R. Orosei | Istituto Nazionale di Astrofisica (original) (raw)

Papers by R. Orosei

Geophysical Research Letters, 2010

Martian Polar Layered Deposits, most likely dusty ice are transparent to radar waves. The reflect... more Martian Polar Layered Deposits, most likely dusty ice are transparent to radar waves. The reflected signal is generally above the noise and can be used to estimate the dielectric properties of the bedrock. Assuming the permittivity of the first layer is known, we use a simplified inversion method to extract the bedrock relative dielectric permittivity below the North PLD's.

Geophysical Research Letters, 2010

Martian Polar Layered Deposits, most likely dusty ice are transparent to radar waves. The reflect... more Martian Polar Layered Deposits, most likely dusty ice are transparent to radar waves. The reflected signal is generally above the noise and can be used to estimate the dielectric properties of the bedrock. Assuming the permittivity of the first layer is known, we use a simplified inversion method to extract the bedrock relative dielectric permittivity below the North PLD's.

Nature, Jan 13, 2016

Although water vapour is the main species observed in the coma of comet 67P/Churyumov-Gerasimenko... more Although water vapour is the main species observed in the coma of comet 67P/Churyumov-Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. The nucleus of 67P/Churyumov-Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. Although micrometre-sized water-ice grains are the usual result of v...

MARSIS, a low frequency nadir looking sounding radar, payload of the Mars Express mission ,is cur... more MARSIS, a low frequency nadir looking sounding radar, payload of the Mars Express mission ,is currently under development by a team of Italian and US researchers and industrial partners; its primary scientific objective is to map the distribution of water, both liquid and solid, in the upper portions of the crust of Mars. Detection of such reservoirs of water will

Planetary and Space Science, 2014

The Mars Express mission was launched in June 2003 and was inserted into orbit around Mars in Dec... more The Mars Express mission was launched in June 2003 and was inserted into orbit around Mars in December 2003. Its main objective is to study the Mars' subsurface, surface, atmosphere and interaction with the solar wind. A secondary objective is to study the martian moons, in particular the largest one Phobos, thanks to a near polar and elliptical orbit which allows the spacecraft to perform close flybys about every five months. The Mars Express data not only consist of high-resolution 3D color images, but also astrometric images, spectra from 0.18 to 20 μm, radar echoes, Doppler signals from gravity experiments, and ion data. A new view of the moons has emerged from this data set, favoring now the idea that they are not captured asteroids, but rather the result of a re-accretion following a major impact on Mars. This unique set of data is available in the ESA Planetary Science Archive (PSA) and mirror imaged in the NASA Planetary Data System (PDS). This paper presents an overview of the Mars Express Phobos flybys, the specificities of their operations and the scientific achievements.

SHARAD (SHAllow RADar) is a Synthetic Aperture Radar (SAR) and Altimeter provided by ASI as a Fac... more SHARAD (SHAllow RADar) is a Synthetic Aperture Radar (SAR) and Altimeter provided by ASI as a Facility Instrument to NASA's 2005 Mars Reconnaissance Orbiter. Primary objective of this nadir-looking GPR instrument is to map Martian surface and subsurface up to 2 Km depth with vertical resolution of 15 m and horizontal resolution of a few hundred meters (300 m-1 Km).

Science (New York, N.Y.), Jan 23, 2015

The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta s... more The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ(-1)), and the broad absorption feature in the 2.9-to-3.6-micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

Initial SHARAD observations of the uppermost north pole layered deposits show many layers and evi... more Initial SHARAD observations of the uppermost north pole layered deposits show many layers and evidence of both large-scale continuity and disruptive structures.

SHARAD radar signals reveal a subsurface dielectric interface in the south part of the basin that... more SHARAD radar signals reveal a subsurface dielectric interface in the south part of the basin that may correspond with the interface between mid-Amazonian basalts and the underlying sedimentary basin fill.

