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A
Plan for Living on a Restless Planet
Jeffrey
T. Booth,
Diane L. Evans,
Casey Heeg,
Yunjin Kim and
David M. Tralli
Where and when will the next significant earthquake
occur? Can we accurately map risk zones for landslides? Is the
magnetic field of the earth reversing, and what would be its
impact? Answering these and other challenging questions is one
of the goals of the solid Earth science community outlined in
a new report from NASA, “Living on a Restless Planet.” Our planet
is, indeed, a restless one, with volcanic eruptions, earthquakes,
floods, and dozens of other processes constantly reshaping the
“solid” earth. The NASA Earth Science Enterprise is
studying these global phenomena from the unique vantage point
of space. In partnership with U.S. and international agencies,
NASA aims to understand the mechanisms driving these planetary
changes, develop tools to forecast potential natural hazards
and assess their impact, and meet its mandate to “improve life
here,” on our home planet.
NASA chartered the Solid Earth Science Working Group (SESWG),
which includes 19 representatives from universities, research
institutions, NASA, and the U.S. Geological Survey, to develop
a strategy for the highest priority objectives in solid Earth
science for the space agency over the next 25 years. The strategy
developed by SESWG over the course of two years addresses six
challenges that are of fundamental scientific importance, have
strong implications for society, and are amenable to substantial
progress through a concerted series of scientific observations
from space.
The plan recommended by SESWG is cast in terms of five observational
strategies, each of which addresses multiple scientific challenges:
deformation of the land surface, high-resolution topography
and topographic change, variability of the earth’s magnetic
field, variability of the earth’s gravity field, and imaging
spectroscopy of Earth’s changing surface. A key element of these
strategies, according to the Report, is NASA’s development of
a satellite dedicated to Interferometric Synthetic Aperture
Radar (InSAR). An InSAR mission would enable the detection of
very small changes on the earth’s surface, bringing significant
improvements in our knowledge of land surface deformation and
topographic change. It also would provide useful data that could
be used, along with data from other instruments, to address
the other scientific challenges outlined above. All of the recommendations
of the SESWG can be found in the full Report, which is available
at http://solidearth.jpl.nasa.gov.
Scientific Challenges
The six solid Earth science challenges identified by SESWG are:
1 What is the nature of deformation at plate boundaries and
what are the implications for earthquake hazards?
2 How is the land surface changing and producing natural hazards?
3 What are the interactions among ice masses, oceans, and the
solid earth and their implications for sea level change?
4 How do magmatic systems evolve and under what conditions do
volcanoes erupt?
5 What are the dynamics of the mantle and crust and how does
the earth’s surface respond?
6 What are the dynamics of the earth’s magnetic field and its
interactions with the Earth system?
For each of these challenges, SESWG assessed current understanding,
identified outstanding scientific challenges, and suggested
benefits to be obtained from further study.
Deformation of Boundaries
Loss of life and property in Los Angeles, Tokyo, Istanbul, and
other major metropolitan areas could be mitigated by knowledge
of when and where the next major earthquake will occur. Unfortunately,
it is difficult to answer this question without a great deal
of uncertainty. A comprehensive understanding of the underlying
mechanics of earthquakes requires different types of data, from
space-based as well as ground-based instruments.
Deformation of the earth’s surface occurs principally at tectonic
plate boundaries as a result of stresses that are accumulated
and released among fault systems in complex ways. Scientists
will need frequent measurements at fine spatial scales in order
to analyze surface deformation regionally, as well as locally,
and infer the source or sources of the “stress field.” Satellites
devoted to InSAR measurements are needed to provide dense, frequent
sampling and high-accuracy observations for mapping changes
of the earth’s surface. Existing and planned programs for seismic,
strain and GPS measurements, such as the National Science
Foundation’s EarthScope Program, will complement space-based
systems for surface and sub-surface imaging, while more precise
gravity measurements can help to characterize subsurface regions.
