Since the 1960s and 70s, scientists have come to view Mars as something of a “dead planet.” As the first close-up images from orbit and the surface came in, previous speculation about canals, water, and a Martian civilization were dispelled. Subsequent studies also revealed that the geological activity that created features like the Tharsis Mons region (especially Olympus Mons) and Valles Marineris had ceased long ago.
However, in the past few decades, robotic missions have found ample evidence that Mars is still an active place. A recent indication was an image taken by the Mars Reconnaissance Orbiter (MRO), which showed relatively fresh landslides in a crater near Nili Fossae. This area is part of the Syrtis Major region and is located just north of the Jezero Crater (where the Perseverance rover will be landing in six weeks!)
These dark, narrow, 100 meter-long streaks called recurring slope lineae flowing downhill on Mars are inferred to have been formed by contemporary flowing water. However, a new study by planetary scientists indicates that these may actually be the result of dry flows. Credits: NASA/JPL/University of Arizona
When robotic missions first began to land on the surface of Mars in the 1970s, they revealed a harsh, cold and desiccated landscape. This effectively put an end generations of speculation about “Martian canals” and the possibility of life on Mars. But as our efforts to explore the Red Planet have continued, scientists have found ample evidence that the planet once had flowing water on its surface.
In addition, scientists have been encouraged by the appearance of Recurring Slope Lineae (RSL), which were believed to be signs of seasonal water flows. Unfortunately, a new study by researchers from the U.S. Geological Survey indicates that these features may be the result of dry, granular flows. These findings are another indication that the environment could be too dry for microorganisms to survive.
This inner slope of a Martian crater has several of the seasonal dark streaks called “recurrent slope lineae,” or RSL, which were caputred by the HiRISE camera on NASA’s Mars Reconnaissance Orbiter. Credits: NASA/JPL-Caltech/UA/USGS
For the sake of their study, the team consulted data from the High Resolution Image Science Experiment (HiRISE) camera aboard the NASA Mars Reconnaissance Orbiter (MRO). This same instrument was responsible for the 2011 discovery of RSL, which were found in the middle latitudes of Mars’ southern hemisphere. These features were also observed to appear on Martian slopes during late spring through summer and then fade away in winter.
The seasonal nature of these flows was seen as a strong indication that they were the result of flowing salt-water, which was indicated by the detection of hydrated salt at the sites. However, after re-examining the HiRISE data, Dundas and his team concluded that RSLs only occur on slopes that are steep enough for dry grains to descend – in much the same way that they would on the faces of active dunes.
“We’ve thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand. This new understanding of RSL supports other evidence that shows that Mars today is very dry.”
Using pairs of images from HiRISE, Dundas and his colleagues constructed a series of 3-D models of slope steepness. These models incorporated 151 RSL features identified by the MRO at 10 different sites. In almost all cases, they found that the RSL were restricted to slopes that were steeper than 27° and each flow ended on a slope that matched the patterns seen in slumping dry sand dunes on Mars and Earth.
Dark, narrow streaks flowing downhill on Mars at sites like the Horowitz Crater are inferred to be due to seasonal flows of water. Credit: NASA/JPL-Caltech/Univ. of Arizona
Basically, sand flows end where a steep angle gives way to a less-steep “angle of repose”, whereas liquid water flows are known to extend along less steep slopes. As Alfred McEwen, HiRISE’s Principal Investigator at the University of Arizona and a co-author of the study, indicated, “The RSL don’t flow onto shallower slopes, and the lengths of these are so closely correlated with the dynamic angle of repose, it can’t be a coincidence.”
These observations is something of a letdown, since the presence of liquid water in Mars’ equatorial region was seen as a possible indication of microbial life. However, compared to seasonal brine flows, the present of granular flows is a far better fit with what is known of Mars’ modern environment. Given that Mars’ atmosphere is very thin and cold, it was difficult to ascertain how liquid water could survive on its surface.
Nevertheless, these latest findings do not resolve all of the mystery surrounding RSLs. For example, there remains the question of how exactly these numerous flows begin and gradually grow, not to mention their seasonal appearance and the way they rapidly fade when inactive. On top of that, there is the matter of hydrated salts, which have been confirmed to contain traces of water.
To this, the authors of the study offer some possible explanations. For example, they indicate that salts can become hydrated by pulling water vapor from the atmosphere, which might explain why patches along the slopes experience changes in color. They also suggest that seasonal changes in hydration might result in some trigger mechanism for RSL grainflows, where water is absorbed and release, causing the slope to collapse.
