David Rothery

New Discovery: Mars Has Quakes

The results from NASA’s InSight probe’s first 10 months on the martian surface have been published in a series of papers.

Most space missions investigate the surface or atmosphere of a body.

But NASA’s InSight probe, which landed on Mars in November 2018, is different – it is the first mission dedicated to studying the interior structure of the planet and whether it gives rise to “marsquakes”. Now the results from its first ten months on the martian surface have been published in a series of papers in Nature Geoscience and Nature Communications (see overview here).

InSight touched down via parachute and retrorockets on the plains known as Elysium Planitia – lying between the ancient volcano Elysium Mons, and Gale Crater where the Curiosity Rover is exploring. Although it is still trying to find a way to hammer its heat probe adequately into the ground, other aspects of InSight’s surface activities have worked well.

Also read: Five Reasons to Forget Mars for Now and Return to the Moon

For example, it successfully managed the crucial operation of using a robot arm to place a seismometer (a very sensitive vibration detector) on the ground well clear of the landing platform, and then to cover it with a wind and thermal shield. This was to isolate it from vibrations caused by wind or lander itself and that could otherwise drown out any vibrations caused by quakes.

Up to 30 September 2019, 174 seismic events had been recorded. None was stronger than magnitude 4 on the Richter Scale, which is commonly used to measured earthquake sizes on our own planet. There were 24 whose magnitude was in the range 3-4, meaning the energy released would be similar to exploding about a ton of TNT. It might sound a lot, but would pose very little risk for future astronauts on Mars unless they were unfortunate enough to be very close to the epicentre of a quake.

Unlike the Earth, the outer layer of Mars is not broken into moving tectonic plates. It has therefore been suggested that vibrations caused by meteorites hitting the surface may be to blame for any “marsquakes”. But so far as can be told, the quakes were all generated inside the planet itself.

Though less strong than the biggest earthquakes that would be detected on Earth during any ten-month period, these marsquakes are a lot stronger than the moonquakes recorded by Apollo seismometers. And whereas moonquakes appear to result from the slow contraction of the Moon, the situation appears different on Mars.

Locating the sources

In the best determined events it was possible to distinguish two kinds of vibrations: first the compressional “p-waves” and then the slightly slower-travelling sideways-shaking “s-waves”. The time-lag between the two gave an idea of the distance from their source, and in two cases it was also possible to deduce the direction from which the vibrations travelled.

This placed the sites of those two marsquakes at or near Cerberus Fossae. This is an array of fractures where the crust – the outermost layer of a planet – has been stretched, possibly by volcanic activity, and which may also have been the source of some catastrophic water outflow. A previous study had identified boulders in this area that had apparently cascaded down slope as a result of recent marsquakes, already suggesting that this system is active.

There can be no doubt now that Mars is seismically active. But this does not mean that it has plate tectonics like the Earth. Rather, Mars’ crust experiences stresses, caused by local deformation, leading to fractures similar to what occurs in earthquakes in the interiors of Earth’s continents – well away from plate boundaries.

Inside Mars

As additional marsquake data accumulates, we will learn more about the exact locations of each event, and how they relate to what we can see in the surface landscape. In addition, the first evidence-based picture of Mars’s interior is beginning to build up.

We know that Mars has three distinct layers: the relatively thin rocky crust, a mantle and a metal core. But we know little about the state of the material in each of those layers.

The seismic waves from the smaller marsquakes appear to have originated within the planet’s crust, and to have travelled only within the crust. Beneath the lander, there is a few metres of regolith (loose rock) overlying solid rock in what is probably just the upper layer of the crust extending down to 8km to 11km. In an unexpected twist, some properties of the regolith were deduced from the details of how the ground trembles when a mini-tornado known as an atmospheric dust-devil passes by.

In contrast, the larger events probably began in the mantle. The paths of those seismic waves went deeper into the mantle before curving back up to InSight’s seismometer, giving a way to probe how the planet’s mantle varies with depth.

