Space Sunday: the sands of Mars and Planet Nine

The spread of the 2018 dust storm as seen by NASA’s Mars Reconnaissance Orbiter between May 31st and June 10th. The black gaps in the images indicate parts of the planet’s surface not captured in the individual images making up this time-lapse view. Note the position of the Opportunity and Curiosity rovers. Credit: NASA / MSSS

Dust storms on Mars are not unusual event; they occur in both hemispheres with the changing of the seasons, and can even grow to encompass the entire planet.

Just such world-girdling dust storm occurred in 1971, and was caught by the cameras on NASA’s Mariner 9 space vehicle when it arrived in the vicinity of Mars in November of that year. The images Mariner 9 returned from Mar as it entered orbit (becoming in the process the first man-made object to orbit another planet) show the entire surface of Mars totally obscured by a blanket of dust that reached high up into the atmosphere. It took some two months for the storms to abate – although scientists were treated to Mars gradually revealing itself to Mariner 9’s camera as the dust slowly settled, starting with the high peaks of Olympus Mons and the Tharsis Ridge volcanoes, which rise up to 25 kilometres above the mean surface level of Mars.

In 2001, the Hubble Space Telescope (HST) reveal just how all-encompassing these more massive storms on Mars can be, when it took two images of Mars just three months apart. In one, surface features are clearly visible; in the second, Mars appears to be devoid of any detail.

These two images, captured 3 months apart by the Hubble Space Telescope in 2001, shoe how all-encompassing the more extreme dust storms on Mars can be. Note that in both images, south is at the top . Credit: NASA / The Hubble Heritage Team

Now, another dust storm is engulfing a huge swathe of Mars. It grew quickly in the opening week and a half of June, While it has not – as yet – engulfed the entire planet, it is raising massive mounts of dust high into the Martian atmosphere, marking it as the “thickest” dust storm witnessed on Mars.

Of to two rovers currently operating on Mars, the Mars Exploration Rover Opportunity is particularly impacted by storms of this nature as it is solar-powered. Such is the volume of dust lifted into the Martian atmosphere when these more extreme storms occur that they can severely limit Oppy’s ability to gather sunlight to charge its batteries.

This is not the first such dust storm Oppy has encountered; in 2007, a large-scale storm resulted in a severe degradation in the amount of sunlight reaching the Martian surface where the rover was operating. At that time, we were treated to some remarkable images of just how all-pervasive the dust can become when lifted into the tenuous Martian atmosphere.

This series of images show the onset of a severe dust storm, as seen by NASA’s Opportunity rover in 2007. On the left, Sol 1205, is “normal” daylight conditions. The remain shots show the reduction in daylight between Sol 1220 and Sol 12035. Credit: NASA / Cornell University

Even so the current storm is perhaps the most severe Oppy has had to face. So much so that even though the decision was quickly made to suspend all science gathering operations as it explores Endeavour Crater, and so reduce its power output, the rover has since switched itself into a further “safe” mode of power conservation.

This  kind of more massive storm is particularly prone to occurring when summer comes to one of the hemispheres (in this case, the southern hemisphere). At this time, the increased sunlight warming the atmosphere causes an increase in wind activity, which results in more dust being lifted into the atmosphere. For so reason, this dust causes the winds to persist  – and even increase, lifting more dust, and a feedback loop is created, turning the process into a self-driving storm that can take weeks or months to die down.

A couple of interesting points with these dust storms is that firstly, and for those familiar with the Matt Damon vehicle The Martian, the winds are nowhere near as violent as portrayed by that film. While wind speeds during these storms can reach speeds of 96-160 km/h (60-100 mph), the Martian atmosphere is so tenuous, the overall effect of such wind speeds is akin to a stiff breeze here on Earth. The second point is that while they do reduce the amount of sunlight reaching the surface of Mars, the dust is an effective insulator, both reducing the amount of heat being radiated away from Mars whilst simultaneously absorbing solar radiation, both of which serve to raise ambient surface temperatures.

This latter point is in part good news for Oppy, as it helps reduce the rover’s power outlay in keeping itself and its instruments warm. However, given that such intense storms can last for periods of several weeks to months at a time, there is genuine concerns as to how well Opportunity might survive if this storm is particularly drawn out, leaving the MER team on Earth reasonably confident the rover will be able to survive the storm without its systems becoming too cold to be restarted.

By June 10th, the storm had grown to a size where it was starting to make itself felt in Gale Crater, where NASA is operating the Mars Science Laboratory rover Curiosity, although the effects haven’t been as great as around Endeavour Crater, which Opportunity has been exploring. When it comes to dust storms, Curiosity has a significant advantage over Opportunity in that it is nuclear powered and is thus its power systems aren’t affected by any loss of  sunlight.

