Think of the Moon and most people will imagine a barren world pockmarked with craters. The same is likely true of Mars albeit more red in colour than grey! The Earth too has had its fair share of craters, some of them large but most of the evidence has been eroded by centuries of weathering. Surprisingly perhaps, Venus, the second planet from the Sun does not have the same weathering processes as we have on Earth yet there are signs of impact craters, but no large impact basins! A team of astronomers now think they have secured a new view on the hottest planet in the Solar System and revealed the missing impact sites.
Venus is the second planet from the Sun and, whilst it’s often called Earth’s sister planet, the reality is really they differ in many ways. The term comes from similarities in size and composition yet the conditions on Venus are far more hostile. Surface temperatures far exceed the boiling point of water, the dense atmosphere exerts a pressure on the surface equivalent to being 3,000 feet under water and there is sulphuric acid rain in the atmosphere! Most definitely not a nice place to head to for your next vacation.
If you were to stand on the surface of Venus you would see beautifully formed craters. Looking down on the planet from orbit you would see none due to the thick, dense atmosphere. Yet if you could gaze through the obscuring clouds you would see a distinct lack of larger impact basins of the sort we are familiar with on the Moon. Now, a team of researchers mostly from the Planetary Science Institute believe they solve the mystery of the missing craters.
They have mapped a region of Venus known as Haastte-baad Tessera using radar technology and the results were rather surprising. The region is thought to be one of the oldest surfaces on Venus and is classed as tessera terrain. This type of feature is complex and is characterised by rough, intersecting ridges to create a tile like pattern thought to be the result of a thin but strong layer of material forming over a weak layer which can flow and convect energy just like boiling water. Images from the area in question reveal a set of concentric rings over 1,400 km across at their widest. The team propose that the feature is the result of two back-to-back impact events. “Think of pea soup with a scum forming on top,” said Vicki Hansen, Planetary Science Institute Senior Scientist.
Obviously there is no pea soup on Venus but instead, the thin crust layer formed upon a layer of molten lava. Venus of today has a thick outer shell called a lithosphere which is about 112 km thick but when Venus was younger, its thought it was just 9km thick! If an impactor struck the hot young Venus then it’s very likely it would have fractured the lithosphere allowing molten lava to seep through and eventually solidify to create the tesserae we see today.
Confusing things slightly however is that features like this have been seen on top of flat, raised plateaus where the lithosphere is likely much thicker. The researchers have an answer for this though, “When you have vast amounts of partial melt in the mantle that rushes to the surface, what gets left behind is something called residuum. Solid residuum is much stronger than the adjacent mantle, which did not experience partial melting.” said Hansen. “What may be surprising is that the solid residuum is also lower density than all the mantle around it. So, it’s stronger, but it’s also buoyant. You basically have an air mattress sitting in the mantle beneath your lava pond, and it’s just going to rise up and raise that tessera terrain.”
The features found by the time seem to show that two impact events happened one after the other with the first creating the build up of lava and the second creating the ring structure seen today.
Source : Impact craters were hiding in plain sight, say researchers with a new view of Venus