Hot stuff on Venus! Venus Express sees right down to the hell-hot surface

The results, presented today at the American Geophysical Union (AGU) assembly in San Francisco, USA, were obtained thanks to VIRTIS, the Visible and Infrared Thermal Imaging Spectrometer on board Venus Express.
To obtain this fundamental information about the surface temperature, VIRTIS made use of the so-called infrared spectral 'windows' present in the Venusian atmosphere. Through these 'windows' thermal radiation at specific wavelengths can leak from the deepest atmospheric layers, pass through the dense cloud curtain situated at about 60 kilometres altitude, and then escape to space, where it can be detected by instruments like VIRTIS. In this way VIRTIS succeeded in looking through the thick carbon dioxide curtain surrounding Venus and detected the heat directly emitted by the hot rocks on the ground.

"We are very excited about these results, as they represent a very important item in the list of Venus Express' and VIRTIS' scientific objectives at Venus", says Giuseppe Piccioni, one of the Principal Investigators of the VIRTIS experiment, from the Istituto di Astrofisica Spaziale e Fisica Cosmica in Rome, Italy.

The measurements, made in August 2006 over the Themis and Phoebe Regions in the southern hemisphere of Venus, reveal temperature variations of 30 degrees between lowlands and mountain tops, correlating well with existing topographical radar data from previous missions. The Themis Region is a highland plateau located at 270° East longitude and at about 37° South latitude. It is a region that has experienced strong volcanic activity, at least in the geologic past.

On Venus there are no day and night variations of the surface temperature. The heat is globally 'trapped' under the carbon-dioxide atmosphere, with pressure 90 times higher than on Earth. Instead, the main temperature variation is due to topography. Just like on Earth, mountain tops are colder, whereas the lowlands are warmer. The 'only' difference is that on Venus 'cold' means 447° Celsius, while 'warm' means 477° Celsius. Such high temperatures are caused by the strongest greenhouse effect found in the Solar System.

"The VIRTIS results represent a major step forward in our attempt to identify specific surface features on the surface of Venus", said Jörn Helbert from the German Aerospace Center's (DLR) Institute of Planetary Research in Berlin, Germany, and a member of the VIRTIS team. "By 'peeling' off the atmospheric layers from the VIRTIS data, we can finally measure the surface temperature," Helbert added.

Image Credit: ESA/VIRTIS-VenusX Team
High resolution image

The temperature maps of the Venusian surface shown in this image were built thanks to direct measurements obtained by Venus Express' VIRTIS instruments (left), compared with surface temperature predictions based on the Magellan topographic data obtained in the early 1990s (right).

VIRTIS, the Visible and Infrared Thermal Imaging Spectrometer on board ESA's Venus Express, gathered the data combined into this mosaic on 10 August 2006, during a single orbit (orbit 112). The spacecraft was flying over the Themis and Phoebe Regiones in the southern hemisphere of Venus. Themis Regio is a highland plateau located on the 270° East meridian and at about 37° South latitude. In the future, combining data from many orbits will significantly improve the quality and the spatial resolution of the surface images.

This is a region that has experienced strong volcanic activity in the past, and possibly still does today. A series of interconnecting large coronae is running through the highland. Coronae are circular to elongate features, which are surrounded by multiple concentric ridges. These features are thought to be forming by hot spots in the Venusian crust. The coronae in Themis Regio are building a chain of volcanoes and faults called Parga Chasma, which runs from northwest to southeast, and eventually connect Parga with Atla Regio (not covered in the inage). The Phoebe Regio is a highland region of Venus (such regions are called 'tesserae'), where most of the Soviet Venera probes landed.

Large highland regions are clearly visible, as well as single volcanic structures such the Mielikki Mons, in the centre of the mosaic. On the south-eastern part of the mosaic VIRTIS covered a region for which Magellan maps are not available.

The surface-temperature measurements were performed using the atmospheric windows located in the near infrared at 1.02, 1.10, and 1.18 microns, respectively. The radiation coming from the surface is affected by the thick cloud layer, so a 'de-clouding' algorithm had to be applied.




Image Credit: NASA/Magellan
High resolution image (2.4 MB)

On this radar-map of Venus, built with data that NASA's Magellan gathered in the early 1990s, the areas that were over-flown by Venus Express on 10 August 2006 are indicated by the named features. The VIRTIS instrument on board retrieved unprecedented information about the surface temperature, and so provided the first ever large-scale temperature map of the Southern hemisphere of Venus.

Eventually, the VIRTIS team hopes to identify 'hot spots' on the surface of Venus, possibly stemming from active volcanoes. In the Solar System, besides Earth, active volcanoes have been observed only on Io, a satellite of Jupiter, on Neptune's satellite Triton, and on Saturn's moon Enceladus (in the form of the so-called 'cryo-volcanism'). Venus is the most likely planet to host other active volcanoes.

In order to achieve this, the Venus Express scientists started comparing the maps of the Venusian topography obtained by NASA's Magellan orbiter in the early 1990s with the data gathered by VIRTIS. The Magellan topography maps allow for a rough prediction of the surface temperature, too. Comparing these predictions with the measurements made by VIRTIS allows searching for hot spots that show even higher temperatures than the oven-hot surface, possibly indicative of active volcanism.

This direct interdependence between temperature and topography will enable scientists to derive new topography maps of the Venusian surface from temperature measurements. This will help in complementing the Magellan maps.

"Actually, when comparing our temperature map with topographical data from Magellan, we are not only obtaining quite a good agreement, but we can even fill gaps that the Magellan and Venera 15 radar data sets left open", concluded Pierre Drossart, the other Principal Investigator of the VIRTIS experiment, from the Observatoire de Paris Meudon, France.

European Space Agency