Already there are hints of a “low velocity layer” in the mantle where the s-waves are slowed down, possibly because it is not fully solid. There is nothing in the data yet that can help us explore the depth of the iron core that almost certainly lies at the planet’s centre, but that may come.

Ultimately, a planet’s composition is determined by its formation, so uncovering Mars’ structure could one day help us understand how it formed.

David Rothery is professor of Planetary Geosciences, The Open University.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Missing Objects at the Fringe of the Solar System Are Puzzling Astronomers

There’s a mysterious lack of small bodies beyond Neptune, but a ‘snowman-shaped’ object may help explain why.

In the dimly lit spaces of our solar system beyond Neptune’s orbit lies the Kuiper Belt. This a region between about 35 and 50 times further from the sun than the Earth, populated by icy bodies so sparsely distributed that they never had the chance to collide and merge into planet sized objects.

Pluto is the largest that we know of, but only just. And over the past two decades, telescope surveys have found a couple of thousand more ranging down in size to only a few tens of kilometres across. The trouble is that most of the objects of that size or smaller are too faint to be spotted by telescopes. So it will be difficult to ever work out how many small but unseen bodies there actually are in the Kuiper belt. Now a new paper, published in Science, has used an ingenious method to help us find out.

This is important because scientists believe Kuiper Belt objects are survivors from the solar system’s birth, developing from a primordial cloud of dust and gas. That means that their size distribution could have a lot to tell us about how the material from which the planets grew was initially assembled.

Counting craters

Instead of counting the small Kuiper belt objects directly, the researchers behind the new study counted the craters made by the random sample of objects that have impacted the surfaces of Pluto and its largest moon, Charon. There, craters 13 km across would have been made by objects only 1km-2km in size. That is already way below the telescopic detection limit for Kuiper belt objects themselves, but images from the flyby of NASA’s New Horizons mission in 2015 allow craters as small as 1.4km to be mapped. Those must have been made by impacts of Kuiper Belt objects not much bigger than 100 metres in size.

The researchers’ analysis shows that for craters of 13km or larger, on both Pluto and Charon, the frequency of impacts of various sizes seems to match with what would be expected from the known size distribution for Kuiper belt objects. However, for smaller craters the abundance falls off dramatically, and so by implication must the abundance of the Kuiper Belt objects capable of making those craters. The same does not happen for the well-documented asteroids that collide with the bodies in the region of Jupiter, Mars and Earth, nor is it consistent with theoretical models.

Interpretation of the most heavily cratered terrains led the researchers to rule out that the small craters have been erased by geological resurfacing such as cryovolcanic activity (eruptions of icy fluids) during the past four billion years. This reinforces the conclusion that smaller craters were never made in the expected numbers, so there must be a mysterious corresponding deficit of Kuiper belt objects less than about 1-2km in size.

Blorping and flomping

When the researchers, led by Kelsi Singer of the Southwest Research Institute (Boulder, Colorado), wrote their paper no one had yet seen a small Kuiper Belt object in detail. However, New Horizons recently flew past a 30km long object known as 2014 MU₆₉ (more controversially nicknamed “Ultima Thule”) on January 1, and has now transmitted probably the best images we are going to get.

Sometimes described as “snowman-shaped”, it is a two-lobed “contact binary”, almost certainly formed by a merger of two round objects that happened so slowly and gently that neither component was deformed in the process. But what happened before that? If you look at the larger of the two lobes, in particular, you can make out what looks like traces of component parts that merged vigorously enough to squish together into an approximate sphere, but with insufficient violence to smash each other apart.

These ideas have inspired quirky new terms. “Blorping” refers to the collisional merging of material to assemble each of the lobes, and “flomping” describes the coming together when two lobes meet without causing any deformation. More importantly, this could offer an insight into the processes that robbed the Kuiper belt of the smaller objects that would otherwise have impacted to make small craters on Pluto and Charon.

