Category Archives: Planetary geology

Witness to an impact

Facebooktwitterlinkedininstagram

The dinosaurs were snuffed out in a geological instant (well not exactly, but that is a popular image).  The Chicxulub bolide, its girth 10-15 kilometres, collided with Earth 65 million years ago, leaving a 150 kilometre-wide crater and enough dust and aerosols in the upper atmosphere to darken latest Cretaceous skies for decades.

Like all planetary bodies in our Solar System, Earth has received its share of meteorite and comet impacts. We still bear the scars of some. Every day, bits of space dust and rock plummet towards us – most burn up on entering the atmosphere, but a few make it to the surface. Occasionally they even startle us with air-bursts – Tunguska in 1908,  Chelyabinsk (2013), both in Russia. But humanity has never witnessed a decent sized impact, at least in recorded history. It’s all theoretical. Continue reading

Facebooktwitterredditlinkedin
Facebooktwitterlinkedininstagram

“There are more Exoplanets than stars in our galaxy”

Facebooktwitterlinkedininstagram

How many stars are there in our own galaxy, the Milky Way? A number frequently bandied about is 100 billion. This is a nice round number. The number could be as high as 400 billion; also a nice round number. In an interesting coincidence, the number of galaxies is estimated at 100 billion, a number that will no doubt increase as we peer into the farthest recesses of the universe. So, a 100 billion galaxies, and in each 100-400 billion stars – the numbers are getting out of hand (even for a geologist who works in millions of years).

So when astronomers announce that there are probably more exoplanets than stars in our own galaxy (ergo all the other galaxies), our ego-centric view of the universe seems just plain silly.  As of September 25, 2018, there were 3779 confirmed planets, 2737 NASA candidates, and 2819 solar systems. The graph below shows the astonishing rate of discovery over the last two decades; 2016 was a banner year with almost 1500 identifications. The NASA plot shows the estimated planet size and orbital periods, relative to Earth; the majority of exoplanets apparently whiz round their stars in less than 100 days, some in a few hours.

So, how does one discover a new exoplanet?

Having a decent telescope at your disposal is a good start. Land-based telescopes are okay, but the real successes have been with orbiting, satellite-based telescopes. Kepler was launched by NASA in March 2009 and was tasked with watching a swath of sky containing about 150,000 sun-like stars. About 70% of the discoveries so far have been made by Kepler. The second orbiting observatory, TESS (Transiting Exoplanet Survey Satellite), launched April 18, 2018, is also dedicated to finding exoplanets, and in its first few days of operation has made some exciting discoveries.

Kepler and TESS use a method of detection called Transiting – several other methods have been used but the transit method has been the most successful (e.g. Radial Velocity measures apparent changes in the velocity of a star’s own motion, or wobble, caused by the orbiting planet. An observer will see velocity changing as the star move towards and away from Earth. A nice summary of detection methods has been compiled by The Planetary Society). A transit occurs when a planet passes between its star and an observer on (or orbiting) Earth.  The planetary disc will block some of the star’s light, and if the telescope is pointing in the right direction, the reduction in luminosity can be measured. Planetary orbits are periodic. Therefore, an important part of the analysis is observing dimming at regular intervals.

Astronomer Edmund Halley (of Halley’s comet fame), and Captain James Cook provide us with a useful historical analogy. Halley surmised that observations of Venus during its transit of the Sun, from different geographical locations, would permit calculations of astronomical distances, and hence, the size of the solar system (the calculations involved simple trigonometry). Cook was dispatched to Tahiti in time for the June 4, 1769 transit. The disc presented by Venus is small compared to the Sun, but there is a measurable decrease in light during its transit.

A more dramatic example of a transit occurs in our own backyard, when the Moon passes between the Sun and Earth during daylight hours. Partial eclipses produce observable dimming of sunlight, but a full eclipse delivers brief twilight. This principle also applies to exoplanets.

In its first few days of operation, TESS discovered a planet orbiting the star Pi Mensae, a bright dwarf star about 60 light years from Earth. The observed period of starlight dimming indicates that Pi Mensae c has an orbit of only 6.27 days. But what about its size – its radius and mass compared to Earth?

