final photograph from the surface of Mars. The rover’s final resting place is in Endeavour Crater, and barring any statistically unlikely event, it will sit there for centuries, millennia, or even longer. And instead of a tombstone, we have this final image.
Could this be the end of the Opportunity rover? There’s been no signal from the rover since last summer, when a massive global dust storm descended on it. But even though the craft has been silent and unreachable for six-and-a-half months, NASA hasn’t given up.
When Opportunity landed at Meridiani Planum on Mars in January 2004, it’s planned mission length was only 90 days. Since that day, which seems so long ago now, 15 years have passed, and over one billion people have been born on Earth. Six months ago, the rover stopped working, maybe for good. So by every measure, Opportunity has been a stunning success.
A tiny electric motor on the Curiosity rover played a role in identifying a global Martian dust storm. The storm completely enveloped the planet between May and July, 2018. It was the biggest storm since 2007.
With the Scientific Revolution, astronomers became aware of the fact that the Earth and the other planets orbit the Sun. And thanks to Copernicus, Galileo, Kepler, and Newton, the study of their orbits was refined to the point of mathematical precision. And with the subsequent discoveries of Uranus, Neptune, Pluto and the Kuiper Belt Objects, we have come to understand just how varied the orbits of the Solar Planets are.
Consider Mars, Earth’s second-closest neighbor, and a planet that is often referred to as “Earth’s Twin”. While it has many things in common with Earth, one area in which they differ greatly is in terms of their orbits. In addition to being farther from the Sun, Mars also has a much more elliptical orbit, which results in some rather interesting variations in temperature and weather patterns.
Perihelion and Aphelion:
Mars orbits the Sun at an average distance (semi-major axis) of 228 million km (141.67 million mi), or 1.524 astronomical units (over one and a half times the distance between Earth and the Sun). However, Mars also has the second most eccentric orbit of all the planets in the Solar System (0.0934), which makes it a distant second to crazy Mercury (at 0.20563).
This means that Mars’ distance from the Sun varies between perihelion (its closest point) and aphelion (its farthest point). In short, the distance between Mars and the Sun ranges during the course of a Martian year from 206,700,000 km (128.437 million mi) at perihelion and 249,200,000 km (154.8457 million mi) at aphelion – or 1.38 AU and 1.666 AU.
Speaking of a Martian year, with an average orbital speed of 24 km/s, Mars takes the equivalent of 687 Earth days to complete a single orbit around the Sun. This means that a year on Mars is equivalent to 1.88 Earth years. Adjusted for Martian days (aka. sols) – which last 24 hours, 39 minutes, and 35 seconds – that works out to a year being 668.5991 sols long (still almost twice as long).
Mars in also the midst of a long-term increase in eccentricity. Roughly 19,000 years ago, it reached a minimum of 0.079, and will peak again at an eccentricity of 0.105 (with a perihelion distance of 1.3621 AU) in about 24,000 years. In addition, the orbit was nearly circular about 1.35 million years ago, and will be again one million years from now.
Much like Earth, Mars also has a significantly tilted axis. In fact, with an inclination of 25.19° to its orbital plane, it is very close to Earth’s own tilt of 23.439°. This means that like Earth, Mars also experiences seasonal variations in terms of temperature. On average, the surface temperature of Mars is much colder than what we experience here on Earth, but the variation is largely the same.
All told, the average surface temperature on Mars is -46 °C (-51 °F). This ranges from a low of -143 °C (-225.4 °F), which takes place during winter at the poles; and a high of 35 °C (95 °F), which occurs during summer and midday at the equator. This means that at certain times of the year, Mars is actually warmer than certain parts of Earth.
Orbit and Seasonal Changes:
Mars’ variations in temperature and its seasonal changes are also related to changes in the planet’s orbit. Essentially, Mars’ eccentric orbit means that it travels more slowly around the Sun when it is further from it, and more quickly when it is closer (as stated in Kepler’s Three Laws of Planetary Motion).
Mars’ aphelion coincides with Spring in its northern hemisphere, which makes it the longest season on the planet – lasting roughly 7 Earth months. Summer is second longest, lasting six months, while Fall and Winter last 5.3 and just over 4 months, respectively. In the south, the length of the seasons is only slightly different.
