Why is Mars continuing to wobble?
Researchers confirm that the Red Planet is spinning off its axis
Researchers have confirmed that the Red Planet is spinning off its axis
Grab your telescope and gaze at Mars. Are you able to spot something unusual as you focus in on the planet, paying particular attention to its white caps? Not even a little bit? Here’s a clue: we’re referring to a teeny, tiny wobble that occurs as the poles wander from the Red Planet’s axis of rotation. Still can’t see it? Don’t worry – it’s taken scientists decades to spot some rather odd behaviour on the Red Planet.
For the past 18 years, radio tracking observations determined from satellites orbiting the planet have been able to show stark evidence of the Chandler wobble on Mars – a variation of latitude named after American astronomer Seth Carlo Chandler, who discovered the phenomenon in 1891. In simple terms, it means the Red Planet is repeatedly wobbling as it spins, in this case by just ten centimetres (four inches) from the planet’s axis of rotation – that’s why you’re unlikely to see it for yourself. If you’re after a nailed-on explanation of why it’s continuing to happen, then you’re sadly out of luck.
Although scientists have made a breakthrough in determining that the Red Planet wobbles in its rotation, they are not exactly sure what is driving it. What they’ve gleaned from studying the data so far is that the Chandler wobble on Mars occurs in a near-circular, counterclockwise direction, as viewed from its north pole, every 207 days. As a result, the poles don’t always line up perfectly. The Red Planet is the only other body in the universe known to exhibit such behaviour, with the phenomenon only ever discovered and confirmed on Earth before.
It makes the new discovery highly significant, and not just because there are now two planets resembling a spinning top teetering as it loses speed. This latest study shows the sheer importance of gathering and analysing information over a long period of time – a laborious process requiring heaps of patience, but one that has proven ultimately rewarding.
“With 18 years of data, the Chandler wobble signal is very clear,” explains Alex Konopliv, an aerospace engineer at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. And as more data comes through, the better the conclusions will become.
Before we take a closer look at the Chandler wobble on Mars, let’s first briefly examine its effect on Earth by way of comparison. We know that our planet rotates on its axis every 24 hours – every 23 hours, 56 minutes and four seconds, to be exact – but while you may excitedly spin a globe on your desk and smoothly reach a destination when it comes to a halt, real life isn’t quite like that.
“With 18 years of data, the Chandler wobble signal is very clear” Alex Konopliv
On Earth, the poles repeatedly wander away from the average axis of rotation by as much as nine metres (30 feet) every 433 days. We don’t feel it, of course, and the only downside is that the wobble needs to be considered when observing Earth or working with GPS. But one thing is certain: it is much more perceptible than the very slight wobble discovered on Mars, so how did the scientists reach their conclusion given such circumstances?
The answer comes in the form of NASA satellites, which have been orbiting Mars for different reasons. The Mars Global Surveyor, for instance, was used to map the entire planet between 1999 and 2006. Mars Odyssey has been seeking evidence of water and ice while studying the planet’s geology and radiation environment since 2001. And the Mars Reconnaissance Orbiter has been looking for potential future landing sites since 2006.
Konopliv tells All About Space that researchers have been able to collect enough precise information from these satellites to calculate the effects of gravitation on the orbits of the spacecraft. They’ve determined the pole motion from radio tracking observations, and it’s led to a better understanding of the planet. “The Chandler wobble was detected because it affects the orbit of spacecraft at Mars,” Konopliv affirms. “It mostly causes a slight oscillation of the orbital plane of a spacecraft with a period of a Mars day.”
The method has been detailed in a study recently published by the American Geophysical Union in its peer-reviewed scientific journal, Geophysical Research Letters. “The size of the oscillation changes slowly over the months and years, and we see it as a time-varying signature in the Mars gravity field,” Konopliv continues.
“This signature is detected using Doppler tracking data of the spacecraft from the NASA Deep Space Network of stations. [It measures the spacecraft’s speed along the direction from Earth’s tracking station to the spacecraft.] Those measurements help us determine the spacecraft’s orbit and how it changes over time.”
Since the signal is incredibly small, and changes so slowly over time, having many years of highly accurate data has proved vital in the wobble’s detection. “Additionally, there are other timevarying signatures in the gravity field that must be separated from the Chandler wobble signal,” Konopliv says.
“These other signatures are due to the seasonal melting of the polar ice caps and the resulting movement of mass between the north and south poles. Our previous attempts with less data to detect the Chandler wobble were unsuccessful because we could not distinguish between the mass movement and the wobble.”
