Venus Express evidence for recent hot-spot volcanism on Venus
Posted by Emily Lakdawalla
09-04-2010 16:51 CDT
Venus? What? Somebody still studies that planet? Yes, and in fact there's an active spacecraft there: Venus Express, the poor little sister to Mars Express (itself the poor little brother to Rosetta). Venus Express probably gets the least respect of any ESA planetary mission, but if today's announcement stands the test of time, Venus Express will get credit for a major shift in our understanding of Venus' geology.
Today's announcement concerns a paper published by Sue Smrekar and seven coauthors in Science: "Recent Hot-Spot Volcanism on Venus from VIRTIS Emissivity Data." Smrekar et al. outline evidence suggesting that lava flows around several large Venusian volcanoes are fresh and unweathered, implying that they are no older than 2.5 million years, and possibly are younger than 250,000 years old. That is young enough to suggest strongly that Venus is active right now.
The data that they are using come from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument on Venus Express. An imaging spectrometer trades off spatial resolution for spectral resolution, like VIMS on Cassini. Like VIMS on Cassini, VIRTIS can use small spectral "windows" to peer through an otherwise opaque atmosphere to discern features on the surface. VIMS on Cassini uses windows where methane is relatively transparent, especially around 2 microns, to see Titan's surface; VIRTIS on Venus Express uses a window at 1.02 microns in which Venus' atmosphere is somewhat transparent to see through to Venus' surface.
However, there is a key difference between what VIMS sees on Titan and what VIRTIS sees on Venus. Titan's a cold place, so VIMS only sees the brightness of Titan's surface in reflected light. Venus is hot, really hot; it has the hottest surface of all of the terrestrial planets. The hotter a surface is, the shorter the wavelength of the radiation that it emits. In that 1.02 micron wavelength, Venus's surface is blazing away with thermal radiation. This is very useful, because it means VIRTIS doesn't need sunlight to see Venus' surface.
So VIRTIS sees the thermal brightness of Venus' surface. Variations in how much radiation the surface emits -- a quality known as emissivity -- can be very informative about what's going on on the surface. But first you have to correct for various things that can affect emissivity. As on Earth, there is a strong inverse correlation between temperature and altitude (that is, lower elevations are warmer); in order to use the VIRTIS data to understand variations in the surface, Smrekar and her coauthors first had to correct for topography. They also had to perform other corrections for stray light within the instrument, viewing geometry, and cloud opacity, but if I understand it correctly, topography is the biggest contributor to the observed variations.
Let's assume they did their topographic and other corrections well. What does the emissivity map look like? They didn't release the map, so I'll have to describe it. The map they produced has lots of smallish high-emissivity spots on low-emissivity background. The positions of the spots are correlated with the presence of circular tectonic features unique to Venus called coronae, and also with large volcanic edifices that were previously mapped as being similar to Earth's hot spot volcanoes like Hawaii. The Venusian "hot spots" don't just look like Hawaii in that they are broad topographic rises covered by tall volcanoes with lava flows; there are other ways in which they are similar. Specifically, they're surrounded by topographic "moats" (which result from the weight of the volcanic deposits pressing down on crust that's been thinned by heating from below), and they appear similar to Earth hot spots in their gravity signatures as well.
These Venusian hot spots, including Imdr, Themis, and Dione Regiones, have huge lava flows that were already mapped in Magellan images as being some of the youngest features around. But how young? Were they the last gasp of a global overturn that ended 500 million years ago, or did it take longer for them to form? That's what Smrekar's study seeks to answer.
The region with the highest emissivity is on the mountain Idunn Mons in Imdr Regio in Venus' southern hemisphere. What does high emissivity mean? Smrekar and her coauthors explain that weathering of typical volcanic rocks under the high temperature and pressure (460°C and 90 bars) of Venusian surface conditions decreases the emissivity of those rocks. This may happen when carbon dioxide or sulfur dioxide from the atmosphere reacts with the surface of the rock to form a crust of secondary minerals like calcite (CaCO3)or anhydrite (which is basically dehydrated gypsum, CaSO4); iron oxide, that is, hematite, may also form. Smrekar argued that while all three of these minerals have low emissivities, they're also transparent to radiation, so you'd need a crust several micrometers thick to lower the emissivity of the rocks they encrusted.
Thus if the emissivity is high, that would mean we're looking at relatively unweathered materials. Which is interesting, to be sure, but we still don't know how fast the weathering occurs. Smrekar and her coauthors argue that it operates very rapidly, on time scales of days to years. Even applying the most conservative assumptions about how fast the stuff is weathering, they arrived at a maximum possible age of the flows at Idunn to be 2.5 million years old. Given more likely assumptions, though, they estimate the flows' ages to be somewhere in the neighborhood of hundreds to tens of thousands of years.
Flight around Idunn Mons, Venus, using Magellan topography and image information, overlain with emissivity data from Venus Express.
