Cold trap (astronomy)

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A cold trap is a concept in planetary sciences that describes an area cold enough to freeze (trap) volatiles. Cold-traps can exist on the surfaces of airless bodies or in the upper layers of an adiabatic atmosphere. On airless bodies, the ices trapped inside cold-traps can potentially remain there for geologic time periods, allowing us a glimpse into the primordial solar system. In adiabatic atmospheres, cold-traps prevent volatiles (such as water) from escaping the atmosphere into space.

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Cold-traps on airless planetary bodies

The floor of Prokofiev crater near Mercury's north pole never experiences sunlight. Prokofiev crater EW0254741735G.jpg
The floor of Prokofiev crater near Mercury's north pole never experiences sunlight.

The obliquity (axial tilt) of some airless planetary bodies in the Solar System such as Mercury, the Moon and Ceres is very close to zero. Harold Urey first noted that depressions or craters located near the poles of these bodies will cast persistent shadows that can survive for geologic time periods (millions–billions of years). [1] The absence of an atmosphere prevents mixing by convection, rendering these shadows extremely cold. [2] If molecules of volatiles such as water ice travel into these permanent shadows, they will become trapped for geologic time periods. [3]

Studying cold-traps on airless bodies

As these shadows receive no insolation, most of the heat they receive is scattered and emitted radiation from the surrounding topography. Usually, horizontal heat conduction from adjacent warmer areas can be neglected due to the high porosity and therefore low thermal conductivity of the uppermost layers of airless bodies. Consequently, the temperatures of these permanent shadows can be modeled using ray casting or ray tracing algorithms coupled with 1D vertical heat conduction models. [4] [2] In some cases, such as bowl-shaped craters, it is possible to obtain an expression for the equilibrium temperature of these shadows. [5]

Additionally, the temperatures (and therefore the stability) of cold-traps can be remotely sensed by an orbiter. The temperatures of lunar cold-traps have been extensively studied by the Lunar Reconnaissance Orbiter Diviner radiometer. [6] On Mercury, evidence for ice deposits inside cold-traps has been obtained through radar, [7] reflectance [8] [9] and visible imagery. [10] On Ceres, cold-traps have been detected by the Dawn spacecraft. [11]

Atmospheric cold-traps

In atmospheric science, a cold-trap is a layer of the atmosphere that is substantially colder than both the deeper and higher layers. For example, for Earth's troposphere, the temperature of the air drops with increasing height reaching a low point (at about 20 kilometers height). This region is called a cold-trap, because it traps ascending gases with high boiling points, forcing them to drop back into Earth.[ citation needed ]

For biological life-forms on Earth, the most important gas to be kept in that way is water vapor. Without the presence of a cold-trap in the atmosphere, the water content would gradually escape into space, making life impossible. The cold trap retains one-tenth of a percent of the water in the atmosphere in the form of a vapor at high altitudes. Earth's cold-trap is also a layer above which ultraviolet intensity is strong, since higher up the amount of water vapor is negligible. Oxygen screens out ultraviolet intensity.[ citation needed ]

Some astronomers believe that the lack of a cold trap is why the planets Venus and Mars both lost most of their liquid water early in their histories. [12] The Earth's cold trap is located about 12 km above sea level, well below the height in which water vapor would be permanently split apart into hydrogen and oxygen by solar UV rays and the former irreversibly being lost to space. Because of the cold trap in the Earth's atmosphere, the Earth is actually losing water to space at a rate of only 1 millimeter of ocean every 1 million years, which is too slow to affect changes in sea levels on any timescales relevant to humans, compared to the current rate of sea level rise at a rate of 3 millimeters every single year due to ongoing human-caused climate change melting the polar ice caps combined with thermal expansion of seawater. At that rate it would take trillions of years, far longer than Earth's life expectancy, for all of its water to disappear (this is also why, due to human-caused climate change, extreme weather events like hurricanes and floods will intensify in the near term, as a warmer atmosphere can hold more moisture, and therefore increase the amount of said water vapor returning as precipitation, as even then the cold trap will still prevent said water vapor from being lost to space, and therefore Earth's atmosphere is still too cold for such to happen), although the eventual warming of the Sun as it ages will only make the cold trap weaker over the next billion years by making the Earth's atmosphere even warmer, which pushes the cold trap even higher into the atmosphere, and therefore causing it to lose the ability to prevent any water vapor from being dissociated back into hydrogen and oxygen by the Sun's UV rays and the former escaping into space, leading to the Earth ultimately losing its oceans to space in about 1 billion years' time, long before the Sun finally expands into a red giant.