Astronomers start by measuring the size of the star; this they can do quite accurately because stars are fairly predictable. The brighter a star, the hotter it burns. Thus, the colour spectrum emitted provides a good indication of its temperature. Knowing the brightness and temperature it is then possible to calculate the surface area of the star (the larger the surface area, the more light it will emit), and if surface area is known, simple arithmetic will give you the star’s diameter.

During a planet’s transit, a measurable proportion of star light will be dimmed – in other words, the planet will dim the light in proportion to its size. Bingo! We now know the size (radius) of our exoplanet. But we still need to know how ‘heavy’ it is.

This is determined by observing the gravitational tug of war between the planet and its sun. The orbiting exoplanet will cause its sun to wobble about its axis; the degree of wobbling will be proportional to the exoplanet mass (we already know the star’s mass).  These very small gravitational perturbations can be measured by Kepler, TESS, and earth-based telescopes.

Knowing the exoplanet size and mass, gives us all the information we need to calculate its density.  Newly discovered Pi Mensae-c has a radius 2.14 times, and a mass 4.82 times that of Earth. So it is roughly Earth-size, but too close to its sun to be habitable.

The Transit method is not without its drawbacks. Importantly, the exoplanet orbit must be aligned with the observer. There must be as many orbit geometries as there are planetary systems in our galaxy, which means our telescopes can detect only a fraction of all possible exoplanets. Binary stars (i.e. two stars in very close proximity having mutual orbits) can also complicate observations of transit and gravitational wobbles. Celestial bodies the size of Jupiter can also be problematic because this size range can include some dwarf stars.

I don’t know about you, but it seems that new discoveries or exploration events are announced almost on a daily basis. Some of the latest excitement centers on two small robots that are playing leap frog on asteroid Ryugu. The number of questions seems to expand exponentially, the more we delve into our universe. Exoplanet science will go a long way to answering some of the questions.

Facebooktwitterredditlinkedin
Facebooktwitterlinkedininstagram

Tidal waves; prisoners of celestial forces

Facebooktwitterlinkedininstagram

We are told that a tide waits for no one, the impatient cousin of time. In its early 13th century idiom (the oldest known quotation is 1225 AD) the word ‘tide’ was associated with time, as in a season, or an instant. Somehow this has morphed in modern English to mean the daily rise and fall of sea level, although the association with time and periodicity remains; the regular advance and retreat of the water’s edge. In modern usage, the word ‘tide’ also connotes an association with the Moon and Sun. Tides, as we now understand them, have helped shape our world for the last 4.6 billion years: kept the oceans honest, tidied up our coastlines, and defined the character of harbours and estuaries. Ships enter and leave on the tide, recycled from one coastal haven to the next.

Ocean tides are the natural response to the forces of gravity acting on earth, its moon, and the sun. Tidal forces act on other planets and their moons. The sulphurous moon Io develops a distinct bulge during its close approach to Jupiter. It is hypothesized that moons Enceladus (Saturn) and Europa (Jupiter), maintain liquid water oceans beneath their frozen surfaces because of the heat generated by the forces of gravity.

Earth tides cycle through highs and lows depending on the relative positions of the moon and sun. Tidal highs and lows also vary from place to place, for example a high tide on the west coast of New Zealand may occur at the same time as a low tide on its east coast. To explain this phenomenon, we first assume a simple model where earth is covered completely by ocean – once we have established an explanation using this simplification, we can add the continents to create a more complicated, real world explanation of tides.

