Measuring seismic activity on Venus: a real challenge
While acquiring seismic signals on Mars is already extremely difficult, the task is even trickier on Venus. Not only is its surface roasted by temperatures around 460°C, but it is also crushed by a very dense atmosphere that exerts a pressure of 90 bar, which is 90 times that of Earth’s atmospheric pressure, and more than the pressure felt at a depth of 900 metres under water on Earth. In such extreme conditions, spacecraft only survive a few hours.
Venus and Earth could almost be considered twin sisters. They have practically the same diameter and a similar mass. However, a closer look reveals glaring differences that Venus’s closer proximity to the Sun does not explain.
For one thing, on Venus’s surface, temperatures can reach a record 460°C. These unbearable conditions are caused by a very thick, suffocating atmosphere mostly composed of carbon dioxide (over 90 bars). It is the greenhouse effect that leads to these infernal temperatures.
Another major difference is the complete absence of water. Earth is known as the Blue Planet for a very good reason: it contains about 100,000 times more water than Venus. Since water plays a vital role in plate tectonics, the extremely arid nature of Venus would explain why this mechanism never developed despite the planet’s huge reserve of internal heat, dissipated through numerous volcanic structures that punctuate its charred surface.
And as if that were not enough, Venus does not turn in the same direction as the other planets in the solar system, and its rotation is extremely slow. It has no natural satellites to keep it company on its journey around the Sun, whereas the Earth has a massive natural satellite—the Moon—and Mars has two small ones. Finally, and strangely, Venus has no magnetic field.
In the light of the horrendous conditions on Venus, it is near-impossible to even attempt to measure any seismic activity there. For the moment, only the Soviets have tried. In 1982, the Venera 13 and Venera 14 space probes managed to land on Venus, but only survived a few hours in its hellish conditions.
Although the Venera probes are most well-known for the magnificent and troubling colour images that they sent back to Earth, they also flew the Groza 2 instrument, a combination of a microphone and rudimentary single-axis seismometer able to measure vertical displacements of the Venusian surface to micron precision.
Attached to a ring around the probe which was in contact with the surface, the Groza 2 seismometer’s coupling with Venus was less than perfect. The very short lifetime of the two stations was also a major drawback for measuring seismic activity. With a chance of survival limited to a matter of hours, the primary objective was to determine background noise and any micro-seismic activity.
Groza 2 managed to send some data back to Earth, but their interpretation ran up against the same problems as those of the Viking landers, as the measurements were complicated by outdoor disturbances such as small stones whipped up by the violent winds that were unfortunately blowing at the time of landing, or disturbances caused by the spacecraft itself (expansion and cracking of materials due to the heat and pressure). The validity of the data acquired was therefore called into question and no convincing results were obtained.
The spectacular divergence in the respective destinies of Venus and Earth, two celestial bodies formed in similar conditions yet which gave rise to two radically different worlds—one a stifling hell and the other a paradise—fascinates planetary scientists. There is no doubt that the answers to the mysteries of the formation and evolution of Venus are sealed in the depths of the planet, and to explore them, seismology is once again vital.
In the absence of plate tectonics—the phenomenon responsible on Earth for most earthquakes—it is logical to wonder if there is still any seismic activity on Venus. The relative youth of the Venusian surface (one billion years on average, which is more recent than the crust of the Moon, Mars or Mercury), the existence of strangely-shaped volcanic structures and the presence of tectonic structures such as rifts and faults indicate that the ground must tremble, obviously not as much as on Earth, but more often than on Mars.
The challenge lies in recording this seismic activity. The development of motion detection or displacement sensors with a similar performance to the pendulums of SEIS aboard the InSight mission to Mars and capable of operating in the hellish conditions encountered on Venus would keep engineers busy for several years, if not decades.
There are other efficient ways of trying to unveil the mysteries of Venus’s interior structure. The very thick atmosphere around the planet creates efficient coupling with the ground. When a quake occurs, the ground tremors are conveyed to the air and generate infrasonic wavefronts that are inaudible to the human ear but that can be detected by instruments designed for the purpose and which could be flown in the gondola of sounding balloons. The latter could orbit at altitudes where the extreme temperatures and pressure of Venus finally become easier to withstand.
Orbital observations are another possibility. The interaction of infrasonic waves with the upper atmosphere and more especially the ionized layer (ionosphere) could be measured and monitored from space by satellites.
At the current time, all the seismic survey projects focusing on Venus are still on the drawing board. In the field of planetary seismology, the InSight mission—designed to deploy an ultrasensitive seismometer on Mars—is the boldest project to date. Geophysicists interested in the Moon and Venus will be keeping a close eye on the mission. The lessons learned will no doubt serve for future missions back to the Moon or to explore the interior structure of Venus, a planet so familiar, yet so enigmatic.
Last updated: 25 October 2016