Look up on a clear night and you can see some circular formations on the face of our lunar neighbour. These are impact craters, circular depressions found on planetary surfaces.
About a century ago, they were suspected to exist on Earth but the cosmic origin was often met with suspicion and most geologists believed that craters were of volcanic origin.
Around 1960, the American astrogeologist Gene Shoemaker, one of the founders of planetary science, studied the dynamics of crater formation on Earth and planetary surfaces. He investigated why they – including our Moon – are so cratered.
Images from Apollo
By 1970, there were more than 50 craters discovered on Earth but that work was still considered controversial, until pictures of the lunar surface brought by the Apollo missions confirmed that impact cratering is a common geological process outside Earth.
Unlike Earth’s surface, the lunar surface is covered with craters. This is because Earth is a dynamic planet, and tectonics, volcanism, seismicity, wind and oceans all play against the preservation of impact craters on Earth.
In contrast to Earth, our Moon has been inactive over long geological timescales and has no atmosphere, which has allowed the persistent impact cratering to remain over eons. The lunar cratering record spans its entire bombardment history – from the Moon’s very origins to today.
The big ones
The largest and oldest impact crater in the Solar system is believed to be on the Moon, and it is called the South Pole-Aitken basin, but we cannot see it from Earth because it is on the far side of the Moon. The Moon is tidally locked to Earth’s rotation and the same side always faces toward us.
But this crater, more than 2,000km across, is thought to predate any other large impact bombardment that occurred during lunar evolution. Impact simulations suggested it was formed by a 150-250km asteroid hurtling into the Moon at 15-20km per second!
From Earth, the human eye can observe areas of different shades of grey on the surface of the Moon facing us. The dark areas are called maria, and can be up to more than 1,000km across.
They are volcanic deposits that flooded depressions created by the formation of the large impact basins on the Moon. These volcanic eruptions were active for millions of years after these impacts occurred.
No other large impact event has occurred on the Moon since then. This is a good sign, because it implies there were no very large impacts occurring on Earth either after this time in evolutionary history. (The asteroid that wiped out the dinosaurs on Earth 66 million years ago was only about 10-15km in size and left a crater larger than 150km in size, which was substantial enough to cause a mass extinction.)
As seen from Earth
They are called complex craters because they are not entirely bowl-shaped, but are a bit shallower and include a peak in the centre of the crater as a consequence of the material collapsing into the hole made during impact. Tycho and Copernicus are both 80-100km across but have spectacular central peaks and prominent “ejecta rays” – areas where material was ejected across the lunar surface after an impact.
The formation of these craters excavated underlying material that was brighter than the actual surface. This is because lunar surface is subjected to space weathering, which causes surface rocks to darken.
Still a target for impacts
The Apollo 12, 14, 15, and 16 missions placed several seismic stations on the Moon between 1969 and 1972, creating the first extraterrestrial seismic network (ALSEP). During one year of operations, more than 1,000 seismic events were recorded, of which 10% were associated with meteoroids impacts.
So the Moon is still being hit by objects, albeit mostly tiny ones. But as there is no atmosphere on the Moon, there is no gas to help burn up these rocks from space and stop them smashing into the Moon.
The seismic network was functional until it was switched off in 1977, in preparation for new space missions. No one expected that the next fully operational extraterrestrial seismometer would not be placed on a planetary surface (Mars) until 40 years later.
Nowadays, from Earth, using a small telescope (and armed with a little patience), you can see so-called “impact flashes”, which are small meteorite impacts on the lunar surface that is facing us.
Thanks to the atmosphere on Earth, similar-sized rocks from space cannot make an impact here because they tend to predominantly burn up, but on the Moon they crash into the soil and release its kinetic energy of the impact via bright thermal emission.
Clownfish achieved worldwide fame following Finding Nemo, but it turns out these fish don’t do so well in the spotlight.
Our research, published in Biology Letters, found when clownfish eggs were exposed to low levels of light at night – as they would be if laid near a coastal town – not a single egg hatched.
This finding adds to the growing body of research on the health affects of light pollution, a rapidly spreading ecological problem.
What is light pollution?
Light pollution occurs when artificial light interferes with ecological systems or processes, usually at night.
Natural light at night, produced by the moon, stars, and other celestial bodies, is minimal. A full moon creates only 0.05-0.1 lux, which pales in comparison to the artificial light produced by humans, which can range from around 10 lux from an LED or low-pressure sodium streetlight, up to 2,000 lux from something like stadium lighting.
Because nearly all organisms on Earth have evolved with a stable day-night, light and dark cycle, many biological events are now highly attuned to the daily, lunar, and seasonal changes in light produced by the reliable movements of the Earth and Moon around the Sun.
But artificial light can mask these natural light rhythms and interfere with the behaviour and physiology of individual creatures, and ecosystems as a whole.
The ocean is not exempt from these problems. Light pollution is spreading to marine habitats through urbanised coastlines and increasing marine infrastructure such as piers, harbours, cruise ships, and tropical island resorts where bungalows extend out into the lagoon, directly above coral reefs.
Why are clownfish at risk?
Clownfish, like many reef fish, are particularly vulnerable to light pollution because they don’t move around much in their adult stage. Clownfish can travel long distances in the first 2 weeks after hatching, but at the end of this period the young fish will settle in a suitable sea anemone that becomes their forever-home.
This means that if a fish chooses an anemone on a shallow reef in an area that is heavily lit at night, they will experience chronic exposure to light pollution throughout their life; they won’t just move away.
Clownfish also lay their eggs attached to rock or other hard surfaces, so in areas exposed to light pollution the eggs will experience continuous artificial light (as opposed to many fish that lay and fertilise eggs in open water, so they are immediately carried away by ocean currents).
What we found
To test how artificial light affects clownfish reproduction, we examined the common clownfish (Amphiprion ocellaris) in a lab experiment.
Five breeding pairs of fish experienced a normal 12-hour daylight, 12-hour dark cycle, while another five pairs of fish had their “night” period replaced with 12 hours of light at 26.5 lux, mimicking light pollution from an average coastal town.
For 60 days, we monitored how often the fish spawned, how many eggs were fertilised, and how many eggs hatched. While we saw no difference in spawning frequency or fertilisation rates between the two groups of fish, the impact of the artificial light treatment on hatch rate was staggering. None of the eggs hatched, compared with an average of 86% in the control group.
At the end of the experiment we removed the artificial light and monitored the fish for another 60 days to see how they would recover. As soon as the light at night was removed, eggs resumed hatching at normal rates.
Clownfish, like many reef fish, have evolved to hatch after dusk to avoid the threat of being eaten. Newly hatched baby clownfish, like most coral reef fish, are small (about 5mm long) and transparent. Hatching in darkness likely means they are less visible to predators as they emerge from their eggs.
Our findings show that the presence of artificial light, even at relatively low levels, can disrupt this crucial process, by masking the environmental cue – darkness – that triggers hatching. As many reef fish share similar reproductive behaviours to clownfish, it is likely artificial light will similarly interfere with the ability of other fish species to produce viable offspring.
The larger problem
And the problem is only growing. The reach of light pollution across all land and sea is expanding at an estimated rate of 2.2% per year, and this will only increase with the rising global human population.
Although research on the ecological impacts of light pollution is arguably only in its infancy, the evidence for negative consequences for a range of insects, birds, amphibians, reptiles, and mammals, including humans, is stacking up.
Our new research adds another species to the list, and highlights the importance of finding ways to manage or reduce artificial light, on land and below the waves.