The Earth and Moon are in a state of balance; the Moon exerts a gravitational pull on Earth (and vice versa), and because the Moon orbits Earth (the orbit is slightly elliptical), there is an opposing force – the centrifugal force. As a crude analogy, imagine riding a fast-moving ferris wheel; you are moving in a circle. If you release the safety harness, you will fly off at an angle, propelled from the safety of your seat by a strong centrifugal force. In our celestial system, these two opposing forces keep the Moon from crashing into Earth (and likewise, Earth into the Sun).  Centrifugal forces are the same everywhere on Earth, but the Moon’s gravitational pull changes with distance; it is strongest on the side closest to the Moon, and weakest on the opposite side. Thus, at different points on the Earth surface, there is a slight difference between the two forces. The difference is not enough to upset the overall balance between Earth and Moon, but it is strong enough to create a bulge in the ocean mass; one on the side facing the Moon, the other on the opposite side of Earth. The bulges correspond to high tides.  However, Earth rotates on its axis, which means that different parts of Earth experience the bulge at different times – note the bulge itself is always aligned with the moon. In this simple model, the bulges on opposite sides of the earth mean that there are two tides every 24 hours, 12 hours apart.  These are semidiurnal tides.

The Sun exerts a similar effect on Earth, but its influence on tides is about half that of the Moon. Nevertheless, the Sun’s gravitational force will reinforce that of the Moon during full and new phases of the Moon, resulting in spring tides; the opposite effect, neap tides occur when the two gravitational forces oppose each other. Other tweaks to this relatively simple celestial model are the elliptical Earth-Moon-Sun orbits (which results in some changes to the gravitational effects), and the tilt of Earth’s own axis of rotation – hence the monthly lunar cycles, and seasonal solar-Earth cycles. Centuries of sky gazing have taught us that all these cycles are predictable which means we can foretell tides well into the future.

To better understand tides in the real world we now need to complicate our model by adding continents.  These massive landmasses have created a degree of ocean isolation (Pacific, Atlantic, Indian) such that tidal cycles can be considered separately for each ocean. The tidal bulge, or tidal wave, is slowed as it enters shallow coastal waters and is also is deflected, such that it moves, wave-like, around each ocean margin. Tidal waves in the northern Hemisphere move anticlockwise, while those in the south move clockwise.  Thus, high and low tides will also migrate along ocean coasts. These patterns apply to the oceans as a whole, but on a more local scale, the tidal wave can be deflected, amplified, or weakened, depending on the shape of the coastline and variations in water depth. A classic example of tidal amplification is Fundy Bay, a narrow stretch of water between New Brunswick and Nova Scotia (eastern Canada).  Here tidal ranges of 17m are common, in marked contrast to those on the opposite coast (e.g. 2m tides in Halifax). Tides here flood very quickly, initially as a tidal bore that in places may be a 2-4m high wall of water (check out this link to a short video, taken near Moncton, New Brunswick).

The simple Earth ocean model predicts two tides every 24 hours, or semidiurnal tides. Adding continents, coastlines and varying water depths further complicates this picture such that in places only one tide occurs each day; these are diurnal tides (e.g. Gulf of Mexico, the Kamchatka coast), or mixed semidiurnal tides where one high tide is significantly higher than the other (i.e. one high tide is weakened).

Along the New Zealand coast, both lunar and solar tidal waves move anticlockwise (the opposite direction to much of the southern hemisphere). Tidal ranges are highest on the west coast, where both tidal waves reinforce the tidal signal, and are lower on the east coast where the solar tidal wave is weakened. The National Institute of Water and Atmosphere (NIWA) has produced animations of the lunar and solar tidal waves. High tide in the far north of New Zealand is about 6 hours ahead (or behind) that in the far south. It takes 12-13 hours for the tidal wave to traverse around the entire New Zealand coast, which means that a high tide at any location, will have a corresponding low tide somewhere else. This difference is nicely illustrated in Auckland city which lies between two harbours; Auckland Harbour on its eastern coast (Hauraki Gulf – Pacific Ocean) is geographically separated from Manukau Harbour on the west side (connected to Tasman Sea), by an isthmus that in places is barely 1000m wide.  It takes about 3.5 hours for the tidal wave to travel north up the east coast, and south down the west coast to Manukau Harbour.

Tidal range, the elevation difference between high and low tides, is also affected by weather. High pressure weather systems tend to lower sea level, whereas low pressure systems result in higher than normal sea levels. Coastal regions can experience serious problems from flooding, when a spring tide corresponds with the passage of major storms, particularly cyclones and hurricanes. To make matters worse, storms like these usually generate high rainfall. Elevated tides plus storm surges will cause rivers to back-up, flooding low lying areas.