By mapping deformation due to the earthquake cycle (before,
during, and after an earthquake), scientists will be able to
provide enhanced global maps of earthquake hazards, which in
turn will support mitigation strategies. Scientists also will
determine whether a local or regional earthquake precursor exists,
from which predictions of future quakes may be based. These
systematic observations would help to improve models that are
used to assess the probabilities of event occurrences and their
magnitudes, the vulnerabilities of communities, and the impact
of direct or indirect disasters (slides, fires) on life and
property. An additional important benefit of InSAR satellite
data is rapid disaster response. A geosynchronous InSAR satellite,
or a constellation of InSAR satellites in low-Earth orbit, could
quickly map urban areas damaged by an earthquake and identify
neighborhoods with the most damage, regardless of lighting conditions
or cloud cover, through the use of image processing techniques.
The information extracted from such images may be used to help
prioritize disaster relief efforts and federal assistance, easing
the event’s impact on those affected and possibly saving lives.
Land Surface Changes
We know that human-induced changes, such as deforestation, and
natural events such as earthquakes and severe weather, reshape
the land surface. Storms and wind cause erosion, affecting the
extent of floods and likelihood of landslides. In addition to
the macroscopic changes brought about by long-term tectonic
plate movement, the SESWG Report addresses the ways in which
more subtle land surface changes produce natural hazards, and
how this danger could be reduced by more effective modeling.
Topography, soil characteristics, vegetation, and rainfall intensity
determine how sediment will be eroded, transported, and deposited
during a storm or other disturbance. By continuously tracking
these and other factors from Earth orbit, remotely sensed data
can play an important role in reconstructing the sequence of
recent land surface changes, and in estimating the likelihood
that these changes will induce or impact other events, such
as floods or landslides. In particular, the SESWG has identified
InSAR, high-resolution topographic mapping, and imaging spectroscopy
as the tools needed to acquire such data, with a long-range
goal of substantially continuous, global, space-based measurements
using each of these techniques. Our ability to provide real-time
or near-real-time information on land surface changes would
enable the modeling of natural hazards with unprecedented accuracy,
with corresponding gains in our ability to plan effectively
for disaster mitigation and response.
Ice, Oceans and Earth
Geological data and historical records have clearly documented
sea level changes in the past. The 10-20 cm global rise in sea
level recorded over the last century has been broadly attributed
to two effects: the heating and expansion of the oceans (steric
effect), and changes in the relative levels of land and ocean
mass (mass-budget effects) due to glacial melting and other
solid Earth processes. However, very little is known about the
interactions among ice masses, oceans and the solid Earth that
cause these effects. The SESWG has recognized that new scientific
information on the nature and causes of sea level change is
key to our understanding of the mechanisms driving this planetary
change, and what role humanity plays in the process. Global
InSAR mapping, targeted high-resolution topography, imaging
spectroscopy and improved gravity measurements will be needed
to enable scientists to separate the relative contributions
to sea level change from the steric and mass-budget effects,
and enable breakthroughs in our knowledge of the mechanisms
driving this change.
Considering that more than 100 million people live at elevations
within one meter of mean sea level, the value of this research
to the global population is indisputable. Improved estimates
of future sea level changes, better assessments of coastal erosion
caused by sea level rise, and more accurate long-term planning
for affected communities will play an essential role in safeguarding
the lives and property of the world’s coastal regions.
Evolution of Volcanic Systems
The eruptive power of volcanoes, interspersed with often long
intervals of dormancy, makes them both difficult objects to
study and dangerous to neighboring population centers. The most
obvious strategy for advancing our understanding of the mechanisms
driving changes in these magmatic systems is to observe volcanic
activity on a global scale. The current state of thousands of
volcanic centers, including remote terrestrial and undersea
volcanoes, is poorly known. Individual indicators of activity
include surface deformation, seismic vibration, gravitational
changes, gaseous venting, lava flows, and actual eruptions.
Little is known, however, of how these phenomena are interrelated.
The physical mechanisms that cause surface deformation and those
that control the rates and styles of eruptions are poorly understood.
Accurate prediction of the timing, magnitude, and style of volcanic
eruptions is a laudable, but generally unmet goal.
The most effective tools for studying these volcanic systems,
according to the SESWG Report, are InSAR, imaging spectroscopy,
and more sensitive measurements of Earth’s gravity field. As
we improve our understanding of volcanic processes and our ability
to forecast eruptions, we may be able to improve the lives of
not only those living in volcanic regions, but global populations
as well. Certainly advanced planning for high-risk populations
will save lives and reduce economic devastation. But the detection
of ash and plume products is also important in order to provide
warnings for air travel, such as the many international flights
that pass over the Kamchatka Peninsula along the eastern margin
of Russia. This region is dotted with volcanoes, and plumes
have previously damaged aircraft and put air travel at risk.