2007 IEEE International Geoscience and Remote Sensing Symposium, 2007

MARSIS (Mars advanced Radar for subsurface and ionosphere sounding) is a low frequency nadir look... more MARSIS (Mars advanced Radar for subsurface and ionosphere sounding) is a low frequency nadir looking sounding radar selected by ESA as a payload of the Mars Express mission, whose primary Scientific Objective is to map the distribution of water both solid and liquid, at global scale on the Martin crust. MARSIS is the first instrument to be able to detect

This paper describes the Cassini RADAR PAD System, which has been designed and developed in the f... more This paper describes the Cassini RADAR PAD System, which has been designed and developed in the frame of Cassini-Huygens, a joint NASA/ESA/ASI mission to Saturn and its moons, responding to ASI request to process the data collected by the Cassini RADAR Altimeter. The PAD System contains the HW and SW operational tools necessary to evaluate the instrument performances, to process

2006 International Radar Symposium, 2006

The analysis of the surface return echoes in the subsurface data extraction in North Mars Polar r... more The analysis of the surface return echoes in the subsurface data extraction in North Mars Polar region has shown the possibility to utilize simplified surface models, at least, for the purpose of the evaluation of the penetration depth capability. The surface simulation, obtained starting from MOLA data, has been utilized during the planning activity in order to select the MARSIS

This paper presents the Radar for Icy Moons Exploration (RIME) instrument, which has been selecte... more This paper presents the Radar for Icy Moons Exploration (RIME) instrument, which has been selected as payload for the JUpiter Icy moons Explorer (JUICE) mission. JUICE is one of the most important missions chosen as part of the ESA's Cosmic Vision 2015-2025, and is aimed to study Jupiter and to investigate the potentially habitable zones in the Galilean icy satellites. RIME is a radar sounder optimized for the penetration of Ganymede, Europa and Callisto up to a depth of 9 km in order to allow the study of the subsurface geology and geophysics of the icy moons and detect possible subsurface water. In this paper we present the main science goals of RIME, the main technical challenges for its development and for its operations, as well as the expected scientific returns.

Journal of Geophysical Research: Space Physics, 2015

The total electron content (TEC) is one of the most useful parameters to evaluate the behavior of... more The total electron content (TEC) is one of the most useful parameters to evaluate the behavior of the Martian ionosphere because it contains information on the total amount of free electrons, the main component of the Martian ionospheric plasma. The Mars Express Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) radar is able to derive TEC from both of its operation modes: (1) the active ionospheric sounding (AIS) mode and (2) the subsurface mode. TEC estimates from the subsurface sounding mode can be computed from the same raw data independently using different algorithms, which should yield similar results. Significant differences on the dayside, however, have been found from two of the algorithms. Moreover, both algorithms seem also to disagree with the TEC results from the AIS mode. This paper gives a critical, quantitative, and independent assessment of these discrepancies and indicates the possible uncertainty of these databases. In addition, a comparison between the results given by the empirical model of the Martian ionosphere developed by and the different data sets has been performed. The main result is that for solar zenith angles higher than 75°, where the maximum plasma frequency is typically small compared with the radar frequencies, the two subsurface algorithms can be confidently used. For solar zenith angles less than 75°, where the maximum plasma frequency is very close to the radar frequencies, both algorithms suffer limitations. Nevertheless, despite the solar zenith angle restrictions, the dayside TEC of one of the two algorithms is consistent with the modeled TEC.

The Astronomical Journal, 2000

We present the results obtained by the simulations of di †erent thermal models of 5145 Pholus, on... more We present the results obtained by the simulations of di †erent thermal models of 5145 Pholus, one of the known Centaurs. PholusÏs orbit is highly eccentric, similar to that of comets, but its dimension is more similar to larger asteroids. Pholus cannot be clearly numbered in either class. The most likely source of Centaurs is the Edgeworth-Kuiper belt : dynamical studies and physical properties suggest that Pholus recently entered the planetary zone. Here we assume that the nature of Pholus is that of a cometary body made of di †erent ices and dust. We have computed the thermal evolution of this object under di †erent conditions : as a "" new ÏÏ object, namely an undi †erentiated body, and an "" old ÏÏ one, di †erentiated and aged in the Kuiper belt. We have tried to see also the e †ects induced by the presence of an organic dust on the overall evolution. Both the "" new ÏÏ and the "" old ÏÏ object show low, but di †erent, levels of gas activity.

MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) on board MarsExpress is the f... more MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) on board MarsExpress is the first and so far the only space borne radar that observed the Martian moon Phobos. Radar echoes were measured for different flyby trajectories. The primary aim of the low frequency sounding of Phobos is to prove the feasibility of deep sounding, into the crust of Phobos. In this poster we present a numerical method that allows a very precise computation of radar echoes backscattered from the surface of large objects. The software is based on a combination of physical optics calculation of surface scattering of the radar target, and Method of Moments to calculate the radiation pattern of the whole space borne radar system. The calculation of the frequency dependent radiation pattern takes into account all relevant gain variations and coupling effects aboard the space craft. Based on very precise digital elevation models of Phobos, patch models in the resolution of lambda/10 were generated....