NASA’s Mars Reconnaissance Orbiter investigating Martian water cycle. Credit: NASA/JPL/Corby Waste
If atmospheric water vapor is a trigger, then it raises another important question – i.e. why do RSLs appear on some slopes and not others? As Alfred McEwen – HiRISE’s Principal Investigator and a co-author on the study – explained, this could indicate that RSLs on Mars and the mechanisms behind their formation may not be entirely similar to what we see here on Earth.
“RSL probably form by some mechanism that is unique to the environment of Mars,” he said, “so they represent an opportunity to learn about how Mars behaves, which is important for future surface exploration.” Rich Zurek, the MRO Project Scientist of NASA’s Jet Propulsion Laboratory, agrees. As he explained,
“Full understanding of RSL is likely to depend upon on-site investigation of these features. While the new report suggests that RSL are not wet enough to favor microbial life, it is likely that on-site investigation of these sites will still require special procedures to guard against introducing microbes from Earth, at least until they are definitively characterized. In particular, a full explanation of how these enigmatic features darken and fade still eludes us. Remote sensing at different times of day could provide important clues.”
In the coming years, NASA plans to carry out the exploration of several sites on the Martian surface using the Mars 2020 rover, which includes a planned sample-return mission. These samples, after being collected and stored by the rover, are expected to be retrieved by a crewed mission mounted sometime in the 2030s, and then returned to Earth for analysis.
The days when we are finally able to study the Mars’ modern environment up close are fast approaching, and is expected to reveal some pretty Earth-shattering things!
A planetary mass object the size of Mars would be sufficient to produce the observed perturbations in the distant Kuiper Belt. (Image: Heather Roper/LPL)
Astronomers have known about the Kuiper Belt for decades, and were postulating about its existence long before it was even observed. Since that time, many discoveries have been made in this region of space – ranging from numerous minor planets to the fact that the orbital planes of Kuiper Belt Objects (KBOs) are widely dispersed – that have led to new theoretical models of the formation and evolution of the Solar System.
For instance, while conducting measurements of the mean plane of minor planets and KBOs, a team from the Lunar and Planetary Laboratory (LPL) at The University of Arizona discovered a warp in orbits of certain, highly-distant KBOs. According to their study, this warp could be an indication of a planetary-mass object in the area, one which orbits our Sun even closer than the theoretical “Planet 9“.
The study – “The Curiously Warped Mean Plane of the Kuiper Belt” which is scheduled to be published in the Astronomical Journal – was produced by Kathryn Volk and Renu Malhotra (two astronomers with the LPL). As they stated in their study, the presence of this planet was confirmed by examining the orbits of icy bodies in the very outer reaches of the Solar System.
Artist’s impression of the yet-to-be-discovered “planetary mass object”, who’s existence has been theorized based on the orbital plane of distant Kuiper Belt objects. Credit: Heather Roper/LPL
Whereas most KBOs – which are leftover material from the formation of the Solar System – orbit the Sun close to the mean plane of the Solar System itself, the most distant objects do not. To determine why, the researchers analyzed the tilt angles of the orbital planes of more than 600 KBOs to determine the direction of their precession – i.e. the direction in which these rotating objects experience a change in their orientation.
As Malhotra – a Louise Foucar Marshall Science Research Professor and Regents’ Professor of Planetary Sciences at LPL – illustrated, KBOs operate in a way that is analogous to spinning tops:
“Imagine you have lots and lots of fast-spinning tops, and you give each one a slight nudge. If you then take a snapshot of them, you will find that their spin axes will be at different orientations, but on average, they will be pointing to the local gravitational field of Earth… We expect each of the KBOs’ orbital tilt angle to be at a different orientation, but on average, they will be pointing perpendicular to the plane determined by the Sun and the big planets.”
What they found was that the average plane of these objects was tilted away from the solar plane by about eight degrees, which suggests that a powerful gravitational force in the outer Solar System is tugging on them. “The most likely explanation for our results is that there is some unseen mass,” said Volk in UA News press release. “According to our calculations, something as massive as Mars would be needed to cause the warp that we measured.”
Animated diagram showing the spacing of the Solar Systems planet’s, the unusually closely spaced orbits of six of the most distant KBOs, and the possible “Planet 9”. Credit: Caltech/nagualdesign
According to their calculations, this Mars-size body would likely orbit the Sun at a distance of roughly 60 AU, and with an orbital inclination that was tilted eight degrees to the average plane of the known planets (i.e. the same tilt as the “warped” KBOs). Within these parameters, a planet of this size would have sufficient gravitational influence to warp the orbital plane of the distant KBOs to within 10 AU on either side of it.