As pointed out by Peter Ward and Donald Brownlee in their book The Life and Death of Planet Earth , the current process of the actual loss of oceans was only documented twice, first during the Apollo 16 Moon mission (although by accident, which involved the mission's astronauts observing Earth via a unique Carruthers camera that was both created and used only once, for that particular mission, as such a process can only be viewed under UV light and from the Moon, due to it lacking an atmosphere to block out said UV light), and again during the 1990s via studies from astronauts taken while aboard the Space Shuttle.

Saturn's moon Titan has a very weak cold trap which is only able to hang on to some of its atmospheric methane. [13] Thus, it has been suggested that Titan is the closest analog to what Earth's atmosphere will look like as Earth's cold trap fails, with methane instead of water, and hydrocarbon products of photochemistry instead of oxygen and ozone. [14]

Cold traps are thought to function for oxygen on Ganymede. [15]

Related Research Articles

<span class="mw-page-title-main">Mercury (planet)</span> First planet from the Sun

Mercury is the first planet from the Sun and the smallest in the Solar System. In English, it is named after the ancient Roman god Mercurius (Mercury), god of commerce and communication, and the messenger of the gods. Mercury is classified as a terrestrial planet, with roughly the same surface gravity as Mars. The surface of Mercury is heavily cratered, as a result of countless impact events that have accumulated over billions of years. Its largest crater, Caloris Planitia, has a diameter of 1,550 km (960 mi) and one-third the diameter of the planet. Similarly to the Earth's Moon, Mercury's surface displays an expansive rupes system generated from thrust faults and bright ray systems formed by impact event remnants.

<span class="mw-page-title-main">Ganymede (moon)</span> Largest moon of Jupiter and in the Solar System

Ganymede, or Jupiter III, is the largest and most massive natural satellite of Jupiter and in the Solar System. It is the largest Solar System object without a substantial atmosphere, despite being the only moon in the Solar System with a substantial magnetic field. Like Titan, Saturn's largest moon, it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io, or the Moon due to its lower density compared to the three.

<span class="mw-page-title-main">Tropopause</span> The boundary of the atmosphere between the troposphere and stratosphere

The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere, which are the lowest two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer that marks the end of the troposphere, and is approximately 17 kilometres (11 mi) above the equatorial regions, and approximately 9 kilometres (5.6 mi) above the polar regions.

<span class="mw-page-title-main">Shackleton (crater)</span> Lunar impact crater

Shackleton is an impact crater that lies at the lunar south pole. The peaks along the crater's rim are exposed to almost continual sunlight, while the interior is perpetually in shadow. The low-temperature interior of this crater functions as a cold trap that may capture and freeze volatiles shed during comet impacts on the Moon. Measurements by the Lunar Prospector spacecraft showed higher than normal amounts of hydrogen within the crater, which may indicate the presence of water ice. The crater is named after Antarctic explorer Ernest Shackleton.

<span class="mw-page-title-main">Cabeus (crater)</span> Lunar impact crater

Cabeus is a lunar impact crater that is located about 100 km (62 mi) from the south pole of the Moon. At this location the crater is seen obliquely from Earth, and it is almost perpetually in deep shadow due to lack of sunlight. Hence, not much detail can be seen of this crater, even from orbit. Through a telescope, this crater appears near the southern limb of the Moon, to the west of the crater Malapert and to the south-southwest of Newton.

<span class="mw-page-title-main">Lunar water</span> Presence of water on the Moon

Lunar water is water that is present on the Moon. The search for the presence of lunar water has attracted considerable attention and motivated several recent lunar missions, largely because of water's usefulness in making long-term lunar habitation feasible.

<span class="mw-page-title-main">Atmosphere of Mars</span> Layer of gases surrounding the planet Mars

The atmosphere of Mars is the layer of gases surrounding Mars. It is primarily composed of carbon dioxide (95%), molecular nitrogen (2.85%), and argon (2%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen, and noble gases. The atmosphere of Mars is much thinner and colder than Earth's having a max density 20g/m3 with a temperature generally below zero down to -60 Celsius. The average surface pressure is about 610 pascals (0.088 psi) which is 0.6% of the Earth's value.

<span class="mw-page-title-main">Space weathering</span> Type of weathering

Space weathering is the type of weathering that occurs to any object exposed to the harsh environment of outer space. Bodies without atmospheres take on many weathering processes:

In astronomy or planetary science, the frost line, also known as the snow line or ice line, is the minimum distance from the central protostar of a solar nebula where the temperature is low enough for volatile compounds such as water, ammonia, methane, carbon dioxide and carbon monoxide to condense into solid grains, which will allow their accretion into planetesimals. Beyond the line, otherwise gaseous compounds can be quite easily condensed to allow formation of gas and ice giants; while within it, only heavier compounds can be accreted to form the typically much smaller rocky planets.