Mars is near perihelion when it is summer in the southern hemisphere and winter in the north, and near aphelion when it is winter in the southern hemisphere and summer in the north. As a result, the seasons in the southern hemisphere are more extreme and the seasons in the northern are milder. The summer temperatures in the south can be up to 30 K (30 °C; 54 °F) warmer than the equivalent summer temperatures in the north.
It also snows on Mars. In 2008, NASA’s Phoenix Landerfound water ice in the polar regions of the planet. This was an expected finding, but scientists were not prepared to observe snow falling from clouds. The snow, combined with soil chemistry experiments, led scientists to believe that the landing site had a wetter and warmer climate in the past.
And then in 2012, data obtained by the Mars Reconnaissance Orbiter revealed that carbon-dioxide snowfalls occur in the southern polar region of Mars. For decades, scientists have known that carbon-dioxide ice is a permanent part of Mars’ seasonal cycle and exists in the southern polar caps. But this was the first time that such a phenomena was detected, and it remains the only known example of carbon-dioxide snow falling anywhere in our solar system.
These seasonal variations allow Mars to experience some extremes in weather. Most notably, Mars has the largest dust storms in the Solar System. These can vary from a storm over a small area to gigantic storms (thousands of km in diameter) that cover the entire planet and obscure the surface from view. They tend to occur when Mars is closest to the Sun, and have been shown to increase the global temperature.
The first mission to notice this was the Mariner 9 orbiter, which was the first spacecraft to orbit Mars in 1971, it sent pictures back to Earth of a world consumed in haze. The entire planet was covered by a dust storm so massive that only Olympus Mons, the giant Martian volcano that measures 24 km high, could be seen above the clouds. This storm lasted for a full month, and delayed Mariner 9‘s attempts to photograph the planet in detail.
And then on June 9th, 2001, the Hubble Space Telescope spotted a dust storm in the Hellas Basin on Mars. By July, the storm had died down, but then grew again to become the largest storm in 25 years. So big was the storm that amateur astronomers using small telescopes were able to see it from Earth. And the cloud raised the temperature of the frigid Martian atmosphere by a stunning 30° Celsius.
These storms tend to occur when Mars is closest to the Sun, and are the result of temperatures rising and triggering changes in the air and soil. As the soil dries, it becomes more easily picked up by air currents, which are caused by pressure changes due to increased heat. The dust storms cause temperatures to rise even further, leading to Mars’ experiencing its own greenhouse effect.
Viewing orbital images of the rovers as they go about their business on the surface of Mars is pretty cool. Besides being of great interest to anyone keen on space in general, they have scientific value as well. New images from the High Resolution Imaging Science Equipment (HiRise) camera aboard the Mars Reconnaissance Orbiter (MRO) help scientists in a number of ways.
Recent images from HiRise show the Mars Science Laboratory (MSL) Curiosity on a feature called the Naukluft Plateau. The Plateau is named after a mountain range in Namibia, and is the site of Curiosity’s 10th and 11th drill targets.
Orbital imagery of the rovers is used to track the activity of sand dunes in the areas the rovers are working in. In this case, the dune field is called the Bagnold Dunes. HiRise imagery allows a detailed look at how dunes change over time, and how any tracks left by the rover are filled in with sand over time. Knowledge of this type of activity is a piece of the puzzle in understanding the Martian surface.
But the ability to take such detailed images of the Martian surface has other benefits, as well. Especially as we get nearer to a human presence on Mars.
Orbital imaging is turning exploration on its ear. Throughout human history, exploration required explorers travelling by land and sea to reconnoiter an area, and to draw maps and charts later. We literally had no idea what was around the corner, over the mountain, or across the sea until someone went there. There was no way to choose a location for a settlement until we had walked the ground.
From the serious (SpaceX, NASA) to the fanciful (MarsOne), a human mission to Mars, and an eventual established presence on Mars, is a coming fact. The how and the where are all connected in this venture, and orbital images will be a huge part of choosing where.
Tracking the changes in dunes over time will help inform the choice for human landing sites on Mars. The types and density of sand particles may be determined by monitoring rover tracks as they fill with sand. This may be invaluable information when it comes to designing the types of facilities used on Mars. Critical infrastructure in the form of greenhouses or solar arrays will need to be placed very carefully.