The relative lack of small Kuiper Belt objects may be because, instead of breaking each other apart in collisions, they tended to merge by blorping – eventually growing into objects like 2014 MU₆₉. If this is correct, then when we try to count them, we see a record of growth rather than collisional fragmentation.

Orbital speeds are slower the further you get from the sun, so we would expect collisions to be less violent in the Kuiper Belt than in the inner solar system. But even so, a “blorp” event to fuse two lumps together rather than break them apart probably requires the ices that make up the bulk of their substance to be a lot less brittle and more squishy than we might have expected. That is crucial information, as these lumps are made from the raw material that the solar system formed from, shedding important light on its evolution.

David Rothery, Professor of Planetary Geosciences, The Open University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

From Volcanoes on Mars to Scarps on Mercury – How Places on Other Worlds Get Their Names

Naming features on other worlds is a trickier issue than you might think.

The New Horizons spacecraft, which flew past Pluto in 2015, is flew by “Ultima Thule,” the object in the Kuiper belt of bodies beyond Neptune on January 1, 2019. The name Ultima Thule, signifying a distant unknown place, is fitting but it is currently just a nickname pending formal naming. The official names of the body and of the features on its surface will eventually be allocated (this could take years) by the International Astronomical Union (IAU), which celebrates its centenary in 2019.

The IAU’s achievements during its first few decades include resolving contradictory sets of names given to features on the Moon and Mars by rival astronomers during the previous few centuries. The nomenclature working group’s task would then have been largely over, had the space age not dawned – allowing space probes to send back images revealing spectacular landscape details on planets and their moons.

Planetary scientists would find life difficult without names for at least the largest or most prominent features on a body. If there were no names, the only ways to be sure that other investigators could locate the same feature would be by numbering them or specifying map coordinates. Either option would be cumbersome and unmemorable.

Also Read: NASA Deep Space Probe Finds Signs of Water on Nearby Asteroid

The rules

Building on some of the already entrenched lunar and martian names, the IAU imposed order by establishing themes for the names of features on each body. For example, large craters on Mars are named after deceased scientists and writers associated with Mars (there’s an Asimov and a Da Vinci), and craters less than 60 km across are named after towns and villages on Earth (there’s a Bordeaux and a Cadiz).

Apart from craters, most names are in two parts, with a “descriptor term” of Latin origin added to denote the type of feature that has been named. On Mars we find neighbouring valleys called Ares Vallis, Tiu Vallis and Simud Vallis, in which the descriptor term “Vallis” is Latin for valley. This is preceded by the word for “Mars” in a different language – in these examples Greek, Old English/Germanic and Sumerian respectively. Among other descriptor terms are Chasma (a deep, elongated depression), Mons (mountain), Planitia (a low lying plain) and Planum (a high plain or plateau).

Descriptor terms are chosen to avoid implying that we know how any particular feature formed. For example, there are many scarps on Mercury that are currently interpreted as thrust faults (where one region of a planet’s surface has been pushed over another). However, a neutral descriptor term – in this case Rupes (Latin for scarp) – is used so they would not have to be renamed if we were to realise that we’d been misinterpreting them. Similarly, none of the giant mountains on Mars that are almost certainly volcanoes has volcano as a formal part of its name.

The largest volcano on Mars, Olympus Mons, coincides with an ephemeral bright spot that can sometimes be discerned through telescopes. Though this was initially dubbed Nix Olympica (meaning “the snows of Olympus”) by the 19th-century observer, Giovanni Schiaparelli, space probes have since shown that the temporary brightness is not snow but clouds that sometimes gather around the summit. The IAU decided to keep the Olympus part of the name, qualified by the more appropriate descriptor Mons (mountain in Latin).

On the Moon, the IAU retained Mare (Latin for sea) as a descriptor term for dark spots, even though it is clear they have never been water-filled as was once thought. However, Michael van Langren’s Mare Langrenianum, which he immodestly named after himself on his 1655 map, is now Mare Fecunditatis.

