Radioglaciology
Encyclopedia
Radioglaciology is the study of glacier
s and ice sheet
s using radar
. It employs a geophysical
method similar to ground-penetrating radar
that operates at frequencies
in the MF
, HF
and VHF
portions of the radio spectrum
. Radioglaciology is sometimes referred to as "ice-penetrating radar" or "radio-echo sounding".
Glaciers are well suited to investigation by radar because the imaginary part of the permittivity
of ice
is small relative to its real part; this ratio is called the loss tangent
. The conductivity of ice is small at radio frequencies
, so its dielectric absorption is also small.
The primary goal of many radioglaciological surveys is to measure the thickness of a body of ice, which is an important boundary condition for ice-flow models. Ice thicknesses greater than 4 km have been measured in East Antarctica
. Internal reflection
s have also been detected in many alpine glaciers and all modern ice sheets. These layers represent the internal stratigraphy
and can also be used to constrain ice-flow models. The shapes of these internal reflections generally follow the bedrock
topography
and they are often assumed to be isochronous
. Disturbances in these reflections that are unrelated to bedrock topography can be used to understand past ice flow, for example the
anticlines
arising from the Raymond Effect
.
The cause of the observed internal reflections partly depends on the frequency of the radar system used to detect them. There are three primary types of reflections:
Ice-penetrating radar systems, particularly the antenna
e, are often homemade systems made of commercially available components. However, commercial ground-penetrating radar systems are sometimes used.
There are currently two ice-penetrating radars orbiting Mars
: MARSIS
and SHARAD
. An ice-penetrating radar system was planned for the canceled Jupiter Icy Moons Orbiter
, and such systems are also being planned for orbiters that are part of the Europa Jupiter System Mission
.
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s and ice sheet
Ice sheet
An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier...
s using radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
. It employs a geophysical
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
method similar to ground-penetrating radar
Ground-penetrating radar
Ground-penetrating radar is a geophysical method that uses radar pulses to image the subsurface. This nondestructive method uses electromagnetic radiation in the microwave band of the radio spectrum, and detects the reflected signals from subsurface structures...
that operates at frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
in the MF
Medium frequency
Medium frequency refers to radio frequencies in the range of 300 kHz to 3 MHz. Part of this band is the medium wave AM broadcast band. The MF band is also known as the hectometer band or hectometer wave as the wavelengths range from ten down to one hectometers...
, HF
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
and VHF
Very high frequency
Very high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...
portions of the radio spectrum
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
. Radioglaciology is sometimes referred to as "ice-penetrating radar" or "radio-echo sounding".
Glaciers are well suited to investigation by radar because the imaginary part of the permittivity
Permittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
of ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
is small relative to its real part; this ratio is called the loss tangent
Dissipation factor
In physics, the dissipation factor is a measure of loss-rate of energy of a mode of oscillation in a dissipative system. It is the reciprocal of Quality factor, which represents the quality of oscillation....
. The conductivity of ice is small at radio frequencies
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
, so its dielectric absorption is also small.
The primary goal of many radioglaciological surveys is to measure the thickness of a body of ice, which is an important boundary condition for ice-flow models. Ice thicknesses greater than 4 km have been measured in East Antarctica
East Antarctica
East Antarctica, also called Greater Antarctica, constitutes the majority of the Antarctic continent, lying on the Indian Ocean side of the Transantarctic Mountains...
. Internal reflection
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
s have also been detected in many alpine glaciers and all modern ice sheets. These layers represent the internal stratigraphy
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
and can also be used to constrain ice-flow models. The shapes of these internal reflections generally follow the bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
topography
Topography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
and they are often assumed to be isochronous
Isochronous
Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...
. Disturbances in these reflections that are unrelated to bedrock topography can be used to understand past ice flow, for example the
anticlines
Anticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
arising from the Raymond Effect
Raymond Effect
Raymond Effect is a flow effect in ice sheets, occurring at flow divides, which gives rise to disturbances in the stratigraphy, showing unusual arches or anticlines called Raymond Arches. The stratigraphy is detected by radio-echo sounding. The Raymond effect arises from the unusual flow...
.
The cause of the observed internal reflections partly depends on the frequency of the radar system used to detect them. There are three primary types of reflections:
- In the firnFirnFirn is partially-compacted névé, a type of snow that has been left over from past seasons and has been recrystallized into a substance denser than névé. It is ice that is at an intermediate stage between snow and glacial ice...
and at depths where densification is occurring, small changes in densityDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
alter the real part of the permittivity, which can cause reflections. Once densification is complete, changes in density in an ice column are not expected to be large enough to cause radar reflections. - High concentrationConcentrationIn chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
s of volcanicVolcano2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
acidAcidAn acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s, e.g., sulfuric acidSulfuric acidSulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
or hydrochloric acidHydrochloric acidHydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
, increase the conductivity of the surface snowSnowSnow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
over which they are deposited. Acidity increases the conductivity, which produces a reflection. Reflections due to volcanic layers are possible at any depth. - Individual crystalCrystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s of ice display dielectric anisotropyAnisotropyAnisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...
. Layers that have a preferred crystal fabric direction different from that above it can therefore also cause reflections.
Ice-penetrating radar systems, particularly the antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
e, are often homemade systems made of commercially available components. However, commercial ground-penetrating radar systems are sometimes used.
There are currently two ice-penetrating radars orbiting Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
: MARSIS
MARSIS
MARSIS is a low frequency, pulse-limited radar sounder and altimeter used on the ESA Mars Express mission...
and SHARAD
SHARAD
SHARAD is a subsurface sounding radar embarked on the Mars Reconnaissance Orbiter probe. It complements the MARSIS instrument on Mars Express, providing lower penetration capabilities but much finer resolution .SHARAD is developed under the responsibility of the Italian Space Agency SHARAD (Mars...
. An ice-penetrating radar system was planned for the canceled Jupiter Icy Moons Orbiter
Jupiter Icy Moons Orbiter
The Jupiter Icy Moons Orbiter was a proposed spacecraft designed to explore the icy moons of Jupiter. The main target was Europa, the suspected ocean of which is one of the places where simple alien life is a possibility in our solar system...
, and such systems are also being planned for orbiters that are part of the Europa Jupiter System Mission
Europa Jupiter System Mission
The Europa Jupiter System Mission – Laplace was a proposed joint NASA/ESA unmanned space mission slated to launch around 2020 for the in-depth exploration of Jupiter's moons with a focus on Europa, Ganymede and Jupiter's magnetosphere...
.