Sci-Fi writers have exaggerated the strength of sand storms on Mars to great effect, but they are real. We know from orbital monitoring, and from rovers, that Martian sandstorms can be very powerful phenomena. Of course, a 100 km/h wind on Earth is much more dangerous than on Mars because of the density of the atmosphere. Martian air is 1% the density of Earth’s, so on Mars the 100 km/h wind wouldn’t do much.
But it can pick up dust, and that dust can foul important equipment. With all this in mind, we can see how these orbital images give us an important understanding of how sand behaves on Mars.
There’s an unpredictability factor to all this too. We can’t always know in advance how important or valuable orbital imagery will be in the future. That’s part of doing science.
But back to the cool factor.
For the rest of us, who aren’t scientists, it’s just plain cool to be able to watch the rovers from above.
Methane on Mars has long perplexed scientists; the short-lived gas has been measured in surprising quantities in Mars’ atmosphere over several seasons, sometimes in fairly large plumes. Scientists have taken this to be evidence of Mars being an ‘active’ planet, either geologically or biologically. But a group of researchers from Mexico have come up with a different – and rather unexpected – source of methane: dust storms and dust devils.
“We propose a new production mechanism for methane based on the effect of electrical discharges over iced surfaces,” reports a paper published in Geophysical Research letters, written by a team led by Arturo Robledo-Martinez from the Universidad Autónoma Metropolitana, Azcapotzalco, Mexico.
“The discharges, caused by electrification of dust devils and sand storms, ionize gaseous CO2 and water molecules and their byproducts recombine to produce methane.”
In a laboratory simulation, they showed that that pulsed electrical discharges over ice samples in a synthetic Martian atmosphere produced about 1.41×1016 molecules of methane per joule of applied energy. The results of the electrical discharge experiment were compared with photolysis induced with UV laser radiation and it was found that both produce methane, although the efficiency of photolysis is one-third of that of the discharge.
The scientists don’t rule out that methane may indeed come from other sources as well, but the way that dust devils and storms can quickly form means they can also quickly generate methane. “The present mechanism may be acting in parallel with other proposed sources but its main advantage is that it can generate methane very quickly and thus explain the generation of plumes,” the team writes.
Methane has been observed in Mars’ atmosphere since 1999, but in 2009, scientists studying the atmosphere of Mars over several Martian years with telescopes here on Earth announced they had found three regions of active release of methane over areas that had evidence of ancient ground ice or flowing water.
They observed and mapped multiple plumes of methane on Mars, one of which released about 19,000 metric tons of methane. The plumes were emitted during the warmer seasons — spring and summer — which is also when dust devils tend to form.
Methane on Mars is enticing because it only lasts a few hundred years in Mars’ atmosphere, meaning it has to be continually replaced. And in the back of everyone’s minds has been the possibility of some sort of Martian life producing it.
“Methane is quickly destroyed in the Martian atmosphere in a variety of ways, so our discovery of substantial plumes of methane … indicates some ongoing process is releasing the gas,” said Dr. Michael Mumma of NASA’s Goddard Space Flight Center in Greenbelt, Md in 2009. “At northern mid-summer, methane is released at a rate comparable to that of the massive hydrocarbon seep at Coal Oil Point in Santa Barbara, Calif.”
The researchers in 2009 thought that the methane was being released from Mars’ interior, perhaps because the permafrost blocking cracks and fissures vaporized, allowing methane to seep into the Martian air.
The unknown has been where the methane has been coming from; if it is being released from the interior, it could be produced by either geologic processes such as serpentinization, a simple water/rock reaction or biologic processes of microbes (or something bigger) releasing methane as a waste product.
But if dust devils and dust storms can also produce methane, the mystery becomes a little more mundane.
The new research by the team from Mexico also mentioned fissures in the surface, but for a different reason, saying that the electric field of dust devils is amplified by the topology of the soil: “The electrical field produced by a dust devil can not only overcome the weak dielectric strength of the Martian atmosphere, but also penetrate into cracks on the soil and so reach the ice lying at the bottom, with added strength, due to the topography of the terrain,” the team wrote.
At a concentration of about 10 to 50 parts per billion by volume, methane is still a trace element in the Martian atmosphere, and indeed the sharp variations in its concentration that have been observed have been difficult to explain. Hopefully the research teams can coordinate follow-up observations of methane production during the dust devil and dust storm seasons on Mars.