Cultural balance

The IAU is rightly sensitive to achieving cultural and gender balance. The names of lunar craters that the IAU inherited commemorate famous past scientists, which for historical reasons are dominantly male and Western. In partial compensation, the IAU decided that all features on Venus, whose surface was virtually unknown because of its global cloud cover until we got radar spacecraft into orbit, would be named after females (deceased or mythical). For example, there is a Nightingale Corona, a large oval-shaped feature named after Florence Nightingale. The only non-female exceptions are three features that had already been named after being detected by Earth-based radar.

Prior to the first detailed images of Jupiter’s moons by Voyager-1 in 1979, the IAU planned to use names from the myths of peoples in Earth’s equatorial zone for the moon Io. It would use mythical names from the European temperate zone for Europa, names from near-Eastern mythology for Ganymede and names from far northern cultures for Callisto.

They stuck to the latter three, and so Europa has Annwn Regio (a region named after the Welsh “Otherworld”), and Ganymede and Callisto have craters named Anubis (Egyptian jackal-headed god) and Valhalla (Norse warriors’ feast hall).

However, because Io was revealed to be undergoing continual volcanic eruptions, the original naming theme was deemed inappropriate and was replaced by the names of fire, sun, thunder/lightning and volcano deities from across the world’s cultures. For example, the names Ah Peku, Camaxtli, Emakong, Maui, Shamshu, Tawhaki, and Tien Mu (which occur on the map above) come from fire, thunder or Sun myths of the Mayans, the Aztecs, New Britain, Hawaii, Arabia, the Maoris, and China, respectively.

Captain Cook and the Maoris

The IAU has struggled to achieve cultural balance for some features. For example, the theme for Rupes on Mercury is “ships of discovery or scientific expeditions.” By the nature of world history, there is a preponderance of Western ship names. For example, we find Adventure, Discovery, Endeavour, and Resolution – all four ships from Captain Cook’s 18th-century voyages to the Southern Ocean and Pacific.

Personally, I am content that these were primarily journeys of scientific discovery rather than of conquest or colonisation. Cook’s first voyage was undertaken to observe a rare transit of Venus, and his second voyage reached further south than ever before.

That said, it would be nice to redress the balance. In connection with a European planetary mapping project, one of my PhD students and I hope to get at least one of Mercury’s as yet unnamed Rupes named after a canoe in which the Maoris arrived in New Zealand.

Ultimately, space exploration is for all of humanity.

This article is republished from The Conversation. Read the original article here.

Mercury: ‘First Rock From the Sun’ in Transit

In every century there are only 13 or 14 transits of Mercury and you have to be on the right part of the globe if you want to watch a particular transit from beginning to end, which usually lasts for several hours.

Mercuy’s Caloris basin, seen in exaggerated colour. At 1,525km diameter this is the largest impact basin on the planet. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The solar system’s smallest and most remarkable planet, Mercury, will cross the face of the sun on May 9 – offering a great opportunity for people in many places across the world to see it.

Mercury is a dark and enigmatic world, which bears the scars of a strangely long history of volcanic eruptions and tectonic activity. Its crust is unreasonably rich in elements that normally easily evaporate from the surface, such as sulphur, sodium and potassium. This is odd, as these are the kind of substances that are most likely to have been lost during a hot and violent birth such as Mercury’s.

Mercury scoots round the sun in only 88 days, overtaking the more sedately moving Earth every three or four months. Because Mercury’s orbit is tilted at about seven degrees with respect to the Earth’s, it passes directly between us and the sun (a transit) only when both it and the Earth are close to the points where their orbital planes intersect. This can happen only in early May or early November.

In every century there are only 13 or 14 transits of Mercury and you have to be on the right part of the globe if you want to watch a particular transit from beginning to end, which usually lasts for several hours. If it starts soon after sunset it is likely to be finished before dawn, meaning you won’t catch any of it. However the May 9 afternoon transit is perfectly timed for viewing the entire thing from Europe and most of the Americas.