Tidal ebbs and flows have moved ocean water masses, flushed embayments, moved sediment, and helped shape coastlines ever since Earth acquired a Moon. Tides are periodic; they cycle endlessly through their highs and lows. They are also superimposed on the straight arrow of time (borrowing a phrase from Stephen J. Gould), such that we can now predict the times and ranges of tides well into the future.

Note: The name tidal wave is sometimes used to describe a tsunami. Tsunamis have no relationship with tides. Tidal wave should only be used to name long period, gravitationally induced waves.

Facebooktwitterredditlinkedin
Facebooktwitterlinkedininstagram

Martian organics; one more step in the right direction

Facebooktwitterlinkedininstagram

Organic compounds (i.e. molecules that contain carbon bonded to hydrogen) are not the prerogative of earth.  They have been identified (remotely) in interstellar space, stellar gas clouds, and measured directly on comets and meteorites. And now Mars.

We tend to associate earth-bound organic compounds with life forms and processes, past and present. So, any discovery of organics elsewhere – the solar system and beyond – always begs the question; were these too associated in some way with extra-terrestrial life?  The answer is usually ‘No’, although media outlets, frequently prone to exaggeration, tend to stretch the answer beyond credulity. In fact we know that most of the organic molecules identified in deep space and on comets probably had an abiotic-non-biological origin.

One of the technical hurdles when working with Martian or comet samples, is the analysis of small volumes of rock and soil, that might, if we’re lucky, contain organic compounds in even smaller amounts. Curiosity Rover has, among its sophisticated array of instruments, a small electrically heated furnace that basically cooks the samples.  The method, called analytical pyrolysis, is used to identify complex organic compounds of unknown composition. Samples are placed in the furnace and heated gradually through temperatures up to 900oC. As temperature increases, small molecular fragments are thermally severed, or broken from the unknown compound.  Heating is usually done in an inert atmosphere (like Helium), that acts as a carrier for the smaller fragments, so they can be identified by gas chromatogram.  The molecular fragments, once identified, help to fingerprint the unknown organic compound.

Two back-to-back papers in the June 2018 issue of Science, provide grist for the ‘Life on Mars’ mill.  The new data do not show definitively that there was/is Martian life, but it does point to some intriguing possibilities.

Soil samples collected by Curiosity Rover from an ancient lake bed in Gale Crater (3 billion years old) are responsible for the current burst of extra-terrestrial excitement. The samples were heated incrementally to 860oC, releasing a range of organic fragments including aromatic organic compounds containing benzene rings (hexagon-shaped molecules), and sulphur-bearing molecular fragments belonging to chemical groups called thiols and thiophenes.

These discoveries are exciting in themselves, but they do not point definitively to any particular origin – whether biological, geological, or derived from meteorites. One reason for this ambiguity is the potential for organic molecules to be altered over time. Note that the Martian organics have been sitting around for about 3000 million years. Organic molecules are susceptible to chemical change if they come into contact with groundwater, hydrothermal-geothermal fluids (fluids at elevated temperatures), and importantly, ionizing solar radiation that on Mars’ surface is intense because of the thin atmosphere (compare this to the strong filtering attributes of earth’s atmosphere).  In fact, the sulphur-bearing compounds are thought to be alteration products that may have enhanced the overall preservation of Martian organic matter.  The current analyses show that complex organic molecules do exist on Mars, although they were not able to identify any clear biological signals.  Buried organic matter, that is shielded from ionizing radiation, may offer the best opportunity to identify fossil biological molecules

Our current preoccupation with methane is linked to its important role as a greenhouse gas. Methane, like other organic compounds on earth, is largely a by-product of biological processes.  It’s only natural then, to entertain the idea that methane in the Martian atmosphere might also be linked to past life forms.  Such flights of scientific imagination are brought to an abrupt halt, when we are reminded that methane can also be produced by geological processes, such as the chemical alteration of certain igneous rocks, or from melting clathrates – indeed, subsea methane bursts are well documented in polar regions.

