Reorientation of the Moon from its Bombardment History
The gravity field of a planetary body tells us how mass is distributed within it. This information can be used to trace back the orientation of the body in space. Centrifugal forces acting on the non-uniformities in mass distribution, on the surface or within the body, pushes the excess mass towards the equator and mass deficiencies towards the nearest pole. This phenomenon is known as "true polar wander". My team and I used the lunar gravity data from the GRAIL mission to retrace the reorientation history of the Moon over the last ~4.25 billion years from the effect of around 5200 craters and basins in the diameter range of 20 to 1200 km.
We found that the Moon's degree-2 structure (a measure of its flattening) significantly changed as a result of these craters and led to the wandering of the Moon's rotational pole by about 10º (~300 km) along the Earth-Moon tidal axis. These findings have implications for the long-term stability of volatiles in the polar region of the Moon.
First Two-way Laser Ranging to a Lunar Orbiter
I was part of a team that successfully performed two-way laser ranging measurements from a ground station (the lunar laser ranging station in Grasse, France) to a spacecraft at lunar distance, the Lunar Reconnaissance Orbiter (LRO).
I performed some statistical analysis using two-way LLR data during and immediately surrounding the LRO ranging experimental periods to compare the return rate from retroreflectors on the LRO (pristine) vs. surface retroreflectors deposited by the Apollo astronauts (possibly dusty) to ascertain the degradation of the surface retroreflectors due to dust deposition.
Measuring the Size and Shape of the Moon's Fluid Core with Lunar Laser Ranging
The study of the rotation of a body gives access to key information about its interior. Using a set of numerically integrated equations, Earth-Moon distance information from lunar laser ranging (LLR) data, and the knowledge of Moon's gravity from the Gravity Recovery and Interior Laboratory mission, we simulated the rotation and motion of the Moon in the vicinity of Earth, Sun, and other planetary bodies with high accuracy. The expected relaxed shape of the Moon's core was compared with a best-fit adjustment of our simulation parameters to the observed LLR data.
This novel approach allowed us to improve the previous uncertainty in the radius and polar flattening of the Moon's core-mantle boundary (CMB), both by a factor of 3. Limits on the size of the lunar CMB provide significant constraints to important works such as the Earth-Moon formation (e.g., giant impact) hypotheses.
Development of High-accuracy Lunar Orbits and Fundamental Physics Tests
I brought down the INPOP lunar ephemerides to the 1 - 2 cm level in one-way LLR range residual, making it one of the primary references in the world that is capable of providing high-precision lunar ephemerides.
I developed the INPOP LLR data processing pipeline consisting of LLR data reduction, weighting scheme and parameter adjustment during my Ph.D. thesis which was funded by the French Ministry of Education and Research.
The research resulted in new open questions regarding the current state-of-the-art lunar ephemerides, providing best-fit estimates of numerically integrated lunar orbit and rotation along with extended body parameters for the Earth and the Moon like Love numbers, core oblateness, polar moment of inertia, etc. I was also able to perform a precision test of the universality of free-fall using the Earth and the Moon as test bodies in the gravitational field of the Sun.
Lunar Laser Ranging in Infrared at the Grasse Laser Station
During my Ph.D., the lunar laser ranging (LLR) station at Grasse, France was developing ranging capabilities in the infrared (IR: 1064 nm) wavelength; an effort led by Dr. Clément Courde of CNRS-Geoazur. This coincidence resulted in the opportunity to work on the raw full-rate LLR data, explore normal point algorithm improvements, understand historical non-uniformities in the data, identify systematic station-level biases and extract science from the new IR LLR data collected from the Grasse station.
One of the primary advantages of operating LLR in IR was the signal strength gained from the transparency of the atmosphere in this wavelength. This enabled us to address and improve the historical distribution of LLR data both spatially (retroreflector-wise) and temporally (across lunar phases).
Is there a six-year ocean tide? Well..
The El Nino-Southern Oscillation (ENSO) is one of the most important interannual modulation of Earth's climate and efforts to improve ENSO forecasting are most welcome. Recent work by Lin and Qian published in Scientific Reports claim a 6-year variability in the tidal forces acting on the ocean. Their claim is based on their use of raw lunar laser ranging (LLR) data. Note that the data is modulated by a variety of orbital, rotational, geophysical, and relativistic phenomena, which stands unaccounted for in their analysis of the LLR data. My colleagues and I show that significant errors would be incurred by using uncorrected, raw LLR data for analyzing the spectrum of tidal forcing from the Moon, including from well-known phenomena such as lunar libration and tides acting of the Moon, both of which contribute to a strong 6-year variation.
In the figure below, Earth-Moon distance spectra (in black) has no spectral peak between 5-7 years whereas Earth-Lunar Retroreflector distance spectra (in color) contain a peak at 6-year as reported by Lin and Qin. The latter has no direct relationship to tidal potential but is shown here to introduce the lunar orientation signal that would be present in uncorrected LLR data.
Digital Shape Models and Astrometry
Astrometric efforts using images with well-resolved bodies often employ limb-based methods for center-finding; this is proven to be robust for satellites with a uniform ellipsoidal shape. However, systematic biases are of concern for bodies that deviate considerably from tri-axial ellipsoids. To help understand these biases, I am using an image analysis tool developed at NASA Goddard to improve the astrometric reduction of irregular moons (e.g., Phoebe - a moon of Saturn).
(work in progress)