Mercury’s transits are more common than those of Venus, which comes in a pair eight years apart then separated by intervals of more than a century. The previous transit of Mercury (November 8, 2006) happened while the sun was below the horizon from Europe, India and anywhere in between, so many sky watchers are particularly keen to see this one for themselves. Interest has been heightened by the surprises revealed by NASA’s recent MESSENGER mission, including that its earliest crust was made of graphite, unlike the other rocky planets.

Historic observations

The first planetary transit ever to be observed was in fact one of Mercury in 1631, when the French astronomer Pierre Gassendi saw it by using a telescope to project an image of the sun onto the wall of a darkened room. Eight years later Englishman Jeremiah Horrocks used the same technique when he became the first to see a transit of Venus. Projection was the only safe way to do it, because a telescope collects heat as well as light, and even today nobody should try to look at the sun through a telescope unless there is a purpose-built solar filter across the main aperture.

On November 7, 1677, Edmond Halley (he of comet fame) documented a transit of Mercury from the South Atlantic island of St Helena. It dawned on him that the slightly different perspective from vantage points in various parts of the globe would cause a transiting planet to take a slightly different track across the sun in each case. The most precise way to determine this would be to measure exactly how long the transit lasted as seen from each site, and the data could then be used to work out the distance between the Earth and the sun, which had not yet been satisfactorily achieved.

In fact Venus, being larger and closer to the Earth, gives a more precise measure, and thanks to Halley’s insight French and British expeditions were mounted to various remote parts of the globe for its 1761 and 1769 transits. For example, the main impetus behind Captain Cook’s first round the world voyage was to observe the 1769 transit of Venus from Tahiti. A few months later he also observed a transit of Mercury while ashore in New Zealand, at a place that he named Mercury Bay.

There’s little new science that we can get out of observing a transit of Mercury these days, but the European Space Agency is inviting schools to submit their transit timing observations to derive their own measurement of the Earth-sun distance.

Transit viewing

The May 9 transit will begin at 12:12 BST and end at 19:42 BST, which could hardly be more convenient for viewing from western Europe. Those in India will be able to watch for an hour or two before the sun sets whereas people on the east coast of North America will have to rise early to catch the start. However, people living in Japan and Australia will miss the whole thing.

The next transit of Mercury after this will be 12:35 to 18:04 GMT on November 11, 2019, but in the UK sunset happens well over an hour before the end. After that there’s a long wait until November 2032.

Unlike Venus, Mercury is too small to see against the sun without magnification, and it can be dangerous to try due to the sun’s glare. So my advice is to go to an organised transit viewing event – many astronomy clubs and universities are organising these. Another option is to view it online. The European Space Agency will be webstreaming live images from space (no clouds in the way) and from solar telescopes in Spain and Chile.

David Rothery, Professor of Planetary Geosciences, The Open University

This article was originally published on The Conversation. Read the original article.

NASA Mission Brings Pluto into Sharp Focus – but it’s Still Not a Planet

Pluto was embraced as the solar system’s ninth planet upon discovery by Clyde Tombaugh in 1930

The new pictures that NASA’s New Horizons probe has begun to beam back have revealed Pluto and its largest moon, Charon, in ever greater detail from what is the first ever spacecraft fly-by.

Pluto has an atmosphere and five known moons which have been glimpsed by New Horizons as it closes in, and while we can’t predict what we will find, whatever is revealed is sure to lead to renewed cries that Pluto be re-classified as a planet – a status it lost in 2006.