The amount of methane in the Martian atmosphere is really tiny – measured in parts per billion. The concentration is also known to vary over time and geographically, but until Curiosity rover began its adventures in 2012, the data was too sparse to identify any kind of regularity or pattern of variation.  Regular atmospheric gas measurements by Curiosity over the last 3 years, have begun to fill this data gap.

The gas measurements were taken during Curiosity’s residence in Gale Crater, where there is a strong signal of seasonal variation, from 0.24 to 0.65 parts per billion methane by volume (yes, the amounts are tiny). The magnitude of the variation is larger than that expected from ultra violet light degradation of organic compounds delivered by meteorites, or the expected seasonal changes in atmospheric pressures. The authors conclude that the variation is caused by seasonal changes in methane released from local reservoirs at or buried beneath the Martian surface.  One intriguing possibility is methane release from clathrates, analogous to those commonly found at shallow depths beneath earthly sea floors (a clathrate is water ice that contains weakly bound methane molecules). Surface heating during the Martian summer would lead to increased methane release through permeable soils, or via open fractures and faults.

As so often happens in science, the new data provoke more questions about the nature of the original organic matter, than providing definitive answers. But this is a positive outcome. We now know there are diverse, complex organic compounds preserved in Martian mudrocks like those deposited in ancient lakes. Continued exploration will no doubt lead to the discovery of other complex organic molecules, some of which may be the fingerprints of ancient life.

There is reasonable confirmation that atmospheric methane varies with the Martian seasons. The cause of these variations is unknown, but if it is from buried reservoirs like clathrates, then the next question is ‘where did that methane come from?’.

None of the data so far indicate past or present biogenic influences. The data do suggest directions for future exploration, such as a focus on buried lake sediments, or geological structures that provide potential pathways for migrating methane gas.  New data is always exciting, but so too is the next generation of questions.

Facebooktwitterredditlinkedin
Facebooktwitterlinkedininstagram

Archeomagnetic Jerks; our decaying magnetic field

Facebooktwitterlinkedininstagram

“Archeomagnetic Jerks”. This interesting phrase refers, not to people, but to our global magnetic field; the one that protects us from incoming solar radiation and protects all those electrical devices we’ve come to rely on, including satellites. The magnetic field is generated by earth’s solid core; it envelopes our earth. The magnetic poles (not the same as the geographic poles), move around a bit. Measurements of the field over several decades indicate that the north magnetic pole is migrating south, towards Siberia and has moved about 1000 km since it was first pin-pointed in 1831. Geological investigations of ‘fossil’ magnetic fields also demonstrates that the magnetic field has flipped hundreds of times over past millennia, where north becomes south (see an earlier post for details). Disconcerting as this sounds, we can take some comfort in the fact that these polarity reversals do not coincide with any extinctions.  Homo sapiens was around during the last reversal (780,000 years ago) and, I’m happy to report, survived intact. We will survive the next reversal, although some of the electrical accoutrements we have amassed, might not.

Earth’s magnetic field is generated by rotation, or convection of a liquid nickel-iron layer that surrounds the solid iron core; it is referred to as the liquid outer-core. The heat necessary to drive convection comes from the solid inner core; temperatures for the outer core range from about 2700C to 7700C. Movement of the liquid iron is also driven by forces generated by earth’s rotation, called coriolis forces. Convection in the outer core is not uniform, and variations in rotation, perhaps analogous to eddies, produce variations in the magnetic field.  One region of significant variation in the magnetic field is the South Atlantic Anomaly (SAA), a relatively narrow band where magnetic field strength is much lower than expected; this region extends from central South America to central Africa.

