Hopes high for measuring moonquakes
doi:10.1038/nindia.2019.102 Published online 6 August 2019
The seismometer on board India’s moon lander Vikram is set to detect moonquakes, first observed by the passive seismic network installed by Apollo astronauts.
On board Chandrayaan-2, launched by the Indian Space Research Organisation (ISRO) on July 22, Vikram is on course to reach the Lunar South Pole at a latitude of about 70° South on September 7.
The Moon is seismically active, but with a lesser intensity than Earth. According to ISRO the seismometer is designed to detect minute ground displacement, velocity or acceleration caused by lunar quakes.
“We hope the seismometer will record some signals from shallow moonquakes,” planetary geologist Senthil Kumar, a principal scientist at the National Geophysical Research Institute (NGRI) in Hyderabad, told Nature India.
Kumar’s confidence that signals will be recorded stems from a new study1 by his team that analyzed data from the largest shallow moonquake (MW 4.1) recorded by the Apollo mission at Lorentz basin, on 3 January 1975.
Their analysis of high‐resolution satellite images and topographic data from NASA's Lunar Reconnaissance Orbiter mission led to the discovery of seismically active faults at shallow depths, known as ‘lobate scarps’, near the epicentre of the moonquake. In addition, hundreds of boulder falls were observed on the interior walls of two impact craters.
“Our study reveals a set of previously unreported, seismically active, young lobate scarps near the epicentre,” the NGRI authors say. “The coexistence of lobate scarps and distribution of boulder fall sites around them provide a new approach for identifying seismically active zones on the Moon.”
According to Kumar, their observation that similar active faults and co-seismic boulders exist at many localities in the Lunar South Polar region, including near Vikram's landing site, raises hopes of Chandrayaan-2 detecting moonquakes.
Shallow moonquakes are energetic events similar to intra-plate earthquakes, and they occur near lunar surface. “Going by the observations made by Apollo missions, there are just three to seven such events each year,” says Kumar. "So, even if Vikram seismometer picks up one or two, during its short mission life of 14 Earth days, that would be great," says Kumar,
Deep moonquakes are low-energy events occurring at 900-950 km depths at the rate of around 2000 events per year. So Vikram’s seismometer may record tens of deep moonquakes, he says. “Meteoroid impacts are at a rate of around 200 events per year. So, it might record a few impact events as well.”
James Head, a lunar geologist at Brown University in the United States, told Nature India why it is valuable to determine the level of seismic activity on the Moon. Such measurements, he says, are critical to future human and robotic exploration. “The very important paper by Kumar and colleagues shows that there is a very high likelihood that the Chandrayaan-2 lander will detect some of these moonquakes.”
J. J. Rawal, president of the Indian Planetary Society, says the study shows that seismic activity on the moon has been more or less continuous. "The lander and the rover should also save themselves from moonquakes which may occur while they are functioning there,” he told Nature India.
1. Senthil Kumar, P. et al. The seismically active lobate scarps and coseismic lunar boulder avalanches triggered by 3 January 1975 (MW 4.1) shallow moonquake. Geophys. Res. Lett. 46 (2019) doi: 10.1029/2019GL083580