Despite decades of exploration and study, Mars still has its fair share of mysteries. In particular, scientists are still trying to ascertain what happened to the water that once flowed on Mars’ surface. Unfortunately, billions of years ago, the Martian atmosphere began to be stripped away by the solar wind, which also resulted in the loss of its surface water over time – although it was not entirely clear where it went and what mechanisms were involved.
To address this, a team of scientists recently consulted data obtained by three orbiter missions studying the Martian atmosphere. In the process, they found evidence that the smaller regional dust storms that happen almost annually on Mars are making the planet drier over time. These findings suggest that storms are a major driving force behind the evolution of Mars’ atmosphere and its transition to the freezing and desiccated place we know today.
Lightning is one of the most powerful forces in nature. Up to 1 billion volts of electricity can flow into a strike in less than a second. Such a large energy buildup can be created by even a relatively simple cause – two particles rubbing together. A team at the University of Oregon has now studied whether those simple interactions might cause lightning on a place it hasn’t been seen before – on Mars.
Data in the world of astronomy is spread out in so many different places. There are archives for instruments on individual spacecraft and telescopes. Sometimes all that is needed to get new insight out of old data is to collect it all together and analyze a whole set rather than isolated instances. That is exactly what happened recently when a team from the Harvard Center for Astrophysics collected and analyzed data about almost 15,000 dust storms that have taken place on Mars over the last eight Martian years.
When a huge dust storm on Mars—like the one in 2018—reaches its full power, it can turn into a globe-bestriding colossus. This happens regularly on Mars, and these storms usually start out as a series of smaller, runaway storms. NASA scientists say that these storms can spawn massive towers of Martian dust that reach 80 km high.
And that phenomenon might help explain how Mars lost its water.
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.