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Surface tensions

A prayer-field of ice may prevent a craft from alighting on Jupiter’s moon, Europa.

S Ananthanarayanan |

Jupiter’s satellite, Europa, just a little smaller than our moon, is believed to be covered with water and a crust of ice. Its thin atmosphere is also rich in oxygen. Europa is hence a good candidate in the search for extraterrestrial life. There have been detailed observations from Earth and from spacecraft that have flown by. And it would make sense that a mission be launched to land a research module on the surface of the satellite.

Daniel EJ Hobley, Jeffrey M Moore, Alan D Howard and Orkan M Umurhan from Cardiff University, Nasa Ames Research Centre in California and the University of Virginia, however, write in the journal, Nature Geoscience, that there may be problems in getting a research station down to the surface of Europa. A theoretical study of the known conditions on Europa indicates that water on its surface would form an ice-field of spikes and blades, which would be hazardous for a landing craft to approach.

Europa is the smallest of the four satellites of Jupiter that Galileo discovered in 1610, and the sixth from the mother planet of the 79 moons finally discovered. Around 800 million km from the Sun, the average surface temperature is 171°C below freezing. With its low surface gravity, it has lost most of its atmosphere, but what remains is rich in oxygen. Sightings show Europa to be the smoothest object in the Solar System. This and studies of the amount of light that it reflects lead to the idea that there is an ocean of liquid water that lies below its surface of ice. And hence, the idea that it may harbour the components essential for life.

The current study, reported in Nature Geoscience, however, questions the impression that the surface of Europa is smooth. This impression had arisen because it was thought that the onslaught of charged particles that are known to strike Jupiter’s moons, thanks to the gas giant’s magnetic field, would have a diffusive effect, and wear down the surface of ice in a process known as impact gardening.

In arid and low pressure conditions, as could be found on Europa, the diffusive effect of radiation, or sunlight, is not to melt low-temperature ice to water, but to make the ice sublime, or pass directly to the vapour form. This effect, called ablation, is often observed in a snowfield after a cold spell and is one way that glacier ice gets eroded.

On Earth, however, the study says, there are conditions in which the process of sublimation dominates over the diffusive effect, leading to roughness, at the centimetre level or even the scale of metres. The same effect, the paper says, is expected at the equatorial belt on Europa, a conclusion that is in keeping with anomalies that have been observed in radar signals bounced of the equator region.

Roughness arising out of sublimation of ice is clearly seen in the formation of “penitents” or nieves penitentes (Spanish for “penitent snows”) on the plains atop the Andes, at altitudes above 4,000 metres. The name has come from the resemblance of a field of penitentes to a crowd of kneeling monks doing penance.

The spikes of ice give the impression of the white, pointed hoods that the brothers of religious orders wear during Processions of Penance, in the Spanish Holy Week. These spikes, which can be from a few centimetres to five metres tall, were first described in scientific literature, by Charles Darwin, who saw them in 1835, in the course of his travels around the coast of South America.

The reason why penitentes arise is known as “differential ablation”, or erosion in the troughs between the spikes but not of the spikes themselves. Once a hint of a spike and a trough forms, the spike presents a side, rather than a surface, to radiation from overhead and there is greater ablation at the trough.

The glancing angle that the sides of the spike present then directs radiation into the trough, and this accelerates the process of spike formation. The Andes are high altitude, arid and at the equator. The conditions of low pressure, low vapour content of the air, low temperature and sunlight from directly above for much of the time are just what it takes for penitentes formation.

The authors of the Nature Geoscience paper note that the conditions of “bright, sustained sunlight, cold, dry, still air, and a melt-free environment” are there on the surface of the satellite, Europa. It is also “tidally locked” with its parent planet, Jupiter. This means that the tidal forces exerted by the planet have slowed down the rotation of the satellite so that one rotation takes the same time as one revolution around the mother planet. This is just like our own moon, which always shows the same face to the Earth as it goes around.

Further, Europa’s orbit is also almost along Jupiter’s equator, and Jupiter’s own axis has a very slight tilt. The result is that Europa, at the equator, has the sun right overhead for most of the time. The team has worked out the effect of different conditions, like the temperature, radiation, movement of air and the geometry of the spikes that form, on the maximum height of penitentes that form. They work it out that given the age of the surface of Europa and the known conditions, sublimation to a depth of 15 m should have taken place. Assuming that the growth of penitentes is such they are half as far apart as they are tall, the penitentes should be 15 m tall and 7.5 m apart.

This has grave implications for plans of missions to Europa and landing a research station on the surface. “This should motivate further detailed quantitative analysis”, the paper says. The European Space Agency’s “Jupiter Icy Moons Explorer” set to launch in 2022 and Nasa’s “Europa-Clipper” proposed between 2022 and 2025 are two such, which would take high-resolution pictures of Europa’s surface.

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