The SAA is thought to evolve from complex interactions in earth’s liquid outer core beneath Africa and central South America.  And although the SAA is considered by some as a possible harbinger of wholesale magnetic pole reversal, the extent of the anomaly has a more immediate impact because of the interaction between the magnetic field and the Van Allen radiation belts (these radiation belts were one of the first discoveries made by an orbiting satellite).  The radiation belts (there are usually two concentric belts) are doughnut-shaped regions in space where charged particles from the sun are trapped as they interact with the magnetic field. In doing so, they protect us from incoming solar radiation. However, the radiation ‘doughnut’ is not oriented symmetrically with earth’s axis of rotation but is slightly off-kilter. This means that one part of the radiation belt comes very close to earth – in fact about 200-300 km, and this low region is what defines the shape of the SAA.  An important consequence of the SAA is that solar radiation is significantly more intense over the extent of the anomaly; orbiting satellites that transit the region of the anomaly are fitted with protective shields to prevent failure of electrical systems.  For example, Hubble Space Telescope passes through the anomaly 15 times a day.

Globally, the strength of the magnetic field has decreased about 15% in the last 200 years. The current scientific dilemma with the SAA is that it seems to be expanding as the magnetic field weakens. This observation, given voice by several media outlets, has led to some predicting dire consequences during an imminent magnetic field reversal. The problem here is that scientists do not know whether this weakening is an unusual event, or one that anomalies like the SAA cycle through from time to time.  It is also not well understood whether the SAA is a relatively recent phenomenon that has been around for a few hundred years, or has persisted over much longer periods of time, perhaps waxing and waning in its extent.

In a recent study, Jay Shah and other geophysicists looked at the magnetic signatures in 46,000 to 90,000 year-old volcanic rocks from Tristan da Cunha.  These isolated volcanic islands in the South Atlantic lie within the SAA and may provide records of older magnetic anomalies. They discovered at least 4 periods of significantly reduce magnetic intensity, and concluded that the SAA could be a persistent anomaly, or at least one that recurs from time to time.  Although the results are preliminary, they suggest that decreasing field strength in the SAA may have happened before, but without wholesale field reversal (there have been no reversals in the last 90,000 years).

The idea that the SAA is a long-lived phenomenon has received an additional boost in a study of archeological materials by Vincent Hare and colleagues, who measured the preserved magnetic signatures in Iron Age mud from southern Africa.  The archeomagnetic materials used in this study were burnt, or baked mud from various Iron Age facilities such as grain storage and hut floors (perhaps baked by cooking and heating fires).  Mud baked above a certain temperature (known as the Currie Point) will retain the magnetic signatures present at the time, in much the same way as solidified volcanic rocks.  Measurements on these materials show significant changes in the magnetic field intensity, between 1225AD and 1550AD, and an earlier period around 500 to 600 AD.  Abrupt changes in field intensity like these are commonly referred to as archeomagnetic jerks.

Despite the ‘End is nigh’ approach taken by tabloids and other popular media to this scientific phenomenon, the actual science is equivocal.  It appears that the South Atlantic (magnetic field) Anomaly is long lived – at least many 10s of thousands of years, and that the magnetic field intensity of the anomaly has waxed and waned several times.  In this context, the current state of decay of the magnetic field both globally and in the SAA, may be nothing more than a repeat of other historical and prehistorical events.  However, on a more sobering note, we are overdue a complete magnetic pole reversal.  No doubt the geophysicists will keep us posted. In the meantime, if a pole reversal takes place tomorrow, you may have to get used to subtracting (or adding) 180o from your compass bearing to ensure you end up where you want to go.

Facebooktwitterredditlinkedin
Facebooktwitterlinkedininstagram

Jet Streams

Facebooktwitterlinkedininstagram

The Polar Vortex. Sounds like scenes from the apocalyptic movie The Day After Tomorrow; a bit of a down-draft and everything freezes. The real vortex refers to a low-pressure system with a cold, west-to-east flowing (counter clockwise) air mass that hovers over the north pole (there is also a vortex over Antarctica). When stable, the cold air remains in the north, contained by the polar jet stream. When unstable, as sometimes happens in winter, the polar jet stream meanders such that cold air can penetrate much farther south.

