Tuesday, June 26, 2012

New Worlds from Existing Data

Kepler is a planet-hunting telescope that searches for planets by precisely measuring the dip in a star’s brightness when a planet happens to pass in front of its host star in an event known as a transit. The primary goal of Kepler is to determine the frequency of Earth-like planets around Sun-like stars. To date, over 2000 planet candidates have been found by Kepler with a trend towards an increasing number of Earth-size planet candidates at larger distances from their host stars as such planet candidates are harder and take longer to sniff out. An independent project to reanalyse the existing public Kepler dataset revealed 84 new transit signals on 64 star systems. Each transit signal represents a planet candidate and I shall describe some of the more notable ones.

Image Credit: NASA/JPL-Caltech

KOI 435
This system was known to contain two transit signatures with the first having a period of 20.55 days and the other yet to be determined. The four new transit signatures found in this reanalysis have periods of 3.93 days, 33.04 days, 66.30 days and 9.92 days. Previously, the only know star with six transiting planets was the Kepler-11 system.

KOI 1574
A transiting planet candidate with a period of 114.74 days is known to exist around KOI 1574. In this reanalysis, a second signal with a period of 191.51 days was detected. The outer planet candidate seems to be in a 3:5 orbital resonance with the inner one. What is more interesting is that the outer planet candidate is about twice the Earth’s diameter and its equilibrium temperature is estimated to be 281 degrees Kelvin. Given that the average surface temperature of the Earth is 287 degrees Kelvin, the outer planet candidate of KOI 1574 is a promising Earth-like planet.

KOI 277 (Kepler-36)
Previously known to have a single transiting planet candidate with a period of 13.84 days, this reanalysis found a second transit signal with a period of 16.24 days. The two planet candidates around KOI 277 are remarkable in the sense that their orbits bring them within 5 Earth-Moon distances from one another. On the last day of work on the paper detailing this reanalysis of the Kepler dataset, the Kepler team announced their discovery of the two planets around KOI 277 and named the system Kepler-36. The inner planet (Kepler-36b) is a super-Earth and the outer planet (Kepler-36c) is a mini-Neptune. These planets are twenty times more closely spaced than any adjacent pair of planets in the Solar System. At closest approach, an observer on Kepler-36b will see Kepler-36c appear 2.5 times larger than the Moon as seen from Earth.

KOI 1843
With two planet candidates already known with periods of 4.19 days and 6.36 days, a third transit signal with an incredibly short period of 0.176 days (4.25 hours) was revealed in this reanalysis. For each day on Earth, 5.65 years would have elapsed on this planet candidate. KOI 1843 is a cool and small star with an effective temperature of 3673 degrees Kelvin and 52 percent the Sun’s diameter. The third transit signal has a very small transit depth of 120 parts-per-million which means that this planet candidate is just 68 percent the diameter of the Earth, making it one of the smallest exoplanet candidates known to date.

Reference: Aviv Ofir and Stefan Dreizler (2012), “An Independent Planet Search in the Kepler Dataset. I. A hundred new candidates and revised KOIs”, arXiv:1206.5347v1 [astro-ph.EP]

Thursday, June 14, 2012

Venusian Snow

Although Venus has a similar size, mass, gravity and bulk composition as the Earth, the conditions on its surface are unlike anything on Earth. Venus is characterized by a massive carbon dioxide atmosphere which gives a surface pressure that is over 90 times the sea-level pressure here on Earth and a hellish average surface temperature of 740 degrees Kelvin. Near the surface of Venus, the temperature is above the melting points of metals such as lead, tin and zinc. However, at an altitude of 50 kilometres up in the Venusian atmosphere, the atmospheric temperature and pressure are similar to those found on the Earth at sea-level.

Radar observations of the surface of Venus have shown a brightening of the radar reflection from higher elevation regions on Venus. It is believed that the substance responsible for the higher radar reflectivity formed from a process that is similar to the formation of snow on Earth, albeit at a far higher temperature. The furnace-like environment of Venus’ lower atmosphere means that water is not a possible candidate material for this highly reflective substance. Instead, the highly reflective substance is likely to be a heavy metal frost consisting of one or more types of volatile compounds. In this case, the Venusian highlands serve as areas where the temperature is cool enough for these heavy metal compounds to condense and be deposited as frost. The source of these heavy metal compounds is likely to be volcanic in nature. On Earth, these heavy metal compounds are stable solids but the high temperatures on Venus allow many of these compounds to become volatile.

