Later measurements showed that one target contained about 90 per cent silica.
According to NASA’s image and instrument record, measurements on sols 1,189 and 1,190, in May 2007, returned a composition of approximately 90 per cent silica.
Some rock surfaces exposed when the wheel broke them contained between about 63 and 72 per cent silica.
The comparison strengthened the possibility that at least some Home Plate material formed as silica sinter deposited by hot water.
It records water interacting with volcanic material under hydrothermal conditions, an environment that can support microbes and preserve traces of them on Earth.
Spirit’s right-front wheel stopped turning on 13 March 2006, more than two Earth years after the rover reached Mars. Engineers kept the vehicle moving by powering its other five wheels, driving mainly backwards and dragging the failed wheel through the ground.
That compromise cost speed and made slopes harder to cross. It also turned the immobile wheel into an unplanned excavation tool. In late March 2007, while Spirit was travelling beside the volcanic plateau called Home Plate in Gusev Crater, the wheel cut into unusually bright soil. Later measurements showed that one target contained about 90 per cent silica.
The exposure was accidental. Identifying it required several instruments, weeks of follow-up work and a geological argument that remains qualified: the silica records an ancient hydrothermal environment, possibly a hot spring or a volcanic steam vent, but it does not by itself determine which process produced the deposit.
The right-front wheel failed after two years
Spirit landed on 4 January 2004 for a mission planned to last 90 Martian days. By the time its right-front drive motor failed on sol 779, the rover had travelled more than 6.8 kilometres and operated roughly nine times beyond its original surface lifetime.
The fault appeared as an open electrical circuit in the wheel’s drive actuator. The wheel could still be steered, but its motor could no longer make it roll. Tests with a duplicate rover at NASA’s Jet Propulsion Laboratory led the operations team to a five-wheel technique that usually placed Spirit’s rear toward its destination.
Pulling the immobile wheel was generally easier than pushing it into the soil ahead. Spirit’s rear hazard-avoidance cameras allowed the rover to assess terrain in that direction, although every drive required careful planning. Soft ground, wheel slip and the need to reach sun-facing winter slopes all narrowed the available routes.
A JPL mission update from April 2006 reported that Spirit had already travelled about 80 metres on five powered wheels after the failure.
The rover no longer moved neatly, but it moved.
The wheel opened a bright trench beside Home Plate
Nearly a year later, Spirit was working in the Eastern Valley beside Home Plate, a layered formation interpreted as volcanic material. As the rover crossed the area, the locked wheel scraped paths through loose soil and crushed weak rocks.
One disturbed patch was informally named Gertrude Weise, after a player in the All-American Girls Professional Baseball League. The wheel exposed it in late March 2007. The newly opened track was about 20 centimetres wide and far brighter than the reddish surface around it.
Bright soil was not automatically unusual. Spirit had uncovered other pale deposits containing sulfur-rich salts. This patch differed because the rover’s Miniature Thermal Emission Spectrometer recorded a strong silica signature and little sulfur.
The team drove Spirit close enough to place the Alpha Particle X-ray Spectrometer over the soil. According to NASA’s image and instrument record, measurements on sols 1,189 and 1,190, in May 2007, returned a composition of approximately 90 per cent silica. In the later peer-reviewed analysis, the specific target called Kenosha Comets measured 90.1 per cent silicon dioxide by weight.
This was not ordinary Martian dust carrying a thin silica coating.
Several instruments identified hydrated opaline silica
Silica is silicon dioxide, the main chemical component of quartz and window glass. Spirit’s material was not detectable quartz. Its infrared spectrum was consistent with opaline silica, a non-crystalline form that can contain bound water.
In a 2008 paper in Science, Steve Squyres and colleagues reported opaline silica in both light-toned soil and bedrock around Home Plate. Some rock surfaces exposed when the wheel broke them contained between about 63 and 72 per cent silica. The authors treated those measurements as lower limits because Martian soil contaminated the instrument’s field of view.
For the richest loose-soil target, correcting for an estimated 30 per cent contamination implied an underlying composition close to 98 per cent silicon dioxide. The conclusion did not rest on a single reading. Thermal infrared spectra, elemental chemistry, microscopic images and iron-sensitive Mössbauer measurements all supported the identification.
The geological context mattered as much as the concentration. The deposits lay among volcanic rocks and near sulfate-rich materials that had also been altered by water.
Hot springs and steam vents produce silica differently
On Earth, concentrated opaline silica can form through more than one hydrothermal route. Hot neutral or alkaline groundwater dissolves silica from rock underground. When that water reaches the surface, cools and evaporates, silica can precipitate as a sinter deposit around springs or geysers.
A fumarole offers another route. Water vapour and acidic volcanic gases condense around a steam vent, producing low-pH fluid that attacks basalt. Many components are carried away while relatively insoluble silica, along with minerals containing titanium, remains concentrated.
The 2008 Science paper considered both mechanisms. Enrichment in titanium and chromium, together with nearby hydrated sulfates, led the authors to favour acid-sulfate alteration at low pH. They also wrote that the chemistry did not uniquely determine the fluid’s pH. If dissolved material travelled farther from its source before precipitation, a hot-spring sinter origin remained plausible.
“Hot springs or steam vents” therefore names two distinct chemical pathways, not two interchangeable descriptions of the same setting.
Later work strengthened the hot-spring interpretation
Spirit also photographed nodular and finger-like silica outcrops near Home Plate. In 2016, Steven Ruff and Jack Farmer compared those textures with deposits in active hot-spring and geyser channels at El Tatio in northern Chile.
Their open-access study in Nature Communications described similarities in the size and shape of the silica structures, along with infrared spectra affected by salt crusts. The comparison strengthened the possibility that at least some Home Plate material formed as silica sinter deposited by hot water.
The El Tatio structures include features shaped by both microbial and non-biological processes. Ruff and Farmer described the Martian forms as potential biosignatures, not evidence that Spirit had found fossils. Their paper explicitly left fully abiotic formation open.
Spirit did not carry the instruments needed to settle that question, and the rover could not preserve a sample for laboratory work on Earth.
A mechanical failure created scientific access
The dead wheel did not create the silica. It removed the surface cover that had kept some deposits out of sight and broke weak material to expose fresh faces. Once the pale track appeared, the science team recognised an unusual spectrum and changed Spirit’s work plan to examine it at close range.
The discovery was neither pure luck nor the result of an instrument following a predetermined target list. A mechanical failure created access. Repeated observation turned that access into evidence.
The silica does not establish that a lake once occupied Gusev Crater, and it does not demonstrate that anything lived there. It records water interacting with volcanic material under hydrothermal conditions, an environment that can support microbes and preserve traces of them on Earth.
Spirit continued operating until it stopped communicating in March 2010. Its failed wheel remained a mobility constraint, but the trench it cut beside Home Plate gave the rover one of its clearest records of ancient water.