Two sides of Pluto (larger and browner) and Charon (smaller and greyer) seen as New Horizons approaches. Credit: NASA/John Hopkins University APL/SWRI

Pluto was embraced as the solar system’s ninth planet upon discovery by Clyde Tombaugh in 1930. He’d been looking for a planet where faulty data suggested a planet-sized body was perturbing the orbit of Neptune. This, he felt, was it – and the world agreed. Pluto’s mass was at first thought to be roughly the same as the Earth’s, but by 1948 estimates had shrunk it to the size of Mars.

When Pluto’s largest moon Charon was discovered in 1978, Charon’s orbit showed that Pluto’s mass is actually about only 0.2% of the Earth’s (one-sixth that of the Moon), and we now know that its diameter is about 2368km, or two-thirds that of the Moon.

Being so insubstantial, then, should Pluto be classed as a planet? There may seem no obvious reason why not. After all, the Earth is only 0.3% the mass of Jupiter. Planets clearly span a wide range of masses. But the main reasons why delegates to the International Astronomical Union (IAU) voted to demote Pluto from planet status are not based primarily on mass or size.

Pluto is one of many

Since the 1990s, many other roughly Pluto-sized bodies have been discovered beyond Neptune, such as Eris, Haumea and Makemake. There are more than a thousand objects now documented in what is called the Kuiper belt, a region beyond Neptune where it seems no large objects were able to form.

If Pluto had been discovered along with the others rather than 60 years earlier, there can be little doubt that no one would have called it a planet in the first place. There is nothing special about Pluto, other than the accident of having been the first to be discovered.

Eight of the so-called trans-Neptunian objects, including Pluto, and their moons. Credit: Lexicon

The crucial part of the definition of planet adopted by the IAU in 2006 is that a planet should have “cleared the neighbourhood of its own orbit”. Neptune, 8,600 times more massive than Pluto, has achieved this because neither Pluto nor anything else that crosses Neptune’s orbit comes close to rivalling Neptune’s mass. On the other hand Pluto clearly does not comply to this definition – it has rivals of comparable mass in addition to being overshadowed by the vastly more massive Neptune.

While it may be that this definition is hard to apply in other solar systems, it works for ours and is a far neater approach than including every Kuiper belt object as a planet – thousands of them, which would be ridiculous. The alternative of defining a size or mass minimum at which an object ceases to be a planet would suffer from our variable and imperfect ability to measure their size or mass remotely.

The Kuiper belt is a busy place. Credit: NASA/Johns Hopkins University APL/SRI/Alex Parker

A linguistic fudge

Nevertheless, the IAU shied away from completely stripping the Pluto of its appellation of planet by inventing a new term, dwarf planet. This denotes an object orbiting the sun that has not cleared its orbit, but which has sufficient mass for its own gravity to have pulled it into a near-spherical shape (described as hydrostatic equilibrium). This applies to Pluto, Eris and a few other Kuiper belt objects, and also to the largest asteroid, Ceres.

‘Pluto a planet, Jim? You’ve got to be kidding me.’ Credit: NBC Television

I think that was an unnecessary concession to the Pluto-is-a-planet lobby, though it proves that the IAU is not controlled by “a clique of Pluto-haters” as one astronomer has claimed. In fact it’s messy for two reasons. First, shapes cannot be precisely determined for objects that have not been visited by a spacecraft; they have to be assumed on the basis of mechanical models that could easily be wrong. Second, whereas the giant planets (Jupiter, Saturn, Uranus and Neptune) are planets, by the IAU’s own definition the dwarf planets are not planets. As Mr Spock might have said, “That’s illogical, Captain.”

Planetary scientists have a duty to describe the nature of the solar system as clearly as possible, and to lead the public to a clearer understanding of nature – irrespective of how its elements are classified. Appealing to sentiment, seeking celebrity endorsement and posting photos of presidential candidates with “Pluto is a planet” T-shirts is not a good way to advance anyone’s understanding. It’s time to let go of the past, and embrace Pluto a fascinating world and the most interesting member of the Kuiper belt.

David Rothery is Professor of Planetary Geosciences at The Open University.

This article was originally published on The Conversation. Read the original article.