February is deep winter in the Arctic, and yet current temperatures there are hovering around zero degrees C; almost T-shirt weather.  Warm air masses are being allowed to enter this normally frozen domain, while the cold snaps (March 2018) are wedged into southern Canada, USA and Europe. Arctic winter temperatures are abnormally high, significantly higher than past recorded temperature anomalies.  Is something happening to the Polar Vortex; is it in a state of decay? And if so, is this process part of some long-term climate change, or is it just another anomalous spike on the climate record? None of the answers proffered so far are definitive, at least from a scientific point of view (mind you, the media are having a field day). Science will go some way to resolving this problem by observing how jet streams respond over the next few decades. Continue reading

Facebooktwitterredditlinkedin
Facebooktwitterlinkedininstagram

It looks like sea level rise is accelerating; the era of satellite altimetry

Facebooktwitterlinkedininstagram

Sea level. It’s the most common starting point for any kind of elevation measurement, a datum, that for centuries was understood to be an invariant surface. Then some geologists came along and showed that, for eons past, sea level has risen and fallen countless times; coasts were flooded, sea floors exposed. And sea level is still changing, going up in some places, down in others, but on average it is rising; it has been doing this for the last few 1000 years.

The current globally averaged rate of sea level rise is 3.0 +/- 0.4 millimetres per year, based on satellite altimetry. Satellite measurement of sea level is now about 25 years old. Earlier measurements, some dating back to 1700, were made by tide gauges, basically glorified measuring sticks (the initial technology was just that) which over the years, have become sophisticated, automated measuring systems.  Tide gauges measure water levels on a local scale, and in order to make sense of the data in the context of global sea level, all manner of local variables need to be considered: for example, is the location open to the ocean or a sheltered harbour, storm surges, seasonal changes in currents and water mass temperatures, changes in air pressure (sea levels rise during passage of low pressure systems – this is part of the storm surge), and whether the land is rising or subsiding (i.e. local tectonics). In contrast, satellite altimetry gathers data from a much broader swath of the ocean surface. Both Jason 2 and the more recent Jason 3 satellites can cover 95% of the ice-free ocean surface in 10 days. The accuracy of the Jason 3 radar altimeter is currently an impressive 3.3 cm.

 

Rising sea levels and changing climate are inextricably linked because ocean water mass volumes increase or decrease in concert with changes in the volume of land-based ice (primarily the Antarctic and Greenland ice sheets), plus changes in ocean temperature (this is the steric effect) and salinity.  Thus, if there is an acceleration in atmospheric warming or cooling, there will be a reasonably sympathetic acceleration in ocean volume change and therefore, sea level change.  This is the scenario posited by many climate-change model projections – that increased warming will produce an acceleration in sea level rise. A recent publication that analyses the 25 years of satellite altimetry data (Proceedings of the National Academy of Sciences, 2018), concludes that the (global) average sea level rise is accelerating at 0.084 +/- 0.025 mm/year2, which means that the current speed of sea level rise (about 3 mm/year), will increase year upon year.

 

The possibility of accelerating sea level rise during the 20th century, based mainly on tidal gauge data, has been debated although most analyses indicated a degree of ambiguity in the data. In fact, a 1990 ICCP report (page 266) concluded there was little concrete evidence at that time for an acceleration, although re-analysis of 20th century tide gauge data, published in Nature (2015) did show a possible accelerating trend. If that analysis is correct, this is the first time such an acceleration has been demonstrated with reasonable confidence from a single data set.

As the published analysis shows, teasing accurate sea level numbers from the satellite data is not a simple task.  As is the case for any kind of remote sensing or monitoring, there are data corrections and filters.  Some of the corrections include:

  • Terrestrial water storage (rivers, lakes, and groundwater); this is necessary because of natural variability in the exchange between land-based water and the oceans,
  • Natural variability in land-based ice storage and melting, that adds to, or subtracts water from ocean masses,
  • Natural variability in heat exchange between the atmosphere and oceans (the steric contribution to sea level),
  • Multi-year cycles such as ENSO (El Niño Southern Oscillation)
  • One-off events such as volcanic eruptions that affect regional temperatures because of ash and aerosols; in this case the Pinatubo eruption influence was incorporated into the analysis.
  • And the drift in satellite orbits; this variability is much less than that of tide gauges.