This is a radar image from NASA’s Magellan spacecraft centred along the eastern edge of Lakshmi Planum and the western edge of Maxwell Montes. The highlands on the right are covered in bright “snow” and are 5 kilometres above the above the adjacent plains in Lakshmi Planum. Credit: NASA/JPL

Standing 11 kilometres high, Maxwell Montes is the tallest mountain on Venus and with a temperature of 650 degrees Kelvin at its summit, the top of Maxwell Montes is the coolest location on the surface of Venus. Radar observations of Maxwell Montes show that most of the mountain is covered a layer of highly reflective substance. For this reason, Maxwell Montes serves as a good example of a cool highland region that is covered by a layer of heavy metal frost and as a “snow-capped mountain” on Earth’s scorching planetary neighbour.

Sunday, June 10, 2012

Ice Floats on Titan’s Lakes

Seas and lakes of liquid hydrocarbons are known to exist on the surface of Titan beneath a thick atmosphere of nitrogen and methane. It is a common assumption that ice on a liquid hydrocarbon lake is negatively buoyant, causing any ice which forms on the lake’s surface to sink towards the bottom. This is because methane ice is denser than liquid methane and this results in a behaviour that is opposite of water where water ice is less dense than liquid water. A paper by Roe and Grundy (2012) titled “Buoyancy of ice in the CH4-N2 system” suggests that contrary to common assumption, the conditions that exist on Titan can allow ice to float on a liquid hydrocarbon lake.

Cassini delivers this stunning vista showing small, battered Epimetheus and smog-enshrouded Titan, with Saturn's A and F rings stretching across the scene. Credit: NASA/JPL/Space Science Institute

Given that nitrogen is the primary constituent of Titan’s atmosphere and nitrogen is soluble in a hydrocarbon mixture, a substantial presence of dissolved nitrogen is expected in the hydrocarbon lakes on Titan. In this study, heavier hydrocarbons are ignored and a liquid solution with a mole fraction abundance of 30.7 percent nitrogen and 69.3 percent methane at a temperature of 88.2 degrees Kelvin is used. In order for ice to start forming, the liquid solution has to be cooled to 78.1 degrees Kelvin where the ice that is formed will have a mole fraction abundance of 16.5 percent nitrogen and 83.5 percent methane. At 78.1 degrees Kelvin, the liquid solution has a density of 0.574 grams per cubic centimetre.

Two methods are used to estimate the density of the ice that is formed. The first method assumes an ideal solution where the weighted mean of the densities of nitrogen and methane are summed up to give a density of 0.564 grams per cubic centimetre. The second method uses a lattice replacement assumption where the lattice structure of the methane ice remains unchanged while a nitrogen molecule replaces a methane molecule and this gives a density of 0.549 grams per cubic centimetre. In either case, the density of the ice is less than the density of the liquid solution and the ice will float. Hence, the decrease in density from the decrease in nitrogen abundance of the ice is larger than the increase in density from the freezing of the liquid solution.

A liquid solution of nitrogen and methane is a simplification since Titan’s lakes contain a mixture of other hydrocarbons and a more accurate study will need to consider these other hydrocarbons. The seasonal variation in surface temperature on Titan is unlikely to be sufficient to allow ice to form on Titan’s lakes. However, it is still possible for ice to be present on the surface of a lake on Titan after a hail storm or a torrential downpour of methane. A hail storm that is large enough can even provide sufficient cooling to freeze the surface of the lake. Raindrops of methane arriving at the surface of a lake from a torrential downpour are expected to be cooler than the local atmosphere and contain some amount of dissolved nitrogen. As the lake consists of a mixture of heavier hydrocarbons, this newly precipitated liquid solution of nitrogen and methane is expected to be less dense and will form a floating layer on the lake’s surface. The volume of this layer can be greatly boosted by drainage from surrounding terrain. Cooling delivered by winds can cause ice to form on this cooler and less dense surface layer of the lake.

When ice floats on the surface of a lake, it isolates the rest of the lake from the atmosphere and allows it to remain liquid beneath. Floating ice also affects the rate of evaporation of methane back to the atmosphere and this can have huge effects on the climate and atmosphere of Titan if lake evaporation is a significant contributor to the atmospheric abundance of methane.