A New Moon: Chang'E-3's APXS and the Discovery of Young Lunar Volcanoism

How China's Chang'E-3 mission revealed evidence of young lunar volcanism, rewriting the Moon's geological history

December 14, 2013

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Introduction
The Scientist's Toolkit
The Mission
Key Experiment
Implications

Introduction: A Groundbreaking Lunar Touchdown

On December 14, 2013, China's Chang'E-3 spacecraft achieved the first soft landing on the Moon since 1976. Its landing site in northern Mare Imbrium was no accident; it was chosen for its potential to rewrite lunar history^⁴. The Yutu rover, equipped with a sophisticated scientific instrument called an Active Particle-induced X-ray Spectrometer (APXS), would soon provide the first in-situ evidence for a type of young, previously unknown lunar basalt^⁴.

Mission Firsts

First soft landing on the Moon since 1976
First in-situ evidence of young lunar basalt
First APXS measurements on the lunar surface

Landing Site

Mare Imbrium

Near Zi Wei crater (450m diameter)

Northern region of the Moon

The Scientist's Toolkit: Key Technology on the Yutu Rover

To understand the discoveries of the CE-3 mission, one must first be familiar with the primary instrument that made them possible. The APXS was the sole payload mounted on Yutu's robotic arm, designed to determine the chemical composition of the lunar soil and rocks^⁴.

How the APXS Works

The APXS is a spectrometer that excels in space missions due to its low weight, small size, and minimal power consumption^¹. Its operating principle is elegant in its simplicity:

Irradiation

The instrument uses a radioactive source, often Curium-244, to irradiate a sample with alpha particles and X-rays^¹.

Interaction

Particles interact with atoms through backscattering and X-ray fluorescence^¹.

Detection & Analysis

A high-resolution X-ray detector collects data for element identification^¹^².

Key Components of the Yutu Rover's APXS

Component	Function	Key Feature
Radioactive Source (Curium-244)	Emits alpha particles and X-rays to excite the sample material^¹.	Provides irradiation without external power.
X-ray Detector	Measures the energy of characteristic X-rays emitted from the sample^².	High resolution for accurate element identification^².
Robotic Arm	Positions the APXS sensor head directly against the target sample^².	Ensures consistent, close-contact measurement.

The Mission and the Landing Site: A Young and Intriguing Landscape

The Chang'E-3 mission landed in the northern part of the vast Mare Imbrium, several tens of meters from the rim of a young, 450-meter diameter crater named Zi Wei^⁴. This location was strategically selected. The impact that created Zi Wei crater excavated and scattered material from beneath the surface, giving Yutu's APXS access to a fresh cross-section of lunar geology without the need for drilling.

The lunar surface showing impact craters similar to the Zi Wei crater near the Chang'E-3 landing site.

Crucially, the landing site sat on what orbital data suggested were "young" Eratosthenian-era lava flows^⁴. Most lunar samples collected by Apollo and Luna missions dated from 3.1 to 3.8 billion years ago. In contrast, the basalts at the CE-3 site were estimated through crater counting to be only around 2.35 billion years old^⁴. If confirmed, this would mean the Moon experienced volcanic activity for at least a billion years longer than previously thought.

Lunar Timeline

Apollo/Luna Samples

3.1-3.8 billion years

CE-3 Basalts

~2.35 billion years

In-Depth Look at a Key Experiment: Analyzing the Lunar Soil

The Yutu rover traversed the area around the Zi Wei crater, using its APXS to perform in-situ measurements of the lunar soil at multiple points^⁷. The process for each measurement was methodical and precise.

Methodology: A Step-by-Step Measurement

Positioning: Using images from its navigation and hazard avoidance cameras, the rover's operators on Earth calculated the precise positioning for its robotic arm^².
Docking: The rover carefully positioned the APXS sensor head so that its contact ring was docked directly against the surface of the selected sample^².

Irradiation and Data Collection: The APXS irradiated the target for a period of several hours^².
Data Transmission: The collected spectral data was transmitted back to Earth for analysis by scientific teams.

Artist's representation of a rover conducting scientific measurements on the lunar surface.

Results and Analysis: The Geochemical Data

The data from the APXS revealed the concentrations of major elements that make up the lunar soil. The results were striking. When scientists plotted the geochemical data, it occupied a distinct area in compositional plots compared to samples from Apollo, Luna, and even the later Chang'E-5 mission^⁴. This was a clear indication that the soil at the CE-3 landing site was a new type of basalt, not previously sampled.

Major Element Composition from CE-3 Landing Site

Element	Concentration (wt%)	Scientific Implication
TiO₂	~5-10% (Low-Intermediate Titanium)^⁴	Indicates a specific type of mantle source material and crystallization history.
FeO	~18-20%^⁴	Suggests a volcanic origin; supports the young basalt hypothesis.
Total (TiO₂ + FeO)	~23-30%^⁷	High content confirmed by both APXS and independent radar data, explaining the distinct nature of the basalts.

The findings from the APXS were corroborated by another instrument on Yutu, the Lunar Penetrating Radar (LPR). The LPR estimated the loss tangent of the subsurface material, a property directly influenced by the TiO₂ + FeO content. The LPR-derived TiO₂ + FeO content of 23-30 wt% aligned perfectly with the APXS measurements, providing strong, multi-instrument validation of the results^⁷.

Validation

APXS data confirmed by Lunar Penetrating Radar

Multi-instrument validation strengthens findings

Elemental Composition Comparison

Implications and Conclusions: Redrawing the Moon's Volcanic Timeline

The in-situ detection data from the Chang'E-3 APXS has had profound implications for our understanding of the Moon.

Young Volcanism

The unique geochemical signature of the basalts confirmed that the CE-3 landing site is indeed composed of a younger type of mare basalt, validating the estimates from crater counting and extending the known period of lunar volcanic activity by a billion years^⁴.

New Geochemical Province

The discovery of a new type of basalt indicates that the Moon's interior is more geochemically diverse than the existing sample collection suggested. The CE-3 basalts show similar properties to regions in the western Procellarum and Imbrium (WPI), pointing to a distinct volcanic history for this area^⁴.

Value of In-Situ Measurement

The mission demonstrated the critical importance of in-situ measurements. Orbital data provided hints, but the high-resolution, ground-truth data from the APXS was essential for making a definitive identification and advancing our knowledge of lunar geology^⁴^⁷.

Evolution of APXS Instrumentation in Planetary Exploration

Mission	Instrument	Key Achievement
Surveyor 5-7	APS	First use of alpha scattering on the Moon^¹.
Mars Pathfinder	APXS	Included a proton detector (alpha proton X-ray spectrometer)^¹.
Mars Exploration Rovers (Spirit & Opportunity)	APXS	Used Curium-244 sources; advanced detector technology^¹^².
Chang'E-3 (Yutu Rover)	APXS	Provided first in-situ evidence for a new type of young lunar basalt^⁴.
Philae (Rosetta)	APXS	Studied the composition of a comet, 67P/Churyumov–Gerasimenko^¹.

In conclusion, the Alpha Particle X-ray Spectrometer on China's Chang'E-3 mission was far more than a technical marvel; it was a key that unlocked a new chapter in lunar science. By reading the elemental story told by the rocks at its landing site, it provided compelling evidence that the Moon's volcanic fires burned for longer than we ever knew, reminding us that our closest celestial neighbor still holds many secrets waiting to be discovered.