Extending missions beyond their initial goals is much on my mind as we consider the future of New Horizons and its possible flyby past a Kuiper Belt Object. But this morning I’m also reminded of EPOXI, which has given us views of the Earth that help us study what a terrestrial world looks like from a distance, characterizing our own planet as if it were an exoplanet. You’ll recall that EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) is a follow-on to another successful mission, the Deep Impact journey to comet Tempel 1.
As is clear from its acronym, EPOXI combined two extended missions, one following up the Tempel 1 studies with a visit to comet Hartley 2 (this followed an unsuccessful plan to make a flyby past comet 85P/Boethin, which proved to be too faint for accurate orbital calculations). The extrasolar component of EPOXI was called EPOCh (Extrasolar Planet Observation and Characterization), using the craft’s high resolution telescope to make photometric observations of stars with known transiting exoplanets. But the spacecraft produced observations of Earth that have been useful for exoplanet studies, as well as recording some remarkable views.
Image: Four images from a sequence of photos taken by the Deep Impact spacecraft when it was 50 million km from the Earth. Africa is at right. Notice how much darker the moon is compared to Earth. It reflects only as much light as a fresh asphalt road. Credit: Donald J. Lindler, Sigma Space Corporation, GSFC, Univ. Maryland, EPOCh/DIXI Science Teams.
Although communications with EPOXI were lost in the summer of 2013, the mission lives on in the form of the data it produced, some of which are again put to use in a new paper out of the University of Washington. Edward Schwieterman, a doctoral student and lead author on the work in collaboration with the university’s Victoria Meadows, reports on Earth observations from EPOXI that have been compared to three-dimensional planet-modeling data from the university’s Virtual Planet Laboratory. The comparison has allowed confirmation of the signature of nitrogen collisions in our atmosphere, a phenomenon that should have wide implications.
The presence of nitrogen is significant because it can help us determine whether an exoplanet’s surface pressure is suitable for the existence of liquid water. Moreover, if we find nitrogen and oxygen in an atmosphere and are able to measure the nitrogen accurately, we can use the nitrogen as a tool for ruling out non-biological origins for the oxygen. But nitrogen is hard to detect, and the best way to find it in a distant planet’s atmosphere is to measure how nitrogen molecules collide with each other. The paper argues that these ‘collisional pairs’ create a signature we can observe, something the team has modeled and that the EPOXI work has confirmed.
Nitrogen pairs, written as (N2)2, are visible in a spectrum at shorter wavelengths, giving us a useful tool. The paper explains how this works:
A comprehensive study of a planetary atmosphere would require determination of its bulk properties, such as atmospheric mass and composition, which are crucial for ascertaining surface conditions. Because (N2)2 is detectable remotely, it can provide an extra tool for terrestrial planet characterization. For example, the level of (N2)2 absorption could be used as a pressure metric if N2 is the bulk gas, and break degeneracies between the abundance of trace gases and the foreign pressure broadening induced by the bulk atmosphere. If limits can be set on surface pressure, then the surface stability of water may be established if information about surface temperature is available.
It’s interesting as well that for half of Earth’s geological history, there was little oxygen present, despite the presence of life for a substantial part of this time. The paper argues that given Earth’s example, there may be habitable and inhabited planets without O2 we can detect. Moreover, atmospheres with low abundances of gases like N2 and argon are more likely to accumulate O2 abiotically, giving us a false positive for life.
A water dominated atmosphere lacks a cold trap, allowing water to more easily diffuse into the stratosphere and become photo-dissociated, leaving free O2 to build up over time. Direct detection of N2 through (N2)2 could rule out abiotic O2 via this mechanism and, in tandem with detection of significant O2 or O3, potentially provide a robust biosignature. Moreover, the simultaneous detection of N2, O2, and a surface ocean would establish the presence of a significant thermodynamic chemical disequilibrium (Krissansen-Totton et al. 2015) and further constrain the false positive potential.