Mapping plume trajectories and understanding their composition
will also improve our understanding of climate change and our
ability to take preventive actions that mitigate its effects.
The 1991 eruption of Mount Pinatubo in the Philippines forced
enough ash and gas into the atmosphere to produce a small but
measurable cooling for nearly two years, resulting in significant
agricultural losses. An ability to recognize such threats and
provide the affected region with advance warning would yield
immediate returns in safety, quality of life, and international
good will.
Mantle and Crust Dynamics
Mantle convection, the movement of the viscous, rocky material
between the earth’s outer core and the thin rigid shell known
as the lithosphere, is the engine responsible for plate movement,
earthquakes, volcanism, ocean floor formation and mountain building.
The
corresponding changes at the earth’s surface required to accommodate
plate tectonics occur primarily along narrow plate boundary
zones in oceanic regions and across broader zones of deformation
where plate boundaries occur within continents. The geometry
and mechanics of mantle convection are not known in detail.
Variations in the global gravity field provide important clues
to density anomalies associated with mantle convection. However,
interpretation of these factors requires information on the
structure of the tectonic plates and the variation of viscosity
within the mantle. The SESWG Report states that new tools to
measure plate characteristics and mantle viscosity can come,
in turn, from measurements of the time-dependent response of
global gravity, topography, and Earth rotation to loading and
unloading by glaciers, oceans, and other forcings.
Such data will result in long-term benefits to society through
a better understanding of the engine that drives earthquakes
and accompanying surface changes. It can also improve our understanding
of sea level rise by measuring post-glacial rebound, a condition
caused by glacial melting that results in local changes in land
surface height, thus affecting the relative sea level in that
region, as well as the total volume of the ocean.
Earth’s Magnetic Field
Albert Einstein once ranked the problem of explaining the origin
of the earth’s magnetic field as among the three most important
unsolved problems in physics. Although today we widely recognize
that a dynamo operating in the fluid outer core generates the
earth’s magnetic field, the details of how that dynamo
works are poorly understood. Over the past 150 years, the main
component of the earth’s magnetic field has decayed by nearly
10%, a rate ten times faster than if the dynamo were simply
switched off. To that extent, the dynamo today is effectively
operating more as an anti-dynamo that destroys part of the field.
Intriguingly, this decay rate is characteristic of magnetic
reversals, which are believed to occur on average, though with
great variability, about once every half million years.
The recent decay in our global magnetic field is due largely
to field changes in the vicinity of the South Atlantic Ocean.
Within an area known as the South Atlantic Magnetic Anomaly
the magnetic field at Earth’s surface is decreasing, and is
now about 35% weaker than would be expected for a global dipole.
This hole in the field has serious implications for satellites
in low-Earth orbit, since it increases their radiation exposure.
How much longer will the South Atlantic Magnetic Anomaly continue
to grow? How large will it become? Is the field reversing? Long-term
satellite observations combined with numerical dynamo modeling
will advance our understanding of this phenomenon, allowing
us to model its evolution and provide forecasts of the magnetic
field on decadal time scales. Such capabilities will help us
to evaluate the effects of space weather on communications and
satellite operations.
Solid Earth Science
To make substantial progress toward answering each of the six
challenges for solid Earth science, the SESWG Report urges NASA
to develop and implement a broadly conceived program with both
near-term goals and clear steps toward longer-term objectives.
The interconnected nature of solid Earth science means that
the most challenging issues in the field today bridge several
disciplines. As such, defining the measurement requirements
to address these challenges is best done through a unified observational
program. A broad approach incorporates diverse methodologies
(including space-borne, sub-orbital and ground measurements)
and technological advances and capitalizes on the complementary
nature of the observations. SESWG has recommended long-term,
continuous observations in the following five areas to address
the fundamental challenges to solid Earth science:
1 Measuring surface deformation
2 High-resolution topography measurements
3 Tracking variations in Earth’s magnetic field
4 Tracking variations in Earth’s gravity field
5 Imaging spectroscopy of Earth’s changing surface
Dedicated research and analysis activities are
essential to ensure that newly acquired data are fully analyzed,
to provide new ideas for instrument and mission development,
and to foster unexpected scientific discoveries. SESWG
recommendations include significant investments in computation
and modeling for testing theories and predictions. A number
of observations needed over the next two and a half decades
require a continuing investment in advanced technology development.
