What’s at the bottom of the ocean?

By Leanne Gunn

It may seem crazy, but we know more about the surface of Mars than we do about the ground beneath our oceans. Yet beneath the sea is a whole underwater landscape with valleys, mountains and even volcanoes. 

So what is stopping us? Why haven’t we discovered more yet?

CC-BY-Amanda Lohr

Out of sight, out of mind

As a rule, we are more inquisitive about things we often see but do not fully understand, than we are about things we rarely see at all.

It is quite easy to forget that there is a world under our oceans, whereas the Moon shining brightly in the sky taunts us into wanting to find out more.

While this certainly plays a part, it is not the whole story…

The art of measuring topography

Scientists use a technique called radar to work out the Earth land topography. Radar works by sending beams of radio waves from a satellite down to the Earth’s surface and measuring the time taken for them to bounce back. By doing this repeatedly, in slightly different locations scientists can build a detailed picture of elevation.

The Atlantic Ocean at 20 Km resolution. CC-BY-NOAA

When it comes to the topography beneath the Earth’s oceans, things become a little trickier. Seawater blocks the radar waves, stopping them from ever reaching the sea floor at all. This gives scientists details information about the height of the surface of the water, but not the sea floor itself.

However, all is not lost.

The height of the sea surface does provide some clues to what is going on beneath it. For example, a large undersea mountain will cause the sea’s surface above it to bump slightly. In contrast,  the sea above an ocean trench will have a measurable dip.

By using this information scientists have been able to produce a complete map of the sea floor giving us a glimpse of the land beneath our oceans.

 

The devil is in the detail

Something doesn’t quite make sense. If we have a complete map of the sea floor, how can we still know more about Mars?

Well the answer lies in the detail.The resolution of the Earth’s ocean floor map is only about 5 km (5,000 m). This means that any features smaller than 5 km cannot be seen. And if we can’t see it, we do not know it is there. 5 Km is quite a large distance. If you think about how much topography you can cover on a 5 km walk in the countryside, it is clear that there is a lot of missing information in these maps.

Topographic map of Mars. CC-BY-NASA/JPL/USGS

Despite Mars being millions of kilometres away it is easier to measure its topography than it is to measure the topography of the sea floor on our own planet. As a result, a complete map of Mars exists at a resolution of 500 m! It is this level of detail that means we know more about Mars than we do the Earth’s ocean floor.

Is this the best we have?

We do have higher resolution images for specific places on the Earth’s ocean floor. These have been obtained by sending boats out with sonar measuring equipment to directly measure the elevation of the seafloor. This is time consuming and expensive work and is often only conducted to fulfil a specific need.

One of these needs was the search for the missing Boeing 777 aircraft operating Malaysia airlines flight MH370 in 2014. The maps generated from this search are now available to the public and can be found here.

In our world of satellite images, internet access and smartphones and it is easy to think that we have explored every corner of the Earth. But more than 70 % of the Earth is covered in ocean. That is more than 70 % of an underwater world waiting to be discovered.

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Posted in Geology