Bakersfield Night Sky – February 4, 2012
By Nick Strobel
Other places beyond the Earth with liquid water is the subject of this month's public evening show at the William M Thomas Planetarium. On February 23rd, the William M Thomas Planetarium at Bakersfield College will show "Oasis in Space" that first shows where the water molecules in our solar system came from and then where we might find water in its liquid form beyond the Earth. The show begins at 7:30 PM with a tour of the evening sky and then continues with the full-dome presentation of Oasis in Space. Tickets must be purchased ahead of time from the BC Ticket Office. In today's column I'm going to give some background on why we are focussing on water as the liquid medium of choice in our search for life beyond the Earth and also about habitable zones of stars.
The habitable zone is defined as the region around a star where the temperature on the surface of a planet orbiting the star would be between the freezing point (0º C) and boiling point (100º C) of water. First of all, why do we constrain ourselves to water on the surface? That's because life on the surface would be able to affect the chemistry of the atmosphere and it is the planet's atmosphere that we can determine the detailed composition of from far away (as in light years away). By gathering light that has passed through the planet's atmosphere and then spreading the light out into its individual colors to make a spectrum, we may be able to detect spectral signatures of certain compounds in certain proportions that could not be produced by non-biological processes, otherwise known as "bio-markers". Life existing far below the surface out of contact with the surface would require landers to detect and our fastest spacecraft would take over 70,000 years to travel to even the next star system. So life on the surface is what we're looking for and we think life on the surface will need liquid water on the surface.
Why liquid water? First, why a liquid? Bio-chemical reactions will not happen in solids (so the Horta rock creature of the classic Star Trek is out) and they would be very inefficient in a gas because the atoms are so far apart from each other. Now for water. While liquid methane/ethane like what is found on Saturn's moon Titan or liquid ammonia might work as a medium for biochemical reactions, water has several advantages. Liquid water dissolves other compounds better than the other possible common liquids (methane, ethane, and ammonia) and biochemical reactions work better in liquid water than those other liquids. Water is liquid at a wide temperature range and that temperature for liquid water is higher than the very low temperatures needed to liquify methane, ethane, and ammonia. That means that chemical reactions will happen more quickly in the liquid water than in the other liquids. That means that water-based life would have a faster metabolism than methan/ethane/ammonia-based life and easier to detect. Also, frozen water floats! When water freezes it becomes less dense, something that methane, ethane, and ammonia do not do. The other types of liquids sink when they freeze and that could lead to a runaway freezing process where all of the liquid freezes (solidifies) because the liquid would always be forced up to the surface to freeze. With water, the floating ice provides an insulating layer above to protect the liquid water below. That is why it is possible for there to be lakes below the glaciers of Antarctica or the icy surfaces of Jupiter's moon, Europa, and Saturn's moon, Enceladus. Finally, water is able to transfer a large amount of heat without changing its temperature much. That would help keep the inhabited environment's temperature stable, another requirement of a habitable planet. By the way, water's high heat of vaporization is also what is responsible for driving our weather as water condenses and evaporates to distribute energy around the globe. Oh, one last thing: water in some form (mostly either gas or solid) is actually quite abundant in our Galaxy, so we are not limiting ourselves too much with the water bias. Plenty of real estate to keep astronomers happy exploring for centuries.
Last week the Kepler mission announced the discovery of 11 new planetary systems, increasing their number of planets by 26. This particular set of planets orbit inside their stars' habitable zones. Of the total 61 confirmed planets found by Kepler, the smallest confirmed planet that resides in a star's habitable zone is Kepler 22b, a planet about 2.4 times the diameter of the Earth. We don't know its mass yet, so we don't know its density (or is mass/volume). Its density will tell us if it is predominantly rocky, gaseous, or liquid in composition. The planet is about 620 light years away and talkes 290 days to orbit its star. Because Kepler 22 is slightly cooler than the Sun, its habitable zone is slightly smaller than the Sun's habitable zone. There over 2300 planet candidates that await confirmation by follow-up observations and almost 50 of them orbit within their stars' habitable zones.
Back here on the planet orbiting near the inside edge of the Sun's habitable zone in the second month of 2012, we see two bright planets in the evening sky, one that orbits the Sun inside the Sun's habitable zone (Venus) visible low in the southwest after sunset and the other a large planet orbiting well outside the Sun's habitable zone (Jupiter) visible higher in the southwest just after sunset. Jupiter has three large moons with signs of liquid water below their icy surfaces. The smallest of those three moons, Europa, has multiple pieces of evidence for a liquid, salt-water ocean below its icy surface. Furthermore, it appears that there may be giant shallow lakes within the top icy layer that could provide a way of exchanging material between the surface and the ocean beneath. You can learn more about that from the NASA mid-November press conference on YouTube at http://www.youtube.com/watch?v=ldzRCO55cUw. Venus is now below the western (right) side of Pisces (to left of the tipped over Great Square of Pegasus) as it scoots toward Jupiter on the eastern (left) side of Pisces. See the first chart below for the view of Venus and Jupiter at 6:30 PM.
Tonight the Moon is almost full and it will be halfway up in the eastern sky at sunset. It will be full on Tuesday, February 7th. The planet in the outer edge of the Sun's habitable zone, Mars, will become visible in the east a little before 9 PM. It is now below Denebola at the tip of the tail of Leo—see second chart below. Mars is now undergoing retrograde motion so it will be drifting westward (rightward) back toward the front of Leo. Mars will almost reach Regulus at the end of the Sickle of Leo, corresponding to the chest of Leo, by the middle of April. We know that Mars had liquid water in the distant past and it may even have liquid water just below the surface today. Last August the Mars Reconnaissance Orbiter team released high-resolution images of dark streaks that re-form on crater walls every spring and summer on Mars. In an effort to not bias our interpretation of them, the team has called the features "recurring slope lineae" but the 2012 NASA Science calendar I received in January has a caption below the same picture that says "water streaks" (so much for not biasing our interpretation of the streaks). Finally, Saturn will become first visible in the east at about midnight. It is among the stars in the eastern part of Virgo, just a little north and east of the bright star Spica. Its brilliant rings reflect a lot of sunlight because they are made of trillions of bits of frozen water orbiting Saturn. Some of the water bits come from geysers on its small moon, Enceladus, that shoot tiny ice bits with such speeds to escape Enceladus' gravity. The third chart below shows the view of Saturn and Mars at 6 AM.
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com