Credit: NASA / JPL / ESA
Whether its hundreds, thousands, or tens of thousands of years, it might as well be yesterday. For context, Hawaii's oldest subaerial volcano, Kohala, started erupting about 500,000 years ago. That's a currently active hot spot. The conclusion to be drawn from the present paper is that Imdr Regio is a currently active hot spot, producing recent flows at Idunn Mons. There are other Venusian volcanoes that may be currently active as well, including Innini and Hathor Montes in Dione Regio, and several volcanoes including Mielikki Mons within Themis Regio.
This is really exciting. It is not quite as convincing as it would be if we actually somehow saw something spewing actively out of the ground the way we have on Io and Enceladus and Earth. But it is still really convincing. Before Venus Express, I would have said that while I had no particular reason to doubt Venus was presently geologically active, I hadn't seen anything that convinced me that it was. Now you can color me much more convinced.
Here's a little more background in case you're interested in the ongoing debate about Venus' volcanic activity. Our best maps of the surface of Venus come from the Magellan mission, which operated a Synthetic Aperture Radar (SAR) instrument in orbit there from 1989 to 1994. It covered 97% of the planet with high-resolution radar images and lower-resolution altimetry and 95% of the planet with gravity data, a wonderfully complete data set. One of the most active areas of research in the years following Magellan was global mapping -- identifying the sorts of features that exist on the planet and how they are distributed across the surface.
One of the things that had been obvious since the first Russian radar maps of Venus was the fact that the surface lacked many craters. Venus has about 900 craters in total. This is more than have been identified on Earth -- currently, 176 -- but it is far fewer than are visible on Mercury, the Moon, or Mars.
Surely more than 900 objects have hit Venus over its history. The low number of impact craters suggests that Venus has been resurfaced, meaning that its ancient surface has been modified by geologic processes to refresh its appearance. The ancient surface could have been buried (covering the craters) or eroded (wiping them away), or it could have been folded, faulted, broken up enough to make them invisible, we don't know; but whichever one of these it is, it implies active geology on Venus.
But how long ago was that geologic activity? Planetary scientists figure out the ages of surfaces in the solar system by counting and measuring the craters that cover a landform, plotting a "size-frequency distribution" (I'll let you Google that to learn more, if you're interested), and reading from that plot how many years old the landform must be to have accumulated that number of craters over the measured area. This works great on Mars, Mercury, and the Moon for establishing the relative ages of various geologic provinces, but, unfortunately, it doesn't work too well on Venus because although 900 seems like a big number, it's really too small a number for good statistics.
All scientists have managed to do is to say that, on average, the age of Venus' surface is 500 million years. (Interestingly enough, this is not a whole lot different from the average age of Earth's surface.) But the number of craters on Venus is just too small for scientists to figure out whether there are some places on Venus that are much younger than other places on Venus. The distribution of craters appears to be random, but the number of craters is small enough that it would be hard to tell a random distribution from one that was slightly patchy due to the age of the surface varying from place to place.
One thing that is a little bit strange is that almost none of the Venusian craters have been modified since they formed. Look at large craters on the Moon and Mars, and you'll see that, over time, craters degrade; their rims are worn away, the patterns of ejecta fade, they have been filled in by lava flows or (in the case of Mars) by sedimentary deposits of one kind or another. On Venus, almost all of the craters appear to be as fresh as they were the day they formed.
The small number of craters and their lack of modification has suggested to some scientists that although Venus has an Earth-like average surface age, its surface is not experiencing the continual modification that Earth's surface is. Instead, they've proposed that the entire surface of Venus was suddenly destroyed in a global paroxysm of geologic activity about 500 million years ago, and has not been modified much since.
For evidence, they point to the fact that Venus does not show any of the landforms associated with plate tectonics on Earth. There isn't continental crust in some places and oceanic crust in others. Volcanoes is not concentrated in arcs as it is on Earth; on Venus, there are volcanoes of different sizes and shapes everywhere you look. Earth gets rid of internal heat by continuously cycling cool surface rocks into the planet's interior and erupting hot rocks. If Venus doesn't have plate tectonics performing this heat cycling, but instead has a "stagnant lid" of solid rock that prevents hot material from erupting, heat could build up inside the planet until it could suddenly overcome the resistance and the whole "lid" could founder, producing global, catastrophic volcanism.
It's funny how debates in geology always seem to come down to a fight between catastrophists and uniformitarians. It's no different on Venus. There's one camp of people who say that all of Venus suffered a global catastrophe at the same time, leading to different styles of volcanism and tectonism that evolved fairly quickly over time from one style of geologic activity to another, ending a few hundred million years ago. And there's another camp of people who argue that this point of view is ludicrous, that Venusian geology is, like Earth's, local, with different processes operating in different places at different times, and that the whole planet has been geologically active, continuously renewing the crust, leading to an average surface age of a few hundred million years but an actual age that varies from place to place.
That's why people are so interested in looking for active volcanism on Venus. If someone discovered very recent or active volcanism on Venus, it would be a major win for the uniformitarian camp (not to mention the fact that it would be really cool to add Venus to the list of planets we know to be geologically active today). So that's the context for the paper I discussed above.