Climate change in the Solar System
Alongside Earth, our planetary neighbourhood is changing, but it’s not for the better… it’s for the worse
Alongside Earth, our planetary neighbourhood is changing – but it's not for the better, it's for the worse
It’s no secret that Earth is in trouble, and it is largely our fault. Since the Industrial Revolution we have been pumping so much carbon dioxide and other greenhouse gases into the atmosphere that our planet is rapidly warming. The race is on to keep the rise to under 1.5 degrees Celsius (2.7 degrees Fahrenheit), but it is a target we are predicted to miss. The consequences could be dire: rising sea levels, water shortages, increased migration and the possibility of more frequent wars as we battle each other for resources.
It could turn out to be the greatest foe we have ever faced, and it is largely of our own making, yet there is still time to turn things around. Public awareness of the issue has never been higher, and governments and individuals alike are slowly starting to wake up to their responsibilities – but will it all be too late? Part of the trouble is that the climate of a planet is an incredibly complex system with a lot of moving parts. Throughout its history Earth has warmed and cooled all on its own, alternating between ice ages and more temperate phases. How do we tease out our contribution from these background ups and downs? According to Dr Nicholas Attree, a research fellow at the University of Stirling, we could do a lot worse than to look at our neighbours. “What we see on Earth is natural climate cycles, plus human influence,” he says. “Looking at the cycles of other planets means we can better understand our cycles and better understand our influence.”
Attree has been looking closely at Mars’ past climate. It is the most explored planet in the Solar System, with a host of active rovers trawling the surface and satellites whizzing around it examining the ground from on high. We have discovered that, like Earth, Mars cycles through periods with different climatic conditions. The reason is simple: gravity. Unlike Earth, Mars has no large Moon for stability. Combine that with the fact it is closer to the Solar System’s big boys – Jupiter and Saturn – and it gets bullied by its giant neighbours. Being pulled this way and that leads to a change in Mars’ obliquity – the tilt of the axis on which it rotates. It also changes the shape of Mars’ orbit over time, making is successively more and less circular.
The upshot is that the intensity of sunlight falling on Mars is constantly changing, but in a regular way. A single cycle lasts tens of thousands of years. Attree has been looking at whether these climatic mood swings could have left a detectable signature on Mars today. “During warmer periods there would be an increased heat flow under the Martian surface,” he says. “We’ve modelled how that heat would build up over time.” In November last year he published a prediction that NASA’s InSight may be able to detect that excess heat. InSight landed on the Red Planet in November 2018 and is equipped with a self-hammering ‘mole’
designed to burrow into the Martian dirt. Among its instruments is a thermometer – the Heat Flow and Physical Properties Package, or HP3, perfect for looking at Mars’ sub-surface heat.
Unfortunately the mission has been beset with difficulties. On its first attempt the mole reached a depth of just 35 centimetres (13.8 inches) before getting stuck. Mission scientists are still trying to puzzle out the problem and see if it can get as deep as planned, but by their own admission it isn’t looking promising. Detecting Attree’s predicted excess now looks difficult. “We were only likely to find it if the instrument was functioning perfectly,” he says. All is not lost, however. There is another way to keep track of Mars’ past climate cycles: carbon dioxide. Today the gas that’s causing us so many woes on Earth is the main constituent of the Martian atmosphere. Yet the air is so thin that the atmospheric pressure on Mars is just 0.6 per cent of Earth’s. Carbon dioxide is also frozen into the Martian ice caps. When changes to Mars’ orbit and tilt increase the Sun’s intensity, the carbon dioxide ice sublimates – turns straight from a solid to gas – and carbon dioxide is added to the Martian atmosphere. When things turn colder, the gas is deposited back onto the ice caps. In the 1960s it was predicted that the atmospheric pressure on Mars cycles in this way, getting as low as four-times less than today’s level and as high as double. Yet evidence to back this up has remained elusive. Then, in December 2019, a new study claimed to have found it at long last.
It all hinges on the layers of carbon dioxide dry ice and water ice on the planet’s south pole. A kilometre (0.62 miles) deep, it contains as much carbon dioxide as currently exists in the entire atmosphere. Radar measurements from orbiting satellites suggest the cap is formed of alternating layers of dry and water ice. Dry ice trapped under water ice shouldn’t be stable, yet it seems to persist.