“The Chandler wobble affects the orbit of spacecraft at Mars” Alex Konopliv
But why does Mars wobble in the first place? Like Earth, which is 0.3 per cent thicker in the middle, the Red Planet – which is 0.6 per cent thicker – is not a perfect sphere, leading to imbalances that have an impact on both of these planets’ spin. As such, it’s determined that the Chandler wobble happens on planets that are not perfectly round, and this is why the phenomenon has long been thought to take place on planets other than Earth. It’s just that scientists have not had firm evidence.
In the case of Mars, the spinning is understood to have begun due to seasonal atmospheric changes caused by the melting of the polar ice caps. If left alone, however, a planetary wobble of this nature should slow down over time. “The time to die down on Mars is the range of 7 to 63 years,” Konopliv says, but that is not happening. So why is that?
For Earth, past studies suggest the excitation of the wobble is likely due to pressure changes in the atmosphere combined with oceanic processes, keeping things moving. In 2000, for example, JPL geophysicist Richard Gross said that fluctuating pressure at the bottom of the ocean – which is caused by temperature and salinity changes – was the principal cause, along with wind-driven changes in the circulation of the ocean.
“The Chandler wobble of Earth is mainly excited by the oceans and the atmosphere,” affirms Belgian geophysicist Véronique Dehant. Yet Mars does not have oceans, so could other external factors help explain the excitation? Maybe it’s due to the polar ice caps melting…
Konopliv thinks not. He says that the seasonal melting and reforming of the polar ice caps is an annual signal that is nearly repeatable. Taking into account that a Mars year is 687 days and a Chandler wobble period is 207 days, Konopliv says the wobble is shown to take place 3.3 times a year.
As a result, any mass signatures from melting polar caps would show exactly one, two, three or four times a year, and would be distinctly different from the wobble. “That is the reason why an extensive dataset is needed, because we’ve been able to separate the Chandler frequency from any third-annual signature,” Konopliv says.
That just leaves pressure changes as the primary cause of the ongoing wobble. “For Mars, the principal excitation is likely of atmospheric origin,” Konopliv explains. And yet the issue could still run deeper, with the motion driven by the properties of Mars’ mantle, something which is being explored by Dehant. “To be detectable, the Chandler wobble requires the presence of a continuous forcing at a period close to that of the wobble,” recaps Dehant. “The wobble of Earth is mainly excited by the oceans and the atmosphere, and for Mars, which doesn’t have oceans, atmospheric processes are the main driver.
“Without forcing, the wobble would decay away after less than 100 years for Mars and about 350 years for Earth, so the Chandler wobble could also be excited by internal processes, like planetary quakes or flows in the liquid core. Measuring the Chandler wobble on different planets therefore provides not only knowledge about the forcing processes and material properties, but also insights into comparative planetology.”
Whatever the cause, the wobble is providing fresh insights into the interior of Mars, notably its material properties and thermal state. By assessing the amount of time it takes for the pole to complete a wobble cycle, scientists learn the extent to which the Red Planet’s mantle can deform.
”The deformations of the Martian mantle mainly depend on its rigidity, and the rigidity is strongly dependent on temperature,” Attilio Rivoldini, a physicist at the Royal Observatory of Belgium, tells
All About Space.
“By measuring the Chandler wobble period, we can deduce information about the thermal state because we have a good knowledge about the rigidity of candidate Mars mantle materials. This knowledge has mainly been acquired by studying the composition of Martian meteorites and by
“The deformations of the mantle depend on rigidity, and the rigidity is strongly dependent on temperature” Attilio Rivoldini
performing laboratory experiments about the material properties of candidate materials.”
The wobbling of Mars has certainly piqued the interest of scientists, and further studies are sure to be carried out over the years. New knowledge about the planet’s temperature and composition is vital in gaining a better picture of the planet, and Dehant is among those at the forefront of future missions.
“I think the next study of interest will be the measurement of the Mars nutation from the InSight mission,” says Dehant. “Nutations are periodic changes in the orientation of the planet, mainly due to the gravitational interaction with the Sun, and the amplitude of the nutations depends on a well-known forcing and on the interior structure of Mars, in particular on the liquid core.” Ongoing measurements of the rotation of Mars using the RISE experiment on InSight – the robotic lander studying the Red Planet’s deep interior which launched in 2018 – is already proving exciting, and could lead to many breakthroughs.
“By comparing the measured nutation with the external forcing, the core radius can be determined, and constraints on the chemical composition of the core be deduced,” Dehant adds. “Unlike the Chandler wobble, a resonant amplification can only occur if Mars has a liquid core. By measuring the Chandler wobble and the nutations, complementary knowledge about the interior structure of Mars can be obtained.” Scientists are sure to be shaking with excitement at what could be unearthed.