Combining the EPOXI data with the Virtual Planetary Laboratory modeling demonstrates that nitrogen collisions that are apparent in our own atmosphere should likewise be apparent in exoplanet studies by future space telescopes. EPOXI, then, demonstrated that nitrogen collisions could be found in a planetary spectrum, and the VPL work modeling a variety of nitrogen abundances in an exoplanet atmosphere shows how accurately the gas can be measured. “One of the interesting results from our study,” adds Schwieterman, “is that, basically, if there’s enough nitrogen to detect at all, you’ve confirmed that the surface pressure is sufficient for liquid water, for a very wide range of surface temperatures,”
The paper is Schwieterman et al., “Detecting and Constraining N2 Abundances in Planetary Atmospheres Using Collisional Pairs,” The Astrophysical Journal Vol. 810, No. 1 (28 August 2015). Abstract / preprint.
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The Deep Impact spacecraft’s view of Earth from 50 million km is close to the view an imaginary Martian astronomer would have from Mars orbit during a good opposition. Recall H.G. Wells: “… intellects vast and cool and unsympathetic, regarded this earth with envious eyes…” During the very good August 2003 opposition, Mars came as close as 55.7 million km from Earth.
“The presence of nitrogen is significant because it can help us determine whether an exoplanet’s surface pressure is suitable for the existence of liquid water.” […]
“If limits can be set on surface pressure, then the surface stability of water may be established if information about surface temperature is available.”
This is reminiscent of the history of observations and estimates of the thickness and composition of the atmosphere of Mars…
In “Mars as the Abode of Life” (1908), Percival Lowell estimated (pp. 238-40) that the barometric pressure on the surface of Mars is 64 mm of Mercury, compared to the 760 mm Standard Atmosphere on Earth, or 8.4%. This estimate did not change significantly until the Mariner 4 flyby in July 1965.
In “The Planet Mars” (1951), for example, Gérard de Vaucouleurs wrote that the atmospheric pressure of Mars “appears to be in the region of 6 or 7 cm. (2.5 in.) of mercury near the ground (about 90 mb.), that is to say, nearly one-tenth of ours.” (p. 54) The 1961 report of the Space Science Board to NASA (William W. Kellogg and Carl Sagan, “The Atmospheres of Mars and Venus”) came up with an estimate close to that of de Vaucouleurs.
The average atmospheric pressure on the Martian surface is now known to be just 0.6 percent of Earth’s mean sea level pressure. The Martian air is so thin that liquid water cannot exist on the surface of the planet. Water ice sublimes directly to water vapour without passing through a liquid phase.
Despite the inability to detect nitrogen spectroscopically, de Vaucouleurs wrote that “the Martian air is likely to be composed mainly of nitrogen.” (p. 45) It was known that the Martian atmosphere contained a small amount of carbon dioxide. What de Vaucouleurs couldn’t guess was that this small amount of carbon dioxide constituted the bulk of Mars’s atmosphere! The total Martian atmospheric pressure is close to the carbon dioxide pressure determined using a spectroscope from Earth. The Martian atmosphere consists of 96% carbon dioxide, with only 1.9% nitrogen and 1.9% argon.
I like the concept, but looking at the spectral data in the paper, I am somewhat skeptical that this can be used on exoplanets without much better spectral data.
Can anyone familiar with spectroscopy of exoplanets weigh in on the viability of this technique today and with reference to upcoming telescopes?.
This is something that the late Patrick Moore emphasized in his astronomy books, that “nitrogen is very shy about showing itself in a planet’s spectrum.”
The high density of the atmosphere of Titan (a “mere satellite” of Saturn), which gives it a surface pressure ~50% *greater* than Earth’s, surprised astronomers for the same reason. Its atmosphere’s methane was detected spectroscopically from Earth as far back as 1944, and it was thought to be a quite thin atmosphere, but the thick, mostly-nitrogen atmosphere that the Saturn flyby spacecraft found was unexpected. At Neptune, Voyager 2 found that Triton’s atmosphere, while far thinner than even Mars’ atmosphere, was also unexpectedly dense due to the presence of unseen (from Earth) nitrogen.
While the nitrogen collision detection method is welcome news, I suspect that this detection technology will require considerable advancement in lowering the detection threshold before it can provide meaningful results for exoplanets’ spectrograms. But this is also a strong argument for developing more powerful dedicated exoplanet telescopes.
And finding oxygen does not always mean there is life:
Of course, but it also helps to eliminate other causes, e.g. photo dissociation alone.