Modelling by Peter Buhler, a planetary scientist at NASA’s Jet Propulsion Laboratory, is attempting to explain its longevity. Each time Mars warms up, some of the dry ice remains trapped under the water ice. The carbon dioxide that does escape is eventually deposited back on top of the water ice when temperatures plummet. That leads to the layering we see. Studying these layers should allow researchers to more accurately construct a picture of Mars’ climate stretching back billions of years to a time when the planet may have been habitable. Ultimately we may get a better answer to the question of whether there’s ever been life on Mars.
According to Michael Way from NASA’s Goddard Institute for Space Studies, it could also help work out where to land when planning future human missions to the Red Planet. “They’d definitely want to talk to the climate modellers,” he says. “It could tell you where to place your settlements or where the sub-surface water is most likely to be.” Way and his colleagues have been adapting NASA’s model of Earth’s climate and applying it to other bodies in the Solar System, including Mars. It’s known as a general circulation model. “It combines factors such as ocean circulation, wind circulation, cloud dynamics and different types of cloud,” Way says.
“It also estimates how many photons of light enter our atmosphere and are absorbed or reflected.” Porting this model over to other worlds is not an easy task. “Applying it to modern Mars is very challenging,” he says. It should get easier with the passage of time as improvements in computing power allow more intricate models to run in a shorter amount of time.
If Mars is hard, then modelling Venus’ climate is even tougher. The world called Earth’s ‘twin’ is an unforgiving hellhole. Thick clouds of carbon dioxide trap the Sun’s heat, sending temperatures soaring beyond 400 degrees Celsius (752 degrees Fahrenheit). The atmospheric pressure is nearly one hundred times greater than Earth’s and over 15,000-times higher than on Mars. That has severely restricted our ability to land space missions on Venus. Those that did make it to the surface succumbed very quickly to the mayhem. “We have very few data points for Venus,” says Way. Unlike Mars you can’t just run rovers around, taking lots of temperature measurements. “Our models struggle as a result,” he says.
The models that have been devised so far point to two different possible climatic histories for Venus, depending on how long the planet’s early magma ocean hung around. The rocky planets were formed when lumps of rock and metal called planetesimals smashed into one another with such ferocity that the solid materials melted. Being closer to the
Sun, whose light was more intense at the time, combined with the presence of a hot magma ocean, created an atmosphere of steam and carbon dioxide. “The atmospheric pressure would have been one
thousand times greater than the modern Earth,” says Way. A molecule of water is H2O – two atoms of hydrogen bonded to one atom of oxygen. On a hot Venus this bond would have been broken regularly. The hydrogen is lost to space and the oxygen becomes trapped inside the magma ocean. “If that’s the case then Venus has been a dry, desiccated world for most of the last 4 billion years,” says Way. The alternative is that the magma ocean was a much shorter lived phase. “Then it would have been cool enough to condense water into lakes, rivers and oceans,” says Way. In other words, far more Earth-like than today. Perhaps the Solar System had two habitable planets at the same time.
If it’s the latter then Venus has experienced a huge change in climate over the last 4 billion years, largely thanks to the role of carbon dioxide. Given our current climate predicament on Earth, is there anything we can learn from our neighbour? “It’s a compelling idea,” says Way, “but it is a difficult comparison to make.” The driver of Venusian climate change is largely thought to have been large-scale volcanism dumping huge quantities of carbon dioxide into the atmosphere, far more than we’ve added to the Earth’s since the Industrial Revolution. “Unlike Venus, the Earth will eventually adjust to this increase in carbon. We just may not be around to see it,” Way says.
Along with Venus and Mars, Way and his colleagues have also secured funding to model the climate of Titan – Saturn’s largest satellite and the only moon in the Solar System with a thick atmosphere. “Titan is interesting as the density of its atmosphere is only 1.5 times Earth’s,” Way says. “A lot of its climate dynamics are similar too.” Part of the attraction of studying Titan is the wealth of data that came back from the Cassini mission