A recent development in the exploration of Mars has been the deployment of two orbital subsurface... more A recent development in the exploration of Mars has been the deployment of two orbital subsurface radar sounding instruments, MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) on Mars Express, and SHARAD (Shallow Radar) on the Mars Reconnaissance Orbiter. Both experiments are fully operational, MARSIS since July 2005, and SHARAD since October 2006. The two radars are complementary, with MARSIS operating at deep-sounding low frequencies (2-5 MHz) with low vertical resolution (100 m), and SHARAD operating at a shallower-sounding higher frequency (15-25 MHz), but with much higher vertical resolution (10 m). The materials on Mars that are most consistently amenable to subsurface sounding are found in the polar regions. These include the several-km-thick, water-ice-rich polar layered deposits (PLD), and in the south, a surrounding ancient sedimentary terrain known as the Dorsa Argentea Formation. MARSIS sounding of the PLD consistently reaches the base of the deposits,...

The Astronomical Journal, 2000

We discuss the results obtained by simulating the thermal evolution of 2060 Chiron, the largest o... more We discuss the results obtained by simulating the thermal evolution of 2060 Chiron, the largest of the Centaur objects. The study of Chiron is extremely important because of the presence of activity in spite of its large distance from the Sun. In this paper we study the thermal evolution of a Chiron-like body under di †erent assumptions concerning the initial composition and taking into account its evolutionary history. The composition of Centaurs and Kuiper belt objects is almost unknown : we can guess that ChironÏs "" cometary behavior ÏÏ is an indication of a composition dominated by di †erent ices. We have therefore computed the thermal evolution of Chiron using a numerical code developed to study the thermal evolution of cometary nuclei. As in the case of comets, we assume that the ice is mixed with dust and that the body is highly porous. CO is present in the mixture both as ice and as gas trapped in the amorphous ice. Our conclusion is that ChironÏs activity can be explained only by assuming a composition in which CO is present not far from the objectÏs surface as an ice or as a gas trapped in the amorphous ice.

Geophysical Research Letters, 2010

Martian Polar Layered Deposits, most likely dusty ice are transparent to radar waves. The reflect... more Martian Polar Layered Deposits, most likely dusty ice are transparent to radar waves. The reflected signal is generally above the noise and can be used to estimate the dielectric properties of the bedrock. Assuming the permittivity of the first layer is known, we use a simplified inversion method to extract the bedrock relative dielectric permittivity below the North PLD's.

Geophysical Research Letters, 2010

Martian Polar Layered Deposits, most likely dusty ice are transparent to radar waves. The reflect... more Martian Polar Layered Deposits, most likely dusty ice are transparent to radar waves. The reflected signal is generally above the noise and can be used to estimate the dielectric properties of the bedrock. Assuming the permittivity of the first layer is known, we use a simplified inversion method to extract the bedrock relative dielectric permittivity below the North PLD's.

Nature, Jan 13, 2016

Although water vapour is the main species observed in the coma of comet 67P/Churyumov-Gerasimenko... more Although water vapour is the main species observed in the coma of comet 67P/Churyumov-Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. The nucleus of 67P/Churyumov-Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. Although micrometre-sized water-ice grains are the usual result of v...

MARSIS, a low frequency nadir looking sounding radar, payload of the Mars Express mission ,is cur... more MARSIS, a low frequency nadir looking sounding radar, payload of the Mars Express mission ,is currently under development by a team of Italian and US researchers and industrial partners; its primary scientific objective is to map the distribution of water, both liquid and solid, in the upper portions of the crust of Mars. Detection of such reservoirs of water will

Planetary and Space Science, 2014

The Mars Express mission was launched in June 2003 and was inserted into orbit around Mars in Dec... more The Mars Express mission was launched in June 2003 and was inserted into orbit around Mars in December 2003. Its main objective is to study the Mars' subsurface, surface, atmosphere and interaction with the solar wind. A secondary objective is to study the martian moons, in particular the largest one Phobos, thanks to a near polar and elliptical orbit which allows the spacecraft to perform close flybys about every five months. The Mars Express data not only consist of high-resolution 3D color images, but also astrometric images, spectra from 0.18 to 20 μm, radar echoes, Doppler signals from gravity experiments, and ion data. A new view of the moons has emerged from this data set, favoring now the idea that they are not captured asteroids, but rather the result of a re-accretion following a major impact on Mars. This unique set of data is available in the ESA Planetary Science Archive (PSA) and mirror imaged in the NASA Planetary Data System (PDS). This paper presents an overview of the Mars Express Phobos flybys, the specificities of their operations and the scientific achievements.

