Vikram Lander and the mysterious south pole of the moon
It was indeed heart-breaking to watch the silent fall of the Vikram lander, which came all the way across its unprecedented voyage to the Moon and was on a final phase of soft-landing at the surface just about 1.334 km away when communication was lost.
The ISRO, the group of Masters behind the pioneered mission of Chandrayaan, consider the possibility of the tumbling of the lander due to the failure of one of the powering engines. However, any solid confirmation has yet to come from the authority.
From the descending trajectory (red circle in the right-panel of Fig. 1) it is apparent that the lander exhibited a severe deflection from the pre-assigned trajectory when it passed 2.1 km above the surface of the Moon, and beyond that point it was almost trending and tempting at an angle closer to a vertical fall.
It can be observed that from an altitude of 16.1 km to 1.334 km, the horizontal velocity has been significantly reduced from 263 m/s to 48 m/s, however the velocity reduction in the vertical component for the above mentioned altitude range was relatively less (i.e., from 72.3 m/s to 60 m/s).
This observation together with a high-angled descend from the trajectory likely indicate that the vertical velocity component was prominent over the horizontal one. If we attribute the loss of communication due to the crash landing, which likely indicates a sudden acceleration of Vikram from the deviation point.
The most likely cause for this situation could be the failure of the engine as it could not generate the thrust require for its soft-landing. However, it is worth thinking about other possible factors and several mysteries associated with the south pole of the Moon in the expense of the fall of the Vikram lander.
The primary force, which is the natural one, responsible for the acceleration of a falling object in the vertical component is gravity. Although the acceleration due to gravity on the surface of the Moon is negligible (just about 16.6% that on Earth's surface), it shows large scale spatial variations ranging from 1.611 m/s2 to 1.636 m/s2 with the maximum values regionally concentrates at its south pole.
This means that the speed of an object falling freely at the south pole of the Moon in the absence of air resistance will increase by a maximum of about 1.636 m/s2. Because the Moon has a shape of oblate spheroid (i.e., pole-to-pole radius is less than that at the equator), a normal increase in the force of gravity can be expected at the poles.
Another natural force to account is geomagnetism. Generally magnetic force will be strongest at the poles of any dipole magnetic body. However, studies indicate that the Moon currently does not have a dipolar magnetic field since its core-dynamo is no more active. However, the Moon does have a fossilized magnetic crust that was magnetized during its early evolution when the core-dynamo was active.
Despite of these facts, a recent study by James et al. (2019, https://doi.org/10.1029/2019GL082252) discovered a mysterious excess mass anomaly under the Pole-Aitken basin located in the south pole of the Moon, which is recognized perhaps as the largest impact structure in the solar system.
The study estimated a minimum mass of 2.18×1018 kg for the anomaly equivalent to a 95-km-diameter iron-nickel core and that likely extends to depths of more than 300 km into the mantle of the Moon. According to their model study, the excess mass beneath the Pole-Aitken basin was likely resulted either by an ineﬃcient mantle overturn that stranded oxides in the upper mantle or by a huge metal body delivered by an asteroid that formed this crater and is still embedded in the Moon’s mantle. The point is that such a large excess mass anomaly can cause significant gravity effect.
Therefore, an underestimation of gravity value may induce excess acceleration of the object (i.e., Vikram lander) by the neglected gravity pull at the near surface condition integrated with the existing speed of motion (i.e., vertical velocity) of the object particularly in the absence of air resistance.
Furthermore, if the excess mass is of iron-nickel core from a differentiated meteorite as surmised by James et al. (2019), it will certainly have some significant magnetic effect as well, which certainly required to consider for an evaluation. Interestingly, the Vikram’s landing/ missing spot is well within the identified inner rim of this mass anomaly.
However, as I mentioned earlier, these factors could be negligible, and which may not have a major significance in the quiescent fall of the Vikram lander. But still a cross-checking and validation of all the possible parameters would always be better and helpful for the future missions.
Figure. (a) Orthographic projection of lunar topography as collected by the LOLA. The inner rim is outlined in black, and a central topographic depression is indicated with a white dashed circle. (b) Free-air gravity from GRAIL referenced to a radius of 1748 km, with the topographic depression from (a) marked (Source: James et al., Geophysical Research Letters, 2019).
By Dr. Ratheesh-Kumar R.T
The Author is an Assistant Professor (Department of Marine Geology & Geophysics) at Cochin University of Science and Technology and is presently working as a postdoctoral scholar in a NASA project on MARS at University of Kentucky, USA.