<span class="mw-page-title-main">Ocean world</span> Planet containing a significant amount of water or other liquid

An ocean world, ocean planet or water world is a type of planet that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava, ammonia or hydrocarbons. The study of extraterrestrial oceans is referred to as planetary oceanography.

<span class="mw-page-title-main">Climate of Mars</span>

The climate of Mars has been a topic of scientific curiosity for centuries, in part because it is the only terrestrial planet whose surface can be easily directly observed in detail from the Earth with help from a telescope.

<span class="mw-page-title-main">Extraterrestrial atmosphere</span> Area of astronomical research

The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earth's atmosphere. In addition to Earth, many of the other astronomical objects in the Solar System have atmospheres. These include all the giant planets, as well as Mars, Venus and Titan. Several moons and other bodies also have atmospheres, as do comets and the Sun. There is evidence that extrasolar planets can have an atmosphere. Comparisons of these atmospheres to one another and to Earth's atmosphere broaden our basic understanding of atmospheric processes such as the greenhouse effect, aerosol and cloud physics, and atmospheric chemistry and dynamics.

The presence of water on the terrestrial planets of the Solar System varies with each planetary body, with the exact origins remaining unclear. Additionally, the terrestrial dwarf planet Ceres is known to have water ice on its surface.

<span class="mw-page-title-main">Lunar south pole</span> Southernmost point on the Moon

The lunar south pole is the southernmost point on the Moon. It is of interest to scientists because of the occurrence of water ice in permanently shadowed areas around it. The lunar south pole region features craters that are unique in that the near-constant sunlight does not reach their interior. Such craters are cold traps that contain fossil records of hydrogen, water ice, and other volatiles dating from the early Solar System. In contrast, the lunar north pole region exhibits a much lower quantity of similarly sheltered craters.

<span class="mw-page-title-main">Planetary surface</span> Where the material of a planetary masss outer crust contacts its atmosphere or outer space

A planetary surface is where the solid or liquid material of certain types of astronomical objects contacts the atmosphere or outer space. Planetary surfaces are found on solid objects of planetary mass, including terrestrial planets, dwarf planets, natural satellites, planetesimals and many other small Solar System bodies (SSSBs). The study of planetary surfaces is a field of planetary geology known as surface geology, but also a focus on a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences, and astronomy. Land is the term given to non-liquid planetary surfaces. The term landing is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached.

<span class="mw-page-title-main">Permanently shadowed crater</span> Permanently shadowed region of a body in the Solar System

A permanently shadowed crater is a depression on a body in the Solar System within which lies a point that is always in darkness.

Comparative planetary science or comparative planetology is a branch of space science and planetary science in which different natural processes and systems are studied by their effects and phenomena on and between multiple bodies. The planetary processes in question include geology, hydrology, atmospheric physics, and interactions such as impact cratering, space weathering, and magnetospheric physics in the solar wind, and possibly biology, via astrobiology.

<span class="mw-page-title-main">Lunar resources</span> In situ resources on the Moon

The Moon bears substantial natural resources which could be exploited in the future. Potential lunar resources may encompass processable materials such as volatiles and minerals, along with geologic structures such as lava tubes that, together, might enable lunar habitation. The use of resources on the Moon may provide a means of reducing the cost and risk of lunar exploration and beyond.

Lunar Trailblazer is a planned small lunar orbiter, part of NASA's SIMPLEx program, that will detect and map water on the lunar surface to determine how its form, abundance, and location relate to geology. Its mission is to aid in the understanding of lunar water and the Moon's water cycle. Lunar Trailblazer is currently slated to launch in 2024 as a secondary payload on the IM-2 mission. The Principal Investigator (PI) of the mission is Bethany Ehlmann, a professor at Caltech.

<span class="mw-page-title-main">Planetary habitability in the Solar System</span> Habitability of the celestial bodies of the Solar System

Planetary habitability in the Solar System is the study that searches the possible existence of past or present extraterrestrial life in those celestial bodies. As exoplanets are too far away and can only be studied by indirect means, the celestial bodies in the Solar System allow for a much more detailed study: direct telescope observation, space probes, rovers and even human spaceflight.