SHARAD (SHAllow RADar) is a Synthetic Aperture Radar (SAR) and Altimeter provided by ASI as a Fac... more SHARAD (SHAllow RADar) is a Synthetic Aperture Radar (SAR) and Altimeter provided by ASI as a Facility Instrument to NASA's 2005 Mars Reconnaissance Orbiter. Primary objective of this nadir-looking GPR instrument is to map Martian surface and subsurface up to 2 Km depth with vertical resolution of 15 m and horizontal resolution of a few hundred meters (300 m-1 Km).

Science (New York, N.Y.), Jan 23, 2015

The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta s... more The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ(-1)), and the broad absorption feature in the 2.9-to-3.6-micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

Initial SHARAD observations of the uppermost north pole layered deposits show many layers and evi... more Initial SHARAD observations of the uppermost north pole layered deposits show many layers and evidence of both large-scale continuity and disruptive structures.

SHARAD radar signals reveal a subsurface dielectric interface in the south part of the basin that... more SHARAD radar signals reveal a subsurface dielectric interface in the south part of the basin that may correspond with the interface between mid-Amazonian basalts and the underlying sedimentary basin fill.

2007 IEEE International Geoscience and Remote Sensing Symposium, 2007

MARSIS (Mars advanced Radar for subsurface and ionosphere sounding) is a low frequency nadir look... more MARSIS (Mars advanced Radar for subsurface and ionosphere sounding) is a low frequency nadir looking sounding radar selected by ESA as a payload of the Mars Express mission, whose primary Scientific Objective is to map the distribution of water both solid and liquid, at global scale on the Martin crust. MARSIS is the first instrument to be able to detect

This paper describes the Cassini RADAR PAD System, which has been designed and developed in the f... more This paper describes the Cassini RADAR PAD System, which has been designed and developed in the frame of Cassini-Huygens, a joint NASA/ESA/ASI mission to Saturn and its moons, responding to ASI request to process the data collected by the Cassini RADAR Altimeter. The PAD System contains the HW and SW operational tools necessary to evaluate the instrument performances, to process

2006 International Radar Symposium, 2006

The analysis of the surface return echoes in the subsurface data extraction in North Mars Polar r... more The analysis of the surface return echoes in the subsurface data extraction in North Mars Polar region has shown the possibility to utilize simplified surface models, at least, for the purpose of the evaluation of the penetration depth capability. The surface simulation, obtained starting from MOLA data, has been utilized during the planning activity in order to select the MARSIS

This paper presents the Radar for Icy Moons Exploration (RIME) instrument, which has been selecte... more This paper presents the Radar for Icy Moons Exploration (RIME) instrument, which has been selected as payload for the JUpiter Icy moons Explorer (JUICE) mission. JUICE is one of the most important missions chosen as part of the ESA's Cosmic Vision 2015-2025, and is aimed to study Jupiter and to investigate the potentially habitable zones in the Galilean icy satellites. RIME is a radar sounder optimized for the penetration of Ganymede, Europa and Callisto up to a depth of 9 km in order to allow the study of the subsurface geology and geophysics of the icy moons and detect possible subsurface water. In this paper we present the main science goals of RIME, the main technical challenges for its development and for its operations, as well as the expected scientific returns.

Journal of Geophysical Research: Space Physics, 2015

The total electron content (TEC) is one of the most useful parameters to evaluate the behavior of... more The total electron content (TEC) is one of the most useful parameters to evaluate the behavior of the Martian ionosphere because it contains information on the total amount of free electrons, the main component of the Martian ionospheric plasma. The Mars Express Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) radar is able to derive TEC from both of its operation modes: (1) the active ionospheric sounding (AIS) mode and (2) the subsurface mode. TEC estimates from the subsurface sounding mode can be computed from the same raw data independently using different algorithms, which should yield similar results. Significant differences on the dayside, however, have been found from two of the algorithms. Moreover, both algorithms seem also to disagree with the TEC results from the AIS mode. This paper gives a critical, quantitative, and independent assessment of these discrepancies and indicates the possible uncertainty of these databases. In addition, a comparison between the results given by the empirical model of the Martian ionosphere developed by and the different data sets has been performed. The main result is that for solar zenith angles higher than 75°, where the maximum plasma frequency is typically small compared with the radar frequencies, the two subsurface algorithms can be confidently used. For solar zenith angles less than 75°, where the maximum plasma frequency is very close to the radar frequencies, both algorithms suffer limitations. Nevertheless, despite the solar zenith angle restrictions, the dayside TEC of one of the two algorithms is consistent with the modeled TEC.