This modular yet broadly interlinked program architecture offers
flexibility to change as scientific discoveries and programmatic
requirements dictate.
NASA’s role in observations is primarily the development of
satellite missions, but such projects cannot be as effective
as possible without complementary terrestrial observations and
the requisite partnerships with other programs and agencies.
The SESWG strategy builds on current capabilities and resources.
It requires data from missions and instruments that have recently
flown, that are currently flying, and that are planned for the
very near future. It also relies on data from missions that
are currently supported by other scientific disciplines. The
SESWG strategy leverages collaborations and partnerships to
the largest extent possible with other government agencies,
the private sector, and the international community. The focus
in the Report is largely on new observational requirements that
promise the greatest advance toward addressing the key challenges
and on the implications of those requirements for investments
in new technology.
For the next five years, the new space mission of highest priority
for solid Earth science is a satellite dedicated to InSAR measurements
of the land surface at the L-band frequency (about 24 cm wavelength).
Such a mission would address the most urgent objectives in the
areas of plate-boundary deformation, land-surface evolution,
ice and sea-level change, volcanism, and mantle dynamics. Over
the next five to ten years, the scientific challenges facing
solid Earth science can be met by NASA leading or partnering
in space missions involving constellations of satellites dedicated
to InSAR and magnetic field measurements, new instruments for
mapping global topography and its temporal changes and for spectroscopic
imaging across a broad portion of the electromagnetic spectrum,
and a GRACE (Gravity Recovery and Climate Experiment) follow-on
mission to improve the resolution of temporal changes in Earth's
gravity field. In the 10 to 25 year time frame, several techniques
offer the promise to contribute in a major fashion to solid
Earth science but require substantial technology development.
A fully realized program should contain elements that encompass
not only new observations but also sustained investment in research
and analysis, information systems, new technologies, supporting
infrastructure, while continuing to inform the public about
the benefits realized from the study of the solid Earth.
This summer’s Earth Observation Summit, held in Washington,
D.C., between ministers from countries around the globe,
exemplified the type of international collaboration needed to
achieve the SESWG recommendations (see the August/September
issue of EOM). The broad spectrum of SESWG’s scientific goals,
and their potential benefits for society, are best achieved
through joint research programs such as those envisioned at
the Earth Observation Summit (www.earthobservationsummit.gov).
Efforts to coordinate observations of the environment, such
as the International Global Observing Strategy (www.igospartners.org),
already are aligned with some of the SESWG
recommendations. As the international community, both scientific
and political, begins to work together to understand the solid
Earth and natural hazards in the coming years and decades, we
look forward to meeting the challenges and reaping the benefits
set forth in the SESWG Report.
About
the Authors
Jeffrey T. Booth, Diane L. Evans, Casey Heeg, Yunjin Kim and
David M. Tralli; Jet Propulsion Laboratory, California Institute
of Technology
SESWG Members: Sean C. Solomon (chair), Carnegie
Institution of Washington; Victor R. Baker, University of Arizona;
Jeremy Bloxham, Harvard University; Andrea Donnellan, Charles
Elachi, Diane Evans, Eric Rignot, Jet Propulsion Laboratory,
California Institute of Technology; Douglas Burbank, University
of California, Santa Barbara; Benjamin F. Chao, NASA Goddard
Space Flight Center; Alan Chave, Woods Hole Oceanographic Institution;
Alan Gillespie, University of Washington; Thomas Herring, Massachusetts
Institute of Technology; Raymond Jeanloz, University of California,
Berkeley; John LaBrecque, NASA Headquarters; Bernard Minster,
University of California, San Diego; Walter C. Pitman, III,
Lamont Doherty Earth Observatory, Columbia University; Mark
Simons, California Institute of Technology; Donald L. Turcotte,
University of California, Davis; Mary Lou C. Zoback, U.S. Geological
Survey.
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