References

  1. ↑ Lucey, P. G. (2009). "The Poles of the Moon". Elements. 5 (1): 41–6. doi:10.2113/gselements.5.1.41.
  2. 1 2 Rubanenko, Lior; Aharonson, Oded (2017). "Stability of ice on the Moon with rough topography". Icarus. 296: 99–109. Bibcode:2017Icar..296...99R. doi:10.1016/j.icarus.2017.05.028.
  3. ↑ Watson, Kenneth; Murray, Bruce C.; Brown, Harrison (1961). "The behavior of volatiles on the lunar surface" (PDF). Journal of Geophysical Research. 66 (9): 3033–45. Bibcode:1961JGR....66.3033W. doi:10.1029/JZ066i009p03033.
  4. ↑ Vasavada, A; Paige, David A.; Wood, Stephen E. (1999). "Near-Surface Temperatures on Mercury and the Moon and the Stability of Polar Ice Deposits". Icarus. 141 (2): 179–93. Bibcode:1999Icar..141..179V. doi:10.1006/icar.1999.6175.
  5. ↑ Buhl, David; Welch, William J.; Rea, Donald G. (1968). "Reradiation and thermal emission from illuminated craters on the lunar surface". Journal of Geophysical Research. 73 (16): 5281–95. Bibcode:1968JGR....73.5281B. doi:10.1029/JB073i016p05281.
  6. ↑ Paige, D. A.; Siegler, M. A.; Zhang, J. A.; Hayne, P. O.; Foote, E. J.; Bennett, K. A.; Vasavada, A. R.; Greenhagen, B. T.; Schofield, J. T.; McCleese, D. J.; Foote, M. C.; Dejong, E.; Bills, B. G.; Hartford, W.; Murray, B. C.; Allen, C. C.; Snook, K.; Soderblom, L. A.; Calcutt, S.; Taylor, F. W.; Bowles, N. E.; Bandfield, J. L.; Elphic, R.; Ghent, R.; Glotch, T. D.; Wyatt, M. B.; Lucey, P. G. (2010). "Diviner Lunar Radiometer Observations of Cold Traps in the Moon's South Polar Region". Science. 330 (6003): 479–82. Bibcode:2010Sci...330..479P. doi:10.1126/science.1187726. PMID   20966246. S2CID   12612315.
  7. ↑ Harmon, J; Perillat, P. J.; Slade, M. A. (2001). "High-Resolution Radar Imaging of Mercury's North Pole". Icarus. 149 (1): 1–15. Bibcode:2001Icar..149....1H. doi:10.1006/icar.2000.6544.
  8. ↑ Neumann, G. A.; Cavanaugh, J. F.; Sun, X.; Mazarico, E. M.; Smith, D. E.; Zuber, M. T.; Mao, D.; Paige, D. A.; Solomon, S. C.; Ernst, C. M.; Barnouin, O. S. (2012). "Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles". Science. 339 (6117): 296–300. Bibcode:2013Sci...339..296N. doi: 10.1126/science.1229764 . PMID   23196910. S2CID   206544976.
  9. ↑ Rubanenko, L.; Mazarico, E.; Neumann, G. A.; Paige, D. A. (2017). "Evidence for Surface and Subsurface Ice Inside Micro Cold-Traps on Mercury's North Pole". 48th Lunar and Planetary Science Conference. 48 (1964): 1461. Bibcode:2017LPI....48.1461R.
  10. ↑ Chabot, N. L.; Ernst, C. M.; Denevi, B. W.; Nair, H.; Deutsch, A. N.; Blewett, D. T.; Murchie, S. L.; Neumann, G. A.; Mazarico, E.; Paige, D. A.; Harmon, J. K.; Head, J. W.; Solomon, S. C. (2014). "Images of surface volatiles in Mercury's polar craters acquired by the MESSENGER spacecraft". Geology. 42 (12): 1051–4. Bibcode:2014Geo....42.1051C. doi:10.1130/G35916.1.
  11. ↑ Schorghofer, Norbert; Mazarico, Erwan; Platz, Thomas; Preusker, Frank; Schröder, Stefan E.; Raymond, Carol A.; Russell, Christopher T. (2016). "The permanently shadowed regions of dwarf planet Ceres". Geophysical Research Letters. 43 (13): 6783–9. Bibcode:2016GeoRL..43.6783S. doi: 10.1002/2016GL069368 .
  12. ↑ Strow, Thompson (1977). Astronomy: Fundamentals and Frontiers. Quinn & Boden. p. 425.
  13. ↑ "Titan and Earth's Future Atmospheres: Lost to Space". NASA Solar System Exploration. August 26, 2009.
  14. ↑ Lunine, J. I. (Feb 2009). "Titan as an analog of Earth's past and future". EPJ Web of Conferences. 1: 267–274. Bibcode:2009EPJWC...1..267L. doi: 10.1140/epjconf/e2009-00926-7 . S2CID   54545487.
  15. ↑ Vidal, R. A.; Bahr, D.; Baragiola, R. A.; Peters, M. (1997). "Oxygen on Ganymede: Laboratory Studies". Science. 276 (5320): 1839–42. Bibcode:1997Sci...276.1839V. doi:10.1126/science.276.5320.1839. PMID   9188525.
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