The Astronomical Journal, 2000

We present the results obtained by the simulations of di †erent thermal models of 5145 Pholus, on... more We present the results obtained by the simulations of di †erent thermal models of 5145 Pholus, one of the known Centaurs. PholusÏs orbit is highly eccentric, similar to that of comets, but its dimension is more similar to larger asteroids. Pholus cannot be clearly numbered in either class. The most likely source of Centaurs is the Edgeworth-Kuiper belt : dynamical studies and physical properties suggest that Pholus recently entered the planetary zone. Here we assume that the nature of Pholus is that of a cometary body made of di †erent ices and dust. We have computed the thermal evolution of this object under di †erent conditions : as a "" new ÏÏ object, namely an undi †erentiated body, and an "" old ÏÏ one, di †erentiated and aged in the Kuiper belt. We have tried to see also the e †ects induced by the presence of an organic dust on the overall evolution. Both the "" new ÏÏ and the "" old ÏÏ object show low, but di †erent, levels of gas activity.

MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) on board MarsExpress is the f... more MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) on board MarsExpress is the first and so far the only space borne radar that observed the Martian moon Phobos. Radar echoes were measured for different flyby trajectories. The primary aim of the low frequency sounding of Phobos is to prove the feasibility of deep sounding, into the crust of Phobos. In this poster we present a numerical method that allows a very precise computation of radar echoes backscattered from the surface of large objects. The software is based on a combination of physical optics calculation of surface scattering of the radar target, and Method of Moments to calculate the radiation pattern of the whole space borne radar system. The calculation of the frequency dependent radiation pattern takes into account all relevant gain variations and coupling effects aboard the space craft. Based on very precise digital elevation models of Phobos, patch models in the resolution of lambda/10 were generated....

A recent development in the exploration of Mars has been the deployment of two orbital subsurface... more A recent development in the exploration of Mars has been the deployment of two orbital subsurface radar sounding instruments, MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) on Mars Express, and SHARAD (Shallow Radar) on the Mars Reconnaissance Orbiter. Both experiments are fully operational, MARSIS since July 2005, and SHARAD since October 2006. The two radars are complementary, with MARSIS operating at deep-sounding low frequencies (2-5 MHz) with low vertical resolution (100 m), and SHARAD operating at a shallower-sounding higher frequency (15-25 MHz), but with much higher vertical resolution (10 m). The materials on Mars that are most consistently amenable to subsurface sounding are found in the polar regions. These include the several-km-thick, water-ice-rich polar layered deposits (PLD), and in the south, a surrounding ancient sedimentary terrain known as the Dorsa Argentea Formation. MARSIS sounding of the PLD consistently reaches the base of the deposits,...

The Astronomical Journal, 2000

We discuss the results obtained by simulating the thermal evolution of 2060 Chiron, the largest o... more We discuss the results obtained by simulating the thermal evolution of 2060 Chiron, the largest of the Centaur objects. The study of Chiron is extremely important because of the presence of activity in spite of its large distance from the Sun. In this paper we study the thermal evolution of a Chiron-like body under di †erent assumptions concerning the initial composition and taking into account its evolutionary history. The composition of Centaurs and Kuiper belt objects is almost unknown : we can guess that ChironÏs "" cometary behavior ÏÏ is an indication of a composition dominated by di †erent ices. We have therefore computed the thermal evolution of Chiron using a numerical code developed to study the thermal evolution of cometary nuclei. As in the case of comets, we assume that the ice is mixed with dust and that the body is highly porous. CO is present in the mixture both as ice and as gas trapped in the amorphous ice. Our conclusion is that ChironÏs activity can be explained only by assuming a composition in which CO is present not far from the objectÏs surface as an ice or as a gas trapped in the amorphous ice.