In other words, a Mars-sized planet in the outer Kuiper Belt would be able to influence the orbital inclination of KBOs that are between 50 and 70 AUs from the Sun. This is certainly consistent with what we know about the Kuiper Belt, who’s orbital inclination appears to be consistently flat (i.e. consistent with the rest of the Solar System) past a distance of about 50 AU – but changes between a distance of 50 and 80 AU.
As Volk indicated, there is a possibility that this warping could be the result of a statistical fluke. But in the end, their calculations indicated that this is highly unlikely, and that the behavior of distant KBOs is consistent with the existence of a as-yet-unseen gravitational influence:
“But going further out from 50 to 80 AU, we found that the average plane actually warps away from the invariable plane. There is a range of uncertainties for the measured warp, but there is not more than 1 or 2 percent chance that this warp is merely a statistical fluke of the limited observational sample of KBOs… The observed distant KBOs are concentrated in a ring about 30 AU wide and would feel the gravity of such a planetary mass object over time, so hypothesizing one planetary mass to cause the observed warp is not unreasonable across that distance.”
Artist’s impression of Planet Nine, blocking out the Milky Way. The Sun is in the distance, with the orbit of Neptune shown as a ring. Credit: ESO/Tomruen/nagualdesign
Another possibility is that another object entirely could have disturbed the plane of the outer Kuiper Belt – for instance, a star passing through the outer Solar System. But as Malhotra explained, this explanation is also a highly unlikely, as any disturbance caused by a passing star would only be temporary and would have manifested itself differently.
“A passing star would draw all the ‘spinning tops’ in one direction,” he said. “Once the star is gone, all the KBOs will go back to precessing around their previous plane. That would have required an extremely close passage at about 100 AU, and the warp would be erased within 10 million years, so we don’t consider this a likely scenario.”
Moreover, the tilt of these objects could not be attributed to the existence of Planet 9, who’s existence has also been suggested based on the extreme eccentricity of certain populations of KBOs. Compared to this Mars-sized planet that is thought to orbit at 60 AUs from the Sun, Planet 9 is predicted to be much more massive (at around 10 Earth masses) and is believed to orbit at a distance of 500 to 700 AU.
Naturally, one has to ask why this planetary-mass body has not been found yet. According to Volk and Malhotra, the reason has to do with the fact that astronomers have not yet searched the entire sky for distant for Solar System objects. Beyond that, there’s also the likely position of the object (within the galactic plane), which is so densely packed with stars that surveys would have a hard time spotting it.
However, with the construction of instruments like the Large Synoptic Survey Telescope (LSST) in Chile nearly complete, opportunities to spot it may be coming sooner other than later. This wide-field survey reflecting telescope, which is run by a consortium that includes the University of Arizona, is expected to provide some of the deepest and widest views of the Universe to date (which will begin in 2020).
In the meantime, and in response to any possible controversies regarding the so-called “Planet Debate”, it is worth noting that this body (if it exists) is currently being referred to as “planetary-mass object”. This is because, by definition, a body needs to have cleared its orbit in order to be called a planet. What’s more, the study does not rule out the possibility that the warp could be the result of more than one planetary mass object in the area.
Therefore, it would premature to state that astronomers – having not yet even confirmed the existence of Planet 9 – are now talking about the existence of a possible “Planet 10”. In the coming years, more news and information will become available, which will hopefully help us put the debate to rest and agree on just how many planets there are out there!
Gerard Kuiper, founder of the Lunar and Planetary Laboratory. Credit: lpl.arizona.edu
In the outer reaches of the Solar System, beyond the orbit of Neptune, lies a region permeated by celestial objects and minor planets. This region is known as the “Kuiper Belt“, and is named in honor of the 20th century astronomer who speculated about the existence of such a disc decades before it was observed. This disc, he reasoned, was the source of the Solar Systems many comets, and the reason there were no large planets beyond Neptune.
Gerard Kuiper is also regarded by many as being the “father of planetary science”. During the 1960s and 70s, he played a crucial role in the development of infrared airborne astronomy, a technology which led to many pivotal discoveries that would have been impossible using ground-based observatories. At the same time, he helped catalog asteroids, surveyed the Moon, Mars and the outer Solar System, and discovered new moons.