The sum of these errors gives the plus (+) and minus (–) value (0.025 mm/year2) that is attached to the overall result – 0.084 mm/year2 (check the open access publication for details). So, if our current rate of sea level rise is 3mm/year, then in 10 years the rate will have increased (accelerated) to 3.84 mm/year, and after 50 years to 7.2 mm/year (almost double the 2017 rate).

Data correction may seem like a bit of a fudge, but it is a critical part of almost every measurement we take, no matter where or what it is; it is part of the process in science that makes data intelligible and coherent.  Correcting data is part and parcel of any attempt to isolate causes and effects, as well as determining the kinds of error that are inherent in all measurements. The bottom line in this example, and one that is pointed to by the authors, is that the analysis is preliminary, and that some of the corrections might change as our knowledge of climate and other global systems improves.  However, there is confidence that this kind of analysis is on the right track.

Facebooktwitterredditlinkedin
Facebooktwitterlinkedininstagram

Visualizing Mars landscapes in 3 dimensions; stunning images from HiRISE

Facebooktwitterlinkedininstagram

For my 10th birthday my grandparents gave me a massive Collins encyclopedia – a 1960 update of the known universe. Among the collection of images were pictures of various planetary bits and pieces, the moon and Halley’s Comet, with the clarity of the Mt. Palomar Observatory telescope; images that set the imagination reeling. Technology back then was firmly attached to terra firma. Only three years earlier Sputnik had entered the history books.  Six decades later and I still have that sense of excitement, but now there’s a constant pictorial stream, with amazing clarity and detail of a comet’s surface, close encounters with Jupiter, vapour plumes erupting from Enceladus, Saturnian rings, and Mars rovers. We can observe sand grains entrained in dunes that move across the Martian surface. A barrage of images and videos, almost in real-time (not counting the 13 minutes it takes for the signal to reach us). Continue reading

Facebooktwitterredditlinkedin
Facebooktwitterlinkedininstagram

Subcutaneous oceans on distant moons; Enceladus and Europa

Facebooktwitterlinkedininstagram

Our blue Earth, rising above the lunar horizon, is an abiding image of our watery state that must evoke an emotional response in any sensible person. Cloud-swirled, Van Gogh-like. Such a blue – there’s nothing like it, at least in our own solar system.  A visitor to Mars three billion years ago might have also seen a red planet daubed blue, but all those expanses of water have since vanished, replaced by seas of sand.

Earth’s oceans are unique in our corner of the universe. Except for a thin carapace of ice at the poles, they are in a liquid state, and are in direct contact with the atmosphere to the extent that feed-back processes control weather patterns and climates.  Sufficient gravitational pull plus the damping effect of our atmosphere, prevents H2O from being stripped from our planet by solar radiation (again, unlike Mars). Our oceans exist because of this finely tuned balancing act. Continue reading

Facebooktwitterredditlinkedin
Facebooktwitterlinkedininstagram

Near Earth Objects; the database designed to save humanity

Facebooktwitterlinkedininstagram

The media love natural disasters, even those that don’t exist. Last week (early October, 2017), dramatic footage of a (simulated) super-volcano eruption beneath Auckland city was aired by several international media outlets, with headlines announcing the city’s calamitous destruction. But there is no super-volcano beneath Auckland. The excitement was short-lived.  While Auckland smouldered (as if that wasn’t enough), it was announced that New Zealand’s North Island could experience a subduction zone earthquake that, in its aftermath, would leave 1000s dead. An interesting backdrop to New Zealand’s recent election. Having scared the local population to death, our purveyors of science moved on to the next concern; other “what ifs…”.

Asteroid impacts are no longer de rigueur; perhaps it’s the turn of super-volcanoes’, or because NASA and the European Space Agency (ESA) have stated, with some confidence, that no large impacts are expected within the next 100 years. And whereas the media may find this Continue reading

Facebooktwitterredditlinkedin
Facebooktwitterlinkedininstagram