The dust storm reaches Gale crater: on the left, a true colour image from Curiosity’s Mastcam taken of the east-north-east rim of Gale Crater taken on June 7th, 2018 (Sol 2074). On the right, the same view seen three days later, on June 10th, 2018 (Sol 2077). Credit: NASA / MSSS

By the time the dust storm reach Gale Crater, it was blanketing to 35 million square kilometres (14 million sq miles) of the Martian surface – or roughly one-quarter of the entire planet, and it was still growing. As well as bing observed by the two surface rovers, it is also being watched from space by the combined network of NASA’s Mars Reconnaissance Orbiter (MRO), Mars Odyssey and MAVEN space vehicles, as well as Europe’s Mars Express mission and India’s Mars Observer Mission.

Observing and probing this kind of storm is seen as vital on a number of counts. In the first place, the precise mechanism that causes the feedback loop of wind and dust mentioned above isn’t really understood, so seeing storms like the develop and abate can help scientists to fill-in the blanks. In addition, and as NASA’s Mars Programme Office chief scientist Rich Zurek explains:

Studying their physics is critical to understanding the ancient and modern Martian climate. Each observation of these large storms brings us closer to being able to model these events, and maybe, someday, being able to forecast them. That would be like forecasting El Niño events on Earth, or the severity of upcoming hurricane seasons.

This latter point is particularly important in terms of planning for future missions – including any human mission to Mars. Being able to predict the rises and potential scope of these storms could go a long way to ensuring human safety on Mars. However, for the duration of this storm, all eyes are on little Opportunity, caught in the midst of it, with the hope that the rover will come through the storm able to resume its record-breaking 14+ years of operations on Mars.

Doubt Cast on Planet Nine’s Existence

It had been thought that Planet Nine, if it exists, might equal Neptune in size, and orbit the Sun 200 times further away than Earth. However, a new study casts doubt on its existence. Credit: Caltech / R. Hurt

I’ve covered the conundrum of Planet Nine  – or “Planet X”, “George”, “Jehoshaphat”, or “Planet of the Apes”, depending your preference – numerous times in Space Sunday (see here, here, and here for more).

In brief, astronomers have observed a number of bodies in the Kuiper Belt (referred to as Kuiper Belt Objects, or KBOs), occupying highly unusual orbits around the Sun, referred to as “detached” orbits. Sedna, the minor planet discovered in 2003is one of these bodies. It orbits the Sun once every 11,000 years in an highly elliptical ranging from 76 AU at perihelion to 936 AU at aphelion.

As Sedna and the other bodies in similar odd orbits are well beyond the orbit of Neptune, and so free from any known gravitational influence that might “deflect” them into their odd paths around the Sun, the theory is that there must be another relatively large planetary body lurking out in the depths of the solar system which, thanks to its own odd orbit, gradually “teased” these bodies into their orbits. Hence: Planet Nine.

Computer modelling had suggested that a large planetary body in an eccentric orbit around the Sun, far out at the limits of the solar system, could be responsible for influencing a number of trans-Neptunian objects (TNOs) into their own eccentric orbits. Credit: Caltech / R Hurt

However, a new study by a team at the Colorado University, Boulder, suggests that while gravity may well have played a role in pushing these KBOs into their odd orbits – it has nothing to do with the presence of a Neptune-sized planet on the very edge of the solar system; rather it could be gravitational interactions between Kuiper Belt Objects (KBOs) themselves that might be responsible for the strange dynamics of the distant rocky bodies.

The researchers didn’t actually intend to through doubt on the Planet Nine hypothesis; rather they were drawn to studying these eccentric KBOs and what the full range of possible interactions among them might be, as the professor overseeing the study explains:

There are so many of these bodies out there. What does their collective gravity do? We can solve a lot of these problems by just taking into account that question… Once you get further away from Neptune, things don’t make any sense, which is really exciting.

Assistant Professor Anne-Marie Madigan, CU Boulder

In particular, team member Jacob Fleisig developed a computer model mapping the motion of KBOs. This model revealed than many of the smaller KBOs travel in clusters, and much faster – relatively speaking – than larger bodies like Sedna. These small clusters of KBOs might be though of as the minute hand of a clock, sweeping around the Sun, and the larger bodies are like the hour hand.

Every so often these two “hands” intersect – with surprising results. Essentially, over time, the clusters of KBOs interact gravitationally with the larger bodies  the intersect, gradually “pushing” them into their “detached” orbits, and the more the large object’s orbit becomes detached, the further it gets away from the Sun, resulting in the eccentric orbits observed with Sedna and similar bodies.

This doesn’t end the search of Planet Nine, but it does offer an explanation for why bodies like Sedna do occupy some very strange orbits without the need a yet-to-be-discovered mystery planet roaming around the Sun. Whether or not this new study dampens enthusiasm in the search for Planet Nine remains to be seen. However, it is interesting to note that it offers an additional explanation as to why some objects become dislodged from the mass (because they also fall prey to the influence of these clusters of KBOs) to fall towards the Sun as comets.

 

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